msd-core/gsd-core/references/edge-probe.md

Edge-Probe — Spec-Completeness Reference

Shared reference for the spec/requirements phase. Companion to @~/.claude/gsd-core/references/domain-probes.md: domain-probes covers the technology axis (auth, search, caching, deployment); this covers the data/behavior-shape axis (boundaries, adjacency, encoding, ordering…). Walk each requirement against the closed taxonomy below, propose a concrete candidate edge for each applicable category, and resolve each to exactly one state. Adopt the established QA names verbatim — this is decades-old black-box test technique, moved upstream to the spec layer.

This doc is written in generic requirements → checks → verifier terms with no tool-specific vocabulary, so it is portable: copy it into any spec/requirements process. A short mapping table at the end binds it to common host structures.

Why front-of-pipeline

A goal-backward verifier only checks assertions that exist; an assertion only exists for a requirement that was written down. A domain-boundary edge the author never surfaced is invisible to the verifier — and worse, the verifier is confidently wrong about it (measured: ~0.93 confidence while catching 0/12 omitted-edge defects, an expected calibration error of 0.81 — worse than a coin flip — versus 100% / 0.03 on edges the spec did state). The fix is not a better verifier or a confidence gate; confidence is uninformative across these regimes. The fix is spec completeness: surface the omitted edge into an explicit, checkable assertion before any code exists, after which the verifier reliably catches it.

The underlying techniques are classic and should be named as such — Boundary Value Analysis, Equivalence Partitioning, the Category-Partition method, and Metamorphic Relations; property-based testing (PBT) is the academic name for the held-out backstop. The literature applies these at the test layer (back of pipeline, generating checks). The differentiated move here is placement, not technique: apply the same edge taxonomy at the spec layer (front of pipeline, generating requirements), with the explicit goal of extending a verifier's reach. Recent LLM spec work corroborates the core finding that the spec layer is the measured weak point:

• SLD-Spec — program slicing + logical deletion (code→spec): arXiv 2509.09917
• Specine / AutoReSpec / SpecMind — spec alignment & postcondition inference
• CodeSpecBench (2604.12268), OSVBench (2504.20964), VERINA (2505.23135) — benchmarks showing models solve tasks far better than they generate precise behavioral specs
• LLM property-based tests for edge cases: arXiv 2510.25297 (PBT+EBT ≈ 81% edge detection)

Inputs

A list of requirements, each a { id, text, shapes? } record where text is a testable statement and shapes is an optional author-supplied override of the data/behavior shape. The five shapes are: numeric-range, collection, text, stateful, io. When shapes is absent, a heuristic classifier proposes them from the requirement prose (propose-then-confirm) — the author may correct the shape.

Taxonomy (8 categories)

Closed and small by design: a fixed eight the author must explicitly clear beats thirty nobody finishes. The failure mode being eliminated was never "too few categories" — it was that no taxonomy was systematically applied at all. Categories 1, 2, and 4 alone cover the three canonical corpus defects (banker's-rounding ties, touching intervals, grapheme truncation). Growth happens via optional domain packs, not by bloating the core.

id	name (QA term)	applies to shapes	probe question
boundary	Boundary values	numeric-range	What happens exactly at each min/max/threshold — and one step either side?
adjacency	Adjacency / touching	collection	When two things are exactly equal or just touch, do they merge, collide, or separate?
empty	Empty / degenerate	collection, text	What is the result for empty, single-element, or null input?
encoding	Encoding / representation	text	Whose definition of length/equality applies — bytes, code points, grapheme clusters, or normalized form?
ordering	Ordering / stability	collection	When elements compare equal, is output order specified and stable?
precision	Precision / overflow	numeric-range	Where can precision loss, overflow, or rounding/tie-breaking occur — and what is the exact contract (e.g. half-up vs half-to-even, ceil/floor/truncate)?
idempotency	Idempotency / repetition	stateful	What happens if this runs twice on the same input?
concurrency	Concurrency / effect ordering	stateful, io	If interrupted or run in parallel, what is guaranteed?

Relevance filter + resolution states

Two rules keep the probe honest and prevent an "everything is N/A" failure mode:

1. Relevance filter first. Classify each requirement's shape, then raise only the categories whose applies to shapes intersect that requirement's shapes. A pure-text requirement is never asked about overflow. This is what makes an unresolved edge meaningful: it is an edge that applies and was not addressed.
2. Dismissal requires a reason string. "N/A — input is a bounded enum, no boundary exists" is valid; silence is not. The reason string is the audit trail.

