stillsane

A drift canary for deployed LLM apps and agents.

Your LLM app does not crash when it gets worse. It returns 200, latency looks normal, the error rate is zero, and the output is quietly less correct than it was last month. You find out when a user complains.

stillsane runs a small set of prompts against your live endpoint on a schedule, compares each response to a stored baseline, and tells you when behaviour has moved outside the range that probe normally varies by. It observes from outside, over plain HTTP. There is nothing to instrument, no SDK to import, no account, and no hosted service.

Status: early, v0.0.5. Everything described below works. The config format may still change before 0.1. See Status.

---

What it looks like

A probe that extracts a total and a due date as JSON. The model was updated, and it started explaining itself. The numbers are still correct and still present, but every caller doing json.loads(response) is now throwing.

DRIFT  extract_invoice @ prod
  semantic_distance           0.133  band <=0.03745 (floor)   z=+17.3
  length_chars                  106  band 36..52 (floor)      z=+23.2
  completion_tokens              26  band 7..15 (floor)       z=+11.2
  valid_json               0% valid  band >=1
  5 other signal(s) unchanged

  baseline (v1, 2026-07-31):
    {"due_date": "2026-07-01", "total": 1240.50}
  now:
    Sure! I found the following:
    {"total": 1240.5, "due_date": "2026-07-01"}
    Happy to help with more invoices.

------------------------------------------------------------
1 drift   ->  DRIFT

Exit code 1, so this fails a build. Nothing errored, nothing was slower, and no conventional monitor would have noticed.

That band <=0.03745 was not a number anyone chose, and the (floor) beside it is the tool being honest about how it got there. This example runs against a tidy mock whose samples barely vary, so there was too little spread to learn from and the band fell back to a built-in floor. Against a real endpoint it is measured from the probe's own behaviour instead, and stillsane tells you which of the two happened rather than presenting a defaulted number as a measured one. Either way the prose-wrapped version sits seventeen times outside it.

You can run exactly this in about thirty seconds, with no API key, from examples/invoice-extract/.

---

Quickstart

pip install stillsane

stillsane init       # write a starter config, then point it at your endpoint

stillsane baseline   # capture what "normal" looks like. Explicit, never automatic.

stillsane check      # compare against it. Non-zero exit on drift.

Put stillsane check on a schedule in CI and you are done. See In CI for a workflow you can copy.

---

The problem

Three ways an LLM app degrades without ever erroring:

1. The provider changed the model. Hosted providers update the model behind an endpoint without changing the version string. The same call can return meaningfully different output months later, and this is entirely outside your control.
2. Someone edited a prompt. A tweak to a system prompt or a tool description breaks a behaviour three steps downstream that no test covers.
3. Retrieval drifted. RAG context shifts, retrieval quality slides, answers get vaguer.

In all three cases the app keeps working. Latency is fine. Error rate is zero. Quality is worse. Nobody gets paged.

This is the same failure mode as a drifting sensor on unattended infrastructure: a crash is loud and you find out immediately, but a plausible-looking wrong number gets believed. The fix there was synthetic monitoring: walk the whole pipeline on a schedule, validate what comes back, alert before a human notices. stillsane is that, pointed at an LLM.

---

Should you use this?

Probably not, if you already have something:

• You want to know whether a prompt is good before you ship it. Use a pre-ship eval framework. There are several good open-source ones, and stillsane will not help you. That is not false modesty. Pre-ship evaluation is a different problem, and tools built for it solve it better than a tool of this scope ever will.
• You already run a tracing or eval platform. You have evaluator scores on real production traffic. Watch those. Adding stillsane buys you provider-fingerprint watching and not much else.
• You are willing to instrument your app. Then instrument it. Tracing sees every real request; stillsane only ever sees the handful of probes you wrote. That is a genuine and permanent disadvantage.

stillsane is for the case none of those cover: you shipped an agent or an LLM pipeline, quite possibly mostly AI-assisted, you have no evals and no observability, you are never going to add a tracing SDK, and right now you would find out about degradation from a user complaint.

