feat(coder): re-key agent_sessions to (chat_id, agent) + worktrees table (P1.5-b)

The tab (a chat) is the context unit: two opencode tabs in one session are two independent agent contexts sharing one worktree. agent_sessions re-keys from (session_id, agent) to (chat_id, agent) — chat_id FK ON DELETE CASCADE (closing a tab ends its context); worktree_id and session_id become informational SET NULL columns. New worktrees table (one-per-session, survives session delete via session_id SET NULL) supersedes session_worktrees, which is defanged (CASCADE dropped) not yet removed. chat_id is threaded end-to-end: tasks.chat_id added, written 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) so ensureSession never gets a null key. Idempotent migration with a backfill-verify gate (0-row assertion after the test session was deleted). config_hash fingerprint logic preserved; one-worktree-per-session unchanged; runExternalAgent untouched. Column rename worktree_path -> path repointed at all five readers (server delete-guard, risk/stash endpoints, ensureSessionWorktree). Supersedes the earlier (worktree_id) draft.

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

apps/coder/src/routes/messages.ts

@@ -224,8 +224,8 @@ export function registerMessageRoutes(
         // External provider: create a task for the dispatcher
         const projectId = sessionRows[0]!.project_id;
         const [task] = await sql<{ id: string; state: string }[]>`
-          INSERT INTO tasks (project_id, input, agent, model, mode_id, thinking_option_id, session_id)
+          INSERT INTO tasks (project_id, input, agent, model, mode_id, thinking_option_id, session_id, chat_id)
-          VALUES (${projectId}, ${content}, ${provider}, ${model ?? null}, ${mode_id ?? null}, ${thinking_option_id ?? null}, ${sessionId})
+          VALUES (${projectId}, ${content}, ${provider}, ${model ?? null}, ${mode_id ?? null}, ${thinking_option_id ?? null}, ${sessionId}, ${chatId})
           RETURNING id, state
         `;
         reply.code(202);

apps/coder/src/routes/skills.ts

@@ -91,8 +91,8 @@ export function registerSkillRoutes(
 
         const taskInput = `${body}\n\n---\n\n${userText}`;
         const [task] = await sql<{ id: string; state: string }[]>`
-          INSERT INTO tasks (project_id, input, agent, model, mode_id, thinking_option_id, session_id)
+          INSERT INTO tasks (project_id, input, agent, model, mode_id, thinking_option_id, session_id, chat_id)
-          VALUES (${sessionRows[0]!.project_id}, ${taskInput}, ${provider}, ${model ?? null}, ${mode_id ?? null}, ${thinking_option_id ?? null}, ${sessionId})
+          VALUES (${sessionRows[0]!.project_id}, ${taskInput}, ${provider}, ${model ?? null}, ${mode_id ?? null}, ${thinking_option_id ?? null}, ${sessionId}, ${chatId})
           RETURNING id, state
         `;
         await sql`UPDATE chats SET updated_at = clock_timestamp() WHERE id = ${chatId}`;

apps/coder/src/routes/worktree-safety.ts

@@ -18,7 +18,7 @@ export function registerWorktreeSafetyRoutes(app: FastifyInstance, sql: Sql): vo
     '/api/sessions/:sessionId/worktree-risk',
     async (req) => {
       const rows = await sql<{ worktree_path: string }[]>`
-        SELECT worktree_path FROM session_worktrees WHERE session_id = ${req.params.sessionId}
+        SELECT path AS worktree_path FROM worktrees WHERE session_id = ${req.params.sessionId}
       `;
       const reports = [];
       for (const row of rows) {
@@ -33,7 +33,7 @@ export function registerWorktreeSafetyRoutes(app: FastifyInstance, sql: Sql): vo
     '/api/sessions/:sessionId/worktree-stash',
     async (req) => {
       const rows = await sql<{ worktree_path: string }[]>`
-        SELECT worktree_path FROM session_worktrees WHERE session_id = ${req.params.sessionId}
+        SELECT path AS worktree_path FROM worktrees WHERE session_id = ${req.params.sessionId}
       `;
       const results = [];
       for (const row of rows) {

