diff --git a/configuration_sources.go b/configuration_sources.go
index 808412e..1b9ed86 100644
--- a/configuration_sources.go
+++ b/configuration_sources.go
@@ -8,9 +8,25 @@ import (
 	"path/filepath"
 
 	"gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 	"gitea.maximumdirect.net/eric/promptkit/internal/filecatalog"
 )
 
+// CollectionInput mounts source at the selected collection's configuration
+// destination and enables ConfigKit imports for that input. The source is not
+// read or validated until ConfigKit loads it. Callers may adjust the returned
+// input, including its filesystem context or imports setting.
+func CollectionInput(namespace string, source configkit.Source) (configkit.Input, error) {
+	if !domain.IsValidResourceNamespace(namespace) {
+		return configkit.Input{}, fmt.Errorf("%w: invalid collection namespace %q", ErrInvalidConfig, namespace)
+	}
+	return configkit.Input{
+		Source:      source,
+		Destination: configkit.Keys("collections", namespace),
+		Imports:     true,
+	}, nil
+}
+
 // DirectoryInputs selects the YAML files beneath an explicitly chosen host
 // directory as one strict-composition group. The returned order is deterministic;
 // it does not assign precedence among those files. Discovery is recursive over
diff --git a/configuration_sources_test.go b/configuration_sources_test.go
index 824e6e3..030f542 100644
--- a/configuration_sources_test.go
+++ b/configuration_sources_test.go
@@ -1,6 +1,8 @@
 package promptkit_test
 
 import (
+	"context"
+	"errors"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -16,6 +18,78 @@ type sourceDocument struct {
 	Second string `yaml:"second"`
 }
 
+func TestCollectionInputValidatesWithoutReadingSource(t *testing.T) {
+	input, err := promptkit.CollectionInput("docs", configkit.File(filepath.Join(t.TempDir(), "not-read.yaml")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if input.Destination.String() != "/collections/docs" || !input.Imports {
+		t.Fatalf("collection input = %+v, want destination /collections/docs with imports enabled", input)
+	}
+	for _, namespace := range []string{"", " ", "docs/api"} {
+		if _, err := promptkit.CollectionInput(namespace, configkit.NoSource()); !errors.Is(err, promptkit.ErrInvalidConfig) {
+			t.Errorf("namespace %q error = %v, want ErrInvalidConfig", namespace, err)
+		}
+	}
+}
+
+func TestCollectionInputLoadsNativeImportsAndLaterOverride(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "collection.yaml")
+	defaultsPath := filepath.Join(directory, "defaults.yaml")
+	if err := os.WriteFile(mainPath, []byte("configkit:\n  imports:\n    - path: defaults.yaml\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(defaultsPath, []byte("prompts:\n  greeting:\n    version: imported\n    messages:\n      - role: user\n        content: Hello\n    output:\n      format: text\n      validation_mode: none\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	input, err := promptkit.CollectionInput("docs", configkit.File(mainPath))
+	if err != nil {
+		t.Fatal(err)
+	}
+	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{input}}},
+		Main: &configkit.Input{
+			Source:      configkit.Mapping("later collection override", map[string]any{"prompts": map[string]any{"greeting": map[string]any{"version": "overridden"}}}),
+			Destination: configkit.Keys("collections", "docs"),
+		},
+	}})
+	if err != nil {
+		t.Fatalf("construct engine with native collection import: %v", err)
+	}
+	inspection, err := engine.InspectPrompt(context.Background(), "docs/greeting")
+	if err != nil || inspection.PromptVersion == nil || *inspection.PromptVersion != "overridden" {
+		t.Fatalf("inspect imported and overlaid prompt = (%+v, %v)", inspection, err)
+	}
+}
+
+func TestCollectionInputLoadsEmbeddedImportsAndLaterOverride(t *testing.T) {
+	files := fstest.MapFS{
+		"collection.yaml": {Data: []byte("configkit:\n  imports:\n    - path: defaults.yaml\n")},
+		"defaults.yaml":   {Data: []byte("prompts:\n  greeting:\n    version: imported\n    messages:\n      - role: user\n        content: Hello\n    output:\n      format: text\n      validation_mode: none\n")},
+	}
+	filesystem := configkit.NewFilesystem("embedded collection", files)
+	input, err := promptkit.CollectionInput("docs", filesystem.File("collection.yaml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	input.Filesystem = &configkit.FilesystemContext{FS: &filesystem, Root: "."}
+	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{input}}},
+		Main: &configkit.Input{
+			Source:      configkit.Mapping("later collection override", map[string]any{"prompts": map[string]any{"greeting": map[string]any{"version": "overridden"}}}),
+			Destination: configkit.Keys("collections", "docs"),
+		},
+	}})
+	if err != nil {
+		t.Fatalf("construct engine with embedded collection import: %v", err)
+	}
+	inspection, err := engine.InspectPrompt(context.Background(), "docs/greeting")
+	if err != nil || inspection.PromptVersion == nil || *inspection.PromptVersion != "overridden" {
+		t.Fatalf("inspect imported and overlaid prompt = (%+v, %v)", inspection, err)
+	}
+}
+
 func TestDirectoryInputsSelectsSortedFilesWithRootContext(t *testing.T) {
 	root := t.TempDir()
 	if err := os.Mkdir(filepath.Join(root, "nested"), 0o755); err != nil {
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index f6620bf..4a4e740 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -593,6 +593,13 @@ Validation: source-helper contract tests and focused configuration/root tests.
 Verify helper construction itself performs no source read using an existing
 controlled source seam where needed, rather than adding a production interface.
 
+Completion: added `CollectionInput` with shared namespace validation, a
+`collections/<namespace>` destination, and imports enabled without reading the
+source. Contract tests cover invalid namespaces and deferred source loading;
+native-file and embedded-filesystem imports assemble successfully, preserve
+their source context, and accept a later override. Validation passed:
+`GOWORK=off go test ./...`, `GOWORK=off go vet .`, and `git diff --check`.
+
 ## Stage 12: Add Immutable Scopes To Ordinary Execution
 
 Scope: public `Namespaces`/`WithNamespaces` and shared engine-boundary routing.