fix: stabilize session fork duplicate-id errors

packages/session-fork/session-fork/README.md

@@ -8,7 +8,7 @@ Session fork service (`ctx.sessionFork`) for creating seeded child sessions from
 
 | Method | Purpose |
 |---|---|
-| `snapshot(source)` | Resolve a live `Session | SessionId`, reject non-boundary logs, and return a deep-cloned seed plus `parentSession` / `seedLength` metadata. |
+| `snapshot(source)` | Resolve a live `Session \| SessionId`, reject non-boundary logs, and return a deep-cloned seed plus `parentSession` / `seedLength` metadata. |
 | `fork({ source, sessionId? })` | Create a live child session from `snapshot(source)`, using the caller-supplied child id or the session store's generated id. |
 
 ## Boundary Rule

packages/session-fork/session-fork/src/index.ts

@@ -95,10 +95,10 @@ export class SessionForkService extends Service {
    * `ctx.agents.create` instead.
    */
   fork(options: ForkSessionOptions): Session {
-    const snapshot = this.snapshot(options.source)
     if (options.sessionId !== undefined && this.ctx.sessions.get(options.sessionId) !== undefined) {
       throw new SessionForkError(`session "${options.sessionId}" already exists`, 'SESSION_ALREADY_EXISTS')
     }
+    const snapshot = this.snapshot(options.source)
     return this.ctx.sessions.create(options.sessionId, {
       seed: snapshot.seed,
       meta: snapshot.meta,

packages/session-fork/session-fork/tests/session-fork.spec.ts

@@ -203,6 +203,16 @@ describe('SessionForkService', () => {
       .toThrow(new SessionForkError('session "child" already exists', 'SESSION_ALREADY_EXISTS'))
   })
 
+  it('rejects a duplicate child session id before validating the source boundary', async () => {
+    const { ctx, fork } = await setup()
+    const source = ctx.sessions.create(SessionId('open-parent'))
+    source.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    ctx.sessions.create(SessionId('child'))
+
+    expect(() => fork.fork({ source, sessionId: SessionId('child') }))
+      .toThrow(new SessionForkError('session "child" already exists', 'SESSION_ALREADY_EXISTS'))
+  })
+
   it('persists a forked child seed through the existing session write path', async () => {
     const root = await tempRoot()
     const ctx = new Context()