fix(providers): reject unsupported rewind before restoring files

apps/server/src/provider/Layers/AmpAdapter.test.ts

@@ -123,6 +123,7 @@ it.effect("AmpAdapter delegates session startup to the manager", () =>
   Effect.gen(function* () {
     const manager = new FakeAmpManager();
     const adapter = yield* makeAmpAdapter(enabledAmpSettings, { manager });
+    NodeAssert.equal(adapter.capabilities.supportsConversationRollback, false);
 
     const session = yield* adapter.startSession({
       threadId: asThreadId("thread-1"),

apps/server/src/provider/Layers/AmpAdapter.ts

@@ -68,7 +68,7 @@ export const makeAmpAdapter = Effect.fn("makeAmpAdapter")(function* (
 
   const service: AmpAdapterShape = {
     provider: PROVIDER,
-    capabilities: { sessionModelSwitch: "in-session" },
+    capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
     startSession: (input) =>
       Effect.gen(function* () {
         if (!ampSettings.enabled) {

apps/server/src/provider/Layers/CopilotAdapter.test.ts

@@ -138,6 +138,12 @@ const modeLayer = it.layer(
 );
 
 modeLayer("CopilotAdapterLive interaction mode", (it) => {
+  it.effect("declares conversation rollback unsupported", () =>
+    Effect.gen(function* () {
+      const adapter = yield* CopilotAdapter;
+      NodeAssert.equal(adapter.capabilities.supportsConversationRollback, false);
+    }),
+  );
   // Skip: @github/copilot-sdk has broken ESM resolution (vscode-jsonrpc/node) in CI
   it.effect.skip("switches the Copilot session mode when interactionMode changes", () =>
     Effect.gen(function* () {

apps/server/src/provider/Layers/CopilotAdapter.ts

@@ -1743,7 +1743,7 @@ export const makeCopilotAdapter = Effect.fn("makeCopilotAdapter")(function* (
 
   return {
     provider: PROVIDER,
-    capabilities: { sessionModelSwitch: "in-session" },
+    capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
     startSession,
     sendTurn,
     interruptTurn,

apps/server/src/provider/Layers/CursorAdapter.test.ts

@@ -196,6 +196,32 @@ async function withAdapter<A>(
 }
 
 describe("CursorAdapter SDK", () => {
+  it("rejects rollback without changing the retained conversation", async () => {
+    const run = new FakeRun("run-rollback", "agent-rollback", [
+      makeSdkMessage({
+        type: "assistant",
+        message: { role: "assistant", content: [{ type: "text", text: "done" }] },
+      }),
+    ]);
+    const fakeClient = new FakeCursorSdkClient(new FakeAgent("agent-rollback", run));
+    await withAdapter(fakeClient, (adapter) =>
+      Effect.gen(function* () {
+        expect(adapter.capabilities.supportsConversationRollback).toBe(false);
+        const threadId = asThreadId("thread-rollback");
+        yield* adapter.startSession({ threadId, cwd: process.cwd(), runtimeMode: "full-access" });
+        const events = yield* collectThroughTurnCompleted(adapter);
+        yield* adapter.sendTurn({ threadId, input: "hello", attachments: [] });
+        yield* Fiber.join(events);
+        const before = yield* adapter.readThread(threadId);
+        expect(before.turns).toHaveLength(1);
+        const rollback = yield* adapter.rollbackThread(threadId, 1).pipe(Effect.exit);
+        expect(rollback._tag).toBe("Failure");
+        expect(yield* adapter.readThread(threadId)).toEqual(before);
+        expect((yield* adapter.readThread(threadId)).turns).toHaveLength(1);
+      }),
+    );
+  });
+
   it("creates local SDK agents, applies model params, and emits canonical runtime events", async () => {
     const run = new FakeRun("run-1", "agent-1", [
       makeSdkMessage({ type: "status", status: "RUNNING" }),

