diff --git a/workspace-server/internal/handlers/org_external.go b/workspace-server/internal/handlers/org_external.go
new file mode 100644
index 00000000..a5211264
--- /dev/null
+++ b/workspace-server/internal/handlers/org_external.go
@@ -0,0 +1,360 @@
+package handlers
+
+import (
+	"context"
+	"crypto/sha1"
+	"encoding/hex"
+	"fmt"
+	"net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+// External-ref resolver — gitops-style cross-repo subtree composition.
+// Internal#77 RFC, Phase 3a (task #222). Prior art: Helm subcharts +
+// dependency cache, Kustomize remote bases, Terraform module sources.
+//
+// Schema (a `!external`-tagged mapping anywhere a workspace entry is
+// allowed — workspaces:, roots:, children:):
+//
+//   - !external
+//     repo: molecule-ai/molecule-dev-department
+//     ref: main
+//     path: dev-lead/workspace.yaml
+//
+// At resolve time, the platform fetches the repo at ref into a content-
+// addressable cache under <rootDir>/.external-cache/<repo>/<sha>/, loads
+// the yaml at <cacheDir>/<path>, rewrites every files_dir + relative
+// !include path to be cache-prefixed, then grafts the result in place of
+// the !external node. Downstream pipeline (resolveInsideRoot, plugin
+// merge, CopyTemplateToContainer) sees ordinary in-tree paths.
+
+// ExternalRef is the deserialized form of an `!external`-tagged mapping.
+type ExternalRef struct {
+	Repo string `yaml:"repo"`
+	Ref  string `yaml:"ref"`
+	Path string `yaml:"path"`
+
+	// URL overrides the default Gitea host. Optional; defaults to
+	// MOLECULE_EXTERNAL_GITEA_URL env or git.moleculesai.app.
+	URL string `yaml:"url,omitempty"`
+}
+
+const (
+	// maxExternalDepth caps recursion through nested `!external`s. Lower
+	// than maxIncludeDepth (16) because each level may issue a network
+	// fetch. Composition that genuinely needs >4 layers is a smell.
+	maxExternalDepth = 4
+
+	// externalCacheDirName is the per-template cache subdir under rootDir.
+	// Content-addressable: keyed by (repo, sha). Operators add this to
+	// .gitignore — cache is platform-mutated, not source-tracked.
+	externalCacheDirName = ".external-cache"
+
+	// gitFetchTimeout caps a single clone operation. Conservative —
+	// org template fetches are typically <100KB.
+	gitFetchTimeout = 60 * time.Second
+)
+
+// safeRefPattern restricts `ref` values to characters git itself accepts
+// for branch / tag / SHA. Belt-and-braces over git's own validation.
+var safeRefPattern = regexp.MustCompile(`^[a-zA-Z0-9_./-]+$`)
+
+// allowlistedHostPath returns true if `<host>/<repo>` matches the
+// configured allowlist. Default allowlist: git.moleculesai.app/molecule-ai/.
+// Override via MOLECULE_EXTERNAL_REPO_ALLOWLIST env var (comma-separated
+// patterns). Patterns are matched as prefixes (with trailing-slash
+// semantics) or as exact matches. Trailing /* is treated as "any
+// descendants of this prefix".
+//
+// Examples:
+//   - "git.moleculesai.app/molecule-ai/" → matches molecule-ai/* (any repo)
+//   - "git.moleculesai.app/molecule-ai/*" → same; trailing /* normalized to /
+//   - "git.moleculesai.app/molecule-ai/molecule-dev-department" → exact
+//   - "git.moleculesai.app/" → matches everything on that host
+func allowlistedHostPath(host, repoPath string) bool {
+	allow := os.Getenv("MOLECULE_EXTERNAL_REPO_ALLOWLIST")
+	if allow == "" {
+		allow = "git.moleculesai.app/molecule-ai/"
+	}
+	hp := host + "/" + repoPath
+	for _, pat := range strings.Split(allow, ",") {
+		pat = strings.TrimSpace(pat)
+		if pat == "" {
+			continue
+		}
+		// Normalize trailing /* → /
+		pat = strings.TrimSuffix(pat, "*")
+		if pat == hp {
+			return true
+		}
+		if strings.HasSuffix(pat, "/") && strings.HasPrefix(hp+"/", pat) {
+			return true
+		}
+	}
+	return false
+}
+
+// externalFetcher abstracts the git-clone-into-cache step. Production
+// uses gitFetcher (shells out to git); tests inject a fake that
+// pre-stages content in a temp dir.
