Anthropic path 502s when the system field contains a newline (json.RawMessage marshal error) #86

Summary

On the Anthropic-compatible path (/v1/messages — e.g. minimax-* / qwen-* on the opencode-go provider), the proxy returns 502 all models failed whenever the request's top-level system field contains a newline. The upstream request is never sent — the proxy fails to marshal the request first.

Error (debug log)

level=WARN msg="non-retryable error (skipping circuit breaker), trying fallback"
  error="failed to marshal anthropic request: json: error calling MarshalJSON for type json.RawMessage: invalid character '\n' in string literal"
level=ERROR msg="request error" status=502 message="all models failed" error="all models failed (1 attempts)"

Minimal repro

Against the proxy's /v1/messages (Anthropic format), with an Anthropic-path model:

# single-line system → 200 OK
curl -s http://127.0.0.1:PORT/v1/messages \
  -H "anthropic-version: 2023-06-01" -H "content-type: application/json" \
  -d '{"model":"minimax-m3","max_tokens":512,"system":"be terse","messages":[{"role":"user","content":"hi"}]}'

# system WITH a newline → 502 (marshal error above)
curl -s http://127.0.0.1:PORT/v1/messages \
  -H "anthropic-version: 2023-06-01" -H "content-type: application/json" \
  -d '{"model":"minimax-m3","max_tokens":512,"system":"be terse\nreply in one word","messages":[{"role":"user","content":"hi"}]}'

Reproduces on minimax-m3 and qwen3.7-plus. Trigger is purely the newline in system; payload otherwise minimal (no tools, no claude-code involvement).

Suspected root cause

MessageRequest.System is json.RawMessage (pkg/types/anthropic.go, cf. #36). On the Anthropic→Anthropic transform the proxy appears to rebuild that RawMessage from a decoded string without re-JSON-escaping, so a literal \n yields invalid JSON that MarshalJSON rejects.

Impact

• Deterministic: any Anthropic-path model + a system containing a newline → 502. Every real agent (e.g. Claude Code) sends a multi-line system prompt, so these models fail 100% in practice.
• OpenAI-path models (deepseek-* / mimo-* / glm-* / kimi-*) are unaffected — different transform path.
• OpenCode's API is fine: the identical payload sent directly to https://opencode.ai/zen/go/v1/messages (with x-api-key) returns 200. The defect is in the proxy's request transformation, not upstream.

Environment

• routatic-proxy 0.3.6 (Homebrew, macOS arm64)
• Models confirmed: minimax-m3, qwen3.7-plus