## Why
External agent migration detection parsed and hashed every JSONL session
file. For users with many large conversations, launching migration could
consume substantial CPU and disk resources.
Detection only needs the most recent sessions for the migration UI, so
full-content work should be bounded.
## What
- Use file modification metadata to select the 50 most recent eligible
sessions before parsing JSONL content.
- Skip unchanged imported sessions using metadata stored in the import
ledger.
- Preserve content hashing when metadata indicates a session may have
changed.
- Stream SHA-256 calculation through a 64 KiB buffer instead of loading
an entire session into memory.
- Continue detecting older sessions in subsequent batches after newer
sessions are imported.
## Validation
- `RUST_MIN_STACK=8388608 cargo nextest run --no-fail-fast -p
codex-external-agent-sessions`
- 20 tests passed.
- Benchmarked release builds against 250 valid JSONL sessions totaling
501 MiB:
- Median detection time decreased from 1,138.8 ms to 47.0 ms.
- CPU instructions decreased by 95.8%.
- Both versions returned the expected 50 sessions.
The benchmark used warm filesystem caches and measured the reduction in
parsing, hashing, and CPU work.
stefanstokic-oai
cbf62f64cb