diff --git a/.speakeasy/in.openapi.yaml b/.speakeasy/in.openapi.yaml
index 8cb8137f..e93072e8 100644
--- a/.speakeasy/in.openapi.yaml
+++ b/.speakeasy/in.openapi.yaml
@@ -12186,7 +12186,9 @@ components:
             message: Rate limit exceeded
             code: 429
         usage:
-          $ref: '#/components/schemas/ChatUsage'
+          nullable: true
+          allOf:
+            - $ref: '#/components/schemas/ChatUsage'
       required:
         - id
         - choices
diff --git a/src/models/chatstreamchunk.ts b/src/models/chatstreamchunk.ts
index f1503f3f..ab17073c 100644
--- a/src/models/chatstreamchunk.ts
+++ b/src/models/chatstreamchunk.ts
@@ -70,7 +70,7 @@ export type ChatStreamChunk = {
   /**
    * Token usage statistics
    */
-  usage?: ChatUsage | undefined;
+  usage?: ChatUsage | null | undefined;
 };
 
 /** @internal */
@@ -107,7 +107,7 @@ export const ChatStreamChunk$inboundSchema: z.ZodType<
   system_fingerprint: z.string().optional(),
   service_tier: z.nullable(z.string()).optional(),
   error: z.lazy(() => ErrorT$inboundSchema).optional(),
-  usage: ChatUsage$inboundSchema.optional(),
+  usage: z.nullable(ChatUsage$inboundSchema).optional(),
 }).transform((v) => {
   return remap$(v, {
     "system_fingerprint": "systemFingerprint",
diff --git a/tests/unit/issue-452-streaming-usage-null.test.ts b/tests/unit/issue-452-streaming-usage-null.test.ts
new file mode 100644
index 00000000..e1b7fe31
--- /dev/null
+++ b/tests/unit/issue-452-streaming-usage-null.test.ts
@@ -0,0 +1,120 @@
+/**
+ * Regression test for GitHub issue #452
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/issues/452
+ * (Moved from OpenRouterTeam/typescript-sdk#146)
+ *
+ * Reported error: ZodError invalid_union — "Invalid input: expected object, received null"
+ * on response.usage path when streaming
+ *
+ * This test verifies that streaming schemas accept usage: null,
+ * since early streaming events (e.g. response.created) and intermediate
+ * chat completion chunks may have usage: null before final usage is available.
+ */
+import { describe, it, expect } from 'vitest';
+import { ChatStreamChunk$inboundSchema } from '../../src/models/chatstreamchunk.js';
+import { StreamEvents$inboundSchema } from '../../src/models/streamevents.js';
+
+describe('Issue #452: streaming usage field accepts null', () => {
+  describe('ChatStreamChunk schema', () => {
+    it('should accept usage: null in a streaming chat completion chunk', () => {
+      const chunk = {
+        id: 'chatcmpl-test123',
+        choices: [
+          {
+            index: 0,
+            delta: { content: 'Hello' },
+            finish_reason: null,
+          },
+        ],
+        created: 1712345678,
+        model: 'anthropic/claude-sonnet-4.5',
+        object: 'chat.completion.chunk',
+        usage: null,
+      };
+
+      const result = ChatStreamChunk$inboundSchema.parse(chunk);
+      expect(result.id).toBe('chatcmpl-test123');
+      expect(result.usage).toBeNull();
+    });
+
+    it('should accept usage: undefined in a streaming chat completion chunk', () => {
+      const chunk = {
+        id: 'chatcmpl-test456',
+        choices: [
+          {
+            index: 0,
+            delta: { content: 'World' },
+            finish_reason: null,
+          },
+        ],
+        created: 1712345678,
+        model: 'anthropic/claude-sonnet-4.5',
+        object: 'chat.completion.chunk',
+      };
+
+      const result = ChatStreamChunk$inboundSchema.parse(chunk);
+      expect(result.id).toBe('chatcmpl-test456');
+      expect(result.usage).toBeUndefined();
+    });
+
+    it('should accept a valid usage object in a streaming chat completion chunk', () => {
+      const chunk = {
+        id: 'chatcmpl-test789',
+        choices: [
+          {
+            index: 0,
+            delta: {},
+            finish_reason: 'stop',
+          },
+        ],
+        created: 1712345678,
+        model: 'anthropic/claude-sonnet-4.5',
+        object: 'chat.completion.chunk',
+        usage: {
+          prompt_tokens: 10,
+          completion_tokens: 20,
+          total_tokens: 30,
+        },
+      };
+
+      const result = ChatStreamChunk$inboundSchema.parse(chunk);
+      expect(result.usage).toBeDefined();
+      expect(result.usage?.promptTokens).toBe(10);
+      expect(result.usage?.completionTokens).toBe(20);
+      expect(result.usage?.totalTokens).toBe(30);
+    });
+  });
+
+  describe('StreamEvents schema (Responses API)', () => {
+    it('should accept usage: null in a response.created event', () => {
+      const event = {
+        type: 'response.created',
+        response: {
+          id: 'resp_test123',
+          object: 'response',
+          created_at: 1712345678,
+          model: 'anthropic/claude-sonnet-4.5',
+          status: 'in_progress',
+          completed_at: null,
+          output: [],
+          error: null,
+          incomplete_details: null,
+          usage: null,
+          temperature: 1,
+          top_p: 1,
+          presence_penalty: 0,
+          frequency_penalty: 0,
+          instructions: null,
+          metadata: null,
+          tools: [],
+          tool_choice: 'auto',
+          parallel_tool_calls: true,
+        },
+        sequence_number: 0,
+      };
+
+      const result = StreamEvents$inboundSchema.parse(event);
+      expect(result.type).toBe('response.created');
+    });
+  });
+});