feat: Native Anthropic pass-through for Claude models (no translation)

[HXYerror]

🔬 Research Finding (live-tested with copilot-models-litellm, enterprise endpoint — 2026-05-10)

GitHub Copilot upstream (api.enterprise.githubcopilot.com) natively supports the Anthropic Messages API at /v1/messages for all Claude models — no translation needed.

Model × Endpoint matrix (10 Claude models tested)

Model	/v1/messages (Anthropic native)	/chat/completions (OpenAI)
claude-sonnet-4.5	✅	✅
claude-sonnet-4.6	✅	✅
claude-opus-4.5	✅	✅
claude-opus-4.6	✅	✅
claude-opus-4.6-1m	✅	✅
claude-opus-4.7	✅	✅
claude-opus-4.7-high	✅	✅
claude-opus-4.7-xhigh	✅	✅
claude-opus-4.7-1m-internal	✅	✅
claude-haiku-4.5	✅	✅

Feature matrix (all tested on /v1/messages native path)

Feature	Status	Notes
Basic chat	✅	Native {type:"message", content:[...]} response
Streaming SSE	✅	message_start / content_block_* / message_delta / message_stop
thinking blocks (4.5/4.6)	✅	Full thinking content + signature returned
thinking blocks (4.7+)	⚠️	New API: "type": "adaptive" + output_config.effort; "enabled" rejected with 400
tools / tool_use	✅	stop_reason: "tool_use", correct input
thinking + tools together	✅	Both blocks in response simultaneously
system prompt	✅	String and array form
cache_control (ephemeral)	✅	cache_creation_input_tokens in usage
top_k	✅	Accepted without error
URL images	❌	"external image URLs are not supported"
Base64 images	✅	(verified in prior tests)
Multi-turn	✅	History forwarded correctly

New API format for claude-opus-4.7+ thinking

{
  "thinking": {"type": "adaptive"},
  "output_config": {"effort": "medium"}
}

Old "type": "enabled" format rejected with:

"thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort"

Native usage fields (richer than OpenAI translation path)

{
  "usage": {
    "input_tokens": 47,
    "output_tokens": 136,
    "cache_creation_input_tokens": 0,
    "cache_read_input_tokens": 0,
    "cache_creation": {
      "ephemeral_1h_input_tokens": 0,
      "ephemeral_5m_input_tokens": 0
    }
  }
}

---

Problem: Current copilot-api translates everything through OpenAI

All /v1/messages requests go through:
non-stream-translation.ts → Copilot /chat/completions → stream-translation.ts

This loses:

• thinking blocks entirely (dropped on input, never returned on output)
• top_k (no OpenAI equivalent → dropped)
• cache_control, cache_creation_input_tokens
• signature on thinking blocks (required for multi-turn reasoning)
• New claude-opus-4.7+ thinking API (adaptive + output_config.effort)

Additionally non-stream-translation.ts:305 has a stale comment: "GitHub Copilot doesn't generate thinking blocks" — factually wrong.

---

Proposed fix: Direct Anthropic pass-through for all Claude models

When the model is Claude (vendor=Anthropic), forward the request directly to ${copilotBaseUrl}/v1/messages and pass the response back unchanged.

Client (Claude Code / Anthropic SDK)
  POST /v1/messages  {model: "claude-sonnet-4.6", thinking: ...}
        ↓
  route.ts → detect Claude model
        ↓
  create-messages-native.ts
  → POST api.enterprise.githubcopilot.com/v1/messages (native Anthropic format)
  ← response passed through unchanged (thinking blocks, signature, rich usage all intact)
        ↓
Client receives native Anthropic response

Non-Claude models (gpt-4o etc.) keep the existing translation path.

---

Sub-issues

• feat: create-messages-native.ts — Anthropic pass-through service client #39 — create-messages-native.ts: Anthropic pass-through service client
• feat: Route dispatch — detect native-capable models, branch to pass-through #40 — Route dispatch: detect Claude models (vendor=Anthropic) and branch to pass-through
• feat: thinking blocks round-trip via native pass-through (signature ↔ multi-turn) #41 — thinking round-trip: signature field types + multi-turn test
• feat: Dynamic NATIVE_ANTHROPIC_MODELS detection from /models endpoint (vendor-based) #42 — Handle claude-opus-4.7+ new thinking API (adaptive + output_config.effort)
• fix: Remove stale 'no thinking blocks' comment; update anthropic-types with signature + richer usage #43 — Dynamic NATIVE_ANTHROPIC_MODELS detection from /models endpoint (vendor=Anthropic)
• test: Native Anthropic pass-through — feature coverage tests #44 — Remove stale comment; update anthropic-types.ts with signature, richer usage fields
• feat: Handle claude-opus-4.7+ new thinking API (adaptive + output_config.effort) #45 — Tests for native pass-through vs translation path

[HXYerror]

Implementation complete

All sub-issues implemented:

Issue	File	Status
#39	src/services/copilot/create-messages-native.ts	✅
#40	src/routes/messages/handler.ts	✅
#41	anthropic-types.ts (signature field)	✅
#42	src/services/copilot/native-models.ts	✅
#43	anthropic-types.ts + non-stream-translation.ts	✅
#44	Tests — see #44 (open)	🔲
#45	create-messages-native.ts adaptive thinking	✅

What changed

New files:

• src/services/copilot/create-messages-native.ts — passes Anthropic payloads directly to /v1/messages upstream; handles adaptive thinking upgrade for opus-4.7+
• src/services/copilot/native-models.ts — isNativeAnthropicModel() checks live state.models vendor field

Modified files:

• src/routes/messages/handler.ts — dispatch to native path for Claude models, fall back to translation for others
• src/routes/messages/anthropic-types.ts — signature? on ThinkingBlock; union thinking type; output_config; widened ImageBlock source; widened ToolResultBlock content
• src/routes/messages/non-stream-translation.ts — stale comment removed; image source type narrowing fix; consola import added

#44 (tests) left open for separate PR.

Closes #38