apps/server/src/provider/Layers/CursorAdapter.ts

@@ -1268,19 +1268,19 @@ export function makeCursorAdapter(
 
     const rollbackThread: CursorAdapterShape["rollbackThread"] = (threadId, numTurns) =>
       Effect.gen(function* () {
-        const context = yield* requireSession(threadId);
+        yield* requireSession(threadId);
         if (!Number.isInteger(numTurns) || numTurns < 1) {
           return yield* new ProviderAdapterValidationError({
             provider: PROVIDER,
             operation: "rollbackThread",
             issue: "numTurns must be an integer >= 1.",
           });
         }
-        context.turns.splice(Math.max(0, context.turns.length - numTurns));
-        return {
-          threadId,
-          turns: context.turns,
-        };
+        return yield* new ProviderAdapterRequestError({
+          provider: PROVIDER,
+          method: "thread/rollback",
+          detail: "Cursor SDK does not support provider-side conversation rollback.",
+        });
       });
 
     const stopSession: CursorAdapterShape["stopSession"] = (threadId) =>
@@ -1317,7 +1317,7 @@ export function makeCursorAdapter(
 
     return {
       provider: PROVIDER,
-      capabilities: { sessionModelSwitch: "in-session" },
+      capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
       startSession,
       sendTurn,
       interruptTurn,

apps/server/src/provider/Layers/DroidAdapter.test.ts

@@ -1124,6 +1124,7 @@ it.effect("reads Droid thread snapshots and rejects unsupported rollback", () =>
 
       const before = yield* adapter.readThread(threadId);
       NodeAssert.equal(before.turns.length, 2);
+      NodeAssert.equal(adapter.capabilities.supportsConversationRollback, false);
       const rollback = yield* adapter.rollbackThread(threadId, 1).pipe(Effect.exit);
       NodeAssert.equal(rollback._tag, "Failure");
       if (rollback._tag === "Failure") {

apps/server/src/provider/Layers/DroidAdapter.ts

@@ -429,7 +429,7 @@ export function makeDroidAdapter(settings: DroidSettings, options?: DroidAdapter
 
     return {
       provider: DROID_PROVIDER,
-      capabilities: { sessionModelSwitch: "in-session" },
+      capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
       startSession,
       sendTurn,
       interruptTurn: (threadId) =>

apps/server/src/provider/Layers/GeminiCliAdapter.test.ts

@@ -125,6 +125,7 @@ it.effect("delegates session startup to the manager", () =>
   Effect.gen(function* () {
     const manager = new FakeGeminiCliManager();
     const adapter = yield* makeGeminiCliAdapter(enabledConfig, { manager });
+    NodeAssert.equal(adapter.capabilities.supportsConversationRollback, false);
 
     const session = yield* adapter.startSession({
       threadId: asThreadId("thread-1"),

apps/server/src/provider/Layers/GeminiCliAdapter.ts

@@ -96,7 +96,7 @@ export const makeGeminiCliAdapter = Effect.fn("makeGeminiCliAdapter")(function*
 
   const adapter: GeminiCliAdapterShape = {
     provider: PROVIDER,
-    capabilities: { sessionModelSwitch: "in-session" },
+    capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
     startSession: (input) =>
       Effect.gen(function* () {
         if (!config.enabled) {

apps/server/src/provider/Layers/StandardAcpAdapter.ts

@@ -2153,7 +2153,7 @@ export function makeStandardAcpAdapter<UserInputParams = never, UserInputEncoded
 
     return {
       provider: PROVIDER,
-      capabilities: { sessionModelSwitch: "in-session" },
+      capabilities: { sessionModelSwitch: "in-session", supportsConversationRollback: false },
       startSession,
       sendTurn,
       interruptTurn,

apps/server/src/provider/Layers/StandardAcpProviders.test.ts

@@ -92,6 +92,7 @@ it.layer(testLayer)("standard ACP provider adapters", (it) => {
               : provider === "ohMyPi"
                 ? decodeOhMyPiSettings({ binaryPath }).pipe(Effect.flatMap(makeOhMyPiAdapter))
                 : decodePiSettings({ binaryPath }).pipe(Effect.flatMap(makePiAdapter));
+        assert.strictEqual(adapter.capabilities.supportsConversationRollback, false);
         const threadId = ThreadId.make(`${provider}-mock-thread`);
         const providerKind = ProviderDriverKind.make(provider);
         const events: ProviderRuntimeEvent[] = [];