diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index a309832..24d6a1d 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -269,7 +269,7 @@ unchanged until Stage 11 so collection is independently reviewable.
 Validate: `go test ./internal/engine -run 'Secret|Sensitivity|Inherited|Effective|Explain'`,
 engine race tests, and the everyday check. No new predicate order/count guarantee.
 
-## Stage 10: Build and test indexed sensitivity triggers
+## Stage 10: Build and test indexed sensitivity triggers — Complete
 
 Roadmap candidate 6, isolated algorithm. Add private
 `internal/engine/secret_propagation.go` and focused tests.
diff --git a/internal/engine/secret_propagation.go b/internal/engine/secret_propagation.go
new file mode 100644
index 0000000..b45a450
--- /dev/null
+++ b/internal/engine/secret_propagation.go
@@ -0,0 +1,69 @@
+package engine
+
+type secretTrigger struct {
+	target   string
+	ordinary bool
+}
+
+func closeSecretDependencies(paths map[string]Path, sensitive map[string]bool, dependencies, inherited []secretDependency) map[string]bool {
+	result := make(map[string]bool, len(sensitive))
+	queue := make([]string, 0, len(sensitive))
+	for pointer, classified := range sensitive {
+		if classified {
+			result[pointer] = true
+			queue = append(queue, pointer)
+		}
+	}
+
+	var triggers pathPrefixIndex
+	triggerValues := make([]secretTrigger, 0, len(dependencies)+2*len(inherited))
+	insert := func(dependency Path, target string, ordinary bool) {
+		if ordinary && !dependency.valid() {
+			return
+		}
+		id := len(triggerValues)
+		triggerValues = append(triggerValues, secretTrigger{target: target, ordinary: ordinary})
+		triggers.insert(dependency.String(), id)
+	}
+	for _, dependency := range dependencies {
+		insert(dependency.dependency, dependency.target.String(), true)
+	}
+	for _, link := range inherited {
+		insert(link.dependency, link.target.String(), false)
+		insert(link.target, link.dependency.String(), false)
+	}
+
+	fired := make([]bool, len(triggerValues))
+	activate := func(id int) {
+		if fired[id] {
+			return
+		}
+		fired[id] = true
+		target := triggerValues[id].target
+		if !result[target] {
+			result[target] = true
+			queue = append(queue, target)
+		}
+	}
+	for head := 0; head < len(queue); head++ {
+		pointer := queue[head]
+		if classified, ok := paths[pointer]; ok && classified.valid() {
+			triggers.visitAncestors(classified.String(), func(id int) {
+				if triggerValues[id].ordinary {
+					activate(id)
+				}
+			})
+			triggers.visitDescendants(classified.String(), func(id int) {
+				if triggerValues[id].ordinary {
+					activate(id)
+				}
+			})
+		}
+		triggers.visitDescendants(pointer, func(id int) {
+			if !triggerValues[id].ordinary {
+				activate(id)
+			}
+		})
+	}
+	return result
+}
diff --git a/internal/engine/secret_propagation_test.go b/internal/engine/secret_propagation_test.go
new file mode 100644
index 0000000..a51415b
--- /dev/null
+++ b/internal/engine/secret_propagation_test.go
@@ -0,0 +1,127 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSecretDependencyClosureMatchesFixedPointScan(t *testing.T) {
+	tests := []struct {
+		name         string
+		paths        map[string]Path
+		seed         map[string]bool
+		dependencies []secretDependency
+		inherited    []secretDependency
+	}{
+		{
+			name:  "forward reverse chains and cycles",
+			paths: secretPaths(Keys("input"), Keys("derived"), Keys("output"), Keys("cycle")),
+			seed:  secretSet(Keys("input")),
+			dependencies: []secretDependency{
+				{dependency: Keys("input"), target: Keys("derived")},
+				{dependency: Keys("derived"), target: Keys("output")},
+				{dependency: Keys("output"), target: Keys("cycle")},
+				{dependency: Keys("cycle"), target: Keys("derived")},
+				{dependency: Keys("input"), target: Keys("derived")},
+			},
+		},
+		{
+			name:  "ordinary ancestor descendant and root relations",
+			paths: secretPaths(Path{}, Keys("branch", "leaf"), Keys("parent", "child")),
+			seed:  secretSet(Keys("branch", "leaf"), Keys("parent")),
+			dependencies: []secretDependency{
+				{dependency: Keys("branch"), target: Keys("from-descendant")},
+				{dependency: Keys("parent", "child"), target: Keys("from-ancestor")},
+				{dependency: Path{}, target: Keys("from-root")},
+			},
+		},
+		{
+			name:  "unindexed seed only activates inherited relations",
+			paths: secretPaths(Keys("ordinary"), Keys("copy")),
+			seed:  map[string]bool{"untracked": true},
+			dependencies: []secretDependency{
+				{dependency: Keys("untracked"), target: Keys("ordinary")},
+			},
+			inherited: []secretDependency{
+				{dependency: Keys("untracked"), target: Keys("copy")},
+			},
+		},
+		{
+			name:  "inheritance direction root escapes and numeric spellings",
+			paths: secretPaths(Path{}, Keys("a/b", "01"), Keys("source", "value")),
+			seed:  secretSet(Path{}),
+			dependencies: []secretDependency{
+				{dependency: Keys("a/b", "01"), target: Keys("ordinary-copy")},
+			},
+			inherited: []secretDependency{
+				{dependency: Keys("a/b", "01"), target: Keys("destination", "01")},
+				{dependency: Keys("source"), target: Keys("destination")},
+			},
+		},
+		{
+			name:  "inherited links do not trigger from classified descendants",
+			paths: secretPaths(Keys("parent", "secret", "nested")),
+			seed:  secretSet(Keys("parent", "secret", "nested")),
+			inherited: []secretDependency{
+				{dependency: Keys("parent", "secret"), target: Keys("child", "secret")},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			want := referenceSecretDependencyClosure(test.paths, test.seed, test.dependencies, test.inherited)
+			got := closeSecretDependencies(test.paths, test.seed, test.dependencies, test.inherited)
+			if !reflect.DeepEqual(got, want) {
+				t.Fatalf("closure = %#v, want %#v", got, want)
+			}
+		})
+	}
+}
+
+func referenceSecretDependencyClosure(paths map[string]Path, seed map[string]bool, dependencies, inherited []secretDependency) map[string]bool {
+	secret := make(map[string]bool, len(seed))
+	for pointer, classified := range seed {
+		if classified {
+			secret[pointer] = true
+		}
+	}
+	for {
+		changed := false
+		for _, dependency := range dependencies {
+			if secretIntersectsPath(paths, secret, dependency.dependency) && !secret[dependency.target.String()] {
+				secret[dependency.target.String()] = true
+				changed = true
+			}
+		}
+		for _, link := range inherited {
+			if pathIsSecret(secret, link.dependency) && !secret[link.target.String()] {
+				secret[link.target.String()] = true
+				changed = true
+			}
+			if pathIsSecret(secret, link.target) && !secret[link.dependency.String()] {
+				secret[link.dependency.String()] = true
+				changed = true
+			}
+		}
+		if !changed {
+			return secret
+		}
+	}
+}
+
+func secretPaths(paths ...Path) map[string]Path {
+	result := make(map[string]Path, len(paths))
+	for _, path := range paths {
+		result[path.String()] = path
+	}
+	return result
+}
+
+func secretSet(paths ...Path) map[string]bool {
+	result := make(map[string]bool, len(paths))
+	for _, path := range paths {
+		result[path.String()] = true
+	}
+	return result
+}