Langfuse MCP Server — Java / Spring AI

A production-hardened, read-only MCP server that connects any MCP-compatible AI agent to your Langfuse observability data.
Query traces, debug errors, inspect sessions, analyze prompts, and explore datasets — all through natural language.

Transport: HTTP/SSE on port 8080, compatible with Cursor and Claude Desktop out of the box.

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Why this server?

Capability	This server	Official Langfuse MCP
Traces & Observations	✅	❌
Sessions & Users	✅	❌
Exception tracking	✅	❌
Prompt management	✅	✅
Dataset management	✅	❌
Scores & metrics	✅	❌
Schema introspection	✅	❌
Java / Spring AI	✅	❌ (Python)
Read-only by design	✅	N/A

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Tools (24 total)

Category	Count	Tools
Traces	6	fetch_traces, fetch_trace, find_exceptions, find_exceptions_in_file, get_exception_details, get_error_count
Observations	2	fetch_observations, fetch_observation
Sessions	3	fetch_sessions, get_session_details, get_user_sessions
Prompts	2	list_prompts, get_prompt
Datasets	4	list_datasets, get_dataset, list_dataset_items, get_dataset_item
Scores	4	get_scores, get_score, get_score_configs, get_score_config
Users	1	get_user_traces
Comments	1	get_comments
Schema	1	get_data_schema

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Quick start

# 1. Build
mvn clean package

# 2. Set credentials
export LANGFUSE_PUBLIC_KEY=pk-lf-...
export LANGFUSE_SECRET_KEY=sk-lf-...
export LANGFUSE_HOST=https://cloud.langfuse.com

# 3. Run (SSE transport — port 8080)
java -jar target/langfuse-mcp-1.0.0.jar

# 4. Verify
curl http://localhost:8080/actuator/health

# 5. Inspect all tools
npx @modelcontextprotocol/inspector http://localhost:8080/sse

Get credentials from Langfuse Cloud → Settings → API Keys.
Self-hosted Langfuse? Set LANGFUSE_HOST to your instance URL.

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Architecture

MCP Client (Cursor / Claude Desktop / other)
    │   HTTP/SSE transport (/sse + /mcp/message)
    ▼
Tool class  (@McpTool — thin delegation layer, validates required params)
    ▼
Service interface + impl  (business logic, error mapping, server-side filtering)
    ▼
LangfuseApiClient  (GET-only HTTP gateway, typed exceptions)
    ▼
Langfuse Public REST API

The architecture is strictly layered:

• client/ — Langfuse integration boundary (HTTP, Basic-Auth, error handling)
• service/ — domain logic (filtering, mapping, pagination)
• tools/ — MCP-facing surface (descriptions, param validation, delegation)
• Spring Boot — runtime and transport wrapper only

Read-only by design: LangfuseApiClient exposes only GET methods. No POST, PATCH, or DELETE operations exist anywhere in the codebase. The langfuse.read-only=true flag is enforced at the properties level.

Every tool returns a consistent ApiResponse<T> envelope:

{ "success": true,  "data": { ... }, "timestamp": "2025-01-15T10:30:00Z" }
{ "success": false, "errorCode": "TRACE_NOT_FOUND", "errorMessage": "...", "timestamp": "..." }

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Configuration

Property	Env var	Required	Default	Description
langfuse.public-key	LANGFUSE_PUBLIC_KEY	✅	—	Langfuse project public key
langfuse.secret-key	LANGFUSE_SECRET_KEY	✅	—	Langfuse project secret key
langfuse.host	LANGFUSE_HOST	✅	—	Langfuse base URL
langfuse.timeout	LANGFUSE_TIMEOUT	❌	30s	HTTP request timeout (Spring Duration format, e.g. 30s, 1m)
langfuse.read-only	—	❌	true	Safety flag — always true, no writes are possible

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Client config

Cursor (.cursor/mcp.json)

{
  "mcpServers": {
    "langfuse": {
      "url": "http://localhost:8080/sse"
    }
  }
}

Claude Desktop (claude_desktop_config.json)

{
  "mcpServers": {
    "langfuse": {
      "url": "http://localhost:8080/sse"
    }
  }
}

On macOS: ~/Library/Application Support/Claude/claude_desktop_config.json

VS Code / GitHub Copilot

URL mode (if your client supports it):

