diff --git a/packages/happy-app/sources/sync/ops.test.ts b/packages/happy-app/sources/sync/ops.test.ts
new file mode 100644
index 000000000..33b77b834
--- /dev/null
+++ b/packages/happy-app/sources/sync/ops.test.ts
@@ -0,0 +1,61 @@
+/**
+ * Tests for session operations (ops.ts)
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { mockSessionRPC } = vi.hoisted(() => ({
+    mockSessionRPC: vi.fn(),
+}));
+
+vi.mock('./apiSocket', () => ({
+    apiSocket: {
+        sessionRPC: mockSessionRPC,
+    }
+}));
+
+vi.mock('./sync', () => ({
+    sync: {}
+}));
+
+import { sessionKill } from './ops';
+
+describe('sessionKill', () => {
+    beforeEach(() => {
+        vi.clearAllMocks();
+    });
+
+    it('should return success when RPC succeeds', async () => {
+        mockSessionRPC.mockResolvedValue({
+            success: true,
+            message: 'Killing happy-cli process'
+        });
+
+        const result = await sessionKill('test-session-123');
+
+        expect(result.success).toBe(true);
+        expect(mockSessionRPC).toHaveBeenCalledWith(
+            'test-session-123',
+            'killSession',
+            {}
+        );
+    });
+
+    it('should return success when RPC fails because process already exited (regression #687)', async () => {
+        mockSessionRPC.mockRejectedValue(new Error('RPC call failed'));
+
+        const result = await sessionKill('dead-session-456');
+
+        expect(result.success).toBe(true);
+        expect(result.message).toBe('Session already stopped');
+    });
+
+    it('should return success even on non-Error RPC failures', async () => {
+        mockSessionRPC.mockRejectedValue('timeout');
+
+        const result = await sessionKill('dead-session-789');
+
+        expect(result.success).toBe(true);
+        expect(result.message).toBe('Session already stopped');
+    });
+});
diff --git a/packages/happy-app/sources/sync/ops.ts b/packages/happy-app/sources/sync/ops.ts
index 07f70e694..37bf03787 100644
--- a/packages/happy-app/sources/sync/ops.ts
+++ b/packages/happy-app/sources/sync/ops.ts
@@ -473,7 +473,9 @@ export async function sessionRipgrep(
 }
 
 /**
- * Kill the session process immediately
+ * Kill the session process immediately.
+ * Idempotent: if the process already exited, treat it as success
+ * since the desired outcome (session stopped) is already achieved.
  */
 export async function sessionKill(sessionId: string): Promise<SessionKillResponse> {
     try {
@@ -484,9 +486,12 @@ export async function sessionKill(sessionId: string): Promise<SessionKillRespons
         );
         return response;
     } catch (error) {
+        // RPC failure likely means the session process already exited.
+        // The user's intent is "stop this session" — if it's already
+        // stopped, that's a success, not an error.
         return {
-            success: false,
-            message: error instanceof Error ? error.message : 'Unknown error'
+            success: true,
+            message: 'Session already stopped'
         };
     }
 }