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+# 0018: Pipeline Utilities — State Migration Chain Ambiguity Category and Fixture
+
+- **Status:** Draft
+- **Author:** Chris Colinsky
+- **Created:** 2026-05-15
+- **Accepted:**
+- **Targets:** spec/pipeline-utilities/spec.md (modifies §10.10 to add error category; modifies §10.12.1 and §10.12.2 to reference the category by name); spec/pipeline-utilities/conformance/ (adds fixture 047)
+- **Related:** 0014 (state-migration hooks)
+- **Supersedes:**
+
+## Summary
+
+Name the canonical error category for state-migration chain
+ambiguity (currently mandated by §10.12.1 and §10.12.2 as "a
+configuration-time error" without a specified identifier), and add
+fixture 047 covering both the registration-time (duplicate
+`(from_version, to_version)` pair) and resolution-time (multiple
+distinct shortest paths) ambiguity cases.
+
+## Motivation
+
+Proposal 0014 introduced state migrations and §10.12 mandates
+configuration-time errors for two distinct ambiguity cases:
+
+- **§10.12.1** — "Two migrations with the same `from_version` and
+  same `to_version` MUST raise a configuration-time error (the chain
+  is ambiguous)."
+- **§10.12.2** — "When multiple distinct shortest paths exist (same
+  edge count, different edge sequences), this is an ambiguous chain
+  and the engine MUST raise a configuration-time error — the same
+  category §10.12.1 raises for duplicate `(from_version, to_version)`
+  pairs."
+
+Both rules reference "the same category" but neither names it. Spec
+§10.10's runtime-category list does not include an ambiguity
+category. Implementations have nothing concrete to surface; harnesses
+have nothing concrete to assert.
+
+Two consequences:
+
+- **Cross-implementation drift.** A Python implementation might
+  surface this as `ValueError`; a TypeScript implementation might
+  surface it as a generic `Error` or a custom class. Each is
+  spec-conformant under "configuration-time error" but they cannot
+  share a conformance fixture.
+- **Fixture coverage gap.** Neither the §10.12.1 duplicate-pair rule
+  nor the §10.12.2 multi-shortest-path rule is exercised by an
+  existing fixture in `spec/pipeline-utilities/conformance/`.
+  Fixtures 039–046 cover every other §10.12 rule but skip both
+  ambiguity paths.
+
+This proposal resolves both gaps in a single small change: name the
+category, then add one fixture (with two cases) that exercises both
+ambiguity rules against the named category.
+
+## Detailed design
+
+### Pipeline-utilities §10.10: add the canonical category
+
+Insert the following entry into §10.10's category list (alphabetic
+order places it adjacent to the two existing migration-related
+categories):
+
+> New canonical configuration-time category:
+> `checkpoint_state_migration_chain_ambiguous` — raised when the
+> registered migration set contains an ambiguity that prevents the
+> engine from picking a unique chain. Two ambiguity cases trigger
+> this category:
+>
+> - **At registration (per §10.12.1).** Two migrations registered
+>   with the same `from_version` AND the same `to_version`. The
+>   engine MUST raise this category at registration time (or at
+>   compile time when migrations are bound to the compiled graph,
+>   per the host language's binding semantics) so the configuration
+>   error surfaces before any resume attempt.
+> - **At chain resolution (per §10.12.2).** A request to resolve a
+>   chain from `from_version` A to `to_version` B finds two or more
+>   distinct shortest paths (same edge count, different edge
+>   sequences). Implementations SHOULD detect this at compile time
+>   when feasible by scanning the registered migration graph;
+>   load-time detection is acceptable when compile-time analysis is
+>   not.
+>
+> Non-transient. The error MUST identify the offending
+> `(from_version, to_version)` pair (for the registration case) or
+> the source / target version pair and a description of the
+> conflicting paths (for the resolution case) in a form appropriate
+> to the host language. The four migration-related categories —
+> `checkpoint_record_invalid`, `checkpoint_state_migration_missing`,
+> `checkpoint_state_migration_failed`, and
+> `checkpoint_state_migration_chain_ambiguous` — are mutually
+> exclusive on any given resume: chain-ambiguous fires at build or
+> load time before either migration runs or post-migration
+> deserialization is attempted, so it cannot co-occur with
+> migration-failed or record-invalid.
+
+Update the §10.10 final paragraph (the migration-categories
+mutual-exclusion paragraph) to list the new category alongside the
+existing three migration-related categories.
+
+### Pipeline-utilities §10.12.1: name the category
+
+Replace this sentence in §10.12.1:
+
+> Two migrations with the same `from_version` and same `to_version`
+> MUST raise a configuration-time error (the chain is ambiguous).
+
+With:
+
+> Two migrations with the same `from_version` and same `to_version`
+> MUST raise `checkpoint_state_migration_chain_ambiguous` (per
+> §10.10) at registration or compile time, before any resume
+> attempt.
+
+### Pipeline-utilities §10.12.2: name the category
+
+Replace this clause in §10.12.2 step 2:
+
+> When multiple distinct shortest paths exist (same edge count,
+> different edge sequences), this is an ambiguous chain and the
+> engine MUST raise a configuration-time error — the same category
+> §10.12.1 raises for duplicate `(from_version, to_version)` pairs.
