fix: replay Opus reasoning, stream thinking, keep inline system messages

[voidsteed]

Summary

Opus 5.5 through the proxy was much slower than it needed to be, and Claude Code looked frozen during long turns. Three fixes:

• Replay prior reasoning to Copilot. The proxy dropped earlier thinking blocks, so Opus worked out its plan again from scratch on every tool call. Signed thinking now goes back as reasoning_text and reasoning_opaque, which is what VS Code's Copilot client does. Unsigned blocks are still dropped.
• Stream thinking to Claude Code. Copilot's reasoning_text and reasoning_opaque are forwarded as thinking_delta and signature_delta events, or as a thinking block when not streaming, so long turns show progress.
• Keep inline role: "system" messages. Claude Code sends SessionStart hook context and reminders inside messages[]. They were stripped as an "assistant prefill" or attributed to the model. They now reach Copilot as user context, on both the chat-completions and Responses paths.

Results

Same real Claude Code 2.1.281 task (Opus 5.5, read 3 files and grep tests), run one at a time:

Effort	Before	After
xhigh	209 s / 266 s / 379 s	98 s / 99 s / 105 s
max	didn't finish in 15 min	702 s

Tradeoffs

• Prompts are 15–40% larger on long tool chains because reasoning is kept. Copilot still caches them.
• The first 3–12 s of a turn stay silent. Copilot sends nothing, not even headers, before its first token.

Testing

• bun test: 219 pass. New tests cover reasoning replay, signature-only blocks, thinking/signature streaming, and inline system messages.
• Two of the new replay tests fail when replay is disabled. The third checks that unsigned reasoning is never replayed, which holds either way.
• Typecheck and lint are clean.

🤖 Generated with Claude Code