apps/coder/src/schema.sql

@@ -83,16 +83,20 @@ CREATE TABLE IF NOT EXISTS session_worktrees (
   base_commit TEXT,
   created_at TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp()
 );
--- Migrate existing FK to CASCADE (idempotent: drops the old constraint if present).
+-- P1.5-b: DEFANG the CASCADE — a session delete must no longer wipe its worktree
+-- row. This table is SUPERSEDED by `worktrees` below; all readers are repointed
+-- this phase, so the row just persists (dead) on session delete until a later
+-- cleanup drops the table. session_id is this table's PRIMARY KEY, so it cannot be
+-- nullable → SET NULL is invalid and NO ACTION/RESTRICT would block deletes; the
+-- only valid defang is to drop the FK with no replacement. Idempotent: only fires
+-- while the FK is still ON DELETE CASCADE ('c').
 DO $$ BEGIN
   IF EXISTS (
     SELECT 1 FROM pg_constraint
     WHERE conname = 'session_worktrees_session_id_fkey'
-      AND confdeltype <> 'c'
+      AND confdeltype = 'c'
   ) THEN
     ALTER TABLE session_worktrees DROP CONSTRAINT session_worktrees_session_id_fkey;
-    ALTER TABLE session_worktrees ADD CONSTRAINT session_worktrees_session_id_fkey
-      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE CASCADE;
   END IF;
 END $$;
 
@@ -127,6 +131,80 @@ END $$;
 -- v2.6: config fingerprint for stale-session detection (auto-recover on model change).
 ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS config_hash TEXT;
 
