Server tools

This page covers the shared mechanics of server-executed tools: the server_tool_use block, pause_turn continuation, ZDR considerations, and domain filtering. For individual tools, see the tool reference.

The server_tool_use block

The server_tool_use block appears in Claude's response when a server-executed tool runs. Its id field uses the srvtoolu_ prefix to distinguish it from client tool calls:

{
  "type": "server_tool_use",
  "id": "srvtoolu_01A2B3C4D5E6F7G8H9",
  "name": "web_search",
  "input": { "query": "latest quantum computing breakthroughs" }
}

The API executes the tool internally. You see the call and its result in the response, but you don't handle execution. Unlike client tool_use blocks, you don't need to respond with a tool_result. The result block appears immediately after the server_tool_use block in the same assistant turn.

The server-side loop and pause_turn

When using server tools like web search, the API may return a pause_turn stop reason, indicating that the API has paused a long-running turn.

Here's how to handle the pause_turn stop reason:

# Initial request with web search
response = client.messages.create(
    model="claude-opus-4-6",
    max_tokens=1024,
    messages=[
        {
            "role": "user",
            "content": "Search for comprehensive information about quantum computing breakthroughs in 2025",
        }
    ],
    tools=[{"type": "web_search_20250305", "name": "web_search", "max_uses": 10}],
)

# Check if the response has pause_turn stop reason
if response.stop_reason == "pause_turn":
    # Continue the conversation with the paused content
    messages = [
        {
            "role": "user",
            "content": "Search for comprehensive information about quantum computing breakthroughs in 2025",
        },
        {"role": "assistant", "content": response.content},
    ]

    # Send the continuation request
    continuation = client.messages.create(
        model="claude-opus-4-6",
        max_tokens=1024,
        messages=messages,
        tools=[{"type": "web_search_20250305", "name": "web_search", "max_uses": 10}],
    )

    print(continuation)
else:
    print(response)

When handling pause_turn:

• Continue the conversation: Pass the paused response back as-is in a subsequent request to let Claude continue its turn
• Modify if needed: You can optionally modify the content before continuing if you want to interrupt or redirect the conversation
• Preserve tool state: Include the same tools in the continuation request to maintain functionality

ZDR and allowed_callers

The basic versions of web search (web_search_20250305) and web fetch (web_fetch_20250910) are eligible for Zero Data Retention (ZDR).

The _20260209 versions with dynamic filtering are not ZDR-eligible by default because dynamic filtering relies on code execution internally.

To use a _20260209 server tool with ZDR, disable dynamic filtering by setting "allowed_callers": ["direct"] on the tool:

{
  "type": "web_search_20260209",
  "name": "web_search",
  "allowed_callers": ["direct"]
}

This restricts the tool to direct invocation only, bypassing the internal code execution step.

Domain filtering

Server tools that access the web accept allowed_domains and blocked_domains parameters to control which domains Claude can reach.

When using domain filters:

• Domains should not include the HTTP/HTTPS scheme (use example.com instead of https://example.com)
• Subdomains are automatically included (example.com covers docs.example.com)
• Specific subdomains restrict results to only that subdomain (docs.example.com returns only results from that subdomain, not from example.com or api.example.com)
• Subpaths are supported and match anything after the path (example.com/blog matches example.com/blog/post-1)
• You can use either allowed_domains or blocked_domains, but not both in the same request

Wildcard support:

• Only one wildcard (*) is allowed per domain entry, and it must appear after the domain part (in the path)
• Valid: example.com/*, example.com/*/articles
• Invalid: *.example.com, ex*.com, example.com/*/news/*

Invalid domain formats return an invalid_tool_input tool error.

Dynamic filtering with code execution

The _20260209 versions of web search and web fetch use code execution internally to apply dynamic filters against search results.

Streaming server-tool events

Server-tool events stream as part of the normal SSE flow. The server_tool_use block and its result arrive as content_block_start and content_block_delta events, the same way text and client tool calls stream.

See Streaming for the full event reference. Individual tool pages document tool-specific event names where they differ.

Batch requests

All server tools support batch processing. See Batch processing.

Next steps