docs(changelog): v2.6.5-panes-tabs-composer

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 All notable changes per release tag. Most recent on top, ordered by tag creation date (which matches the git history). Tag names follow `vMAJOR.MINOR.PATCH-slug` — the slug describes what shipped, so the tag name alone is enough to recall the batch.
 
+## v2.6.5-panes-tabs-composer — 2026-05-31
+
+A workspace UX batch across BooChat panes, tabs, and the composer, plus the persistence model that backs them. **Panes & tabs:** a chat can be opened in a fresh pane (the ChatTabBar tab context menu's "Open in new pane", and the fork button — which now lands the fork beside the original via a new `open_chat_in_new_pane` event instead of replacing the active pane); the per-pane "+" became a New BooChat/BooTerm/BooCode menu; closing a chat pane relocates its tabs (in order) into the oldest chat/empty pane instead of discarding them, and reopen strips the restored chatIds from every live pane first so a relocated-then-reopened pane never duplicates a tab (no stack-shape change); each tab carries a stable session-scoped number assigned on open and retired on close (never reused), rendered map-keyed rather than positional. The per-message "Open in pane" artifact button was removed, and the empty/landing pane became a real session history — the session's open chats plus separately-fetched archived chats, click to open or restore-and-open. **Persistence:** `sessions.workspace_panes` was widened from a bare `WorkspacePane[]` to a `WorkspaceState` envelope (`panes` + `tabNumbers`/`nextTabNumber` + `closedPaneStack`) so tab numbers and the reopen stack survive reload; the PATCH validator accepts the legacy array or the envelope (zod union) and migrates on write, and the `session_workspace_updated` WS-frame schema was widened on both web and server (byte-identical, parity test green) — the same schema-drift class as `v2.6.4-agent-sessions-fk`. **Composer:** the send button morphs Send → Stop → Queue with generation state (BooCoder keys on `sending || activeTaskId`, which also corrected its queue gates and added `cancelTask`), the standalone "Stop generating" pill was folded into it, and pasted chips now trail the typed text so a leading slash command stays first. **Tooling:** adds the read-only `read_tab_by_number` tool — resolves a session-scoped tab number to its chat via the persisted `tabNumbers` map and returns that chat's transcript; tools gained an optional `ToolExecCtx` (`{ sql, sessionId }`) on `execute` to support DB-reading tools. Builds on `v2.6.4-agent-sessions-fk`.
+
+## v2.6.4-agent-sessions-fk — 2026-05-31
+
+Follow-up to `v2.6.3-chatkey-and-skills` (P1.5-b): the live `agent_sessions.session_id` foreign key is converged from `ON DELETE CASCADE` to `ON DELETE SET NULL`, matching the schema's stated intent. The P1.5-b re-key block re-adds `session_id_fkey` as `SET NULL`, but the whole block is guarded on `chat_id_fkey`'s absence — so a database already re-keyed to `(chat_id, agent)` while `session_id_fkey` was still `CASCADE` never re-enters it, leaving the live FK at `CASCADE` and diverging from both `worktree_id` (already `SET NULL`) and the `v2.6.3` changelog's own claim that `session_id` is informational `SET NULL`. The fix adds a standalone `confdeltype`-guarded `DO` block (mirroring the `session_worktrees` defang) that flips `session_id_fkey` `CASCADE → SET NULL` independently of the re-key gate; it is idempotent — fires only while the FK is still `'c'`, a no-op on a fresh deploy (already `'n'`) and on every re-run. The live DB was converged by hand with the identical statements, so `applySchema` and the hand-applied state match (`\d agent_sessions` now shows `session_id ... ON DELETE SET NULL`). Also bundles a CLAUDE.md doc-sync (committed separately): per-session SSE (P1.5-a) and the `(chat_id, agent)` re-key reflected in the engineering notes, the stale root `AGENTS.md` navigation pointer dropped, and new conventions for `data/AGENTS.md` parsing and the `data/skills/<vendor>/` layout.
+
 ## v2.6.3-chatkey-and-skills — 2026-05-31
 
 Three threads. **agent_sessions re-keyed to `(chat_id, agent)` (P1.5-b):** the tab (a chat) is now the agent-context unit, so two opencode tabs in one BooCode session are two independent contexts that share one worktree. `chat_id` is threaded end-to-end — `tasks.chat_id` added, stamped by the coder message + skills routes from the frontend tab, read by `runOpenCodeServerTask` which falls back to resolve-or-create a chat for session-less creators (arena/MCP/new_task/generic `/api/tasks`) so `ensureSession` never receives a degenerate `(null, agent)` key. A new first-class `worktrees` table (one-per-session, survives session delete via `session_id ON DELETE SET NULL`) supersedes `session_worktrees`, which is defanged (CASCADE dropped, not yet removed); `agent_sessions.chat_id` CASCADEs from `chats` (closing a tab ends its context) while `worktree_id`/`session_id` are informational `SET NULL`. The migration is idempotent with a backfill-verify gate; the live re-key was applied against an empty table after the 35-chat test session `20d28876` was deleted (backed up first). This corrects and supersedes an earlier draft that wrongly keyed on `(worktree_id, agent)`; the delete-guard from `v2.6.2-delete-guard-and-sse` is repointed here from `session_worktrees` to `worktrees` (`worktree_path`→`path`). **dcp-strip cross-chunk fix:** the `<dcp-message-id>` tag streams split across SSE deltas, which the per-chunk strip from `v2.6.1-phase1-opencode` missed — a stateful `makeDcpStreamStripper` at the dispatcher boundary holds back partial-tag tails so neither live frames nor persisted content carry the tag (11 unit tests). **Agent-judgment skills:** `committing-changes` (segment by concern, stage explicitly, present-and-stop, never push) and `using-worktrees` (the when-to-isolate heuristic, autonomous-when-clear vs committing's command-gate) land in `data/skills/boocode/` with eval.yamls, plus a parser-safe `data/AGENTS.md` preamble pointing at both.

CLAUDE.md

@@ -2,7 +2,7 @@
 
 This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
 
-**Cursor agents:** start with `AGENTS.md` (navigation) and `docs/ARCHITECTURE.md` (diagram). This file is the deep engineering reference.
+**Cursor agents:** start with `docs/ARCHITECTURE.md` (diagram). This file is the deep engineering reference. (Note: the root navigation `AGENTS.md` was removed in v1.12; `data/AGENTS.md` is the agent *registry*, not navigation.)
 
 ## What is BooCode
 
@@ -90,9 +90,9 @@ Route registration: all routes registered in `index.ts` via `register*Routes(app
 - **Provider snapshot lifecycle** (`apps/coder/src/services/`): `provider-config.ts` (Zod config, never-throws on bad input) → `provider-config-registry.ts` (`buildResolvedRegistry`, singleton) → `provider-snapshot.ts` (two-tier probe: tier-1 fast presence, tier-2 cold ACP probe skipped unless force / stale `PROVIDER_PROBE_TTL_MS` 24h / dbEmpty; cached). Verify live: `curl http://100.114.205.53:9502/api/providers/snapshot` — returns providers + models + commands, the exact shape `AgentComposerBar` renders.
 - `PATCH /api/providers/config` replaces a provider id's override object **wholesale** (per-id shallow merge) — to flip one field send `{...existing, enabled}`, or a custom ACP entry's `command`/`label` is wiped and it drops out of the resolved registry. `data/coder-providers.json` is **gitignored** (it's live runtime config — the coder reads AND writes it on UI toggles); the tracked reference is `data/coder-providers.example.json`. The loader falls back to `{providers:{}}` (built-ins only) when the live file is absent, so a fresh checkout needs no copy.
 - **opencode** runs as a warm HTTP server (v2.6 Phase 1, `services/backends/opencode-server.ts` — `opencode serve` per BooCoder process, one opencode session per BooCode session, resumed via `agent_sessions`). goose/qwen/claude still dispatch **one-shot** ACP/PTY with no ctx/token usage; only native `boocode` (llama-swap engine) tracks ctx. Paseo's per-provider native clients (design §12) deliberately not ported.
-- **opencode SSE** (`opencode-server.ts`): live streaming arrives as `session.next.text.delta` / `session.next.reasoning.delta` / `session.next.tool.{called,success,failed}` — NOT `message.part.*` (those are terminal/post-hoc). `client.event.subscribe({ directory })` MUST pass the session's worktree directory; omit it and opencode scopes events to the server's `process.cwd()` → zero session events (empty turns, 180s watchdog timeout). One SSE stream at a time scoped to the last session's dir — concurrent opencode sessions in different worktrees collide (known Phase 1 limit, warns). Turn completes on `session.idle`; `promptAsync` is fire-and-forget (204).
+- **opencode SSE** (`opencode-server.ts`): live streaming arrives as `session.next.text.delta` / `session.next.reasoning.delta` / `session.next.tool.{called,success,failed}` — NOT `message.part.*` (those are terminal/post-hoc). `client.event.subscribe({ directory })` MUST pass the session's worktree directory; omit it and opencode scopes events to the server's `process.cwd()` → zero session events (empty turns, 180s watchdog timeout). Per-session SSE (P1.5-a): each live session owns its own `event.subscribe({directory})` loop + AbortController, so concurrent sessions in different worktrees stream independently; a `sessionID` demux guard drops cross-session events when two share a dir. Turn completes on `session.idle`; `promptAsync` is fire-and-forget (204).
 - **opencode model strings** must be provider-prefixed (`llama-swap/<model>`) AND exist in `~/.config/opencode/opencode.json` `provider.llama-swap.models` — not merely loadable by llama-swap. `parseModel` infers `llama-swap/` for a bare id; the dispatcher coalesces empty→DEFAULT_MODEL then prefixes. `agent-probe` populates opencode's `available_agents.models` via `mergeLlamaSwap` (fetches `/v1/models`); empty model list → frontend sends `''` → no inference (`input:0`, empty turn).
-- **agent_sessions resume**: `config_hash = sha256('opencode_server|<model>')` — must NOT include the server port (random per boot; including it breaks cross-restart resume). `session_worktrees` + `agent_sessions` FKs to `sessions(id)` are `ON DELETE CASCADE` (else DELETE /api/sessions/:id 500s on FK violation). The `@opencode-ai/sdk` v2 client takes flattened params (`{sessionID, directory, parts, model:{providerID,modelID}}`), imports `createOpencodeClient` from `@opencode-ai/sdk/v2/client`.
+- **agent_sessions resume**: `config_hash = sha256('opencode_server|<model>')` — must NOT include the server port (random per boot; including it breaks cross-restart resume). P1.5-b: `agent_sessions` is keyed `(chat_id, agent)` — the tab/chat is the context unit (two opencode tabs in one session = two contexts sharing one worktree). `chat_id` CASCADEs from `chats`; `session_id`/`worktree_id` are informational `SET NULL`. The `worktrees` table (one-per-session, `session_id` SET NULL so it survives session delete) supersedes the defanged `session_worktrees`. `tasks.chat_id` threads the tab id to the dispatcher; `runOpenCodeServerTask` falls back to resolve-or-create a chat when it's null (arena/MCP/new_task). The `@opencode-ai/sdk` v2 client takes flattened params (`{sessionID, directory, parts, model:{providerID,modelID}}`), imports `createOpencodeClient` from `@opencode-ai/sdk/v2/client`.
 
