Summary

When web_fetch returns an error (e.g., url_not_allowed or url_not_accessible), the SDK crashes with:

SyntaxError: JSON Parse error: Unexpected identifier "object"

Root Cause

In convert-to-anthropic-messages-prompt.ts, the error handling assumes output.value is always a JSON string:

const errorValue = JSON.parse(output.value as string);

However, output.value is already an object:

{"type":"error-json","value":{"type":"web_fetch_tool_result_error","errorCode":"url_not_allowed"}}

When JSON.parse() receives an object, it gets coerced to "[object Object]" and fails.

Fix

Check if the value is a string before parsing:

const errorValue =
  typeof output.value === 'string'
    ? JSON.parse(output.value)
    : output.value;

Testing

Tested locally with @ai-sdk/[email protected] and [email protected]. The agent now handles web_fetch errors gracefully instead of crashing.