diff --git a/open-sse/executors/index.ts b/open-sse/executors/index.ts
index f6af45bb5e..f67c2f63e1 100644
--- a/open-sse/executors/index.ts
+++ b/open-sse/executors/index.ts
@@ -49,6 +49,7 @@ import { QwenWebExecutor } from "./qwen-web.ts";
 import { KimiExecutor } from "./kimi.ts"
 import { TheOldLlmExecutor } from "./theoldllm.ts";
 import { ChipotleExecutor } from "./chipotle.ts";
+import { LMArenaExecutor } from "./lmarena.ts";
 
 const executors = {
   antigravity: new AntigravityExecutor(),
@@ -140,6 +141,8 @@ const executors = {
   tllm: new TheOldLlmExecutor(), // Alias
   chipotle: new ChipotleExecutor(),
   pepper: new ChipotleExecutor(), // Alias
+  lmarena: new LMArenaExecutor(),
+  lma: new LMArenaExecutor(), // Alias
 };
 
 const defaultCache = new Map();
@@ -198,3 +201,4 @@ export { InnerAiExecutor } from "./inner-ai.ts";
 export { QwenWebExecutor } from "./qwen-web.ts";
 export { TheOldLlmExecutor } from "./theoldllm.ts";
 export { ChipotleExecutor } from "./chipotle.ts";
+export { LMArenaExecutor } from "./lmarena.ts";
diff --git a/open-sse/executors/lmarena.ts b/open-sse/executors/lmarena.ts
new file mode 100644
index 0000000000..de4c18c3d0
--- /dev/null
+++ b/open-sse/executors/lmarena.ts
@@ -0,0 +1,414 @@
+/**
+ * LMArenaExecutor — LMArena Web Session Provider
+ *
+ * Routes requests through LMArena's web API using session credentials.
+ * LMArena is a model comparison platform with 100+ models (GPT, Claude, Gemini, Llama).
+ *
+ * API Structure:
+ *   Endpoint: https://arena.ai/nextjs-api/stream
+ *   Method: POST
+ *   Content-Type: application/json
+ *   Accept: text/event-stream
+ *
+ * Auth pipeline (per request):
+ *   1. Extract session cookie from credentials
+ *   2. Build request with model and messages
+ *   3. Make authenticated POST request to LMArena API
+ *   4. Handle SSE response stream with custom prefixes (a0:, ag:, a3:, ae:, ad:)
+ *
+ * SSE Format:
+ *   a0: - Text content (concatenate)
+ *   ag: - Thinking/reasoning content
+ *   a2: - Heartbeat (ignore)
+ *   a3: - Model error
+ *   ae: - Platform error
+ *   ad: - Done marker
+ */
+import { BaseExecutor, type ExecuteInput } from "./base.ts";
+import { sanitizeErrorMessage } from "../utils/error.ts";
+
+const LMARENA_API_BASE = "https://arena.ai";
+const LMARENA_STREAM_URL = `${LMARENA_API_BASE}/nextjs-api/stream`;
+
+const LMARENA_USER_AGENT =
+  "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36";
+
+function readLMArenaCookie(credentials: unknown): string {
+  if (!credentials || typeof credentials !== "object") return "";
+  const c = credentials as Record<string, unknown>;
+  const direct = typeof c.cookie === "string" ? c.cookie : "";
+  if (direct.trim()) return direct;
+  const apiKey = typeof c.apiKey === "string" ? c.apiKey : "";
+  if (apiKey.trim()) return apiKey;
+  const psd = c.providerSpecificData;
+  if (psd && typeof psd === "object") {
+    const nested = (psd as Record<string, unknown>).cookie;
+    if (typeof nested === "string" && nested.trim()) return nested;
+  }
+  return "";
+}
+
+interface ArenaSSEEvent {
+  type: "text" | "thinking" | "error" | "done" | "heartbeat";
+  content?: string;
+}
+
+export function parseArenaSSE(line: string): ArenaSSEEvent | null {
+  if (line.startsWith("a0:")) {
+    try {
+      const content = JSON.parse(line.substring(3));
+      return { type: "text", content: typeof content === "string" ? content : content.text || "" };
+    } catch {
+      return null;
+    }
+  } else if (line.startsWith("ag:")) {
+    try {
+      const content = JSON.parse(line.substring(3));
+      return { type: "thinking", content: typeof content === "string" ? content : content.thinking || "" };
+    } catch {
+      return null;
+    }
+  } else if (line.startsWith("a3:") || line.startsWith("ae:")) {
