fix: infer EIP-712 primaryType in viem adapter

src/provider/viem-adapter.ts

@@ -29,6 +29,23 @@ export interface ViemAdapterParams {
   rpcUrl?: string
 }
 
+/** Infer the EIP-712 root struct: the named type no other struct references. */
+function inferPrimaryType(types: Record<string, Array<{ type: string }>>): string {
+  const namedTypes = Object.keys(types).filter(key => key !== "EIP712Domain")
+  const referencedTypes = new Set<string>()
+
+  for (const fields of Object.values(types)) {
+    for (const field of fields) {
+      const referencedType = field.type.replace(/\[[^\]]*\]$/g, "")
+      if (namedTypes.includes(referencedType)) {
+        referencedTypes.add(referencedType)
+      }
+    }
+  }
+
+  return namedTypes.find(type => !referencedTypes.has(type)) ?? namedTypes[0] ?? ""
+}
+
 /**
  * Creates an OpenSeaWallet from viem clients.
  */
@@ -95,9 +112,7 @@ function createViemSigner(
           verifyingContract: domain.verifyingContract as `0x${string}`,
         },
         types,
-        primaryType:
-          Object.keys(types).find(key => key !== "EIP712Domain") ||
-          Object.keys(types)[0],
+        primaryType: inferPrimaryType(types),
         message: value,
         account: walletClient.account,
       })

test/provider/viem-adapter.spec.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test, vi } from "vitest"
+import { createViemWallet } from "../../src/provider/viem-adapter"
+
+describe("createViemWallet", () => {
+  test("infers the EIP-712 root primaryType instead of using the first type key", async () => {
+    const signTypedData = vi.fn().mockResolvedValue("0xsignature")
+    const walletClient = {
+      account: { address: "0x0000000000000000000000000000000000000001" },
+      chain: { id: 1 },
+      signTypedData,
+    }
+    const publicClient = {
+      waitForTransactionReceipt: vi.fn(),
+    }
+
+    const wallet = createViemWallet({
+      publicClient: publicClient as never,
+      walletClient: walletClient as never,
+    })
+    if (!("signer" in wallet)) {
+      throw new Error("expected signer")
+    }
+
+    const types = {
+      Person: [{ name: "wallet", type: "address" }],
+      Mail: [
+        { name: "from", type: "Person" },
+        { name: "contents", type: "string" },
+      ],
+    }
+
+    await wallet.signer.signTypedData(
+      {
+        name: "Example",
+        version: "1",
+        chainId: 1,
+        verifyingContract: "0x0000000000000000000000000000000000000002",
+      },
+      types,
+      {
+        from: { wallet: walletClient.account.address },
+        contents: "hello",
+      },
+    )
+
+    expect(signTypedData).toHaveBeenCalledWith(
+      expect.objectContaining({ primaryType: "Mail" }),
+    )
+  })
+})