diff --git a/.pr/live_priority_fast_mode.ts b/.pr/live_priority_fast_mode.ts
new file mode 100644
index 00000000..3a361bf0
--- /dev/null
+++ b/.pr/live_priority_fast_mode.ts
@@ -0,0 +1,106 @@
+import fs from "node:fs";
+import path from "node:path";
+import {pathToFileURL} from "node:url";
+
+const sourceRoot = process.env.SOURCE_ROOT;
+if (!sourceRoot) {
+    throw new Error("SOURCE_ROOT must point to the codex-acp checkout under test");
+}
+
+const importSource = async (relativePath: string) => {
+    return import(pathToFileURL(path.join(sourceRoot, relativePath)).href);
+};
+
+const {CodexAcpClient} = await importSource("src/CodexAcpClient.ts");
+const {CodexAppServerClient} = await importSource("src/CodexAppServerClient.ts");
+const {startCodexConnection} = await importSource("src/CodexJsonRpcConnection.ts");
+const {CodexAcpServer} = await importSource("src/CodexAcpServer.ts");
+
+const codexPath = path.join(
+    sourceRoot,
+    "node_modules",
+    ".bin",
+    process.platform === "win32" ? "codex.cmd" : "codex",
+);
+if (!fs.existsSync(codexPath)) {
+    throw new Error(`Codex binary not found at ${codexPath}`);
+}
+
+const transportEvents: any[] = [];
+const acpEvents: Array<{method: string; args: unknown[]}> = [];
+const acpConnection = new Proxy({} as any, {
+    get(_, property) {
+        return (...args: unknown[]) => {
+            acpEvents.push({method: String(property), args});
+            return Promise.resolve({});
+        };
+    },
+});
+
+const codexConnection = startCodexConnection(codexPath);
+const appServer = new CodexAppServerClient(codexConnection.connection);
+appServer.onClientTransportEvent((event: any) => transportEvents.push(event));
+
+// Exercise the same production configuration path as CODEX_CONFIG.
+const client = new CodexAcpClient(appServer, {service_tier: "priority"});
+const agent = new CodexAcpServer(
+    acpConnection,
+    client,
+    undefined,
+    () => codexConnection.process.exitCode,
+);
+
+try {
+    await agent.initialize({protocolVersion: 1});
+    if (await client.authRequired()) {
+        throw new Error("Codex authentication is required");
+    }
+
+    const session = await agent.newSession({cwd: sourceRoot, mcpServers: []});
+    const fastOption = session.configOptions?.find((option: any) => option.id === "fast-mode");
+    const stateAfterStart = agent.getSessionState(session.sessionId);
+    const promptResponse = await agent.prompt({
+        sessionId: session.sessionId,
+        prompt: [{type: "text", text: "Reply with exactly: priority-fast-live-ok"}],
+    });
+
+    const threadStartRequest = transportEvents.find(
+        (event) => event.eventType === "request" && event.method === "thread/start",
+    );
+    const threadStartIndex = transportEvents.indexOf(threadStartRequest);
+    const threadStartResponse = transportEvents
+        .slice(threadStartIndex + 1)
+        .find((event) => event.eventType === "response" && event.thread?.id === session.sessionId);
+    const turnStartRequest = transportEvents.find(
+        (event) => event.eventType === "request" && event.method === "turn/start",
+    );
+    const finalMessage = transportEvents
+        .filter(
+            (event) =>
+                event.eventType === "notification" &&
+                event.method === "item/completed" &&
+                event.params?.item?.type === "agentMessage",
+        )
+        .at(-1)?.params?.item?.text;
+
+    console.log(JSON.stringify({
+        testedCommit: process.env.TESTED_COMMIT ?? null,
+        codexVersion: threadStartResponse?.thread?.cliVersion ?? null,
+        configuredServiceTier: threadStartRequest?.params?.config?.service_tier ?? null,
+        appServerServiceTier: threadStartResponse?.serviceTier ?? null,
+        fastConfigOption: fastOption
+            ? {type: fastOption.type, currentValue: fastOption.currentValue}
+            : null,
+        fastModeEnabledAfterStart: stateAfterStart.fastModeEnabled,
+        firstPromptServiceTier: turnStartRequest?.params?.serviceTier ?? null,
+        promptStopReason: promptResponse.stopReason,
+        finalMessage,
+        requestSequence: transportEvents
+            .filter((event) => event.eventType === "request")
+            .map((event) => event.method),
+        acpEventCount: acpEvents.length,
+    }, null, 2));
+} finally {
+    codexConnection.connection.end();
+    codexConnection.process.kill();
+}
diff --git a/src/CodexAcpClient.ts b/src/CodexAcpClient.ts
index 12593a65..27befc67 100644
--- a/src/CodexAcpClient.ts
+++ b/src/CodexAcpClient.ts
@@ -842,7 +842,7 @@ export type SessionMetadata = {
     currentModelId: string,
     models: Model[],
     modelProvider?: string | null,
-    currentServiceTier?: ServiceTier | null,
+    currentServiceTier?: string | null,
     additionalDirectories: string[],
 }
 
