diff --git a/docs/internal/sources.md b/docs/internal/sources.md
index cac79ee..cdc4a69 100644
--- a/docs/internal/sources.md
+++ b/docs/internal/sources.md
@@ -57,6 +57,14 @@ and permits inherited target fields only when a base is named. File-backed
 bounded JSON-value owner before a profile is published. OpenAI-compatible
 reserved-field policy remains with the model-client and backend-registry owners.
 
+`LoadFSRepository` is the eager immutable loading boundary for internal
+catalog consumers. It discovers and strictly validates every raw profile once,
+preserves safe source metadata including explicitly present YAML fields, and
+publishes independently copied values from memory. It does not resolve profile
+inheritance. Configured consumer sources continue to use the lazy point lookup
+repositories described above; engine assembly still uses the embedded built-in
+catalog at this point.
+
 The overlay repository consults the next repository only when the
 higher-precedence repository reports that a profile is absent. A reliably
 selected malformed profile stops fallback, while an unrelated malformed file
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index fd50071..1c616fb 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -305,7 +305,7 @@ Inspect the copied profile against Promptkit's Stage 1 fixture before release.
 
 ## Stage 4: Add Eager Immutable Profile Loading To Promptkit
 
-**Status:** Pending
+**Status:** Complete
 
 ### Repository
 
diff --git a/internal/profile/eager_repository.go b/internal/profile/eager_repository.go
new file mode 100644
index 0000000..76d7f1f
--- /dev/null
+++ b/internal/profile/eager_repository.go
@@ -0,0 +1,95 @@
+package profile
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"sort"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/filecatalog"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+)
+
+// LoadedProfileMetadata identifies one profile accepted by LoadFSRepository.
+type LoadedProfileMetadata struct {
+	ID             string
+	Path           string
+	ExplicitFields []string
+}
+
+// LoadFSRepository eagerly validates every profile under root and returns an
+// immutable raw repository and independently owned source metadata.
+func LoadFSRepository(ctx context.Context, fsys fs.FS, root string) (Repository, []LoadedProfileMetadata, error) {
+	if fsys == nil {
+		return nil, nil, fmt.Errorf("failed to read profile directory: filesystem is nil")
+	}
+	paths, err := filecatalog.FindFSYAMLFiles(ctx, fsys, root)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to read profile directory: %w", err)
+	}
+
+	repository := &loadedRepository{profiles: make(map[string]domain.ExecutionProfile, len(paths))}
+	metadata := make([]LoadedProfileMetadata, 0, len(paths))
+	for _, path := range paths {
+		if err := ctx.Err(); err != nil {
+			return nil, nil, err
+		}
+		data, err := fs.ReadFile(fsys, path)
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to read profile file %s: %w", filecatalog.DisplayPath(root, path), err)
+		}
+		fileMetadata, err := readProfileFileMetadata(data)
+		if err != nil {
+			return nil, nil, fmt.Errorf("%w: %s: %v", ErrInvalidYAML, filecatalog.DisplayPath(root, path), err)
+		}
+		if fileMetadata.hasRawAPIKey {
+			return nil, nil, fmt.Errorf("%w: %s", ErrRawAPIKeyNotAllowed, filecatalog.DisplayPath(root, path))
+		}
+		definition, err := decodeProfile(data)
+		if err != nil {
+			return nil, nil, fmt.Errorf("%w: %s: %v", ErrInvalidYAML, filecatalog.DisplayPath(root, path), err)
