diff --git a/apps/server/scripts/acp-mock-agent.ts b/apps/server/scripts/acp-mock-agent.ts
index 5548bebac84c0e949c8eef6b1635eae24a04eb87..dc85161ad351982ef64dfab5a0c275b0bc7971d1 100644
--- a/apps/server/scripts/acp-mock-agent.ts
+++ b/apps/server/scripts/acp-mock-agent.ts
@@ -94,6 +94,7 @@ const emitOverlappingXAiPromptCompleteOutOfOrder =
   process.env.T3_ACP_EMIT_OVERLAPPING_XAI_PROMPT_COMPLETE_OUT_OF_ORDER === "1";
 const failPrompt = process.env.T3_ACP_FAIL_PROMPT === "1";
 const failSetConfigOption = process.env.T3_ACP_FAIL_SET_CONFIG_OPTION === "1";
+const failSetModeConfigOption = process.env.T3_ACP_FAIL_SET_MODE_CONFIG_OPTION === "1";
 const exitOnSetConfigOption = process.env.T3_ACP_EXIT_ON_SET_CONFIG_OPTION === "1";
 const omitModelConfigOption = process.env.T3_ACP_OMIT_MODEL_CONFIG_OPTION === "1";
 const promptResponseText = process.env.T3_ACP_PROMPT_RESPONSE_TEXT;
@@ -775,7 +776,7 @@ const program = Effect.gen(function* () {
           process.exit(7);
         });
       }
-      if (failSetConfigOption) {
+      if (failSetConfigOption || (failSetModeConfigOption && request.configId === "mode")) {
         return yield* AcpError.AcpRequestError.invalidParams(
           "Mock invalid params for session/set_config_option",
           {
diff --git a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
index 369f1c29a569cd52cc22bb5c04777966d5f4c64c..e8ae5a0c9dd072c5d504da87b189fb40d29fc2eb 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
@@ -58,6 +58,7 @@ import type * as EffectAcpSchema from "effect-acp/compat";
 import * as TestProviderHost from "@t3tools/provider-testing/TestProviderHost";
 import * as McpProviderSessions from "@t3tools/provider-core/server/McpProviderSessions";
 import * as AcpSessionRuntime from "@t3tools/provider-acp/server/AcpSessionRuntime";
+import { ACP_SESSION_MODE_OPTION_ID } from "@t3tools/provider-acp/server/sessionConfig";
 import {
   extractXAiAcpSubagentEndNotice,
   extractXAiAcpSubagentUpdate,
@@ -880,6 +881,95 @@ describe("AcpAdapterV2", () => {
     }).pipe(Effect.provide(layerTest), Effect.scoped),
   );
 
+  it.live("reapplies a selection after a switch away from it failed partway", () =>
+    Effect.gen(function* () {
+      const childProcessSpawner = yield* ChildProcessSpawner.ChildProcessSpawner;
+      const path = yield* Path.Path;
+      const mockAgentPath = yield* path.fromFileUrl(
+        new URL("../../../scripts/acp-mock-agent.ts", import.meta.url),
+      );
+      const protocolEvents = yield* Queue.unbounded<EffectAcpProtocol.AcpProtocolLogEvent>();
+      const instanceId = ProviderInstanceId.make("acp-test-partial-selection-reapply");
+      const threadId = ThreadId.make("thread-acp-partial-selection-reapply");
+      const adapter = yield* makeAcpAdapterV2({
+        instanceId,
+        flavor: {
+          driver: ACP_TEST_DRIVER,
+          capabilities: AcpProviderCapabilitiesV2,
+          makeRuntime: makeMockRuntime({
+            childProcessSpawner,
+            mockAgentPath,
+            protocolEvents,
+            // The model switch lands, then the agent rejects the mode step.
+            environment: { T3_ACP_FAIL_SET_MODE_CONFIG_OPTION: "1" },
+          }),
+        },
+        selfInvocation: yield* resolveSelfInvocation(),
+      });
+      const runtimePolicy = ProviderAdapter.ProviderAdapterV2RuntimePolicy.make({
+        runtimeMode: "full-access",
+        interactionMode: "default",
+        cwd: process.cwd(),
+      });
+      const selectionA = { instanceId, model: "composer-2" } as const;
+      const selectionB = {
+        instanceId,
+        model: "composer-2[fast=true]",
+        options: [{ id: ACP_SESSION_MODE_OPTION_ID, value: "code" }],
+      } as const;
+      const runtime = yield* adapter.openSession({
+        threadId,
+        providerSessionId: ProviderSessionId.make("provider-session-acp-partial-selection"),
+        modelSelection: selectionA,
+        runtimePolicy,
+      });
+      const providerThread = yield* runtime.ensureThread({
+        threadId,
+        modelSelection: selectionA,
+        runtimePolicy,
+      });
+      const now = yield* DateTime.now;
+      const turnInput = (ordinal: number, modelSelection: ModelSelection) =>
+        makeTurnInput({
+          threadId,
+          providerThread,
+          instanceId,
+          runtimePolicy,
+          now,
+          ordinal,
+          modelSelection,
+        });
+
+      const switchToB = yield* Effect.exit(runtime.startTurn(turnInput(1, selectionB)));
+      assert.isTrue(Exit.isFailure(switchToB), "the agent rejected B's mode step");
+      yield* Queue.clear(protocolEvents);
+
+      yield* runtime.startTurn(turnInput(2, selectionA));
+      yield* runtime.events.pipe(
+        Stream.takeUntil((event) => event.type === "turn.terminal"),
+        Stream.runDrain,
+      );
+      const modelSetsBeforePrompt: Array<unknown> = [];
+      while (true) {
+        const event = yield* Queue.take(protocolEvents);
+        if (event.direction !== "outgoing") continue;
+        const method = rawProtocolMethod(event);
+        if (method === "session/prompt") break;
+        if (
+          method === "session/set_config_option" &&
+          rawProtocolRequestParam(event, "configId") === "model"
+        ) {
+          modelSetsBeforePrompt.push(rawProtocolRequestParam(event, "value"));
+        }
+      }
+      assert.deepEqual(
+        modelSetsBeforePrompt,
+        ["composer-2"],
+        "the native model was left on B, so the turn on A must set it back first",
+      );
+    }).pipe(Effect.provide(layerTest), Effect.scoped),
+  );
+
   it.effect("starts a new replay message after ACP v2 plan boundaries", () =>
     Effect.gen(function* () {
       const childProcessSpawner = yield* ChildProcessSpawner.ChildProcessSpawner;
diff --git a/packages/provider-acp/src/server/adapter.ts b/packages/provider-acp/src/server/adapter.ts
index c1208b49556e80f6b4a83288232d0167b75aea69..6e125b2318cfc34d8d6d63696e44f9aab649e434 100644
--- a/packages/provider-acp/src/server/adapter.ts
+++ b/packages/provider-acp/src/server/adapter.ts
@@ -6283,6 +6283,10 @@ export const makeAcpAdapterV2 = Effect.fn("makeAcpAdapterV2")(function* (
           modelSelection: ModelSelection,
           runtimePolicy: ProviderAdapter.ProviderAdapterV2RuntimePolicy,
         ) {
+          // A step failing below leaves the native config partly changed, so
+          // the previously applied selection no longer describes the session.
+          // Callers record the new selection only once this completes.
+          yield* Ref.set(activeSelection, null);
           const requestedModel = flavor.resolveModelId?.(modelSelection) ?? modelSelection.model;
           let appliedModel: string | undefined;
           if (flavor.applyModelSelection !== undefined) {