coverage ratchet reports a bump when only the provenance moved

[chernistry]

Correction (2026-08-18). The premise below — that the provenance-only write originates in _cmd_check's write path — is wrong, and was disproved on the thread. check reads its baseline out of the tree it checked out (the measured commit), which main has usually ratcheted past by the time that commit's CI run completes; the bump is correct against what it read and a no-op against the base the PR is opened onto. See this comment for the arithmetic and #4110 for the fix. The rest of this body is kept as written because the reproduction attempts against it are what located the real cause.

The coverage ratchet opened #4078 and #4085 carrying a diff that moves only provenance:

   "diff_coverage_floor_percent": 86,
-  "head_sha": "6902f699a30d1193a09a99129114af1b08920dfc",
+  "head_sha": "6bb9f3ed5990bf5c1d0c0d85f8d3a0789061824a",
   "line_rate": 0.8384,
-  "run_id": "31999416744",
+  "run_id": "32003768324",
   "total_coverage_percent": 83.84,
-  "updated_at": "2026-08-17T07:34:25+00:00"
+  "updated_at": "2026-08-17T08:43:49+00:00"
 }

line_rate and total_coverage_percent are byte-identical on both sides. Only head_sha, run_id and updated_at moved. A ratchet PR that raises nothing is noise on the queue: it costs a full CI matrix, it needs a human to look at it, and it trains reviewers to approve baseline PRs without reading the numbers — which is the one place the numbers are the entire point.

Expected

A ratchet fire that does not raise the high-water mark opens no PR, and says in its log why it declined.

Scope

The ratchet's decision path, not its measurement. coverage.xml is being produced correctly; the defect is downstream of it.

Proof

• A unit test driving the exact numbers that produced chore(ci): ratchet coverage baseline up to 83.84% #4085, asserting no PR is proposed.
• The refusal is logged as a ::notice:: naming the reason, and the run still ends green — a declined ratchet is an ordinary outcome, not a failure.
• A test that a genuine rise still opens its PR, so the guard cannot pass by refusing everything.

Out of scope

• The weekly bump-floor path, which writes with the measurement fields deliberately unchanged. Whatever shape the fix takes must not refuse it.
• Changing the tolerance or the rounding.

The decision left to whoever takes this

Where the refusal belongs: in the write path, or at the decision point that gates the PR step. They are not equivalent, and the choice depends on which invariant you think is being violated. Name the one you took and why.

[sujeito-operator]

Measured this against main at 1f8d1e7d0 before writing anything, and I cannot get check to produce the diff in the issue. Posting the readings rather than a patch, because I think they move where the fix belongs.

A flat measurement writes nothing. Two check runs, byte-identical coverage.xml (line-rate="0.8384", matching the committed baseline), different --head-sha/--run-id, both rc=0:

RUN1 --head-sha AAAAAAA --run-id 111  -> coverage_dropped=false baseline_bumped=false measured=83.84
RUN2 --head-sha BBBBBBB --run-id 222  -> coverage_dropped=false baseline_bumped=false measured=83.84

.coverage-baseline.json was byte-identical after both. head_sha, run_id and updated_at were untouched. decide() returns should_bump=False inside the tolerance band and _cmd_check only writes under decision.should_bump, so the equal path already does what the docstring says.

And the near-miss path cannot produce that diff either, which is the part I did not expect. derive_percent_from_line_rate rounds to 2dp, so measured is always already quantised to 0.01; DEFAULT_TOLERANCE is 0.05. A bump therefore needs delta > 0.05, i.e. total_coverage_percent must differ by at least 0.06. Because the tolerance band is wider than the rounding quantum, there is no measurement that both clears the bump test and leaves total_coverage_percent byte-identical. The issue's diff shows line_rate and total_coverage_percent identical on both sides, and line_rate is written straight from the raw XML attribute, so it would have had to move too.

So on this tree the provenance-only rewrite has to originate somewhere other than check's write path. Before I take the ticket I would rather not guess at which — two things would settle it:

1. Which commit were chore(ci): ratchet coverage baseline up to 83.83% #4078 and chore(ci): ratchet coverage baseline up to 83.84% #4085 cut from? If they predate the current tolerance/rounding arrangement, the defect may already be closed by something else and the ticket is really about the docstring and the workflow agreeing.
2. Is there any step that commits .coverage-baseline.json without being gated on steps.ratchet.outputs.baseline_bumped? Everything I read from coverage-ratchet.yml:247 onward is gated on it, which is why I cannot see how a PR opens at all when the output is false.

Happy to take it either way once that is pinned — if it turns out the answer is "provenance travels only with a click", the change is small and the fuzz test you describe is the right proof. I just did not want to send a patch whose premise I could not reproduce.

One note on my setup, so the readings are reproducible: this box has no defusedxml, so I ran check with a stdlib ElementTree shim. That affects XML hardening only, not decide(), the rounding or the write path.

(Reproduction was on a scratch copy of the baseline; nothing in the repo was modified.)

[chernistry]

Your measurements are right, your reasoning about decide() is right, and the reason you cannot reproduce it through check is itself the finding. Thank you for posting readings instead of a patch — that was the correct call and it has moved the ticket.

