fix: handle bigint chainId in TypedData.extractEip712DomainTypes (#98)

* fix: handle bigint chainId in TypedData.extractEip712DomainTypes

Fixes #97 where TypedData.getSignPayload produced invalid EIP-712 hash
when chainId was passed as bigint instead of number.

The extractEip712DomainTypes function now correctly handles both number
and bigint types for chainId field.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* Update late-kings-promise.md

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: jxom <[email protected]>

Author: 3 people

.changeset/late-kings-promise.md

@@ -0,0 +1,5 @@
+---
+"ox": patch
+---
+
+Added handling for `bigint` chain IDs in `TypedData.extractEip712DomainTypes`.

src/core/TypedData.ts

@@ -394,7 +394,8 @@ export function extractEip712DomainTypes(
   return [
     typeof domain?.name === 'string' && { name: 'name', type: 'string' },
     domain?.version && { name: 'version', type: 'string' },
-    typeof domain?.chainId === 'number' && {
+    (typeof domain?.chainId === 'number' ||
+      typeof domain?.chainId === 'bigint') && {
       name: 'chainId',
       type: 'uint256',
     },

src/core/_test/TypedData.test.ts

@@ -780,6 +780,37 @@ describe('getSignPayload', () => {
     )
   })
 
+  test('domain: bigint chainId produces same hash as number chainId', () => {
+    const typedDataWithNumberChainId = {
+      domain: {
+        name: 'Test',
+        version: '1',
+        chainId: 1,
+        verifyingContract: '0x0000000000000000000000000000000000000000',
+      },
+      types: {
+        Message: [{ name: 'content', type: 'string' }],
+      },
+      primaryType: 'Message',
+      message: {
+        content: 'Hello World',
+      },
+    } as const
+
+    const typedDataWithBigintChainId = {
+      ...typedDataWithNumberChainId,
+      domain: {
+        ...typedDataWithNumberChainId.domain,
+        chainId: 1n,
+      },
+    }
+
+    const hashWithNumber = TypedData.getSignPayload(typedDataWithNumberChainId)
+    const hashWithBigint = TypedData.getSignPayload(typedDataWithBigintChainId)
+
+    expect(hashWithNumber).toBe(hashWithBigint)
+  })
+
   test('minimal valid typed message', () => {
     const payload = TypedData.getSignPayload({
       types: {