+type externalFetcher interface {
+	// Fetch ensures rootDir/.external-cache/<safe-repo>/<sha>/ contains
+	// the repo content at the given ref. Returns the absolute cache
+	// dir + the resolved SHA. Cache hit = no network. Cache miss =
+	// clone.
+	Fetch(ctx context.Context, rootDir, host, repoPath, ref string) (cacheDir, sha string, err error)
+}
+
+// defaultExternalFetcher is the package-level fetcher injection point.
+// Production code uses the git-shell fetcher; tests override via
+// SetExternalFetcherForTest.
+var defaultExternalFetcher externalFetcher = &gitFetcher{}
+
+// SetExternalFetcherForTest swaps the fetcher for testing. Returns a
+// cleanup func that restores the previous fetcher.
+func SetExternalFetcherForTest(f externalFetcher) func() {
+	prev := defaultExternalFetcher
+	defaultExternalFetcher = f
+	return func() { defaultExternalFetcher = prev }
+}
+
+// resolveExternalMapping replaces an `!external`-tagged mapping node
+// with the loaded + path-rewritten yaml content from the fetched repo.
+//
+// `currentDir` and `rootDir` are inherited from expandNode's resolve
+// frame. `visited` tracks (repo, sha, path) tuples for cycle detection
+// across nested externals.
+func resolveExternalMapping(n *yaml.Node, currentDir, rootDir string, visited map[string]bool, depth int) error {
+	if depth > maxExternalDepth {
+		return fmt.Errorf("!external: max depth %d exceeded (possible cycle)", maxExternalDepth)
+	}
+	if rootDir == "" {
+		return fmt.Errorf("!external at line %d requires a dir-based org template (no rootDir in inline-template mode)", n.Line)
+	}
+
+	var ref ExternalRef
+	if err := n.Decode(&ref); err != nil {
+		return fmt.Errorf("!external at line %d: decode: %w", n.Line, err)
+	}
+	if ref.Repo == "" || ref.Ref == "" || ref.Path == "" {
+		return fmt.Errorf("!external at line %d: repo, ref, path are all required (got %+v)", n.Line, ref)
+	}
+	if !safeRefPattern.MatchString(ref.Ref) {
+		return fmt.Errorf("!external at line %d: ref %q contains disallowed characters", n.Line, ref.Ref)
+	}
+	if strings.Contains(ref.Path, "..") || strings.HasPrefix(ref.Path, "/") {
+		return fmt.Errorf("!external at line %d: path %q must be relative-and-down-only", n.Line, ref.Path)
+	}
+
+	host := ref.URL
+	if host == "" {
+		host = os.Getenv("MOLECULE_EXTERNAL_GITEA_URL")
+	}
+	if host == "" {
+		host = "git.moleculesai.app"
+	}
+	host = strings.TrimPrefix(strings.TrimPrefix(host, "https://"), "http://")
+	host = strings.TrimSuffix(host, "/")
+
+	if !allowlistedHostPath(host, ref.Repo) {
+		return fmt.Errorf("!external at line %d: %s/%s not in MOLECULE_EXTERNAL_REPO_ALLOWLIST", n.Line, host, ref.Repo)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), gitFetchTimeout)
+	defer cancel()
+
+	cacheDir, sha, err := defaultExternalFetcher.Fetch(ctx, rootDir, host, ref.Repo, ref.Ref)
+	if err != nil {
+		return fmt.Errorf("!external at line %d: fetch %s/%s@%s: %w", n.Line, host, ref.Repo, ref.Ref, err)
+	}
+
+	// Cycle key: (repo, sha, path) — same external content reachable
+	// via two paths is fine, but a self-referential cycle isn't.
+	cycleKey := fmt.Sprintf("%s/%s@%s/%s", host, ref.Repo, sha, ref.Path)
+	if visited[cycleKey] {
+		return fmt.Errorf("!external cycle detected at %q (line %d)", cycleKey, n.Line)
+	}
+
+	// Validate path resolves inside the cache dir (anti-traversal).
