diff --git a/server/src/__tests__/adapter-routes.test.ts b/server/src/__tests__/adapter-routes.test.ts
index b90869e6..ccc0edcb 100644
--- a/server/src/__tests__/adapter-routes.test.ts
+++ b/server/src/__tests__/adapter-routes.test.ts
@@ -46,6 +46,7 @@ const overridingConfigSchemaAdapter: ServerAdapterModule = {
 let registerServerAdapter: typeof import("../adapters/registry.js").registerServerAdapter;
 let unregisterServerAdapter: typeof import("../adapters/registry.js").unregisterServerAdapter;
 let findServerAdapter: typeof import("../adapters/registry.js").findServerAdapter;
+let findActiveServerAdapter: typeof import("../adapters/registry.js").findActiveServerAdapter;
 let setOverridePaused: typeof import("../adapters/registry.js").setOverridePaused;
 let adapterRoutes: typeof import("../routes/adapters.js").adapterRoutes;
 let errorHandler: typeof import("../middleware/index.js").errorHandler;
@@ -109,6 +110,7 @@ describe("adapter routes", () => {
     registerServerAdapter = registry.registerServerAdapter;
     unregisterServerAdapter = registry.unregisterServerAdapter;
     findServerAdapter = registry.findServerAdapter;
+    findActiveServerAdapter = registry.findActiveServerAdapter;
     setOverridePaused = registry.setOverridePaused;
     adapterRoutes = routes.adapterRoutes;
     errorHandler = middleware.errorHandler;
@@ -339,4 +341,54 @@ describe("adapter routes", () => {
 
     unregisterServerAdapter(HOT_INSTALL_TYPE);
   });
+
+  it("POST /api/adapters/install allows an external adapter to override a builtin type", async () => {
+    const builtin = findServerAdapter("codex_local");
+    expect(builtin).not.toBeNull();
+
+    const externalModule: ServerAdapterModule = {
+      type: "codex_local",
+      execute: async () => ({ exitCode: 0, signal: null, timedOut: false }),
+      testEnvironment: async () => ({
+        adapterType: "codex_local",
+        status: "pass",
+        checks: [],
+        testedAt: new Date(0).toISOString(),
+      }),
+      models: [{ id: "plugin-codex", label: "Plugin Codex" }],
+    };
+    mockPluginLoader.loadExternalAdapterPackage.mockResolvedValue(externalModule);
+
+    const app = createApp({ isInstanceAdmin: true });
+    const res = await request(app)
+      .post("/api/adapters/install")
+      .send({ packageName: "/tmp/fake-codex-override", isLocalPath: true });
+
+    expect(res.status, JSON.stringify(res.body)).toBe(201);
+    expect(res.body.type).toBe("codex_local");
+    const registeredOverride = findServerAdapter("codex_local");
+    expect(registeredOverride).toMatchObject({
+      type: "codex_local",
+      models: [{ id: "plugin-codex", label: "Plugin Codex" }],
+    });
+
+    setOverridePaused("codex_local", true);
+    expect(findActiveServerAdapter("codex_local")).toBe(builtin);
+
+    mockAdapterPluginStore.getAdapterPluginByType.mockReturnValue({
+      type: "codex_local",
+      packageName: undefined,
+      localPath: "/tmp/fake-codex-override",
+      installedAt: new Date(0).toISOString(),
+    });
+    mockAdapterPluginStore.removeAdapterPlugin.mockReturnValue(true);
+
+    const removed = await request(app).delete("/api/adapters/codex_local");
+    expect(removed.status, JSON.stringify(removed.body)).toBe(200);
+    expect(removed.body).toMatchObject({ type: "codex_local", removed: true });
+
+    unregisterServerAdapter("codex_local");
+    expect(findServerAdapter("codex_local")).toBe(builtin);
+    setOverridePaused("codex_local", false);
+  });
 });
diff --git a/server/src/routes/adapters.ts b/server/src/routes/adapters.ts
index afdc6762..55d766f4 100644
--- a/server/src/routes/adapters.ts
+++ b/server/src/routes/adapters.ts
@@ -296,17 +296,16 @@ export function adapterRoutes() {
       // Load and register the adapter (use canonicalName for path resolution)
       const adapterModule = await loadExternalAdapterPackage(canonicalName, moduleLocalPath);
 
-      // Check if this type conflicts with a built-in adapter
-      if (BUILTIN_ADAPTER_TYPES.has(adapterModule.type)) {
-        res.status(409).json({
-          error: `Adapter type "${adapterModule.type}" is a built-in adapter and cannot be overwritten.`,
-        });
-        return;
-      }
+      // External adapters may intentionally override built-in adapter types.
+      // registerServerAdapter preserves the built-in as a fallback so pausing or
+      // removing the override restores the original implementation.
 
-      // Check if already registered (indicates a reinstall/update)
+      // Check if already registered (indicates a reinstall/update).
+      // For built-in types the registry always returns the built-in, so we
+      // additionally require an existing external plugin record to
+      // distinguish a true reinstall from a first-time override.
       const existing = findServerAdapter(adapterModule.type);
-      const isReinstall = existing !== null;
+      const isReinstall = existing !== null && !!getAdapterPluginByType(adapterModule.type);
       if (existing) {
         unregisterServerAdapter(adapterModule.type);
         logger.info({ type: adapterModule.type }, "Unregistered existing adapter for replacement");
@@ -446,8 +445,9 @@ export function adapterRoutes() {
       return;
     }
 
-    // Prevent removal of built-in adapters
-    if (BUILTIN_ADAPTER_TYPES.has(adapterType)) {
+    // Prevent removal of built-in adapters, unless this built-in type is
+    // currently backed by an external adapter plugin override.
+    if (BUILTIN_ADAPTER_TYPES.has(adapterType) && !getAdapterPluginByType(adapterType)) {
       res.status(403).json({
         error: `Cannot remove built-in adapter "${adapterType}".`,
       });