 ### Frontend (`apps/web/src/`)
 
@@ -192,6 +192,8 @@ BooCoder at port 9502: `curl http://100.114.205.53:9502/api/health`. Runs as `bo
 - **ReadableStream test stubs** use `pull()` (not `start()`) so chunks are produced lazily — `start()` enqueues everything and calls `controller.close()` before the consumer reads, so a subsequent `reader.cancel()` finds the stream already closed and the `cancel()` callback never fires. Also provide MORE chunks than the test will consume so the source stays in 'readable' state when cancel runs (e.g. cap test reads ~6 chunks, stub provides 10).
 - Tool-name whitelists must derive from `ALL_TOOLS` in `services/tools.ts`, never hardcoded. `services/agents.ts` `ALL_TOOL_NAMES` had this drift class until v1.12 — same pattern applies to any future tool-aware code.
 - Agent registry lives at `data/AGENTS.md` (global, bind-mounted at `/data/AGENTS.md`). No per-project `AGENTS.md` in this repo — removed in v1.12 to eliminate the two-files-must-stay-in-sync drift. The `getAgentsForProject` per-project override mechanism remains for *other* projects.
+- `data/AGENTS.md` is PARSED (`agents.ts` `splitSections`/`parseAgentSection`): each `## <Name>` is one agent and must be followed by a `---` frontmatter fence or the block throws; content before the first `## ` is discarded. Do NOT add free-form `## ` rule sections — they break the registry. Cross-cutting agent rules go in CLAUDE.md or a parser-ignored preamble.
+- Skills live in `data/skills/<vendor>/`; Sam's own namespace is `boocode/` (`committing-changes`, `using-worktrees`, `improving-boocode-guidance`) — `SKILL.md` + optional `eval.yaml` (gerund names; eval = `skill:` + `tasks:` of `prompt`+`grader`, incl. a negative-trigger task). `data/skills/` is canonical; a divergent mirror at `/opt/skills/` exists.
 - MCP stdio transport uses newline-delimited JSON (NDJSON), NOT LSP-style `Content-Length` headers. The `codecontext/shim.go` framing implementation is the reference; per the MCP spec (modelcontextprotocol.io/specification/server/transports).
 - **Workspace dependency pattern** (`apps/coder` → `@boocode/server`): the consuming package adds `"@boocode/server": "workspace:*"` in `package.json`. The provider's `package.json` needs `exports` with `types` + `default` conditions per subpath: `"./inference": { "types": "./dist/.../index.d.ts", "default": "./dist/.../index.js" }`. Without the `types` condition, NodeNext resolution can't find `.d.ts` files and tsc fails with "Cannot find module" in the consumer.
 - **JSONB columns**: use `sql.json(value as never)` — NOT `${JSON.stringify(value)}::jsonb` which double-serializes (stores a JSON string instead of a JSON object/array). Pattern established in `parts.ts`, `settings.ts`.

apps/coder/src/schema.sql

@@ -205,6 +205,27 @@ DO $$ BEGIN
   END IF;
 END $$;
 
+-- P1.5-b follow-up: converge agent_sessions.session_id FK CASCADE → SET NULL.
+-- The re-key block above re-adds session_id_fkey as SET NULL, but it is guarded on
+-- chat_id_fkey's ABSENCE — so a DB already re-keyed to (chat_id, agent) while
+-- session_id_fkey was still ON DELETE CASCADE never re-enters that block and stays
+-- 'c'. This standalone guard flips it to SET NULL ('n'), matching worktree_id.
+-- Idempotent (mirrors the session_worktrees defang's confdeltype check): only fires
+-- while the FK is still CASCADE — a no-op on a fresh deploy (already 'n' from the
+-- re-key block) and on every re-run thereafter.
+DO $$ BEGIN
+  IF EXISTS (
+    SELECT 1 FROM pg_constraint
+    WHERE conname = 'agent_sessions_session_id_fkey'
+      AND confdeltype = 'c'
+  ) THEN
+    ALTER TABLE agent_sessions ALTER COLUMN session_id DROP NOT NULL;
+    ALTER TABLE agent_sessions DROP CONSTRAINT agent_sessions_session_id_fkey;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_session_id_fkey
+      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE SET NULL;
+  END IF;
+END $$;
+
 -- v2.6: attribution for DiffPanel badges (Phase 1 UX reads this).
 ALTER TABLE pending_changes ADD COLUMN IF NOT EXISTS agent TEXT;
 

apps/server/src/routes/sessions.ts

@@ -28,9 +28,7 @@ const HtmlArtifactStateZ = z.object({
   title: z.string().max(500),
 });
 
-const WorkspacePaneZ = z.object({
-  id: z.string().min(1).max(200),
-  kind: z.enum([
+const PaneKindZ = z.enum([
   'chat',
   'terminal',
   'coder',
@@ -39,7 +37,11 @@ const WorkspacePaneZ = z.object({
   'settings',
   'markdown_artifact',
   'html_artifact',
-  ]),
+]);
+
+const WorkspacePaneZ = z.object({
+  id: z.string().min(1).max(200),
+  kind: PaneKindZ,
   chatId: z.string().min(1).max(200).optional(),
   chatIds: z.array(z.string().min(1).max(200)).max(50),
   activeChatIdx: z.number().int(),
@@ -47,8 +49,27 @@ const WorkspacePaneZ = z.object({
   html_artifact_state: HtmlArtifactStateZ.optional(),
 });
 
+// v2.6.x: workspace_panes column widened from a bare WorkspacePane[] to a
+// WorkspaceState envelope (panes + stable session-scoped tab numbering +
+// reopen stack). closedPaneStack entries are lighter than full panes — just
+// the kind + chat ids needed to recreate a closed pane on reopen.
+const ClosedPaneEntryZ = z.object({
+  kind: PaneKindZ,
+  chatIds: z.array(z.string().min(1).max(200)).max(50),
+  activeChatIdx: z.number().int(),
+});
+
+const WorkspaceStateZ = z.object({
+  panes: z.array(WorkspacePaneZ).max(10),
+  tabNumbers: z.record(z.string(), z.number().int()).default({}),
+  nextTabNumber: z.number().int().default(1),
+  closedPaneStack: z.array(ClosedPaneEntryZ).max(10).default([]),
+});
+
+// Accept either the legacy bare array OR the envelope. The handler normalizes
+// to a full envelope before storing (see MIGRATION rule in the PATCH handler).
 const WorkspacePanesBody = z.object({
-  workspace_panes: z.array(WorkspacePaneZ).max(10),
+  workspace_panes: z.union([z.array(WorkspacePaneZ).max(10), WorkspaceStateZ]),
 });
 
 const PatchBody = z.object({
@@ -308,12 +329,20 @@ export function registerSessionRoutes(
         reply.code(400);
         return { error: 'invalid body', details: parsed.error.flatten() };
       }
-      const workspacePanes = parsed.data.workspace_panes.map((pane) =>
+      // v2.6.x MIGRATION: the body is either a legacy bare WorkspacePane[] or
+      // the WorkspaceState envelope. Normalize to a full envelope so the column
+      // always stores the envelope shape going forward.
+      const body = parsed.data.workspace_panes;
+      const envelope = Array.isArray(body)
+        ? { panes: body, tabNumbers: {}, nextTabNumber: 1, closedPaneStack: [] }
+        : body;
+      // agent → coder normalization on the panes array (unchanged write rule).
+      envelope.panes = envelope.panes.map((pane) =>
         pane.kind === 'agent' ? { ...pane, kind: 'coder' as const } : pane,
       );
       const rows = await sql<Session[]>`
         UPDATE sessions
-        SET workspace_panes = ${sql.json(workspacePanes as never)},
+        SET workspace_panes = ${sql.json(envelope as never)},
             updated_at = clock_timestamp()
         WHERE id = ${req.params.id}
         RETURNING id, project_id, name, model, system_prompt, status, created_at, updated_at,

apps/server/src/services/inference/tool-phase.ts

@@ -2,6 +2,7 @@ import type { Agent, Session, ToolCall } from '../../types/api.js';
 import * as modelContext from '../model-context.js';
 import { PathScopeError } from '../path_guard.js';
 import { TOOLS_BY_NAME } from '../tools.js';
+import type { ToolExecCtx } from '../tools.js';
 import { matchToolGlob } from '../agents.js';
 import { maybeFlagForCompaction } from './payload.js';
 import { insertParts, partsFromAssistantMessage, partsFromToolMessage } from './parts.js';
@@ -31,6 +32,7 @@ async function executeToolCall(
   projectRoot: string,
   toolCall: ToolCall,
   extraRoots: readonly string[],
+  toolCtx?: ToolExecCtx,
 ): Promise<{ output: unknown; truncated: boolean; error?: string }> {
   const tool = TOOLS_BY_NAME[toolCall.name];
   if (!tool) {
@@ -65,7 +67,7 @@ async function executeToolCall(
     };
   }
   try {
-    const output = await tool.execute(parsed.data, projectRoot, extraRoots);
+    const output = await tool.execute(parsed.data, projectRoot, extraRoots, toolCtx);
     const truncated =
       typeof output === 'object' && output !== null && 'truncated' in output
         ? Boolean((output as { truncated: unknown }).truncated)
@@ -289,7 +291,10 @@ export async function executeToolPhase(
         });
         return;
       }
-      const tres = await executeToolCall(projectRoot, tc, session.allowed_read_paths);
+      const tres = await executeToolCall(projectRoot, tc, session.allowed_read_paths, {
+        sql: ctx.sql,
+        sessionId,
+      });
       if (SYNTHESIS_TOOLS.has(tc.name)) {
         synthEntries.push({ tc, output: tres.output, ...(tres.error ? { error: tres.error } : {}) });
       }

