From f5546efbaae982bd6c353df500b5a398cc470ce2 Mon Sep 17 00:00:00 2001
From: Dotta <bippadotta@protonmail.com>
Date: Tue, 29 Sep 2026 16:24:48 -0500
Subject: [PATCH] Reject Cursor mode drift in native configuration updates

Co-Authored-By: Paperclip <noreply@paperclip.ing>
---
 .../src/drivers/acpx/cursor-mode.test.ts       | 18 ++++++++++++++++++
 .../src/drivers/acpx/cursor-mode.ts            | 10 ++++++++++
 2 files changed, 28 insertions(+)

diff --git a/packages/paperclip-runner/src/drivers/acpx/cursor-mode.test.ts b/packages/paperclip-runner/src/drivers/acpx/cursor-mode.test.ts
index d941429f44..34b7fcb986 100644
--- a/packages/paperclip-runner/src/drivers/acpx/cursor-mode.test.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/cursor-mode.test.ts
@@ -60,6 +60,24 @@ describe("Cursor native mode admission", () => {
     const other = opened("plan").guard;
     expect(() => other("outbound", { id: 3, method: "session/set_config_option", params: { sessionId: "other", configId: "mode", value: "plan" } })).toThrow(/different session/);
   });
+  it.each([false, true])("rejects config-option mode drift with active prompt=%s", active => {
+    const { admission, guard } = opened("plan");
+    if (active) guard("outbound", prompt);
+    expect(() => guard("inbound", { method: "session/update", params: { sessionId: "native", update: { sessionUpdate: "config_option_update", configOptions: config("agent") } } })).toThrow(/drifted/);
+    expect(admission.assertReady).toThrow(/drifted/);
+  });
+  it.each(["foreign", "duplicate", "invalid"])("rejects %s mode configuration notifications", kind => {
+    const { guard } = opened("plan");
+    const options = kind === "duplicate" ? [...config("plan"), ...config("plan")] : config(kind === "invalid" ? "architect" : "plan");
+    expect(() => guard("inbound", { method: "session/update", params: { sessionId: kind === "foreign" ? "other" : "native", update: { sessionUpdate: "config_option_update", configOptions: options } } })).toThrow(/mode admission/);
+  });
+  it("ignores model-only partial updates and cannot admit from a mode notification", () => {
+    const { admission, guard } = opened("plan", "agent");
+    for (const options of [[{ id: "model", currentValue: "fixture" }], config("plan")]) {
+      guard("inbound", { method: "session/update", params: { sessionId: "native", update: { sessionUpdate: "config_option_update", configOptions: options } } });
+      expect(admission.isReady()).toBe(false);
+    }
+  });
   it("does not mistake a bare set_mode response for configuration proof", () => {
     const { admission, guard } = opened("plan", "agent");
     guard("outbound", { id: 4, method: "session/set_mode", params: { sessionId: "native", modeId: "plan" } });
diff --git a/packages/paperclip-runner/src/drivers/acpx/cursor-mode.ts b/packages/paperclip-runner/src/drivers/acpx/cursor-mode.ts
index 358fd86281..80b5de0189 100644
--- a/packages/paperclip-runner/src/drivers/acpx/cursor-mode.ts
+++ b/packages/paperclip-runner/src/drivers/acpx/cursor-mode.ts
@@ -69,6 +69,16 @@ export function createCursorModeAdmission(expected: CursorSessionMode) {
             }
             return;
           }
+          if (message.method === "session/update" && object(params.update).sessionUpdate === "config_option_update") {
+            const update = object(params.update);
+            // Config updates may contain only model options. A mode entry is
+            // authoritative observation, but never an admission acknowledgement.
+            if (Array.isArray(update.configOptions) && update.configOptions.some(value => object(value).id === "mode")) {
+              if (!state.sessionId || params.sessionId !== state.sessionId) throw failure("mode update belongs to a different session");
+              if (readMode(update, false) !== expected) throw failure("native mode drifted from the selected mode");
+            }
+            return;
+          }
           if (message.method === "session/update" && object(params.update).sessionUpdate === "current_mode_update") {
             if (!state.sessionId || params.sessionId !== state.sessionId) throw failure("mode update belongs to a different session");
             if (object(params.update).currentModeId !== expected) throw failure("native mode drifted from the selected mode");