+		}
+		definition.ExtraParams, err = jsonvalue.CopyMap(definition.ExtraParams)
+		if err != nil {
+			return nil, nil, fmt.Errorf("%w: %s: %v", ErrInvalidProfile, filecatalog.DisplayPath(root, path), err)
+		}
+		if err := NormalizeAndValidateDefinition(definition); err != nil {
+			return nil, nil, fmt.Errorf("%w: %s: %v", ErrInvalidProfile, filecatalog.DisplayPath(root, path), err)
+		}
+		if _, exists := repository.profiles[definition.ID]; exists {
+			return nil, nil, fmt.Errorf("%w: duplicate execution profile id %q", ErrInvalidProfile, definition.ID)
+		}
+		repository.profiles[definition.ID] = *definition
+		fields := append([]string(nil), fileMetadata.explicitFields...)
+		sort.Strings(fields)
+		metadata = append(metadata, LoadedProfileMetadata{
+			ID:             definition.ID,
+			Path:           filecatalog.DisplayPath(root, path),
+			ExplicitFields: fields,
+		})
+	}
+	sort.Slice(metadata, func(left, right int) bool { return metadata[left].ID < metadata[right].ID })
+	return repository, metadata, nil
+}
+
+type loadedRepository struct {
+	profiles map[string]domain.ExecutionProfile
+}
+
+func (r *loadedRepository) GetProfile(ctx context.Context, id string) (*domain.ExecutionProfile, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	definition, found := r.profiles[strings.TrimSpace(id)]
+	if !found {
+		return nil, ErrProfileNotFound
+	}
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
+	if err != nil {
+		return nil, fmt.Errorf("copy loaded profile %q: %w", definition.ID, err)
+	}
+	definition.ExtraParams = extraParams
+	return &definition, nil
+}
diff --git a/internal/profile/eager_repository_test.go b/internal/profile/eager_repository_test.go
new file mode 100644
index 0000000..0b5ba60
--- /dev/null
+++ b/internal/profile/eager_repository_test.go
@@ -0,0 +1,110 @@
+package profile
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"reflect"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLoadFSRepositoryLoadsSortedIndependentProfiles(t *testing.T) {
+	fsys := fstest.MapFS{
+		"profiles/z.yml": {Data: []byte("id: z\nbackend: local\nmodel: z-model\nextra_params:\n  nested:\n    value: one\n")},
+		"profiles/a.yml": {Data: []byte("id: a\nbackend: local\nmodel: a-model\nendpoint: ''\n")},
+	}
+
+	repository, metadata, err := LoadFSRepository(context.Background(), fsys, "profiles")
+	if err != nil {
+		t.Fatalf("load repository: %v", err)
+	}
+	if len(metadata) != 2 || metadata[0].ID != "a" || metadata[1].ID != "z" || metadata[0].Path != "a.yml" {
+		t.Fatalf("unexpected metadata: %#v", metadata)
+	}
+	if len(metadata[0].ExplicitFields) != 4 || metadata[0].ExplicitFields[0] != "backend" || metadata[0].ExplicitFields[1] != "endpoint" {
+		t.Fatalf("expected explicitly empty endpoint metadata, got %#v", metadata[0].ExplicitFields)
+	}
+	metadata[1].ExplicitFields[0] = "changed"
+	first, err := repository.GetProfile(context.Background(), "z")
+	if err != nil {
+		t.Fatalf("load profile: %v", err)
+	}
+	first.ExtraParams["nested"].(map[string]any)["value"] = "changed"
+	second, err := repository.GetProfile(context.Background(), "z")
+	if err != nil {
+		t.Fatalf("reload profile: %v", err)
+	}
+	if second.ExtraParams["nested"].(map[string]any)["value"] != "one" {
+		t.Fatalf("profile value was not defensively copied: %#v", second)
+	}
+}
+
+func TestLoadFSRepositoryRejectsInvalidProfiles(t *testing.T) {
+	tests := map[string]fstest.MapFS{
+		"duplicate IDs": {
+			"profiles/one.yml": {Data: []byte("id: duplicate\nbackend: local\nmodel: one\n")},