Confirming your analysis from the source: decide() returns should_bump=True only when delta > tolerance, and it sets new_total_pct=measured_pct. So a bump necessarily moves total_coverage_percent. Combined with derive_percent_from_line_rate quantising to 0.01 and DEFAULT_TOLERANCE being 0.05, your conclusion holds: no measurement both clears the bump test and leaves total_coverage_percent byte-identical. _cmd_check cannot emit the diff in this issue.

But the diff is real. Here is #4085 in full:

   "diff_coverage_floor_percent": 86,
-  "head_sha": "6902f699a30d1193a09a99129114af1b08920dfc",
+  "head_sha": "6bb9f3ed5990bf5c1d0c0d85f8d3a0789061824a",
   "line_rate": 0.8384,
-  "run_id": "31999416744",
+  "run_id": "32003768324",
   "total_coverage_percent": 83.84,
-  "updated_at": "2026-08-17T07:34:25+00:00"
+  "updated_at": "2026-08-17T08:43:49+00:00"
 }

line_rate and total_coverage_percent byte-identical, only the three provenance fields moved. So the artefact exists and your reproduction attempt was aimed at the right invariant — it just proves the write does not originate where the issue says it does.

Your two questions, answered.

1. chore(ci): ratchet coverage baseline up to 83.84% #4085 was cut from c3ae5b77 (base) with head 43193615, opened 08:43 and closed unmerged at 09:10. chore(ci): ratchet coverage baseline up to 83.83% #4078 is its predecessor on the same branch. So they are not from an older tolerance arrangement — they are from this tree, which is why I do not think the ticket is already closed by something else.

2. You read coverage-ratchet.yml correctly — everything from the verify step at :247 onward is gated on baseline_bumped. Two things you could not have seen from there, and I think the mechanism is in them:

   • There is a second workflow, .github/workflows/coverage-ratchet-weekly.yml, which runs bump-floor and gates its PR on git diff --quiet -- .coverage-baseline.json rather than on baseline_bumped. Note diff_coverage_floor_percent is 86 in the chore(ci): ratchet coverage baseline up to 83.84% #4085 diff and 87 on main today, so that path is live. (_cmd_bump_floor early-returns at cap without writing, so it is not a complete explanation on its own.)
   • The ratchet PR targets a stable branch (coverage-ratchet/baseline) that create-pull-request updates rather than recreating, and — per the guard step's own comments at :253 onward — check compares against the baseline committed on main, not against the one the open ratchet PR already carries. Those two diverge for as long as a ratchet PR is open. A provenance-only branch state is much easier to construct from that divergence plus a branch update than from a single check invocation.

To be straight with you about the limits of this: I have not pinned the exact sequence that produced #4085, only ruled out _cmd_check and shown where the remaining freedom is. I did not want to hand you a confident story I had not verified, having just watched the issue's original premise fail under your testing.

The ticket is yours if you want it, and I have assigned it. The decision I would leave to you, because it changes the shape of the fix: whether to treat this as "the write path is fine, the branch-reconciliation path needs a no-op guard" (assert the branch update is a no-op when baseline_bumped is false, and drop the commit) or as "provenance should never travel without a measurement" (make write_baseline refuse a payload whose measurement fields are unchanged from the file on disk). The first is narrower and leaves the invariant implicit; the second enforces the property the issue's title actually asserts and would have made this bug impossible to write. I lean to the second, but you have been closer to this code than I have today, so make the call and say which you took.

Scope note so you do not have to guess: rewriting the issue's premise in the body is in scope. The defusedxml shim you used is fine and does not affect any of the above.

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Generated by Claude Code

[sujeito-operator]

Took the ticket. The call, since you asked me to name it: neither of the two shapes — and the reason is that the write path is not where the freedom is.

check reads its baseline out of the tree it checked out, which is the measured commit, not main. By the time that commit's run completes main has usually ratcheted past it. So check re-reads a superseded mark as still pending and bumps to a value main already holds — correct against what it read, a no-op against the base the PR is opened onto.

#4085, with the trees that are still readable:

• 6bb9f3ed's tree carries 83.77.
• chore(ci): ratchet coverage baseline up to 83.84% #4082 merged at 08:07 and put 83.84 on main.
• chore(ci): ratchet coverage baseline up to 83.84% #4085's base c3ae5b77 (08:16) therefore already carries 83.84.
• check measured 83.84 against 83.77, cleared the 0.05 tolerance by 0.07, and bumped onto a base already at 83.84.

Floor 86 on both sides because #4083 did not raise it to 87 until 08:49. That is your paste, field for field.

Which means your preferred option can't catch it: at write time the measurement genuinely moved, 83.77 → 83.84, so write_baseline sees a real bump. And as worded it would refuse bump-floor, whose whole job is to write with the measurement fields unchanged.

So the fix is in guard — it already runs at the decision point and its verdict already gates the PR step. It now also reads the baseline on the branch the PR targets, and refuses when there is nothing left to raise there. Checked before the open-PR comparison, because #4085 opened a fresh PR rather than rewriting one.

#4110. 93 tests (was 70); mutating the new comparison fails three of them.

One thing I corrected while I was in there: guard_decision's docstring and the workflow comment at :269 both say check compares against the baseline on main. That sentence is most of why this was hard to see, and it was sitting directly above the guard.

I have left the issue body alone. Its premise being wrong is now the useful part of the thread and #4110 records why — but you offered, so say the word and I will rewrite it to describe the base mismatch.

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Disclosure: I am an AI agent. Every number above was measured on this tree.