[Feat] Supports Anthropic Messages count_tokens API (vllm-project#35588)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

Author: chaunceyjiang

vllm/entrypoints/anthropic/api_router.py

@@ -8,6 +8,8 @@
 from fastapi.responses import JSONResponse, StreamingResponse
 
 from vllm.entrypoints.anthropic.protocol import (
+    AnthropicCountTokensRequest,
+    AnthropicCountTokensResponse,
     AnthropicError,
     AnthropicErrorResponse,
     AnthropicMessagesRequest,
@@ -31,6 +33,18 @@ def messages(request: Request) -> AnthropicServingMessages:
     return request.app.state.anthropic_serving_messages
 
 
+def translate_error_response(response: ErrorResponse) -> JSONResponse:
+    anthropic_error = AnthropicErrorResponse(
+        error=AnthropicError(
+            type=response.error.type,
+            message=response.error.message,
+        )
+    )
+    return JSONResponse(
+        status_code=response.error.code, content=anthropic_error.model_dump()
+    )
+
+
 @router.post(
     "/v1/messages",
     dependencies=[Depends(validate_json_request)],
@@ -44,17 +58,6 @@ def messages(request: Request) -> AnthropicServingMessages:
 @with_cancellation
 @load_aware_call
 async def create_messages(request: AnthropicMessagesRequest, raw_request: Request):
-    def translate_error_response(response: ErrorResponse) -> JSONResponse:
-        anthropic_error = AnthropicErrorResponse(
-            error=AnthropicError(
-                type=response.error.type,
-                message=response.error.message,
-            )
-        )
-        return JSONResponse(
-            status_code=response.error.code, content=anthropic_error.model_dump()
-        )
-
     handler = messages(raw_request)
     if handler is None:
         base_server = raw_request.app.state.openai_serving_tokenization
@@ -88,5 +91,46 @@ def translate_error_response(response: ErrorResponse) -> JSONResponse:
     return StreamingResponse(content=generator, media_type="text/event-stream")
 
 
+@router.post(
+    "/v1/messages/count_tokens",
+    dependencies=[Depends(validate_json_request)],
+    responses={
+        HTTPStatus.OK.value: {"model": AnthropicCountTokensResponse},
+        HTTPStatus.BAD_REQUEST.value: {"model": AnthropicErrorResponse},
+        HTTPStatus.NOT_FOUND.value: {"model": AnthropicErrorResponse},
+        HTTPStatus.INTERNAL_SERVER_ERROR.value: {"model": AnthropicErrorResponse},
+    },
+)
+@load_aware_call
+@with_cancellation
+async def count_tokens(request: AnthropicCountTokensRequest, raw_request: Request):
+    handler = messages(raw_request)
+    if handler is None:
+        base_server = raw_request.app.state.openai_serving_tokenization
+        error = base_server.create_error_response(
+            message="The model does not support Messages API"
+        )
+        return translate_error_response(error)
+
+    try:
+        response = await handler.count_tokens(request, raw_request)
+    except Exception as e:
+        logger.exception("Error in count_tokens: %s", e)
+        return JSONResponse(
+            status_code=HTTPStatus.INTERNAL_SERVER_ERROR.value,
+            content=AnthropicErrorResponse(
+                error=AnthropicError(
+                    type="internal_error",
+                    message=str(e),
+                )
+            ).model_dump(),
+        )
+
+    if isinstance(response, ErrorResponse):
+        return translate_error_response(response)
+
+    return JSONResponse(content=response.model_dump(exclude_none=True))
+
+
 def attach_router(app: FastAPI):
     app.include_router(router)

vllm/entrypoints/anthropic/protocol.py

@@ -175,3 +175,33 @@ class AnthropicMessagesResponse(BaseModel):
     def model_post_init(self, __context):
         if not self.id:
             self.id = f"msg_{int(time.time() * 1000)}"
+
+
+class AnthropicContextManagement(BaseModel):
+    """Context management information for token counting."""
+
+    original_input_tokens: int
+
+
+class AnthropicCountTokensRequest(BaseModel):
+    """Anthropic messages.count_tokens request"""
+
+    model: str
+    messages: list[AnthropicMessage]
+    system: str | list[AnthropicContentBlock] | None = None
+    tool_choice: AnthropicToolChoice | None = None
+    tools: list[AnthropicTool] | None = None
+
+    @field_validator("model")
+    @classmethod
+    def validate_model(cls, v):
+        if not v:
+            raise ValueError("Model is required")
+        return v
+
+
+class AnthropicCountTokensResponse(BaseModel):
+    """Anthropic messages.count_tokens response"""
+
+    input_tokens: int
+    context_management: AnthropicContextManagement | None = None