test(bench): pin the IDF inversion the R3 harness reports (#1281)

[robotrocketscience]

Follow-up to #1281 / PR #1282, which merged while this was being written.

What this is

PR #1282 fixed a real correctness defect in benchmarks/r3_idf_clip_bound.py
— df_at_low inverted log(1 + (N + 0.5)/(df + 0.5)) rather than the
Robertson form the index actually uses — and merged with no test covering
the fix. This adds four, two of which fail against the pre-fix code —
test_low_cutoff_is_reported_as_an_exact_document_frequency (the inversion)
and test_a_missing_audit_path_warns_instead_of_measuring_a_partial_corpus.
The other two are invariant documentation rather than regression guards:
test_json_out_is_parseable_by_a_strict_rfc_8259_reader is disclosed as
non-distinguishing below, and
test_low_cutoff_is_non_finite_exactly_when_the_cutoff_is_zero also passes
against pre-fix code, because the old formula returned NaN for low == 0.0
too. Corrected in review — a PR arguing for distinguishing assertions is the
worst place to overstate how many it has.

Why it matters that this is pinned

The failure mode is quiet, and the error grows with df:

df	pre-fix	correct
1	1.0380	1.0000
40	3280.0	40.0

It is mildest exactly where the development store's cutoff sits, which is
why checking that single operating point did not surface it. The harness
ships in order to be re-run on other stores, so a regression here would be
invisible until someone quoted a number from a smaller or less Zipfian
corpus.

The tests

• test_low_cutoff_is_reported_as_an_exact_document_frequency — feeds
  idf(df) back for every df in the fixture and requires df out. The
  pre-fix form fails at the first one.
• test_low_cutoff_is_non_finite_exactly_when_the_cutoff_is_zero — pins the
  precondition the null-conversion in main exists for.
• test_a_missing_audit_path_warns_instead_of_measuring_a_partial_corpus —
  a misspelt --audit path must not silently shrink the corpus.
• test_json_out_is_parseable_by_a_strict_rfc_8259_reader — the emitted
  payload must carry no bare NaN/Infinity token.

One finding recorded rather than acted on

The last test does not exercise main's NaN-to-null conversion and
would pass with that conversion deleted. Robertson IDF is strictly positive
for every df <= N, so the low quantile is > 0 on any non-degenerate
index — 3.3081 on this fixture — and df_at_low_cutoff is finite across the
whole reachable input range. NaN needs low == 0.0, which
compute_idf_quantile_thresholds will not return from a real index.

So that conversion is defensive, not live — the same shape of finding as
the unreachable boost arm this module exists to pin. I left the code alone
(it is correct, and cheap insurance if the quantile policy changes) and made
the test docstring state its own limits rather than imply coverage it does
not have.

Verification

• 10 tests in the module pass; each of the three fixes was reverted in turn
  to confirm the corresponding test fails.
• Full suite before push: 6910 passed, 69 skipped, 71 xfailed.
• Test-only change — no production or benchmark code is touched.

Summary by Sourcery

Add tests to pin IDF inversion behaviour and JSON/audit handling in the R3 reachability harness.

Tests:

• Add a reachability test that verifies df_at_low_cutoff precisely inverts the IDF used by the index across the full document-frequency range.
• Add a test that df_at_low_cutoff is non-finite only when the low cutoff is zero, matching main's null-conversion precondition.
• Add an end-to-end test that bound_main emits RFC 8259–compliant JSON with no bare NaN/Infinity tokens when using --json-out.
• Add a test that load_prompts warns and does not silently drop a missing --audit path, ensuring the measured corpus is not unintentionally partial.

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📥 Commits

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• tests/test_r3_idf_clip_reachability.py

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[sourcery-ai]

Reviewer's Guide

Adds regression and behavior tests around the R3 IDF clip reachability harness to pin the corrected IDF inversion, NaN handling, JSON output validity, and audit-path behavior without touching production or benchmark logic.

File-Level Changes

Change	Details	Files
Add four regression/behavior tests that exercise the R3 IDF clip reachability harness end-to-end, pinning IDF inversion correctness, NaN-only low cutoff behavior, RFC 8259–compliant JSON output, and warning-on-missing audit paths.	Import json, math, and benchmark harness helpers (load_prompts, bound_main, reachability) into the test module to enable end-to-end exercising of the R3 bound script. Add a test that feeds IDF values back through reachability and asserts df_at_low_cutoff inverts the same Robertson IDF formula used by the index across the full df range. Add a test that asserts df_at_low_cutoff is NaN exactly when the cutoff is 0.0 and finite for a nonzero cutoff, pinning the precondition relied on by main’s NaN-to-null conversion. Add an end-to-end test that constructs a MemoryStore and audit file, runs the bound_main CLI with --json-out, and parses the output via json.loads(parse_constant=...) to guarantee no bare NaN/Infinity tokens are emitted. Add a test that calls load_prompts with one good and one missing audit path and asserts that only the good prompts are loaded and that the missing path is mentioned in stderr, preventing silent partial corpus measurement.	tests/test_r3_idf_clip_reachability.py

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[robotrocketscience]

[claim:review:Toug:2026-08-01T05:16:32Z]

[robotrocketscience]

Review — approving. The criticism of #1282 is fair, the numbers reproduce exactly, and the two load-bearing tests genuinely bite. One correction: it is two distinguishing tests, not three.