If that is you, this is a config file and one command.

How it compares

	stillsane	Pre-ship eval frameworks	Tracing platforms
Answers "will this prompt work?"	no	yes	partly
Answers "is what I shipped still fine?"	yes	no	yes
Scores real production traffic	no	no	yes
Requires instrumenting your app	no	no	yes
Requires an account / hosted service	no	usually not	usually
Learns per-probe variance	yes	no	no
Alerts on provider fingerprint change	yes	no	rarely
Breadth of providers	narrow	broad	broad
Assertion / eval library	minimal	large	large

Both other columns cover a range of tools that differ considerably from each other; they are a rough shape, not a specification.

Where the alternatives win outright: provider coverage, assertion breadth, dataset-driven evaluation, red teaming, and maturity. Several eval frameworks also document a drift workflow (save a baseline, re-run on a schedule, compare), and if you are happy writing the comparison logic yourself, that gets you a good deal of what stillsane does.

What stillsane adds is that comparison logic: a variance model so the thing does not cry wolf, a baseline that refuses to update itself, and fingerprint watching. As far as I can tell nothing in the pre-ship category ships a command that compares a run against a stored baseline and tells you what moved. That gap is the entire reason this exists.

---

The interesting part: variance

Most drift tools compare the new output to one stored output and alert past a fixed threshold. That fails immediately, because probes do not share a variance.

A temperature-0 JSON extraction returns near-identical text every time. A summarisation probe legitimately rewords itself on every single call. One fixed threshold either misses real drift on the first or fires constantly on the second, and a tool that fires constantly gets uninstalled inside a week.

So stillsane learns the band per probe, from the probe's own behaviour:

• At baseline it captures N samples and measures the distances among them. That distribution is the probe's intrinsic variance.
• At check time it captures M samples and measures the distance from each baseline sample to each new one.
• With no drift, those two sets of distances are drawn from the same distribution. Both are "how far apart are two independent draws". With drift, the second set shifts up.

The obvious objection is that you could just guess: tight threshold for the JSON probe, loose one for the summariser. Here is what happened when I actually measured it, against a live model, with two probes picked to make exactly that point:

extract_invoice     JSON, ~60 chars     within-baseline distance  0.128
summarise_quarter   prose, ~230 chars   within-baseline distance  0.066

The "deterministic" extraction probe is twice as variable as the open-ended summariser, which is the opposite of what I expected when I wrote the probes.

The reason is visible in the samples. The extraction output is about sixty characters, and the model sometimes wraps it in a markdown fence and sometimes does not, sometimes writing "1240.50 USD" and sometimes 1240.50. Three distinct outputs across five samples. On text that short, formatting variation dominates. The summaries are all different sentences but all around 230 characters saying the same thing, so they stay close together.

That is the argument for learning the band rather than setting it. I guessed confidently and was wrong by a factor of two on my own tool; the measurement was right and both probes passed. Anyone hand-tuning a threshold from intuition would have set it too tight on the probe that looked deterministic and too loose on the one that looked chatty.

The headline number is z: how far behaviour moved in units of that probe's own normal variation. "Moved 6.2x further than this probe usually varies" is a sentence you can act on. A p-value is not.

On calling it z. It is computed as (observed − median) / (1.4826 × MAD), which is a robust analogue of a standard score, not a standard score. The 1.4826 makes a MAD comparable to a standard deviation for normally distributed data, and a distribution of distances is emphatically not normal. It is bounded below at zero and right-skewed. So z here assumes no distribution at all: it is a scale-free measure of how far outside normal something sits, and it does not convert to a probability. z=6 is not a one-in-a-billion event. The thresholds (warn_k: 3, drift_k: 6) are chosen empirically rather than derived from Gaussian tails, and they are config knobs precisely because the right values are an open question. Being straight about it: they were tuned against constructed drift scenarios, not yet against a long run on a real provider, so treat them as sensible starting points rather than settled numbers. The Mann-Whitney p-value reported alongside is distribution-free and does carry its usual meaning, which is exactly why it is supporting evidence and never the gate.