+    try {
+      const content = JSON.parse(line.substring(3));
+      return { type: "error", content: typeof content === "string" ? content : content.error || JSON.stringify(content) };
+    } catch {
+      return { type: "error", content: line.substring(3) };
+    }
+  } else if (line.startsWith("ad:")) {
+    return { type: "done" };
+  } else if (line.startsWith("a2:")) {
+    return { type: "heartbeat" };
+  }
+  return null;
+}
+
+export class LMArenaExecutor extends BaseExecutor {
+  constructor(providerConfig = {}) {
+    super("lmarena", { format: "openai", ...providerConfig });
+  }
+
+  protected buildUrl(_model: string, _credentials: unknown): string {
+    return LMARENA_STREAM_URL;
+  }
+
+  protected buildHeaders(
+    _model: string,
+    credentials: unknown,
+    _body: unknown
+  ): Record<string, string> {
+    const cookie = readLMArenaCookie(credentials);
+    const headers: Record<string, string> = {
+      "Content-Type": "application/json",
+      Accept: "text/event-stream",
+      "User-Agent": LMARENA_USER_AGENT,
+      Origin: LMARENA_API_BASE,
+      Referer: `${LMARENA_API_BASE}/`,
+    };
+
+    if (cookie) {
+      headers.Cookie = cookie;
+    }
+
+    return headers;
+  }
+
+  protected transformRequest(body: unknown, model: string): unknown {
+    const openaiBody = body as Record<string, unknown>;
+    const messages = openaiBody.messages as Array<{ role: string; content: string }>;
+    
+    return {
+      messages: messages.map(m => ({
+        role: m.role,
+        content: m.content,
+      })),
+      model,
+      stream: openaiBody.stream || false,
+    };
+  }
+
+  async execute(input: ExecuteInput): Promise<Response> {
+    const { model, body, stream, credentials, signal, log } = input;
+
+    const cookie = readLMArenaCookie(credentials);
+    if (!cookie) {
+      return new Response(
+        JSON.stringify({
+          error: {
+            message: "LMArena requires a session cookie. Please provide cookie in credentials.",
+            type: "authentication_error",
+            code: "missing_cookie",
+          },
+        }),
+        {
+          status: 401,
+          headers: { "Content-Type": "application/json" },
+        }
+      );
+    }
+
+    const url = this.buildUrl(model, credentials);
+    const headers = this.buildHeaders(model, credentials, body);
+    const transformedBody = this.transformRequest(body, model);
+
+    log?.info?.("LMArenaExecutor", `Executing request for model: ${model}`);
+
+    try {
+      const response = await fetch(url, {
+        method: "POST",
+        headers,
+        body: JSON.stringify(transformedBody),
+        signal,
+      });
+
+      if (!response.ok) {
+        const errorText = await response.text();
+        let errorMessage = `LMArena API error: ${response.status}`;
+        try {
+          const errorJson = JSON.parse(errorText);
+          errorMessage = errorJson.error?.message || errorJson.message || errorMessage;
+        } catch {
+          errorMessage = errorText || errorMessage;
+        }
+
+        return new Response(
+          JSON.stringify({
+            error: {
+              message: sanitizeErrorMessage(errorMessage),
+              type: "api_error",
+              code: String(response.status),
+            },
+          }),
+          {
+            status: response.status,
+            headers: { "Content-Type": "application/json" },
+          }
+        );
+      }
+
+      if (stream) {
+        return this.handleStreamingResponse(response, model, log);
+      } else {
+        return this.handleNonStreamingResponse(response, model, log);
+      }
+    } catch (error) {
+      const message = error instanceof Error ? error.message : String(error);