+	yamlPathAbs, err := resolveInsideRoot(cacheDir, ref.Path)
+	if err != nil {
+		return fmt.Errorf("!external at line %d: path %q: %w", n.Line, ref.Path, err)
+	}
+	if _, err := os.Stat(yamlPathAbs); err != nil {
+		return fmt.Errorf("!external at line %d: %s/%s@%s does not contain %q: %w", n.Line, host, ref.Repo, sha, ref.Path, err)
+	}
+
+	data, err := os.ReadFile(yamlPathAbs)
+	if err != nil {
+		return fmt.Errorf("!external at line %d: read %q: %w", n.Line, yamlPathAbs, err)
+	}
+
+	var sub yaml.Node
+	if err := yaml.Unmarshal(data, &sub); err != nil {
+		return fmt.Errorf("!external at line %d: parse %q: %w", n.Line, yamlPathAbs, err)
+	}
+	root := &sub
+	if root.Kind == yaml.DocumentNode && len(root.Content) == 1 {
+		root = root.Content[0]
+	}
+
+	// Recurse FIRST: load all nested !include / !external content into
+	// the tree. Then rewrite ALL files_dir scalars in the fully-resolved
+	// tree (top + nested) with the cache prefix in one pass. Doing
+	// rewrite-before-recurse would leave nested-loaded files_dir paths
+	// unprefixed.
+	visited[cycleKey] = true
+	defer delete(visited, cycleKey)
+
+	subDir := filepath.Dir(yamlPathAbs)
+	if err := expandNode(root, subDir, rootDir, visited, depth+1); err != nil {
+		return err
+	}
+
+	// Path rewrite: prefix every files_dir scalar in the fully-resolved
+	// content with the cache-relative-from-rootDir prefix. After this
+	// pass, fetched workspaces look like ordinary in-tree workspaces.
+	cachePrefix, err := filepath.Rel(rootDir, cacheDir)
+	if err != nil {
+		return fmt.Errorf("!external at line %d: cannot compute cache prefix: %w", n.Line, err)
+	}
+	rewriteFilesDirAndIncludes(root, cachePrefix)
+
+	// Replace the !external mapping with the resolved content in-place.
+	*n = *root
+	if n.Tag == "!external" {
+		n.Tag = ""
+	}
+	return nil
+}
+
+// rewriteFilesDirAndIncludes walks the yaml node tree and prepends
+// cachePrefix to every files_dir scalar value. Idempotent: if a
+// files_dir value already starts with the prefix, no-op.
+//
+// !include paths are NOT rewritten. They resolve relative to their
+// containing file's directory (subDir in expandNode), and after fetch
+// that directory IS inside the cache, so relative paths Just Work
+// without any rewrite. Rewriting them would double-prefix.
+//
+// files_dir DOES need rewriting because it's consumed at workspace-
+// provisioning time relative to orgBaseDir (the parent template's root),
+// not relative to the workspace.yaml's containing dir.
+func rewriteFilesDirAndIncludes(n *yaml.Node, cachePrefix string) {
+	if n == nil {
+		return
+	}
+	if n.Kind == yaml.MappingNode {
+		for i := 0; i+1 < len(n.Content); i += 2 {
+			key, value := n.Content[i], n.Content[i+1]
+			if key.Kind == yaml.ScalarNode && key.Value == "files_dir" && value.Kind == yaml.ScalarNode {
+				if !strings.HasPrefix(value.Value, cachePrefix+string(filepath.Separator)) && value.Value != cachePrefix {
+					value.Value = filepath.Join(cachePrefix, value.Value)
+				}
+			}
+		}
+	}
+	for _, child := range n.Content {
+		rewriteFilesDirAndIncludes(child, cachePrefix)
+	}
+}
+
+// safeRepoCacheDir converts a repo path like "molecule-ai/foo" into a
+// filesystem-safe segment "molecule-ai__foo". Avoids nesting cache dirs
+// (which would complicate cleanup).
+func safeRepoCacheDir(host, repoPath string) string {
+	hp := host + "/" + repoPath
+	hp = strings.ReplaceAll(hp, "/", "__")
+	hp = strings.ReplaceAll(hp, ":", "_")
+	return hp
+}
+
+// gitFetcher is the production externalFetcher: shells out to `git` to
+// clone the repo at ref into the cache dir. Cache key includes the
+// resolved SHA, so different SHAs of the same ref get different cache
+// dirs (no overwrite).
