diff --git a/src/agents/openai-ws-connection.test.ts b/src/agents/openai-ws-connection.test.ts
index 57f4ff896a4..7bc143942db 100644
--- a/src/agents/openai-ws-connection.test.ts
+++ b/src/agents/openai-ws-connection.test.ts
@@ -9,11 +9,12 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import type { ClientOptions } from "ws";
 import type {
   ClientEvent,
+  ErrorEvent,
   OpenAIWebSocketEvent,
   ResponseCompletedEvent,
   ResponseCreateEvent,
 } from "./openai-ws-connection.js";
-import { OpenAIWebSocketManager } from "./openai-ws-connection.js";
+import { getOpenAIWebSocketErrorDetails, OpenAIWebSocketManager } from "./openai-ws-connection.js";
 
 // ─────────────────────────────────────────────────────────────────────────────
 // Mock WebSocket (hoisted so vi.mock factory can reference it)
@@ -661,6 +662,27 @@ describe("OpenAIWebSocketManager", () => {
   // ─── Error handling ─────────────────────────────────────────────────────────
 
   describe("error handling", () => {
+    it("normalizes nested websocket error payloads", () => {
+      const details = getOpenAIWebSocketErrorDetails({
+        type: "error",
+        status: 400,
+        error: {
+          type: "invalid_request_error",
+          code: "previous_response_not_found",
+          message: "Previous response with id 'resp_abc' not found.",
+          param: "previous_response_id",
+        },
+      } satisfies ErrorEvent);
+
+      expect(details).toEqual({
+        status: 400,
+        type: "invalid_request_error",
+        code: "previous_response_not_found",
+        message: "Previous response with id 'resp_abc' not found.",
+        param: "previous_response_id",
+      });
+    });
+
     it("emits error event on malformed JSON message", async () => {
       const manager = buildManager();
       const sock = await connectManagerAndGetSocket(manager);
diff --git a/src/agents/openai-ws-connection.ts b/src/agents/openai-ws-connection.ts
index c044531f820..52d6f8623ff 100644
--- a/src/agents/openai-ws-connection.ts
+++ b/src/agents/openai-ws-connection.ts
@@ -34,6 +34,7 @@ export interface ResponseObject {
   output: OutputItem[];
   usage?: UsageInfo;
   error?: { code: string; message: string };
+  incomplete_details?: { reason?: string };
 }
 
 export interface UsageInfo {
@@ -160,9 +161,16 @@ export interface RateLimitUpdatedEvent {
 
 export interface ErrorEvent {
   type: "error";
-  code: string;
-  message: string;
+  status?: number;
+  code?: string;
+  message?: string;
   param?: string;
+  error?: {
+    type?: string;
+    code?: string;
+    message?: string;
+    param?: string;
+  };
 }
 
 export type OpenAIWebSocketEvent =
@@ -568,3 +576,19 @@ export class OpenAIWebSocketManager extends EventEmitter<InternalEvents> {
     this.send(event);
   }
 }
+
+export function getOpenAIWebSocketErrorDetails(event: ErrorEvent): {
+  status?: number;
+  type?: string;
+  code?: string;
+  message?: string;
+  param?: string;
+} {
+  return {
+    status: typeof event.status === "number" ? event.status : undefined,
+    type: event.error?.type,
+    code: event.error?.code ?? event.code,
+    message: event.error?.message ?? event.message,
+    param: event.error?.param ?? event.param,
+  };
+}
diff --git a/src/agents/openai-ws-stream.test.ts b/src/agents/openai-ws-stream.test.ts
index f82e54e6116..cfe25254ea8 100644
--- a/src/agents/openai-ws-stream.test.ts
+++ b/src/agents/openai-ws-stream.test.ts
@@ -1455,6 +1455,40 @@ describe("createOpenAIWebSocketStreamFn", () => {
     expect(doneEvent?.message?.content?.[0]?.text).toBe("http fallback response");
   });
 
