Web Fetch tool

Upcoming release — Zylon 1.58These docs are published ahead of release so you can prepare your API integrations. The release image is not yet available. For installations running 1.57, use the 1.57 documentation.

Use Web Fetch when you need the assistant to retrieve content from a URL.

Prerequisites

An API token. See Backoffice Developer Console.

Requirements

Tool spec: { "name": "web_extract", "type": "web_extract_v1" }
No tool_context required

Basic request and response

This example validates a request that enables the web fetch tool:

curl -X POST "https://{BASE_URL}/api/gpt/v1/messages/validate" \
  -H "Authorization: Bearer {API_TOKEN}" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "default",
    "max_tokens": 64,
    "messages": [
      { "role": "user", "content": "Summarize https://example.com/" }
    ],
    "tools": [
      { "name": "web_extract", "type": "web_extract_v1" }
    ],
    "tool_choice": { "type": "auto" }
  }'

Example response:

{ "valid": true, "errors": null }

To run the request, send the same body to POST /messages.

Response blocks

Zylon runs this tool on the server. The response contains a server_tool_use block with the call, followed by a web_fetch_tool_result block with the result. The call ID starts with srvtoolu_. Don’t send a tool_result back for it.

[
  { "type": "server_tool_use", "id": "srvtoolu_3c4d", "name": "web_extract", "input": { "url": "https://example.com/" } },
  {
    "type": "web_fetch_tool_result",
    "tool_use_id": "srvtoolu_3c4d",
    "content": {
      "type": "web_fetch_result",
      "url": "https://example.com/",
      "retrieved_at": "2026-10-08T10:00:00Z",
      "content": {
        "type": "document",
        "title": "Example Domain",
        "source": { "type": "text", "media_type": "text/plain", "data": "# Example Domain\n\nThis domain is for use in examples." }
      }
    }
  }
]

Error:

{
  "type": "web_fetch_tool_result",
  "tool_use_id": "srvtoolu_3c4d",
  "content": { "type": "web_fetch_tool_result_error", "error_code": "url_not_accessible" }
}

These blocks have no is_error field. To detect a failure, check whether content.type ends in _error, then read content.error_code. With the Zylon extension enabled (system.extensions includes zylon), the result is returned as a plain tool_result block with text content instead. See Migrating from 1.57 to 1.58 for the full list of server tool blocks.

Tips

The URL must appear in the user message.
Keep the prompt concise to reduce extraction time.

Next steps

Learn how messages return tool blocks: Messages

First Steps

Build with Zylon

Coding Agents

Web Fetch tool

Prerequisites

Requirements

Basic request and response

Response blocks

Tips

Next steps

​Prerequisites

​Requirements

​Basic request and response

​Response blocks

​Tips

​Next steps

Prerequisites

Requirements

Basic request and response

Response blocks

Tips

Next steps