fix: Map priority mode to fast

src/CodexAcpClient.ts

@@ -715,7 +715,7 @@ export class CodexAcpClient {
             threadStatus: response.thread.status,
             sessionTitle: normalizeSessionTitle(listedSessionTitle(response.thread)),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier ?? null,
             additionalDirectories,
             skippedMcpServers,
             ...(instructions ? {instructions} : {}),
@@ -798,7 +798,7 @@ export class CodexAcpClient {
             // Codex sends no thread/settings/updated on a resume. The response holds the mode.
             collaborationMode: response.collaborationMode ?? this.getCollaborationMode(response.thread.id),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier ?? null,
             thread,
             history,
             additionalDirectories,
@@ -883,7 +883,7 @@ export class CodexAcpClient {
             models: codexModels,
             collaborationMode: this.getCollaborationMode(response.thread.id),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier ?? null,
             additionalDirectories,
             skippedMcpServers,
             ...(instructions ? {instructions} : {}),

src/CodexAcpServer.ts

@@ -156,6 +156,7 @@ import {
     FAST_MODE_ON,
     modelSupportsFast,
     resolveFastServiceTier,
+    serviceTierEnablesFast,
 } from "./FastModeConfig";
 import packageJson from "../package.json";
 import {isJetBrains2026_1Client} from "./JBUtils";
@@ -1412,7 +1413,7 @@ export class CodexAcpServer {
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
             mcpServers: requestedMcpServers,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: serviceTierEnablesFast(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             clientCapabilities: this.capabilities,
@@ -3645,7 +3646,7 @@ export class CodexAcpServer {
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
             mcpServers: requestedMcpServers,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: serviceTierEnablesFast(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             clientCapabilities: this.capabilities,

src/FastModeConfig.ts

@@ -14,6 +14,11 @@ export function modelSupportsFast(model: Model | undefined): boolean {
     return model?.additionalSpeedTiers?.includes("fast") ?? false;
 }
 
+export function serviceTierEnablesFast(serviceTier: string | null | undefined): boolean {
+    // Codex returns the canonical "priority" tier even when configured with "fast".
+    return serviceTier === "fast" || serviceTier === "priority";
+}
+
 export function resolveFastServiceTier(fastModeEnabled: boolean, currentModelSupportsFast: boolean): ServiceTier | null {
     return fastModeEnabled && currentModelSupportsFast ? "fast" : null;
 }

src/SessionFork.ts

@@ -4,7 +4,6 @@ import {RequestError} from "@agentclientprotocol/sdk/unstable";
 import type {SessionConfig} from "./CodexAcpClient";
 import type {CodexAppServerClient} from "./CodexAppServerClient";
 import type {ModeKind} from "./app-server/ModeKind";
-import type {ServiceTier} from "./app-server/ServiceTier";
 import type {Model} from "./app-server/v2";
 import type {SessionMetadata} from "./SessionMetadata";
 import type {AcpMcpServer, WithAcpMcpServers} from "./McpServerConfig";
@@ -68,7 +67,7 @@ export async function forkSession(
         models,
         collaborationMode: dependencies.getCollaborationMode(response.thread.id),
         modelProvider: response.modelProvider,
-        currentServiceTier: response.serviceTier as ServiceTier ?? null,
+        currentServiceTier: response.serviceTier ?? null,
         additionalDirectories,
         skippedMcpServers,
         activePermissionProfileId: response.activePermissionProfile?.id ?? null,

src/SessionMetadata.ts

@@ -1,5 +1,4 @@
 import type {ModeKind} from "./app-server/ModeKind";
-import type {ServiceTier} from "./app-server/ServiceTier";
 import type {
     ApprovalsReviewer,
     AskForApproval,
@@ -17,7 +16,7 @@ export type SessionMetadata = {
     models: Model[],
     collaborationMode: ModeKind,
     modelProvider?: string | null,
-    currentServiceTier?: ServiceTier | null,
+    currentServiceTier?: string | null,
     threadStatus?: ThreadStatus,
     additionalDirectories: string[],
     skippedMcpServers?: string[],

src/__tests__/CodexACPAgent/data/session-lifecycle-v2-fast-mode-off.json

@@ -0,0 +1,8 @@
+{
+  "currentValue": false,
+  "type": "boolean",
+  "configId": "fast-mode",
+  "name": "Fast mode",
+  "description": "1.5x speed, increased usage",
+  "category": "model_config"
+}

src/__tests__/CodexACPAgent/data/session-lifecycle-v2-fast-mode-on.json

@@ -0,0 +1,8 @@
+{
+  "currentValue": true,
+  "type": "boolean",
+  "configId": "fast-mode",
+  "name": "Fast mode",
+  "description": "1.5x speed, increased usage",
+  "category": "model_config"
+}

src/__tests__/CodexACPAgent/fast-mode-config.test.ts

@@ -24,7 +24,7 @@ describe("Fast mode session config", () => {
     };
 
     async function createSession(
-        currentServiceTier: "fast" | "flex" | null = null,
+        currentServiceTier: string | null = null,
         clientInfo: acp.Implementation | null = null,
         clientCapabilities?: acp.ClientCapabilities,
     ) {
@@ -99,11 +99,23 @@ describe("Fast mode session config", () => {
         expect(response.configOptions).toContainEqual(createFastModeConfigOption(false));
     });
 
