fix: Map priority mode to fast

src/CodexAcpClient.ts

@@ -44,6 +44,7 @@ import packageJson from "../package.json";
 import type {AuthenticationStatusResponse} from "./AcpExtensions";
 import {createCodexCollaborationMode} from "./CollaborationModeConfig";
 import type {ModeKind} from "./app-server/ModeKind";
+import type {SupportedServiceTier} from "./FastModeConfig";
 
 /**
  * Well-known provider id for the client-configurable custom LLM gateway.
@@ -345,7 +346,7 @@ export class CodexAcpClient {
             models: codexModels,
             collaborationMode: this.getCollaborationMode(response.thread.id),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier as SupportedServiceTier ?? null,
             additionalDirectories,
         }
     }
@@ -373,7 +374,7 @@ export class CodexAcpClient {
             models: codexModels,
             collaborationMode: this.getCollaborationMode(response.thread.id),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier as SupportedServiceTier ?? null,
             thread: historyResponse.thread,
             additionalDirectories,
         };
@@ -400,7 +401,7 @@ export class CodexAcpClient {
             models: codexModels,
             collaborationMode: this.getCollaborationMode(response.thread.id),
             modelProvider: response.modelProvider,
-            currentServiceTier: response.serviceTier as ServiceTier ?? null,
+            currentServiceTier: response.serviceTier as SupportedServiceTier ?? null,
             additionalDirectories,
         };
     }
@@ -899,7 +900,7 @@ export type SessionMetadata = {
     models: Model[],
     collaborationMode: ModeKind,
     modelProvider?: string | null,
-    currentServiceTier?: ServiceTier | null,
+    currentServiceTier?: SupportedServiceTier | null,
     additionalDirectories: string[],
 }
 

src/CodexAcpServer.ts

@@ -75,6 +75,7 @@ import {
     FAST_MODE_ON,
     modelSupportsFast,
     resolveFastServiceTier,
+    serviceTierEnablesFast,
 } from "./FastModeConfig";
 import packageJson from "../package.json";
 import {isJetBrains2026_1Client} from "./JBUtils";
@@ -460,7 +461,7 @@ export class CodexAcpServer {
             authProvider: authProvider,
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: serviceTierEnablesFast(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             terminalOutputMode: this.terminalOutputMode,
@@ -1302,7 +1303,7 @@ export class CodexAcpServer {
             authProvider: authProvider,
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: serviceTierEnablesFast(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             terminalOutputMode: this.terminalOutputMode,

src/FastModeConfig.ts

@@ -3,6 +3,8 @@ import type * as acp from "@agentclientprotocol/sdk";
 import type {ServiceTier} from "./app-server";
 import type {Model} from "./app-server/v2";
 
+export type SupportedServiceTier = ServiceTier | "priority";
+
 export const FAST_MODE_CONFIG_ID = "fast-mode";
 export const FAST_MODE_CATEGORY = "model_config";
 export const FAST_MODE_ON = "on";
@@ -14,6 +16,10 @@ export function modelSupportsFast(model: Model | undefined): boolean {
     return model?.additionalSpeedTiers?.includes("fast") ?? false;
 }
 
+export function serviceTierEnablesFast(serviceTier: SupportedServiceTier | null | undefined): boolean {
+    return serviceTier === "fast" || serviceTier === "priority";
+}
+
 export function resolveFastServiceTier(fastModeEnabled: boolean, currentModelSupportsFast: boolean): ServiceTier | null {
     return fastModeEnabled && currentModelSupportsFast ? "fast" : null;
 }

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: "fast" | "priority" | "flex" | null = null,
         clientInfo: acp.Implementation | null = null,
         clientCapabilities?: acp.ClientCapabilities,
     ) {
@@ -106,6 +106,20 @@ describe("Fast mode session config", () => {
         expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
     });
 
+    it("initializes Fast mode as On when the app-server session tier is priority", async () => {
+        const {fixture, response, codexAcpAgent} = await createSession("priority");
+        const turnStartSpy = mockPromptTurn(fixture, "session-id");
+
+        expect(response.configOptions).toContainEqual(createFastModeConfigOption(true));
+        expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
+
+        await codexAcpAgent.prompt({sessionId: "session-id", prompt: [{type: "text", text: "test"}]});
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({
+            serviceTier: "fast",
+        }));
+    });
+
     it("omits Fast mode config options for JetBrains 2026.1 IntelliJ clients", async () => {
         const {response} = await createSession(null, {
             name: "JetBrains.WebStorm",