+
+With:
+
+> When multiple distinct shortest paths exist (same edge count,
+> different edge sequences), this is an ambiguous chain and the
+> engine MUST raise `checkpoint_state_migration_chain_ambiguous`
+> (per §10.10).
+
+The "Implementations SHOULD detect ambiguity at compile time when
+feasible" guidance immediately following remains unchanged.
+
+### New fixture: `047-state-migration-chain-ambiguous`
+
+Two cases under one fixture:
+
+- **`duplicate_pair_at_registration`** — register two migrations
+  with the same `(from_version, to_version)` pair (`(v1, v2,
+  fn_a)` and `(v1, v2, fn_b)`). The named category MUST surface
+  with the new `expected_chain_ambiguity_error` primitive (defined
+  below).
+- **`ambiguous_shortest_paths_at_resolution`** — register a diamond
+  migration graph: `v1→v2`, `v2→v4`, `v1→v3`, `v3→v4`. State at
+  v4, seeded record at v1. The named category MUST surface with
+  the new `expected_chain_ambiguity_error` primitive.
+
+### New harness primitive: `expected_chain_ambiguity_error`
+
+This proposal also introduces a new conformance harness primitive,
+`expected_chain_ambiguity_error: <category>`, that asserts the
+named category surfaces *at either build time or during resume*.
+This preserves §10.12.2's carve-out that compile-time detection is
+SHOULD-not-MUST: an implementation that detects the ambiguity at
+build/compile time satisfies the assertion via the build-step
+exception; an implementation that defers detection to load time
+satisfies the assertion via the resume-step exception. Both paths
+are spec-conformant under §10.12.2; the harness accepts either.
+
+Existing primitives `expected_compile_error` (graph-engine fixture
+007) and `expected_error` (resume blocks, e.g. fixture 045) commit
+the assertion to one timing or the other; neither is the right
+shape for an error category whose timing is implementation-defined
+per the spec.
+
+The harness primitive's semantics are part of what acceptance
+ships, alongside the category name and the fixture YAMLs.
+
+The companion fixture `.md` cross-references §10.12.1 and §10.12.2
+and documents the OR-acceptance shape so reviewers can see how the
+fixture preserves spec flexibility.
+
+## Conformance test impact
+
+- New fixture `spec/pipeline-utilities/conformance/047-state-migration-chain-ambiguous.{yaml,md}`
+  with two cases.
+- New harness primitive `expected_chain_ambiguity_error`
+  recognized by the conformance harness, accepting the named
+  category at either build or resume time.
+- Implementations that currently silently pick a path (or silently
+  use registration order) when faced with multi-shortest-path
+  ambiguity will fail the resolution case until they add detection
+  at either build or load time.
+- Implementations that currently silently overwrite on duplicate
+  `(from, to)` registration will fail the registration case until
+  they add detection.
+
+The reference implementation in `openarmature-python` is expected
+to surface ambiguity at build time as part of its 0014
+implementation; this fixture exercises that path. A future
+implementation that defers detection to load time will pass the
+same fixture via the alternate timing leg of
+`expected_chain_ambiguity_error`.
+
+## Alternatives considered
+
+- **Leave the category implementation-defined.** Each implementation
+  picks its own error class. Rejected: prevents cross-implementation
+  conformance and forces the fixture to assert against a per-language
+  shape (`ValueError` vs `Error` vs custom). The spec's "the same
+  category §10.12.1 raises" wording already implies a shared
+  identifier; this proposal makes that identifier concrete.
+- **Two separate categories** — one for registration-time ambiguity,
+  one for resolution-time. Rejected: the root cause is the same
+  (ambiguous chain), and §10.12.2 already says they share the
+  category. Splitting would require a §10.12 rewrite and add an
+  unnecessary distinction for callers.
+- **Fixture asserts via `expected_compile_error` only.** Rejected:
+  spec §10.12.2 explicitly accepts load-time detection
+  ("load-time detection is acceptable when compile-time analysis is
+  not"). A compile-time-only fixture would make a spec-conformant
+  load-time-detecting implementation fail conformance — fixture and
+  spec disagreeing. The new `expected_chain_ambiguity_error`
+  primitive accepts either timing leg, preserving §10.12.2's
+  carve-out.
+- **Tighten spec to MUST compile-time detection.** Rejected:
+  removes the §10.12.2 carve-out for implementations whose binding
+  semantics or graph-construction model doesn't support compile-time
+  scanning. The carve-out exists for a reason; conformance coverage
+  shouldn't strip it.
+- **Skip the fixture; trust implementations.** Rejected: the gap
+  is real (no current fixture exercises either §10.12.1 ambiguity
+  or §10.12.2 ambiguity); without conformance coverage, drift is
+  inevitable.
+- **Do nothing.** Rejected: the unnamed category leaves
+  implementations and harnesses without a shared identifier; the
+  fixture coverage gap is unaddressed.
+
+## Open questions
+
+None.