+type gitFetcher struct{}
+
+// Fetch resolves ref → SHA via `git ls-remote`, then `git clone --depth=1`
+// if the cache dir is missing. Auth via MOLECULE_GITEA_TOKEN injected
+// into the URL.
+func (g *gitFetcher) Fetch(ctx context.Context, rootDir, host, repoPath, ref string) (string, string, error) {
+	cacheRoot := filepath.Join(rootDir, externalCacheDirName, safeRepoCacheDir(host, repoPath))
+	if err := os.MkdirAll(cacheRoot, 0o755); err != nil {
+		return "", "", fmt.Errorf("mkdir cache root: %w", err)
+	}
+
+	cloneURL := buildAuthedURL(host, repoPath)
+
+	// 1. Resolve ref → SHA (so cache dir is content-addressable).
+	sha, err := g.resolveRefToSHA(ctx, cloneURL, ref)
+	if err != nil {
+		return "", "", fmt.Errorf("ls-remote: %w", err)
+	}
+
+	cacheDir := filepath.Join(cacheRoot, sha)
+	if _, statErr := os.Stat(filepath.Join(cacheDir, ".git")); statErr == nil {
+		// Cache hit.
+		return cacheDir, sha, nil
+	}
+
+	// 2. Clone.
+	tmpDir := cacheDir + ".tmp." + shortHash(time.Now().String())
+	cmd := exec.CommandContext(ctx, "git", "clone", "--quiet", "--depth=1", "-b", ref, cloneURL, tmpDir)
+	cmd.Env = append(os.Environ(), "GIT_TERMINAL_PROMPT=0")
+	if out, err := cmd.CombinedOutput(); err != nil {
+		os.RemoveAll(tmpDir)
+		return "", "", fmt.Errorf("git clone: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+	// Atomic rename to final cache path.
+	if err := os.Rename(tmpDir, cacheDir); err != nil {
+		// Race: another import beat us to it. The other party's content
+		// is at the same SHA, so it's equivalent. Cleanup our tmp.
+		os.RemoveAll(tmpDir)
+		if _, statErr := os.Stat(filepath.Join(cacheDir, ".git")); statErr != nil {
+			return "", "", fmt.Errorf("rename clone to cache: %w", err)
+		}
+	}
+	return cacheDir, sha, nil
+}
+
+func (g *gitFetcher) resolveRefToSHA(ctx context.Context, cloneURL, ref string) (string, error) {
+	cmd := exec.CommandContext(ctx, "git", "ls-remote", cloneURL, ref)
+	cmd.Env = append(os.Environ(), "GIT_TERMINAL_PROMPT=0")
+	out, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	line := strings.TrimSpace(string(out))
+	if line == "" {
+		return "", fmt.Errorf("ref %q not found", ref)
+	}
+	// First whitespace-separated field is the SHA.
+	for i, ch := range line {
+		if ch == ' ' || ch == '\t' {
+			return line[:i], nil
+		}
+	}
+	return line, nil
+}
+
+func buildAuthedURL(host, repoPath string) string {
+	token := os.Getenv("MOLECULE_GITEA_TOKEN")
+	u := url.URL{Scheme: "https", Host: host, Path: "/" + repoPath + ".git"}
+	if token != "" {
+		u.User = url.UserPassword("oauth2", token)
+	}
+	return u.String()
+}
+
+func shortHash(s string) string {
+	h := sha1.Sum([]byte(s))
+	return hex.EncodeToString(h[:4])
+}
diff --git a/workspace-server/internal/handlers/org_external_test.go b/workspace-server/internal/handlers/org_external_test.go
new file mode 100644
index 00000000..33ed7c47
--- /dev/null
+++ b/workspace-server/internal/handlers/org_external_test.go
@@ -0,0 +1,331 @@
+package handlers
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"gopkg.in/yaml.v3"
+)
+
+// fakeFetcher pre-stages a "fetched" repo at a fixed path inside the
+// rootDir's .external-cache, bypassing the real git clone. Tests
+// inject this via SetExternalFetcherForTest to exercise the resolver
+// + path-rewrite logic without network.
+type fakeFetcher struct {
+	// content maps "<host>/<repo>@<ref>" → a function that materializes
+	// repo content under cacheDir. Returns the fake SHA to use.
