diff --git a/changes/unreleased/implicit-parameter-position-from-memo.fixed.md b/changes/unreleased/implicit-parameter-position-from-memo.fixed.md
new file mode 100644
index 0000000000..58862a2ee5
--- /dev/null
+++ b/changes/unreleased/implicit-parameter-position-from-memo.fixed.md
@@ -0,0 +1 @@
+- The features a feature redefines, directly or through them, are computed once per model, and a parameter's implicit redefinitions take its position from its owner's memoized parameter list instead of rebuilding it on every query; same redefinitions, fewer allocations per query — and per execution root frame, which asks for every feature it holds.
diff --git a/internal/semantic/semantics/implicit_parameter_memo_test.go b/internal/semantic/semantics/implicit_parameter_memo_test.go
new file mode 100644
index 0000000000..13ed5a4f51
--- /dev/null
+++ b/internal/semantic/semantics/implicit_parameter_memo_test.go
@@ -0,0 +1,28 @@
+package semantics
+
+import "testing"
+
+// The redefinition closure of a feature is computed once per model, and a
+// parameter's implicit redefinitions take its position from the owner's
+// memoized parameter list instead of rebuilding it: repeated queries allocate
+// nothing beyond what the owner's implied generals cost.
+func TestRedefinedFeaturesAreComputedOncePerSymbol(t *testing.T) {
+	m, root := buildModel(t, `package P {
+		action def Base { in x; in y; out z; }
+		action def Proc :> Base { in a; in b; out c; }
+	}`)
+	p := sym(t, root, "P")
+	proc := nested(t, p.Scope, "Proc")
+	a := nested(t, p.Scope, "Proc", "a")
+	wantX := nested(t, p.Scope, "Base", "x")
+	if got := m.AllRedefinedFeatures(a); len(got) != 1 || got[0] != wantX {
+		t.Fatalf("AllRedefinedFeatures(Proc::a) = %v, want [Base::x]", got)
+	}
+	if got := testing.AllocsPerRun(20, func() { m.AllRedefinedFeatures(a) }); got != 0 {
+		t.Fatalf("a repeated AllRedefinedFeatures query allocates %.0f times, want 0", got)
+	}
+	generals := testing.AllocsPerRun(20, func() { m.ImplicitGenerals(proc) })
+	if got := testing.AllocsPerRun(20, func() { m.ImplicitParameterRedefinitions(a) }); got > generals+1 {
+		t.Fatalf("a repeated ImplicitParameterRedefinitions query allocates %.0f times, want at most its result beyond the owner's implied generals (%.0f)", got, generals)
+	}
+}
diff --git a/internal/semantic/semantics/masking.go b/internal/semantic/semantics/masking.go
index d02c4dc117..45c223f47c 100644
--- a/internal/semantic/semantics/masking.go
+++ b/internal/semantic/semantics/masking.go
@@ -74,6 +74,11 @@ func (m *Model) AllRedefinedFeatures(sym *symbols.Symbol) []*symbols.Symbol {
 	if m == nil || sym == nil {
 		return nil
 	}
+	defer m.own(sym).LeaveDoc()
+	if cached, ok := m.allRedefined[sym]; ok {
+		return cached
+	}
+	m.resolver.Enter()
 	var out []*symbols.Symbol
 	seen := map[*symbols.Symbol]bool{sym: true}
 	var visit func(*symbols.Symbol)
@@ -88,6 +93,13 @@ func (m *Model) AllRedefinedFeatures(sym *symbols.Symbol) []*symbols.Symbol {
 		}
 	}
 	visit(sym)
+	// A closure walked while a guard cut a query short may lack a target, so it
+	// is recomputed on the next query, not memoized.
+	if !m.resolver.Leave() {
+		return out
+	}
+	journal(m, m.allRedefined, sym, sym.Decl)
+	m.allRedefined[sym] = out
 	return out
 }
 
diff --git a/internal/semantic/semantics/model.go b/internal/semantic/semantics/model.go
index bfdf8e7558..37c8cb049a 100644
--- a/internal/semantic/semantics/model.go
+++ b/internal/semantic/semantics/model.go
@@ -34,20 +34,22 @@ type Model struct {
 	computingSupers map[*symbols.Symbol]int
 	// valuing holds the features whose typing value is being judged, so a value
 	// that leads back to its own feature is not followed again.