+      log?.error?.("LMArenaExecutor", `Request failed: ${message}`);
+
+      return new Response(
+        JSON.stringify({
+          error: {
+            message: sanitizeErrorMessage(message),
+            type: "network_error",
+            code: "request_failed",
+          },
+        }),
+        {
+          status: 502,
+          headers: { "Content-Type": "application/json" },
+        }
+      );
+    }
+  }
+
+  private async handleStreamingResponse(
+    response: Response,
+    model: string,
+    log?: ExecuteInput["log"]
+  ): Promise<Response> {
+    const reader = response.body?.getReader();
+    if (!reader) {
+      throw new Error("No response body for streaming");
+    }
+
+    const decoder = new TextDecoder();
+    let buffer = "";
+    let fullText = "";
+    let fullThinking = "";
+
+    const stream = new ReadableStream({
+      async start(controller) {
+        try {
+          while (true) {
+            const { done, value } = await reader.read();
+            if (done) break;
+
+            buffer += decoder.decode(value, { stream: true });
+            const lines = buffer.split("\n");
+            buffer = lines.pop() || "";
+
+            for (const line of lines) {
+              if (!line.trim()) continue;
+
+              const sseLine = line.startsWith("data: ") ? line.substring(6) : line;
+              const event = parseArenaSSE(sseLine);
+
+              if (!event) continue;
+
+              if (event.type === "text" && event.content) {
+                fullText += event.content;
+                const chunk = {
+                  id: `chatcmpl-${Date.now()}`,
+                  object: "chat.completion.chunk",
+                  created: Math.floor(Date.now() / 1000),
+                  model,
+                  choices: [
+                    {
+                      index: 0,
+                      delta: { content: event.content },
+                      finish_reason: null,
+                    },
+                  ],
+                };
+                controller.enqueue(`data: ${JSON.stringify(chunk)}\n\n`);
+              } else if (event.type === "thinking" && event.content) {
+                fullThinking += event.content;
+              } else if (event.type === "error") {
+                const errorChunk = {
+                  id: `chatcmpl-${Date.now()}`,
+                  object: "chat.completion.chunk",
+                  created: Math.floor(Date.now() / 1000),
+                  model,
+                  choices: [
+                    {
+                      index: 0,
+                      delta: {},
+                      finish_reason: "stop",
+                    },
+                  ],
+                  error: { message: event.content },
+                };
+                controller.enqueue(`data: ${JSON.stringify(errorChunk)}\n\n`);
+                controller.close();
+                return;
+              } else if (event.type === "done") {
+                const finalChunk = {
+                  id: `chatcmpl-${Date.now()}`,
+                  object: "chat.completion.chunk",
+                  created: Math.floor(Date.now() / 1000),
+                  model,
+                  choices: [
+                    {
+                      index: 0,
+                      delta: {},
+                      finish_reason: "stop",
+                    },
+                  ],
+                };
+                controller.enqueue(`data: ${JSON.stringify(finalChunk)}\n\n`);
+                controller.enqueue("data: [DONE]\n\n");
+                controller.close();
+                return;
+              }
+            }
+          }
+
+          const finalChunk = {
+            id: `chatcmpl-${Date.now()}`,
+            object: "chat.completion.chunk",
+            created: Math.floor(Date.now() / 1000),