-    it("initializes Fast mode as On when the app-server session tier is fast", async () => {
-        const {response, codexAcpAgent} = await createSession("fast");
+    it.each([
+        {serviceTier: "fast", enabled: true},
+        {serviceTier: "priority", enabled: true},
+        {serviceTier: "flex", enabled: false},
+        {serviceTier: null, enabled: false},
+        {serviceTier: "unknown", enabled: false},
+    ])("preserves Fast mode from the app-server tier $serviceTier through the first prompt", async ({serviceTier, enabled}) => {
+        const {fixture, response, codexAcpAgent} = await createSession(serviceTier);
+        const turnStartSpy = mockPromptTurn(fixture, "session-id");
 
-        expect(response.configOptions).toContainEqual(createFastModeConfigOption(true));
-        expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
+        expect(response.configOptions).toContainEqual(createFastModeConfigOption(enabled));
+
+        await codexAcpAgent.prompt({sessionId: "session-id", prompt: [{type: "text", text: "test"}]});
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({
+            serviceTier: enabled ? "fast" : null,
+        }));
     });
 
     it("omits Fast mode config options for JetBrains 2026.1 IntelliJ clients", async () => {

src/__tests__/CodexACPAgent/session-lifecycle-v2.test.ts

@@ -8,6 +8,7 @@ import {CodexAcpClient} from '../../CodexAcpClient';
 import {CodexAppServerClient} from '../../CodexAppServerClient';
 import type {PermissionProfileSummary, SkillsListParams, SkillsListResponse, Thread} from '../../app-server/v2';
 import {MODE_CONFIG_ID} from '../../AgentMode';
+import {FAST_MODE_CONFIG_ID} from '../../FastModeConfig';
 import {createTestModel} from '../acp-test-utils';
 import {createMockConnections} from './test-utils';
 import {checkV2SessionUpdate, expectConformingV2SessionUpdates} from './v2-session-update-guard';
@@ -115,6 +116,7 @@ async function connectV2Client(options?: {
     permissionProfiles?: PermissionProfileSummary[];
     activePermissionProfileId?: string;
     defaultPermissionProfileId?: string;
+    serviceTier?: string | null;
 }) {
     const mocks = createMockConnections();
     const codexRequests: Array<{method: string, params: unknown}> = [];
@@ -136,6 +138,14 @@ async function connectV2Client(options?: {
             return {config: {default_permissions: options.defaultPermissionProfileId}, origins: {}, layers: []};
         }
         const response = codexResponse(method, options?.replayTurns, options?.thread);
+        if (options?.serviceTier !== undefined) {
+            if (["thread/start", "thread/resume", "thread/fork"].includes(method)) {
+                response["serviceTier"] = options.serviceTier;
+            }
+            if (method === "model/list") {
+                response["data"] = [createTestModel({id: "gpt-5", additionalSpeedTiers: ["fast"]})];
+            }
+        }
         if (options?.activePermissionProfileId && ["thread/start", "thread/resume", "thread/fork"].includes(method)) {
             return {...response, activePermissionProfile: {id: options.activePermissionProfileId, extends: null}};
         }
@@ -243,6 +253,31 @@ describe('Session lifecycle over ACP v2', () => {
             .toMatchObject({type: "select", currentValue: "permission-profile:team-default"});
     });
 
+    describe.each([
+        {operation: "new", replayFrom: undefined},
+        {operation: "resume", replayFrom: null},
+        {operation: "resume", replayFrom: {type: "start"}},
+        {operation: "fork", replayFrom: undefined},
+    ] as const)('Fast mode over v2 $operation with replayFrom=$replayFrom', ({operation, replayFrom}) => {
+        it.each([
+            {serviceTier: "fast", enabled: true},
+            {serviceTier: "priority", enabled: true},
+            {serviceTier: "flex", enabled: false},
+            {serviceTier: null, enabled: false},
+            {serviceTier: "unknown", enabled: false},
+        ])('restores Fast mode from the app-server tier $serviceTier', async ({serviceTier, enabled}) => {
+            const {connection} = await connectV2Client({serviceTier});
+            closeClient = () => connection.close();
+
+            const response = await connection.agent.request(acpV2.methods.agent.session[operation], {
+                sessionId, cwd, ...(replayFrom === undefined ? {} : {replayFrom}),
+            });
+
+            await expect(dump(response.configOptions?.find(option => option.configId === FAST_MODE_CONFIG_ID)))
+                .toMatchFileSnapshot(`data/session-lifecycle-v2-fast-mode-${enabled ? "on" : "off"}.json`);
+        });
+    });
+
     it.each([null, {type: "start"}] as const)(
         'preserves the default profile on an unprompted v2 resume with replayFrom=%s', async replayFrom => {
             const {connection} = await connectV2Client({