define-tools changedagents-and-tools/tool-use/define-tools
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## Availability ### API support #### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) #### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
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22title: Define tools
33url: https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools
44description: Specify tool schemas, write effective descriptions, and control when Claude calls your tools.
5featureMetadata:
6 status: ga
7 supportedModels:
8 - claude-fable-5-1
9 - claude-mythos-5-1
10 - claude-fable-5
11 - claude-mythos-5
12 - claude-mythos-preview
13 - claude-opus-5-5
14 - claude-opus-5
15 - claude-sonnet-5-5
16 - claude-sonnet-5
17 - claude-haiku-5-5
18 - claude-haiku-4-5-20251001
19 - claude-opus-4-8
20 - claude-opus-4-7
21 - claude-opus-4-6
22 - claude-sonnet-4-6
23 - claude-opus-4-5-20251101
24 - claude-sonnet-4-5-20250929
25 supportedPlatforms:
26 Claude API: ga
27 Claude Platform on AWS: ga
28 Claude Models on Google Cloud (Anthropic managed): ga
29 Amazon Bedrock: ga
30 Amazon Bedrock (InvokeModel): ga
31 Claude Models on Google Cloud (Google managed): ga
32 Microsoft Foundry: ga
533---
634
35**API support**: Usage differs by product.
36
37* **Messages API**: Define them in the request’s `tools`, and send the results as `tool_result` blocks.
38* **Managed Agents**: Declare them on the agent with `type` `"custom"`; calls arrive as `agent.custom_tool_use` events.
39
740## Prerequisites
841
942* Familiarity with the [tool use overview](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview)
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906939 Directory of Anthropic-provided tools and optional properties.
907940 </Card>
908941</CardGroup>
942
943## Availability
944
945Custom tools are available on every model.
946
947### API support
948
949#### [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages)
950
951* Availability: Available
952* Usage: Define them in the request’s `tools`. Claude’s response contains `tool_use` blocks, and you send the results as `tool_result` blocks in the next request.
953
954| Status | Cloud platforms |
955| --------- | ----------------------- |
956| Available | All supported platforms |
957
958#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
959
960* Availability: Available
961* Usage: Declare them on the agent with `type` `"custom"`. Calls arrive as `agent.custom_tool_use` events, and you reply with `user.custom_tool_result` events.
962
963| Status | Cloud platforms |
964| --------- | ------------------------------------------------------------------------------------------------------------------ |
965| Available | Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws) |
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