+            model,
+            choices: [
+              {
+                index: 0,
+                delta: {},
+                finish_reason: "stop",
+              },
+            ],
+          };
+          controller.enqueue(`data: ${JSON.stringify(finalChunk)}\n\n`);
+          controller.enqueue("data: [DONE]\n\n");
+          controller.close();
+        } catch (error) {
+          const message = error instanceof Error ? error.message : String(error);
+          log?.error?.("LMArenaExecutor", `Streaming error: ${message}`);
+          controller.error(error);
+        }
+      },
+    });
+
+    return new Response(stream, {
+      status: 200,
+      headers: {
+        "Content-Type": "text/event-stream",
+        "Cache-Control": "no-cache",
+        Connection: "keep-alive",
+      },
+    });
+  }
+
+  private async handleNonStreamingResponse(
+    response: Response,
+    model: string,
+    log?: ExecuteInput["log"]
+  ): Promise<Response> {
+    const text = await response.text();
+    const lines = text.split("\n");
+    let fullText = "";
+    let fullThinking = "";
+    let error: string | null = null;
+
+    for (const line of lines) {
+      if (!line.trim()) continue;
+
+      const sseLine = line.startsWith("data: ") ? line.substring(6) : line;
+      const event = parseArenaSSE(sseLine);
+
+      if (!event) continue;
+
+      if (event.type === "text" && event.content) {
+        fullText += event.content;
+      } else if (event.type === "thinking" && event.content) {
+        fullThinking += event.content;
+      } else if (event.type === "error") {
+        error = event.content || "Unknown error";
+        break;
+      } else if (event.type === "done") {
+        break;
+      }
+    }
+
+    if (error) {
+      return new Response(
+        JSON.stringify({
+          error: {
+            message: sanitizeErrorMessage(error),
+            type: "api_error",
+            code: "lmarena_error",
+          },
+        }),
+        {
+          status: 502,
+          headers: { "Content-Type": "application/json" },
+        }
+      );
+    }
+
+    const result = {
+      id: `chatcmpl-${Date.now()}`,
+      object: "chat.completion",
+      created: Math.floor(Date.now() / 1000),
+      model,
+      choices: [
+        {
+          index: 0,
+          message: {
+            role: "assistant",
+            content: fullText,
+          },
+          finish_reason: "stop",
+        },
+      ],
+      usage: {
+        prompt_tokens: 0,
+        completion_tokens: 0,
+        total_tokens: 0,
+      },
+    };
+
+    return new Response(JSON.stringify(result), {
+      status: 200,
+      headers: { "Content-Type": "application/json" },
+    });
+  }
+}
diff --git a/src/shared/constants/providers.ts b/src/shared/constants/providers.ts
index de54566eee..b937becf80 100644
--- a/src/shared/constants/providers.ts
+++ b/src/shared/constants/providers.ts
@@ -470,6 +470,20 @@ export const WEB_COOKIE_PROVIDERS = {
     authHint:
       "Paste your __client cookie value from .clerk.agent.adapta.one (DevTools → Application → Cookies)",
   },
+  lmarena: {
+    id: "lmarena",
+    alias: "lma",
+    name: "LMArena (Free)",
+    icon: "auto_awesome",
+    color: "#FF6B6B",
+    textIcon: "LMA",
+    website: "https://lmarena.ai",
+    hasFree: true,
+    freeNote: "Free model comparison platform — 40+ models (GPT, Claude, Gemini, Llama). No subscription required.",
+    authHint:
+      "Paste your session cookie from lmarena.ai (DevTools → Application → Cookies). Optional — works with free tier for basic comparisons.",
+    riskNoticeVariant: "webCookie",
+  },
   huggingchat: {
     id: "huggingchat",
     // "hc" belongs to the hackclub provider; huggingchat uses its own id as alias.
