diff --git a/cmd/add/module.go b/cmd/add/module.go
index bcc30496..d4425b2c 100644
--- a/cmd/add/module.go
+++ b/cmd/add/module.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
 
 	"github.com/codefly-dev/cli/cmd/common"
 	modulesync "github.com/codefly-dev/cli/cmd/sync"
@@ -27,13 +28,65 @@ var ModuleCmd = &cobra.Command{
 		if interactive {
 			return fmt.Errorf("interactive mode not implemented yet")
 		}
+		if moduleSource != "" {
+			return addReferencedModule(args[0])
+		}
 		return addModule(args[0])
 	},
 }
 
 var moduleAgentInput string
+var moduleSource string
 var moduleWithDefault bool
 
+// addReferencedModule registers a module by reference — pointing at an
+// out-of-repo directory — instead of vendoring a copy under modules/<name>/.
+// This lets a solution repo be the composition root that references the host
+// and runtime modules it does not own; `codefly run` boots them alongside
+// local modules because resolution flows through the reference's path override.
+func addReferencedModule(name string) error {
+	ctx, done := common.NewContext()
+	defer done()
+
+	workspace, err := common.LoadWorkspace(ctx)
+	if err != nil {
+		return fmt.Errorf("cannot load workspace: %w", err)
+	}
+	if workspace.ExistsModule(name) {
+		return fmt.Errorf("module <%s> already exists", name)
+	}
+
+	source, err := filepath.Abs(moduleSource)
+	if err != nil {
+		return fmt.Errorf("cannot resolve source path %q: %w", moduleSource, err)
+	}
+	mod, err := resources.LoadModuleFromDir(ctx, source)
+	if err != nil {
+		return fmt.Errorf("cannot load referenced module at %s: %w", source, err)
+	}
+	if mod.Name != name {
+		return fmt.Errorf("referenced module at %s is named <%s>, not <%s>", source, mod.Name, name)
+	}
+
+	// Store a workspace-relative override when the source lives inside the
+	// workspace so the reference stays portable; fall back to the absolute
+	// path for the out-of-repo case, which is the whole point of a reference.
+	stored := source
+	if rel, relErr := filepath.Rel(workspace.Dir(), source); relErr == nil && filepath.IsLocal(rel) {
+		stored = rel
+	}
+
+	if err := workspace.AddModuleReference(&resources.ModuleReference{Name: name, PathOverride: &stored}); err != nil {
+		return fmt.Errorf("cannot add module reference: %w", err)
+	}
+	if err := workspace.Save(ctx); err != nil {
+		return fmt.Errorf("cannot save workspace: %w", err)
+	}
+
+	cli.Header(2, "Referenced module <%s> added from %s.", name, source)
+	return nil
+}
+
 func addModule(name string) (result error) {
 	ctx, done := common.NewContext()
 	defer done()
@@ -149,5 +202,7 @@ func addModule(name string) (result error) {
 func init() {
 	ModuleCmd.PersistentFlags().BoolVarP(&interactive, "interactive", "i", false, "interactive mode")
 	ModuleCmd.Flags().StringVar(&moduleAgentInput, "agent", "", "Module template agent (e.g. user-management, rag)")
+	ModuleCmd.Flags().StringVar(&moduleSource, "source", "", "Reference an existing out-of-repo module by path instead of vendoring a copy")
+	ModuleCmd.MarkFlagsMutuallyExclusive("agent", "source")
 	ModuleCmd.Flags().BoolVar(&moduleWithDefault, "yes", false, "Skip confirmation prompts (non-interactive/MCP mode)")
 }
diff --git a/cmd/add/module_test.go b/cmd/add/module_test.go
index ac240cbe..7397c3f6 100644
--- a/cmd/add/module_test.go
+++ b/cmd/add/module_test.go
@@ -175,6 +175,77 @@ func TestAddModuleRollsBackScaffoldWhoseBytesDoNotMatchPinnedSource(t *testing.T
 	}
 }
 
