fix(ai-gateway): always rewrite proxied model responses

[chrarnoldus]

Summary

Removes the passthrough shortcut in rewriteModelResponse so every proxied model response goes through the processing pipeline (previously only Kilo-exclusive free models and request-logged traffic did).

Why

Stream errors (upstream timeouts/disconnects) are common, and for the traffic that skipped the rewrite — paid models without request logging — they were undebuggable: the raw stream was passed through untouched, so mid-stream failures surfaced only as connection errors with no captured context. Processing the full stream is the only way to surface and log them.

Performance is not a concern here: token generation is inherently slow, and ~50% of traffic (free models + internal traffic with logging enabled) already goes through the rewrite.

Changes

• rewriteModelResponse now always returns a NextResponse (never null); the route returns its result directly instead of falling back to wrapInSafeNextResponse.
• The unrecognized-API-kind branch now throws (fail loud on an implementation error) instead of silently passing the response through.
• Tests updated for the new contract: the route mock passes responses through via wrapInSafeNextResponse, and the removed skip-behavior test now asserts paid-model traffic is rewritten with cost preserved.

Verification

• rewriteModelResponse.test.ts: 58/58 pass
• openrouter/[...path]/route.test.ts: 20/20 pass
• tsgo --noEmit (web): clean
• oxlint on the 4 changed files: 0 warnings, 0 errors
• pnpm format:changed applied; git diff --check clean

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the removal of the passthrough shortcut in rewriteModelResponse and the corresponding route/test updates; the change is internally consistent, the single caller was updated correctly, and no security, correctness, or reliability issues were found in the changed lines.

Files Reviewed (4 files)

• apps/web/src/app/api/openrouter/[...path]/route.ts
• apps/web/src/app/api/openrouter/[...path]/route.test.ts
• apps/web/src/lib/rewriteModelResponse.ts
• apps/web/src/lib/rewriteModelResponse.test.ts

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_{Reviewed by claude-sonnet-5 · Input: 14 · Output: 2.2K · Cached: 249.8K}

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