apps/server/src/services/read_tab_by_number.ts

@@ -0,0 +1,142 @@
+// v2.6.x: read_tab_by_number tool. Reads the conversation transcript of the
+// chat that occupies a given session-scoped tab number. Stable tab numbers are
+// stored in the session's workspace_panes envelope (WorkspaceState.tabNumbers),
+// keyed by chat id. Lives in its own file (not appended to tools.ts) so tests
+// can import the executor directly without dragging in the whole tool registry.
+// Registered in tools.ts ALL_TOOLS + READ_ONLY_TOOL_NAMES.
+
+import { z } from 'zod';
+import type { Sql } from '../db.js';
+// type-only import to dodge the runtime cycle (tools.ts re-exports this tool
+// via ALL_TOOLS; importing ToolDef/ToolExecCtx at type level keeps the dep
+// one-way).
+import type { ToolDef, ToolExecCtx } from './tools.js';
+
+const ReadTabByNumberInput = z.object({
+  number: z.number().int().positive(),
+});
+export type ReadTabByNumberInputT = z.infer<typeof ReadTabByNumberInput>;
+
+// Cap total transcript size so a long conversation can't blow the context
+// window. The model gets a clear truncation marker when the cap is hit.
+const MAX_TRANSCRIPT_CHARS = 20_000;
+
+// WorkspaceState envelope shape (panes omitted — we only need tabNumbers here).
+interface WorkspaceStateLike {
+  panes?: unknown;
+  tabNumbers?: Record<string, number>;
+  nextTabNumber?: number;
+  closedPaneStack?: unknown[];
+}
+
+// MIGRATION: the stored workspace_panes value may be the legacy bare
+// WorkspacePane[] OR the WorkspaceState envelope. Normalize to an envelope so
+// tabNumbers is always available (empty for the legacy shape — no tab numbers
+// were tracked before the envelope landed).
+function normalizeWorkspaceState(v: unknown): {
+  tabNumbers: Record<string, number>;
+} {
+  if (Array.isArray(v)) {
+    return { tabNumbers: {} };
+  }
+  if (v && typeof v === 'object' && Array.isArray((v as WorkspaceStateLike).panes)) {
+    const env = v as WorkspaceStateLike;
+    return { tabNumbers: env.tabNumbers ?? {} };
+  }
+  return { tabNumbers: {} };
+}
+
+// Pure executor split out from the ToolDef wrapper so tests can call it with a
+// mocked Sql. Returns a transcript string (read-only — never writes).
+export async function executeReadTabByNumber(
+  input: ReadTabByNumberInputT,
+  sql: Sql,
+  sessionId: string,
+): Promise<string> {
+  const sessionRows = await sql<{ workspace_panes: unknown }[]>`
+    SELECT workspace_panes FROM sessions WHERE id = ${sessionId}
+  `;
+  if (sessionRows.length === 0) {
+    return `Session not found.`;
+  }
+  const { tabNumbers } = normalizeWorkspaceState(sessionRows[0]!.workspace_panes);
+
+  // Reverse-lookup: find the chat id whose stable tab number equals the input.
+  let chatId: string | null = null;
+  for (const [cid, num] of Object.entries(tabNumbers)) {
+    if (num === input.number) {
+      chatId = cid;
+      break;
+    }
+  }
+  if (chatId === null) {
+    return `No tab is numbered ${input.number} in this session.`;
+  }
+
+  // Read the conversation: skip system sentinels (role='system') and empty
+  // content rows. Oldest first.
+  const messages = await sql<{ role: string; content: string }[]>`
+    SELECT role, content
+      FROM messages
+     WHERE chat_id = ${chatId}
+       AND role <> 'system'
+       AND content <> ''
+     ORDER BY created_at ASC
+  `;
+  if (messages.length === 0) {
+    return `Tab ${input.number} (chat ${chatId}) has no messages yet.`;
+  }
+
+  // Format a compact transcript, capping total output size.
+  const parts: string[] = [];
+  let total = 0;
+  let truncated = false;
+  for (const m of messages) {
+    const block = `### ${m.role}\n${m.content}`;
+    // +2 accounts for the "\n\n" joiner between blocks.
+    if (total + block.length + 2 > MAX_TRANSCRIPT_CHARS) {
+      truncated = true;
+      break;
+    }
+    parts.push(block);
+    total += block.length + 2;
+  }
+
+  let out = parts.join('\n\n');
+  if (truncated) {
+    out += `\n\n[transcript truncated at ${MAX_TRANSCRIPT_CHARS} chars]`;
+  }
+  return out;
+}
+
+export const readTabByNumber: ToolDef<ReadTabByNumberInputT> = {
+  name: 'read_tab_by_number',
+  description:
+    'Read the conversation transcript of the tab with the given session-scoped tab number. Tab numbers are stable per session (shown in the workspace tab strip). Returns the messages of that tab oldest-first as a compact transcript. Read-only.',
+  inputSchema: ReadTabByNumberInput,
+  jsonSchema: {
+    type: 'function',
+    function: {
+      name: 'read_tab_by_number',
+      description:
+        'Read the conversation transcript of the tab with the given session-scoped tab number. Read-only.',
+      parameters: {
+        type: 'object',
+        properties: {
+          number: {
+            type: 'integer',
+            description: 'The session-scoped tab number (positive integer).',
+          },
+        },
+        required: ['number'],
+        additionalProperties: false,
+      },
+    },
+  },
+  async execute(input, _projectRoot, _extraRoots, toolCtx?: ToolExecCtx) {
+    if (!toolCtx) {
+      return 'read_tab_by_number unavailable: no session context';
+    }
+    return await executeReadTabByNumber(input, toolCtx.sql, toolCtx.sessionId);
+  },
+};

apps/server/src/services/tools.ts

@@ -1,6 +1,7 @@
 import { readFile, readdir, stat } from 'node:fs/promises';
 import { resolve, basename, relative } from 'node:path';
 import { z } from 'zod';
+import type { Sql } from '../db.js';
 import { pathGuard, PathScopeError } from './path_guard.js';
 import { isSecretPath, SecretBlockedError, filterSecretEntries } from './secret_guard.js';
 import { grep as fileOpsGrep, findFiles as fileOpsFindFiles } from './file_ops.js';
@@ -30,6 +31,9 @@ import {
 // with the pause-on-pending-grant branch in inference/tool-phase.ts and the
 // POST /api/chats/:id/grant_read_access endpoint in routes/messages.ts.
 import { requestReadAccess } from './request_read_access.js';
+// v2.6.x: read-only tool that reads a tab's transcript by its session-scoped
+// tab number. Needs DB/session context (ToolExecCtx 4th arg).
+import { readTabByNumber } from './read_tab_by_number.js';
 
 const MAX_FILE_BYTES = 5 * 1024 * 1024;
 const DEFAULT_VIEW_LINES = 200;
@@ -48,6 +52,16 @@ export interface ToolJsonSchema {
   };
 }
 
+// v2.6.x: optional DB/session context threaded into a tool's execute(). Only
+// tools that need to read session-scoped DB state (e.g. read_tab_by_number)
+// use it; every other tool ignores the 4th arg. Kept optional so existing
+// 3-arg execute() implementations stay assignable (apps/coder consumes this
+// type from the compiled dist — the optional param keeps it backward-compatible).
+export interface ToolExecCtx {
+  sql: Sql;
+  sessionId: string;
+}
+
 export interface ToolDef<TInput> {
   name: string;
   description: string;
@@ -59,7 +73,15 @@ export interface ToolDef<TInput> {
   // view_truncated_output) forward it to pathGuard; other tools accept the
   // arg and ignore it. The execute signature stays compatible with
   // pre-v1.13.17 callsites because the parameter is optional.
-  execute(input: TInput, projectRoot: string, extraRoots?: readonly string[]): Promise<unknown>;
+  // v2.6.x: optional 4th param toolCtx carries DB/session context for tools
+  // that read session-scoped state (read_tab_by_number). Optional so 3-arg
+  // implementations remain assignable.
+  execute(
+    input: TInput,
+    projectRoot: string,
+    extraRoots?: readonly string[],
+    toolCtx?: ToolExecCtx,
+  ): Promise<unknown>;
 }
 
 const ViewFileInput = z.object({
@@ -694,6 +716,9 @@ export let ALL_TOOLS: ToolDef<unknown>[] = [
   // state change is appending to sessions.allowed_read_paths via the
   // grant endpoint, gated by user consent.
   requestReadAccess as ToolDef<unknown>,
+  // v2.6.x: read a tab's transcript by its session-scoped tab number.
+  // Read-only; uses the ToolExecCtx 4th arg for DB/session access.
+  readTabByNumber as ToolDef<unknown>,
 ].sort((a, b) => a.name.localeCompare(b.name));
 
 // v1.8.2: forward-compatible read-only whitelist. An agent whose `tools` is
@@ -734,6 +759,9 @@ export const READ_ONLY_TOOL_NAMES = [
   // state directly (the grant endpoint appends to sessions.allowed_read_paths
   // only with user consent). Belongs in the read-only budget tier.
   'request_read_access',
+  // v2.6.x: reads a tab's transcript from session-scoped DB state; never
+  // writes. Belongs in the read-only budget tier.
+  'read_tab_by_number',
 ] as const;
 
 export let TOOLS_BY_NAME: Record<string, ToolDef<unknown>> = Object.fromEntries(

apps/server/src/types/ws-frames.ts

@@ -203,7 +203,12 @@ export const SessionDeletedFrame = z.object({
 export const SessionWorkspaceUpdatedFrame = z.object({
   type: z.literal('session_workspace_updated'),
   session_id: Uuid,
-  workspace_panes: z.array(OpaqueObject),
+  // v2.6.x: widened from z.array — the payload is now either the legacy bare
+  // WorkspacePane[] OR the WorkspaceState envelope object (panes + tabNumbers +
+  // nextTabNumber + closedPaneStack). z.array alone would fail-closed and drop
+  // every envelope frame at validation. MUST be mirrored in the server's
+  // byte-identical copy (parity test).
+  workspace_panes: z.union([z.array(OpaqueObject), z.record(z.unknown())]),
 });
 
 export const ChatCreatedFrame = z.object({

apps/web/src/api/client.ts

@@ -22,6 +22,7 @@ import type {
   CoderTaskDetail,
   PermissionPrompt,
   AgentCommand,
+  WorkspaceState,
 } from './types';
 
 export class ApiError extends Error {
@@ -175,10 +176,10 @@ export const api = {
       ),
     openChatsCount: (id: string) =>
       request<{ count: number }>(`/api/sessions/${id}/chats/open-count`),
-    updateWorkspacePanes: (id: string, panes: Session['workspace_panes']) =>
+    updateWorkspacePanes: (id: string, state: WorkspaceState) =>
       request<Session>(`/api/sessions/${id}/workspace`, {
         method: 'PATCH',
-        body: JSON.stringify({ workspace_panes: panes }),
+        body: JSON.stringify({ workspace_panes: state }),
       }),
   },
 
