webSearch_20250305 with streamText throws "BaseError: text part X not found"

[michaelfuest]

Description
When using Anthropic's webSearch_20250305 tool with streamText, the stream consistently fails with a "text part X not found" error after the web search returns results. The error occurs 100% of the time when streaming.

Environment
ai: ^5.0.65
@ai-sdk/anthropic: ^2.0.25
Node.js: v22.x
Model: claude-sonnet-4-5 (also tested with other Claude models)

Steps to Reproduce

import { anthropic } from "@ai-sdk/anthropic";
import { streamText } from "ai";

const result = streamText({  model: anthropic("claude-sonnet-4-5"),  prompt: "What's the weather in Munich today?",  tools: {    web_search: anthropic.tools.webSearch_20250305({ maxUses: 5 }),  },});

for await (const part of result.fullStream) {
  console.log(part);
}

Expected Behavior
Stream completes successfully with web search results incorporated into the response.

Actual Behavior
Stream fails with error:
BaseError: text part 2 not found

The web search tool executes successfully (returns encrypted results with URLs/titles), but the error occurs when Claude tries to cite/reference the results in its response.

Additional Context
The tool result contains encryptedContent fields - the error seems related to Claude's citation system referencing text parts that don't exist