+-- ─── P1.5-b (corrected): worktrees entity + re-key agent_sessions to (chat_id, agent) ───
+-- The TAB (a chat) is the context unit: two opencode tabs in one session = two
+-- independent contexts sharing one worktree. So agent_sessions keys on
+-- (chat_id, agent), NOT (worktree_id, agent) or (session_id, agent). The
+-- `worktrees` table is one-per-session (selectable later) and only referenced
+-- informationally by agent_sessions.worktree_id (SET NULL); chat_id is the key.
+--
+-- PREREQUISITE: the unmigratable test session (35 chats, 1 agent_sessions row that
+-- maps to no single chat) is DELETED before this runs, so agent_sessions is empty
+-- and the chat_id backfill is N/A. If a row with NULL chat_id remains, the verify
+-- gate below RAISEs and aborts — delete the offending session first.
+
+-- worktree as a first-class entity; survives session delete (session_id SET NULL).
+CREATE TABLE IF NOT EXISTS worktrees (
+  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  session_id  UUID REFERENCES sessions(id) ON DELETE SET NULL,
+  project_id  UUID,
+  path        TEXT NOT NULL,
+  branch      TEXT,
+  base_commit TEXT,
+  slug        TEXT,
+  status      TEXT NOT NULL DEFAULT 'active' CHECK (status IN ('active','archived')),
+  created_at  TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp()
+);
+CREATE UNIQUE INDEX IF NOT EXISTS worktrees_active_path_uidx ON worktrees(path) WHERE status='active';
+
+-- Migrate any surviving session_worktrees rows → worktrees (idempotent; 0 rows
+-- after the test-session delete, kept for generality / fresh-DB safety).
+INSERT INTO worktrees (session_id, path, branch, base_commit, status)
+SELECT sw.session_id, sw.worktree_path, 'session-' || sw.session_id, sw.base_commit, 'active'
+FROM session_worktrees sw
+WHERE NOT EXISTS (SELECT 1 FROM worktrees w WHERE w.session_id = sw.session_id AND w.status='active');
+
+-- Dispatch hint: which chat (tab) a task belongs to. The coder message route and
+-- skills route set it from the frontend tab; session-less creators (arena, MCP,
+-- new_task, generic /api/tasks) leave it NULL and the dispatcher creates a chat.
+ALTER TABLE tasks ADD COLUMN IF NOT EXISTS chat_id UUID REFERENCES chats(id) ON DELETE SET NULL;
+
+-- Re-key columns on agent_sessions.
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS chat_id UUID;
+ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS worktree_id UUID;
+
+-- BACKFILL-VERIFY GATE: the new PK is (chat_id, agent), so chat_id must be
+-- non-null on every row before the swap. With the test session deleted this is a
+-- 0-row assertion; if any row has NULL chat_id (an unmigratable pre-existing row),
+-- abort loudly rather than create a degenerate (NULL, agent) key.
+DO $$
+DECLARE n int;
+BEGIN
+  SELECT count(*) INTO n FROM agent_sessions WHERE chat_id IS NULL;
+  IF n > 0 THEN
+    RAISE EXCEPTION 'P1.5-b: % agent_sessions row(s) have NULL chat_id — delete the unmigratable session(s) before applying', n;
+  END IF;
+END $$;
+
+-- Swap PK (session_id,agent) → (chat_id,agent) + FKs (run-once, guarded on the new
+-- FK's absence). chat_id CASCADEs from chats (closing a tab ends its context);
+-- worktree_id is informational SET NULL; session_id defanged to nullable SET NULL.
+DO $$ BEGIN
+  IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'agent_sessions_chat_id_fkey') THEN
+    ALTER TABLE agent_sessions DROP CONSTRAINT IF EXISTS agent_sessions_pkey;
+    ALTER TABLE agent_sessions DROP CONSTRAINT IF EXISTS agent_sessions_session_id_fkey;
+    ALTER TABLE agent_sessions ALTER COLUMN session_id DROP NOT NULL;
+    ALTER TABLE agent_sessions ALTER COLUMN chat_id SET NOT NULL;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_pkey PRIMARY KEY (chat_id, agent);
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_chat_id_fkey
+      FOREIGN KEY (chat_id) REFERENCES chats(id) ON DELETE CASCADE;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_session_id_fkey
+      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE SET NULL;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_worktree_id_fkey
+      FOREIGN KEY (worktree_id) REFERENCES worktrees(id) ON DELETE SET NULL;
+  END IF;
+END $$;
+
 -- v2.6: attribution for DiffPanel badges (Phase 1 UX reads this).
 ALTER TABLE pending_changes ADD COLUMN IF NOT EXISTS agent TEXT;
 

apps/coder/src/services/agent-backend.ts

@@ -37,8 +37,15 @@ export interface EnsureSessionOpts {
   agent: string;
   /** Resolved model id. */
   model: string;
+  /** P1.5-b: the chat (tab) this turn belongs to. agent_sessions is keyed
+   *  (chat_id, agent) — the tab/chat is the context unit. Always non-null:
+   *  the dispatcher creates a chat for session-less tasks before calling. */
+  chatId: string;
   /** Shared per-session worktree (one per `sessions.id`, not per pane). */
   worktreePath: string;
+  /** P1.5-b: the `worktrees.id` for this session's worktree — stored on the
+   *  agent_sessions row informationally (NOT the key). */
+  worktreeId: string;
   projectId: string;
 }
 
@@ -47,6 +54,10 @@ export interface AgentSessionHandle {
   sessionId: string;
   agent: string;
   backend: AgentBackendKind;
+  /** P1.5-b: the chat (tab) this session is keyed on (with agent). */
+  chatId: string;
+  /** P1.5-b: the worktree this session's chat runs in (informational link). */
+  worktreeId: string;
   /** Provider's own session id (resume token); null until the backend assigns one. */
   agentSessionId: string | null;
   /** opencode HTTP server port; null for ACP backends. */

apps/coder/src/services/backends/opencode-server.ts

@@ -423,9 +423,12 @@ export class OpenCodeServerBackend implements AgentBackend {
     if (!this.client) throw new Error('opencode-server: client not ready after ensureServer');
 