{
  "github.copilot.chat.mcp.servers": {
    "langfuse": {
      "url": "http://localhost:8080/sse"
    }
  }
}

Command mode (stdio-only clients):

{
  "github.copilot.chat.mcp.servers": {
    "langfuse": {
      "command": "java",
      "args": ["-jar", "/path/to/langfuse-mcp-1.0.0.jar"],
      "env": {
        "LANGFUSE_PUBLIC_KEY": "pk-lf-...",
        "LANGFUSE_SECRET_KEY": "sk-lf-...",
        "LANGFUSE_HOST": "https://cloud.langfuse.com"
      }
    }
  }
}

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Docker

# Build image
docker build -t langfuse-mcp:latest .

# Run
docker run --rm -p 8080:8080 \
  -e LANGFUSE_PUBLIC_KEY=pk-lf-... \
  -e LANGFUSE_SECRET_KEY=sk-lf-... \
  -e LANGFUSE_HOST=https://cloud.langfuse.com \
  langfuse-mcp:latest

If Langfuse runs in Docker on the same host, use host.docker.internal:

-e LANGFUSE_HOST=http://host.docker.internal:3000

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Running tests

mvn test

Test coverage includes:

• LangfusePropertiesBindingTest — config binding from application-test.yml and constructor-level validation
• RawJsonResponseSerializationTest — Jackson serialization round-trips for RawJsonBackedResponse

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Package structure

com.langfuse.mcp
├── LangfuseMcpApplication.java          @SpringBootApplication @ConfigurationPropertiesScan
├── config/
│   ├── LangfuseProperties.java          @ConfigurationProperties — publicKey, secretKey, host, timeout, readOnly
│   ├── LangfuseClientConfig.java        RestClient bean with Basic-Auth and JdkClientHttpRequestFactory
│   └── JacksonConfig.java               Primary ObjectMapper (JSR310, ignore unknowns)
├── client/
│   └── LangfuseApiClient.java           GET-only HTTP gateway; typed exceptions; UriComponentsBuilder queries
├── exception/
│   ├── LangfuseApiException.java        Wraps HTTP errors — statusCode + endpoint on all error paths
│   └── ResourceNotFoundException.java   Thrown on HTTP 404
├── dto/
│   ├── common/    ApiResponse · PagedResponse · PaginationMeta
│   ├── request/   *FilterRequest · PromptGetRequest (9 filter classes + 1 get class)
│   └── response/  *Response (14 classes — JsonNode for open-schema fields)
├── service/       Interfaces (9): Trace · Observation · Session · Prompt · Dataset · Score · User · Comment · Schema
│   └── impl/      *ServiceImpl (9) — business logic, server-side filtering, error mapping
├── tools/         @McpTool classes (9) — param validation, delegation, agent-friendly descriptions
│   ├── TraceTools.java        (6 tools)
│   ├── ObservationTools.java  (2 tools)
│   ├── SessionTools.java      (3 tools)
│   ├── PromptTools.java       (2 tools)
│   ├── DatasetTools.java      (4 tools)
│   ├── ScoreTools.java        (4 tools)
│   ├── UserTools.java         (1 tool)
│   ├── CommentTools.java      (1 tool)
│   └── SchemaTools.java       (1 tool)
└── util/
    └── JsonPageMapper.java    Centralised JSON → PagedResponse mapper (no duplication)

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Troubleshooting

TRACE_FETCH_ERROR: HTTP/1.1 header parser received no bytes

Connectivity issue — not a code bug. Check:

1. LANGFUSE_HOST points to a running Langfuse instance
2. The host is reachable from the JVM process
3. For Docker: use host.docker.internal instead of localhost
4. The HTTP/HTTPS scheme matches your server (http:// vs https://)
5. Confirm the API is up: curl $LANGFUSE_HOST/api/public/health

INVALID_INPUT: <param> is required

A required parameter was not provided by the agent. All required = true parameters are enforced at the tool layer before any HTTP call is made.

Connection timeouts

Increase the timeout via the env var:

export LANGFUSE_TIMEOUT=60s

Copilot / Claude can't see the server

1. Confirm the server is running: curl http://localhost:8080/actuator/health
2. Confirm the MCP SSE endpoint is alive: curl http://localhost:8080/sse
3. Check that the URL in the client config points to http://localhost:8080/sse

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License