+func TestAddReferencedModuleRegistersPathWithoutVendoring(t *testing.T) {
+	source := t.TempDir()
+	writeAddTestFile(t, filepath.Join(source, "module.codefly.yaml"), "kind: module\nname: host\nservices:\n  - name: api\n")
+	writeAddTestFile(t, filepath.Join(source, "services", "api", "service.codefly.yaml"),
+		"name: api\nversion: 0.0.0\nagent:\n  kind: codefly:service\n  name: fixture\n  publisher: codefly.dev\n  version: 1.0.0\nendpoints: []\n")
+
+	root := t.TempDir()
+	workspace := &resources.Workspace{Name: "solution", Layout: resources.LayoutKindModules}
+	if err := workspace.SaveToDirUnsafe(context.Background(), root); err != nil {
+		t.Fatal(err)
+	}
+	t.Chdir(root)
+
+	previousSource, previousDefault := moduleSource, moduleWithDefault
+	moduleSource, moduleWithDefault = source, true
+	defer func() { moduleSource, moduleWithDefault = previousSource, previousDefault }()
+
+	if err := addReferencedModule("host"); err != nil {
+		t.Fatal(err)
+	}
+
+	// No vendored copy is materialized.
+	if _, err := os.Stat(filepath.Join(root, "modules", "host")); !os.IsNotExist(err) {
+		t.Fatalf("referenced module was vendored: %v", err)
+	}
+
+	reloaded, err := resources.FindWorkspaceUp(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reloaded.ExistsModule("host") {
+		t.Fatal("referenced module was not registered")
+	}
+	// The reference resolves back to the out-of-repo source, which is what
+	// `codefly run` relies on to boot the module.
+	mod, err := reloaded.LoadModuleFromName(context.Background(), "host")
+	if err != nil {
+		t.Fatalf("referenced module does not resolve: %v", err)
+	}
+	if mod.Dir() != source {
+		t.Fatalf("resolved dir = %s, want %s", mod.Dir(), source)
+	}
+}
+
+func TestAddReferencedModuleRejectsNameMismatch(t *testing.T) {
+	source := t.TempDir()
+	writeAddTestFile(t, filepath.Join(source, "module.codefly.yaml"), "kind: module\nname: host\nservices: []\n")
+
+	root := t.TempDir()
+	workspace := &resources.Workspace{Name: "solution", Layout: resources.LayoutKindModules}
+	if err := workspace.SaveToDirUnsafe(context.Background(), root); err != nil {
+		t.Fatal(err)
+	}
+	t.Chdir(root)
+
+	previousSource, previousDefault := moduleSource, moduleWithDefault
+	moduleSource, moduleWithDefault = source, true
+	defer func() { moduleSource, moduleWithDefault = previousSource, previousDefault }()
+
+	if err := addReferencedModule("runtime"); err == nil {
+		t.Fatal("referencing a module under a mismatched name returned success")
+	}
+	reloaded, err := resources.FindWorkspaceUp(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if reloaded.ExistsModule("runtime") {
+		t.Fatal("mismatched reference was registered")
+	}
+}
+
 func TestResourceCommandsReturnErrorsThroughCobra(t *testing.T) {
 	for _, command := range []*cobra.Command{
 		ModuleCmd,
diff --git a/cmd/doctor_workspace.go b/cmd/doctor_workspace.go
index 5738099e..3c90af15 100644
--- a/cmd/doctor_workspace.go
+++ b/cmd/doctor_workspace.go
@@ -42,6 +42,7 @@ const (
 	codeProviderResolutionFailed  = "provider_resolution_failed"
 	codePlaintextNotAllowed       = "plaintext_not_allowed"
 	codeReferenceSchemeUnknown    = "reference_scheme_unknown"
+	codeModuleReferenceUnresolved = "module_reference_unresolved"
 	codeTimeout                   = "timeout"
 )
 
@@ -112,6 +113,8 @@ func workspaceReadiness(ctx context.Context, opts workspaceReadinessOptions) *wo
 		return report
 	}
 
+	checkReferencedModules(ctx, ws, report)
+
 	env := checkEnvironment(ws, opts.env, report)
 	if env == nil {
 		return report
@@ -194,6 +197,28 @@ func checkWorkspace(ctx context.Context, opts workspaceReadinessOptions, report
 	return ws
 }
 
+// checkReferencedModules reports every module declared by reference — a `path:`
+// override pointing outside the vendored modules/<name>/ layout — and flags any
+// whose target cannot be loaded. Without it an unresolved reference only
+// surfaces later as an opaque "cannot load workspace services" failure; here the
+// module and its resolved path are named directly. Vendored modules (no path
+// override) and the implicit flat-layout module are skipped.
+func checkReferencedModules(ctx context.Context, ws *resources.Workspace, report *workspaceReadinessReport) {
+	for _, ref := range ws.Modules {
+		if ref.PathOverride == nil {
+			continue
+		}
+		resolved := ws.ModulePath(ctx, ref)
+		if _, err := resources.LoadModuleFromDir(ctx, resolved); err != nil {
+			report.add(codeModuleReferenceUnresolved, "referenced module "+ref.Name, "fail",
+				fmt.Sprintf("referenced module %q does not resolve at %s: %v", ref.Name, resolved, err),
+				fmt.Sprintf("fix the `path:` of module %q in %s, or vendor it with `codefly sync module`", ref.Name, resources.WorkspaceConfigurationName))
+			continue
+		}
+		report.add("", "referenced module "+ref.Name, "ok", fmt.Sprintf("%s → %s", ref.Name, resolved), "")
+	}
+}
+
 func checkEnvironment(ws *resources.Workspace, name string, report *workspaceReadinessReport) *resources.Environment {
 	env := ws.FindEnvironment(name)
 	if env == nil {
@@ -737,11 +762,12 @@ With --json, a versioned report is printed to stdout:
    remediation?}]}
 
