server-tools changedagents-and-tools/tool-use/server-tools
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99## The server\_tool\_use block
1010
11The `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:
11The `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:
1212
1313```json
1414{
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1919}
2020```
2121
22The API runs 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 tool's result block (for example, `web_search_tool_result` for web search) follows the `server_tool_use` block in the same assistant turn, paired by `tool_use_id`. If Claude calls one of your client tools at the same time, the `server_tool_use` block appears without its result, and the response ends with `stop_reason: "tool_use"`. The API runs the tool when you return the client `tool_result` blocks in your next request.
22The API runs 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 tool’s result block (for example, `web_search_tool_result` for web search) follows the `server_tool_use` block in the same assistant turn, paired by `tool_use_id`. If Claude calls one of your client tools at the same time, the `server_tool_use` block appears without its result, and the response ends with `stop_reason: "tool_use"`. The API runs the tool when you return the client `tool_result` blocks in your next request.
2323
2424## The server-side loop and pause\_turn
2525
2626When using server tools such as web search, the API executes tool calls in a server-side agentic loop. On a long-running turn, the API might pause that loop and return a `pause_turn` stop reason.
2727
28Here's how to handle the `pause_turn` stop reason:
28Here’s how to handle the `pause_turn` stop reason:
2929
3030<CodeGroup>
3131 ```bash cURL
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405405* There is no other marker. Detect the state by looking for a `server_tool_use` block whose `id` has no matching result block in the response. An `mcp_tool_use` block from the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) behaves the same way. Server tool calls that already have their result block in the same response are complete and need nothing from you.
406406
407407<Note>
408 With [programmatic tool calling](https://platform.claude.com/docs/en/agents-and-tools/tool-use/programmatic-tool-calling), the same response shape means something different. The client `tool_use` block comes from code that is running in the `code_execution` tool rather than from Claude directly, and its `caller` field names the `code_execution` block that called it. That code has already started: it is paused waiting for your `tool_result` blocks, and sending them resumes the execution instead of starting a deferred tool. The `code_execution` block's own result block arrives once the code completes, which can take more than one round of tool results. The follow-up user message itself is the same in both cases; with programmatic tool calling, also pass back the `id` from the response's `container` field, as that page shows.
408 With [programmatic tool calling](https://platform.claude.com/docs/en/agents-and-tools/tool-use/programmatic-tool-calling), the same response shape means something different. The client `tool_use` block comes from code that is running in the `code_execution` tool rather than from Claude directly, and its `caller` field names the `code_execution` block that called it. That code has already started: it is paused waiting for your `tool_result` blocks, and sending them resumes the execution instead of starting a deferred tool. The `code_execution` block’s own result block arrives once the code completes, which can take more than one round of tool results. The follow-up user message itself is the same in both cases; with programmatic tool calling, also pass back the `id` from the response’s `container` field, as that page shows.
409409</Note>
410410
411411```json
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447447}
448448```
449449
450The API attaches your results to the still-open assistant turn, runs the deferred server tool (for paused code execution, resumes it), and then lets Claude continue. For a server tool Claude called directly, the next response starts with the result block that answers the previous response's `server_tool_use` `id`, followed by the newly generated content and a fresh `stop_reason`:
450The API attaches your results to the still-open assistant turn, runs the deferred server tool (for paused code execution, resumes it), and then lets Claude continue. For a server tool Claude called directly, the next response starts with the result block that answers the previous response’s `server_tool_use` `id`, followed by the newly generated content and a fresh `stop_reason`:
451451
452452```json
453453{
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10301030
10311031This restricts the tool to direct calls only, bypassing the internal code execution step.
10321032
1033`allowed_callers` controls how a tool can be called: directly by Claude (`"direct"`), from inside a code execution container (for example, `"code_execution_20260120"`), or both. The `_20260209` versions of the web tools default to the code execution caller only; earlier versions default to `["direct"]`. On models that don't support programmatic tool calling, these versions require `allowed_callers: ["direct"]`; without it the API returns a validation error that says to set it.
1033`allowed_callers` controls how a tool can be called: directly by Claude (`"direct"`), from inside a code execution container (for example, `"code_execution_20260120"`), or both. The `_20260209` versions of the web tools default to the code execution caller only; earlier versions default to `["direct"]`. On models that don’t support programmatic tool calling, these versions require `allowed_callers: ["direct"]`; without it the API returns a validation error that says to set it.
10341034
10351035<Note>
10361036 Even when web fetch is used in a ZDR-eligible configuration, website publishers might retain any parameters passed to the URL if Claude fetches content from their site.
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10731073 Unicode characters in domain names can bypass domain filters through homograph attacks: `аmazon.com` (with a Cyrillic `а`) looks identical to `amazon.com` but is a different domain. Use ASCII-only domain names in allow and block lists, and audit existing entries for non-ASCII characters.
10741074</Warning>
10751075
1076For domain lists and your organization's settings in Claude Managed Agents sessions, see [Restrict web search and web fetch domains](https://platform.claude.com/docs/en/managed-agents/tools-web-restrictions).
1076For domain lists and your organization’s settings in Claude Managed Agents sessions, see [Restrict web search and web fetch domains](https://platform.claude.com/docs/en/managed-agents/tools-web-restrictions).
10771077
10781078## Dynamic filtering with code execution
10791079
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10801080The `_20260209` and later versions of web search and web fetch use code execution internally to apply dynamic filters against search results.
10811081
10821082<Note>
1083 You don't need to add a `code_execution` tool for these versions: when dynamic filtering runs, the API provisions code execution for the request automatically, and both tools share a single execution container. If you do include one, use `code_execution_20260120` or later; the API rejects older code execution versions alongside these web tool versions.
1083 You don’t need to add a `code_execution` tool for these versions: when dynamic filtering runs, the API provisions code execution for the request automatically, and both tools share a single execution container. If you do include one, use `code_execution_20260120` or later; the API rejects older code execution versions alongside these web tool versions.
10841084</Note>
10851085
10861086## Streaming server-tool events
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11071107 </Card>
11081108
11091109 <Card title="Web fetch tool" icon="download" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool">
1110 Fetch and read content from specific URLs to augment Claude's context with live web content.
1110 Fetch and read content from specific URLs to augment Claude’s context with live web content.
11111111 </Card>
11121112
11131113 <Card title="Code execution tool" icon="terminal" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool">
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