diff --git a/.changeset/nervous-lemons-admire.md b/.changeset/nervous-lemons-admire.md
new file mode 100644
index 00000000..8fbcb54a
--- /dev/null
+++ b/.changeset/nervous-lemons-admire.md
@@ -0,0 +1,5 @@
+---
+"@openai/agents-extensions": patch
+---
+
+fix: #791 support DeepSeek thinking tool calls
diff --git a/packages/agents-extensions/src/aiSdk.ts b/packages/agents-extensions/src/aiSdk.ts
index 627dce24..318ff226 100644
--- a/packages/agents-extensions/src/aiSdk.ts
+++ b/packages/agents-extensions/src/aiSdk.ts
@@ -42,13 +42,22 @@ import { encodeUint8ArrayToBase64 } from '@openai/agents/utils';
 export function itemsToLanguageV2Messages(
   model: LanguageModelV2,
   items: protocol.ModelItem[],
+  modelSettingsProviderData?: Record<string, any>,
 ): LanguageModelV2Message[] {
   const messages: LanguageModelV2Message[] = [];
   let currentAssistantMessage: LanguageModelV2Message | undefined;
+  const isDeepSeekThinkingMode = isDeepSeekModel(
+    model,
+    modelSettingsProviderData,
+  );
 
   for (const item of items) {
     if (item.type === 'message' || typeof item.type === 'undefined') {
       const { role, content, providerData } = item;
+      if (currentAssistantMessage && role !== 'assistant') {
+        messages.push(currentAssistantMessage);
+        currentAssistantMessage = undefined;
+      }
       if (role === 'system') {
         messages.push({
           role: 'system',
@@ -226,6 +235,29 @@ export function itemsToLanguageV2Messages(
       item.content.length > 0 &&
       typeof item.content[0].text === 'string'
     ) {
+      if (isDeepSeekThinkingMode) {
+        if (!currentAssistantMessage) {
+          currentAssistantMessage = {
+            role: 'assistant',
+            content: [],
+            providerOptions: {
+              ...(item.providerData ?? {}),
+            },
+          };
+        }
+        if (
+          Array.isArray(currentAssistantMessage.content) &&
+          currentAssistantMessage.role === 'assistant'
+        ) {
+          currentAssistantMessage.content.push({
+            type: 'reasoning',
+            text: item.content[0].text,
+            providerOptions: { ...(item.providerData ?? {}) },
+          } as any);
+        }
+        continue;
+      }
+
       messages.push({
         role: 'assistant',
         content: [
@@ -262,6 +294,26 @@ export function itemsToLanguageV2Messages(
   return messages;
 }
 
+function isDeepSeekModel(
+  model: LanguageModelV2,
+  modelSettingsProviderData?: Record<string, any>,
+): boolean {
+  const provider = typeof model.provider === 'string' ? model.provider : '';
+  if (!provider.toLowerCase().startsWith('deepseek')) {
+    return false;
+  }
+
+  const modelId = typeof model.modelId === 'string' ? model.modelId : '';
+  if (modelId === 'deepseek-reasoner') {
+    return true;
+  }
+
+  return (
+    modelSettingsProviderData?.providerOptions?.deepseek?.thinking?.type ===
+    'enabled'
+  );
+}
+
 /**
  * @internal
  * Converts a handoff to a language model V2 tool.
@@ -612,7 +664,11 @@ export class AiSdkModel implements Model {
                   content: [{ type: 'text', text: request.input }],
                 },
               ]
-            : itemsToLanguageV2Messages(this.#model, request.input);
+            : itemsToLanguageV2Messages(
+                this.#model,
+                request.input,
+                request.modelSettings.providerData,
+              );
 
         if (request.systemInstructions) {
           input = [
@@ -867,7 +923,11 @@ export class AiSdkModel implements Model {
                 content: [{ type: 'text', text: request.input }],
               },
             ]
-          : itemsToLanguageV2Messages(this.#model, request.input);
+          : itemsToLanguageV2Messages(
+              this.#model,
+              request.input,
+              request.modelSettings.providerData,
+            );
 
       if (request.systemInstructions) {
         input = [
diff --git a/packages/agents-extensions/test/aiSdk.test.ts b/packages/agents-extensions/test/aiSdk.test.ts
index 89714015..e51f2507 100644
--- a/packages/agents-extensions/test/aiSdk.test.ts
+++ b/packages/agents-extensions/test/aiSdk.test.ts
@@ -155,6 +155,176 @@ describe('itemsToLanguageV2Messages', () => {
     ]);
   });
 
