Streaming Responses API fails Zod validation - usage field is null instead of object/undefined

[mattapperson]

Moved from OpenRouterTeam/typescript-sdk#146

Description

When using the Responses API with streaming, the response.created event fails Zod validation because the usage field is null, but the SDK schema only allows object | undefined, not null.

SDK Version

@openrouter/sdk@0.3.12

Steps to Reproduce

1. Make a streaming request to the Responses API
2. The first event (response.created) triggers a ZodError

import { OpenRouter } from "@openrouter/sdk";

const client = new OpenRouter();

const response = client.callModel({
    model: 'anthropic/claude-sonnet-4.5',
    input: 'Hello',
});

for await (const event of response.getTextStream()) {
    // Throws ZodError on first event
    console.log(event);
}

Error

ZodError invalid_union

ZodError: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": [
            "response",
            "usage"
          ],
          "message": "Invalid input: expected object, received null"
        }
      ],
      ...
    ]
  }
]

Expected Behavior

The SDK should handle null values for usage in streaming events, since initial events like response.created don't have usage data yet.

Workaround

None currently — the error prevents consuming the stream entirely.