overview changed

The first capture of this source. The page was already there, and this is what it said.

---
title: Tool use with Claude
url: https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview
description: Connect Claude to external tools and APIs. See where tools execute, when Claude calls them, and which tool fits your task.
---

Tool use 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 executes (client tools) or that Anthropic executes (server tools).

Here'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 executes for you:

<CodeGroup>
  ```bash cURL
  curl https://api.anthropic.com/v1/messages \
    -H "x-api-key: $ANTHROPIC_API_KEY" \
    -H "anthropic-version: 2023-06-01" \
    -H "content-type: application/json" \
    -d '{
      "model": "claude-opus-5",
      "max_tokens": 1024,
      "tools": [{"type": "web_search_20260209", "name": "web_search"}],
      "messages": [{"role": "user", "content": "What'\''s the latest on the Mars rover?"}]
    }'
  ```

  ```bash CLI
  ant messages create --transform content --format yaml \
    --model claude-opus-5 \
    --max-tokens 1024 \
    --tool '{type: web_search_20260209, name: web_search}' \
    --message '{role: user, content: "What is the latest on the Mars rover?"}'
  ```

  ```python Python
  client = anthropic.Anthropic()
  response = client.messages.create(
      model="claude-opus-5",
      max_tokens=1024,
      tools=[{"type": "web_search_20260209", "name": "web_search"}],
      messages=[{"role": "user", "content": "What's the latest on the Mars rover?"}],
  )
  print(response.content)
  ```

  ```typescript TypeScript
  const client = new Anthropic();
  const response = await client.messages.create({
    model: "claude-opus-5",
    max_tokens: 1024,
    tools: [{ type: "web_search_20260209", name: "web_search" }],
    messages: [{ role: "user", content: "What's the latest on the Mars rover?" }]
  });
  console.log(response.content);
  ```

  ```csharp C#
  AnthropicClient client = new();

  var parameters = new MessageCreateParams
  {
      Model = Model.ClaudeOpus5,
      MaxTokens = 1024,
      Tools = [new ToolUnion(new WebSearchTool20260209())],
      Messages = [new() { Role = Role.User, Content = "What's the latest on the Mars rover?" }]
  };

  var message = await client.Messages.Create(parameters);
  Console.WriteLine(message.Content);
  ```

  ```go Go
  client := anthropic.NewClient()

  response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
  	Model:     anthropic.ModelClaudeOpus5,
  	MaxTokens: 1024,
  	Tools: []anthropic.ToolUnionParam{
  		{OfWebSearchTool20260209: &anthropic.WebSearchTool20260209Param{}},
  	},
  	Messages: []anthropic.MessageParam{
  		anthropic.NewUserMessage(anthropic.NewTextBlock("What's the latest on the Mars rover?")),
  	},
  })
  if err != nil {
  	log.Fatal(err)
  }
  fmt.Println(response.Content)
  ```

  ```java Java
  import com.anthropic.models.messages.WebSearchTool20260209;

  void main() {
      AnthropicClient client = AnthropicOkHttpClient.fromEnv();

      MessageCreateParams params = MessageCreateParams.builder()
          .model(Model.CLAUDE_OPUS_5)
          .maxTokens(1024L)
          .addTool(WebSearchTool20260209.builder().build())
          .addUserMessage("What's the latest on the Mars rover?")
          .build();

      Message response = client.messages().create(params);
      IO.println(response.content());
  }
  ```

  ```php PHP
  $client = new Client();

  $message = $client->messages->create(
      model: 'claude-opus-5',
      maxTokens: 1024,
      tools: [
          ['type' => 'web_search_20260209', 'name' => 'web_search'],
      ],
      messages: [
          ['role' => 'user', 'content' => "What's the latest on the Mars rover?"],
      ],
  );

  echo $message;
  ```

  ```ruby Ruby
  client = Anthropic::Client.new

  message = client.messages.create(
    model: "claude-opus-5",
    max_tokens: 1024,
    tools: [{ type: "web_search_20260209", name: "web_search" }],
    messages: [{ role: "user", content: "What's the latest on the Mars rover?" }]
  )
  puts message.content
  ```
</CodeGroup>

Claude 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 execute the call when Claude returns a `tool_use` block. [How tool use works](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).

## How tool use works

Tools differ primarily by where the code executes. **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 executes 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)).

Here'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.