@@ -354,6 +355,10 @@ export const api = {
       request<{ taskId: string; commands: AgentCommand[] }>(`/api/coder/tasks/${taskId}/commands`),
     getTask: (taskId: string) =>
       request<CoderTaskDetail>(`/api/coder/tasks/${taskId}`),
+    // Cancel a pending/running coder task (cancels permission wait + inference;
+    // server sets state='cancelled'). Used by CoderPane's stop button.
+    cancelTask: (taskId: string) =>
+      request<{ cancelled: boolean }>(`/api/coder/tasks/${taskId}/cancel`, { method: 'POST' }),
     listMessages: (sessionId: string, chatId?: string) =>
       request<CoderMessageWire[]>(
         `/api/coder/sessions/${sessionId}/messages${chatId ? `?chat_id=${encodeURIComponent(chatId)}` : ''}`,

apps/web/src/api/types.ts

@@ -60,7 +60,10 @@ export interface Session {
   // v1.9: null = inherit from project.default_web_search_enabled.
   web_search_enabled: boolean | null;
   // v1.12.1: server-authoritative pane layout, replaces localStorage.
-  workspace_panes: WorkspacePane[];
+  // A value may be the legacy bare WorkspacePane[] (older rows) OR the new
+  // WorkspaceState envelope (panes + tab numbering + reopen stack). Normalize
+  // on read via useWorkspacePanes' toWorkspaceState.
+  workspace_panes: WorkspacePane[] | WorkspaceState;
   // v1.13.17: paths the agent has been granted read access to via the
   // request_read_access tool. Empty by default. Settings UI surfaces the
   // list with per-row revoke; the grant flow itself appends through the
@@ -511,6 +514,30 @@ export interface WorkspacePane {
   html_artifact_state?: HtmlArtifactState;
 }
 
+// Reopen LIFO stack entry. Shape unchanged from the prior module-level stack;
+// now persisted inside the WorkspaceState envelope so the reopen-pane stack
+// survives a reload / cross-device sync.
+export interface ClosedPaneEntry {
+  kind: WorkspacePane['kind'];
+  chatIds: string[];
+  activeChatIdx: number;
+}
+
+// Envelope persisted to sessions.workspace_panes. Supersedes the bare
+// WorkspacePane[] shape (still accepted on read for legacy rows — see the
+// migration in useWorkspacePanes.toWorkspaceState). The server accepts either
+// shape; the frontend always emits this envelope going forward.
+export interface WorkspaceState {
+  panes: WorkspacePane[];
+  // Stable, session-scoped tab number per chat id. Numbers only ever increase
+  // and are never reused (retired entries are pruned on tab close).
+  tabNumbers: { [chatId: string]: number };
+  // Next number to hand out; starts at 1; ONLY increments.
+  nextTabNumber: number;
+  // Reopen LIFO stack, max 10, most-recent last.
+  closedPaneStack: ClosedPaneEntry[];
+}
+
 export type WsFrame =
   | { type: 'snapshot'; messages: Message[] }
   | { type: 'message_started'; message_id: string; chat_id?: string; role: MessageRole }

apps/web/src/api/ws-frames.ts

@@ -203,7 +203,12 @@ export const SessionDeletedFrame = z.object({
 export const SessionWorkspaceUpdatedFrame = z.object({
   type: z.literal('session_workspace_updated'),
   session_id: Uuid,
-  workspace_panes: z.array(OpaqueObject),
+  // v2.6.x: widened from z.array — the payload is now either the legacy bare
+  // WorkspacePane[] OR the WorkspaceState envelope object (panes + tabNumbers +
+  // nextTabNumber + closedPaneStack). z.array alone would fail-closed and drop
+  // every envelope frame at validation. MUST be mirrored in the server's
+  // byte-identical copy (parity test).
+  workspace_panes: z.union([z.array(OpaqueObject), z.record(z.unknown())]),
 });
 
 export const ChatCreatedFrame = z.object({

apps/web/src/components/ChatInput.tsx

@@ -1,5 +1,5 @@
 import { useCallback, useEffect, useMemo, useRef, useState, type DragEvent, type KeyboardEvent } from 'react';
-import { Check, Plus, Send } from 'lucide-react';
+import { Check, ListPlus, Plus, Send, Square } from 'lucide-react';
 import { toast } from 'sonner';
 import { Textarea } from '@/components/ui/textarea';
 import { Button } from '@/components/ui/button';
@@ -51,6 +51,11 @@ interface Props {
   webSearchEnabled?: boolean | null;
   onSend: (content: string) => void | Promise<void>;
   onForceSend?: (content: string) => void | Promise<void>;
+  // When the assistant/agent is generating, the send button morphs: empty draft
+  // → Stop (calls onStop); non-empty draft → Queue (submits, which the caller
+  // queues while busy). Omitting onStop falls back to a (disabled) Send button.
+  generating?: boolean;
+  onStop?: () => void | Promise<void>;
   // Batch 9.6: slash-command dispatch. When the input parses to a known skill,
   // ChatInput calls this with the skill name + the post-name args (possibly
   // empty). Callers wire this to api.chats.skillInvoke. Omitting the prop
@@ -78,7 +83,7 @@ interface Props {
   modelContextLimit?: number | null;
 }
 
-export function ChatInput({ disabled, projectId, agentId, onAgentChange, sessionId, webSearchEnabled, onSend, onForceSend, onSlashCommand, slashGroups, chatId, chatLabel, messages, modelContextLimit }: Props) {
+export function ChatInput({ disabled, projectId, agentId, onAgentChange, sessionId, webSearchEnabled, onSend, onForceSend, generating, onStop, onSlashCommand, slashGroups, chatId, chatLabel, messages, modelContextLimit }: Props) {
   const { isMobile } = useViewport();
   const [value, setValue] = useState('');
   const [busy, setBusy] = useState(false);
@@ -651,14 +656,38 @@ export function ChatInput({ disabled, projectId, agentId, onAgentChange, session
           rows={3}
           className="resize-none min-h-[68px] max-h-[240px]"
         />
+        {(() => {
+          const hasContent = value.trim().length > 0 || attachments.length > 0;
+          // While generating with an empty draft, the button stops generation.
+          if (generating && onStop && !hasContent) {
+            return (
+              <Button
+                onClick={() => void onStop()}
+                size="icon-lg"
+                variant="outline"
+                aria-label="Stop generating"
+                title="Stop generating"
+              >
+                <Square className="fill-current size-3.5" />
+              </Button>
+            );
+          }
+          // With a draft, submit. While generating the caller queues it, so the
+          // button reads as Queue; otherwise it's a normal Send.
+          const queueing = !!generating && hasContent;
+          return (
             <Button
               onClick={() => void submit()}
-          disabled={disabled || busy || (!value.trim() && attachments.length === 0)}
+              disabled={disabled || busy || !hasContent}
               size="icon-lg"
-          aria-label="Send"
+              variant={queueing ? 'secondary' : 'default'}
+              aria-label={queueing ? 'Queue message' : 'Send'}
+              title={queueing ? 'Queue message' : 'Send'}
             >
-          <Send />
+              {queueing ? <ListPlus /> : <Send />}
             </Button>
+          );
+        })()}
       </div>
       </div>
       <AttachmentPreviewModal

apps/web/src/components/ChatTabBar.tsx

@@ -16,11 +16,15 @@ import {
   DropdownMenuTrigger,
 } from '@/components/ui/dropdown-menu';
 import { useLongPress } from '@/hooks/useLongPress';
+import { sessionEvents } from '@/hooks/sessionEvents';
 import { cn } from '@/lib/utils';
 
 interface Props {
   pane: WorkspacePane;
   tabs: Chat[];
+  // v2.6.x (Batch 3a): stable session-scoped tab number per chat id. Keyed by
+  // chat.id, NEVER by tab position.
+  tabNumbers: Record<string, number>;
   onSwitchTab: (tabIdx: number) => void;
   onRemoveTab: (chatId: string) => void;
   onCloseOthers: (chatId: string) => void;
@@ -37,6 +41,7 @@ interface Props {
 export function ChatTabBar({
   pane,
   tabs,
+  tabNumbers,
   onSwitchTab,
   onRemoveTab,
   onCloseOthers,
@@ -83,6 +88,9 @@ export function ChatTabBar({
         const isLast = tabIdx === tabs.length - 1;
         const onlyTab = tabs.length === 1;
         const label = chat.name ?? 'New chat';
+        // v2.6.x: stable tab number keyed by chat.id (NOT tab position).
+        // Omit gracefully when not yet assigned.
+        const tabNumber = tabNumbers[chat.id];
         return (
           <ContextMenu key={chat.id}>
             <ContextMenuTrigger asChild>
@@ -117,8 +125,11 @@ export function ChatTabBar({
                     className="bg-transparent border-b border-border text-xs outline-none w-28"
                   />
                 ) : (
-                  <span className="truncate max-w-[140px]" title={label}>
-                    {label}
+                  <span
+                    className="truncate max-w-[140px]"
+                    title={tabNumber !== undefined ? `${tabNumber} · ${label}` : label}
+                  >
+                    {tabNumber !== undefined ? `${tabNumber} · ${label}` : label}
                   </span>
                 )}
                 <button
@@ -138,6 +149,13 @@ export function ChatTabBar({
               <ContextMenuItem onSelect={onNewTab}>
                 New chat
               </ContextMenuItem>
+              <ContextMenuItem
+                onSelect={() =>
+                  sessionEvents.emit({ type: 'open_chat_in_new_pane', chat_id: chat.id })
+                }
+              >
+                Open in new pane
+              </ContextMenuItem>
               <ContextMenuSeparator />
               <ContextMenuItem onSelect={() => startRename(chat.id, chat.name)}>
                 Rename
@@ -174,15 +192,31 @@ export function ChatTabBar({
       )}
 
       <div className="flex items-center ml-auto gap-0.5 px-1 shrink-0">
+        <DropdownMenu>
+          <DropdownMenuTrigger asChild>
             <button
               type="button"
-          onClick={onNewTab}
               className="inline-flex items-center justify-center p-1 rounded text-muted-foreground hover:bg-muted hover:text-foreground max-md:min-h-[44px] max-md:min-w-[44px]"
-          aria-label="New tab"
-          title="New tab"
+              aria-label="New chat, terminal, or coder"
+              title="New chat / terminal / coder"
             >
               <Plus size={12} />
             </button>
+          </DropdownMenuTrigger>
+          <DropdownMenuContent align="end" className="w-fit">
+            {/* New BooChat opens a tab in THIS pane; terminal/coder can't be
+                tabs, so they split into a new pane (matches the Split menu). */}
+            <DropdownMenuItem onSelect={onNewTab}>
+              <MessageSquare size={14} /> New BooChat
+            </DropdownMenuItem>
+            <DropdownMenuItem onSelect={() => onSplitPane('terminal')}>
+              <Terminal size={14} /> New BooTerm
+            </DropdownMenuItem>
+            <DropdownMenuItem onSelect={() => onSplitPane('coder')}>
+              <Code size={14} /> New BooCode
+            </DropdownMenuItem>
+          </DropdownMenuContent>
+        </DropdownMenu>
         <DropdownMenu>
           <DropdownMenuTrigger asChild>
             <button

apps/web/src/components/MessageBubble.tsx

@@ -1,9 +1,9 @@
 import { useEffect, useState } from 'react';
 import type { ReactNode } from 'react';
-import { ChevronDown, ChevronRight, Copy, RefreshCw, Check, Share2, RotateCw, GitFork, Trash2, PanelRightOpen, Brain } from 'lucide-react';
+import { ChevronDown, ChevronRight, Copy, RefreshCw, Check, Share2, RotateCw, GitFork, Trash2, Brain } from 'lucide-react';
 import { toast } from 'sonner';
 import type { Chat, ErrorReason, Message } from '@/api/types';
-import { api, ApiError } from '@/api/client';
+import { api } from '@/api/client';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { sendToTerminal, terminalsRegistry, type TerminalRegistration } from '@/lib/events';
 import { CapHitSentinel } from './CapHitSentinel';
@@ -105,18 +105,6 @@ const ERROR_REASON_LABELS: Record<ErrorReason, string> = {
 // moved to apps/web/src/components/MarkdownRenderer.tsx so the new artifact
 // panes can render assistant content with the same Shiki + remark-gfm setup.
 
