fix(session): tolerate contentless synthetic messages

packages/core/session/src/index.ts

@@ -366,10 +366,10 @@ function assertMessageEventShape(event: Record<string, unknown>, subject: string
 /** Reject the request-only image marker from durable message content at every nesting depth. */
 function assertNoLoggedImageOffloadMarker(type: string, data: unknown, subject: string): void {
   if (type !== 'user/message' && type !== 'assistant/message' && type !== 'tool/result') return
-  // Both callers supply a message envelope with array content: append by type and seed by validation.
   const dataRecord = data as Record<string, unknown>
   const message = (type === 'user/message' ? dataRecord : dataRecord['message']) as Record<string, unknown>
-  const content = message['content'] as unknown[]
+  const content = message['content']
+  if (!Array.isArray(content)) return
   const visit = (blocks: readonly unknown[]): void => {
     for (const value of blocks) {
       if (typeof value !== 'object' || value === null || Array.isArray(value)) continue

packages/core/session/tests/image-offload.spec.ts

@@ -92,6 +92,11 @@ describe('image/offload append validation', () => {
 
   it('rejects request-only offload markers from appended and restored messages', () => {
     const session = seeded()
+    expect(() => session.append('assistant/message', {
+      turn: 1,
+      step: 1,
+      message: {},
+    } as never, { surfaceOp: 'append' })).not.toThrow()
     expect(() => session.append('user/message', createUserMessage({
       content: [{ ...image('marked'), offloaded: true }], source: { kind: 'user' },
     }), { surfaceOp: 'append' }))