 Stable diagnostic codes: workspace_not_found, workspace_invalid,
-environment_not_found, service_not_found, configuration_directory_missing,
-configuration_missing, configuration_invalid, configuration_duplicate,
-provider_not_configured, provider_executable_missing,
-provider_authentication_required, provider_resolution_failed,
-plaintext_not_allowed, reference_scheme_unknown, timeout. External provider
+environment_not_found, service_not_found, module_reference_unresolved,
+configuration_directory_missing, configuration_missing,
+configuration_invalid, configuration_duplicate, provider_not_configured,
+provider_executable_missing, provider_authentication_required,
+provider_resolution_failed, plaintext_not_allowed, reference_scheme_unknown,
+timeout. External provider
 binding checks add external_provider.* codes (bindings_unreadable,
 bindings_schema_unknown, and the per-binding validation codes).`,
 	RunE: func(cmd *cobra.Command, _ []string) error {
diff --git a/cmd/doctor_workspace_test.go b/cmd/doctor_workspace_test.go
index ec9f0b69..f8060cb2 100644
--- a/cmd/doctor_workspace_test.go
+++ b/cmd/doctor_workspace_test.go
@@ -145,6 +145,38 @@ func TestDoctorWorkspaceOutsideWorkspace(t *testing.T) {
 	requireCode(t, report, codeWorkspaceNotFound, "fail")
 }
 
+func TestDoctorWorkspaceReportsResolvedReferencedModule(t *testing.T) {
+	source := t.TempDir()
+	if err := os.WriteFile(filepath.Join(source, "module.codefly.yaml"), []byte("kind: module\nname: host\nservices: []\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	dir := writeTestWorkspace(t, map[string]string{
+		"workspace.codefly.yaml": "name: solution\nlayout: modules\nmodules:\n    - name: host\n      path: " + source + "\n",
+	})
+	report := runReadiness(t, workspaceReadinessOptions{dir: dir})
+	requireNoCode(t, report, codeModuleReferenceUnresolved)
+	found := false
+	for _, d := range report.Checks {
+		if d.Name == "referenced module host" && d.Status == "ok" {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatalf("resolved referenced module not reported: %s", reportJSON(t, report))
+	}
+}
+
+func TestDoctorWorkspaceFlagsUnresolvedReferencedModule(t *testing.T) {
+	dir := writeTestWorkspace(t, map[string]string{
+		"workspace.codefly.yaml": "name: solution\nlayout: modules\nmodules:\n    - name: host\n      path: /nonexistent/host\n",
+	})
+	report := runReadiness(t, workspaceReadinessOptions{dir: dir})
+	requireCode(t, report, codeModuleReferenceUnresolved, "fail")
+	if report.Status != readinessStatusNotReady {
+		t.Fatalf("status = %q, want not_ready", report.Status)
+	}
+}
+
 func TestDoctorWorkspaceMalformedWorkspace(t *testing.T) {
 	dir := writeTestWorkspace(t, map[string]string{
 		"workspace.codefly.yaml": "name: [unclosed\n  bad yaml::\n",
diff --git a/docs/commands.md b/docs/commands.md
index bc19e50c..ce27145a 100644
--- a/docs/commands.md
+++ b/docs/commands.md
@@ -307,6 +307,7 @@ Add resources to the workspace.
 ```bash
 codefly add module backend                                     # Add a module
 codefly add module saas --agent=saas-starter                   # Scaffold and pin a module template
+codefly add module host --source=../saas-host/modules/host     # Reference an out-of-repo module (no vendored copy)
 codefly add service api --agent=go-grpc                        # Add a service with an agent
 codefly add service-dependency api --dependency=backend/db     # Add a service dependency
 codefly add library utils                                      # Add a library
@@ -323,6 +324,15 @@ behind. Inventory-only scaffolds may omit the base manifest and service code;
 their first `sync module` treats the missing manifest as an empty base and
 populates the pinned source without rerunning the agent.
 
+`add module --source <path>` declares a module **by reference** rather than
+vendoring a copy: the workspace entry records a `path:` to an out-of-repo module
+directory, and `codefly run` boots it alongside local modules. This is the
+composition mode for multi-repo solutions (a solution repo referencing the host
+and runtime modules it does not own); it is distinct from `sync module`, which
+vendors a hash-pinned base. `codefly doctor workspace` reports each referenced
+module and flags an unresolved reference with the `module_reference_unresolved`
+diagnostic.
+
 **`add service` flags:**
 
 | Flag | Description |