Does the SDK support Fine-grained tool streaming?

[Shane-12138]

The "include_partial_messages=True" parameter setting cannot enable streaming of tool usage parameter values. Could you please explain how to achieve the fine-grained tool streaming effect when using the claude-agent-sdk-python?

https://platform.claude.com/docs/en/agents-and-tools/tool-use/fine-grained-tool-streaming

[jolee-quora]

+1, this used to work for me in the past when fine-grained-tool-streaming-2025-05-14 was enabled by default, but doesn't seem to work anymore potentially due to eager_input_streaming being added as the standard way to do this at the API level

[qing-ant]

Root cause and fix

The fine-grained-tool-streaming-2025-05-14 beta header was removed in CLI v2.1.40 (Feb 2026) and replaced with an opt-in env var. This caused include_partial_messages=True to stop delivering input_json_delta events for tool inputs starting in SDK v0.1.36.

Workaround (until the fix ships):

Set the env var before running your script:

CLAUDE_CODE_ENABLE_FINE_GRAINED_TOOL_STREAMING=1 python your_script.py

Or set it in Python before the SDK is called:

import os
os.environ["CLAUDE_CODE_ENABLE_FINE_GRAINED_TOOL_STREAMING"] = "1"

Then use include_partial_messages=True as normal — input_json_delta stream events will flow through.

Permanent fix: Upcoming fix will inject this env var automatically whenever include_partial_messages=True is set, so no manual workaround will be needed after that ships. ETA - early next week.