fix: forward client headers in native passthrough mode

[zerray]

Summary

• In native passthrough mode (e.g. Claude Code → Claude API), 9router was rebuilding request headers from static config + cached values instead of forwarding the client's original headers
• This caused extra anthropic-beta flags (e.g. token-efficient-tools-2026-03-28) to be injected into upstream API requests via the union merge logic in DefaultExecutor.buildHeaders, even though the client never sent them
• The extra beta flags altered model behavior, causing unicode encoding anomalies in tool call arguments (e.g. u8fd9 instead of 这 in partial_json deltas)

Changes

open-sse/executors/base.js

• Added buildPassthroughHeaders() method that starts from the client's original headers, strips hop-by-hop headers, swaps auth credentials, and ensures the oauth-2025-04-20 beta flag when using OAuth
• Updated execute() to accept passthrough and clientHeaders params, using buildPassthroughHeaders when both are present

open-sse/handlers/chatCore.js

• Passes passthrough flag and clientRawRequest.headers to executor.execute() at both call sites (initial request + retry-after-refresh)

Root cause

The static Anthropic-Beta header in providers.js includes flags like token-efficient-tools-2026-03-28 that are not present in Claude Code's own requests. The header merge logic (union of static + cached flags) injected these into every upstream request. With this fix, passthrough requests forward the client's exact headers — only auth is swapped.

Test plan

• Verified with Claude Code 2.1.97 → cc/claude-opus-4-6 passthrough: Chinese unicode characters now render correctly in tool call arguments
• Non-passthrough requests continue to use buildHeaders() with static config as before
• OAuth access token requests get oauth-2025-04-20 beta flag injected automatically

🤖 Generated with Claude Code