diff --git a/modules/abstract-utxo/src/address/fixedScript.ts b/modules/abstract-utxo/src/address/fixedScript.ts
index 6c77abbf9e..fd3002124a 100644
--- a/modules/abstract-utxo/src/address/fixedScript.ts
+++ b/modules/abstract-utxo/src/address/fixedScript.ts
@@ -78,6 +78,10 @@ export function generateAddress(coinName: UtxoCoinName, params: GenerateFixedScr
 
   const { keychains, chain, segwit = false, bech32 = false } = params as GenerateFixedScriptAddressOptions;
 
+  if (_.isNumber(chain) && _.isInteger(chain) && !fixedScriptWallet.ChainCode.is(chain)) {
+    throw new InvalidAddressDerivationPropertyError(`address validation failure: unrecognised chain code (${chain})`);
+  }
+
   let derivationChain: ChainCode = fixedScriptWallet.ChainCode.value('p2sh', 'external');
   if (_.isNumber(chain) && _.isInteger(chain) && fixedScriptWallet.ChainCode.is(chain)) {
     derivationChain = chain;
diff --git a/modules/abstract-utxo/test/unit/address.ts b/modules/abstract-utxo/test/unit/address.ts
index 3e64975ba0..8a66840f64 100644
--- a/modules/abstract-utxo/test/unit/address.ts
+++ b/modules/abstract-utxo/test/unit/address.ts
@@ -1,6 +1,7 @@
 import * as assert from 'assert';
 
 import { InvalidAddressDerivationPropertyError, UnexpectedAddressError } from '@bitgo/sdk-core';
+import { fixedScriptWallet } from '@bitgo/wasm-utxo';
 
 import { assertFixedScriptWalletAddress, generateAddress } from '../../src';
 
@@ -8,6 +9,10 @@ import { keychainsBase58 } from './util';
 
 const keychains = keychainsBase58.map((k) => ({ pub: k.pub }));
 
+// A chain code that no released SDK version has ever defined — simulates a future
+// server-side address type unknown to an older client.
+const unknownChainCode = 99;
+
 describe('assertFixedScriptWalletAddress', function () {
   describe('input validation', function () {
     it('throws InvalidAddressDerivationPropertyError when both chain and index are undefined', function () {
@@ -121,6 +126,40 @@ describe('assertFixedScriptWalletAddress', function () {
       );
     });
 
+    // Regression guard for T1-3386 / T1-3385: an unknown chain code must throw
+    // immediately with a clear message rather than silently falling back to P2SH
+    // and producing a confusing "expected <P2SH> but got <P2TR>" error.
+    it('throws InvalidAddressDerivationPropertyError for an unknown chain code', function () {
+      assert.ok(!fixedScriptWallet.ChainCode.is(unknownChainCode), 'test prerequisite: chain must be unknown');
+      const serverAddress = generateAddress('btc', { keychains, chain: 40 });
+      assert.throws(
+        () =>
+          assertFixedScriptWalletAddress('btc', {
+            chain: unknownChainCode,
+            index: 0,
+            keychains,
+            format: 'base58',
+            address: serverAddress,
+          }),
+        (err: unknown) => {
+          assert.ok(err instanceof InvalidAddressDerivationPropertyError);
+          assert.ok(
+            err.message.includes(String(unknownChainCode)),
+            `expected error to name the unrecognised chain code, got: ${err.message}`
+          );
+          return true;
+        }
+      );
+    });
+
+    it('generateAddress throws InvalidAddressDerivationPropertyError for an unknown chain code', function () {
+      assert.ok(!fixedScriptWallet.ChainCode.is(unknownChainCode), 'test prerequisite: chain must be unknown');
+      assert.throws(
+        () => generateAddress('btc', { keychains, chain: unknownChainCode }),
+        InvalidAddressDerivationPropertyError
+      );
+    });
+
     it('succeeds for bch cashaddr (chain 0)', function () {
       const address = generateAddress('bch', { keychains, chain: 0, format: 'cashaddr' });
       assert.doesNotThrow(() =>