diff --git a/src/shared/providers/webSessionCredentials.ts b/src/shared/providers/webSessionCredentials.ts
index bf4c4a190a..f3f159c554 100644
--- a/src/shared/providers/webSessionCredentials.ts
+++ b/src/shared/providers/webSessionCredentials.ts
@@ -192,6 +192,13 @@ export const WEB_SESSION_CREDENTIAL_REQUIREMENTS = {
     acceptsFullCookieHeader: true,
     storageKeys: ["cookie", "manus_session"],
   },
+  lmarena: {
+    kind: "cookie",
+    credentialName: "session",
+    placeholder: "session=... or full Cookie header from lmarena.ai",
+    acceptsFullCookieHeader: true,
+    storageKeys: ["cookie", "session"],
+  },
 } satisfies Record<keyof typeof WEB_COOKIE_PROVIDERS, WebSessionCredentialRequirement>;
 
 export function getWebSessionCredentialRequirement(
diff --git a/tests/unit/lmarena-provider.test.ts b/tests/unit/lmarena-provider.test.ts
new file mode 100644
index 0000000000..72982bfda7
--- /dev/null
+++ b/tests/unit/lmarena-provider.test.ts
@@ -0,0 +1,280 @@
+/**
+ * LMArena Provider — Unit Tests (Phase 2A of issue #3368)
+ *
+ * Run: node --import tsx/esm --test tests/unit/lmarena-provider.test.ts
+ */
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { WEB_COOKIE_PROVIDERS } from "../../src/shared/constants/providers.ts";
+import {
+  getWebSessionCredentialRequirement,
+  requiresWebSessionCredential,
+  hasUsableWebSessionCredential,
+} from "../../src/shared/providers/webSessionCredentials.ts";
+import { LMArenaExecutor, parseArenaSSE } from "../../open-sse/executors/lmarena.ts";
+
+describe("LMArena Provider Definition", () => {
+  it("is registered in WEB_COOKIE_PROVIDERS", () => {
+    assert.ok(WEB_COOKIE_PROVIDERS.lmarena, "lmarena should be in WEB_COOKIE_PROVIDERS");
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.id, "lmarena");
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.alias, "lma");
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.name, "LMArena (Free)");
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.website, "https://lmarena.ai");
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.hasFree, true);
+    assert.equal(WEB_COOKIE_PROVIDERS.lmarena.riskNoticeVariant, "webCookie");
+  });
+
+  it("has correct metadata", () => {
+    const provider = WEB_COOKIE_PROVIDERS.lmarena;
+    assert.ok(provider.freeNote, "Should have freeNote");
+    assert.ok(provider.authHint, "Should have authHint");
+    assert.ok(provider.icon, "Should have icon");
+    assert.ok(provider.color, "Should have color");
+    assert.ok(provider.textIcon, "Should have textIcon");
+  });
+});
+
+describe("LMArena Credential Requirements", () => {
+  it("requires web session credential", () => {
+    assert.equal(requiresWebSessionCredential("lmarena"), true);
+  });
+
+  it("has correct credential requirement", () => {
+    const req = getWebSessionCredentialRequirement("lmarena");
+    assert.ok(req, "Should have credential requirement");
+    assert.equal(req.kind, "cookie");
+    assert.equal(req.credentialName, "session");
+    assert.ok(req.placeholder.includes("lmarena.ai"));
+    assert.equal(req.acceptsFullCookieHeader, true);
+    assert.ok(req.storageKeys.includes("cookie"));
+    assert.ok(req.storageKeys.includes("session"));
+  });
+
+  it("validates usable credentials correctly", () => {
+    assert.equal(
+      hasUsableWebSessionCredential("lmarena", { cookie: "session=abc123" }),
+      true
+    );
+    assert.equal(
+      hasUsableWebSessionCredential("lmarena", { session: "abc123" }),
+      true
+    );
+    assert.equal(
+      hasUsableWebSessionCredential("lmarena", { cookie: "" }),
+      false
+    );
+    assert.equal(
+      hasUsableWebSessionCredential("lmarena", {}),
+      false
+    );
+  });
+});
+
+describe("LMArena Executor", () => {
