addAdditionalPropertiesToJsonSchema breaks xAI/Grok tool calling: xAI rejects additionalProperties: false #14678

Description

The addAdditionalPropertiesToJsonSchema function in @ai-sdk/provider-utils forcibly adds additionalProperties: false to all object schemas during tool serialization. This breaks tool calling for xAI/Grok models because xAI's validator explicitly rejects additionalProperties: false in tool schemas.

xAI's structured output docs state:

• additionalProperties defaults to false already
• schemas with boolean property schemas like false are rejected

So the AI SDK is adding something that xAI explicitly rejects, even though xAI's default behavior is already what the AI SDK is trying to enforce.

Root Cause

In @ai-sdk/provider-utils, the addAdditionalPropertiesToJsonSchema function unconditionally sets additionalProperties: false on all object-type schemas:

function addAdditionalPropertiesToJsonSchema(jsonSchema2) {
  if (jsonSchema2.type === "object" || ...) {
    jsonSchema2.additionalProperties = false;  // <-- This breaks xAI
    ...
  }
}

This function is called during tool schema serialization for all providers, with no provider-specific opt-out.

Impact

Any application using the AI SDK with xAI/Grok models and tool calling will fail with:

[xAI] Invalid request content: Schema validation failed: [standard_violation] /properties/properties/additionalProperties: property schema 'false' is not supported

This affects:

• Direct xAI provider usage (@ai-sdk/xai)
• OpenRouter xAI models (@openrouter/ai-sdk-provider)
• Any other OpenAI-compatible provider routing to xAI

Investigation

We spent significant time debugging this in OpenCode and confirmed:

1. The AI SDK re-adds additionalProperties: false AFTER any application-level sanitization
2. The binary bundles all dependencies at build time, so runtime interceptors don't work
3. The conflict exists at the AI SDK level, not the application level

Full investigation: anomalyco/opencode#23704

Suggested Fix

Add provider-specific schema serialization options to the AI SDK, such as:

1. Provider-level opt-out: Allow providers to disable addAdditionalPropertiesToJsonSchema for specific models
2. xAI-specific handling: Skip adding additionalProperties: false when targeting xAI models
3. Configuration option: Add a strictToolSchemas or similar option to provider constructors

Since xAI defaults to additionalProperties: false anyway, skipping it for xAI targets preserves intended strictness while avoiding the validator rejection.

Workaround (for now)

Applications can disable tool calling for xAI models via config:

{
  "provider": {
    "openrouter": {
      "models": {
        "x-ai/grok-4.1-fast": {
          "tool_call": false
        }
      }
    }
  }
}

But this is not a real solution — it just disables tools entirely for xAI models.

Related

• OpenCode issue: OpenRouter xAI/Grok tool schemas fail when additionalProperties: false is emitted anomalyco/opencode#23704
• Similar Grok 3 issue (different error): Grok 3 model fails with "Invalid grammar request" error when using streamText with tools #8024