diff --git a/src/agents/openai-transport-stream.test.ts b/src/agents/openai-transport-stream.test.ts
index 59aae01b080..4971112bc55 100644
--- a/src/agents/openai-transport-stream.test.ts
+++ b/src/agents/openai-transport-stream.test.ts
@@ -19,6 +19,74 @@ import {
 import { SYSTEM_PROMPT_CACHE_BOUNDARY } from "./system-prompt-cache-boundary.js";
 
 describe("openai transport stream", () => {
+  it("moves Azure OpenAI completions api-version headers into default query params", () => {
+    const config = __testing.buildOpenAICompletionsClientConfig(
+      {
+        id: "gpt-4o-mini",
+        name: "GPT-4o Mini",
+        api: "openai-completions",
+        provider: "azure-custom",
+        baseUrl: "https://example.openai.azure.com/openai/deployments/gpt-4o-mini?existing=1",
+        headers: {
+          "api-key": "azure-key",
+          "api-version": "2024-10-21",
+          "X-Tenant": "acme",
+        },
+        reasoning: false,
+        input: ["text"],
+        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+        contextWindow: 128000,
+        maxTokens: 4096,
+      } satisfies Model<"openai-completions">,
+      { systemPrompt: "", messages: [] } as never,
+    );
+
+    expect(config).toEqual({
+      baseURL: "https://example.openai.azure.com/openai/deployments/gpt-4o-mini",
+      defaultHeaders: {
+        "api-key": "azure-key",
+        "X-Tenant": "acme",
+      },
+      defaultQuery: {
+        existing: "1",
+        "api-version": "2024-10-21",
+      },
+    });
+  });
+
+  it("preserves configured base URL query params without moving non-Azure headers", () => {
+    const config = __testing.buildOpenAICompletionsClientConfig(
+      {
+        id: "proxy-model",
+        name: "Proxy Model",
+        api: "openai-completions",
+        provider: "custom-proxy",
+        baseUrl: "https://proxy.example.com/v1?tenant=acme",
+        headers: {
+          "api-version": "proxy-header",
+          "X-Tenant": "acme",
+        },
+        reasoning: false,
+        input: ["text"],
+        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+        contextWindow: 128000,
+        maxTokens: 4096,
+      } satisfies Model<"openai-completions">,
+      { systemPrompt: "", messages: [] } as never,
+    );
+
+    expect(config).toEqual({
+      baseURL: "https://proxy.example.com/v1",
+      defaultHeaders: {
+        "api-version": "proxy-header",
+        "X-Tenant": "acme",
+      },
+      defaultQuery: {
+        tenant: "acme",
+      },
+    });
+  });
+
   it("reports the supported transport-aware APIs", () => {
     expect(isTransportAwareApiSupported("openai-responses")).toBe(true);
     expect(isTransportAwareApiSupported("openai-codex-responses")).toBe(true);
diff --git a/src/agents/openai-transport-stream.ts b/src/agents/openai-transport-stream.ts
index d98f97560f0..9e3e8869643 100644
--- a/src/agents/openai-transport-stream.ts
+++ b/src/agents/openai-transport-stream.ts
@@ -963,15 +963,73 @@ function createOpenAICompletionsClient(
   apiKey: string,
   optionHeaders?: Record<string, string>,
 ) {
+  const clientConfig = buildOpenAICompletionsClientConfig(model, context, optionHeaders);
   return new OpenAI({
     apiKey,
-    baseURL: model.baseUrl,
+    baseURL: clientConfig.baseURL,
     dangerouslyAllowBrowser: true,
-    defaultHeaders: buildOpenAIClientHeaders(model, context, optionHeaders),
+    defaultHeaders: clientConfig.defaultHeaders,
+    defaultQuery: clientConfig.defaultQuery,
     fetch: buildGuardedModelFetch(model),
   });
 }
 
+function isAzureOpenAICompatibleHost(hostname: string): boolean {
+  return (
+    hostname.endsWith(".openai.azure.com") ||
+    hostname.endsWith(".services.ai.azure.com") ||
+    hostname.endsWith(".cognitiveservices.azure.com")
+  );
+}
+
+function buildOpenAICompletionsClientConfig(
+  model: Model<Api>,
+  context: Context,
+  optionHeaders?: Record<string, string>,
+): {
+  baseURL: string;
+  defaultHeaders: Record<string, string>;
+  defaultQuery?: Record<string, string>;
+} {
+  const headers = buildOpenAIClientHeaders(model, context, optionHeaders);
+  const defaultQuery: Record<string, string> = {};
+  let baseURL = model.baseUrl;
+  let isAzureHost = false;
+
+  try {
+    const parsed = new URL(model.baseUrl);
+    isAzureHost = isAzureOpenAICompatibleHost(parsed.hostname.toLowerCase());
+    parsed.searchParams.forEach((value, key) => {
+      if (value) {
+        defaultQuery[key] = value;
+      }
+    });
+    parsed.search = "";
+    baseURL = parsed.toString().replace(/\/$/, "");
+  } catch {
+    // Keep the configured base URL unchanged; the OpenAI SDK will surface invalid URLs.
+  }
+
+  if (isAzureHost) {
+    const apiVersionHeader = Object.keys(headers).find(
+      (key) => key.toLowerCase() === "api-version",
+    );
+    if (apiVersionHeader) {
+      const apiVersion = headers[apiVersionHeader]?.trim();
+      delete headers[apiVersionHeader];
+      if (apiVersion && !defaultQuery["api-version"]) {
+        defaultQuery["api-version"] = apiVersion;
+      }
+    }
+  }
+
+  return {
+    baseURL,
+    defaultHeaders: headers,
+    defaultQuery: Object.keys(defaultQuery).length > 0 ? defaultQuery : undefined,
+  };
+}
+
 export function createOpenAICompletionsTransportStreamFn(): StreamFn {
   return (model, context, options) => {
     const eventStream = createAssistantMessageEventStream();
@@ -1577,5 +1635,6 @@ function mapStopReason(reason: string | null) {
 }
 
 export const __testing = {
+  buildOpenAICompletionsClientConfig,
   processOpenAICompletionsStream,
 };