diff --git a/.changeset/cool-cloths-fry.md b/.changeset/cool-cloths-fry.md
new file mode 100644
index 00000000..140e5fea
--- /dev/null
+++ b/.changeset/cool-cloths-fry.md
@@ -0,0 +1,6 @@
+---
+'@openai/agents-openai': patch
+'@openai/agents-core': patch
+---
+
+fix: support snake_case usage fields from OpenAI responses
diff --git a/packages/agents-core/src/usage.ts b/packages/agents-core/src/usage.ts
index 82bd9c72..e9894d2c 100644
--- a/packages/agents-core/src/usage.ts
+++ b/packages/agents-core/src/usage.ts
@@ -1,5 +1,15 @@
 import { UsageData } from './types/protocol';
 
+type UsageInput = Partial<
+  UsageData & {
+    input_tokens: number;
+    output_tokens: number;
+    total_tokens: number;
+    input_tokens_details: object;
+    output_tokens_details: object;
+  }
+> & { requests?: number };
+
 /**
  * Tracks token usage and request counts for an agent run.
  */
@@ -34,7 +44,7 @@ export class Usage {
    */
   public outputTokensDetails: Array<Record<string, number>> = [];
 
-  constructor(input?: Partial<UsageData> & { requests?: number }) {
+  constructor(input?: UsageInput) {
     if (typeof input === 'undefined') {
       this.requests = 0;
       this.inputTokens = 0;
@@ -44,14 +54,18 @@ export class Usage {
       this.outputTokensDetails = [];
     } else {
       this.requests = input?.requests ?? 1;
-      this.inputTokens = input?.inputTokens ?? 0;
-      this.outputTokens = input?.outputTokens ?? 0;
-      this.totalTokens = input?.totalTokens ?? 0;
-      this.inputTokensDetails = input?.inputTokensDetails
-        ? [input.inputTokensDetails]
+      this.inputTokens = input?.inputTokens ?? input?.input_tokens ?? 0;
+      this.outputTokens = input?.outputTokens ?? input?.output_tokens ?? 0;
+      this.totalTokens = input?.totalTokens ?? input?.total_tokens ?? 0;
+      const inputTokensDetails =
+        input?.inputTokensDetails ?? input?.input_tokens_details;
+      this.inputTokensDetails = inputTokensDetails
+        ? [inputTokensDetails as Record<string, number>]
         : [];
-      this.outputTokensDetails = input?.outputTokensDetails
-        ? [input.outputTokensDetails]
+      const outputTokensDetails =
+        input?.outputTokensDetails ?? input?.output_tokens_details;
+      this.outputTokensDetails = outputTokensDetails
+        ? [outputTokensDetails as Record<string, number>]
         : [];
     }
   }
diff --git a/packages/agents-core/test/usage.test.ts b/packages/agents-core/test/usage.test.ts
index 1a3d864a..dc215a93 100644
--- a/packages/agents-core/test/usage.test.ts
+++ b/packages/agents-core/test/usage.test.ts
@@ -26,6 +26,24 @@ describe('Usage', () => {
     expect(usage.totalTokens).toBe(15);
   });
 
+  it('falls back to snake_case fields', () => {
+    const usage = new Usage({
+      requests: 2,
+      input_tokens: 7,
+      output_tokens: 3,
+      total_tokens: 10,
+      input_tokens_details: { foo: 1 },
+      output_tokens_details: { bar: 2 },
+    });
+
+    expect(usage.requests).toBe(2);
+    expect(usage.inputTokens).toBe(7);
+    expect(usage.outputTokens).toBe(3);
+    expect(usage.totalTokens).toBe(10);
+    expect(usage.inputTokensDetails).toEqual([{ foo: 1 }]);
+    expect(usage.outputTokensDetails).toEqual([{ bar: 2 }]);
+  });
+
   it('adds other Usage instances correctly', () => {
     const usageA = new Usage({
       inputTokens: 1,
diff --git a/packages/agents-openai/test/openaiChatCompletionsModel.test.ts b/packages/agents-openai/test/openaiChatCompletionsModel.test.ts
index 18ed14de..eade88cc 100644
--- a/packages/agents-openai/test/openaiChatCompletionsModel.test.ts
+++ b/packages/agents-openai/test/openaiChatCompletionsModel.test.ts
@@ -79,6 +79,32 @@ describe('OpenAIChatCompletionsModel', () => {
     ]);
   });
 
+  it('parses usage tokens from snake_case fields', async () => {
+    const client = new FakeClient();
+    const response = {
+      id: 'r',
+      choices: [{ message: { content: 'hi' } }],
+      usage: { prompt_tokens: 11, completion_tokens: 7, total_tokens: 18 },
+    } as any;
+    client.chat.completions.create.mockResolvedValue(response);
+
+    const model = new OpenAIChatCompletionsModel(client as any, 'gpt');
+    const req: any = {
+      input: 'u',
+      modelSettings: {},
+      tools: [],
+      outputType: 'text',
+      handoffs: [],
+      tracing: false,
+    };
+
+    const result = await withTrace('t', () => model.getResponse(req));
+
+    expect(result.usage.inputTokens).toBe(11);
+    expect(result.usage.outputTokens).toBe(7);
+    expect(result.usage.totalTokens).toBe(18);
+  });
+
   it('outputs message when content is empty string', async () => {
     const client = new FakeClient();
     const response = {