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+# Slice 6b: Capability Selection Boundary
+
+*2026-09-06, from 05f79173*
+
+## 1. CapabilitySelection / selector contract
+
+```go
+type CapabilitySelection struct {
+    Fn         string
+    Args       []string
+    Resolved   bool
+    Method     ActionResolutionMethod
+    InputKind  SelectionInputKind
+    Producer   RouteProducer
+    Confidence float64
+}
+
+type SelectionInputKind string
+
+const (
+    SelectionRaw          SelectionInputKind = "raw"
+    SelectionLLMText      SelectionInputKind = "llm_text"
+    SelectionDeterministic SelectionInputKind = "deterministic"
+)
+
+func SelectCapability(dec Decision, m ActMatcher) CapabilitySelection
+func applyCapabilityToSlots(dec *Decision, sel CapabilitySelection)
+```
+
+`SelectCapability` is the single entry point for capability selection. It sits
+between route resolution and action candidate production.
+
+`applyCapabilityToSlots` propagates the selection into `Decision.Slots.Fn/Args/HasFn`
+for backward compatibility. `CapabilitySelection` is the authoritative record.
+
+## 2. Where the selector lives
+
+`internal/router/capability.go` — new file, 115 lines.
+
+Called from `Router.Route` in `internal/router/router.go` after each cascade path:
+- Stage-0 grammar path (line ~108)
+- Stage-0b heads path (line ~148)
+- Stage-1a LLM path (line ~190)
+- Stage-1 classifier path (line ~250)
+
+## 3. Old vs new ownership
+
+| Concern | Before | After |
+|---|---|---|
+| Raw capability extraction | `Extractor.Extract(IntentAct)` inside `fillMatchedSlots` | Same extractor, but `CapabilitySelection` is the authoritative record |
+| LLM text capability backfill | Embedded in `fillSlots` (lines 319-324) | Moved to `SelectCapability` |
+| Fallback matcher | `ResolveActionCandidate` (lines 160-171) | Same location, unchanged |
+| Resolution method tracking | `Slots.ResolvedBy` only | `CapabilitySelection.Method` (authoritative), `Slots.ResolvedBy` (compatibility) |
+
+## 4. Compatibility fields
+
+`Decision.Slots.Fn/Args/HasFn/ResolvedBy` are still populated by
+`applyCapabilityToSlots` from the `CapabilitySelection` result. They exist for
+backward compatibility with code that reads slots directly (tests, rebuilt
+decisions). The ownership distinction is documented on the `Decision` struct.
+
+## 5. Before/after flow
+
+**Before:**
+```
+Route → Decision (Slots.Fn set by extractor/fillSlots)
+    ↓
+resolveAction → ResolveActionCandidate(dec, matcher)
+    → if Slots.HasFn: candidate from slots
+    → else: fallback matcher
+```
+
+**After:**
+```
+Route → Decision
+    ↓
+fillMatchedSlots (time, key extraction)
+    ↓
+SelectCapability(dec, matcher) → CapabilitySelection
+    ↓
+applyCapabilityToSlots(dec, sel) → populates Slots.Fn/Args/HasFn for compat
+    ↓
+resolveAction → ResolveActionCandidate(dec, matcher)
+    → if CapabilitySelection.Resolved: candidate from selection
+    → else if Slots.HasFn: backward compat path
+    → else: fallback matcher
+```
+
+## 6. Fixture matrix before/after
+
+```
+Routing fixture (11 act cases):
+  grammar_fixed=2, grammar_matcher=2, extractor_raw=4, extractor_llm_text=0, fallback_matcher=0
+  resolved: 8, unresolved: 3
+
+Ecosystem fixture (24 act cases):
+  grammar_fixed=12, grammar_matcher=0, extractor_raw=11, extractor_llm_text=0, fallback_matcher=0
+  resolved: 23, unresolved: 1
+```
+
+Identical before and after. Zero distribution change.
+
+## 7. Fallback matcher usage
+
+The fallback matcher in `ResolveActionCandidate` is unchanged. It still runs
+when `CapabilitySelection.Resolved == false` and `Slots.HasFn == false`. The
+fallback matcher fires zero times in the eval fixture, consistent with previous
+measurements.
+
+## 8. Tests
+
+New test file: `internal/router/capability_test.go` — 14 tests.
+
+| Test | What it pins |
+|---|---|
+| `TestSelectCapability_DeterministicBypass` | Grammar-fixed acts bypass selector |
+| `TestSelectCapability_ExtractorRawBypass` | Extractor-raw acts bypass selector |
+| `TestSelectCapability_GrammarMatcherBypass` | Grammar-matcher acts bypass selector |
+| `TestSelectCapability_LLMTextMatch` | LLM cleaned text resolves when raw misses |
+| `TestSelectCapability_LLMTextSameAsUtterance` | No double-match when Text == Utterance |
+| `TestSelectCapability_Unresolved` | No match on any input |
+| `TestSelectCapability_NonActIntent` | Non-act returns empty selection |
+| `TestSelectCapability_NilMatcher` | Nil matcher does not panic |
+| `TestApplyCapabilityToSlots_PopulatesCompatibilityFields` | Compat fields populated from selection |
+| `TestApplyCapabilityToSlots_UnresolvedDoesNotSetSlots` | Unresolved selection does not overwrite slots |
+| `TestResolveActionCandidate_CapabilitySelectionSource` | Selection produces route-sourced candidate |
+| `TestResolveActionCandidate_CapabilitySelectionOverSlots` | CapabilitySelection takes precedence over Slots.HasFn |
+| `TestResolveActionCandidate_BackwardCompatSlotsHasFn` | Decisions with HasFn but no CapabilitySelection still work |
+| `TestRouterRoute_CapabilitySelectionPopulated` | Router.Route sets CapabilitySelection on Decision |
+| `TestSelectionInputKindConstants` | Three input kind constants are distinct |
+
+Modified tests: `actioncandidate_test.go` — 8 tests updated to set
+`CapabilitySelection` alongside `Slots.HasFn`.
+
+## 9. Fn/Args result confirmation
+
+Every `Fn` and `Args` value is unchanged:
+- Routing fixture: identical distribution (8 resolved, 3 unresolved)
+- Ecosystem fixture: identical distribution (23 resolved, 1 unresolved)
+- All 57 router tests pass
+- All cmd/mavend tests pass
+- All eval fixture tests pass
+
+## 10. Newly exposed architectural problems
+
+None. The extraction is clean and mechanical. The backward compatibility
+path in `ResolveActionCandidate` (checking `Slots.HasFn` when `CapabilitySelection`
+is not set) is a temporary bridge that should be removed when all call sites
+produce decisions through the router.
+
+## 11. Commit hash
+
+Pending commit on branch `task/slice-6b-capability-selection`.