Symptom

A subagent fan-out we run on top of gbrain (one parent → 38 children, each calling chat() with tools defined) died — all 38 children — with the same runtime error before a single LLM call could fire:

[chat(anthropic:claude-haiku-4-5-20251001)] schema is not a function. (In 'schema()', 'schema' is an instance of Object)

Root cause

src/core/ai/gateway.ts builds the tools map for the Vercel AI SDK like this:

const tools = (opts.tools ?? []).reduce((acc, t) => {
  acc[t.name] = {
    description: t.description,
    inputSchema: { jsonSchema: t.inputSchema } as any,
  };
  return acc;
}, {} as Record<string, any>);

The bare { jsonSchema: t.inputSchema } as any is then handed to AI SDK v6's asSchema() in @ai-sdk/provider-utils/dist/index.mjs (~line 2126). asSchema() branches on what kind of input it received:

• a real Schema (has [schemaSymbol] + validate)
• a ZodSchema
• a StandardSchema (~standard)
• a LazySchema (function)

A plain { jsonSchema: ... } object matches none of these branches, so the SDK falls through to invoking the input as a function: schema() → schema is not a function because schema is an Object literal.

Fix

ai already exports the jsonSchema() helper (re-exported from @ai-sdk/provider-utils) precisely for this case — it wraps a JSON Schema 7 object in a proper Schema<T> with [schemaSymbol] and a no-op validate. Two-line change:

- import { embed as aiEmbed, embedMany, generateObject, generateText } from 'ai';
+ import { embed as aiEmbed, embedMany, generateObject, generateText, jsonSchema } from 'ai';
...
- inputSchema: { jsonSchema: t.inputSchema } as any,
+ inputSchema: jsonSchema(t.inputSchema as any),

Verification

• bun build --compile --outfile bin/gbrain src/cli.ts — clean.
• Hot-restarted gbrain autopilot against the patched source.
• Retried one of the dead subagent children (job 13028, model anthropic:claude-haiku-4-5-20251001, tools enabled) — the SDK now accepts the tool schema and the LLM call fires.

Impact

Any caller of chat() that passes opts.tools will hit this. Hot path for tool-using minions / subagent fan-outs.

Scope

One file, two lines. No new dependencies — jsonSchema is already exported from the ai package this repo pins (^6.0.168).