Golem Configure MCP Server

Configuring MCP Server Deployments

Overview

Any Golem agent can be automatically exposed as an MCP (Model Context Protocol) server without writing any extra code. MCP is enabled by adding an mcp section to golem.yaml — the same way HTTP API deployments are configured under httpApi.

The MCP server uses the Streamable HTTP transport. Once deployed, any MCP-compatible client (Claude Desktop, MCP Inspector, Cursor, custom clients) can connect to the server and interact with your agents through MCP tools, resources, and prompts.

Adding an MCP Deployment

Add an mcp section to the root golem.yaml:

mcp:
  deployments:
    local:
      - subdomain: my-app  # resolves to my-app.localhost:9007 by default
        agents:
          CounterAgent: {}
          TaskAgent: {}

Structure

• mcp.deployments is a map keyed by environment name (e.g., local, cloud, staging)
• Each environment contains a list of deployment objects
• Each deployment has:
  • subdomain: a single DNS label (lowercase letters, digits, and hyphens only — no dots, port, or URL scheme) resolved through the target environment server. Local MCP deployments resolve to <subdomain>.localhost:9007 by default, or <subdomain>.localhost:<mcpPort> when localServer.mcpPort is set to a stable nonzero port. Cloud MCP deployments resolve to <subdomain>.mcps.golem.cloud.
  • domain: a full domain such as mcp.example.com for custom registered domains or custom server environments.
  • agents: a map of agent type names (PascalCase) to their deployment options

Define exactly one of subdomain or domain on each deployment. Prefer subdomain for built-in server: local and server: cloud environments. Use domain when you need a full custom domain. subdomain cannot be used with custom server environments — those must use domain.

Do not use localServer.mcpPort: 0 in the manifest. Port 0 is only allowed when passed directly as --mcp-port 0 to golem server run; manifest deployment domains require stable nonzero ports.

Local Development

For local development, the MCP server listens on port 9007 by default (separate from the HTTP API gateway on port 9006). Use *.localhost:9007 domains:

mcp:
  deployments:
    local:
      - subdomain: my-app  # resolves to my-app.localhost:9007 by default
        agents:
          MyAgent: {}

After deploying with golem deploy --yes, the MCP server is available at:

http://my-app.localhost:9007/mcp

Cloud Deployment

For Golem Cloud's built-in server, configure the cloud environment and use subdomain:

mcp:
  deployments:
    local:
      - subdomain: my-app  # resolves to my-app.localhost:9007 by default
        agents:
          MyAgent: {}
    cloud:
      - subdomain: my-app  # resolves to my-app.mcps.golem.cloud
        agents:
          MyAgent:
            securityScheme: my-oauth

environments:
  local:
    server: local
  cloud:
    server: cloud

Deploy to cloud with:

golem deploy --yes --cloud

For custom DNS or custom server environments, use domain with a full domain:

mcp:
  deployments:
    prod:
      - domain: mcp.example.com
        agents:
          MyAgent:
            securityScheme: my-oauth

Agent Options

Each agent entry accepts an optional securityScheme field:

agents:
  PublicAgent: {}                          # No authentication
  SecureAgent:
    securityScheme: my-oidc                # Require OAuth authentication

Security Schemes

MCP deployments support OAuth-based authentication through security schemes. Create a security scheme using the CLI:

Creating a Security Scheme

golem api security-scheme create my-oidc \
  --provider-type google \
  --client-id "YOUR_CLIENT_ID" \
  --client-secret "YOUR_CLIENT_SECRET" \
  --redirect-url "http://my-app.localhost:9007/mcp/oauth/callback" \
  --scope openid --scope email --scope profile

For a custom OAuth provider:

golem api security-scheme create my-custom-oidc \
  --provider-type custom \
  --custom-provider-name "my-provider" \
  --custom-issuer-url "https://auth.example.com/realm" \
  --client-id "CLIENT_ID" \
  --client-secret "CLIENT_SECRET" \
  --redirect-url "http://my-app.localhost:9007/mcp/oauth/callback" \
  --scope openid --scope email --scope profile

Supported Providers

Referencing in golem.yaml

After creating a security scheme, reference it by name:

mcp:
  deployments:
    local:
      - subdomain: my-app  # resolves to my-app.localhost:9007 by default
        agents:
          SecureAgent:
            securityScheme: my-oidc

Important: The OAuth callback URL in the security scheme must match the MCP server domain. The callback path is always /mcp/oauth/callback.

