docs: publish canonical API reference for /anthropic/v1/messages compatibility endpoint #129

Summary

Moonshot's agent-support docs document the env var
ANTHROPIC_BASE_URL=https://api.moonshot.ai/anthropic for Claude Code integration,
and we can confirm the endpoint POST /anthropic/v1/messages works end-to-end.
However, the request/response contract, supported headers, and feature matrix for
this endpoint are not published anywhere we can find. We're asking for canonical
reference docs so we (and other agentic-tool builders) can pin against a stable
contract rather than empirical probing.

What works today (observed)

• POST https://api.moonshot.ai/anthropic/v1/messages with an Anthropic-shape body
  accepts: model, messages, system, tools, tool_choice, max_tokens,
  temperature, stream.
• Authorization: Bearer <moonshot_api_key> is accepted (claude-code-router's
  anthropic transformer uses UseBearer: true against this base URL).
• Streaming (SSE) returns Anthropic event types (message_start, content_block_delta,
  message_delta, message_stop) compatible enough for Claude Code's SDK to consume.
• Temperature is documented as rescaled: real_temperature = request_temperature * 0.6.
• kimi-k2.5 and kimi-k2.6 both respond on this endpoint.

What is undocumented

1. Endpoint contract. No published spec for request/response shape, only the
   env-var setup blurb. Is the contract guaranteed to match Anthropic Messages API
   v1? Which version?
2. Header support. Is anthropic-version honored / required / ignored? Does
   anthropic-beta (e.g. tools-2024-04-04, prompt-caching-2024-07-31,
   extended-cache-ttl-2025-04-11) do anything?
3. Feature matrix. Which Anthropic features are supported / translated /
   silently dropped?
   • cache_control blocks (prompt caching)
   • Vision ({type: image, source: ...} content blocks)
   • Document content blocks ({type: document, ...}) — we observe 400s and
     have to strip these client-side
   • thinking: {type: enabled} (we observe Kimi K2.6 supports thinking but
     400s in multi-turn tool-calling when prior assistant tool_use lacks
     reasoning_content — what's the contract here?)
   • tool_choice: {type: any|auto|tool} semantics
   • Stop sequences, parallel tool use
4. Version skew + deprecation. Where is the changelog for this endpoint?
   How are breaking changes announced? Is there a way to pin to a specific
   API version (header)?
5. Rate limits + error shapes. Are Anthropic-style error bodies
   ({type: error, error: {type, message}}) guaranteed, or is OpenAI-shape
   leakage possible?

Why it matters

We operate a federated AI control plane (SemanticGate fleet) where Kimi K2 is a
first-class cascade tier alongside Anthropic-native and DeepSeek's
/anthropic/v1/messages compat endpoint. Provider switches in our cascade
need a stable contract so we can:

• Detect feature-availability deterministically (rather than empirical probing
  that's brittle across model upgrades).
• Pin clients against documented behavior so model-version bumps on Moonshot's
  side don't silently break our agents.
• Author transformers/shims with confidence (we currently maintain
  kimi-document-strip and kimi-tool-thinking-strip based on empirical
  probing — would prefer to map these to documented contracts).

What would help

A reference page under platform.kimi.ai/docs/api/anthropic/ covering:

1. Endpoint URL + auth (Authorization: Bearer) + accepted headers
2. Request body fields supported / unsupported / partial (with a clean
   "Anthropic feature → Kimi support" table)
3. Response shape + streaming event types
4. Error format
5. Versioning policy + changelog link
6. Known divergences from upstream Anthropic spec (e.g. the temperature
   rescaling note already in your migrating-from-anthropic-claude page)

Happy to contribute a docs PR if you'd accept community input on the page
structure.

References

• Existing setup docs: https://platform.kimi.ai/docs/guide/agent-support
• Migration page (mentions temperature rescaling):
  https://platform.kimi.ai/docs/guide/migrating-from-anthropic-claude
• claude-code-router transformer using this endpoint:
  https://github.com/musistudio/claude-code-router (transformers/anthropic.ts)