Related decisions, since they are the ones that determine whether this is usable:

• Robust statistics throughout. Median and MAD, not mean and standard deviation. At the sample counts anyone will actually pay for, one weird sample dominates a standard deviation and barely moves a MAD. The known cost: MAD collapses to zero when over half the samples are identical, which a low-temperature model does often. The band then falls to its floor and can end up tighter than the probe's own baseline. stillsane bands finds that case and says so; it is not yet fixed in the estimator.
• Baselines never update themselves. Only stillsane baseline replaces one. A monitor that silently re-baselines has defined drift out of existence.
• Clean runs tighten the band, but widening is capped. Passing runs feed back into the variance estimate, so the tool gets more sensitive over time at no extra cost. Widening is measured against the original baseline rather than against yesterday, so drift arriving a little at a time cannot slowly stretch the band around itself.
• Editing a prompt invalidates its baseline. The config hash covers the prompt, system message, checks and model. Change any of them and check refuses to compare rather than reporting your own edit as provider drift.
• A transport error is not drift. A dead endpoint exits with a different code than a quality regression, because they call for different responses.

---

Usage

Python 3.10+, five dependencies, no torch. Everything needed to run is installed; the embedding model itself is fetched once on first use (~32MB) and cached. See Design constraints if you need to stay fully offline.

Beyond the three commands in the quickstart there is stillsane watch, a sleep loop that is honest about being one. cron or CI does this better: they survive reboots, they log, and they can tell you when the job itself stopped running, which a bare process cannot do for itself. There is also stillsane bands, below.

Inspecting the bands

check tells you whether a probe moved. stillsane bands answers the question underneath it: is the band it would be judged against a measurement at all?

stillsane bands

It reads only what is already on disk, so it costs nothing, needs no API key, and touches no network. It reports every band, including the pointwise ones that never appear in the capture-time warning, and names the ones that will misreport:

extract_invoice @ claude   (v1, 8 sample(s), captured 2026-08-04)
  semantic_distance      band <=0.02 (floor)          31 pairs   spread 0..0.1276
    COLLAPSED: the median and MAD are both zero, so the scale could not be
    measured and the band fell to its floor. The baseline itself spans
    0..0.1276, and 14 of 31 pairs (45%) fall outside the band. A check drawing
    those reports drift against an endpoint that has not changed. Typically
    the output is bimodal: identical on most runs, formatted differently on
    the rest. More samples will not help while one form dominates, because the
    median stays put and the MAD stays zero.
  length_chars           band 60..76 (floor)           8 values  spread 56..68
    COLLAPSED: ...
  defaulted to a floor, samples agree: latency_ms
  3 other band(s) look sound

summarise_incident @ claude   (v1, 8 sample(s), captured 2026-08-04)
  3 other band(s) look sound

2 band(s) will misreport: length_chars, semantic_distance
Recapture will not help a collapsed band. Consider a probe whose output
varies less arbitrarily, or pin the band explicitly in config.

(The second COLLAPSED paragraph is elided above; it repeats the first with that signal's own numbers.)

That is a real baseline against a real provider, and it is the failure worth knowing about. When a probe returns byte-identical output most of the time and a different formatting the rest, the median pair distance is zero and so is the MAD. The scale collapses, the band drops to its floor, and the result looks exactly like every other band. It is not one: 45% of the baseline it was built from already sits outside it.

More samples do not fix that one, which is why it gets a different message from an ordinary floored band. While one formatting dominates, the median stays put and the MAD stays zero however many you take.

--strict exits 2 when any band will misreport, for a CI job that should fail on a baseline this shape. -v shows every band rather than only the interesting ones.

