Fine-grained tool streaming

Fine-grained tool streaming delivers a tool's input to your client as Claude generates it, without server-side buffering or JSON validation. Skipping the buffering step reduces the time to the first fragment of a large parameter, such as a document or a block of code, and the fragments arrive through the same Streaming messages events as standard tool use.

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How to use fine-grained tool streaming

All models support fine-grained tool streaming on the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry. To use it, set eager_input_streaming to true on any user-defined tool where you want fine-grained streaming enabled, and enable streaming on your request.

The eager_input_streaming field is optional. Setting it to true turns on fine-grained streaming for that tool, and omitting it gives you standard buffered streaming, in which the API buffers and validates each parameter value before streaming it back. The exception is a request that still sends the legacy fine-grained-tool-streaming-2025-05-14 beta header, which turns fine-grained streaming on for tools that leave the field unset. The per-tool field replaces that header, and an explicit false keeps buffered streaming for a tool even when a request still sends it. See Tool reference for the field definition.

The following example turns on fine-grained streaming for a make_file tool and asks Claude for a long poem, so the tool input is large enough to watch it stream in:

client = anthropic.Anthropic()

with client.messages.stream(
    max_tokens=65536,
    model="claude-opus-5",
    tools=[
        {
            "name": "make_file",
            "description": "Write text to a file",
            "eager_input_streaming": True,
            "input_schema": {
                "type": "object",
                "properties": {
                    "filename": {
                        "type": "string",
                        "description": "The filename to write text to",
                    },
                    "lines_of_text": {
                        "type": "array",
                        "description": "An array of lines of text to write to the file",
                    },
                },
                "required": ["filename", "lines_of_text"],
            },
        }
    ],
    messages=[
        {
            "role": "user",
            "content": "Can you write a long poem and make a file called poem.txt?",
        }
    ],
) as stream:
    for event in stream:
        if event.type == "input_json":
            print(event.partial_json, end="", flush=True)
    final_message = stream.get_final_message()