[billxc]

Summary

• Preserve images returned inside Anthropic tool_result blocks as Chat Completions image_url input instead of serializing their Base64 data into tool-message text.
• Keep tool-result text and call IDs intact, and emit images after contiguous parallel tool outputs, including outputs split across adjacent user messages.
• Add regressions for Base64/URL images, mixed text and images, parallel result ordering, trailing images, and unsupported file-only references.

Why

The previous conversion used JSON.stringify() for non-text tool results. Six screenshots in a real Gemini request consequently became approximately 1.37 million characters of ordinary Base64 text rather than multimodal input. The preceding accepted request already consumed 1,048,204 input tokens; the next tool turn was rejected by Copilot with HTTP 400 invalid_request_body.

A controlled one-field text-size reduction restored the rejected request, while restoring or removing reasoning signatures did not. This change fixes the image conversion itself rather than altering signatures, retries, or feature flags.

Verification

• The new image regressions failed before the fix; all 62 tests across the three named request/stream test files passed afterward.
• ESLint passed for the two changed files.
• TypeScript passed for @floway-dev/translate.
• git diff --check passed.
• An independent Node gateway process with its own migrated SQLite database and loopback HTTP listener replayed the original failing source request unchanged against Copilot:
  • HTTP 200, real tool output, and a completed SSE stream without errors.
  • All six screenshots reached the upstream as actual image_url parts and used the default WebP compression.
  • No image Base64 remained in tool-message text.
  • Actual input usage fell from 1,048,204 to 104,154 tokens, approximately 90%.
  • No token exchange or feature-flag changes were used.

Scope

This PR contains only the translation fix and its regression tests. It does not include playground changes, the separate Copilot image-generation feature, dependency changes, or database migrations.