+  it("falls back to HTTP when OpenAI sends a nested websocket error payload", async () => {
+    const streamFn = createOpenAIWebSocketStreamFn("sk-test", "sess-runtime-fallback-nested");
+    const stream = streamFn(
+      modelStub as Parameters<typeof streamFn>[0],
+      contextStub as Parameters<typeof streamFn>[1],
+      { transport: "auto" } as Parameters<typeof streamFn>[2],
+    );
+
+    await new Promise((r) => setImmediate(r));
+    const manager = MockManager.lastInstance!;
+    manager.simulateEvent({
+      type: "error",
+      status: 400,
+      error: {
+        type: "invalid_request_error",
+        code: "previous_response_not_found",
+        message: "Previous response with id 'resp_abc' not found.",
+        param: "previous_response_id",
+      },
+    });
+
+    const events: Array<{ type?: string; message?: { content?: Array<{ text?: string }> } }> = [];
+    for await (const ev of await resolveStream(stream)) {
+      events.push(ev as { type?: string; message?: { content?: Array<{ text?: string }> } });
+    }
+
+    expect(streamSimpleCalls.length).toBeGreaterThanOrEqual(1);
+    expect(manager.closeCallCount).toBeGreaterThanOrEqual(1);
+    expect(events.filter((event) => event.type === "start")).toHaveLength(1);
+    expect(events.some((event) => event.type === "error")).toBe(false);
+    const doneEvent = events.find((event) => event.type === "done");
+    expect(doneEvent?.message?.content?.[0]?.text).toBe("http fallback response");
+  });
+
   it("tracks previous_response_id across turns (incremental send)", async () => {
     const sessionId = "sess-incremental";
     const streamFn = createOpenAIWebSocketStreamFn("sk-test", sessionId);
diff --git a/src/agents/openai-ws-stream.ts b/src/agents/openai-ws-stream.ts
index 24b16ef94bf..5114f41e292 100644
--- a/src/agents/openai-ws-stream.ts
+++ b/src/agents/openai-ws-stream.ts
@@ -30,6 +30,7 @@ import type {
 } from "@mariozechner/pi-ai";
 import * as piAi from "@mariozechner/pi-ai";
 import {
+  getOpenAIWebSocketErrorDetails,
   OpenAIWebSocketManager,
   type FunctionToolDefinition,
   type OpenAIWebSocketManagerOptions,
@@ -231,6 +232,37 @@ function resetWsSession(params: { sessionId: string; session: WsSession }): void
   wsRegistry.delete(params.sessionId);
 }
 
+function formatOpenAIWebSocketError(
+  event: Parameters<OpenAIWebSocketManager["onMessage"]>[0] extends (arg: infer T) => void
+    ? Extract<T, { type: "error" }>
+    : never,
+): string {
+  const details = getOpenAIWebSocketErrorDetails(event);
+  const code = details.code ?? "unknown";
+  const message = details.message ?? "Unknown error";
+  const extras = [
+    typeof details.status === "number" ? `status=${details.status}` : null,
+    details.type ? `type=${details.type}` : null,
+    details.param ? `param=${details.param}` : null,
+  ].filter(Boolean);
+  return extras.length > 0
+    ? `${message} (code=${code}; ${extras.join(", ")})`
+    : `${message} (code=${code})`;
+}
+
+function formatOpenAIWebSocketResponseFailure(response: {
+  error?: { code?: string; message?: string };
+  incomplete_details?: { reason?: string };
+}): string {
+  if (response.error) {
+    return `${response.error.code || "unknown"}: ${response.error.message || "no message"}`;
+  }
+  if (response.incomplete_details?.reason) {
+    return `incomplete: ${response.incomplete_details.reason}`;
+  }
+  return "Unknown error (no error details in response)";
+}
+
 async function runWarmUp(params: {
   manager: OpenAIWebSocketManager;
   modelId: string;
@@ -261,11 +293,12 @@ async function runWarmUp(params: {
         resolve();
       } else if (event.type === "response.failed") {
         cleanup();
-        const errMsg = event.response?.error?.message ?? "Response failed";
-        reject(new Error(`warm-up failed: ${errMsg}`));
+        reject(
+          new Error(`warm-up failed: ${formatOpenAIWebSocketResponseFailure(event.response)}`),
+        );
       } else if (event.type === "error") {
         cleanup();
-        reject(new Error(`warm-up error: ${event.message} (code=${event.code})`));
+        reject(new Error(`warm-up error: ${formatOpenAIWebSocketError(event)}`));
       }
     });
 
@@ -609,11 +642,14 @@ export function createOpenAIWebSocketStreamFn(
               resolve();
             } else if (event.type === "response.failed") {
               cleanup();
-              const errMsg = event.response?.error?.message ?? "Response failed";
-              reject(new Error(`OpenAI WebSocket response failed: ${errMsg}`));
+              reject(
+                new Error(
+                  `OpenAI WebSocket response failed: ${formatOpenAIWebSocketResponseFailure(event.response)}`,
+                ),
+              );
             } else if (event.type === "error") {
               cleanup();
-              reject(new Error(`OpenAI WebSocket error: ${event.message} (code=${event.code})`));
+              reject(new Error(`OpenAI WebSocket error: ${formatOpenAIWebSocketError(event)}`));
             } else if (event.type === "response.output_text.delta") {
               // Stream partial text updates for responsive UI
               const partialMsg: AssistantMessage = buildAssistantMessageWithZeroUsage({