-// Pane-header title derivation for a markdown artifact. Order matches the
-// server slug logic in services/artifacts.ts: first `# ` heading → first 6
-// words of the body → 'Markdown artifact'. Truncated to keep the pane header
-// readable.
-function deriveMarkdownTitle(content: string): string {
-  const headingMatch = content.match(/^\s*#\s+(.+?)\s*$/m);
-  if (headingMatch && headingMatch[1]) return headingMatch[1].slice(0, 80);
-  const words = content.trim().split(/\s+/).slice(0, 6).join(' ');
-  if (words) return words.slice(0, 80);
-  return 'Markdown artifact';
-}
-
 export interface MessageActions {
   onRegenerate?: (chatId: string, messageId: string) => Promise<void>;
   onResend?: (chatId: string, content: string) => Promise<void>;
@@ -129,8 +117,8 @@ interface Props {
   sessionChats?: Chat[];
   capHitInfo?: { position: number; isLatest: boolean };
   actions?: MessageActions;
-  /** Hide actions that don't apply (fork, delete, open-in-pane). */
-  hideActions?: ('fork' | 'delete' | 'openInPane')[];
+  /** Hide actions that don't apply (fork, delete). */
+  hideActions?: ('fork' | 'delete')[];
 }
 
 function StatsLine({ message }: { message: Message }) {
@@ -226,7 +214,7 @@ function ActionRow({
       } else {
         const chat = await api.chats.fork(message.chat_id, { messageId: message.id });
         sessionEvents.emit({ type: 'refetch_messages' });
-        sessionEvents.emit({ type: 'open_chat_in_active_pane', chat_id: chat.id });
+        sessionEvents.emit({ type: 'open_chat_in_new_pane', chat_id: chat.id });
       }
     } catch (err) {
       toast.error(err instanceof Error ? err.message : 'fork failed');
@@ -258,54 +246,6 @@ function ActionRow({
   const canResend = isUser && message.status === 'complete' && !!message.content?.trim();
   const canFork = message.status === 'complete';
   const canDelete = message.status !== 'streaming';
-  const [openingPane, setOpeningPane] = useState(false);
-
-  // v1.14.x-html-artifact-panes: probe for an html_artifact part. If present,
-  // open the HTML pane variant; otherwise fall back to the markdown variant.
-  // Title derivation for markdown: first `# ` heading → first 6 words of the
-  // body → 'Markdown artifact' (mirrors the slug logic in
-  // services/artifacts.ts).
-  async function openInPane() {
-    if (openingPane || message.status === 'streaming') return;
-    setOpeningPane(true);
-    try {
-      try {
-        const payload = await api.messages.getHtmlArtifact(
-          message.chat_id,
-          message.id,
-        );
-        sessionEvents.emit({
-          type: 'open_html_artifact_pane',
-          state: {
-            chat_id: message.chat_id,
-            message_id: message.id,
-            title: payload.title,
-          },
-        });
-        return;
-      } catch (err) {
-        // 404 (no html_artifact part) is the expected fall-through path —
-        // markdown variant opens below. Any other error (network, 500) is
-        // a real failure; toast and bail rather than masquerading as markdown.
-        const status = err instanceof ApiError ? err.status : null;
-        if (status !== 404) {
-          toast.error(err instanceof Error ? err.message : 'open in pane failed');
-          return;
-        }
-      }
-      const title = deriveMarkdownTitle(message.content);
-      sessionEvents.emit({
-        type: 'open_markdown_artifact_pane',
-        state: {
-          chat_id: message.chat_id,
-          message_id: message.id,
-          title,
-        },
-      });
-    } finally {
-      setOpeningPane(false);
-    }
-  }
 
   return (
     <>
@@ -330,18 +270,6 @@ function ActionRow({
             <RefreshCw className="size-3" />
           </button>
         )}
-        {isAssistant && !hiddenSet.has('openInPane') && (
-          <button
-            type="button"
-            onClick={() => void openInPane()}
-            disabled={openingPane || message.status === 'streaming'}
-            className="inline-flex items-center justify-center size-6 rounded text-muted-foreground hover:bg-muted hover:text-foreground disabled:opacity-40 disabled:cursor-not-allowed max-md:min-h-[44px] max-md:min-w-[44px]"
-            aria-label="Open in pane"
-            title="Open in pane"
-          >
-            <PanelRightOpen className="size-3" />
-          </button>
-        )}
         {isAssistant && (
           <button
             type="button"

apps/web/src/components/SessionLandingPage.tsx

@@ -1,6 +1,9 @@
-import { useCallback, useState } from 'react';
+import { useCallback, useEffect, useState } from 'react';
+import { Archive, MessageSquare, RotateCcw } from 'lucide-react';
 import { toast } from 'sonner';
 import { ChatInput } from '@/components/ChatInput';
+import { api } from '@/api/client';
+import type { Chat } from '@/api/types';
 
 interface Props {
   projectId: string;
@@ -13,6 +16,30 @@ interface Props {
   // the skill — same transition the text send uses. See useSessionChats.
   onSkillInvoke: (skillName: string, userMessage: string | null) => void;
   createChat: () => Promise<{ id: string }>;
+  // Session history: the session's open chats (live), and callbacks to open one
+  // in THIS pane / restore an archived one. Archived chats are fetched here
+  // (the default open-only list excludes them).
+  chats: Chat[];
+  onOpenChat: (chatId: string) => void;
+  onUnarchiveChat: (chatId: string) => Promise<void>;
+}
+
+function formatRelative(iso: string): string {
+  const then = new Date(iso).getTime();
+  if (Number.isNaN(then)) return '';
+  const s = Math.max(0, Math.round((Date.now() - then) / 1000));
+  if (s < 60) return 'just now';
+  const m = Math.round(s / 60);
+  if (m < 60) return `${m}m ago`;
+  const h = Math.round(m / 60);
+  if (h < 24) return `${h}h ago`;
+  const d = Math.round(h / 24);
+  if (d < 7) return `${d}d ago`;
+  return new Date(iso).toLocaleDateString();
+}
+
+function byRecent(a: Chat, b: Chat): number {
+  return (b.updated_at ?? '').localeCompare(a.updated_at ?? '');
 }
 
 export function SessionLandingPage({
@@ -23,8 +50,24 @@ export function SessionLandingPage({
   onSend,
   onSkillInvoke,
   createChat,
+  chats,
+  onOpenChat,
+  onUnarchiveChat,
 }: Props) {
   const [chatId, setChatId] = useState<string | null>(null);
+  const [archived, setArchived] = useState<Chat[]>([]);
+
+  // Archived chats aren't in the default (open-only) list, so fetch them. One
+  // shot on session change — the history view is transient (pick a chat and
+  // it's gone), so slight staleness is fine; reopening the pane refetches.
+  useEffect(() => {
+    let cancelled = false;
+    api.chats
+      .listForSession(sessionId, { status: 'archived' })
+      .then((list) => { if (!cancelled) setArchived(list); })
+      .catch(() => {});
+    return () => { cancelled = true; };
+  }, [sessionId]);
 
   const ensureChat = useCallback(async (): Promise<string> => {
     if (chatId) return chatId;
@@ -57,12 +100,87 @@ export function SessionLandingPage({
     onSkillInvoke(skillName, userMessage.length > 0 ? userMessage : null);
   }, [onSkillInvoke]);
 
+  const restoreAndOpen = useCallback(async (id: string) => {
+    try {
+      await onUnarchiveChat(id);
+      onOpenChat(id);
+    } catch {
+      // onUnarchiveChat surfaces its own toast.
+    }
+  }, [onUnarchiveChat, onOpenChat]);
+
+  const openChats = [...chats.filter((c) => c.status === 'open')].sort(byRecent);
+  const openIds = new Set(openChats.map((c) => c.id));
+  const archivedChats = archived.filter((c) => !openIds.has(c.id)).sort(byRecent);
+  const isEmpty = openChats.length === 0 && archivedChats.length === 0;
+
   return (
     <div className="flex flex-col h-full min-h-0">
-      <div className="flex-1 flex items-center justify-center px-6">
-        <p className="text-sm text-muted-foreground">
-          Send a message to start.
+      <div className="flex-1 min-h-0 overflow-y-auto">
+        <div className="max-w-[760px] mx-auto w-full px-4 py-4">
+          {isEmpty ? (
+            <p className="text-sm text-muted-foreground text-center py-8">
+              No conversations yet. Send a message to start.
             </p>
+          ) : (
+            <>
+              {openChats.length > 0 && (
+                <>
+                  <h3 className="text-xs font-medium uppercase tracking-wide text-muted-foreground px-1 mb-1.5">
+                    Conversations
+                  </h3>
+                  <div className="space-y-0.5 mb-4">
+                    {openChats.map((c) => (
+                      <button
+                        key={c.id}
+                        type="button"
+                        onClick={() => onOpenChat(c.id)}
+                        className="w-full flex items-center gap-2 text-left px-2 py-1.5 rounded hover:bg-muted text-sm max-md:min-h-[44px]"
+                      >
+                        <MessageSquare size={14} className="shrink-0 text-muted-foreground" />
+                        <span className="truncate shrink-0 max-w-[45%]">{c.name ?? 'New chat'}</span>
+                        {c.last_message_preview && (
+                          <span className="truncate flex-1 text-xs text-muted-foreground hidden sm:block">
+                            {c.last_message_preview}
+                          </span>
+                        )}
+                        <span className="shrink-0 ml-auto text-xs text-muted-foreground">
+                          {formatRelative(c.updated_at)}
+                        </span>
+                      </button>
+                    ))}
+                  </div>
+                </>
+              )}
+              {archivedChats.length > 0 && (
+                <>
+                  <h3 className="text-xs font-medium uppercase tracking-wide text-muted-foreground px-1 mb-1.5">
+                    Archived
+                  </h3>
+                  <div className="space-y-0.5">
+                    {archivedChats.map((c) => (
+                      <button
+                        key={c.id}
+                        type="button"
+                        onClick={() => void restoreAndOpen(c.id)}
+                        title="Restore and open"
+                        className="group/arch w-full flex items-center gap-2 text-left px-2 py-1.5 rounded hover:bg-muted text-sm text-muted-foreground max-md:min-h-[44px]"
+                      >
+                        <Archive size={14} className="shrink-0" />
+                        <span className="truncate flex-1">{c.name ?? 'New chat'}</span>
+                        <span className="shrink-0 text-xs">{formatRelative(c.updated_at)}</span>
+                        <RotateCcw
+                          size={13}
+                          className="shrink-0 opacity-0 group-hover/arch:opacity-100"
+                        />
+                      </button>
+                    ))}
+                  </div>
+                </>
+              )}
+            </>
+          )}
+        </div>
       </div>
       <ChatInput
         disabled={false}