diff --git a/src/CodexAcpServer.ts b/src/CodexAcpServer.ts
index 18fdf195..19a569f8 100644
--- a/src/CodexAcpServer.ts
+++ b/src/CodexAcpServer.ts
@@ -61,6 +61,7 @@ import {
     FAST_MODE_CONFIG_ID,
     FAST_MODE_OFF,
     FAST_MODE_ON,
+    isFastServiceTier,
     modelSupportsFast,
     resolveFastServiceTier,
 } from "./FastModeConfig";
@@ -412,7 +413,7 @@ export class CodexAcpServer {
             authProvider: authProvider,
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: isFastServiceTier(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             terminalOutputMode: this.terminalOutputMode,
@@ -941,7 +942,7 @@ export class CodexAcpServer {
             authProvider: authProvider,
             cwd: request.cwd,
             additionalDirectories: sessionMetadata.additionalDirectories,
-            fastModeEnabled: sessionMetadata.currentServiceTier === "fast",
+            fastModeEnabled: isFastServiceTier(sessionMetadata.currentServiceTier),
             currentModelSupportsFast: currentModelSupportsFast,
             sessionMcpServers: sessionMcpServers,
             terminalOutputMode: this.terminalOutputMode,
diff --git a/src/FastModeConfig.ts b/src/FastModeConfig.ts
index 8e681c24..d40593b5 100644
--- a/src/FastModeConfig.ts
+++ b/src/FastModeConfig.ts
@@ -18,6 +18,10 @@ export function resolveFastServiceTier(fastModeEnabled: boolean, currentModelSup
     return fastModeEnabled && currentModelSupportsFast ? "fast" : null;
 }
 
+export function isFastServiceTier(serviceTier: string | null | undefined): boolean {
+    return serviceTier === "fast" || serviceTier === "priority";
+}
+
 export function clientSupportsBooleanConfigOptions(clientCapabilities?: acp.ClientCapabilities | null): boolean {
     return clientCapabilities?.session?.configOptions?.boolean != null;
 }
diff --git a/src/__tests__/CodexACPAgent/fast-mode-config.test.ts b/src/__tests__/CodexACPAgent/fast-mode-config.test.ts
index 07d42444..16fa5302 100644
--- a/src/__tests__/CodexACPAgent/fast-mode-config.test.ts
+++ b/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,
     ) {
@@ -98,12 +98,15 @@ 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(["fast", "priority"] as const)(
+        "initializes Fast mode as On when the app-server session tier is %s",
+        async (serviceTier) => {
+            const {response, codexAcpAgent} = await createSession(serviceTier);
 
-        expect(response.configOptions).toContainEqual(createFastModeConfigOption(true));
-        expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
-    });
+            expect(response.configOptions).toContainEqual(createFastModeConfigOption(true));
+            expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
+        },
+    );
 
     it("omits Fast mode config options for JetBrains 2026.1 IntelliJ clients", async () => {
         const {response} = await createSession(null, {