anomaly-investigation

Imported from xlinh2301/EditCTC (.agents/skills/anomaly-investigation/SKILL.md). Install upstream with npx skills add xlinh2301/EditCTC --skill anomaly-investigation. Copyright stays with the author.

Anomaly Investigation Loop

A form → test → eliminate → confirm loop — root-cause analysis as a search. The artifact is an investigation log; the feedback signal is the count of live candidate explanations, driven down toward a single cause that is confirmed, not merely consistent. Each iteration you test one candidate against the data and drop the ones the data refutes, narrowing the field until one survives.

The discipline this enforces: a cause is "root" only when it both survives an honest attempt to refute it and makes a positive prediction that checks out (e.g. "if this is the cause, removing it restores normal" — and it does). A story that merely could explain the anomaly is a hypothesis, not a finding.

When to use

Use this when an anomaly is already in hand — you know roughly what looks wrong and want the cause diagnosed by elimination against the data. Default to a broad initial slate of mutually distinguishable causes, then test the one that splits the field fastest; if the anomaly is vague, your first job is to make it precise (iteration 0). Not for open-ended exploration of a dataset with no anomaly to chase (use data-analysis), and not for verifying an external claim against the literature (use claim-verify).

Setup

Resolve bindings interactively. If loop.run.yaml exists in the working dir, load it, confirm the values in one line, and skip to the loop. Otherwise: on Claude Code (the AskUserQuestion tool is available) infer a likely value for each binding and present it as the recommended option; on other hosts ask each as a quoted plain-text prompt. Then write loop.run.yaml (format: examples/run.example.yaml) and confirm the values before creating any other files.

Analysis snippets run in the user's environment via <analysis_cmd>, so they may use whatever the user has installed. Keep helper code stdlib-first (csv, statistics): if a snippet needs pandas/numpy, probe with try/except ImportError and degrade to a stdlib path, or offer a consented uv pip install "pandas==<ver>" — never assume the package is installed.

The loop

Copy this checklist and tick items off:

• Iteration 0 — characterize the anomaly precisely; form an initial slate of candidate causes in <log>.
• Pick a candidate to test (the one whose test most cleanly splits the remaining field).
• Test it: write <sandbox_root>/iter<N>/test.py, run with <analysis_cmd>, redirect to out.txt.
• Eliminate (data refutes it → drop from the live set) or advance (data supports it → keep it live).
• Confirm the survivor: when one candidate leads, run a positive test only it predicts, after a refutation attempt.
• Append a ledger row; stop when one cause is confirmed or at <budget>.

Iteration 0 — characterize. Quantify the anomaly precisely: write and run a snippet that pins down what deviated, where/when, and how big the deviation is against the normal baseline (the same metric on surrounding periods/segments). Then form an initial slate of candidate causes — mutually distinguishable explanations, broad enough to contain the truth (a real change, a composition/mix shift, a data-quality bug, a measurement change, seasonality, an outlier segment). List them in <log> as the live candidates. Record nothing as confirmed yet.

Then, until stop (one confirmed cause, or budget):

1. Pick a candidate to test. Ideally the one whose test most cleanly splits the remaining field, so each iteration removes as many live candidates as possible.
2. Test it. Write <sandbox_root>/iter<N>/test.py that computes the thing that would refute or support it (slice by segment/source/time, recompute the metric, compare distributions). Run it with <analysis_cmd>, redirecting output to <sandbox_root>/iter<N>/out.txt (never flood your context).
3. Eliminate or advance.
   • Data refutes it → mark it eliminated in <log> with the evidence; drop it from the live set.
   • Data supports it → keep it live, and if it is now the leading candidate run a confirming test: a positive prediction it uniquely makes (e.g. "remove / seasonally-adjust the suspected factor → the anomaly disappears"). Also try to refute it — a leading candidate that survives a genuine refutation attempt and passes its confirming test is the root cause.
4. Log one ledger row and continue, narrowing the live set.

Observational equivalence. Two mechanistically different candidates can make identical predictions in the data you have (e.g. a bot flood and a pipeline double-count both look like "sessions spike, conversions flat" in daily aggregates). When that happens you cannot separate them here — do not pick one arbitrarily. Report them as a single confirmed cause at the resolution of the available data, and name the additional data that would distinguish them (finer-grained logs, raw event records, an upstream check). Distinguish, too, the mechanism (how the metric moved) from the root cause (why the inputs were wrong) — confirming the mechanism is progress, but is not the cause.

Ledger

<sandbox_root>/ledger.tsv, tab-separated, never commas in the text. Header:

iter	candidate_tested	verdict	live_candidates

verdict ∈ {characterize, refuted, supported, confirmed}. Example:

iter	candidate_tested	verdict	live_candidates
0	characterize anomaly + slate	characterize	5
1	real drop across all segments	refuted	4
2	one segment's conversions fell	refuted	3
3	one source's sessions inflated	supported	2
4	removing that source restores normal	confirmed	1

Report the confirmed root cause with its confirming evidence, the alternatives and how each was ruled out, and — if you stop without a single confirmed cause — the remaining live candidates and the test that would separate them.

Constraints

• Confirm, don't just fit. The root cause must survive an honest refutation attempt and pass a positive confirming test; "consistent with the data" is not enough, since several stories usually are.
• Test against the data, not intuition — every elimination and the final confirmation is backed by a computation you ran, recorded in <log>.
• Keep candidates distinguishable and prefer the test that splits the field fastest, so the live count falls; do not chase one pet theory while leaving alternatives untested.
• Only read <dataset> — never modify it, because it is the ground truth every test is checked against. The sandbox is self-contained (no ../ escapes).
• Do not pause the loop to ask whether to continue; run until a cause is confirmed or the budget is hit.

Stops

• Confirmed — exactly one candidate survived refutation and passed a positive confirming test.
• Budget — <budget> iterations reached without a single confirmed cause; report the live set.