<CodeGroup>
  ```bash cURL
  # Claude replies with a tool_use block naming the tool and its arguments.
  TOOLS='[
    {
      "name": "get_weather",
      "description": "Get the current weather for a given location.",
      "input_schema": {
        "type": "object",
        "properties": {
          "location": {"type": "string", "description": "City and state, e.g. San Francisco, CA"}
        },
        "required": ["location"]
      }
    }
  ]'
  USER_MSG="What's the weather in San Francisco?"
  RESPONSE=$(curl -s https://api.anthropic.com/v1/messages \
    -H "x-api-key: $ANTHROPIC_API_KEY" \
    -H "anthropic-version: 2023-06-01" \
    -H "content-type: application/json" \
    -d "$(jq -n --argjson tools "$TOOLS" --arg msg "$USER_MSG" '{
      model: "claude-opus-5",
      max_tokens: 1024,
      tools: $tools,
      # Ask for at most one tool call per turn.
      tool_choice: {type: "auto", disable_parallel_tool_use: true},
      messages: [{role: "user", content: $msg}]
    }')")
  TOOL_USE=$(echo "$RESPONSE" | jq '.content[] | select(.type == "tool_use")')
  echo "Claude called $(echo "$TOOL_USE" | jq -r '.name') with $(echo "$TOOL_USE" | jq -c '.input')"

  # Run the tool, then send the result back in a tool_result block.
  WEATHER="15 degrees Celsius, partly cloudy"
  FOLLOWUP=$(curl -s https://api.anthropic.com/v1/messages \
    -H "x-api-key: $ANTHROPIC_API_KEY" \
    -H "anthropic-version: 2023-06-01" \
    -H "content-type: application/json" \
    -d "$(jq -n \
      --argjson tools "$TOOLS" \
      --arg msg "$USER_MSG" \
      --argjson assistant "$(echo "$RESPONSE" | jq '.content')" \
      --arg tool_use_id "$(echo "$TOOL_USE" | jq -r '.id')" \
      --arg weather "$WEATHER" \
      '{
        model: "claude-opus-5",
        max_tokens: 1024,
        tools: $tools,
        tool_choice: {type: "auto", disable_parallel_tool_use: true},
        messages: [
          {role: "user", content: $msg},
          {role: "assistant", content: $assistant},
          {role: "user", content: [
            {type: "tool_result", tool_use_id: $tool_use_id, content: $weather}
          ]}
        ]
      }')")

  # Claude uses the result to answer the original question.
  echo "$FOLLOWUP" | jq -r '.content[] | select(.type == "text") | .text'
  ```

  ```bash CLI
  # ant reads the request body as YAML on stdin; jq carries the conversation
  # state into the second request.
  USER_MSG="What's the weather in San Francisco?"
  MESSAGES=$(jq -n --arg msg "$USER_MSG" '[{role: "user", content: $msg}]')
  call_api() {
    {
      cat <<'YAML'
  model: claude-opus-5
  max_tokens: 1024
  # Ask for at most one tool call per turn.
  tool_choice: {type: auto, disable_parallel_tool_use: true}
  tools:
    - name: get_weather
      description: Get the current weather for a given location.
      input_schema:
        type: object
        properties:
          location: {type: string, description: "City and state, e.g. San Francisco, CA"}
        required: [location]
  YAML
      printf 'messages: %s\n' "$MESSAGES"
    } | ant messages create --format json
  }

  # Claude replies with a tool_use block naming the tool and its arguments.
  RESPONSE=$(call_api)
  TOOL_USE=$(jq '.content[] | select(.type == "tool_use")' <<<"$RESPONSE")
  echo "Claude called $(jq -r '.name' <<<"$TOOL_USE") with $(jq -c '.input' <<<"$TOOL_USE")"

  # Run the tool, then send the result back in a tool_result block.
  WEATHER="15 degrees Celsius, partly cloudy"
  MESSAGES=$(jq \
    --argjson assistant "$(jq '.content' <<<"$RESPONSE")" \
    --arg tool_use_id "$(jq -r '.id' <<<"$TOOL_USE")" \
    --arg weather "$WEATHER" \
    '. + [
      {role: "assistant", content: $assistant},
      {role: "user", content: [
        {type: "tool_result", tool_use_id: $tool_use_id, content: $weather}
      ]}
    ]' <<<"$MESSAGES")
  FOLLOWUP=$(call_api)

  # Claude uses the result to answer the original question.
  jq -r '.content[] | select(.type == "text") | .text' <<<"$FOLLOWUP"
  ```

  ```python Python
  client = anthropic.Anthropic()

  tools = [
      {
          "name": "get_weather",
          "description": "Get the current weather for a given location.",
          "input_schema": {
              "type": "object",
              "properties": {
                  "location": {
                      "type": "string",
                      "description": "City and state, e.g. San Francisco, CA",
                  }
              },
              "required": ["location"],
          },
      }
  ]
  messages = [{"role": "user", "content": "What's the weather in San Francisco?"}]

  # Claude replies with a tool_use block naming the tool and its arguments.
  response = client.messages.create(
      model="claude-opus-5",
      max_tokens=1024,
      tools=tools,
      # Ask for at most one tool call per turn.
      tool_choice={"type": "auto", "disable_parallel_tool_use": True},
      messages=messages,
  )
  tool_use = next(block for block in response.content if block.type == "tool_use")
  print(f"Claude called {tool_use.name} with {json.dumps(tool_use.input)}")

  # Run the tool, then send the result back in a tool_result block.
  weather = "15 degrees Celsius, partly cloudy"  # your weather lookup goes here
  messages += [
      {"role": "assistant", "content": response.content},
      {
          "role": "user",
          "content": [
              {"type": "tool_result", "tool_use_id": tool_use.id, "content": weather}
          ],
      },
  ]
  followup = client.messages.create(
      model="claude-opus-5",

Cut at 300 lines. The page has the rest.