+  it("can be instantiated", () => {
+    const executor = new LMArenaExecutor();
+    assert.ok(executor, "Executor should be instantiated");
+  });
+
+  it("has correct provider ID", () => {
+    const executor = new LMArenaExecutor();
+    assert.equal((executor as any).provider, "lmarena");
+  });
+
+  it("builds correct URL (arena.ai/nextjs-api/stream)", () => {
+    const executor = new LMArenaExecutor();
+    const url = (executor as any).buildUrl("gpt-4", {});
+    assert.ok(url.includes("arena.ai"), "URL should include arena.ai");
+    assert.ok(url.includes("/nextjs-api/stream"), "URL should include /nextjs-api/stream");
+  });
+
+  it("builds headers with cookie", () => {
+    const executor = new LMArenaExecutor();
+    const headers = (executor as any).buildHeaders("gpt-4", { cookie: "session=abc123" }, {});
+    assert.ok(headers.Cookie, "Should have Cookie header");
+    assert.equal(headers.Cookie, "session=abc123");
+    assert.equal(headers["Content-Type"], "application/json");
+    assert.equal(headers.Accept, "text/event-stream");
+  });
+
+  it("builds headers without cookie when not provided", () => {
+    const executor = new LMArenaExecutor();
+    const headers = (executor as any).buildHeaders("gpt-4", {}, {});
+    assert.ok(!headers.Cookie, "Should not have Cookie header when no cookie provided");
+  });
+
+  it("reads cookie from credentials correctly", () => {
+    const executor = new LMArenaExecutor();
+
+    // Direct cookie field
+    let headers = (executor as any).buildHeaders("gpt-4", { cookie: "session=abc" }, {});
+    assert.equal(headers.Cookie, "session=abc");
+
+    // apiKey field (dashboard form)
+    headers = (executor as any).buildHeaders("gpt-4", { apiKey: "session=def" }, {});
+    assert.equal(headers.Cookie, "session=def");
+
+    // providerSpecificData.cookie
+    headers = (executor as any).buildHeaders(
+      "gpt-4",
+      { providerSpecificData: { cookie: "session=ghi" } },
+      {}
+    );
+    assert.equal(headers.Cookie, "session=ghi");
+
+    // Priority: direct > apiKey > providerSpecificData
+    headers = (executor as any).buildHeaders(
+      "gpt-4",
+      { cookie: "session=abc", apiKey: "session=def" },
+      {}
+    );
+    assert.equal(headers.Cookie, "session=abc");
+  });
+
+  it("parses LMArena SSE text events (a0: prefix)", () => {
+    const textEvent = 'a0:{"text":"Hello, world!"}';
+    const result = parseArenaSSE(textEvent);
+    
+    assert.ok(result, "Should parse text event");
+    assert.equal(result.type, "text");
+    assert.equal(result.content, "Hello, world!");
+  });
+
+  it("parses LMArena SSE thinking events (ag: prefix)", () => {
+    const thinkingEvent = 'ag:{"thinking":"Let me analyze this..."}';
+    const result = parseArenaSSE(thinkingEvent);
+    
+    assert.ok(result, "Should parse thinking event");
+    assert.equal(result.type, "thinking");
+    assert.equal(result.content, "Let me analyze this...");
+  });
+
+  it("parses LMArena SSE error events (a3: and ae: prefixes)", () => {
+    const errorEvent1 = 'a3:{"error":"Rate limit exceeded"}';
+    const result1 = parseArenaSSE(errorEvent1);
+    assert.ok(result1, "Should parse a3: error event");
+    assert.equal(result1.type, "error");
+    assert.equal(result1.content, "Rate limit exceeded");
+
+    const errorEvent2 = 'ae:{"error":"Invalid session"}';
+    const result2 = parseArenaSSE(errorEvent2);
+    assert.ok(result2, "Should parse ae: error event");
+    assert.equal(result2.type, "error");
+    assert.equal(result2.content, "Invalid session");
+  });
+
+  it("parses LMArena SSE done event (ad: prefix)", () => {
+    const doneEvent = 'ad:{}';
+    const result = parseArenaSSE(doneEvent);
+    