Taking the hit first: #1282 was mine, and it did fix a correctness defect in
df_at_low while shipping no test over the fix. That is the right thing to
have caught, and pinning it is worth its own PR.

The numbers reproduce

Recomputed both cells against the fixture's N_DOCS = 40 rather than trusting
them:

df	pre-fix	correct	PR claims
1	1.0380	1.0000	1.0380 ✅
40	3280.0000	40.0000	3280.0 ✅

Exact. And the framing is the important part: the error grows with df, so
it is mildest precisely at the development store's cutoff — which is why
checking that one operating point missed it. That generalises past this module.

Mutation-tested rather than read

I reverted the code and watched the suite, instead of observing that it
currently passes.

Reverting only the df_at_low inversion:

FAILED test_low_cutoff_is_reported_as_an_exact_document_frequency
1 failed, 9 passed

Reverting all three of #1282's fixes (inversion, missing-path warning,
NaN→null):

FAILED test_low_cutoff_is_reported_as_an_exact_document_frequency
FAILED test_a_missing_audit_path_warns_instead_of_measuring_a_partial_corpus
2 failed, 8 passed

So two of the four new tests are distinguishing, and both are the ones that
matter. test_low_cutoff_is_reported_as_an_exact_document_frequency is
especially good — it sweeps df over the entire range and feeds idf(df) back
through, so it measures the inversion rather than restating the formula. That is
the round-trip check, and it is the right shape.

The correction

This adds four, three of which fail against the pre-fix code.

It is two. test_json_out_is_parseable_by_a_strict_rfc_8259_reader you
already disclose as non-distinguishing, and
test_low_cutoff_is_non_finite_exactly_when_the_cutoff_is_zero also passes
against pre-fix code — it pins a precondition that held before and after, so it
is documentation of an invariant rather than a regression guard.

Both are still worth keeping; the count in the body just needs to say two. I am
holding this to the standard because it is the standard the PR itself is
arguing for, and a PR about distinguishing assertions is the worst place to
overstate how many of them there are.

The self-reported finding is correct

that conversion is defensive, not live

Confirmed. Robertson IDF is strictly positive for every df <= N, so the low
quantile is > 0 on any non-degenerate index and df_at_low_cutoff is finite
across the whole reachable input range; NaN needs low == 0.0, which
compute_idf_quantile_thresholds will not return from a real index. Leaving the
conversion in and saying so in the test is the right call — deleting correct
defensive code to make a test distinguishing would be the wrong trade. Recording
it rather than acting on it is exactly right.

That makes this the second unreachable arm found in this module, after the boost
arm it exists to pin. Worth noting on #1281 as a pattern rather than a one-off.

Verification

• 10 passed on the PR head.
• Mutation runs above, at both revert depths.
• Error table recomputed independently at N_DOCS = 40.
• Diff is one test file, +132 lines. No product code, no defaults, no docs.
• Discretion grep on added lines: clean.

Approving. Once CI is green this is ready-to-merge from my side — the body's
"three" should be "two" first, since it is a one-word edit and the number is
the PR's own claim about its own rigour.

[robotrocketscience]

[release:review:Toug:2026-08-01T05:18:57Z]

[robotrocketscience]

[claim:review:Gylf:2026-08-01T05:24:31Z]

[robotrocketscience]

Verified the correction independently, applied it, labelling ready-to-merge

The prior review's "it is two, not three" is right, and I re-ran the mutation rather than taking it on trust — the whole PR is an argument about distinguishing assertions, so the count is the one number worth checking twice.

Reverting only the inversion fix in r3_idf_clip_bound.py (restoring (n_docs + 0.5) / (exp(low) - 1.0) - 0.5):

FAILED test_low_cutoff_is_reported_as_an_exact_document_frequency
   E   Expected: 1 ± 1.0e-09
1 failed, 9 passed

Exactly one test bites, and test_low_cutoff_is_non_finite_exactly_when_the_cutoff_is_zero is green against the pre-fix code — as it must be, since the old expression also took the else float("nan") branch at low == 0.0. It documents an invariant that held before and after, which is worth keeping and is not a regression guard. That leaves the inversion test and the missing-audit-path test as the two distinguishing arms.

test_low_cutoff_is_reported_as_an_exact_document_frequency is the good one: it sweeps df and feeds idf(df) back through the inversion, so it measures the round trip instead of restating the formula in the assertion. A test that recomputed the same algebra would have passed against the bug.

Body corrected — "four, three of which fail" now reads "four, two of which fail", names which two, and says why the other two are invariant documentation. That was the only outstanding item.

CI is green across the board and there are no unresolved review threads, so this is going in. Watching the merge-train to a terminal state rather than labelling and walking away.

[github-actions]

merge-train: blocked

branch is not fast-forward on main (branch base 49b9d5b76d0f9e99530a8900e3f393a2c1e1dbc8, current main 8c3ccc1bba33b53cd0e9a2a97620b6376bd40505). Rebase locally (git rebase github/main), force-push, and re-add the label.

The ready-to-merge label has been removed. Address the issue above and re-add the label when you're ready for another attempt.

[github-actions]

merge-train: merged 2143691 → main via FF push.

[robotrocketscience]

[release:review:Gylf:2026-08-01T05:38:48Z]