fix(ai-gateway): distinguish client disconnects from upstream disconnects

[chrarnoldus]

Problem

When a client disconnects mid-request, the request signal is forwarded to the upstream fetch, so the fetch aborts too. The gateway logged AI gateway client disconnected and then immediately returned The upstream provider disconnected before sending a response. — implying an upstream fault that never happened.

The same message was also used for gateway-side timeouts (no response headers within 15 minutes), which is a different failure.

Changes

• upstream-request.ts: when the fetch fails while the caller's signal is aborted (and the failure is an abort, or classification itself failed), return a dedicated client_disconnect response with HTTP 499 and the message "The client disconnected before the upstream provider responded, so the request was cancelled. The upstream provider did not fail." 499 keeps these cancellations out of upstream 5xx error rates.
• Genuine upstream failures now get two distinct messages: "The upstream provider did not send response headers before the gateway timeout." for the timeout families, and "The upstream provider closed the connection before sending a response." otherwise. Both keep error_type: upstream_disconnect and HTTP 503.
• The 15-minute timeout listener log now states that it is a gateway timeout waiting for response headers, including the timeout value.
• route.ts: the client-disconnect log now says it is aborting in-flight upstream work, with a comment recording that any abort logged afterwards for that request is a consequence of the disconnect.
• Added client_disconnect to ProxyErrorType.

Sentry/console reporting behaviour is unchanged: client-caused aborts are still not captured as upstream fetch failures.

Verification

• pnpm --filter web test -- src/tests/openrouterApi.timeout.test.ts (extended with client-disconnect, gateway-timeout, and connection-close response assertions)
• pnpm --filter web test -- src/app/api/openrouter
• pnpm --filter web typecheck, pnpm --filter web lint, pnpm format

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the client-disconnect vs. upstream-disconnect classification changes across the gateway route, upstream fetch handling, error-type enum, and tests, and found no high-confidence bugs, security issues, or logic errors in the changed lines.

Files Reviewed (4 files)

• apps/web/src/app/api/openrouter/[...path]/route.ts
• apps/web/src/lib/ai-gateway/providers/upstream-request.ts
• apps/web/src/lib/proxy-error-types.ts
• apps/web/src/tests/openrouterApi.timeout.test.ts

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_{Reviewed by claude-sonnet-5 · Input: 16 · Output: 5K · Cached: 404.7K}

_{Review guidance: REVIEW.md from base branch main}