chore(openspec): propose DGSEM multipatch compatibility semantics

docs/exec-plans/active/20260407-formdsl-dgsem-multipatch-compatibility-semantics.md

@@ -0,0 +1,40 @@
+# Form DSL DG-SEM Multipatch Compatibility Semantics
+
+## Objective
+
+Add deterministic compatibility diagnostics for DG-SEM multipatch requests while
+keeping DG-SEM multipatch execution unsupported.
+
+## Scope
+
+- Preserve strict fail-fast behavior for multipatch on backend `dgsem`.
+- Add deterministic permissive diagnostics describing unsupported multipatch
+  semantics.
+- Cover orientation variants and per-interface penalty controls.
+- Add tests and docs updates for diagnostic stability.
+
+## Non-Goals
+
+- DG-SEM multipatch execution.
+- Changes to IGA multipatch execution semantics.
+- New DG flux/operator execution paths.
+
+## Acceptance Criteria
+
+- Strict mode still raises `unsupported_multipatch_interface`.
+- Permissive mode includes deterministic DG-SEM compatibility diagnostics.
+- Orientation and per-interface penalty controls each produce stable
+  compatibility diagnostics.
+- Regression tests and docs are updated.
+
+## Implementation Checklist
+
+- [x] Add OpenSpec proposal/design/spec/tasks artifacts.
+- [ ] Implement DG-SEM compatibility diagnostics for multipatch descriptors.
+- [ ] Add strict/permissive regression tests.
+- [ ] Update docs and progress tracking.
+
+## Risks / Open Questions
+
+- Diagnostic code set should stay minimal and deterministic to avoid future
+  churn when DG-SEM multipatch execution is introduced.

docs/formdsl-nurbs-multipatch-evaluation.md

@@ -56,6 +56,10 @@ Implication:
 
 ## Tracking
 
+- Active OpenSpec change:
+  `openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/`
+- Active execution plan:
+  `docs/exec-plans/active/20260407-formdsl-dgsem-multipatch-compatibility-semantics.md`
 - Per-interface controls archive:
   `openspec/changes/archive/2026-04-07-formdsl-iga-multipatch-interface-controls/`
 - Per-interface controls completed plan:

docs/ufl-backend-support.md

@@ -184,3 +184,7 @@ These are tracked outcomes from the Phase 3 backlog tasks:
   `openspec/changes/archive/2026-04-07-formdsl-iga-multipatch-interface-controls/`
   and
   `docs/exec-plans/completed/20260407-formdsl-iga-multipatch-interface-controls.md`.
+  DG-SEM multipatch compatibility diagnostics are tracked in active change
+  `openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/` and
+  active plan
+  `docs/exec-plans/active/20260407-formdsl-dgsem-multipatch-compatibility-semantics.md`.

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-04-07

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/design.md

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+## Context
+
+Multipatch parsing and IGA execution are implemented, including per-interface
+controls. For backend `dgsem`, multipatch remains unsupported by design, but
+diagnostics currently provide only a single generic capability error.
+
+This phase introduces deterministic DG-SEM compatibility diagnostics so callers
+can understand exactly which multipatch semantics block lowering.
+
+## Goals / Non-Goals
+
+**Goals**
+
+- Preserve strict unsupported behavior for DG-SEM multipatch requests.
+- Add deterministic compatibility diagnostics in permissive mode.
+- Distinguish base unsupported descriptor, orientation variants, and
+  per-interface penalty controls.
+- Keep diagnostics stable and easy to assert in tests.
+
+**Non-Goals**
+
+- DG-SEM multipatch execution.
+- Changes to IGA multipatch behavior.
+- Flux-family or trace-operator redesign.
+
+## Decisions
+
+1. Keep `unsupported_multipatch_interface` as the primary strict failure code.
+   - Rationale: avoids compatibility break for existing strict-mode callers.
+
+2. Append additional DG-SEM compatibility diagnostics in deterministic order.
+   - Rationale: permissive users get actionable context while retaining stable
+     payload semantics.
+
+3. Detect unsupported descriptor features directly from IR multipatch
+   interfaces (orientation and penalty controls).
+   - Rationale: keeps diagnostics backend-agnostic but DG-SEM scoped.
+
+## Risks / Trade-offs
+
+- [Risk] Additional diagnostics may be noisy for simple unsupported requests.
+  - Mitigation: keep code set compact and deterministic.
+- [Risk] Future DG-SEM execution phases may require code deprecation.
+  - Mitigation: use clearly scoped `dgsem_*` diagnostic codes.
+
+## Migration Plan
+
+1. Extend OpenSpec requirements for DG-SEM multipatch compatibility diagnostics.
+2. Implement deterministic diagnostic generation in capability checks.
+3. Add strict/permissive regression assertions and update docs.

