Functions as MCP Tools

Expose a Fission function as an MCP tool so any LLM agent that speaks MCP can discover and invoke it, with no hand-written adapter code. The Model Context Protocol (MCP) is an open protocol that lets LLM agents discover and call external tools. Starting with Fission v1.27.0, an agent that speaks MCP (for example Claude) can list an advertised function and invoke it over Fission’s existing internal invocation path.

Exposing a function as a tool is opt-in per function and additive — functions you don’t mark stay private.

The request travels from agent to MCP server to router to function pod:

flowchart TB
  agent["LLM Agent"]:::user -->|"<b>1.</b> list / call tool (JWT)"| mcp["MCP Server"]:::fission
  mcp -->|"<b>2.</b> watch Function CRDs"| fns["Tool-exposed Functions"]:::store
  mcp -->|"<b>3.</b> invoke via internal path"| router["Router"]:::fission
  router -->|"<b>4.</b> HTTP request"| pod["Function Pod"]:::pod
  classDef user fill:#ffffff,stroke:#94a3b8,color:#1f2a43
  classDef fission fill:#e8f0fe,stroke:#2d70de,color:#1f2a43
  classDef pod fill:#e6f7f1,stroke:#11a37f,color:#1f2a43,stroke-dasharray:5 3
  classDef store fill:#fff7e0,stroke:#dba514,color:#1f2a43,stroke-dasharray:5 3

Enable the MCP server

The MCP server is off by default. Enable it with a Helm value:

helm upgrade --install fission fission-charts/fission-all \
  --namespace fission \
  --set mcp.enabled=true

This deploys a stateless MCP server that watches Function resources and hot-updates its advertised tool list. It listens on a ClusterIP Service on port 8890 by default (mcp.port).

In production, leave mcp.allowInsecure: false and run with authentication enabled. Agents then authenticate with a signed JWT whose allowed_namespaces claim scopes which functions they may see and call.

Expose a function as a tool

Create (or update) a function with --expose-as-mcp and a required agent-facing description:

fission fn create --name weather --env nodejs --code weather.js \
  --expose-as-mcp \
  --tool-description "Return the current weather for a city" \
  --tool-input-schema weather-schema.json

--tool-input-schema points at a JSON Schema (draft 2020-12) file describing the tool’s arguments; it is advertised verbatim as the MCP tool inputSchema. When omitted, the tool advertises an open object schema ({"type":"object"}).

List exposed tools

fission function tools lists every tool-exposed function in the current namespace:

$ fission function tools
NAME              FUNCTION   NAMESPACE   DESCRIPTION
default-weather   weather    default     Return the current weather for a city

Use -o wide, -o json, or -o yaml for more detail.

Declarative spec

Exposing a function as a tool is controlled by the presence of spec.tool on the Function resource — there is no separate enabled flag:

apiVersion: fission.io/v1
kind: Function
metadata:
  name: weather
spec:
  environment:
    name: nodejs
  package:
    packageref:
      name: weather
  tool:
    description: Return the current weather for a city   # required
    # Optional JSON Schema (draft 2020-12) for the tool's arguments.
    inputSchema:
      type: object
      properties:
        city:
          type: string
      required: [city]
    # Optional: override the advertised tool name (defaults to <namespace>-<name>).
    toolName: get-weather

How agents reach the tools

Point an MCP-capable agent at the MCP server’s endpoint (the mcp Service on port 8890, exposed however you route in-cluster traffic — for example through an HTTP trigger or your ingress/gateway). With authentication enabled, the agent presents a bearer JWT and only sees tools in its allowed_namespaces. When the agent calls a tool, the MCP server invokes the underlying function through Fission’s internal invocation path and returns the response.

Related

• Create and run functions — the everyday function workflow.
• Internal Service Authentication — the signing used to scope agent access.
• Functions — the Function resource and the tool field.
• Custom Resource Definition Specification — the ToolConfig spec fields.