--json writes the same inspection as structured output. Unlike the human report it always includes every band, sound ones included, since a consumer diffing bands between runs needs to tell "still sound" from "no longer reported":

{
  "signal": "semantic_distance",
  "finding": "collapsed",
  "suspect": true,
  "unit": "pairs",
  "n": 31,
  "observed_min": 0.0,
  "observed_max": 0.1276024580001831,
  "raw_scale": 0.0,
  "outside": 14,
  "outside_pct": 45.16,
  "band": {"center": 0.0, "scale": 0.006666666666666667,
           "lower": null, "upper": 0.02, "n": 31, "floored": true}
}

Config

Plain YAML, meant to live in git and be diffed like code.

targets:
  - name: prod
    type: openai_compatible
    base_url: https://api.example.com/v1
    model: some-model-id
    api_key_env: PROVIDER_API_KEY   # the variable name, never the key itself
    watch_fingerprint: true

probes:
  - id: extract_invoice
    prompt: "Extract the total and due date as JSON from: ..."
    baseline_samples: 5             # paid once, this is where variance comes from
    check_samples: 3                # paid every run, only needs to find a median
    checks:
      - valid_json
      - has_keys: [total, due_date]
      - semantic_similarity: auto   # learned band, not a fixed number

alerts:
  webhook: https://hooks.example.com/...

samples: 5 also works and sets the baseline count.

On cost. Sampling is the whole mechanism, so it is worth being explicit: the expensive part is the baseline, and you pay it once. Routine checks need only enough samples to locate a median, because the variance estimate already lives in the baseline. Embeddings run locally and cost nothing. The LLM judge is opt-in and only fires when a band has already been crossed, so a normal run spends nothing beyond the probe calls themselves.

Checks

Check	Meaning
valid_json	The whole response parses as JSON. A markdown fence is allowed; surrounding prose is not, because that is what breaks a caller's json.loads.
has_keys: [a, b]	Those keys are present in the JSON, found leniently. Deliberately separate from valid_json, so the report can say the data survived even when the envelope broke.
semantic_similarity: auto	Learn the band. A number instead of auto pins a fixed threshold.
max_length: 2000	Hard cap on response length.

Several signals are always on and need no configuration: semantic distance, JSON shape, tool-call shape, length, completion tokens, cost, latency, provider fingerprint and model id. Signals that do not apply to a probe stay silent, so tool-call drift says nothing about a probe that never calls a tool.

Generating probes from logs

Writing twenty probes by hand is the reason most people never start, and the ones you would write are the ones you already think about. The prompts actually hitting your endpoint are a better sample, and you already have them:

stillsane init --from-logs requests.jsonl

Read requests.jsonl
  61 distinct prompt(s), clustered into 3

That number is the point. Real logs are enormously repetitive: a thousand requests are usually a handful of shapes with different payloads stuffed into them. Near duplicates are clustered by meaning using the embedder that already ships for drift detection, and the most frequent variant of each cluster becomes the probe, annotated with how often it appeared.

Reads JSONL, a JSON array, or a directory of .json files. Each record can be an OpenAI-style request body, a bare {"prompt": ...}, or either of those wrapped under request, body or payload. Malformed lines are skipped, because refusing a 10,000-line log over one line truncated mid-write would make the feature useless on exactly the files it exists for.

Checks are emitted commented out. Guessing that a probe returns JSON and being wrong would fail your first baseline and teach you the tool is broken. You get the prompts and a suggestion; you decide what holds.

Flag
--limit N	Most probes to emit, most frequent first. Default 20.
--merge-distance D	How aggressively to cluster. Higher merges more. Default 0.12.
--probes-only	Emit just the probes: block, for pasting into a config you already have.
--embedder hashing	Cluster without the embedding model, fully offline.

The judge (optional)

Add a judge block and a probe that crosses its band gets one extra call, to explain in a sentence what changed:

judge:
  base_url: https://api.openai.com/v1
  model: gpt-4o-mini
  api_key_env: OPENAI_API_KEY

DRIFT  extract_invoice @ prod
  semantic_distance           0.133  band <=0.03745       z=+17.3
  valid_json               0% valid  band >=1
  ...
  -> breaking: Still valid JSON, but now wrapped in conversational prose.

