[chengrm (chengwudi1)]

Summary

Fixes #9682.

In FileSessionStore.set(), the temporary file was created using:

temp_path = file_path.with_name(f".{file_path.name}.{uuid.uuid4().hex}.tmp")

This redundant inclusion of {file_path.name} appended 38 characters (. + 32-char hex + .tmp + .) to an already long filename. For long session IDs (up to MAX_SESSION_ID_LENGTH = 128) residing in moderately deep directory trees on Windows where LongPathsEnabled is not set, this caused atomic writes to exceed the Windows MAX_PATH limit (260 characters) and fail with FileNotFoundError, even when the final snapshot file path was well within limits.

Changes

• Shorten the atomic write temporary filename in FileSessionStore.set() to .{uuid.uuid4().hex}.tmp.
  • Its length is a constant 37 characters, matching the compact, ID-independent temp file naming strategy used in _workflows/_checkpoint.py and _filesystem.py.
  • It stays significantly shorter than long session snapshot filenames (which are 133+ characters for 128-char session IDs), guaranteeing that temporary paths will never overflow MAX_PATH if the destination path is valid.
  • Matches the .*.tmp glob pattern expected by cleanup assertions in tests.
• Added regression test test_set_long_session_id_atomic_write_does_not_overflow_path in python/packages/core/tests/core/test_sessions.py.

Verification

• Run core session tests:
  uv run pytest packages/core/tests/core/test_sessions.py -v -> 188 passed (including new regression test).
• Run ruff syntax formatting and check:
  uv run poe syntax -P core -> All tasks passed.
• Run type check:
  uv run poe pyright -P core -> 0 errors, 0 warnings.

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[Copilot]

🟡 Changes recommended

The regression test must enforce the path-length constraint independently of host configuration to reliably detect the original bug.

1 open issue

• Enforce path-length limit at filesystem write boundary · New

What changed in this PR

Addresses #9682 by shortening temporary filenames used by Python’s FileSessionStore, avoiding Windows path-limit failures when saving long session IDs.

Changes:

• Uses a compact, session-ID-independent temporary filename while preserving atomic replacement.
• Adds a regression test covering long session IDs, round-trip persistence, and temporary-file cleanup.

File	Description
python/​packages/​core/​tests/​core/​test_sessions.py	Adds a long-session-ID regression test.
python/​packages/​core/​agent_framework/​_sessions.py	Shortens atomic-write temporary filenames.

_{Review effort: Balanced}

[Simin Yuan (simin-yuan)]

Ran this where the issue's condition holds: Windows 11, LongPathsEnabled = 0, Python 3.14.9.

Method: the repro from #9682 unchanged — session ID at MAX_SESSION_ID_LENGTH, storage path sized so the snapshot path is exactly 240 characters (fits, while the old temp name did not).

agent-framework-core at	set() result
67ba500 (this PR's base)	FileNotFoundError, errno 2
d1c38940 (this PR)	saved, no error

Same script, same machine, same storage path both times.

With the change, the temp name is .<uuid>.tmp (33 characters) instead of .<session id>.json.<uuid>.tmp (up to 171), so the temp path stays shorter than the snapshot path regardless of session ID length.

tests/core/test_sessions.py with this PR applied on the same machine: all tests pass, no failures (exit code 0).

One caveat on scope: this covers only the long-paths-disabled configuration that CI does not exercise (windows-latest images set LongPathsEnabled = 1). I did not run this on POSIX.

Reporting the measurement only.