     const configHash = sessionConfigHash(opts.model);
+    // P1.5-b: agent_sessions is keyed (chat_id, agent) — the tab/chat is the
+    // context unit (two tabs in one session = two contexts sharing one worktree).
+    // session_id + worktree_id are retained as informational (SET NULL) columns.
     const [row] = await this.sql<{ agent_session_id: string | null; status: string; config_hash: string | null }[]>`
       SELECT agent_session_id, status, config_hash FROM agent_sessions
-      WHERE session_id = ${sessionId} AND agent = ${opts.agent}
+      WHERE chat_id = ${opts.chatId} AND agent = ${opts.agent}
     `;
     let agentSessionId = row?.agent_session_id ?? null;
 
@@ -447,10 +450,12 @@ export class OpenCodeServerBackend implements AgentBackend {
       agentSessionId = created.data.id;
       await this.sql`
         INSERT INTO agent_sessions
-          (session_id, agent, backend, agent_session_id, server_port, status, last_active_at, config_hash)
+          (chat_id, session_id, worktree_id, agent, backend, agent_session_id, server_port, status, last_active_at, config_hash)
         VALUES
-          (${sessionId}, ${opts.agent}, 'opencode_server', ${agentSessionId}, ${this.port}, 'active', clock_timestamp(), ${configHash})
+          (${opts.chatId}, ${sessionId}, ${opts.worktreeId}, ${opts.agent}, 'opencode_server', ${agentSessionId}, ${this.port}, 'active', clock_timestamp(), ${configHash})
-        ON CONFLICT (session_id, agent) DO UPDATE SET
+        ON CONFLICT (chat_id, agent) DO UPDATE SET
+          session_id = EXCLUDED.session_id,
+          worktree_id = EXCLUDED.worktree_id,
           backend = 'opencode_server',
           agent_session_id = EXCLUDED.agent_session_id,
           server_port = EXCLUDED.server_port,
@@ -462,7 +467,7 @@ export class OpenCodeServerBackend implements AgentBackend {
       await this.sql`
         UPDATE agent_sessions
         SET status = 'active', last_active_at = clock_timestamp(), server_port = ${this.port}, config_hash = ${configHash}
-        WHERE session_id = ${sessionId} AND agent = ${opts.agent}
+        WHERE chat_id = ${opts.chatId} AND agent = ${opts.agent}
       `;
     }
 
@@ -498,6 +503,8 @@ export class OpenCodeServerBackend implements AgentBackend {
       sessionId,
       agent: opts.agent,
       backend: 'opencode_server',
+      chatId: opts.chatId,
+      worktreeId: opts.worktreeId,
       agentSessionId: ocSessionId,
       serverPort: this.port,
     };
@@ -593,7 +600,7 @@ export class OpenCodeServerBackend implements AgentBackend {
     }
     await this.sql`
       UPDATE agent_sessions SET status = 'closed'
-      WHERE session_id = ${handle.sessionId} AND agent = ${handle.agent}
+      WHERE chat_id = ${handle.chatId} AND agent = ${handle.agent}
     `.catch(() => {});
   }
 

apps/coder/src/services/dispatcher.ts

@@ -78,8 +78,9 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
         mode_id: string | null;
         thinking_option_id: string | null;
         session_id: string | null;
+        chat_id: string | null;
       }[]>`
-        SELECT id, project_id, input, agent, model, mode_id, thinking_option_id, session_id
+        SELECT id, project_id, input, agent, model, mode_id, thinking_option_id, session_id, chat_id
         FROM tasks
         WHERE state = 'pending'
         ORDER BY created_at
@@ -110,6 +111,7 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
     mode_id: string | null;
     thinking_option_id: string | null;
     session_id: string | null;
+    chat_id: string | null;
   }): Promise<void> {
     const taskId = task.id;
 
