Summary

completeSubagentRun can be invoked in parallel from two completion paths for the same runId in embedded mode:

• In-process listener on phase: "end" — src/agents/subagent-registry.ts (via onAgentEvent).
• Gateway waitForSubagentCompletion RPC — src/agents/subagent-registry-run-manager.ts.

registerSubagentRun pairs both unconditionally. Both callers hit cleanupBrowserSessionsForLifecycleEnd(...) at src/agents/subagent-registry-lifecycle.ts for the same childSessionKey, so the wrapper runs twice.

Scope

Actual CDP browserCloseTab IPC is already idempotent: extensions/browser/src/browser/session-tab-registry.ts's takeTrackedTabsForSessionKeys drains trackedTabsBySession.delete(sessionKey), and closeTrackedBrowserTabsForSessions short-circuits with tabs.length === 0 on the second call.

The remaining duplication is at the wrapper layer:

• cleanupBrowserSessionsForLifecycleEnd wrapper call (runs twice)
• normalizeSessionKeys + runBestEffortCleanup (runs twice)
• plugin-sdk facade dispatch (runs twice; resolution itself is cached)
• onWarn logging path in the non-empty window

Why it matters

• Sibling race 48042c3875 fix(agents): avoid duplicate subagent ended hook loads already introduced endedHookEmittedAt in the same file for the same dual-dispatch pattern. Extending that pattern to browser cleanup keeps the dedup surface consistent.
• Closes a defense-in-depth gap if the bundled browser extension ever removes or replaces the take-and-drain idempotency.
• Trims wrapper overhead on every embedded subagent completion.

Reproduction

Promise.all([completeSubagentRun(p), completeSubagentRun(p)]) for the same runId + triggerCleanup: true calls cleanupBrowserSessionsForLifecycleEnd twice without a guard. See accompanying PR's regression test.

Proposed fix

Add a dedicated dispatch flag browserCleanupDispatchedAt?: number on SubagentRunRecord and a sync check-then-set guard in completeSubagentRun that matches the endedHookEmittedAt pattern. Reset the flag on rearm alongside endedHookEmittedAt in subagent-registry-run-manager.ts.

PR: coming.

[AI-assisted]