Zero-classification surfaces an unclassified candidate (#1110). The relevance filter is a heuristic over prose cues, so a requirement whose wording is edge-relevant but matches no shape cue would otherwise classify to zero shapes → zero edges and vanish from coverage with no signal — the same silent blind spot the probe exists to catch. Instead, a requirement with non-empty prose, no authored shapes, and zero matched shapes surfaces exactly one soft unclassified — review manually candidate (category: "unclassified", status: "unresolved"). It is a dismissible nudge — resolve it, or dismiss it with a reason (e.g. a genuinely edge-free static-asset requirement) — never a hard block. unclassified is a review signal, not a ninth taxonomy category: the closed eight above are unchanged, and an explicit shapes: [] opt-out stays silent (the author's deliberate "no edge surface").

Each raised edge carries two orthogonal axes — a resolution lifecycle and, when resolved, a verification tier (ADR-550 Decision 7, the shared probe-core model):

• status — resolved | dismissed | unresolved:
  • resolved — the edge is addressed; how it is addressed is the verification tier.
  • dismissed — not applicable, accompanied by a required, non-empty reason string.
  • unresolved — carried forward and flagged; the author chose not to resolve it yet.
• verification (only when status is resolved; null otherwise) — explicit | backstop:
  • explicit — a checkable assertion for the edge is written (a SPEC acceptance criterion).
  • backstop — a held-out / property-based test stands in for an edge the author knows but cannot fully articulate in prose (records intent; the test body is authored later).

Splitting these axes keeps the lifecycle enum free of a verification fact (the old single enum smuggled covered/backstop — both resolved — into one flat list) and lets sibling probes add their own verification tiers (e.g. test | judgment) without a parallel enum.

coverage.resolved is the count of closed edges — resolved + dismissed (the pre-re-cut "covered + dismissed + backstop" set, count-preserved). An unresolved applicable edge is the precise signal a soft completeness gate raises.

Output schema

The probe emits, per edge, an item of the form:

{ requirement_id, category, status, verification, resolution, reason, probe }

plus a coverage summary:

coverage: { applicable, resolved, unresolved, byVerification: { explicit, backstop } }

applicable is the number of raised edges, resolved = closed (resolved + dismissed) status edges, unresolved is the remainder, and byVerification breaks the resolved-status edges down by tier (probe-agnostic in core; the edge adapter declares { explicit, backstop }). This JSON is the stable contract both the reference implementation and any third-party port emit.

Generic mapping (requirements → checks → verifier)

Host structure	"requirement"	a resolved/explicit edge becomes	a resolved/backstop edge becomes
GSD SPEC	a SPEC Requirement	an Acceptance Criterion that plan-phase lifts into must_haves.truths	a non-inferable check in must_haves.truths (needs a held-out/PBT test)
Gherkin feature	a Scenario	an additional Then assertion / Scenario Outline row	a tagged scenario routed to a property test
OpenAPI operation	an operation	a response/constraint example + schema rule	a contract/property test on the operation
Docstring contract	a documented behavior	an assertion in the contract test	a property-based test for the function

The portable invariant: a resolved/explicit edge produces the unit your verifier iterates over (GSD: a must_haves.truth); a resolved/backstop edge produces a test added to that same set as a non-inferable check. An unresolved edge is an explicit assumption the downstream planner must surface, not silently drop.

Worked example (merge-intervals)

Given a single requirement with no resolutions yet:

[{ "id": "R1", "text": "Merge a list of overlapping intervals into the minimal set" }]

the requirement classifies as a collection, which raises adjacency, empty, and ordering (but not boundary/precision/encoding/idempotency/concurrency). With no resolutions supplied, every applicable edge is unresolved:

{
  "items": [
    { "requirement_id": "R1", "category": "adjacency", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "When two things are exactly equal or just touch, do they merge, collide, or separate?" },
    { "requirement_id": "R1", "category": "empty", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What is the result for empty, single-element, or null input?" },
    { "requirement_id": "R1", "category": "ordering", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "When elements compare equal, is output order specified and stable?" }
  ],
  "coverage": { "applicable": 3, "resolved": 0, "unresolved": 3, "byVerification": { "explicit": 0, "backstop": 0 } }
}

The adjacency row is the one that catches the canonical defect: [[1,2],[2,3]] intervals that only touch — does the spec say they merge? Resolving it resolved/explicit writes that assertion, and the verifier can then enforce it. This worked-example block is kept byte-for-byte (parsed-JSON) identical to its fixture by edge-probe-docs-fixtures.test.cjs, so the doc and the reference implementation cannot silently drift.

