diff --git a/marketplace/builder.go b/marketplace/builder.go
index 448e9a8..6ff7fc6 100644
--- a/marketplace/builder.go
+++ b/marketplace/builder.go
@@ -22,6 +22,10 @@ const IndexVersion = 1
 // Builder constructs a marketplace Index by crawling directories for
 // core.json (compiled manifests) or .core/manifest.yaml files.
 type Builder struct {
+	// Medium is the filesystem abstraction used for manifest reads.
+	// If nil, io.Local is used.
+	Medium coreio.Medium
+
 	// BaseURL is the prefix for constructing repository URLs, e.g.
 	// "https://forge.lthn.ai". When set, module Repo is derived as
 	// BaseURL + "/" + org + "/" + code + ".git".
@@ -147,9 +151,14 @@ func WriteIndex(m coreio.Medium, path string, idx *Index) error {
 
 // loadFromDir tries core.json first, then falls back to .core/manifest.yaml.
 func (b *Builder) loadFromDir(dir string) (*manifest.Manifest, error) {
+	medium := b.Medium
+	if medium == nil {
+		medium = coreio.Local
+	}
+
 	// Prefer compiled manifest (core.json).
 	coreJSON := filepath.Join(dir, "core.json")
-	if raw, err := coreio.Local.Read(coreJSON); err == nil {
+	if raw, err := medium.Read(coreJSON); err == nil {
 		cm, err := manifest.ParseCompiled([]byte(raw))
 		if err != nil {
 			return nil, coreerr.E("marketplace.Builder.loadFromDir", "parse core.json", err)
@@ -159,7 +168,7 @@ func (b *Builder) loadFromDir(dir string) (*manifest.Manifest, error) {
 
 	// Fall back to source manifest.
 	manifestYAML := filepath.Join(dir, ".core", "manifest.yaml")
-	raw, err := coreio.Local.Read(manifestYAML)
+	raw, err := medium.Read(manifestYAML)
 	if err != nil {
 		return nil, nil // No manifest — skip silently.
 	}
diff --git a/marketplace/builder_test.go b/marketplace/builder_test.go
index 60dd2e5..abcc040 100644
--- a/marketplace/builder_test.go
+++ b/marketplace/builder_test.go
@@ -108,6 +108,34 @@ func TestBuildFromDirs_Good_CoreJSON_Good(t *testing.T) {
 	assert.Equal(t, "Compiled Module", idx.Modules[0].Name)
 }
 
+func TestBuildFromDirs_Good_UsesInjectedMedium_Good(t *testing.T) {
+	root := t.TempDir()
+	modDir := filepath.Join(root, "virtual-mod")
+	require.NoError(t, os.MkdirAll(modDir, 0755))
+
+	medium := io.NewMockMedium()
+	cm := manifest.CompiledManifest{
+		Manifest: manifest.Manifest{
+			Code:    "virtual-mod",
+			Name:    "Virtual Module",
+			Version: "9.9.9",
+			Sign:    "sig-virtual",
+		},
+		Commit: "commit-virtual",
+	}
+	data, err := json.Marshal(cm)
+	require.NoError(t, err)
+	require.NoError(t, medium.Write(filepath.Join(modDir, "core.json"), string(data)))
+
+	b := &Builder{Medium: medium}
+	idx, err := b.BuildFromDirs(root)
+	require.NoError(t, err)
+
+	require.Len(t, idx.Modules, 1)
+	assert.Equal(t, "virtual-mod", idx.Modules[0].Code)
+	assert.Equal(t, "sig-virtual", idx.Modules[0].SignKey)
+}
+
 func TestBuildFromDirs_Good_PrefersCompiledOverSource_Good(t *testing.T) {
 	root := t.TempDir()
 	modDir := filepath.Join(root, "dual-mod")
diff --git a/marketplace/discovery.go b/marketplace/discovery.go
index a2b3d84..0bb98aa 100644
--- a/marketplace/discovery.go
+++ b/marketplace/discovery.go
@@ -29,9 +29,16 @@ type DiscoveredProvider struct {
 // Only manifests with provider fields (namespace + binary) are returned.
 // Usage: DiscoverProviders(...)
 func DiscoverProviders(dir string) ([]DiscoveredProvider, error) {
-	entries, err := os.ReadDir(dir)
+	return DiscoverProvidersWithMedium(coreio.Local, dir)
+}
+
+// DiscoverProvidersWithMedium scans the given directory for runtime provider
+// manifests using the supplied filesystem medium.
+// Usage: DiscoverProvidersWithMedium(...)
+func DiscoverProvidersWithMedium(medium coreio.Medium, dir string) ([]DiscoveredProvider, error) {
+	entries, err := medium.List(dir)
 	if err != nil {
-		if os.IsNotExist(err) {
+		if !medium.Exists(dir) {
 			return nil, nil // No providers directory — not an error.
 		}
 		return nil, coreerr.E("marketplace.DiscoverProviders", "read directory", err)
@@ -46,7 +53,7 @@ func DiscoverProviders(dir string) ([]DiscoveredProvider, error) {
 		providerDir := filepath.Join(dir, e.Name())
 		manifestPath := filepath.Join(providerDir, ".core", "manifest.yaml")
 
