Fine-grained tool streaming is available on all models and all platforms. It enables streaming of tool use parameter values without buffering or JSON validation, reducing the latency to begin receiving large parameters.

How to use fine-grained tool streaming

Fine-grained tool streaming is supported on the Claude API, Claude Platform on AWS, Amazon Bedrock, Vertex AI, 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.

Here's an example of how to use fine-grained tool streaming with the API:

client = anthropic.Anthropic()

with client.messages.stream(
 max_tokens=65536,
 model="claude-opus-4-8",
 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:
 final_message = stream.get_final_message()

print(f"Input tokens: {final_message.usage.input_tokens}")
print(f"Output tokens: {final_message.usage.output_tokens}")

In this example, fine-grained tool streaming enables Claude to stream the lines of a long poem into the tool call make_file without buffering to validate if the lines_of_text parameter is valid JSON. This means you can see the parameter stream as it arrives, without having to wait for the entire parameter to buffer and validate.

Accumulating tool input deltas

When a tool_use content block streams, the initial content_block_start event contains input: {} (an empty object). This is a placeholder. The actual input arrives as a series of input_json_delta events, each carrying a partial_json string fragment. To assemble the full input, concatenate these fragments and parse the result when the block closes.

Where your SDK provides an accumulator helper (as used in the first example on this page), it handles this for you. The manual pattern is for SDKs without a helper, or when you need to react to partial input before the block closes.

The accumulation contract:

1. On content_block_start with type: "tool_use", initialize an empty string: input_json = ""
2. For each content_block_delta with type: "input_json_delta", append: input_json += event.delta.partial_json
3. On content_block_stop, parse the accumulated string: json.loads(input_json)

The type mismatch between the initial input: {} (object) and partial_json (string) is by design. The empty object marks the slot in the content array; the delta strings build the real value.

Handling invalid JSON in tool responses

When using fine-grained tool streaming, you may receive invalid or incomplete JSON from the model. If you need to pass this invalid JSON back to the model in an error response block, you may wrap it in a JSON object to ensure proper handling (with a reasonable key). For example:

{
 "INVALID_JSON": "<your invalid json string>"
}

This approach helps the model understand that the content is invalid JSON while preserving the original malformed data for debugging purposes.

Next steps