+  test('merges reasoning into assistant tool-call message for DeepSeek thinking mode (deepseek-reasoner)', () => {
+    const deepseekModel = {
+      ...stubModel({}),
+      provider: 'deepseek.chat',
+      modelId: 'deepseek-reasoner',
+    } as any;
+    const items: protocol.ModelItem[] = [
+      { type: 'reasoning', content: [{ text: 'thinking' }] } as any,
+      {
+        type: 'function_call',
+        callId: '1',
+        name: 'foo',
+        arguments: '{}',
+      } as any,
+      {
+        type: 'function_call_result',
+        callId: '1',
+        name: 'foo',
+        output: { type: 'text', text: 'out' },
+      } as any,
+    ];
+
+    const msgs = itemsToLanguageV2Messages(deepseekModel, items);
+    expect(msgs).toEqual([
+      {
+        role: 'assistant',
+        content: [
+          { type: 'reasoning', text: 'thinking', providerOptions: {} },
+          {
+            type: 'tool-call',
+            toolCallId: '1',
+            toolName: 'foo',
+            input: {},
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+      {
+        role: 'tool',
+        content: [
+          {
+            type: 'tool-result',
+            toolCallId: '1',
+            toolName: 'foo',
+            output: { type: 'text', value: 'out' },
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+    ]);
+  });
+
+  test('does not merge reasoning into tool-call message for DeepSeek without thinking mode', () => {
+    const deepseekModel = {
+      ...stubModel({}),
+      provider: 'deepseek.chat',
+      modelId: 'deepseek-chat',
+    } as any;
+    const items: protocol.ModelItem[] = [
+      { type: 'reasoning', content: [{ text: 'thinking' }] } as any,
+      {
+        type: 'function_call',
+        callId: '1',
+        name: 'foo',
+        arguments: '{}',
+      } as any,
+      {
+        type: 'function_call_result',
+        callId: '1',
+        name: 'foo',
+        output: { type: 'text', text: 'out' },
+      } as any,
+    ];
+
+    const msgs = itemsToLanguageV2Messages(deepseekModel, items);
+    expect(msgs).toEqual([
+      {
+        role: 'assistant',
+        content: [{ type: 'reasoning', text: 'thinking', providerOptions: {} }],
+        providerOptions: {},
+      },
+      {
+        role: 'assistant',
+        content: [
+          {
+            type: 'tool-call',
+            toolCallId: '1',
+            toolName: 'foo',
+            input: {},
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+      {
+        role: 'tool',
+        content: [
+          {
+            type: 'tool-result',
+            toolCallId: '1',
+            toolName: 'foo',
+            output: { type: 'text', value: 'out' },
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+    ]);
+  });
+
+  test('merges reasoning into tool-call message for DeepSeek thinking mode (providerOptions)', () => {
+    const deepseekModel = {
+      ...stubModel({}),
+      provider: 'deepseek.chat',
+      modelId: 'deepseek-chat',
+    } as any;
+    const items: protocol.ModelItem[] = [
+      { type: 'reasoning', content: [{ text: 'thinking' }] } as any,
+      {
+        type: 'function_call',
+        callId: '1',
+        name: 'foo',
+        arguments: '{}',
+      } as any,
+      {
+        type: 'function_call_result',
+        callId: '1',
+        name: 'foo',
+        output: { type: 'text', text: 'out' },
+      } as any,
+    ];
+
+    const providerData = {
+      providerOptions: { deepseek: { thinking: { type: 'enabled' } } },
+    };
+
+    const msgs = itemsToLanguageV2Messages(deepseekModel, items, providerData);
+    expect(msgs).toEqual([
+      {
+        role: 'assistant',
+        content: [
+          { type: 'reasoning', text: 'thinking', providerOptions: {} },
+          {
+            type: 'tool-call',
+            toolCallId: '1',
+            toolName: 'foo',
+            input: {},
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+      {
+        role: 'tool',
+        content: [
+          {
+            type: 'tool-result',
+            toolCallId: '1',
+            toolName: 'foo',
+            output: { type: 'text', value: 'out' },
+            providerOptions: {},
+          },
+        ],
+        providerOptions: {},
+      },
+    ]);
+  });
+
   test('throws on built-in tool calls', () => {
     const items: protocol.ModelItem[] = [
       { type: 'hosted_tool_call', name: 'search' } as any,