web_fetch fails for some links

[svashish305]

web_fetch fails for some links

Summary

The web_fetch tool fails silently when fetching large PDFs (400KB+), providing no error codes, error events, or failure indication. The stream simply ends prematurely, leaving the SDK to throw a generic exception. This makes it impossible to handle errors gracefully or provide meaningful feedback to users.

Reproduction

Full reproduction with comparative test: https://gist.github.com/svashish305/eb008ad3112f4e71575a4977d2a0f057

Quick Test Results

✅ Small PDF (1-2 KB): Works perfectly

• Tool result received
• All events fire normally

❌ Large PDF (400 KB): Fails silently

• web_fetch_tool_error: ❌ NO
• stream_error event: ❌ NO
• abort event: ❌ NO
• SDK throws: "stream ended without producing a Message with role=assistant"

One-Minute Reproduction

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic();

// Fails silently - no error code!
const largePDF = 'https://absl.net/wp-content/uploads/2025/09/Business-Services-Industry-in-Europe-2025-Report.pdf';

let toolErrorReceived = false;

const stream = client.beta.messages.stream({
model: 'claude-sonnet-4-5-20250929',
max_tokens: 2048,
messages: [{ role: 'user', content: Analyze: ${largePDF} }],
tools: [{ type: 'web_fetch_20250910', name: 'web_fetch' }],
betas: ['web-fetch-2025-09-10'],
});

stream.on('streamEvent', (event) => {
if (event.content_block?.type === 'web_fetch_tool_error') {
toolErrorReceived = true; // Never happens!
}
});

try {
await stream.finalMessage();
} catch (error) {
console.log(error.message); // "stream ended without producing a Message"
console.log('API sent error code:', toolErrorReceived); // false
}## Environment

• SDK: @anthropic-ai/sdk@0.38.0
• Model: claude-sonnet-4-5-20250929
• Beta: web-fetch-2025-09-10
• Node.js: 20+

Expected vs Actual

Expected (per docs)

When fetch fails, API returns web_fetch_tool_error block with error code

Actual

Stream ends silently with:

• No error block
• No error events
• No error indication
• Only SDK exception

Impact

Makes web_fetch unusable in production:

• ❌ Cannot detect failures
• ❌ Cannot handle errors
• ❌ Cannot provide user feedback
• ❌ Cannot debug issues

Requested Fix

Return explicit error codes (as documented for other error scenarios):

{
"type": "web_fetch_tool_error",
"error_code": "content_too_large",
"message": "PDF size (400KB) exceeds limit (100KB)"
}Even if the underlying size limit can't be changed immediately, proper error codes would make the feature usable by allowing error handling.

Test Details

Full comparative test in gist shows:

• Small PDF (1-2 KB): ✅ Success with all events
• Large PDF (400 KB): ❌ Silent failure with no error indication

To run:
git clone https://gist.github.com/eb008ad3112f4e71575a4977d2a0f057.git
cd eb008ad3112f4e71575a4977d2a0f057
npm install
export ANTHROPIC_API_KEY=your-key
npm run test:compare## References

• Web Fetch Errors Documentation
• Reproduction Gist

---

Bottom Line: Feature works for small files but fails silently for large ones. Adding proper error codes (already documented for other scenarios) would make this production-ready.

[svashish305]

For now I've added this problematic domain to web_fetch's blocked_domains for now, but let me know when its fixed to remove the blacklist.

[rikuohirasawa]

For reference, I believe we've run into a similar issue with MCP tool calls. Providing a tool via an MCP that returns a large payload results in the following error being thrown:

Error: stream ended without producing a Message with role=assistant

I've created a minimal repro here:

rikuohirasawa/anthropic-error-min-repro@9d201c8