User-defined cost tracker contexts

[sidnarayanan]

Allows a user to scope certain operations in a cost-tracking context, instead of adding it to the global total:

my_tracker = CostTracker()
with my_tracker:
    await some_expensive_operation()
print(my_tracker.lifetime_cost_usd)

Up for discussion: if inside a custom tracker context, should we still add to GLOBAL_COST_TRACKER?

[Copilot]

Pull Request Overview

This PR introduces user-defined cost tracking contexts, enabling users to scope expensive operations to custom CostTracker instances instead of the global tracker. Custom trackers are enabled by default, while the global tracker is now disabled by default. Users can instantiate a CostTracker and use it as a context manager to isolate cost tracking for specific operations.

Key Changes:

• Added context manager protocol (__enter__/__exit__) to CostTracker for scoped cost tracking
• Introduced module-level _active_tracker context variable to manage active tracker state
• Changed default behavior: custom trackers enabled by default, global tracker disabled by default

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

File	Description
packages/lmi/src/lmi/cost_tracker.py	Core implementation of custom tracker contexts, context variable management, and updated default behavior
packages/lmi/tests/test_cost_tracking.py	Comprehensive test suite covering custom tracker defaults, basic usage, LLM integration, nested/sequential trackers, and parallel async task isolation
packages/lmi/tests/cassettes/*.yaml	VCR cassettes for test_custom_tracker_with_llm_calls with OpenAI and Anthropic models in streaming and non-streaming modes

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[jamesbraza]

Suggested change

	def __enter__(self):
	def __enter__(self) -> Self:

[jamesbraza]

Based on this and the PR description, this is where we have to choose between a temporary cost tracker and the global cost tracker?

I can see arguments for both sides (switching between global cost tracking vs always adding to it).

For me, I think having global cost tracking always on is the most easy/predictable. To do a sum across different cost trackers could get arduous.

What did you think?

[sidnarayanan]

Yeah after talking about it, I'm on the same page. Will do.

[jamesbraza]

Suggested change

	@pytest.mark.vcr(match_on=VCR_DEFAULT_MATCH_ON)
	@pytest.mark.vcr

I think this is basically re-specifying the defaults, unless I am missing something