diff --git a/apps/server/src/mcp/toolkits/project/handlers.test.ts b/apps/server/src/mcp/toolkits/project/handlers.test.ts
index b77df3ac2075e..810436712a873 100644
--- a/apps/server/src/mcp/toolkits/project/handlers.test.ts
+++ b/apps/server/src/mcp/toolkits/project/handlers.test.ts
@@ -53,6 +53,7 @@ const clientLaunchHarness = (input: {
   readonly hostedAppUrl?: string;
   /** The orchestrator refuses the new thread's mode, as under a managed ceiling. */
   readonly refusedByCeiling?: true;
+  readonly isScratchProject?: true;
 }) => {
   const projectId = ProjectId.make("project:client-target");
   const modelSelection = {
@@ -159,7 +160,10 @@ const clientLaunchHarness = (input: {
         ),
     }),
     Layer.mock(PeerForwarding.PeerForwarding)({}),
-    Layer.mock(ManagedProjectFolders.ManagedProjectFolders)({ namedProjectsRoot: "/projects" }),
+    Layer.mock(ManagedProjectFolders.ManagedProjectFolders)({
+      namedProjectsRoot: "/projects",
+      isScratchProject: (id) => Effect.succeed(input.isScratchProject === true && id === projectId),
+    }),
     DocumentText.layer.pipe(Layer.provideMerge(NodeServices.layer)),
   ).pipe(
     Layer.provideMerge(GitVcsDriver.layer),
@@ -665,6 +669,41 @@ it.effect("deleting a project through MCP also discards its clone record", () =>
   }),
 );
 
+it.effect("a forced MCP delete refuses the built-in No project and keeps its threads", () =>
+  Effect.gen(function* () {
+    const deleted: Array<ProjectId> = [];
+    const harness = clientLaunchHarness({
+      runtimeModeCeiling: "full-access",
+      launched: [],
+      isScratchProject: true,
+    });
+    const dependencies = Layer.merge(
+      harness.dependencies,
+      Layer.mergeAll(
+        Layer.mock(Project.ProjectService)({
+          delete: (input) =>
+            Effect.sync(() => void deleted.push(input.projectId)).pipe(
+              Effect.andThen(Effect.die("deleted")),
+            ),
+        }),
+        Layer.mock(ProjectCloneTracker.ProjectCloneTracker)({}),
+      ),
+    );
+    const toolkit = yield* ProjectToolkit.pipe(
+      Effect.provide(
+        McpToolAccess.HandlersLayer.layer(ProjectHandlers.layer).pipe(Layer.provide(dependencies)),
+      ),
+    );
+
+    const refused = yield* toolkit
+      .handle("t3_project_delete", { projectId: harness.projectId, force: true })
+      .pipe(Stream.unwrap, Stream.runCollect, Effect.provide(dependencies));
+
+    expect(refused.at(-1)?.result).toMatchObject({ code: "invalid_request" });
+    expect(deleted).toEqual([]);
+  }),
+);
+
 it.effect("an update the project refuses tells the agent why", () =>
   Effect.gen(function* () {
     const harness = clientLaunchHarness({ runtimeModeCeiling: "full-access", launched: [] });
diff --git a/apps/server/src/mcp/toolkits/project/handlers.ts b/apps/server/src/mcp/toolkits/project/handlers.ts
index ab27b6b5bc4b4..f60542aef60a6 100644
--- a/apps/server/src/mcp/toolkits/project/handlers.ts
+++ b/apps/server/src/mcp/toolkits/project/handlers.ts
@@ -366,6 +366,14 @@ export const layer = McpToolAccess.toLayer(ProjectToolkit, {
   ),
   t3_project_delete: McpToolAccess.writesEnvironment((input) =>
     Effect.gen(function* () {
+      const folders = yield* ManagedProjectFolders.ManagedProjectFolders;
+      // The server recreates No project on demand, so deleting it only deletes every thread in it.
+      if (yield* folders.isScratchProject(input.projectId))
+        return yield* new OrchestratorMcpFailure({
+          code: "invalid_request",
+          message:
+            "The built-in No project cannot be deleted. Delete or archive its threads individually instead.",
+        });
       const projects = yield* Project.ProjectService;
       const clones = yield* ProjectCloneTracker.ProjectCloneTracker;
       return yield* mutateProjectWithClone(projects, clones, {
diff --git a/apps/server/src/mcp/toolkits/project/tools.ts b/apps/server/src/mcp/toolkits/project/tools.ts
index 56799959f6cd8..14d2fb2efbc02 100644
--- a/apps/server/src/mcp/toolkits/project/tools.ts
+++ b/apps/server/src/mcp/toolkits/project/tools.ts
@@ -103,7 +103,11 @@ const ProjectDeleteTool = Tool.make("t3_project_delete", {
     projectId: ProjectId,
     force: Schema.optionalKey(Schema.Boolean),
   }),
-  dependencies: [...shared.dependencies, ProjectCloneTracker.ProjectCloneTracker],
+  dependencies: [
+    ...shared.dependencies,
+    ProjectCloneTracker.ProjectCloneTracker,
+    ManagedProjectFolders.ManagedProjectFolders,
+  ],
 }).annotate(Tool.Destructive, true);
 const ProjectCloneTool = Tool.make("t3_project_clone", {
   ...shared,