apps/web/src/components/Workspace.tsx

@@ -54,6 +54,7 @@ export function Workspace({
 }: Props) {
   const {
     panes,
+    tabNumbers,
     activePaneIdx,
     setActivePaneIdx,
     openChatInPane,
@@ -204,6 +205,7 @@ export function Workspace({
                 <ChatTabBar
                   pane={pane}
                   tabs={chatsForPane(pane)}
+                  tabNumbers={tabNumbers}
                   onSwitchTab={(tabIdx) => switchTab(idx, tabIdx)}
                   onRemoveTab={(chatId) => removeTab(idx, chatId)}
                   onCloseOthers={(chatId) => closeOtherTabs(idx, chatId)}
@@ -390,6 +392,9 @@ export function Workspace({
                   createChat={() => api.chats.create(sessionId)}
                   onSend={(content) => void handleLandingSend(idx, content)}
                   onSkillInvoke={(skillName, userMessage) => void handleLandingSkill(idx, skillName, userMessage)}
+                  chats={chats}
+                  onOpenChat={(chatId) => openChatInPane(idx, chatId)}
+                  onUnarchiveChat={unarchiveChat}
                 />
               )}
             </div>

apps/web/src/components/panes/ChatPane.tsx

@@ -1,5 +1,5 @@
 import { useCallback, useEffect, useRef, useState } from 'react';
-import { Pencil, Send, Square, X } from 'lucide-react';
+import { Pencil, Send, X } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import { useSessionStream } from '@/hooks/useSessionStream';
@@ -248,22 +248,6 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
         </div>
       )}
 
-      {/* Stop button when streaming */}
-      {streaming && (
-        <div className="border-t py-1">
-          <div className="max-w-[1000px] mx-auto w-full flex justify-center">
-          <button
-            type="button"
-            onClick={() => void handleStop()}
-            className="flex items-center gap-1.5 text-xs px-3 py-1 rounded-full border hover:bg-muted text-muted-foreground hover:text-foreground max-md:min-h-[44px] max-md:px-5"
-          >
-            <Square size={10} className="fill-current" />
-            Stop generating
-          </button>
-          </div>
-        </div>
-      )}
-
       {stale && streamingId && (
         <StaleStreamBanner
           onRetry={() => void handleRetryStale()}
@@ -280,6 +264,8 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
         webSearchEnabled={webSearchEnabled}
         onSend={handleSend}
         onForceSend={streaming ? handleForceSend : undefined}
+        generating={streaming}
+        onStop={handleStop}
         onSlashCommand={handleSlashCommand}
         chatId={chatId}
         chatLabel={sessionChats?.find((c) => c.id === chatId)?.name ?? 'Chat'}

apps/web/src/components/panes/CoderMessageList.tsx

@@ -149,7 +149,7 @@ interface Props {
   actions?: MessageActions;
 }
 
-const CODER_HIDDEN_ACTIONS: ('fork' | 'delete' | 'openInPane')[] = ['fork', 'openInPane'];
+const CODER_HIDDEN_ACTIONS: ('fork' | 'delete')[] = ['fork'];
 
 export function CoderMessageList({ messages, chatId, footer, actions }: Props) {
   const endRef = useRef<HTMLDivElement>(null);

apps/web/src/components/panes/CoderPane.tsx

@@ -581,6 +581,10 @@ export function CoderPane({
   const [queue, setQueue] = useState<string[]>([]);
   const queueProcessing = useRef(false);
   const inputRef = useRef<HTMLTextAreaElement>(null);
+  // The agent is "generating" during the dispatch POST (sending) AND while its
+  // task runs (activeTaskId). sending alone is too brief — it clears the moment
+  // dispatch returns — so queueing/stop must key on this combined signal.
+  const generating = sending || activeTaskId !== null;
 
   // Refresh pending changes when a message_complete arrives
   useEffect(() => {
@@ -760,24 +764,35 @@ export function CoderPane({
     }
   }, [sessionId, paneId, chatId, agentConfig, setMessages]);
 
-  // Drain queue when not busy
+  // Drain queue once the agent is idle (not just past the dispatch POST).
   useEffect(() => {
-    if (sending || queue.length === 0 || queueProcessing.current) return;
+    if (generating || queue.length === 0 || queueProcessing.current) return;
     queueProcessing.current = true;
     const next = queue[0]!;
     setQueue((prev) => prev.slice(1));
     sendOneMessage(next).finally(() => { queueProcessing.current = false; });
-  }, [sending, queue, sendOneMessage]);
+  }, [generating, queue, sendOneMessage]);
 
   const handleChatInputSend = useCallback(async (content: string) => {
     const text = content.trim();
     if (!text || !chatId) return;
-    if (sending) {
+    if (generating) {
       setQueue((prev) => [...prev, text]);
       return;
     }
     await sendOneMessage(text);
-  }, [sending, chatId, sendOneMessage]);
+  }, [generating, chatId, sendOneMessage]);
+
+  const handleStop = useCallback(async () => {
+    const taskId = activeTaskId;
+    if (!taskId) return;
+    try {
+      await api.coder.cancelTask(taskId);
+      setActiveTaskId(null); // optimistic; WS/poll terminal-state also clears it
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'stop failed');
+    }
+  }, [activeTaskId]);
 
   const handleChatInputSlash = useCallback(async (skillName: string, userMessage: string) => {
     if (!chatId) return;
@@ -867,9 +882,11 @@ export function CoderPane({
       {/* Composer + input */}
       <div className="shrink-0 border-t border-border">
         <ChatInput
-          disabled={sending || !chatId || chatPending}
+          disabled={!chatId || chatPending}
           projectId={projectPath ?? ''}
           onSend={handleChatInputSend}
+          generating={generating}
+          onStop={handleStop}
           onSlashCommand={handleChatInputSlash}
           slashGroups={slashGroups}
           chatId={chatId ?? undefined}

apps/web/src/hooks/sessionEvents.ts

@@ -51,7 +51,11 @@ export interface SessionUpdatedEvent {
 export interface SessionWorkspaceUpdatedEvent {
   type: 'session_workspace_updated';
   session_id: string;
-  workspace_panes: import('@/api/types').WorkspacePane[];
+  // Legacy bare array OR the new envelope — useWorkspacePanes normalizes both
+  // via toWorkspaceState.
+  workspace_panes:
+    | import('@/api/types').WorkspacePane[]
+    | import('@/api/types').WorkspaceState;
 }
 
 export interface SessionLoadedEvent {
@@ -75,6 +79,14 @@ export interface OpenChatInActivePaneEvent {
   chat_id: string;
 }
 
+// Open a whole chat in a fresh split pane (vs the active pane). Emitted by the
+// ChatTabBar tab context menu ("Open in new pane") and by MessageBubble.fork()
+// so a fork lands beside the original. useWorkspacePanes subscribes.
+export interface OpenChatInNewPaneEvent {
+  type: 'open_chat_in_new_pane';
+  chat_id: string;
+}
+
 // v1.14.x-html-artifact-panes: ActionRow's "Open in pane" button emits one of
 // these; useWorkspacePanes subscribes and inserts the corresponding artifact
 // pane (or focuses an existing one keyed by message_id).
@@ -178,6 +190,7 @@ export type SessionEvent =
   | OpenFileInBrowserEvent
   | AttachChatFileEvent
   | OpenChatInActivePaneEvent
+  | OpenChatInNewPaneEvent
   | OpenMarkdownArtifactPaneEvent
   | OpenHtmlArtifactPaneEvent
   | OpenSettingsPaneEvent

apps/web/src/hooks/useSidebar.ts

@@ -152,6 +152,7 @@ function applyEvent(prev: SidebarResponse, event: import('./sessionEvents').Sess
     case 'attach_chat_file':
       return prev;
     case 'open_chat_in_active_pane':
+    case 'open_chat_in_new_pane':
       // Consumed by Workspace; sidebar has no business with pane state.
       return prev;
     case 'open_markdown_artifact_pane':

apps/web/src/hooks/useWorkspacePanes.ts

@@ -3,9 +3,11 @@ import type { DragEvent } from 'react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import type {
+  ClosedPaneEntry,
   HtmlArtifactState,
   MarkdownArtifactState,
   WorkspacePane,
+  WorkspaceState,
 } from '@/api/types';
 import { setActivePaneInfo, clearActivePane } from '@/hooks/useActivePane';
 import { sessionEvents } from '@/hooks/sessionEvents';
@@ -32,19 +34,35 @@ function chatPane(chatId: string): WorkspacePane {
   return { id: generateId(), kind: 'chat', chatId, chatIds: [chatId], activeChatIdx: 0 };
 }
 
-interface ClosedPaneEntry {
-  kind: WorkspacePane['kind'];
-  chatIds: string[];
-  activeChatIdx: number;
-}
+// v2.6.x: reopen stack cap. The stack now lives in React state (persisted in
+// the WorkspaceState envelope), not a module-level array. `appendClosed` is the
+// pure state-updater helper.
 const MAX_CLOSED = 10;
-const closedPaneStack: ClosedPaneEntry[] = [];
 
