overview changedagents-and-tools/tool-use/overview
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44description: Connect Claude to external tools and APIs. See where tools run, when Claude calls them, and which tool fits your task.
55---
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7Tool use (also called function calling) lets Claude call functions that you define or that Anthropic provides. Claude determines when to call a tool based on the user's request and the tool's description. It then returns a structured call that your application runs (client tools) or that Anthropic runs (server tools).
7Tool use (also called function calling) lets Claude call functions that you define or that Anthropic provides. Claude determines when to call a tool based on the user’s request and the tool’s description. It then returns a structured call that your application runs (client tools) or that Anthropic runs (server tools).
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9Here's a minimal example using a server tool, the [Web search tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-search-tool), which Anthropic runs for you:
9Here’s a minimal example using a server tool, the [Web search tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-search-tool), which Anthropic runs for you:
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1111<CodeGroup>
1212 ```bash cURL
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134134 ```
135135</CodeGroup>
136136
137Claude runs the search on Anthropic's infrastructure and returns the cited results in the same response. To have Claude call a function that you define, pass a tool with an `input_schema`, then run the call when Claude returns a `tool_use` block. The [client tool example](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview#how-tool-use-works) shows that round trip end to end. Learn more about [defining tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools) and [handling tool calls](https://platform.claude.com/docs/en/agents-and-tools/tool-use/handle-tool-calls).
137Claude runs the search on Anthropic’s infrastructure and returns the cited results in the same response. To have Claude call a function that you define, pass a tool with an `input_schema`, then run the call when Claude returns a `tool_use` block. The [client tool example](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview#how-tool-use-works) shows that round trip end to end. Learn more about [defining tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools) and [handling tool calls](https://platform.claude.com/docs/en/agents-and-tools/tool-use/handle-tool-calls).
138138
139139## How tool use works
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141Tools differ primarily by where the code runs. **Client tools** (including user-defined tools and tools with Anthropic-defined schemas, such as `bash` and `text_editor`) run in your application. Claude responds with `stop_reason: "tool_use"` and one or more `tool_use` blocks. Your code runs the operation and sends back a `tool_result`. **Server tools** (such as `web_search`, `web_fetch`, `code_execution`, and `tool_search`) run on Anthropic's infrastructure: you see the results directly without handling execution, unless Claude calls the tool in the same group of parallel tool calls as one of your client tools (see [Stop reasons and fallback](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons#tool-use)).
141Tools differ primarily by where the code runs. **Client tools** (including user-defined tools and tools with Anthropic-defined schemas, such as `bash` and `text_editor`) run in your application. Claude responds with `stop_reason: "tool_use"` and one or more `tool_use` blocks. Your code runs the operation and sends back a `tool_result`. **Server tools** (such as `web_search`, `web_fetch`, `code_execution`, and `tool_search`) run on Anthropic’s infrastructure: you see the results directly without handling execution, unless Claude calls the tool in the same group of parallel tool calls as one of your client tools (see [Stop reasons and fallback](https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons#tool-use)).
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143Here's that round trip in full for a client tool. The first request defines a `get_weather` tool, and Claude answers the question by calling it: the response carries a `tool_use` block, your code runs the lookup, and a second request sends the result back in a `tool_result` block so Claude can reply with the answer.
143Here’s that round trip in full for a client tool. The first request defines a `get_weather` tool, and Claude answers the question by calling it: the response carries a `tool_use` block, your code runs the lookup, and a second request sends the result back in a `tool_result` block so Claude can reply with the answer.
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145145<CodeGroup exclude="shell:cURL, shell:CLI">
146146 <CodeGroupItem language="cURL">
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635635
636636## When Claude uses tools
637637
638With the default `tool_choice` of `{"type": "auto"}`, Claude determines on each turn whether to call a tool or respond directly. It calls a tool when the request maps to that tool's described capability and the answer isn't already in context. It responds directly for stable knowledge, creative tasks, and conversational turns.
638With the default `tool_choice` of `{"type": "auto"}`, Claude determines on each turn whether to call a tool or respond directly. It calls a tool when the request maps to that tool’s described capability and the answer isn’t already in context. It responds directly for stable knowledge, creative tasks, and conversational turns.
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640This boundary is steerable through your system prompt. If Claude isn't calling tools when you expect, a light instruction such as `"Use the tools to investigate before responding."` increases tool use. A stronger form such as `"Always call a tool first before responding."` pushes further. Conversely, `"Use your judgment about whether to call a tool or respond directly."` keeps triggering behavior conservative.
640This boundary is steerable through your system prompt. If Claude isn’t calling tools when you expect, a light instruction such as `"Use the tools to investigate before responding."` increases tool use. A stronger form such as `"Always call a tool first before responding."` pushes further. Conversely, `"Use your judgment about whether to call a tool or respond directly."` keeps triggering behavior conservative.
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642642To require a tool call rather than rely on prompting, set [`tool_choice`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools#forcing-tool-use).
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644644<Tip>
645645 **Guarantee schema conformance with strict tool use**
646646
647 Add `strict: true` to your custom tool definitions to ensure Claude's tool calls always match your schema exactly. See [Strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use).
647 Add `strict: true` to your custom tool definitions to ensure Claude’s tool calls always match your schema exactly. See [Strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use).
648648</Tip>
649649
650Each server tool's page describes its own trigger boundary in more detail.
650Each server tool’s page describes its own trigger boundary in more detail.
651651
652652<Accordion title="When required parameters are missing">
653 If the user's prompt doesn't include enough information to fill all the required parameters for a tool, Claude Opus is much more likely to recognize that a parameter is missing and ask for it. Claude Sonnet might ask, especially when prompted to think before outputting a tool request. But it might also infer a reasonable value.
653 If the user’s prompt doesn’t include enough information to fill all the required parameters for a tool, Claude Opus is much more likely to recognize that a parameter is missing and ask for it. Claude Sonnet might ask, especially when prompted to think before outputting a tool request. But it might also infer a reasonable value.
654654
655 For example, given a `get_weather` tool that requires a `location` parameter, if you ask Claude "What's the weather?" without specifying a location, Claude (particularly Claude Sonnet) might guess values you didn't supply:
655 For example, given a `get_weather` tool that requires a `location` parameter, if you ask Claude “What’s the weather?” without specifying a location, Claude (particularly Claude Sonnet) might guess values you didn’t supply:
656656
657657 ```json JSON
658658 {
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712712
713713### Server tools
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715Server tools run on Anthropic's infrastructure, with no handler code in your application. See [Server tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools) for the mechanics they share.
715Server tools run on Anthropic’s infrastructure, with no handler code in your application. See [Server tools](https://platform.claude.com/docs/en/agents-and-tools/tool-use/server-tools) for the mechanics they share.
716716
717717<CardGroup cols={2}>
718718 <Card title="Web search tool" icon="magnifying-glass" href="https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-search-tool">
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