Best possible solution:

Use #84306 as the canonical review and landing path for the Gateway abort race, and close this duplicate branch to avoid splitting review across two similar fixes.

Do we have a high-confidence way to reproduce the issue?

Yes from source inspection: current main awaits attachment/model preparation before registering the abort controller, and chat.abort returns aborted:false if the run id is absent. I did not execute the repro because this review was read-only.

Is this the best way to solve the issue?

No for this branch as-is: early registration is the right direction, but the canonical sibling is the safer solution because it also handles aborted prep failures, cleans pre-dispatch media, and supplies live Gateway proof.

Security review:

Security review cleared: The diff only touches Gateway TypeScript behavior and tests, with no concrete security or supply-chain concern found.

What I checked:

• Current main race window: Current main still awaits attachment/model preparation in chat.send before calling registerChatAbortController, while chat.abort returns aborted:false when context.chatAbortControllers.get(runId) is absent. (src/gateway/server-methods/chat.ts:1927, 6048cd43a5ab)
• This PR central implementation: The branch registers activeRunAbort before awaited preparation and then checks activeRunAbort.controller.signal.aborted before dispatch, which addresses the missing-controller window. (src/gateway/server-methods/chat.ts:2190, df74a84ad03f)
• This PR remaining cancellation gap: The branch still responds with an attachment parse/stage failure if preparation throws after the run was already aborted, and its pre-dispatch-aborted path returns without media cleanup. (src/gateway/server-methods/chat.ts:2309, df74a84ad03f)
• Canonical sibling PR: Fix chat.abort during attachment send preprocessing #84306 is open, covers the same chat.abort during attachment preprocessing issue, and adds explicit handling for aborted prep errors plus cleanup of offloaded/staged media before returning the accepted send ack. (src/gateway/server-methods/chat.ts:2342, eb0ef797fcb0)
• Canonical cleanup helper: The canonical sibling adds cleanupChatSendPreDispatchMedia, which deletes inbound offloaded refs and sandbox-staged paths when abort wins before dispatch. (src/gateway/server-methods/chat.ts:954, eb0ef797fcb0)
• Related item search: GitHub search for the central problem returns Fix chat.abort during attachment send preprocessing #84306 as an open PR for chat.abort attachment preprocessing, matching the related-item context.

Likely related people:

• steipete: Recent GitHub path history shows repeated work on the central Gateway chat handler and tests, including session route projection, stop matching, transcript IO, and duplicate attachment send races. (role: recent area contributor; confidence: high; commits: 02f8fb7147c0, 8477a67fafc6, 9f7d606469ec; files: src/gateway/server-methods/chat.ts, src/gateway/server.chat.gateway-server-chat-b.test.ts)
• BunsDev: Path history ties BunsDev to active WebChat send dedupe and sessions abort wait semantics, both adjacent to this race and active-run behavior. (role: adjacent behavior contributor; confidence: medium; commits: bd2f8560fee6, 1f1f70a23f86; files: src/gateway/server-methods/chat.ts, src/gateway/server.chat.gateway-server-chat-b.test.ts)
• frankekn: Recent history shows work routing WebChat images through imageModel, which is the same attachment/model-preparation path involved in this race. (role: recent attachment-path contributor; confidence: medium; commits: e6d04550cac7; files: src/gateway/server-methods/chat.ts, src/gateway/server.chat.gateway-server-chat-b.test.ts)
• VACInc: Recent path history includes stop-active-turns work in the same Gateway chat handler, making this a plausible routing candidate for abort lifecycle review. (role: adjacent abort contributor; confidence: medium; commits: b72f0081836e; files: src/gateway/server-methods/chat.ts)

Codex review notes: model gpt-5.5, reasoning high; reviewed against 6048cd43a5ab.