perf: optimize tracer hot path with string-based path ops and caching

[KRRT7]

Summary

• Cache path resolution and validity checks to avoid redundant os.path.realpath and os.path.exists calls on repeated function entries
• Replace Path.is_relative_to and Path.resolve with os.path.realpath + str.startswith in the hot path for lower overhead
• Defer Path construction to first-seen functions only, keeping the per-call path as string-based
• Restore tests-root to tests in pyproject.toml

Test plan

• Existing tracer tests should pass
• Profile with a real project to verify reduced overhead in profile_callback

[KRRT7]

this was causing 10-15% overhead

[claude]

PR Review Summary

Prek Checks

All checks passed (ruff check, ruff format) — no issues found.

Mypy

48 pre-existing mypy errors in codeflash/tracing/tracing_new_process.py (missing type annotations, untyped dicts, etc.) and 1 in tests/test_worktree.py (missing return type on test function). No new mypy errors introduced by this PR.

Code Review

This PR makes solid performance optimizations to the tracer hot path:

1. String-based path operations — replaces Path().resolve() with os.path.realpath() and Path.is_relative_to() with str.startswith() in the trace callback. This is the right call since tracer_logic is called on every traced function invocation.

2. Cached validity checks — path_cache now stores (resolved_path, is_valid) tuples, avoiding repeated Path construction and filesystem checks for already-seen filenames. Invalid paths short-circuit immediately on cache hit.

3. Deferred Path construction — Path(file_name) is only constructed when seeing a function for the first time (for filter_files_optimized and FunctionModules), not on every call.

Minor issue found:

• tests/test_worktree.py:63 — Stale comment says tests_root is configured as "codeflash" but pyproject.toml now has "tests" (inline comment posted with suggestion).

No critical bugs, security issues, or breaking API changes detected.

Test Coverage

File	Stmts (PR)	Miss (PR)	Cover (PR)	Cover (main)	Delta
codeflash/tracing/tracing_new_process.py	508	422	17%	17%	0%
tests/test_worktree.py	48	0	100%	100%	0%
Overall	50042	10810	78%	78%	0%

• No coverage regression
• tracing_new_process.py has low coverage (17%) but this is pre-existing — the tracer requires subprocess execution and is difficult to unit test in isolation. The changed lines follow the same coverage pattern as the code they replaced.
• test_worktree.py has 100% coverage
• 8 test failures are pre-existing on both main and PR branch (same test_tracer_with_complex_arguments assertion error)

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Last updated: 2026-02-27T14:50 UTC

[claude]

The tests-root change from "codeflash" to "tests" breaks tests/test_worktree.py::test_mirror_paths_for_worktree_mode which hardcodes the old value:

# tests_root is configured as "codeflash" in pyproject.toml
assert optimizer.args.tests_root == worktree_dir / "codeflash"

The test at lines 63-67 needs to be updated to expect worktree_dir / "tests" instead of worktree_dir / "codeflash".

[claude]

Stale comment: says tests_root is configured as "codeflash" in pyproject.toml but it's now "tests".

Suggested change

	# tests_root is configured as "codeflash" in pyproject.toml
	# tests_root is configured as "tests" in pyproject.toml