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/proposal.md

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+## Why
+
+`dgsem` remains multipatch execution unsupported, but compatibility diagnostics
+are currently coarse (`unsupported_multipatch_interface` only). Callers cannot
+reliably distinguish descriptor features that block DG-SEM lowering (for
+example interface orientation variants or per-interface penalty controls).
+
+We need deterministic diagnostic semantics so DG-SEM users get actionable,
+structured feedback while execution remains out of scope.
+
+## What Changes
+
+- Keep DG-SEM multipatch execution unsupported.
+- Add deterministic compatibility-profile diagnostics for DG-SEM when
+  multipatch descriptors are present.
+- Emit structured diagnostics for orientation variants and per-interface penalty
+  controls under DG-SEM.
+- Add regression tests for strict/permissive behavior and diagnostics payload
+  stability.
+
+## Capabilities
+
+### Modified Capabilities
+
+- `ufl-dgsem-form-dsl`: expands diagnostics from one generic unsupported code
+  to a deterministic compatibility profile for multipatch descriptors.
+
+### Unchanged Capabilities
+
+- `ufl-iga-form-dsl`: unchanged execution semantics.
+- DG-SEM multipatch execution remains unsupported in this phase.
+
+## Impact
+
+- Affected code:
+  - `src/cutkit/formdsl/capabilities.py`
+  - tests under `tests/formdsl/`
+  - support docs under `docs/`

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/specs/form-dsl-backend-assembly/spec.md

@@ -0,0 +1,27 @@
+## ADDED Requirements
+
+### Requirement: DG-SEM multipatch strict mode remains fail-fast
+The system SHALL continue to reject multipatch descriptors on backend `dgsem`
+in strict mode.
+
+#### Scenario: Strict DG-SEM multipatch request
+- **WHEN** a form with multipatch descriptors is assembled on `dgsem` with
+  `strict=True`
+- **THEN** capability validation fails with
+  `unsupported_multipatch_interface`
+
+### Requirement: DG-SEM multipatch permissive diagnostics are compatibility-aware
+The system SHALL emit deterministic compatibility diagnostics for DG-SEM
+multipatch requests in permissive mode.
+
+#### Scenario: Permissive DG-SEM multipatch request with orientation variants
+- **WHEN** a multipatch descriptor on `dgsem` includes `reversed` orientation
+  interfaces and `strict=False`
+- **THEN** diagnostics include a DG-SEM compatibility code describing
+  unsupported orientation semantics
+
+#### Scenario: Permissive DG-SEM multipatch request with per-interface penalties
+- **WHEN** a multipatch descriptor on `dgsem` includes per-interface `penalty`
+  controls and `strict=False`
+- **THEN** diagnostics include a DG-SEM compatibility code describing
+  unsupported per-interface coupling controls

openspec/changes/formdsl-dgsem-multipatch-compatibility-semantics/tasks.md

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+## 1. OpenSpec Artifacts
+
+- [x] 1.1 Add proposal/design/spec/tasks artifacts for DG-SEM multipatch
+  compatibility diagnostics.
+
+## 2. Capability Diagnostics
+
+- [ ] 2.1 Keep strict unsupported behavior and add deterministic compatibility
+  profile diagnostics in permissive mode.
+- [ ] 2.2 Emit explicit DG-SEM diagnostics for orientation variants and
+  per-interface penalty controls.
+
+## 3. Validation + Docs
+
+- [ ] 3.1 Add regression tests for strict/permissive diagnostics stability.
+- [ ] 3.2 Update docs and execution-plan progress.