From 1b1ed4f5e19a5482f47d6642f0ef4fa0d841eee8 Mon Sep 17 00:00:00 2001
From: Amp <amp@ampcode.com>
Date: Wed, 11 Nov 2026 03:35:56 +0000
Subject: [PATCH] fix(0.x): derive chain ID from `v` in `TxEnvelopeLegacy`
 presign hash

Amp-Thread-ID: https://ampcode.com/threads/T-01a1c85c-51cb-75e9-8269-b23f8649f1c8
Co-authored-by: jxom <jake@tempo.xyz>
---
 .../tx-envelope-legacy-presign-chain-id.md    |  5 +++
 src/core/TxEnvelopeLegacy.ts                  |  7 +++
 .../_test/TransactionEnvelopeLegacy.test.ts   | 44 +++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 .changeset/tx-envelope-legacy-presign-chain-id.md

diff --git a/.changeset/tx-envelope-legacy-presign-chain-id.md b/.changeset/tx-envelope-legacy-presign-chain-id.md
new file mode 100644
index 000000000..a63a867a3
--- /dev/null
+++ b/.changeset/tx-envelope-legacy-presign-chain-id.md
@@ -0,0 +1,5 @@
+---
+"ox": patch
+---
+
+Fixed `TxEnvelopeLegacy.hash` (with `presign`) and `TxEnvelopeLegacy.getSignPayload` to derive the chain ID from an EIP-155 `v` when the envelope has no `chainId`.
diff --git a/src/core/TxEnvelopeLegacy.ts b/src/core/TxEnvelopeLegacy.ts
index 65560ab90..94fb48d5d 100644
--- a/src/core/TxEnvelopeLegacy.ts
+++ b/src/core/TxEnvelopeLegacy.ts
@@ -380,11 +380,18 @@ export function hash<presign extends boolean = false>(
   options: hash.Options<presign> = {},
 ): hash.ReturnType {
   const { presign } = options
+  const { chainId, v } = envelope
   return Hash.keccak256(
     serialize({
       ...envelope,
       ...(presign
         ? {
+            // An EIP-155 `v` carries the chain ID when `chainId` is absent.
+            chainId:
+              chainId ??
+              (typeof v === 'number' && v >= 37
+                ? Math.floor((v - 35) / 2)
+                : undefined),
             r: undefined,
             s: undefined,
             yParity: undefined,
diff --git a/src/core/_test/TransactionEnvelopeLegacy.test.ts b/src/core/_test/TransactionEnvelopeLegacy.test.ts
index 94e387e47..5c30b4ca1 100644
--- a/src/core/_test/TransactionEnvelopeLegacy.test.ts
+++ b/src/core/_test/TransactionEnvelopeLegacy.test.ts
@@ -509,6 +509,50 @@ describe('hash', () => {
       `"0x5e427e088ae00b084b41e198c52440aa43b2d6f8f1f01246fd25d4e9b6ebfdab"`,
     )
   })
+
+  test('options: presign: chain ID carried by `v`', () => {
+    const unsigned = {
+      gas: 21000n,
+      gasPrice: 1000000000n,
+      nonce: 665n,
+      to: '0x70997970c51812dc3a010c7d01b50e0d17dc79c8',
+      value: 1000000000000000000n,
+    } as const
+    const payload = TxEnvelopeLegacy.getSignPayload(
+      TxEnvelopeLegacy.from({ ...unsigned, chainId: 1 }),
+    )
+    const { r, s, yParity } = Secp256k1.sign({
+      payload,
+      privateKey: accounts[0].privateKey,
+    })
+
+    // EIP-155 `v`, no `chainId`
+    const envelope = TxEnvelopeLegacy.from({
+      ...unsigned,
+      r,
+      s,
+      v: 37 + yParity,
+    })
+    expect(
+      TxEnvelopeLegacy.hash(envelope, { presign: true }),
+    ).toMatchInlineSnapshot(
+      `"0x4c1dec0d90aa1a17cb0aa735b5550e43a1bd27cd1cacb8987522576fa6220e46"`,
+    )
+    expect(TxEnvelopeLegacy.hash(envelope, { presign: true })).toBe(payload)
+    expect(TxEnvelopeLegacy.getSignPayload(envelope)).toBe(
+      TxEnvelopeLegacy.getSignPayload(
+        TxEnvelopeLegacy.deserialize(TxEnvelopeLegacy.serialize(envelope)),
+      ),
+    )
+
+    // pre-EIP-155 `v`, no `chainId`
+    expect(
+      TxEnvelopeLegacy.hash(
+        TxEnvelopeLegacy.from({ ...unsigned, r, s, v: 27 + yParity }),
+        { presign: true },
+      ),
+    ).toBe(TxEnvelopeLegacy.getSignPayload(TxEnvelopeLegacy.from(unsigned)))
+  })
 })
 
 describe('serialize', () => {