diff --git a/src/proxy/__tests__/codex-api-headers.test.ts b/src/proxy/__tests__/codex-api-headers.test.ts
index 263f88a..8b73930 100644
--- a/src/proxy/__tests__/codex-api-headers.test.ts
+++ b/src/proxy/__tests__/codex-api-headers.test.ts
@@ -112,13 +112,37 @@ describe("codex-api headers", () => {
       expect(transport.lastHeaders!["x-codex-turn-state"]).toBeUndefined();
     });
 
-    it("excludes turnState and service_tier from JSON body", async () => {
+    it("excludes turnState from JSON body", async () => {
       const api = await createApi();
       await api.createResponse(
-        makeRequest({ turnState: "abc", service_tier: "fast" }),
+        makeRequest({ turnState: "abc" }),
       );
       const body = JSON.parse(transport.lastBody!) as Record<string, unknown>;
       expect(body.turnState).toBeUndefined();
+    });
+
+    it("maps service_tier 'fast' to 'priority' in JSON body", async () => {
+      const api = await createApi();
+      await api.createResponse(
+        makeRequest({ service_tier: "fast" }),
+      );
+      const body = JSON.parse(transport.lastBody!) as Record<string, unknown>;
+      expect(body.service_tier).toBe("priority");
+    });
+
+    it("passes non-fast service_tier through unchanged", async () => {
+      const api = await createApi();
+      await api.createResponse(
+        makeRequest({ service_tier: "flex" }),
+      );
+      const body = JSON.parse(transport.lastBody!) as Record<string, unknown>;
+      expect(body.service_tier).toBe("flex");
+    });
+
+    it("omits service_tier from body when null", async () => {
+      const api = await createApi();
+      await api.createResponse(makeRequest({ service_tier: null }));
+      const body = JSON.parse(transport.lastBody!) as Record<string, unknown>;
       expect(body.service_tier).toBeUndefined();
     });
   });
@@ -148,5 +172,45 @@ describe("codex-api headers", () => {
       );
       expect(headers["x-codex-turn-state"]).toBe("ws_turn_abc");
     });
+
+    it("maps service_tier 'fast' to 'priority' in WS request", async () => {
+      mockCreateWebSocketResponse.mockResolvedValue(
+        new Response("data: {}\n\n", {
+          headers: { "content-type": "text/event-stream" },
+        }),
+      );
+
+      const api = await createApi();
+      await api.createResponse(
+        makeRequest({
+          previous_response_id: "resp_prev",
+          useWebSocket: true,
+          service_tier: "fast",
+        }),
+      );
+
+      const wsRequest = mockCreateWebSocketResponse.mock.calls[0][2] as Record<string, unknown>;
+      expect(wsRequest.service_tier).toBe("priority");
+    });
+
+    it("passes non-fast service_tier through in WS request", async () => {
+      mockCreateWebSocketResponse.mockResolvedValue(
+        new Response("data: {}\n\n", {
+          headers: { "content-type": "text/event-stream" },
+        }),
+      );
+
+      const api = await createApi();
+      await api.createResponse(
+        makeRequest({
+          previous_response_id: "resp_prev",
+          useWebSocket: true,
+          service_tier: "flex",
+        }),
+      );
+
+      const wsRequest = mockCreateWebSocketResponse.mock.calls[0][2] as Record<string, unknown>;
+      expect(wsRequest.service_tier).toBe("flex");
+    });
   });
 });
diff --git a/src/proxy/codex-api.ts b/src/proxy/codex-api.ts
index 8846a8b..c706cf4 100644
--- a/src/proxy/codex-api.ts
+++ b/src/proxy/codex-api.ts
@@ -215,7 +215,7 @@ export class CodexApi {
     if (request.tools?.length) wsRequest.tools = request.tools;
     if (request.tool_choice) wsRequest.tool_choice = request.tool_choice;
     if (request.text) wsRequest.text = request.text;
-    // service_tier is stripped — Codex backend rejects it ("Unsupported service_tier")
+    if (request.service_tier) wsRequest.service_tier = request.service_tier === "fast" ? "priority" : request.service_tier;
     if (request.prompt_cache_key) wsRequest.prompt_cache_key = request.prompt_cache_key;
     if (request.include?.length) wsRequest.include = request.include;
 
@@ -243,8 +243,10 @@ export class CodexApi {
     headers["x-client-request-id"] = crypto.randomUUID();
     if (request.turnState) headers["x-codex-turn-state"] = request.turnState;
 
-    const { previous_response_id: _pid, useWebSocket: _ws, turnState: _ts, service_tier: _st, ...bodyFields } = request;
-    const body = JSON.stringify(bodyFields);
+    const { previous_response_id: _pid, useWebSocket: _ws, turnState: _ts, service_tier, ...bodyFields } = request;
+    const body = JSON.stringify(
+      service_tier ? { ...bodyFields, service_tier: service_tier === "fast" ? "priority" : service_tier } : bodyFields,
+    );
 
     let transportRes;
     try {
diff --git a/src/proxy/ws-transport.ts b/src/proxy/ws-transport.ts
index 1618648..3d93e08 100644
--- a/src/proxy/ws-transport.ts
+++ b/src/proxy/ws-transport.ts
@@ -51,6 +51,7 @@ export interface WsCreateRequest {
       strict?: boolean;
     };
   };
+  service_tier?: string;
   prompt_cache_key?: string;
   include?: string[];
   // NOTE: `store` and `stream` are intentionally omitted.