Worked example (round-half-even)

Given a requirement to round floating-point values using the banker's-rounding (half-even) rule, the requirement classifies as numeric-range, which raises boundary and precision (but not adjacency/empty/ordering/encoding/idempotency/concurrency):

{
  "items": [
    { "requirement_id": "R1", "category": "boundary", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What happens exactly at each min/max/threshold — and one step either side?" },
    { "requirement_id": "R1", "category": "precision", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "Where can precision loss, overflow, or rounding/tie-breaking occur — and what is the exact contract (e.g. half-up vs half-to-even, ceil/floor/truncate)?" }
  ],
  "coverage": { "applicable": 2, "resolved": 0, "unresolved": 2, "byVerification": { "explicit": 0, "backstop": 0 } }
}

The precision row surfaces the canonical defect for this requirement: IEEE 754 floating-point arithmetic rounds 2.5 to 2 (not 3) under half-even — the probe asks whether the spec states that contract explicitly, so the verifier can enforce it.

Worked example (truncate-graphemes)

Given a requirement to truncate a string to N grapheme clusters (not bytes or code points), the requirement classifies as text, which raises empty and encoding:

{
  "items": [
    { "requirement_id": "R1", "category": "empty", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What is the result for empty, single-element, or null input?" },
    { "requirement_id": "R1", "category": "encoding", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "Whose definition of length/equality applies — bytes, code points, grapheme clusters, or normalized form?" }
  ],
  "coverage": { "applicable": 2, "resolved": 0, "unresolved": 2, "byVerification": { "explicit": 0, "backstop": 0 } }
}

The encoding row is the load-bearing one: a requirement that says "truncate to 10 characters" is ambiguous — the spec must state whether "character" means bytes, UTF-16 code units, Unicode code points, or grapheme clusters (emoji sequences are 1 grapheme, multiple code points).

Worked example (money-rounding)

Given a requirement to round monetary amounts to two decimal places, the requirement classifies as numeric-range, which raises boundary and precision:

{
  "items": [
    { "requirement_id": "R1", "category": "boundary", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What happens exactly at each min/max/threshold — and one step either side?" },
    { "requirement_id": "R1", "category": "precision", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "Where can precision loss, overflow, or rounding/tie-breaking occur — and what is the exact contract (e.g. half-up vs half-to-even, ceil/floor/truncate)?" }
  ],
  "coverage": { "applicable": 2, "resolved": 0, "unresolved": 2, "byVerification": { "explicit": 0, "backstop": 0 } }
}

The boundary row asks what the minimum and maximum representable values are (negative amounts? fractional cents?). The precision row asks whether IEEE 754 binary rounding can produce $0.30000000000000004 — the spec must commit to a representation.

Worked example (list-dedupe)

Given a requirement to deduplicate a list of items, the requirement classifies as collection, which raises adjacency, empty, and ordering:

{
  "items": [
    { "requirement_id": "R1", "category": "adjacency", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "When two things are exactly equal or just touch, do they merge, collide, or separate?" },
    { "requirement_id": "R1", "category": "empty", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What is the result for empty, single-element, or null input?" },
    { "requirement_id": "R1", "category": "ordering", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "When elements compare equal, is output order specified and stable?" }
  ],
  "coverage": { "applicable": 3, "resolved": 0, "unresolved": 3, "byVerification": { "explicit": 0, "backstop": 0 } }
}

The ordering row is the non-obvious one: dedupe removes duplicates, but which copy is kept — first occurrence, last, or implementation-defined? The spec must commit.

Worked example (resolved-mixed)

The same merge-intervals requirement after a resolution session where adjacency was resolved with an explicit acceptance criterion, ordering was dismissed, and empty was left unresolved:

{
  "items": [
    { "requirement_id": "R1", "category": "adjacency", "status": "resolved", "verification": "explicit", "resolution": "AC#6: touching intervals merge", "reason": null, "probe": "When two things are exactly equal or just touch, do they merge, collide, or separate?" },
    { "requirement_id": "R1", "category": "empty", "status": "unresolved", "verification": null, "resolution": null, "reason": null, "probe": "What is the result for empty, single-element, or null input?" },
    { "requirement_id": "R1", "category": "ordering", "status": "dismissed", "verification": null, "resolution": null, "reason": "output is canonically sorted; no tie possible", "probe": "When elements compare equal, is output order specified and stable?" }
  ],
  "coverage": { "applicable": 3, "resolved": 2, "unresolved": 1, "byVerification": { "explicit": 1, "backstop": 0 } }
}

coverage.resolved is 2 — the closed set (adjacency=resolved/explicit + ordering=dismissed); unresolved is 1 (empty); byVerification.explicit is 1 (the single explicitly-verified edge), backstop 0. The soft gate raises on this example because one applicable edge remains unresolved — the author must either specify, dismiss, or backstop it before writing the SPEC.