diff --git a/bindings/node-adapters/__test__/viem.spec.mjs b/bindings/node-adapters/__test__/viem.spec.mjs
index 8748047fb..2668edeed 100644
--- a/bindings/node-adapters/__test__/viem.spec.mjs
+++ b/bindings/node-adapters/__test__/viem.spec.mjs
@@ -3,7 +3,8 @@ import assert from 'node:assert/strict';
 import { mkdtempSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
-import { createWallet, getWallet, signMessage as owsSignMessage } from '@open-wallet-standard/core';
+import { createWallet, getWallet, signMessage as owsSignMessage, importWalletPrivateKey } from '@open-wallet-standard/core';
+import { privateKeyToAccount } from 'viem/accounts';
 import { owsToViemAccount } from '../src/viem.js';
 
 describe('@open-wallet-standard/adapters — viem', () => {
@@ -59,6 +60,27 @@ describe('@open-wallet-standard/adapters — viem', () => {
     const td = { domain: { name: 'T', version: '1', chainId: '1', verifyingContract: '0x0000000000000000000000000000000000000001' }, types: { EIP712Domain: [{ name: 'name', type: 'string' }, { name: 'version', type: 'string' }, { name: 'chainId', type: 'uint256' }, { name: 'verifyingContract', type: 'address' }], M: [{ name: 'c', type: 'string' }] }, primaryType: 'M', message: { c: 'D' } };
     assert.equal(await account.signTypedData(td), await account.signTypedData(td));
   });
+  it('signTypedData matches viem for a uint256-bearing message', async () => {
+    // A wallet imported from a known key so the signature can be compared to
+    // viem's own privateKeyToAccount. The message carries uint256 bigints and
+    // omits EIP712Domain, matching how a real viem walletClient signs.
+    const pkVault = mkdtempSync(join(tmpdir(), 'ows-viem-pk-'));
+    const pk = '0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80';
+    importWalletPrivateKey('viem-pk-test', pk.slice(2), undefined, pkVault, 'evm');
+    const account = owsToViemAccount('viem-pk-test', { chain: 'eip155:1', vaultPath: pkVault });
+    const reference = privateKeyToAccount(pk);
+    const typedData = {
+      domain: { name: 'Test', version: '1', chainId: 1, verifyingContract: '0x0000000000000000000000000000000000000001' },
+      types: { Msg: [{ name: 'id', type: 'uint256' }, { name: 'ids', type: 'uint256[]' }] },
+      primaryType: 'Msg',
+      message: { id: 2n ** 200n, ids: [0n, 1n, 2n ** 256n - 1n] },
+    };
+    try {
+      assert.equal(await account.signTypedData(typedData), await reference.signTypedData(typedData));
+    } finally {
+      rmSync(pkVault, { recursive: true, force: true });
+    }
+  });
   it('signTransaction returns RLP-encoded signed transaction', async () => {
     const account = owsToViemAccount(walletName, { vaultPath: vaultDir });
     const tx = { to: '0x0000000000000000000000000000000000000001', value: 0n, chainId: 1, type: 'eip1559', maxFeePerGas: 1000000000n, maxPriorityFeePerGas: 1000000000n };
diff --git a/bindings/node-adapters/src/viem.js b/bindings/node-adapters/src/viem.js
index 4fbc51218..da73150bf 100644
--- a/bindings/node-adapters/src/viem.js
+++ b/bindings/node-adapters/src/viem.js
@@ -1,6 +1,15 @@
 const { getWallet, signMessage, signTypedData, signTransaction } = require("@open-wallet-standard/core");
 const { toAccount } = require("viem/accounts");
 
+// Encode a bigint as the core's EIP-712 parser expects a uint value: even-length
+// hex. Decimal uints above 2^128 are rejected, and hex must have an even number
+// of digits. Negative values fall back to decimal (int types).
+function bigintToOwsHex(value) {
+  if (value < 0n) return value.toString();
+  const hex = value.toString(16);
+  return `0x${hex.length % 2 === 1 ? `0${hex}` : hex}`;
+}
+
 function owsToViemAccount(walletNameOrId, options = {}) {
   const chain = options.chain ?? "eip155:1";
   const wallet = getWallet(walletNameOrId, options.vaultPath);
@@ -33,7 +42,23 @@ function owsToViemAccount(walletNameOrId, options = {}) {
       return serializeTransaction(transaction, { r, s, yParity });
     },
     async signTypedData(typedData) {
-      const result = signTypedData(walletNameOrId, chain, JSON.stringify(typedData), options.passphrase, options.index, options.vaultPath);
+      const { getTypesForEIP712Domain } = require("viem");
+      // viem's signTypedData action adds EIP712Domain to `types` before calling an
+      // account; a direct account.signTypedData() call does not. Add it when absent
+      // so the core resolves the domain type. A caller-supplied EIP712Domain wins.
+      const payload = {
+        ...typedData,
+        types: {
+          EIP712Domain: getTypesForEIP712Domain({ domain: typedData.domain }),
+          ...typedData.types,
+        },
+      };
+      // The core parses the JSON payload and expects uint values as even-length hex.
+      // JSON.stringify cannot serialize bigints, so encode them here.
+      const json = JSON.stringify(payload, (_key, value) =>
+        typeof value === "bigint" ? bigintToOwsHex(value) : value
+      );
+      const result = signTypedData(walletNameOrId, chain, json, options.passphrase, options.index, options.vaultPath);
       return result.signature.startsWith("0x") ? result.signature : `0x${result.signature}`;
     },
   });