Mcp.Tools

Examples below assume mcp-server is imported as Mcp and javascript as JS.

A passive bundle declaring tools that any Mcp.StdioServer or Mcp.HttpEndpoint can advertise. Bundles are referenced by name from a transport's tools: array — multiple bundles compose into a single transport, and one bundle can serve multiple transports.

Schema

kind: Mcp.Tools
metadata: { name: <BundleName> }
entries:
  - name: <tool_id>                   # advertised to clients via tools/list
    description: <text>
    argumentsSchema:                  # JSON Schema validated by the MCP SDK
      type: object
      properties: { ... }
      required: [...]
    handler: <ref to a Telo.Invocable>
    inputs:                           # CEL: MCP request → handler input
      <handlerField>: ${{ request.arguments.<x> }}
    result:                           # CEL: handler output → CallToolResult
      content:
        - type: text
          text: ${{ result.<y> }}
      isError: ${{ result.<bool>? }}  # optional
    catches:                          # optional — InvokeError → JSON-RPC error
      - code: <telo_error_code>       # match by code, omit for catch-all
        when: ${{ <bool> }}           # optional CEL predicate
        error:
          code: <jsonrpc_int>
          message: ${{ error.message }}
          data: ${{ error.data }}     # optional

CEL scopes

Each entry exposes three runtime scopes:

Field	Available
inputs:	request.{name, arguments, meta, session.{id, clientInfo, capabilities}}
result:	result (handler output), request.{name, arguments}
catches:	error.{code, message, data}, request.{name, arguments}

request.session is per-connection metadata: for Mcp.HttpEndpoint it's the session ID minted on initialize; for Mcp.StdioServer it's a synthetic UUID minted at process start (stdio has no transport-level session).

Result envelope

result: is the full MCP CallToolResult, not just the content array:

Field	Required	Notes
content	yes	Array of content blocks ({type, text}, {type, image}, …).
isError	no	Soft tool failure — content describes what went wrong.
structuredContent	no	Optional structured object alongside content.
_meta	no	MCP metadata passthrough.

isError: true and catches: are distinct:

• isError — handler ran successfully but the result describes an upstream failure (e.g. a third-party 404). The LLM reads natural-language failure text.
• catches: — handler threw an InvokeError. Maps Telo error codes to JSON-RPC error responses with negative integer codes (convention: -32001 and below for application errors).

Cross-bundle composition

kind: Mcp.HttpEndpoint
metadata: { name: McpHttp }
serverInfo: { name: my-mcp, version: 1.0.0 }
tools: [WeatherTools, DatabaseTools]   # entries from both bundles are merged

Duplicate name across bundles → init-time error naming both bundles. Same bundle referenced twice → init-time error.