MCP Configuration Reference

Define MCP servers under mcp_servers in $HERMES_HOME/config.yaml. Discovery, tool calls, reconnection, and shutdown run as tasks on the agent's event loop.

Stdio server

mcp_servers:
  filesystem:
    command: npx
    args: ["-y", "@modelcontextprotocol/server-filesystem", "/workspace"]
    env: {}
    timeout: 300
    connect_timeout: 60

command selects stdio transport. args is optional. The subprocess receives a filtered environment plus the explicit env map; it does not inherit every host secret automatically.

Streamable HTTP server

mcp_servers:
  remote:
    url: https://example.test/mcp
    headers:
      Authorization: "Bearer ${REMOTE_MCP_TOKEN}"
    timeout: 180
    connect_timeout: 30

An entry with url uses Streamable HTTP by default. Remote URL validation is applied before connection.

SSE server

mcp_servers:
  legacy_sse:
    url: https://example.test/sse
    transport: sse
    timeout: 180

Use transport: sse only for a server implementing the older MCP SSE transport.

Common fields

Field	Meaning
enabled	Enable the server; defaults to true
command	Stdio executable
args	Stdio argument list
env	Explicit stdio environment additions; values may reference secrets
url	Remote MCP endpoint
transport	http by default for URLs, or sse
headers	Remote request headers
timeout	Per-tool-call timeout in seconds; default 300
connect_timeout	Initial connection timeout; default 60
protocol	auto (default), stateless, or legacy protocol negotiation mode
lazy	Register from a valid schema cache and connect on first call when possible
supports_parallel_tool_calls	Opt this server's tools into parallel-safe scheduling; default false
keepalive_interval	Liveness-ping interval; default 180, minimum 5 seconds
idle_timeout_seconds	Recycle an idle stdio server; 0 disables
max_lifetime_seconds	Recycle an aged stdio server; 0 disables
skip_preflight	Skip the remote content-type probe for a known valid endpoint

Lifecycle limits may also be nested below lifecycle.

Tool filtering

mcp_servers:
  github:
    command: npx
    args: ["-y", "@modelcontextprotocol/server-github"]
    tools:
      include: ["get_*", "search_repositories"]
      exclude: []
      resources: false
      prompts: true

• tools.include is a whitelist and takes precedence.
• tools.exclude is a blacklist used only when no include list is present.
• Exact names and case-sensitive fnmatch patterns are accepted.
• resources and prompts control generated utility tools, subject to the capabilities actually advertised by the server.

Registered names

An MCP tool is exposed as:

mcp__<sanitized_server>__<sanitized_tool>

For example, server github tool search-repositories becomes mcp__github__search_repositories. The server also contributes the dynamic toolset mcp-github; its raw server name is accepted as an alias by toolset resolution.

Name collisions caused by sanitization fail closed rather than selecting an arbitrary handler.

Parallel calls

MCP tools are sequential by default. Set supports_parallel_tool_calls: true only when the server and every relevant operation are safe to overlap. Results still enter the model transcript in the original tool-call order.

Sampling

Servers that request MCP sampling can be configured under sampling:

sampling:
  enabled: true
  model: example/model
  max_tokens_cap: 4096
  timeout: 30
  max_rpm: 10
  allowed_models: []
  max_tool_rounds: 5

Sampling uses host-owned model configuration and remains bounded by these limits. Disable it for servers that should never invoke an LLM.

Lifecycle

The first awaited agent boundary discovers configured servers. MCP ownership is reference-counted across active agents, and await agent.close() releases the agent's ownership. Always close agents so stdio subprocesses and remote sessions terminate cleanly.

See MCP and Use MCP with Hermes.