diff --git a/apps/coder/src/services/backends/__tests__/turn-guard.test.ts b/apps/coder/src/services/backends/__tests__/turn-guard.test.ts
new file mode 100644
index 0000000..87f73ce
--- /dev/null
+++ b/apps/coder/src/services/backends/__tests__/turn-guard.test.ts
@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import {
+  armAbortGuard,
+  noteTurnActivity,
+  consumeTerminal,
+  type AbortTerminalGuard,
+} from '../turn-guard.js';
+
+describe('post-abort terminal guard (F.1)', () => {
+  it('swallows the orphan terminal that follows an abort, then settles the next real one', () => {
+    // Reproduces the v2.6.5 Stop-button bug: abort turn A, then opencode emits a
+    // trailing session.idle for A. That orphan must NOT settle the next turn.
+    const g: AbortTerminalGuard = { swallowNextTerminal: false };
+
+    armAbortGuard(g); // user aborts turn A
+    expect(consumeTerminal(g)).toBe('swallow'); // opencode's orphan idle for A → dropped
+    expect(consumeTerminal(g)).toBe('settle'); // turn B's real idle → settles B
+  });
+
+  it('settles a terminal when no abort happened', () => {
+    const g: AbortTerminalGuard = { swallowNextTerminal: false };
+    expect(consumeTerminal(g)).toBe('settle');
+  });
+
+  it('self-heals if the orphan never arrives: new-turn activity clears the guard', () => {
+    // If opencode emits no orphan idle (e.g. abort-before-prompt), the next turn's
+    // real terminal must still settle rather than being swallowed forever.
+    const g: AbortTerminalGuard = { swallowNextTerminal: false };
+
+    armAbortGuard(g); // abort A, but no orphan idle arrives
+    noteTurnActivity(g); // turn B produces its first delta
+    expect(consumeTerminal(g)).toBe('settle'); // turn B's idle settles, not swallowed
+  });
+});
diff --git a/apps/coder/src/services/backends/opencode-server.ts b/apps/coder/src/services/backends/opencode-server.ts
index a3d7d8c..4b62456 100644
--- a/apps/coder/src/services/backends/opencode-server.ts
+++ b/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' });
       };
 
diff --git a/apps/coder/src/services/backends/turn-guard.ts b/apps/coder/src/services/backends/turn-guard.ts
new file mode 100644
index 0000000..3bf469e
--- /dev/null
+++ b/apps/coder/src/services/backends/turn-guard.ts
@@ -0,0 +1,38 @@
+/**
+ * Guard against opencode's post-abort "orphan" terminal event (F.1).
+ *
+ * When a turn is aborted (`client.session.abort`), opencode emits one trailing
+ * `session.idle` / `session.error` for the cancelled turn. Without a guard that
+ * orphan settles whatever turn currently holds the session slot — which, after
+ * the user immediately sends another message, is the NEXT turn, settling it early
+ * as success (the v2.6.5 Stop-button bug). opencode terminal events carry only a
+ * `sessionID` (no turn id), so we can't match by id; instead we swallow exactly
+ * one terminal per abort, and self-heal if that orphan never arrives.
+ */
+export interface AbortTerminalGuard {
+  /** True between an abort and the orphan terminal event that follows it. */
+  swallowNextTerminal: boolean;
+}
+
+/** Arm on abort: the next terminal event for this session is the orphan. */
+export function armAbortGuard(g: AbortTerminalGuard): void {
+  g.swallowNextTerminal = true;
+}
+
+/**
+ * A new turn produced activity (delta) → the orphan window is over. Self-heals
+ * the case where opencode emits no orphan idle (e.g. abort-before-prompt), so a
+ * real terminal still settles instead of being swallowed forever.
+ */
+export function noteTurnActivity(g: AbortTerminalGuard): void {
+  g.swallowNextTerminal = false;
+}
+
+/** Decide a terminal (idle/error): swallow the post-abort orphan once, else settle. */
+export function consumeTerminal(g: AbortTerminalGuard): 'swallow' | 'settle' {
+  if (g.swallowNextTerminal) {
+    g.swallowNextTerminal = false;
+    return 'swallow';
+  }
+  return 'settle';
+}