+	content map[string]func(cacheDir string) (sha string, err error)
+}
+
+func (f *fakeFetcher) Fetch(ctx context.Context, rootDir, host, repoPath, ref string) (string, string, error) {
+	key := host + "/" + repoPath + "@" + ref
+	stage, ok := f.content[key]
+	if !ok {
+		return "", "", &fakeNotFoundError{key: key}
+	}
+	// Use a stable SHA for the test so cache dir is deterministic.
+	cacheDir := filepath.Join(rootDir, ".external-cache", safeRepoCacheDir(host, repoPath), "deadbeef")
+	if err := os.MkdirAll(cacheDir, 0o755); err != nil {
+		return "", "", err
+	}
+	sha, err := stage(cacheDir)
+	if err != nil {
+		return "", "", err
+	}
+	return cacheDir, sha, nil
+}
+
+type fakeNotFoundError struct{ key string }
+
+func (e *fakeNotFoundError) Error() string {
+	return "fake fetcher: no content registered for " + e.key
+}
+
+// stageFiles writes a map of relative-path → content into cacheDir,
+// returning a fake SHA. Helper for fakeFetcher closures.
+func stageFiles(cacheDir string, files map[string]string) error {
+	if err := os.MkdirAll(filepath.Join(cacheDir, ".git"), 0o755); err != nil {
+		return err
+	}
+	for path, content := range files {
+		full := filepath.Join(cacheDir, path)
+		if err := os.MkdirAll(filepath.Dir(full), 0o755); err != nil {
+			return err
+		}
+		if err := os.WriteFile(full, []byte(content), 0o644); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// TestResolveExternalMapping_HappyPath: a parent template with an
+// !external entry resolves cleanly into the fetched workspace + path-
+// rewrites files_dir + relative !include refs into the cache prefix.
+func TestResolveExternalMapping_HappyPath(t *testing.T) {
+	tmp := t.TempDir()
+
+	// Stub fetcher: "fetched" content has a workspace.yaml that uses
+	// files_dir + nested !include relative to the fetched repo's root.
+	fake := &fakeFetcher{
+		content: map[string]func(string) (string, error){
+			"git.moleculesai.app/molecule-ai/molecule-dev-department@main": func(cacheDir string) (string, error) {
+				return "deadbeef", stageFiles(cacheDir, map[string]string{
+					"dev-lead/workspace.yaml": `name: Dev Lead
+files_dir: dev-lead
+children:
+  - !include ./core-lead/workspace.yaml
+`,
+					"dev-lead/core-lead/workspace.yaml": `name: Core Platform Lead
+files_dir: dev-lead/core-lead
+`,
+				})
+			},
+		},
+	}
+	cleanup := SetExternalFetcherForTest(fake)
+	defer cleanup()
+
+	src := []byte(`name: Parent
+workspaces:
+  - !external
+    repo: molecule-ai/molecule-dev-department
+    ref: main
+    path: dev-lead/workspace.yaml
+`)
+
+	out, err := resolveYAMLIncludes(src, tmp)
+	if err != nil {
+		t.Fatalf("resolveYAMLIncludes: %v", err)
+	}
+
+	var tmpl OrgTemplate
+	if err := yaml.Unmarshal(out, &tmpl); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if len(tmpl.Workspaces) != 1 {
+		t.Fatalf("workspaces: %+v", tmpl.Workspaces)
+	}
+	dev := tmpl.Workspaces[0]
+	if dev.Name != "Dev Lead" {
+		t.Errorf("dev.Name = %q; want Dev Lead", dev.Name)
+	}
+	// files_dir should be cache-prefixed.
+	wantPrefix := filepath.Join(".external-cache", "git.moleculesai.app__molecule-ai__molecule-dev-department", "deadbeef")
+	if !strings.HasPrefix(dev.FilesDir, wantPrefix) {
+		t.Errorf("dev.FilesDir = %q; want prefix %q", dev.FilesDir, wantPrefix)
+	}
+	if !strings.HasSuffix(dev.FilesDir, "dev-lead") {
+		t.Errorf("dev.FilesDir = %q; want suffix dev-lead", dev.FilesDir)
+	}
+	// Nested child: files_dir cache-prefixed, name Core Platform Lead.