-function pushClosed(pane: WorkspacePane): void {
-  if (pane.kind === 'empty' || pane.kind === 'settings') return;
-  if (pane.chatIds.length === 0) return;
-  closedPaneStack.push({ kind: pane.kind, chatIds: [...pane.chatIds], activeChatIdx: pane.activeChatIdx });
-  if (closedPaneStack.length > MAX_CLOSED) closedPaneStack.shift();
+// Pure helper: append a closed-pane entry derived from `pane` to `stack`,
+// capped at MAX_CLOSED (most-recent last). Returns the SAME reference when the
+// pane is not eligible (empty/settings/no chats) so callers can skip setState.
+function appendClosed(stack: ClosedPaneEntry[], pane: WorkspacePane): ClosedPaneEntry[] {
+  if (pane.kind === 'empty' || pane.kind === 'settings') return stack;
+  if (pane.chatIds.length === 0) return stack;
+  const entry = { kind: pane.kind, chatIds: [...pane.chatIds], activeChatIdx: pane.activeChatIdx };
+  // Dedupe a value-identical top entry. This is called via setClosedPaneStack
+  // inside the setPanes updater in removePane; React StrictMode double-invokes
+  // that updater in dev, which would otherwise push two identical entries.
+  // Real closes never collide (one chat lives in at most one pane).
+  const top = stack[stack.length - 1];
+  if (
+    top &&
+    top.kind === entry.kind &&
+    top.activeChatIdx === entry.activeChatIdx &&
+    top.chatIds.length === entry.chatIds.length &&
+    top.chatIds.every((id, i) => id === entry.chatIds[i])
+  ) {
+    return stack;
+  }
+  const next = [...stack, entry];
+  if (next.length > MAX_CLOSED) next.splice(0, next.length - MAX_CLOSED);
+  return next;
 }
 
 function chatNameForPaneKind(kind: 'coder' | 'terminal'): string {
@@ -110,6 +128,26 @@ function persistablePanes(panes: WorkspacePane[]): WorkspacePane[] {
   return normalizePanes(panes).filter((p) => p.kind !== 'settings');
 }
 
+// v2.6.x: LOCKED migration — a value read from session.workspace_panes (or the
+// session_workspace_updated frame) may be EITHER the legacy bare
+// WorkspacePane[] OR the new WorkspaceState envelope. Normalize to the
+// envelope. Must match the server's normalization byte-for-byte.
+function toWorkspaceState(raw: unknown): WorkspaceState {
+  if (Array.isArray(raw)) {
+    return { panes: raw as WorkspacePane[], tabNumbers: {}, nextTabNumber: 1, closedPaneStack: [] };
+  }
+  if (raw && typeof raw === 'object' && Array.isArray((raw as WorkspaceState).panes)) {
+    const env = raw as WorkspaceState;
+    return {
+      panes: env.panes,
+      tabNumbers: env.tabNumbers ?? {},
+      nextTabNumber: env.nextTabNumber ?? 1,
+      closedPaneStack: env.closedPaneStack ?? [],
+    };
+  }
+  return { panes: [], tabNumbers: {}, nextTabNumber: 1, closedPaneStack: [] };
+}
+
 // v1.9: per recon decision (c), settings panes don't count toward MAX_PANES.
 // Helper used at every pane-insertion site so the rule lives in one place.
 function nonSettingsCount(panes: WorkspacePane[]): number {
@@ -132,6 +170,9 @@ function readLegacyPanes(sessionId: string): WorkspacePane[] | null {
 
 export interface UseWorkspacePanesResult {
   panes: WorkspacePane[];
+  // v2.6.x: stable session-scoped tab number per chat id (Batch 3a). Keyed by
+  // chat.id, NEVER by tab position.
+  tabNumbers: Record<string, number>;
   activePaneIdx: number;
   setActivePaneIdx: React.Dispatch<React.SetStateAction<number>>;
   activePaneIdxRef: React.MutableRefObject<number>;
@@ -171,6 +212,12 @@ export interface UseWorkspacePanesResult {
 export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
   const [panes, setPanes] = useState<WorkspacePane[]>(() => [emptyPane()]);
   const [activePaneIdx, setActivePaneIdx] = useState(0);
+  // v2.6.x envelope state. Persisted alongside `panes` in the WorkspaceState
+  // envelope. `tabNumbers` is the stable session-scoped tab number per chat id;
+  // `nextTabNumber` only ever increments; `closedPaneStack` is the reopen LIFO.
+  const [tabNumbers, setTabNumbers] = useState<Record<string, number>>({});
+  const [nextTabNumber, setNextTabNumber] = useState(1);
+  const [closedPaneStack, setClosedPaneStack] = useState<ClosedPaneEntry[]>([]);
   const draggingIdxRef = useRef<number | null>(null);
   const [dragOverIdx, setDragOverIdx] = useState<number | null>(null);
   // v1.12.1: skip PATCH while hydrating from the server. Without this, the
@@ -237,27 +284,42 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
       try {
         const session = await api.sessions.get(sessionId);
         if (cancelled) return;
-        let initial: WorkspacePane[] = Array.isArray(session.workspace_panes)
-          ? normalizePanes(session.workspace_panes)
-          : [];
+        let env = toWorkspaceState(session.workspace_panes);
+        let initial: WorkspacePane[] = normalizePanes(env.panes);
         // One-time migration: if server is empty but legacy localStorage has
-        // a layout, seed the server and delete the local key.
+        // a layout, seed the server (as an envelope) and delete the local key.
         if (initial.length === 0) {
           const legacy = readLegacyPanes(sessionId);
           if (legacy && legacy.length > 0) {
             try {
-              const updated = await api.sessions.updateWorkspacePanes(sessionId, legacy);
+              const seedState: WorkspaceState = {
+                panes: persistablePanes(legacy),
+                tabNumbers: {},
+                nextTabNumber: 1,
+                closedPaneStack: [],
+              };
+              const updated = await api.sessions.updateWorkspacePanes(sessionId, seedState);
               if (cancelled) return;
-              initial = updated.workspace_panes;
+              env = toWorkspaceState(updated.workspace_panes);
+              initial = normalizePanes(env.panes);
               localStorage.removeItem(`${LEGACY_STORAGE_KEY}.${sessionId}`);
             } catch {
-              initial = legacy;
+              env = { ...env, panes: legacy };
+              initial = normalizePanes(legacy);
             }
           }
         }
         const next = initial.length > 0 ? initial : [emptyPane()];
-        lastRemoteJsonRef.current = JSON.stringify(persistablePanes(next));
+        lastRemoteJsonRef.current = JSON.stringify({
+          panes: persistablePanes(next),
+          tabNumbers: env.tabNumbers,
+          nextTabNumber: env.nextTabNumber,
+          closedPaneStack: env.closedPaneStack,
+        });
         setPanes(next);
+        setTabNumbers(env.tabNumbers);
+        setNextTabNumber(env.nextTabNumber);
+        setClosedPaneStack(env.closedPaneStack);
         setActivePaneIdx(0);
         seedEmptyScopedPanes(next);
       } finally {
@@ -273,15 +335,25 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     return sessionEvents.subscribe((ev) => {
       if (ev.type !== 'session_workspace_updated') return;
       if (ev.session_id !== sessionId) return;
-      const incoming = normalizePanes(
-        Array.isArray(ev.workspace_panes) ? ev.workspace_panes : [],
-      );
-      const json = JSON.stringify(incoming);
+      const env = toWorkspaceState(ev.workspace_panes);
+      const incoming = normalizePanes(env.panes);
+      // Echo-dedup on the FULL envelope so tabNumber / stack-only changes are
+      // not mistaken for our own write echo.
+      const json = JSON.stringify({
+        panes: persistablePanes(incoming),
+        tabNumbers: env.tabNumbers,
+        nextTabNumber: env.nextTabNumber,
+        closedPaneStack: env.closedPaneStack,
+      });
       if (json === lastRemoteJsonRef.current) return;
       lastRemoteJsonRef.current = json;
-      setPanes(incoming.length > 0 ? incoming : [emptyPane()]);
+      const nextPanes = incoming.length > 0 ? incoming : [emptyPane()];
+      setPanes(nextPanes);
+      setTabNumbers(env.tabNumbers);
+      setNextTabNumber(env.nextTabNumber);
+      setClosedPaneStack(env.closedPaneStack);
       setActivePaneIdx((prev) => Math.min(prev, Math.max(0, incoming.length - 1)));
-      seedEmptyScopedPanes(incoming.length > 0 ? incoming : [emptyPane()]);
+      seedEmptyScopedPanes(nextPanes);
     });
   }, [sessionId, seedEmptyScopedPanes]);
 
@@ -333,18 +405,75 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
   // before saving (ephemeral per v1.9).
   useEffect(() => {
     if (!hydratedRef.current) return;
-    const payload = persistablePanes(panes);
-    const json = JSON.stringify(payload);
+    // v2.6.x: persist the full WorkspaceState envelope. The dedup ref compares
+    // the whole envelope so tabNumber / reopen-stack changes also persist.
+    const envelope: WorkspaceState = {
+      panes: persistablePanes(panes),
+      tabNumbers,
+      nextTabNumber,
+      closedPaneStack,
+    };
+    const json = JSON.stringify(envelope);
     if (json === lastRemoteJsonRef.current) return;
     const timer = setTimeout(() => {
       lastRemoteJsonRef.current = json;
-      api.sessions.updateWorkspacePanes(sessionId, payload).catch(() => {
+      api.sessions.updateWorkspacePanes(sessionId, envelope).catch(() => {
         // Non-fatal: next change retries. Persistent failures surface via
         // the network layer's existing reconnect toast.
       });
     }, SAVE_DEBOUNCE_MS);
     return () => clearTimeout(timer);
-  }, [sessionId, panes]);
+  }, [sessionId, panes, tabNumbers, nextTabNumber, closedPaneStack]);
+
+  // v2.6.x (Batch 3a): maintain stable, session-scoped tab numbers. Collect the
+  // chat ids that appear in CHAT-kind panes in deterministic order (pane index,
+  // then tab index). Assign numbers to any without one (global per session,
+  // only ever increasing, never reused) and prune entries whose chat is no
+  // longer in any chat-kind pane. Guarded against render loops: only setState
+  // when something actually changed.
+  useEffect(() => {
+    const liveChatIds: string[] = [];
+    const liveSet = new Set<string>();
+    for (const pane of panes) {
+      if (pane.kind !== 'chat') continue;
+      for (const id of pane.chatIds) {
+        if (!liveSet.has(id)) {
+          liveSet.add(id);
+          liveChatIds.push(id);
+        }
+      }
+    }
+
+    // Assign: walk live ids in deterministic order, handing out numbers.
+    let counter = nextTabNumber;
+    const additions: Record<string, number> = {};
+    for (const id of liveChatIds) {
+      if (tabNumbers[id] === undefined && additions[id] === undefined) {
+        additions[id] = counter;
+        counter += 1;
+      }
+    }
+
+    // Prune: retire numbers for chats no longer in any chat-kind pane.
+    const removals: string[] = [];
+    for (const id of Object.keys(tabNumbers)) {
+      if (!liveSet.has(id)) removals.push(id);
+    }
+
+    const hasAdditions = Object.keys(additions).length > 0;
+    const hasRemovals = removals.length > 0;
+    if (!hasAdditions && !hasRemovals) return;
+
+    setTabNumbers((prev) => {
+      const next: Record<string, number> = {};
+      for (const [id, n] of Object.entries(prev)) {
+        if (!removals.includes(id)) next[id] = n;
+      }
+      Object.assign(next, additions);
+      return next;
+    });
+    if (hasAdditions) setNextTabNumber(counter);
+  }, [panes, tabNumbers, nextTabNumber]);
 
   useEffect(() => {
     const active = panes[activePaneIdx];
@@ -391,6 +520,37 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     setActivePaneIdx(paneIdx);
   }, []);
 