-	valuing       map[*symbols.Symbol]bool
-	referenced    map[*symbols.Symbol]*symbols.Symbol
-	resolvingRef  map[*symbols.Symbol]bool
-	memberSources map[*symbols.Symbol][]*symbols.Symbol
-	lookupOrder   map[*symbols.Symbol][]lookupSource    // name-lookup order
-	contributed   map[*symbols.Symbol][]*symbols.Symbol // memoized contributors
-	nestedRedefs  map[*symbols.Symbol][]NestedRedefinition
-	primTypes     map[*symbols.Symbol]PrimType
-	scalars       map[*symbols.Symbol]PrimType // stdlib scalar symbols, resolved once
-	params        map[*symbols.Symbol]behaviorParameters
-	invocations   map[invocationKey]*InvocationSelection
-	arguments     ArgumentTyper // the checker's argument typing, nil when no checker runs
-	sourceText    source.Lookup // notation of the loaded documents, nil when unavailable
-	sourceFile    source.Locate // the files the documents were read from, nil for source.FileNamed
+	valuing         map[*symbols.Symbol]bool
+	referenced      map[*symbols.Symbol]*symbols.Symbol
+	resolvingRef    map[*symbols.Symbol]bool
+	memberSources   map[*symbols.Symbol][]*symbols.Symbol
+	lookupOrder     map[*symbols.Symbol][]lookupSource    // name-lookup order
+	contributed     map[*symbols.Symbol][]*symbols.Symbol // memoized contributors
+	nestedRedefs    map[*symbols.Symbol][]NestedRedefinition
+	allRedefined    map[*symbols.Symbol][]*symbols.Symbol // memoized redefinition closure
+	primTypes       map[*symbols.Symbol]PrimType
+	scalars         map[*symbols.Symbol]PrimType // stdlib scalar symbols, resolved once
+	params          map[*symbols.Symbol]behaviorParameters
+	computingParams map[*symbols.Symbol]int // lookup depth of a params entry being computed
+	invocations     map[invocationKey]*InvocationSelection
+	arguments       ArgumentTyper // the checker's argument typing, nil when no checker runs
+	sourceText      source.Lookup // notation of the loaded documents, nil when unavailable
+	sourceFile      source.Locate // the files the documents were read from, nil for source.FileNamed
 	// typingArgs holds the calls whose arguments are being typed, so an argument
 	// whose type leads back to its own call is not typed again.
 	typingArgs map[*ast.InvocationExpr]bool
@@ -168,8 +170,10 @@ func NewModel(resolver *resolve.Resolver) *Model {
 		lookupOrder:         make(map[*symbols.Symbol][]lookupSource),
 		contributed:         make(map[*symbols.Symbol][]*symbols.Symbol),
 		nestedRedefs:        make(map[*symbols.Symbol][]NestedRedefinition),
+		allRedefined:        make(map[*symbols.Symbol][]*symbols.Symbol),
 		primTypes:           make(map[*symbols.Symbol]PrimType),
 		params:              make(map[*symbols.Symbol]behaviorParameters),
+		computingParams:     make(map[*symbols.Symbol]int),
 		invocations:         make(map[invocationKey]*InvocationSelection),
 		typingArgs:          make(map[*ast.InvocationExpr]bool),
 		composed:            make(map[composedKey][]*symbols.Symbol),
diff --git a/internal/semantic/semantics/parameters_guard_test.go b/internal/semantic/semantics/parameters_guard_test.go
new file mode 100644
index 0000000000..9271fd59d8
--- /dev/null
+++ b/internal/semantic/semantics/parameters_guard_test.go
@@ -0,0 +1,28 @@
+package semantics
+
+import "testing"
+
+// A parameter list read while its owner's list is still being computed cuts
+// the reading query short, so a closure built on it is not memoized either.
+func TestParameterListGuardCutsDependentQueriesShort(t *testing.T) {
+	m, root := buildModel(t, `package P {
+		action def Base { in x; out z; }
+		action def Proc :> Base { in a; out c; }
+	}`)
+	p := sym(t, root, "P")
+	proc := nested(t, p.Scope, "Proc")
+	a := nested(t, p.Scope, "Proc", "a")
+	wantX := nested(t, p.Scope, "Base", "x")
+	// As parametersOf stands mid-computation for the owner.
