[zichen0116]

Problem

@observe(capture_input=True) silently drops the first positional argument of a free function whenever any parameter is named self or cls — including keyword-only ones:

@observe(capture_input=True)
def classify(value, *, cls="news"): ...

classify("hello", cls="news")
# captured input: {"args": (), "kwargs": {"cls": "news"}}  -- "hello" is gone

The function itself still receives the argument and returns the correct value; only the traced input is wrong. Both sync and async functions are affected.

Root cause

LangfuseDecorator._is_method in langfuse/_client/observe.py treated a function as a method if "self" or "cls" appeared anywhere in the signature:

return (
    "self" in inspect.signature(func).parameters
    or "cls" in inspect.signature(func).parameters
)

_get_input_from_func_args then strips func_args[0] as the implicit receiver — which doesn't exist for free functions.

Fix

Only the first parameter indicates an unbound instance/class method, so _is_method now checks just that:

first_parameter = next(iter(inspect.signature(func).parameters), None)
return first_parameter in ("self", "cls")

Genuine instance methods (def m(self, ...)) and unbound classmethods (def m(cls, ...)) keep their existing behavior; bound methods never reach this code with self/cls visible (inspect.signature already drops the bound argument), so there is no behavior change there.

Verification

• The issue's self-contained reproducer fails on main (args captured as ()) and passes with this fix for both sync and async paths.
• Added 5 regression tests in tests/unit/test_observe.py:
  • free function with keyword-only cls keeps its first positional arg (sync + async),
  • instance method still drops self,
  • classmethod still drops cls,
  • free function with a later positional self is not treated as a method.
• tests/unit/test_observe.py: 20/20 passed.
• ruff check, ruff format --check, and mypy are clean on the changed files.

Closes #18859

Confidence Score: 5/5

The PR appears safe to merge.

Summary

Fixes @observe dropping the first argument of free functions that have a later parameter named self or cls.

• Observed free functions keep their first argument in captured input.

No actionable issues found. Tests were not run during this review.

_{Reviews (1) · Last reviewed commit: "fix(observe): detect methods by first pa..." · Reviewed by Greptile}

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