+  // Open a whole chat in its own fresh pane (focused). Detaches the chat from
+  // any pane currently showing it so it lives in exactly one pane (preserves
+  // the one-chat-per-pane model), dropping a source pane left with no tabs. For
+  // fork the chat isn't in any pane yet, so the detach is a no-op (pure append).
+  const openChatInNewPane = useCallback((chatId: string) => {
+    setPanes((prev) => {
+      const detached = prev.flatMap((p) => {
+        if (!p.chatIds.includes(chatId)) return [p];
+        const nextIds = p.chatIds.filter((id) => id !== chatId);
+        if (nextIds.length === 0) return [];
+        const ai = Math.min(p.activeChatIdx, nextIds.length - 1);
+        return [{ ...p, kind: 'chat' as const, chatId: nextIds[ai], chatIds: nextIds, activeChatIdx: ai }];
+      });
+      if (nonSettingsCount(detached) >= MAX_PANES) {
+        toast.error(`Maximum ${MAX_PANES} panes`);
+        return prev;
+      }
+      const next = [...detached, chatPane(chatId)];
+      setActivePaneIdx(next.length - 1);
+      return next;
+    });
+  }, []);
+
+  // ChatTabBar's "Open in new pane" + MessageBubble.fork() emit this.
+  useEffect(() => {
+    return sessionEvents.subscribe((ev) => {
+      if (ev.type !== 'open_chat_in_new_pane') return;
+      openChatInNewPane(ev.chat_id);
+    });
+  }, [openChatInNewPane]);
+
   const switchTab = useCallback((paneIdx: number, tabIdx: number) => {
     setPanes((prev) => {
       const next = [...prev];
@@ -411,7 +571,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
         if (next.length > 1) {
           // Last tab closed and other panes exist — remove the whole pane
           // instead of leaving an orphaned empty panel.
-          pushClosed(pane); setHasClosedPanes(true);
+          setClosedPaneStack((stack) => appendClosed(stack, pane));
           const spliced = next.filter((_, i) => i !== paneIdx);
           setActivePaneIdx((ai) => Math.min(ai, spliced.length - 1));
           return spliced;
@@ -547,7 +707,8 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     setPanes((prev) => {
       if (prev.length <= 1) {
         // Settings is the only kind that can be the last pane and still need
-        // closing (X / Esc / sidebar toggle). Fall back to empty.
+        // closing (X / Esc / sidebar toggle). Fall back to empty. One-pane
+        // edge: no relocation — there is no other pane.
         if (prev[idx]?.kind === 'settings') {
           setActivePaneIdx(0);
           return [emptyPane()];
@@ -559,35 +720,101 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
       // The endpoint is idempotent (404 on missing session) so a strict-mode
       // double-invoke of the updater is safe.
       const removed = prev[idx];
-      if (removed) { pushClosed(removed); setHasClosedPanes(true); }
+      // Push the original pane (with its chatIds intact) to the reopen stack.
+      if (removed) setClosedPaneStack((stack) => appendClosed(stack, removed));
       if (removed?.kind === 'terminal') {
         api.terminals.kill(sessionId, removed.id).catch(() => { /* non-fatal */ });
       }
-      const next = prev.filter((_, i) => i !== idx);
+
+      // v2.6.x (Batch 1): relocate a closing CHAT pane's tabs to the oldest
+      // remaining pane that can host chat tabs, so chats aren't lost on close.
+      // Only chat panes relocate — terminal/coder panes own a scoped chat bound
+      // to the pane, so those close exactly as before (no relocation).
+      let working = prev;
+      if (removed && removed.kind === 'chat' && removed.chatIds.length > 0) {
+        // "Oldest remaining": lowest index, excluding `idx`, that is a chat or
+        // empty pane (the only kinds that can host arbitrary chat tabs). Skip
+        // terminal/coder/settings/artifact panes.
+        let targetIdx = -1;
+        for (let i = 0; i < prev.length; i += 1) {
+          if (i === idx) continue;
+          const p = prev[i]!;
+          if (p.kind === 'chat' || p.kind === 'empty') {
+            targetIdx = i;
+            break;
+          }
+        }
+        if (targetIdx >= 0) {
+          working = prev.map((p, i) => {
+            if (i !== targetIdx) return p;
+            const mergedIds = [...p.chatIds, ...removed.chatIds];
+            // Preserve the target's existing focus — append, don't force-focus
+            // the moved tabs. Clamp only when the target had no active tab.
+            const ai = p.activeChatIdx >= 0 ? p.activeChatIdx : 0;
+            return {
+              ...p,
+              kind: 'chat' as const,
+              chatIds: mergedIds,
+              activeChatIdx: ai,
+              chatId: mergedIds[ai],
+            };
+          });
+        }
+      }
+
+      const next = working.filter((_, i) => i !== idx);
       setActivePaneIdx((ai) => Math.min(ai, next.length - 1));
       return next;
     });
   }, [sessionId]);
 
-  const [hasClosedPanes, setHasClosedPanes] = useState(closedPaneStack.length > 0);
+  const hasClosedPanes = closedPaneStack.length > 0;
 
   const reopenPane = useCallback(() => {
-    const entry = closedPaneStack.pop();
-    setHasClosedPanes(closedPaneStack.length > 0);
-    if (!entry) return;
+    // Read the top entry from the current render's stack (not inside the
+    // updater) so a StrictMode double-invoke can't pop two entries. The pop
+    // setState is idempotent: filtering by reference removes exactly this entry.
+    const e = closedPaneStack[closedPaneStack.length - 1];
+    if (!e) return;
+    setClosedPaneStack((stack) => (stack[stack.length - 1] === e ? stack.slice(0, -1) : stack));
     setPanes((prev) => {
+      // v2.6.x (Batch 4): reversible reopen. The closed tabs may have been
+      // relocated into another pane on close (Batch 1). Strip e.chatIds from
+      // every existing pane first so reopening never duplicates a tab —
+      // whether or not it was relocated (a no-op strip when it wasn't). Mirror
+      // removeTab's emptiness handling: a chat pane emptied by the strip is
+      // dropped when other panes remain, else turned empty.
+      const stripped: WorkspacePane[] = [];
+      for (const p of prev) {
+        const idxs = p.chatIds.filter((id) => !e.chatIds.includes(id));
+        if (idxs.length === p.chatIds.length) {
+          stripped.push(p);
+          continue;
+        }
+        if (idxs.length === 0) {
+          if (p.kind === 'chat') {
+            // Drop the now-empty chat pane (we still have the restored pane plus
+            // possibly others). If it would leave zero panes, turn it empty.
+            continue;
+          }
+          stripped.push({ ...p, chatId: undefined, chatIds: [], activeChatIdx: -1 });
+          continue;
+        }
+        const ai = Math.min(p.activeChatIdx, idxs.length - 1);
+        stripped.push({ ...p, chatIds: idxs, activeChatIdx: ai < 0 ? 0 : ai, chatId: idxs[ai < 0 ? 0 : ai] });
+      }
       const restored: WorkspacePane = {
         id: generateId(),
-        kind: entry.kind,
-        chatId: entry.chatIds[entry.activeChatIdx] ?? entry.chatIds[0],
-        chatIds: entry.chatIds,
-        activeChatIdx: Math.min(entry.activeChatIdx, entry.chatIds.length - 1),
+        kind: e.kind,
+        chatId: e.chatIds[e.activeChatIdx] ?? e.chatIds[0],
+        chatIds: e.chatIds,
+        activeChatIdx: Math.min(e.activeChatIdx, e.chatIds.length - 1),
       };
-      const next = [...prev, restored];
+      const next = [...stripped, restored];
       setActivePaneIdx(next.length - 1);
       return next;
     });
-  }, []);
+  }, [closedPaneStack]);
 
   // Replaces a single empty default pane with a chat pane. Used by the initial
   // chat fetch to land on the most-recent open chat if no saved pane state.
@@ -705,6 +932,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
 
   return {
     panes,
+    tabNumbers,
     activePaneIdx,
     setActivePaneIdx,
     activePaneIdxRef,

apps/web/src/lib/attachments.ts

@@ -56,19 +56,26 @@ export function inferLanguage(filename: string): string | null {
 
 export function flattenToMessage(attachments: Attachment[], text: string): string {
   if (attachments.length === 0) return text;
-  const blocks = attachments.map(a => {
-    // Pasted text is raw context, not code from a file — insert it verbatim with
-    // no ``` fence or provenance header. The chip only exists to keep the textarea
-    // tidy while composing; on send it should be exactly what the user pasted.
+  // Pasted text is raw context, not code from a file — insert it verbatim with no
+  // ``` fence or provenance header. It trails the typed text with a leading space
+  // so a leading slash command / prompt stays first and the paste reads as its
+  // continuation. File/line chips stay fenced provenance blocks, appended after.
+  const pasteBlocks: string[] = [];
+  const fencedBlocks: string[] = [];
+  for (const a of attachments) {
     if (a.kind === 'paste') {
-      return a.content;
+      pasteBlocks.push(a.content);
+      continue;
     }
     const fence = '```' + (a.language ?? '');
     const header =
       a.kind === 'lines'
         ? `// from: ${a.filename}:${a.range?.[0] ?? '?'}-${a.range?.[1] ?? '?'}`
         : `// from: ${a.filename}`;
-    return `${fence}\n${header}\n${a.content}\n\`\`\``;
-  });
-  return [...blocks, text].filter(Boolean).join('\n\n');
+    fencedBlocks.push(`${fence}\n${header}\n${a.content}\n\`\`\``);
+  }
+  // Typed text + pasted content on the same logical line (space-joined), then
+  // any fenced file blocks as separate paragraphs.
+  const lead = [text, ...pasteBlocks].filter(Boolean).join(' ');
+  return [lead, ...fencedBlocks].filter(Boolean).join('\n\n');
 }