fix(coder): F.1 post-interrupt stale-terminal guard (opencode warm server)

opencode emits one trailing session.idle/error for a turn cancelled via client.session.abort(), carrying only a sessionID (no turn id). The warm-server backend settled activeTurn on that event, so after Stop + an immediate new message the orphan idle settled the NEXT turn early as success (one-click reachable since v2.6.5's Send->Stop composer).

Adds a pure per-session guard (backends/turn-guard.ts: armAbortGuard / noteTurnActivity / consumeTerminal over swallowNextTerminal) wired into opencode-server.ts: abort arms it, the next terminal is swallowed once, and a new turn's first delta self-heals so a never-arriving orphan can't strand a real turn. Test-first; 3 regression tests in turn-guard.test.ts. Paseo parallel: 1d38aac.

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

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

@@ -37,6 +37,7 @@ import {
 import type { ToolCallStatus } from '@agentclientprotocol/sdk';
 import type { Sql } from '../../db.js';
 import type { AcpToolSnapshot } from '../acp-tool-snapshot.js';
+import { armAbortGuard, noteTurnActivity, consumeTerminal } from './turn-guard.js';
 import type {
   AgentBackend,
   AgentEvent,
@@ -78,6 +79,9 @@ interface SessionState {
   /** Per-session SSE subscription handle. Non-null while the loop is running;
    *  aborting it tears down the underlying fetch and exits the loop. */
   sseAbort: AbortController | null;
+  /** F.1 post-abort orphan-terminal guard: swallow the one session.idle/error
+   *  opencode emits for an aborted turn so it can't settle the next turn. */
+  swallowNextTerminal: boolean;
 }
 
 export interface OpenCodeServerBackendDeps {
@@ -305,13 +309,19 @@ export class OpenCodeServerBackend implements AgentBackend {
       }
       // ─── lifecycle ─────────────────────────────────────────────────────────
       case 'session.idle': {
-        this.byOpencodeId.get(ev.properties.sessionID)?.activeTurn?.settle({ ok: true });
+        const st = this.byOpencodeId.get(ev.properties.sessionID);
+        if (!st) return;
+        if (consumeTerminal(st) === 'swallow') return; // F.1: drop the post-abort orphan
+        st.activeTurn?.settle({ ok: true });
         return;
       }
       case 'session.error': {
         const sid = ev.properties.sessionID;
         if (!sid) return;
-        this.byOpencodeId.get(sid)?.activeTurn?.settle({ ok: false, error: errToString(ev.properties.error) });
+        const st = this.byOpencodeId.get(sid);
+        if (!st) return;
+        if (consumeTerminal(st) === 'swallow') return; // F.1: drop the post-abort orphan
+        st.activeTurn?.settle({ ok: false, error: errToString(ev.properties.error) });
         return;
       }
       default:
@@ -358,6 +368,8 @@ export class OpenCodeServerBackend implements AgentBackend {
   /** Reset the inactivity backstop on any event routed to a session's active turn. */
   private bumpActivity(st: SessionState): void {
     if (!st.activeTurn) return;
+    // A live turn is producing → the post-abort orphan-terminal window is over.
+    noteTurnActivity(st);
     if (st.watchdog) clearTimeout(st.watchdog);
     st.watchdog = setTimeout(() => {
       void this.onTurnStall(st);
@@ -490,6 +502,7 @@ export class OpenCodeServerBackend implements AgentBackend {
         activeTurn: null,
         watchdog: null,
         sseAbort: null,
+        swallowNextTerminal: false,
       };
       this.byOpencodeId.set(ocSessionId, state);
     }
@@ -528,6 +541,7 @@ export class OpenCodeServerBackend implements AgentBackend {
         activeTurn: null,
         watchdog: null,
         sseAbort: null,
+        swallowNextTerminal: false,
       };
       this.byOpencodeId.set(oc, state);
     }
@@ -561,6 +575,9 @@ export class OpenCodeServerBackend implements AgentBackend {
       const onAbort = () => {
         // Abort the turn only — never the server.
         client.session.abort({ sessionID: oc, directory: ctx.worktreePath }).catch(() => {});
+        // F.1: opencode emits one trailing session.idle/error for the cancelled
+        // turn — arm the guard so it's swallowed, not used to settle the next turn.
+        armAbortGuard(session);
         settle({ ok: false, error: 'aborted' });
       };