+    assert.ok(result, "Should parse done event");
+    assert.equal(result.type, "done");
+  });
+
+  it("handles malformed SSE events gracefully", () => {
+    const malformedEvent = 'invalid:data';
+    const result = parseArenaSSE(malformedEvent);
+    
+    assert.equal(result, null, "Should return null for malformed events");
+  });
+
+  it("transforms OpenAI messages to LMArena format", () => {
+    const executor = new LMArenaExecutor();
+    const transformRequest = (executor as any).transformRequest.bind(executor);
+    
+    const openaiBody = {
+      messages: [
+        { role: "system", content: "You are a helpful assistant." },
+        { role: "user", content: "Hello!" },
+        { role: "assistant", content: "Hi there!" },
+        { role: "user", content: "How are you?" }
+      ],
+      model: "gpt-4",
+      stream: true
+    };
+    
+    const arenaBody = transformRequest(openaiBody, "gpt-4");
+    
+    assert.ok(arenaBody, "Should transform request body");
+    assert.ok(arenaBody.messages, "Should have messages array");
+    assert.equal(arenaBody.model, "gpt-4", "Should preserve model");
+    assert.equal(arenaBody.stream, true, "Should preserve stream flag");
+  });
+
+  it("returns 401 when cookie is missing", async () => {
+    const executor = new LMArenaExecutor();
+    
+    const response = await executor.execute({
+      model: "gpt-4",
+      body: { messages: [{ role: "user", content: "Hello" }] },
+      credentials: {},
+      signal: new AbortController().signal,
+      log: console
+    });
+    
+    assert.equal(response.status, 401, "Should return 401 for missing cookie");
+    const errorBody = await response.json();
+    assert.ok(errorBody.error, "Should have error object");
+    assert.ok(errorBody.error.message.includes("cookie"), "Error should mention cookie");
+  });
+
+  it("handles streaming response correctly", async () => {
+    const executor = new LMArenaExecutor();
+    
+    const mockSSE = [
+      'data: a0:{"text":"Hello"}\n\n',
+      'data: a0:{"text":", world!"}\n\n',
+      'data: ad:{}\n\n'
+    ].join('');
+    
+    const originalFetch = global.fetch;
+    global.fetch = async () => new Response(mockSSE, {
+      status: 200,
+      headers: { "Content-Type": "text/event-stream" }
+    });
+    
+    try {
+      const response = await executor.execute({
+        model: "gpt-4",
+        body: { messages: [{ role: "user", content: "Hello" }], stream: true },
+        credentials: { cookie: "session=test" },
+        signal: new AbortController().signal,
+        log: console
+      });
+      
+      assert.equal(response.status, 200, "Should return 200 for successful streaming");
+      assert.ok(response.body, "Should have response body for streaming");
+    } finally {
+      global.fetch = originalFetch;
+    }
+  });
+
+  it("handles error response from LMArena API", async () => {
+    const executor = new LMArenaExecutor();
+    
+    const originalFetch = global.fetch;
+    global.fetch = async () => new Response(JSON.stringify({
+      error: { message: "Rate limit exceeded" }
+    }), {
+      status: 429,
+      headers: { "Content-Type": "application/json" }
+    });
+    
+    try {
+      const response = await executor.execute({
+        model: "gpt-4",
+        body: { messages: [{ role: "user", content: "Hello" }] },
+        credentials: { cookie: "session=test" },
+        signal: new AbortController().signal,
+        log: console
+      });
+      
+      assert.equal(response.status, 429, "Should return 429 for rate limit");
+      const errorBody = await response.json();
+      assert.ok(errorBody.error, "Should have error object");
+    } finally {
+      global.fetch = originalFetch;
+    }
+  });
+});