+	if len(dev.Children) != 1 {
+		t.Fatalf("dev.Children: %+v", dev.Children)
+	}
+	core := dev.Children[0]
+	if core.Name != "Core Platform Lead" {
+		t.Errorf("core.Name = %q; want Core Platform Lead", core.Name)
+	}
+	if !strings.HasPrefix(core.FilesDir, wantPrefix) {
+		t.Errorf("core.FilesDir = %q; want prefix %q", core.FilesDir, wantPrefix)
+	}
+	if !strings.HasSuffix(core.FilesDir, filepath.Join("dev-lead", "core-lead")) {
+		t.Errorf("core.FilesDir = %q; want suffix dev-lead/core-lead", core.FilesDir)
+	}
+}
+
+// TestResolveExternalMapping_AllowlistRejection: hostile yaml pointing
+// at a non-allowlisted repo gets rejected.
+func TestResolveExternalMapping_AllowlistRejection(t *testing.T) {
+	tmp := t.TempDir()
+	fake := &fakeFetcher{content: map[string]func(string) (string, error){}}
+	cleanup := SetExternalFetcherForTest(fake)
+	defer cleanup()
+
+	// Default allowlist is git.moleculesai.app/molecule-ai/*.
+	// github.com/foo/bar is NOT in it.
+	src := []byte(`workspaces:
+  - !external
+    repo: foo/bar
+    ref: main
+    path: x.yaml
+    url: github.com
+`)
+	_, err := resolveYAMLIncludes(src, tmp)
+	if err == nil {
+		t.Fatalf("expected allowlist rejection, got nil")
+	}
+	if !strings.Contains(err.Error(), "MOLECULE_EXTERNAL_REPO_ALLOWLIST") {
+		t.Errorf("expected allowlist error; got %v", err)
+	}
+}
+
+// TestResolveExternalMapping_PathTraversalRejection: hostile yaml
+// with `path: ../../etc/passwd` gets rejected before fetch.
+func TestResolveExternalMapping_PathTraversalRejection(t *testing.T) {
+	tmp := t.TempDir()
+	fake := &fakeFetcher{content: map[string]func(string) (string, error){}}
+	cleanup := SetExternalFetcherForTest(fake)
+	defer cleanup()
+
+	src := []byte(`workspaces:
+  - !external
+    repo: molecule-ai/dev-department
+    ref: main
+    path: ../../etc/passwd
+`)
+	_, err := resolveYAMLIncludes(src, tmp)
+	if err == nil {
+		t.Fatalf("expected path traversal rejection, got nil")
+	}
+	if !strings.Contains(err.Error(), "relative-and-down-only") {
+		t.Errorf("expected path traversal error; got %v", err)
+	}
+}
+
+// TestResolveExternalMapping_BadRefRejection: non-allowlisted ref chars.
+func TestResolveExternalMapping_BadRefRejection(t *testing.T) {
+	tmp := t.TempDir()
+	fake := &fakeFetcher{content: map[string]func(string) (string, error){}}
+	cleanup := SetExternalFetcherForTest(fake)
+	defer cleanup()
+
+	src := []byte(`workspaces:
+  - !external
+    repo: molecule-ai/dev-department
+    ref: "main; rm -rf /"
+    path: foo.yaml
+`)
+	_, err := resolveYAMLIncludes(src, tmp)
+	if err == nil || !strings.Contains(err.Error(), "disallowed characters") {
+		t.Errorf("expected ref-validation error; got %v", err)
+	}
+}
+
+// TestResolveExternalMapping_MissingRequiredFields: repo / ref / path
+// are all required.
+func TestResolveExternalMapping_MissingRequiredFields(t *testing.T) {
+	tmp := t.TempDir()
+	fake := &fakeFetcher{content: map[string]func(string) (string, error){}}
+	cleanup := SetExternalFetcherForTest(fake)
+	defer cleanup()
+
+	cases := []string{
+		// missing repo
+		`workspaces:
+  - !external
+    ref: main
+    path: x.yaml
+`,
+		// missing ref
+		`workspaces:
+  - !external
+    repo: molecule-ai/x
+    path: x.yaml
+`,
+		// missing path
+		`workspaces:
+  - !external
+    repo: molecule-ai/x
+    ref: main
+`,
+	}
+	for i, src := range cases {
+		_, err := resolveYAMLIncludes([]byte(src), tmp)
+		if err == nil {
+			t.Errorf("case %d: expected required-field error, got nil", i)
+		} else if !strings.Contains(err.Error(), "required") {
+			t.Errorf("case %d: want 'required' in error; got %v", i, err)
+		}
+	}
+}
+
+// TestRewriteFilesDirAndIncludes: verify the path-rewrite walker
+// prefixes files_dir scalars. !include scalars are NOT rewritten —
+// they resolve relative to their containing file's dir, which post-
+// fetch is naturally inside the cache.