+			"profiles/two.yml": {Data: []byte("id: duplicate\nbackend: local\nmodel: two\n")},
+		},
+		"raw API key": {
+			"profiles/one.yml": {Data: []byte("id: one\nbackend: local\nmodel: one\napi_key: forbidden\n")},
+		},
+		"multiple documents": {
+			"profiles/one.yml": {Data: []byte("id: one\nbackend: local\nmodel: one\n---\nid: two\n")},
+		},
+	}
+	for name, fsys := range tests {
+		t.Run(name, func(t *testing.T) {
+			_, _, err := LoadFSRepository(context.Background(), fsys, "profiles")
+			if err == nil {
+				t.Fatal("expected load failure")
+			}
+		})
+	}
+}
+
+func TestLoadedRepositoryHonorsCancellationAndMissingProfiles(t *testing.T) {
+	repository, _, err := LoadFSRepository(context.Background(), fstest.MapFS{
+		"profiles/one.yml": {Data: []byte("id: one\nbackend: local\nmodel: one\n")},
+	}, "profiles")
+	if err != nil {
+		t.Fatalf("load repository: %v", err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if _, err := repository.GetProfile(ctx, "one"); !errors.Is(err, context.Canceled) {
+		t.Fatalf("expected cancellation, got %v", err)
+	}
+	if _, err := repository.GetProfile(context.Background(), "missing"); !errors.Is(err, ErrProfileNotFound) {
+		t.Fatalf("expected missing profile, got %v", err)
+	}
+}
+
+func TestLoadFSRepositoryMatchesPointLookupForRawProfiles(t *testing.T) {
+	fsys := fstest.MapFS{
+		"profiles/base.yml":    {Data: []byte("id: base\nbackend: local\nmodel: base-model\n")},
+		"profiles/derived.yml": {Data: []byte("id: derived\nbase_profile: base\nreasoning_effort: high\n")},
+	}
+	eager, _, err := LoadFSRepository(context.Background(), fsys, "profiles")
+	if err != nil {
+		t.Fatalf("load eager repository: %v", err)
+	}
+	pointLookup := NewFSRepository(fsys, "profiles")
+	for _, id := range []string{"base", "derived"} {
+		t.Run(id, func(t *testing.T) {
+			got, err := eager.GetProfile(context.Background(), id)
+			if err != nil {
+				t.Fatalf("load eager profile: %v", err)
+			}
+			want, err := pointLookup.GetProfile(context.Background(), id)
+			if err != nil {
+				t.Fatalf("load point-in-time profile: %v", err)
+			}
+			if !reflect.DeepEqual(got, want) {
+				t.Fatalf("raw profiles differ: got %#v, want %#v", got, want)
+			}
+		})
+	}
+}
+
+var _ fs.FS = fstest.MapFS{}
diff --git a/internal/profile/filesystem_repository.go b/internal/profile/filesystem_repository.go
index 83b1cd3..a93d79b 100644
--- a/internal/profile/filesystem_repository.go
+++ b/internal/profile/filesystem_repository.go
@@ -164,8 +164,9 @@ type profileMatch struct {
 }
 
 type profileFileMetadata struct {
-	ids          []string
-	hasRawAPIKey bool
+	ids            []string
+	hasRawAPIKey   bool
+	explicitFields []string
 }
 
 func readProfileFileMetadata(data []byte) (profileFileMetadata, error) {
@@ -206,6 +207,7 @@ func profileMetadataFromNode(node *yaml.Node) profileFileMetadata {
 	for i := 0; i+1 < len(mapping.Content); i += 2 {
 		key := mapping.Content[i]
 		value := mapping.Content[i+1]
+		metadata.explicitFields = append(metadata.explicitFields, key.Value)
 		switch key.Value {
 		case "id":
 			metadata.ids = append(metadata.ids, strings.TrimSpace(value.Value))
@@ -228,6 +230,7 @@ func (m profileFileMetadata) matchesID(id string) bool {
 func (m *profileFileMetadata) merge(other profileFileMetadata) {
 	m.ids = append(m.ids, other.ids...)
 	m.hasRawAPIKey = m.hasRawAPIKey || other.hasRawAPIKey
+	m.explicitFields = append(m.explicitFields, other.explicitFields...)
 }
 
 func decodeProfile(data []byte) (*domain.ExecutionProfile, error) {