fix(agent): close two run-teardown races in ReusableReadableStream and async drain

[abhinav-pola]

Summary

Fixes two reachable run-teardown races. Each has a Vitest reproduction that fails on main and passes here.

1. ReusableReadableStream.cancel() vs. a late createConsumer() (active-consumers replay)

cancel() is two-phase: drop backlog, await sourceReader.cancel(), drop backlog again. A chunk the pump already read lands in the buffer during the await and the second drop advances trimOffset past it and clears the slot. A consumer created during the await is registered at the pre-drop watermark, so its first next() hits the cleared slot and throws ReusableReadableStream buffer invariant violated: consumed slot was cleared.

 export class ReusableReadableStream<T> {
+  private cancelled = false;

   createConsumer() {
     const consumerId = this.nextConsumerId++;
-    this.consumers.set(consumerId, {...}); startPump();
+    if (!this.cancelled) { this.consumers.set(consumerId, {...}); startPump(); }
     // iterator: a consumer not in the map is done immediately
   }

   async cancel() {
+    this.cancelled = true;
     ...
     if (this.sourceReader) await this.cancelSourceReader(this.sourceReader);
+    else if (!this.pumpStarted) await this.cancelUnstartedSource();

The cancelUnstartedSource() branch covers cancel() before any consumer exists: with the terminal flag, no later consumer can start the pump, so the wrapper cancels the unlocked source itself.

Reachable from ModelResult: any getFullResponsesStream()/getUiStream() call racing a run-level abort or cancel().

2. onRunEnd: 'drain' loses a settlement that lands during the last drain turn

     if (registry.hasInFlight()) {
       registry.abortAll('Async tool drain budget exhausted at run end');
-      await this.dropSettledTasks();
     }
+    await this.dropSettledTasks();

When the loop exits by maxDrainTurns (or the loop-top settledQueue check misses a settlement that arrives during the following model turn), the task is no longer in flight, so the old conditional skipped the final drop. The settlement stayed in settledQueue, no tool.async_settled event was broadcast, and the persisted task record remained working. Now it is broadcast with delivery: 'dropped' and persisted as terminal, matching the documented drain contract.

Verification

• pnpm run lint, pnpm run typecheck, pnpm run build pass.
• pnpm run test: agent 1352 tests pass, mcp 12 pass.
• sentrux gate reports no degradation (createConsumer kept under cc 15 by extracting registerConsumer).
• With src/lib/reusable-stream.ts and src/lib/model-result.ts reverted to main, the three new tests are the only failures.
• The e2e-tests failure (toolStream deltas, BadRequestResponseError from the live provider) also fails on main's scheduled run https://github.com/OpenRouterTeam/typescript-agent/actions/runs/35694019034.

Searched open PRs; #122 and #117 touch model-result.ts but not these paths.

Link to Devin session: https://openrouter.devinenterprise.com/sessions/751c0e35dd354e94ba0d8889f6f1cefb
Open in Devin Desktop: https://openrouter.devinenterprise.com/desktop/session/751c0e35dd354e94ba0d8889f6f1cefb?variant=devin
Requested by: @abhinav-pola

---

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'DevinAI' or '@devin'.

• Disable automatic comment, CI, and merge conflict monitoring

---

Original prompt from Abhinav

SYSTEM:
<latest_message>
Abhinav Pola (U090K0G7JF3) [ts=1790126362.734519]: @Devin do this to openrouter agents sdk <https://x.com/bcherny/status/2102543349102338309|x.com/bcherny/status/2102543349102338309>
</latest_message>

=== BEGIN THREAD HISTORY (in #brain-abhinav) ===
Abhinav Pola (U090K0G7JF3) [ts=1790126362.734519]: @Devin do this to openrouter agents sdk <https://x.com/bcherny/status/2102543349102338309|x.com/bcherny/status/2102543349102338309>
=== END THREAD HISTORY ===
Channel ID: C09JH4HR3DH
Thread URL: https://openrouter.slack.com/archives/C09JH4HR3DH/p1790126362734519?thread_ts=1790126362.734519&amp;cid=C09JH4HR3DH

The <latest_message> is the message that you should use to guide your goals + task for this session, and you should use the rest of the slack thread as context.
A [ts=...] marker on a Slack message is that message's timestamp. To act on a specific message with the slack tool (e.g. adding an emoji reaction via the reaction command), pass that value as timestamp along with the Channel ID — no extra lookup call is needed.