@@ -511,6 +513,7 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
       mode_id: string | null;
       thinking_option_id: string | null;
       session_id: string | null;
+      chat_id: string | null;
     },
     installPath: string | null,
   ): Promise<void> {
@@ -543,10 +546,18 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
         WHERE id = ${taskId}
       `;
 
-      // Resolve session + chat (mirrors runExternalAgent).
+      // Resolve session + chat. P1.5-b: the chat (tab) is the context key, so the
+      // chat_id MUST be non-null and stable before ensureSession. The coder message
+      // route + skills route stamp task.chat_id with the frontend tab's chat — use
+      // it directly. Session-less creators (arena, MCP, new_task, generic
+      // /api/tasks) leave it null; fall back to resolving/creating a real chat so
+      // ensureSession never receives a degenerate (null, agent) key.
       let sessionId: string;
       let chatId: string;
-      if (task.session_id) {
+      if (task.chat_id && task.session_id) {
+        sessionId = task.session_id;
+        chatId = task.chat_id;
+      } else if (task.session_id) {
         sessionId = task.session_id;
         const chats = await sql<{ id: string }[]>`
           SELECT id FROM chats WHERE session_id = ${sessionId} AND status = 'open' ORDER BY created_at DESC LIMIT 1
@@ -587,7 +598,7 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
 
       // Persistent, session-keyed worktree (shared across turns; NOT torn down
       // per turn — Phase 3 reaps it). Captures base_commit for a stable diff.
-      const { worktreePath, baseCommit } = await ensureSessionWorktree(sql, projectPath, sessionId, {
+      const { worktreeId, worktreePath, baseCommit } = await ensureSessionWorktree(sql, projectPath, sessionId, {
         signal: ac.signal,
       });
       log.info({ taskId, worktreePath }, 'dispatcher: session worktree ready');
@@ -680,7 +691,9 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
       const handle = await backend.ensureSession(sessionId, {
         agent,
         model,
+        chatId,
         worktreePath,
+        worktreeId,
         projectId: task.project_id,
       });
       const result = await backend.prompt(handle, task.input, {

apps/coder/src/services/worktrees.ts

@@ -119,16 +119,18 @@ export async function cleanupWorktree(
 // ─── v2.6: session-keyed persistent worktree ────────────────────────────────
 
 export interface SessionWorktree {
+  /** P1.5-b: the `worktrees.id` — stored on agent_sessions informationally. */
+  worktreeId: string;
   worktreePath: string;
   baseCommit: string | null;
 }
 
 /**
- * v2.6: create-or-reuse ONE worktree per BooCode session (shared across all
+ * v2.6 / P1.5-b: create-or-reuse ONE worktree per BooCode session (shared across
- * agents/turns in the session), recorded in `session_worktrees`. Unlike the
+ * all tabs/agents in the session), recorded in `worktrees` (was the superseded
- * per-task `createWorktree`, this persists — it is NOT torn down per turn
+ * `session_worktrees`). Persists — NOT torn down per turn (cleanup is Phase 3) —
- * (cleanup is Phase 3). Captures the project's current HEAD as `base_commit`
+ * and now survives session delete (`worktrees.session_id` is ON DELETE SET NULL).
- * so the accumulating diff has a stable baseline across turns.
+ * Captures the project's current HEAD as `base_commit` for a stable diff baseline.
  *
  * Distinct path namespace (`session-<id>` branch, `/sess-<id>` dir) so it never
  * collides with the per-task worktrees that arena/new_task/MCP still use.
@@ -139,11 +141,13 @@ export async function ensureSessionWorktree(
   sessionId: string,
   opts?: { signal?: AbortSignal },
 ): Promise<SessionWorktree> {
-  const [existing] = await sql<{ worktree_path: string; base_commit: string | null }[]>`
+  const [existing] = await sql<{ id: string; path: string; base_commit: string | null }[]>`
-    SELECT worktree_path, base_commit FROM session_worktrees WHERE session_id = ${sessionId}
+    SELECT id, path, base_commit FROM worktrees
+    WHERE session_id = ${sessionId} AND status = 'active'
+    LIMIT 1
   `;
   if (existing) {
-    return { worktreePath: existing.worktree_path, baseCommit: existing.base_commit };
+    return { worktreeId: existing.id, worktreePath: existing.path, baseCommit: existing.base_commit };
   }
 
   const worktreePath = `${WORKTREE_BASE}/sess-${sessionId}`;
@@ -167,17 +171,28 @@ export async function ensureSessionWorktree(
     throw new Error(`Failed to create session worktree: ${result.stderr.trim() || result.stdout.trim()}`);
   }
 
