DF0019: Agent Requires JSON-Serializable RPC

Message

RPC function "{name}" has agent set but jsonSerializable is not true — MCP requires JSON-serializable data.

Cause

The agent field exposes an RPC function as an MCP tool. MCP and the underlying schema-conversion path (@valibot/to-json-schema) only consume JSON-shaped data. Functions whose payloads can include Map, Set, Date, BigInt, circular references, or class instances cannot be safely advertised to agents.

A registered function is rejected when agent is present and jsonSerializable is not explicitly true.

Example

defineRpcFunction({
  name: 'my-plugin:summary',
  agent: { description: 'Returns a summary' },
  // missing `jsonSerializable: true` → registration throws DF0019
  handler: () => ({ items: [1, 2, 3] }),
})

Fix

Either declare the payload as JSON-safe:

defineRpcFunction({
  name: 'my-plugin:summary',
  jsonSerializable: true,
  agent: { description: 'Returns a summary' },
  handler: () => ({ items: [1, 2, 3] }),
})

Or remove agent to keep the function as an internal RPC (no agent exposure):

defineRpcFunction({
  name: 'my-plugin:summary',
  handler: () => new Map([['a', 1]]),
})

Source

• packages/devframe/src/rpc/collector.ts — RpcFunctionsCollectorBase.register() throws DF0019 when a definition has agent set but is not declared jsonSerializable: true.