Merge branch 'codex/simp-unify-agent-session-id' into codex/simp-ui-identity-residue

# Conflicts:
#	docs/event-producer-consumer.md

packages/subagent/tool-subagent/README.md

@@ -24,6 +24,10 @@ With `run_in_background: true`, the tool registers the parent-owned task before
 | `toolFilter` | Per-child global-tool restriction; requires `toolFilter` capability. |
 | `maxDepth` | Absolute delegation-depth cap; requires `depthLimit` capability. |
 
+## Concurrency
+
+Foreground and background calls are exclusive. Children may share the parent's workspace or external resources, and a unary classifier cannot prove that sibling delegations have disjoint effects. See the [parallel tool-call RFC](../../../docs/rfc/implemented/feature/2026-07-10-parallel-tool-call-execution.md).
+
 ## Model Experience
 
 ### Tool schema

packages/subagent/tool-subagent/tests/tool-subagent.spec.ts

@@ -97,6 +97,20 @@ describe('dsh-tool-subagent', () => {
     expect(foreground.isError).toBe(false)
   })
 
+  it('keeps foreground and background calls exclusive', async () => {
+    const ctx = await setup({ provider: 'mock' })
+    expect(ctx.tools.executionMode({
+      callId: CallId('subagent-foreground'),
+      name: 'subagent',
+      arguments: { description: 'do work', prompt: 'Reply OK' },
+    })).toEqual({ kind: 'exclusive' })
+    expect(ctx.tools.executionMode({
+      callId: CallId('subagent-background'),
+      name: 'subagent',
+      arguments: { description: 'do work', prompt: 'Reply OK', run_in_background: true },
+    })).toEqual({ kind: 'exclusive' })
+  })
+
   it.each([
     { stopReason: 'aborted' as const, fragment: 'cancelled' },
     { stopReason: 'error' as const, fragment: 'failed' },