Automatic MCP Mapping

Agent methods are automatically mapped to MCP entities based on these rules:

Tool and Resource Naming

MCP tool and resource names are constructed as {AgentTypeName}-{method_name}, where the method name preserves the source language naming convention (not kebab-case):

Constructor parameters (which identify the agent instance) are automatically included in the tool's input schema alongside the method's own parameters.

Agent and Method Metadata

Add description and prompt metadata to improve MCP discoverability:

Rust:

#[description("Increments the counter by n")]
#[prompt("Increment by a given number")]
fn increment_by(&mut self, n: u32) -> u32;

TypeScript:

incrementBy: method({
  input: { n: z.number() },
  returns: z.number(),
  description: "Increments the counter by n",
  promptHint: "Increment by a given number",
}),

Scala:

@description("Increments the counter by n")
@prompt("Increment by a given number")
def incrementBy(n: Int): Future[Int]

Both are optional and are included in the MCP metadata sent to clients.

Special Data Types for MCP

Golem supports special data types that map well to MCP concepts. These are not MCP-specific — agents using them can still be invoked through HTTP, RPC, etc.

Unstructured Text

Accept free-form text input, optionally constrained by language:

Rust:

fn summarize(&self, text: UnstructuredText) -> String;

Unstructured Binary

Accept binary data, optionally constrained by MIME type:

Rust:

#[derive(Debug, Clone, AllowedMimeTypes)]
enum Image {
    #[mime_type("image/png")] Png,
    #[mime_type("image/jpeg")] Jpeg,
}
fn process_image(&self, image: UnstructuredBinary<Image>) -> String;

Multimodal

Accept mixed text, binary, or structured data:

Rust:

fn analyze(&self, input: Multimodal) -> String;

Multi-Environment Deployments

Configure different domains and security settings per environment:

mcp:
  deployments:
    local:
      - subdomain: my-app  # resolves to my-app.localhost:9007 by default
        agents:
          MyAgent: {}
    cloud:
      - subdomain: my-app  # resolves to my-app.mcps.golem.cloud
        agents:
          MyAgent:
            securityScheme: prod-google-oidc

environments:
  local:
    server: local
  cloud:
    server: cloud

Deploying

After configuring golem.yaml, deploy. Always use --yes to avoid interactive prompts:

golem deploy --yes               # Deploy to the default environment (usually local)
golem deploy --yes --cloud       # Deploy to the cloud environment

Testing with MCP Inspector

You can verify your MCP server using the MCP Inspector:

npx @modelcontextprotocol/inspector

Then connect to http://my-app.localhost:9007/mcp using the Streamable HTTP transport.

Complete Example

# golem.yaml

mcp:
  deployments:
    local:
      - subdomain: counter-app  # resolves to counter-app.localhost:9007 by default
        agents:
          CounterAgent: {}
    cloud:
      - subdomain: counter-app  # resolves to counter-app.mcps.golem.cloud
        agents:
          CounterAgent:
            securityScheme: google-oidc

environments:
  local:
    server: local
  cloud:
    server: cloud

Key Constraints

• Agent type names in golem.yaml use PascalCase (matching the class/trait name in code)
• The MCP server listens on port 9007 by default for local development (separate from the HTTP API gateway port 9006)
• The MCP endpoint path is always /mcp (e.g., http://my-app.localhost:9007/mcp)
• Security schemes must be created via golem api security-scheme create before they can be referenced
• The resolved domain must be unique per environment
• After changing golem.yaml, run golem deploy --yes to apply changes
• The OAuth callback path for MCP security schemes is /mcp/oauth/callback