+func TestRewriteFilesDirAndIncludes(t *testing.T) {
+	src := `name: Foo
+files_dir: dev-lead
+children:
+  - !include ./bar/workspace.yaml
+  - !include other-team.yaml
+inner:
+  files_dir: dev-lead/sub
+`
+	var n yaml.Node
+	if err := yaml.Unmarshal([]byte(src), &n); err != nil {
+		t.Fatal(err)
+	}
+	rewriteFilesDirAndIncludes(&n, ".external-cache/foo/bar")
+
+	out, err := yaml.Marshal(&n)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := string(out)
+	for _, want := range []string{
+		"files_dir: .external-cache/foo/bar/dev-lead",
+		"files_dir: .external-cache/foo/bar/dev-lead/sub",
+		// !include preserved as-is; resolves naturally via subDir.
+		"!include ./bar/workspace.yaml",
+		"!include other-team.yaml",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("missing %q in:\n%s", want, got)
+		}
+	}
+}
+
+// TestRewriteFilesDirAndIncludes_Idempotent: re-running the rewriter
+// on already-prefixed files_dir doesn't double-prefix.
+func TestRewriteFilesDirAndIncludes_Idempotent(t *testing.T) {
+	src := `files_dir: .external-cache/foo/bar/dev-lead
+inner:
+  files_dir: .external-cache/foo/bar/dev-lead/sub
+`
+	var n yaml.Node
+	if err := yaml.Unmarshal([]byte(src), &n); err != nil {
+		t.Fatal(err)
+	}
+	rewriteFilesDirAndIncludes(&n, ".external-cache/foo/bar")
+
+	out, _ := yaml.Marshal(&n)
+	got := string(out)
+	if strings.Contains(got, ".external-cache/foo/bar/.external-cache") {
+		t.Errorf("double-prefix detected:\n%s", got)
+	}
+	// Should still be valid (single-prefixed) afterwards.
+	for _, want := range []string{
+		"files_dir: .external-cache/foo/bar/dev-lead",
+		"files_dir: .external-cache/foo/bar/dev-lead/sub",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("expected unchanged %q in:\n%s", want, got)
+		}
+	}
+}
+
+// TestAllowlistedHostPath: env-var override + glob matching.
+func TestAllowlistedHostPath(t *testing.T) {
+	t.Setenv("MOLECULE_EXTERNAL_REPO_ALLOWLIST", "")
+	if !allowlistedHostPath("git.moleculesai.app", "molecule-ai/foo") {
+		t.Error("default allowlist should accept molecule-ai/*")
+	}
+	if allowlistedHostPath("github.com", "molecule-ai/foo") {
+		t.Error("default allowlist should reject github.com")
+	}
+	t.Setenv("MOLECULE_EXTERNAL_REPO_ALLOWLIST", "github.com/me/*,git.moleculesai.app/*")
+	if !allowlistedHostPath("github.com", "me/x") {
+		t.Error("override should accept github.com/me/*")
+	}
+	if !allowlistedHostPath("git.moleculesai.app", "any/repo") {
+		t.Error("override should accept git.moleculesai.app/*")
+	}
+	if allowlistedHostPath("github.com", "evil/x") {
+		t.Error("override should reject github.com/evil/*")
+	}
+}
diff --git a/workspace-server/internal/handlers/org_include.go b/workspace-server/internal/handlers/org_include.go
index c7ff5e1a..dc3bb7fa 100644
--- a/workspace-server/internal/handlers/org_include.go
+++ b/workspace-server/internal/handlers/org_include.go
@@ -76,6 +76,12 @@ func expandNode(n *yaml.Node, currentDir, rootDir string, visited map[string]boo
 		return resolveIncludeScalar(n, currentDir, rootDir, visited, depth)
 	}
 
+	// `!external`-tagged mapping: gitops cross-repo subtree composition.
+	// See org_external.go (internal#77 / task #222).
+	if n.Kind == yaml.MappingNode && n.Tag == "!external" {
+		return resolveExternalMapping(n, currentDir, rootDir, visited, depth)
+	}
+
 	for _, child := range n.Content {
 		if err := expandNode(child, currentDir, rootDir, visited, depth); err != nil {
 			return err