fix(step): using a Step instance as a decorator always raises TypeError

[Anai-Guo]

The bug

Step.__call__ lets a Step instance be used as a decorator. It forwards thread_id to the step() decorator factory:

backend/chainlit/step.py:447-455

def __call__(self, func):
    return step(
        original_function=func,
        type=self.type,
        name=self.name,
        id=self.id,
        parent_id=self.parent_id,
        thread_id=self.thread_id,   # <-- step() has no such parameter
    )

…but step() does not declare thread_id. So any use of a Step instance as a decorator raises before the wrapped function ever runs:

TypeError: step() got an unexpected keyword argument 'thread_id'

Step.__init__ does accept thread_id (and tests/test_step.py::test_step_with_custom_thread_id already covers it there), so the parameter is real everywhere except on the decorator path that tries to propagate it.

The fix

Add thread_id: Optional[str] = None to step() and pass it to the Step(...) it constructs — in both the async and the sync wrapper, so the two branches stay symmetric. Step.__init__ already does self.thread_id = thread_id or context.session.thread_id, so the default None preserves today's behaviour for every existing caller.

+3 lines in step.py.

Tests

Two tests added to TestStepDecorator:

• test_step_instance_used_as_decorator — the crash itself: @Step(name=..., type="tool") on an async function.
• test_step_decorator_forwards_thread_id — @step(thread_id=...) actually reaches the Step that gets created, so the new parameter is wired up rather than merely accepted.

Both fail on main with the exact TypeError above, and pass with the fix:

# before
tests/test_step.py::TestStepDecorator::test_step_instance_used_as_decorator
E  TypeError: step() got an unexpected keyword argument 'thread_id'  (chainlit/step.py:449)
tests/test_step.py::TestStepDecorator::test_step_decorator_forwards_thread_id
E  TypeError: step() got an unexpected keyword argument 'thread_id'
2 failed

# after
47 passed, 3 warnings in 0.53s     # 45 pre-existing + 2 new

ruff check and ruff format --check are clean on both files using the repo's root pyproject.toml config.

🤖 Generated with Claude Code

---

Summary by cubic

Fixes using a Step instance as a decorator, which always raised TypeError because step() didn't accept thread_id; now it does and forwards it, so @Step(...) works again. Adds tests for decorator usage and thread_id forwarding.

^{Written for commit f301816. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="backend/chainlit/step.py">

<violation number="1" location="backend/chainlit/step.py:113">
P3: The sync decorator path now forwards `thread_id` to `Step(...)` but has no test. Since the PR explicitly aims for sync/async symmetry and only exercises async in the new tests, add a sync-variant of `test_step_decorator_forwards_thread_id` (a plain `def` wrapped with `@step(thread_id=...)`) to cover `sync_wrapper`.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P3: The sync decorator path now forwards thread_id to Step(...) but has no test. Since the PR explicitly aims for sync/async symmetry and only exercises async in the new tests, add a sync-variant of test_step_decorator_forwards_thread_id (a plain def wrapped with @step(thread_id=...)) to cover sync_wrapper.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At backend/chainlit/step.py, line 113:

<comment>The sync decorator path now forwards `thread_id` to `Step(...)` but has no test. Since the PR explicitly aims for sync/async symmetry and only exercises async in the new tests, add a sync-variant of `test_step_decorator_forwards_thread_id` (a plain `def` wrapped with `@step(thread_id=...)`) to cover `sync_wrapper`.</comment>

<file context>
@@ -109,6 +110,7 @@ async def async_wrapper(*args, **kwargs):
                     name=name,
                     id=id,
                     parent_id=parent_id,
+                    thread_id=thread_id,
                     tags=tags,
                     language=language,
</file context>