-		raw, err := coreio.Local.Read(manifestPath)
+		raw, err := medium.Read(manifestPath)
 		if err != nil {
 			core.Warn(core.Sprintf("marketplace: skipping %s: %v", e.Name(), err))
 			continue
@@ -90,7 +97,14 @@ type ProviderRegistryFile struct {
 // Returns an empty registry if the file does not exist.
 // Usage: LoadProviderRegistry(...)
 func LoadProviderRegistry(path string) (*ProviderRegistryFile, error) {
-	raw, err := coreio.Local.Read(path)
+	return LoadProviderRegistryWithMedium(coreio.Local, path)
+}
+
+// LoadProviderRegistryWithMedium reads a registry.yaml file using the supplied
+// filesystem medium.
+// Usage: LoadProviderRegistryWithMedium(...)
+func LoadProviderRegistryWithMedium(medium coreio.Medium, path string) (*ProviderRegistryFile, error) {
+	raw, err := medium.Read(path)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return &ProviderRegistryFile{
@@ -116,7 +130,14 @@ func LoadProviderRegistry(path string) (*ProviderRegistryFile, error) {
 // SaveProviderRegistry writes the registry to the given path.
 // Usage: SaveProviderRegistry(...)
 func SaveProviderRegistry(path string, reg *ProviderRegistryFile) error {
-	if err := coreio.Local.EnsureDir(filepath.Dir(path)); err != nil {
+	return SaveProviderRegistryWithMedium(coreio.Local, path, reg)
+}
+
+// SaveProviderRegistryWithMedium writes the registry using the supplied
+// filesystem medium.
+// Usage: SaveProviderRegistryWithMedium(...)
+func SaveProviderRegistryWithMedium(medium coreio.Medium, path string, reg *ProviderRegistryFile) error {
+	if err := medium.EnsureDir(filepath.Dir(path)); err != nil {
 		return coreerr.E("marketplace.SaveProviderRegistry", "ensure directory", err)
 	}
 
@@ -125,7 +146,7 @@ func SaveProviderRegistry(path string, reg *ProviderRegistryFile) error {
 		return coreerr.E("marketplace.SaveProviderRegistry", "marshal failed", err)
 	}
 
-	return coreio.Local.Write(path, string(data))
+	return medium.Write(path, string(data))
 }
 
 // Add adds or updates a provider entry in the registry.
diff --git a/marketplace/discovery_test.go b/marketplace/discovery_test.go
index a08dfa8..42e8a2e 100644
--- a/marketplace/discovery_test.go
+++ b/marketplace/discovery_test.go
@@ -7,6 +7,7 @@ import (
 	os "dappco.re/go/core/scm/internal/ax/osx"
 	"testing"
 
+	"dappco.re/go/core/io"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -163,6 +164,31 @@ binary: ./test-prov
 	assert.Equal(t, filepath.Join(dir, "test-prov"), providers[0].Dir)
 }
 
+func TestDiscoverProvidersWithMedium_Good(t *testing.T) {
+	medium := io.NewMockMedium()
+	medium.Dirs["/providers"] = true
+	medium.Dirs["/providers/cool-widget"] = true
+	medium.Dirs["/providers/data-viz"] = true
+	medium.Files["/providers/cool-widget/.core/manifest.yaml"] = `
+code: cool-widget
+name: Cool Widget
+version: 1.0.0
+namespace: /api/v1/cool-widget
+binary: ./cool-widget
+`
+	medium.Files["/providers/data-viz/.core/manifest.yaml"] = `
+code: data-viz
+name: Data Visualiser
+version: 0.2.0
+namespace: /api/v1/data-viz
+binary: ./data-viz
+`
+
+	providers, err := DiscoverProvidersWithMedium(medium, "/providers")
+	require.NoError(t, err)
+	assert.Len(t, providers, 2)
+}
+
 // -- ProviderRegistryFile tests -----------------------------------------------
 
 func TestProviderRegistry_LoadSave_Good(t *testing.T) {
@@ -201,6 +227,30 @@ func TestProviderRegistry_Load_Good_NonexistentFile_Good(t *testing.T) {
 	assert.Empty(t, reg.Providers)
 }
 
+func TestProviderRegistry_LoadSave_WithMedium_Good(t *testing.T) {
+	medium := io.NewMockMedium()
+	path := "/registry/registry.yaml"
+
+	reg := &ProviderRegistryFile{
+		Version:   1,
+		Providers: map[string]ProviderRegistryEntry{},
+	}
+	reg.Add("cool-widget", ProviderRegistryEntry{
+		Installed: "2026-03-14T12:00:00Z",
+		Version:   "1.0.0",
+		Source:    "forge.lthn.ai/someone/cool-widget",
+		AutoStart: true,
+	})
+
+	err := SaveProviderRegistryWithMedium(medium, path, reg)
+	require.NoError(t, err)
+
+	loaded, err := LoadProviderRegistryWithMedium(medium, path)
+	require.NoError(t, err)
+	assert.Equal(t, 1, loaded.Version)
+	assert.Len(t, loaded.Providers, 1)
+}
+
 func TestProviderRegistry_Add_Good(t *testing.T) {
 	reg := &ProviderRegistryFile{
 		Version:   1,