It only runs on probes that already failed their band, so on a day when nothing drifted it is never called and costs nothing. That tiering is the point: structural checks are free, embeddings are free after the one-time download, and the only paid layer fires when something is already known to be wrong.

Two deliberate limits:

• It is advisory. By default it explains and nothing else. The verdict came from a band learned out of the probe's own measured behaviour, and a model that saw two samples does not get to overrule that. Set can_downgrade: true to let it soften a drift it considers purely cosmetic, once you trust it.
• It gets its own endpoint. Not a flag on a target, because judging with the same deployment you are watching means a provider-side change moves both the thing being measured and the instrument measuring it.

If the judge is unreachable or answers with something unparseable, the run is unaffected: the verdict stands and the explanation is simply absent.

Exit codes

Code	Meaning
0	No drift.
1	Drift.
2	Warning only. Does not fail a build unless fail_on_warn: true.
3	Error. The endpoint failed, or there is no usable baseline.

In CI

Copy examples/invoice-extract/github-actions.yml into .github/workflows/. It runs stillsane check every morning, caches the embedding model between runs, and fails the job on drift.

Kept as one file rather than pasted here as a second copy, because two copies of a workflow drift apart and the one in the README is the one nobody re-tests.

Two things it relies on:

• Commit .stillsane/baselines/. The workflow needs something to compare against. They are plain text and diff like code. Leave .stillsane/history.sqlite out, since it is a binary that changes every run.
• A daily schedule is the point. Provider-side model changes arrive without warning; finding out within a day is the entire product.

---

Status

Working end to end:

• The comparison engine: variance bands, effect sizes, verdict aggregation
• Signals: structural, semantic, JSON shape, tool-call shape, fingerprint, tokens, cost, latency
• Variance pooling, with the caps that stop gradual drift widening its own band
• Targets: OpenAI-compatible and arbitrary HTTP
• Versioned baseline store, SQLite history, and the config hash that refuses a stale comparison
• init, baseline, check, watch, the report renderer, and webhook/Slack alerts
• The optional LLM judge, which only runs on probes that already failed their band
• init --from-logs, which clusters your logged prompts into a probe set

The test suite runs with no network, no API key and no model download. It ships in the sdist, so you can verify the variance model yourself rather than taking this README's word for it:

pip install -e ".[dev]" && pytest

There is a runnable worked example in examples/invoice-extract/, with a committed baseline and a mock provider, so you can watch a real regression get caught without an API key. CI builds the wheel, installs it into an empty environment and runs that example on every push, which is how a broken install gets caught before a release rather than after one.

Expect breakage before 0.1. The config format is not frozen. If a field is renamed you will get a validation error naming it, not a silent misread, but a version pin is wise for now.

---

Non-goals

Each of these turns a finishable project into an unfinished platform, so they are out permanently rather than deferred:

no web dashboard · no hosted service, accounts or billing · no tracing or instrumentation of your app · no SDK to import · no database beyond SQLite · no agent-framework integrations, it speaks plain HTTP · no leaderboards or model benchmarking

And the one that matters most: stillsane is not an eval framework. It does not measure whether your app is good. It measures whether it changed from a known baseline. Existing tools measure quality; this measures change.

---

Design constraints

• Point it at an endpoint with a few prompts and get value in under five minutes.
• Near-zero running cost. Local embeddings by default, judge opt-in and only on suspicion.
• No internet dependency except the target endpoint, with one exception stated plainly: the default embedding model is a 32MB one-time download. Set embedder: hashing to stay fully offline, at the cost of a weaker signal on rewrites that preserve meaning.
• Plain text config, so it lives in git.
• Works with any OpenAI-compatible endpoint, which covers most providers plus local Ollama and vLLM.

---

Licence

MIT.