Claude Code has builtin tools to (e.g.) explore the filesystem.

-- request --
{
	"model": "claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"tools": [
		{
			"name": "Read",
			"description": "Read the contents of a file at the given path.",
			"input_schema": {
				"type": "object",
				"properties": {
					"file_path": {
						"type": "string",
						"description": "The absolute path to the file to read"
					}
				},
				"required": ["file_path"]
			}
		}
	],
	"messages": [
		{
			"role": "user",
			"content": "read the foo file"
		}
	]
}

-- streaming --
event: message_start
data: {"type":"message_start","message":{"id":"msg_015SQewixvT9s4cABCVvUE6g","type":"message","role":"assistant","model":"claude-sonnet-4-20250514","content":[],"stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":2,"cache_creation_input_tokens":22,"cache_read_input_tokens":13993,"output_tokens":5,"service_tier":"standard"}}             }

event: content_block_start
data: {"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":""}}

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":"The user wants me to read"}            }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":" a"}       }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":" file called \""}           }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":"foo\"."}       }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":" Let me find"}         }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":" and"} }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":" read it."}    }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":""}               }

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"signature_delta","signature":"Eu8BCkYICxgCKkBR++kFr7Za2JhF/9OCpjEc46/EcipL75RK+MEbxJ/VBJPWQTWrNGfwb5khWYJtKEpjjkH07cR/MQvThfb7t7CkEgwU4pKwL7NuZXd1/wgaDILyd0bYMqQovWo3dyIw95Ny7yZPljNBDLsvMBdBr7w+RtbU+AlSftjBuBZHp0VzI54/W+9u6f7qfx0JXsVBKldqqOjFvewT8Xm6Qp/77g6/j0zBiuAQABj/6vS1qATjd8KSIFDg9G/tCtzwmV/T/egmzswWd5CBiAhW6lgJgEDRr+gRUrFSOB7o3hypW8FUnUrr1JtzzwMYAQ=="}}

event: content_block_stop
data: {"type":"content_block_stop","index":0}

event: content_block_start
data: {"type":"content_block_start","index":1,"content_block":{"type":"tool_use","id":"toolu_01RX68weRSquLx6HUTj65iBo","name":"Read","input":{}}}

event: ping
data: {"type": "ping"}

event: content_block_delta
data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":""}            }

event: content_block_delta
data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"{\"file_path\": \"/tmp/blah/foo"}       }

event: content_block_delta
data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"\"}"}  }

event: content_block_stop
data: {"type":"content_block_stop","index":1          }

event: message_delta
data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":61}              }

event: message_stop
data: {"type":"message_stop"              }


-- non-streaming --
{
  "id": "msg_01JHKqEmh7wYuPXqUWUvusfL",
  "container": {
    "id": "",
    "expires_at": "0001-01-01T00:00:00Z"
  },
  "content": [
    {
      "type": "thinking",
      "thinking": "The user wants me to read a file called \"foo\". Let me find and read it.",
      "signature": "Eu8BCkYICxgCKkBR++kFr7Za2JhF/9OCpjEc46/EcipL75RK+MEbxJ/VBJPWQTWrNGfwb5khWYJtKEpjjkH07cR/MQvThfb7t7CkEgwU4pKwL7NuZXd1/wgaDILyd0bYMqQovWo3dyIw95Ny7yZPljNBDLsvMBdBr7w+RtbU+AlSftjBuBZHp0VzI54/W+9u6f7qfx0JXsVBKldqqOjFvewT8Xm6Qp/77g6/j0zBiuAQABj/6vS1qATjd8KSIFDg9G/tCtzwmV/T/egmzswWd5CBiAhW6lgJgEDRr+gRUrFSOB7o3hypW8FUnUrr1JtzzwMYAQ=="
    },
    {
      "citations": null,
      "text": "I can see there's a file named `foo` in the `/tmp/blah` directory. Let me read it.",
      "type": "text",
      "id": "",
      "input": null,
      "name": "",
      "content": {
        "OfWebSearchResultBlockArray": null,
        "OfString": "",
        "OfMCPToolResultBlockContent": null,
        "error_code": "",
        "type": "",
        "content": null,
        "return_code": 0,
        "stderr": "",
        "stdout": ""
      },
      "tool_use_id": "",
      "server_name": "",
      "is_error": false,
      "file_id": "",
      "signature": "",
      "thinking": "",
      "data": ""
    },
    {
      "citations": null,
      "text": "",
      "type": "tool_use",
      "id": "toolu_01AusGgY5aKFhzWrFBv9JfHq",
      "input": {
        "file_path": "/tmp/blah/foo"
      },
      "name": "Read",
      "content": {
        "OfWebSearchResultBlockArray": null,
        "OfString": "",
        "OfMCPToolResultBlockContent": null,
        "error_code": "",
        "type": "",
        "content": null,
        "return_code": 0,
        "stderr": "",
        "stdout": ""
      },
      "tool_use_id": "",
      "server_name": "",
      "is_error": false,
      "file_id": "",
      "signature": "",
      "thinking": "",
      "data": ""
    }
  ],
  "model": "claude-sonnet-4-20250514",
  "role": "assistant",
  "stop_reason": "tool_use",
  "stop_sequence": "",
  "type": "message",
  "usage": {
    "cache_creation": {
      "ephemeral_1h_input_tokens": 0,
      "ephemeral_5m_input_tokens": 0
    },
    "cache_creation_input_tokens": 0,
    "cache_read_input_tokens": 23490,
    "input_tokens": 5,
    "output_tokens": 84,
    "server_tool_use": {
      "web_search_requests": 0
    },
    "service_tier": "standard"
  }
}