AI SDK v6: tool().parameters not read by prepareToolsAndToolChoice (uses inputSchema which is undefined)

[LucasCzechia]

Root Cause Found

tool() sets the schema on .parameters, but prepareToolsAndToolChoice and doParseToolCall read .inputSchema — which is undefined. This causes all tool schemas to fall through to the empty default {"properties":{},"additionalProperties":false}.

One-line fix in two locations in ai/dist/index.mjs:

- inputSchema: await asSchema(tool2.inputSchema).jsonSchema,
+ inputSchema: await asSchema(tool2.inputSchema ?? tool2.parameters).jsonSchema,

- const schema = asSchema3(tool2.inputSchema);
+ const schema = asSchema3(tool2.inputSchema ?? tool2.parameters);

How We Found It

const { tool } = require('ai');
const z = require('zod');
const t = tool({ description: 'test', parameters: z.object({ q: z.string() }), execute: async () => {} });

console.log(Object.keys(t));       // ['description', 'parameters', 'execute']
console.log(t.inputSchema);        // undefined
console.log(typeof t.parameters);  // object (zod schema)

tool() stores the zod schema on parameters. But prepareToolsAndToolChoice (line 1779) reads tool2.inputSchema which doesn't exist → asSchema(undefined) → returns the empty fallback {"properties":{},"additionalProperties":false}.

Similarly, doParseToolCall (line 3714) uses asSchema3(tool2.inputSchema) for validation → empty schema → validation against empty schema either fails or produces wrong results.

Impact

This breaks ALL tool calling for ALL providers when using tool() with Zod schemas in AI SDK v6. The model receives no parameter information, so:

1. It guesses parameter names (sometimes query, sometimes q, sometimes {})
2. Even when the model guesses correctly, doParseToolCall validates against the empty schema and may reject valid inputs

Verification

With the patch applied:

• API receives correct tool schema with all properties
• Model returns correct arguments 100% of the time
• parseToolCall validates and passes correctly
• 8/10 → limited only by model non-determinism, not SDK bugs

Without the patch: 0/10 — every tool call gets empty schema, model guesses, SDK rejects.

Environment

• ai: 6.0.116
• zod: 4.3.6
• Tested with: Cloudflare Workers AI, @ai-sdk/openai-compatible
• Both streamText and generateText affected

Previous Report (Updated)

The original report described two separate bugs. After investigation, both symptoms trace to this single root cause:

• "Empty schema sent to API" → tool2.inputSchema is undefined, fallback produces {}
• "SDK drops valid tool arguments" → doParseToolCall validates against empty schema, marks calls as invalid or produces wrong parsed values

The streaming-specific arguments: "{}" from Cloudflare was a red herring — it only happened because the empty schema gave the model no parameter guidance. With the correct schema, Cloudflare's streaming endpoint also returns correct arguments.

Related

This likely supersedes #12020 ("AI SDK v6 + Zod: Tool input_schema is empty when using Anthropic") — same root cause.

[theamodhshetty]

Put up a fix in #13465.

It keeps the current inputSchema behavior as-is and adds a fallback for legacy runtime tool objects that still carry parameters, covering prompt prep, tool-call parsing, and UI message validation. I also added focused regression tests plus a patch changeset for ai.

[lgrammel]

Where is parameters defined? We switched to inputSchema in AI SDK 5

[LucasCzechia]

To answer your question — parameters is defined by your own tool() helper:

const t = tool({ description: 'test', parameters: z.object({ q: z.string() }), execute: async () => {} });
console.log(Object.keys(t));    // ['description', 'parameters', 'execute']
console.log(t.inputSchema);     // undefined

tool() accepts parameters and stores it as parameters. Then prepareToolsAndToolChoice reads tool.inputSchema, which is undefined. This is an internal inconsistency in the SDK — not a user migration issue.

The reproduction, root cause, and fix are all in the issue body above. PR #13465 was closed without review after less than 2 minutes, with no engagement with the actual code or tests.

[lgrammel]

@LucasCzechia this is not true in ai sdk 5 or later. see our tool helper: https://github.com/vercel/ai/blob/main/packages/provider-utils/src/types/tool.ts#L153