+	m.params[proc] = behaviorParameters{}
+	m.computingParams[proc] = m.resolver.Enter()
+	if got := m.AllRedefinedFeatures(a); len(got) != 0 {
+		t.Fatalf("AllRedefinedFeatures(Proc::a) under the guard = %v, want the guard's empty answer", got)
+	}
+	m.resolver.Leave()
+	delete(m.params, proc)
+	delete(m.computingParams, proc)
+	if got := m.AllRedefinedFeatures(a); len(got) != 1 || got[0] != wantX {
+		t.Fatalf("AllRedefinedFeatures(Proc::a) after the guard = %v, want [Base::x]", got)
+	}
+}
diff --git a/internal/semantic/semantics/redefinition.go b/internal/semantic/semantics/redefinition.go
index f5ab8143dc..e76c0cb421 100644
--- a/internal/semantic/semantics/redefinition.go
+++ b/internal/semantic/semantics/redefinition.go
@@ -159,11 +159,17 @@ func (m *Model) BehaviorParametersOf(sym *symbols.Symbol) []BehaviorParameter {
 func (m *Model) parametersOf(sym *symbols.Symbol) behaviorParameters {
 	defer m.own(sym).LeaveDoc()
 	if cached, ok := m.params[sym]; ok {
+		// The seed answers a re-entrant query with nothing, cutting that query short.
+		if depth := m.computingParams[sym]; depth != 0 {
+			m.resolver.CutShort(depth)
+		}
 		return cached
 	}
 	// Guard against re-entrancy on cyclic specialization graphs.
 	journal(m, m.params, sym, sym.Decl)
 	m.params[sym] = behaviorParameters{}
+	m.computingParams[sym] = m.resolver.Enter()
+	defer delete(m.computingParams, sym)
 
 	owned := ownedParameters(sym)
 	out := behaviorParameters{positional: positionalParameters(owned), result: resultParameter(owned)}
@@ -194,6 +200,12 @@ func (m *Model) parametersOf(sym *symbols.Symbol) behaviorParameters {
 		}
 	}
 
+	// A list derived while a guard cut a query short may have missed a
+	// parameter, so it is recomputed on the next query, not memoized.
+	if !m.resolver.Leave() {
+		delete(m.params, sym)
+		return out
+	}
 	m.params[sym] = out
 	return out
 }
@@ -391,7 +403,8 @@ func (m *Model) implicitParameterTargets(sym *symbols.Symbol, matchDirection boo
 		return nil
 	}
 	position := -1
-	for i, p := range positionalParameters(ownedParameters(owner)) {
+	// The owner's memoized parameters list its owned ones first, in order.
+	for i, p := range m.parametersOf(owner).positional {
 		if p.sym == sym {
 			position = i
 			break
diff --git a/internal/semantic/semantics/redefinition_closure_guard_test.go b/internal/semantic/semantics/redefinition_closure_guard_test.go
new file mode 100644
index 0000000000..4cc09d6479
--- /dev/null
+++ b/internal/semantic/semantics/redefinition_closure_guard_test.go
@@ -0,0 +1,28 @@
+package semantics
+
+import "testing"
+
+// A redefinition closure walked while a guard cuts a query short is answered
+// but not memoized: once the guard lifts, the next query sees the target.
+func TestRedefinitionClosureIsNotCachedWhileAGuardCutsItShort(t *testing.T) {
+	m, root := buildModel(t, `package P {
+		part def W { attribute x; }
+		part def A { part w : W; }
+		part def B :> A { attribute z :>> w.x; }
+	}`)
+	p := sym(t, root, "P")
+	wantX := nested(t, p.Scope, "W", "x")
+	bz := nested(t, p.Scope, "B", "z")
+	// As RedefinedFeatures stands mid-computation: seeded, with its lookup open.
+	m.redefined[bz] = nil
+	m.computingRedefined[bz] = m.resolver.Enter()
+	if got := m.AllRedefinedFeatures(bz); len(got) != 0 {
+		t.Fatalf("AllRedefinedFeatures(B::z) under the guard = %v, want the guard's empty answer", got)
+	}
+	m.resolver.Leave()
+	delete(m.redefined, bz)
+	delete(m.computingRedefined, bz)
+	if got := m.AllRedefinedFeatures(bz); len(got) != 1 || got[0] != wantX {
+		t.Fatalf("AllRedefinedFeatures(B::z) after the guard = %v, want [W::x]", got)
+	}
+}