bug(agent): defineAgent mutates the caller's def object in-place

[Sertug17]

Summary

defineAgent modifies the object passed in by the caller: it reassigns def.steps when folding an agent-level inputSchema, and attaches AGENT_DEFINITION_BRAND via Object.defineProperty directly on def. Both mutations are visible on the caller's original reference.

Root Cause

packages/agent/src/agent.ts:

// Mutation 1: reassigns def.steps on the CALLER'S object
(def as { steps: ... }).steps = {
  ...def.steps,
  [def.entry]: { ...entryStep, inputSchema: def.inputSchema },
};

// Mutation 2: brand attached to caller's object
Object.defineProperty(def, AGENT_DEFINITION_BRAND, { value: 1, enumerable: false });

Reproduction

const myDef = {
  name: 'x', entry: 'start',
  inputSchema: z.object({ name: z.string() }),
  steps: { start: myStep },
};
const originalSteps = myDef.steps;
defineAgent(myDef);

console.log(myDef.steps === originalSteps); // false mutated
console.log(isAgentDefinition(myDef));      // true brand on original object

Fix

Operate on a shallow copy instead of mutating the input:

const result = { ...def };
result.steps = { ...def.steps, [def.entry]: { ...entryStep, inputSchema: ... } };
Object.defineProperty(result, AGENT_DEFINITION_BRAND, { value: 1, enumerable: false });
return result;