claude-drift

Replay your own Claude Code sessions against a new model and see what actually changed. Replaying Opus 5 against itself, it reproduced its own next action 80% of the time but matched the action in the recorded session only 35% of the time — so a raw "the model changed" number is mostly measuring your replay setup, and you need the same-model control to tell drift from noise.

What it is

• No config. No API key. Uses your existing claude login and the sessions already in ~/.claude/projects.
• Teacher-forced single-step replay: the new model sees exactly the context the old one saw and proposes one next action. No tools run.
• A same-model control: the old model is replayed against the same cuts, so every number comes with a noise band.
• --self-replays N replays the old model against itself, separating the model's own sampling instability from replay-vs-interactive mismatch.
• Per-transition changes are reported as REAL or NOISE with a bootstrap interval, Bonferroni-corrected over the transitions tested.

The number, and why you need the control

From a real run on the author's machine, 20 turns across 5 sessions, claude-opus-5 recorded and sonnet as the candidate:

next-action agreement: 25%  (noise band 14%-58%)
old-vs-record agreement: 35%
old-vs-old self-agreement (k=5 measured): 80%  (band 63%-88%)
interpretation: the old model mostly reproduces itself; the gap to the record is replay-vs-interactive mismatch, not model instability
verdict: no detectable drift (candidate agreement inside noise band)
agreement by level: tool 30% / target 25% / full 25%

Read naively, the candidate picked a different next action than the record on 75% of turns, which sounds like a large behaviour change. But the old model disagreed with its own recorded action on 65% of turns, and disagreed with itself across five draws on only 20% — most of that gap is the replay regime (claude -p, hooks and plugin state, thinking budget), not the model. Against that band the candidate sits inside the noise, so on these 20 turns there is no detectable drift.

Install

uvx claude-drift scan            # run without installing

or pip install claude-drift / uv tool install claude-drift, which gives you the short drift command used in the examples below. Python 3.11+ and a logged-in claude CLI are required.

Use

drift scan                                   # what is replayable on this machine (no model calls)
drift replay --from opus-5 --to sonnet-5     # replay 30 sampled turns with both models
drift report                                 # re-render the last run (no model calls)
drift resume                                 # finish the replays a cut-off run left pending

30 turns is the default so one run fits inside a subscription window. Use drift resume if a run is cut off.

Example output: see docs/examples/first-report.md.

Options for drift replay

What is stored

Each run writes a directory under ~/.claude-drift/runs/ holding the sampled prompts, the raw tool inputs of both the recorded and the proposed actions, and the rendered report. That is your own session content sitting on your own disk; delete the directory to remove it.

If a replay is interrupted (Ctrl-C or SIGTERM) in-flight replays finish first so temporary session copies are always removed; the run is marked interrupted.

How it works

1. ingest — finds human prompts in your session logs that were followed by a tool call.
2. sample — picks up to 30 turns, stratified by tool, at most 3 per session, seeded.
3. replay — writes a truncated copy of the session next to the original under a temporary id and runs claude -p --resume <id> --fork-session --model <new> --max-turns 1. The primary guarantee that nothing runs is the stop itself: the stream is read only up to the first tool_use block and the process is killed there, before any tool can execute. As a second layer, a PreToolUse hook that blocks every tool is passed via --settings. That hook was never exercised in testing, because the stop always happens first. --settings merges rather than replaces, so your own hooks and plugins still load during a replay. The temporary file is deleted afterwards, and the original session file is never written to.
4. classify — normalises each action to tool/target (e.g. Bash/local-read, Read/local-read, Agent/delegate, Bash/remote).
5. stats — agreement with the recorded action for the new model and for the old model; a bootstrap 95% band of old-vs-record agreement is the noise floor. The bootstrap resamples sessions, not turns, because turns from one session are not independent. Transitions whose new-minus-old count has a bootstrap interval excluding zero are reported as REAL, with the interval Bonferroni-corrected over the number of transitions tested. Transitions are enumerated from what the new model did differently, so a cut where only the old model drifted away from the record is not listed in v1.
6. report — text or markdown.

Cost

Each replayed turn sends the session prefix again (typically 30k–100k tokens, mostly cached). Default settings replay 30 turns twice, once per model. The run quoted above used --turns 20 --per-session 8 --self-replays 5, which is 120 replays (20 candidate, 100 old-model):

That run hit the subscription session limit partway through and was finished with drift resume, so the wall clock excludes the wait for the window to reset. --self-replays 5 is what makes it expensive; the default of 1 costs about a third of this for the same number of turns. drift replay prints an estimate and asks before starting.

Limits

• Only turns after a human-typed prompt are replayed; turns after tool results are skipped in v1.
• Only Claude models, because only Claude Code sessions are read.
• Results describe next-action drift, not end-to-end task outcomes.
• Replays inherit your local plugins and hooks, so reports are not directly comparable across machines.
• The self-replay control measures instability under claude -p; it does not reproduce the interactive session's thinking budget or the plugin state at recording time, so old-vs-record agreement is a lower bound.

Related

• delta-hq/cc-canary — descriptive statistics over Claude Code session logs; it never calls a model.
• sshh12/agent-pr-replay — end-to-end re-execution of agent tasks, without teacher forcing and without a same-model control.

As far as we know, claude-drift is the only tool that replays the old model against itself to separate drift from noise.

Development

uv sync
uv run pytest -q
uv run ruff check .
uv run mypy

The live probe that shells out to a real claude process is opt-in:

DRIFT_LIVE=1 uv run pytest tests/test_replay_live.py -s

Releasing

git tag v0.1.1 && git push --tags   # bump pyproject version first; the workflow checks it

A v* tag runs .github/workflows/publish.yml, which builds the wheel and uploads it to the PyPI project claude-drift through trusted publishing from the pypi environment. No token is stored in the repository.

License

MIT