diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f041e2d6..bbf9e302 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -194,6 +194,8 @@ jobs:
           node-version: 24.16.0
       - run: npm ci
       - run: npm run build
+      - name: Native environment containment tests
+        run: node --test dist/environment.test.js
       - name: Native ACL and credential lifecycle tests
         run: node --test dist/macos-storage.test.js dist/private-storage.test.js dist/storage.test.js dist/github.test.js
 
diff --git a/packages/code/src/environment.test.ts b/packages/code/src/environment.test.ts
index 9080f6a2..a8b5b036 100644
--- a/packages/code/src/environment.test.ts
+++ b/packages/code/src/environment.test.ts
@@ -193,6 +193,68 @@ test('environment roots resolve relative to the definition and fingerprints cove
     );
 });
 
+test('accepts an own root through trusted external symlinks without allowing other root identities', async t => {
+    const directory = await realpath(await mkdtemp(join(tmpdir(), 'code-env-own-alias-')));
+    t.after(() => rm(directory, { recursive: true, force: true }));
+    const root = join(directory, 'project');
+    await mkdir(root);
+    const alias = join(directory, 'alias');
+    await symlink(root, alias);
+    await symlink(alias, join(directory, 'nested-alias'));
+    const path = join(directory, 'environment.yaml');
+    for (const selected of [alias, join(directory, 'nested-alias')]) {
+        await writeFile(path, `name: app\nroot: ${selected}\n`);
+        const loaded = await loadCodeEnvironment(path);
+        assert.equal(loaded.definition.root, root);
+        await assertEnvironmentDefinitionsOutsideRoots([loaded], [{ id: 'app', root }]);
+        await assert.rejects(
+            assertEnvironmentDefinitionsOutsideRoots([loaded], [{ id: 'other', root }]),
+            /root traversal|mount alias/,
+        );
+    }
+});
+
+test('rejects own-root links hidden by parent aliases or filesystem casing', async t => {
+    const directory = await realpath(await mkdtemp(join(tmpdir(), 'code-env-parent-alias-')));
+    t.after(() => rm(directory, { recursive: true, force: true }));
+    const root = join(directory, 'Project');
+    await mkdir(root);
+    await symlink(root, join(root, 'self'));
+    const outside = join(directory, 'outside');
+    await mkdir(outside);
+    await symlink(root, join(outside, 'back'));
+    await symlink(outside, join(root, 'pivot'));
+    const alias = join(directory, 'parent-alias');
+    await symlink(root, alias);
+    const path = join(directory, 'environment.yaml');
+    const selectedRoots = [join(alias, 'self'), join(alias, 'pivot', 'back')];
+    try {
+        if (await realpath(join(directory, 'project')) === root) {
+            selectedRoots.push(join(directory, 'project', 'self'));
+            selectedRoots.push(join(directory, 'project', 'pivot', 'back'));
+        }
+    } catch (error) {
+        if ((error as NodeJS.ErrnoException).code !== 'ENOENT') throw error;
+    }
+    for (const selected of selectedRoots) {
+        await writeFile(path, `name: app\nroot: ${selected}\n`);
+        const loaded = await loadCodeEnvironment(path);
+        await assert.rejects(
+            assertEnvironmentDefinitionsOutsideRoots([loaded], [{ id: 'app', root }]),
+            /root traversal|mount alias/,
+        );
+    }
+    const definition = join(outside, 'environment.yaml');
+    await writeFile(definition, `name: app\nroot: ${root}\n`);
+    for (const selected of selectedRoots.filter(path => path.endsWith('/back'))) {
+        const loaded = await loadCodeEnvironment(selected.replace(/back$/, 'environment.yaml'));
+        await assert.rejects(
+            assertEnvironmentDefinitionsOutsideRoots([loaded], [{ id: 'app', root }]),
+            /outside|mount alias/,
+        );
+    }
+});
+
 test('rejects a trusted definition with an in-workspace hard link', async t => {
     const directory = await mkdtemp(join(tmpdir(), 'code-env-hardlink-'));
     t.after(() => rm(directory, { recursive: true, force: true }));
diff --git a/packages/code/src/environment.ts b/packages/code/src/environment.ts
index 700fc0dc..c2b2cf1f 100644
--- a/packages/code/src/environment.ts
+++ b/packages/code/src/environment.ts
@@ -355,6 +355,25 @@ export async function assertEnvironmentDefinitionsOutsideRoots(
         }
         return result;
     };
+    // The entry's parent, not its symlink target, determines who can replace it.
+    // Compare ancestor identities so casing and directory aliases cannot make a
+    // workspace-controlled entry look external on case-insensitive filesystems.
+    const canonicalParents = new Map<string, Promise<string>>();
+    const controlsEntry = async (component: string, rootIdentity: string): Promise<boolean> => {
+        const directory = dirname(component);
+        let canonical = canonicalParents.get(directory);
+        if (!canonical) {
+            canonical = realpath(directory);
+            canonicalParents.set(directory, canonical);
+        }
+        let parent = await canonical;
+        while (true) {
+            if ((await identity(parent)) === rootIdentity) return true;
+            const next = dirname(parent);
+            if (next === parent) return false;
+            parent = next;
+        }
+    };
     for (const environment of environments) {
         for (const root of roots) {
             const rootIdentity = await identity(root.root);
@@ -362,10 +381,14 @@ export async function assertEnvironmentDefinitionsOutsideRoots(
             {
                 for (const component of environment.rootPaths ?? []) {
                     const path = relative(root.root, component);
-                    if (path === '' && root.id === environment.definition.name)
+                    const sameRoot = (await identity(component)) === rootIdentity;
+                    const controlled = await controlsEntry(component, rootIdentity);
+                    // A trusted external alias may select its own root, but a
+                    // link beneath that root is still writable by the workspace.
+                    if (sameRoot && !controlled && root.id === environment.definition.name)
                         continue;
                     if (
-                        (await identity(component)) === rootIdentity ||
+                        controlled || sameRoot ||
                         path === '' ||
                         (!isAbsolute(path) &&
                             path !== '..' &&
@@ -383,6 +406,7 @@ export async function assertEnvironmentDefinitionsOutsideRoots(
             ]) {
                 const path = relative(root.root, controlPath);
                 if (
+                    (await controlsEntry(controlPath, rootIdentity)) ||
                     (await identity(controlPath)) === rootIdentity ||
                     path === '' ||
                     (!isAbsolute(path) &&