diff --git a/src/CodexAcpServer.ts b/src/CodexAcpServer.ts
index 1903033..80486fe 100644
--- a/src/CodexAcpServer.ts
+++ b/src/CodexAcpServer.ts
@@ -784,7 +784,9 @@ export class CodexAcpServer {
                 createReasoningEffortConfigOption(sessionState.supportedReasoningEfforts, currentModelId.effort),
             );
         }
-        configOptions.push(createFastModeConfigOption(sessionState.fastModeEnabled));
+        if (sessionState.currentModelSupportsFast) {
+            configOptions.push(createFastModeConfigOption(sessionState.fastModeEnabled));
+        }
         return configOptions;
     }
 
diff --git a/src/__tests__/CodexACPAgent/fast-mode-config.test.ts b/src/__tests__/CodexACPAgent/fast-mode-config.test.ts
index 70716c8..ba8a71d 100644
--- a/src/__tests__/CodexACPAgent/fast-mode-config.test.ts
+++ b/src/__tests__/CodexACPAgent/fast-mode-config.test.ts
@@ -12,6 +12,7 @@ import {
     FAST_MODE_OFF,
     FAST_MODE_ON,
 } from "../../FastModeConfig";
+import {MODEL_CONFIG_ID} from "../../ModelConfigOption";
 
 describe("Fast mode session config", () => {
     async function createSession(
@@ -25,13 +26,14 @@ describe("Fast mode session config", () => {
             id: "fast-model",
             additionalSpeedTiers: ["fast"],
         });
+        const slowModel = createTestModel({id: "slow-model"});
 
         vi.spyOn(codexAcpClient, "authRequired").mockResolvedValue(false);
         vi.spyOn(codexAcpClient, "getAccount").mockResolvedValue({account: null, requiresOpenaiAuth: false});
         vi.spyOn(codexAcpClient, "newSession").mockResolvedValue({
             sessionId: "session-id",
             currentModelId: "fast-model[medium]",
-            models: [fastModel],
+            models: [fastModel, slowModel],
             currentServiceTier,
             additionalDirectories: [],
         });
@@ -183,6 +185,24 @@ describe("Fast mode session config", () => {
         }));
     });
 
+    it("removes the Fast mode config option when switching to a non-fast model via session_config", async () => {
+        const {codexAcpAgent} = await createSession("fast");
+
+        const fastResponse = await codexAcpAgent.setSessionConfigOption({
+            sessionId: "session-id",
+            configId: MODEL_CONFIG_ID,
+            value: "fast-model",
+        });
+        expect(fastResponse.configOptions?.some(o => o.id === FAST_MODE_CONFIG_ID)).toBe(true);
+
+        const slowResponse = await codexAcpAgent.setSessionConfigOption({
+            sessionId: "session-id",
+            configId: MODEL_CONFIG_ID,
+            value: "slow-model",
+        });
+        expect(slowResponse.configOptions?.some(o => o.id === FAST_MODE_CONFIG_ID)).toBe(false);
+    });
+
     it("keeps Fast mode selected across model switches but stops applying it for non-fast models", async () => {
         const {codexAcpAgent, codexAcpClient, fixture} = await createSession("fast");
         const slowModel = createTestModel({id: "slow-model"});
diff --git a/src/__tests__/CodexACPAgent/session-config-options.test.ts b/src/__tests__/CodexACPAgent/session-config-options.test.ts
index 391cc04..7377c90 100644
--- a/src/__tests__/CodexACPAgent/session-config-options.test.ts
+++ b/src/__tests__/CodexACPAgent/session-config-options.test.ts
@@ -19,6 +19,7 @@ function buildModels(): {fast: Model; slow: Model} {
         description: "Frontier",
         supportedReasoningEfforts: [lowEffort, mediumEffort, highEffort],
         defaultReasoningEffort: "medium",
+        additionalSpeedTiers: ["fast"],
     });
     const slow = createTestModel({
         id: "slow-model",
@@ -95,7 +96,7 @@ describe("Session config options", () => {
         const {codexAcpAgent, response} = await createSession("custom-model[high]", [fast, slow]);
 
         const ids = response.configOptions?.map(o => o.id);
-        expect(ids).toEqual([MODE_CONFIG_ID, MODEL_CONFIG_ID, "fast-mode"]);
+        expect(ids).toEqual([MODE_CONFIG_ID, MODEL_CONFIG_ID]);
 
         const modelOption = response.configOptions?.find(o => o.id === MODEL_CONFIG_ID);
         expect(modelOption).toMatchObject({