Fuzz run description	Iterations	Pass	Result
Baseline-vs-head parity, JSON/YAML equivalence, and deterministic output (frontmatter target, corrected baseline fa675cf)	97	96/97	FAIL

Case	Template / input	Outcome
FUZZ-006	`Equivalent JSON and YAML inputs must not produce different semantics without a documented reason. Either both reject an out-of-range integer or both preserve it losslessly.`	confirmed product bug (5 reproductions): JSON: exit 0, renders VALUE=18446744073709552000.0 -- the integer u64::MAX+1 is silently rounded through f64 into a different value, and `validate --json` reports {"valid": true} with zero diagnostics. YAML with the same
FUZZ-007	ERR_VAL_DUPLICATE is a documented stable code for duplicate frontmatter variable declarations; once `variables:` is a real declaration source, both spellings feeding one `seen` set	intentional boundary (not a defect) (3 reproductions): HEAD exits 2 with ERR_VAL_DUPLICATE; pre-fix baseline ignored `variables:` so no duplicate was detected. A faithful consequence of intentional fix (c). Accounts for 12 of the 21 baseline-vs-HEAD sweep diffs.

Minimal template / frontmatter: --- required_variables: - top --- VALUE=[[top]]
Input: {"top": 18446744073709551616}
Expected: Equivalent JSON and YAML inputs must not produce different semantics without a documented reason. Either both reject an out-of-range integer or both preserve it losslessly.
Observed: JSON: exit 0, renders VALUE=18446744073709552000.0 -- the integer u64::MAX+1 is silently rounded through f64 into a different value, and `validate --json` reports {"valid": true} with zero diagnostics. YAML with the same value: exit 3, ERR_CONFIG_PARSE. Control at exactly u64::MAX renders losslessly in both formats.
Requirement / ADR: Silent data corruption with an affirmative valid:true is the worst failure mode of the two paths.
Requirement / ADR follow-up: Reject or losslessly preserve JSON integers beyond u64::MAX; never silently round to f64 while reporting valid:true.
Root cause: See observed_result; coordinator-established root cause recorded in campaign-report.json.
Recommended fix: Reject or losslessly preserve JSON integers beyond u64::MAX; never silently round to f64 while reporting valid:true.

Minimal template / frontmatter: --- required_variables: - name variables: name: required: true --- Hello {{name}}
Input: (none)
Expected: ERR_VAL_DUPLICATE is a documented stable code for duplicate frontmatter variable declarations; once `variables:` is a real declaration source, both spellings feeding one `seen` set is consistent.
Observed: HEAD exits 2 with ERR_VAL_DUPLICATE; pre-fix baseline ignored `variables:` so no duplicate was detected. A faithful consequence of intentional fix (c). Accounts for 12 of the 21 baseline-vs-HEAD sweep diffs.
Requirement / ADR: Document the cross-section duplicate rule in requirements.md; currently only implied by code.
Requirement / ADR follow-up: Coordinator classified as non-actionable (intentional_boundary/inconclusive); no new requirement/ADR needed. Document the cross-section duplicate rule in requirements.md; currently only implied by code.
Root cause: See observed_result; coordinator-established root cause recorded in campaign-report.json.
Recommended fix: No further action recommended; see coordinator_note/attribution_note.

differential-probe