LM Studio — Anthropic Messages API

The /v1/messages endpoint in LM Studio mirrors the Anthropic Messages API exactly — same request shape, same response shape. Use it as a local drop-in for any code already calling Anthropic's cloud API.

Endpoint

POST http://localhost:1234/v1/messages

Required headers:

Content-Type: application/json
x-api-key: $LM_API_TOKEN — optional if Require Authentication is disabled in LM Studio

Basic Request

curl http://localhost:1234/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: $LM_API_TOKEN" \
  -d '{
    "model": "ibm/granite-4-micro",
    "max_tokens": 256,
    "messages": [
      {"role": "user", "content": "Say hello from LM Studio."}
    ]
  }'

Streaming

Add "stream": true to receive Server-Sent Events (SSE):

curl http://localhost:1234/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: $LM_API_TOKEN" \
  -d '{
    "model": "ibm/granite-4-micro",
    "messages": [{"role": "user", "content": "Hello"}],
    "max_tokens": 256,
    "stream": true
  }'

SSE event sequence:

message_start
content_block_start
content_block_delta (repeating)
content_block_stop
message_delta
message_stop

Tool Use

Pass a tools array with JSON Schema input definitions and a tool_choice policy:

curl http://localhost:1234/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: $LM_API_TOKEN" \
  -d '{
    "model": "ibm/granite-4-micro",
    "max_tokens": 1024,
    "tools": [
      {
        "name": "get_weather",
        "description": "Get the current weather in a given location",
        "input_schema": {
          "type": "object",
          "properties": {
            "location": {
              "type": "string",
              "description": "The city and state, e.g. San Francisco, CA"
            }
          },
          "required": ["location"]
        }
      }
    ],
    "tool_choice": {"type": "any"},
    "messages": [
      {"role": "user", "content": "What is the weather like in San Francisco?"}
    ]
  }'

tool_choice options (Anthropic-compat): "auto", "any", {"type": "tool", "name": "…"}.

Authentication

Scenario	Header needed
Auth disabled in LM Studio	No `x-api-key` required
Auth enabled	`x-api-key: $LM_API_TOKEN`

Key Takeaways

POST /v1/messages on localhost:1234 is a drop-in for api.anthropic.com/v1/messages
Same request body — swap the base URL and optionally add x-api-key
Streaming uses standard Anthropic SSE event names — existing stream parsers work unchanged
Tool use with input_schema / tool_choice is supported
Auth header is optional when LM Studio's Require Authentication is off
See wiki/claude-code/lmstudio-anthropic-compat for redirecting the full Anthropic SDK via env vars

wiki/claude-code/lmstudio-anthropic-compat — redirect Claude Code / SDK to local server
wiki/claude-code/lmstudio-chat-completions — OpenAI-compatible /v1/chat/completions
wiki/claude-code/lmstudio-rest-api — native v1 endpoints and feature comparison table
wiki/claude-code/lmstudio-idle-ttl-auto-evict — memory management for loaded models

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