-  // Persist. ON CONFLICT keeps the first writer's row if two turns race the create.
+  // Insert-or-get: WHERE NOT EXISTS keeps the first writer's row if two turns race
-  await sql`
+  // the create (the partial unique on active path also backstops it).
-    INSERT INTO session_worktrees (session_id, worktree_path, base_commit)
+  const [inserted] = await sql<{ id: string; path: string; base_commit: string | null }[]>`
-    VALUES (${sessionId}, ${worktreePath}, ${baseCommit})
+    INSERT INTO worktrees (session_id, path, branch, base_commit, status)
-    ON CONFLICT (session_id) DO NOTHING
+    SELECT ${sessionId}, ${worktreePath}, ${branchName}, ${baseCommit}, 'active'
+    WHERE NOT EXISTS (
+      SELECT 1 FROM worktrees WHERE session_id = ${sessionId} AND status = 'active'
+    )
+    RETURNING id, path, base_commit
   `;
-  const [row] = await sql<{ worktree_path: string; base_commit: string | null }[]>`
+  if (inserted) {
-    SELECT worktree_path, base_commit FROM session_worktrees WHERE session_id = ${sessionId}
+    return { worktreeId: inserted.id, worktreePath: inserted.path, baseCommit: inserted.base_commit };
+  }
+  // Lost the race — another turn inserted first; read its row.
+  const [row] = await sql<{ id: string; path: string; base_commit: string | null }[]>`
+    SELECT id, path, base_commit FROM worktrees
+    WHERE session_id = ${sessionId} AND status = 'active'
+    LIMIT 1
   `;
   return {
-    worktreePath: row?.worktree_path ?? worktreePath,
+    worktreeId: row!.id,
+    worktreePath: row?.path ?? worktreePath,
     baseCommit: row?.base_commit ?? baseCommit,
   };
 }

apps/server/src/routes/sessions.ts

@@ -432,16 +432,18 @@ export function registerSessionRoutes(
       const id = req.params.id;
       const force = req.query.force === 'true' || req.query.force === '1';
 
-      // Session-delete work-loss guard. CASCADE on session_worktrees means the
+      // Session-delete work-loss guard. The check MUST run BEFORE the DELETE:
-      // DELETE below auto-wipes the worktree row, so the safety check MUST run
+      // worktrees.session_id is ON DELETE SET NULL (P1.5-b), so once the session
-      // BEFORE it (paths read while the row still exists, pre-CASCADE).
+      // is gone the worktree rows no longer point back to it — read them while
+      // the link still exists.
       //
-      // Optimization: read session_worktrees from our own (shared) DB first.
+      // Optimization: read worktrees (P1.5-b — was session_worktrees) from our
-      // No row => chat-only session => nothing on disk => delete immediately,
+      // own (shared) DB first. No row => chat-only session => nothing on disk =>
-      // zero round-trip. Only worktree-backed sessions pay the host git check.
+      // delete immediately, zero round-trip. Only worktree-backed sessions pay
+      // the host git check.
       if (!force) {
-        const worktrees = await sql<{ worktree_path: string }[]>`
+        const worktrees = await sql<{ path: string }[]>`
-          SELECT worktree_path FROM session_worktrees WHERE session_id = ${id}
+          SELECT path FROM worktrees WHERE session_id = ${id}
         `;
         if (worktrees.length > 0) {
           // Worktree dirs live on the host; only BooCoder can run git on them.