policy: update `AreInputsStandard` to return error string

diff --git a/src/bench/ccoins_caching.cpp b/src/bench/ccoins_caching.cpp
index 35186153..673616a5 100644
--- a/src/bench/ccoins_caching.cpp
+++ b/src/bench/ccoins_caching.cpp
@@ -49,8 +49,7 @@ static void CCoinsCaching(benchmark::Bench& bench)
     // Benchmark.
     const CTransaction tx_1(t1);
     bench.run([&] {
-        bool success{AreInputsStandard(tx_1, coins)};
-        assert(success);
+        assert(ValidateInputsStandardness(tx_1, coins).IsValid());
     });
 }
 
diff --git a/src/policy/policy.cpp b/src/policy/policy.cpp
index 3da6cb74..2f5f9fce 100644
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -18,6 +18,7 @@
 #include <script/solver.h>
 #include <serialize.h>
 #include <span.h>
+#include <tinyformat.h>
 
 #include <algorithm>
 #include <cstddef>
@@ -210,14 +211,16 @@ static bool CheckSigopsBIP54(const CTransaction& tx, const CCoinsViewCache& inpu
  *
  * We also check the total number of non-witness sigops across the whole transaction, as per BIP54.
  */
-bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
+TxValidationState ValidateInputsStandardness(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 {
+    TxValidationState state;
     if (tx.IsCoinBase()) {
-        return true; // Coinbases don't use vin normally
+        return state; // Coinbases don't use vin normally
     }
 
     if (!CheckSigopsBIP54(tx, mapInputs)) {
-        return false;
+        state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", "non-witness sigops exceed bip54 limit");
+        return state;
     }
 
     for (unsigned int i = 0; i < tx.vin.size(); i++) {
@@ -225,27 +228,38 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 
         std::vector<std::vector<unsigned char> > vSolutions;
         TxoutType whichType = Solver(prev.scriptPubKey, vSolutions);
-        if (whichType == TxoutType::NONSTANDARD || whichType == TxoutType::WITNESS_UNKNOWN) {
+        if (whichType == TxoutType::NONSTANDARD) {
+            state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", strprintf("input %u script unknown", i));
+            return state;
+        } else if (whichType == TxoutType::WITNESS_UNKNOWN) {
             // WITNESS_UNKNOWN failures are typically also caught with a policy
             // flag in the script interpreter, but it can be helpful to catch
             // this type of NONSTANDARD transaction earlier in transaction
             // validation.
-            return false;
+            state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", strprintf("input %u witness program is undefined", i));
+            return state;
         } else if (whichType == TxoutType::SCRIPTHASH) {
             std::vector<std::vector<unsigned char> > stack;
+            ScriptError serror;
             // convert the scriptSig into a stack, so we can inspect the redeemScript
-            if (!EvalScript(stack, tx.vin[i].scriptSig, SCRIPT_VERIFY_NONE, BaseSignatureChecker(), SigVersion::BASE))
-                return false;
-            if (stack.empty())
-                return false;
+            if (!EvalScript(stack, tx.vin[i].scriptSig, SCRIPT_VERIFY_NONE, BaseSignatureChecker(), SigVersion::BASE, &serror)) {
+                state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", strprintf("p2sh scriptsig malformed (input %u: %s)", i, ScriptErrorString(serror)));
+                return state;
+            }
+            if (stack.empty()) {
+                state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", strprintf("input %u P2SH redeemscript missing", i));
+                return state;
+            }
             CScript subscript(stack.back().begin(), stack.back().end());
-            if (subscript.GetSigOpCount(true) > MAX_P2SH_SIGOPS) {
-                return false;
+            unsigned int sigop_count = subscript.GetSigOpCount(true);
+            if (sigop_count > MAX_P2SH_SIGOPS) {
+                state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs", strprintf("p2sh redeemscript sigops exceed limit (input %u: %u > %u)", i, sigop_count, MAX_P2SH_SIGOPS));
+                return state;
             }
         }
     }
 
-    return true;
+    return state;
 }
 
 bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
@@ -354,7 +368,7 @@ bool SpendsNonAnchorWitnessProg(const CTransaction& tx, const CCoinsViewCache& p
         }
 
         // For P2SH extract the redeem script and check if it spends a non-Taproot witness program. Note
-        // this is fine to call EvalScript (as done in AreInputsStandard/IsWitnessStandard) because this
+        // this is fine to call EvalScript (as done in ValidateInputsStandardness/IsWitnessStandard) because this
         // function is only ever called after IsStandardTx, which checks the scriptsig is pushonly.
         if (prev_spk.IsPayToScriptHash()) {
             // If EvalScript fails or results in an empty stack, the transaction is invalid by consensus.
diff --git a/src/policy/policy.h b/src/policy/policy.h
index 0131b56b..f10b120d 100644
--- a/src/policy/policy.h
+++ b/src/policy/policy.h
@@ -8,6 +8,7 @@
 
 #include <consensus/amount.h>
 #include <consensus/consensus.h>
+#include <consensus/validation.h>
 #include <primitives/transaction.h>
 #include <script/interpreter.h>
 #include <script/solver.h>
@@ -154,11 +155,12 @@ static constexpr decltype(CTransaction::version) TX_MAX_STANDARD_VERSION{3};
 */
 bool IsStandardTx(const CTransaction& tx, const std::optional<unsigned>& max_datacarrier_bytes, bool permit_bare_multisig, const CFeeRate& dust_relay_fee, std::string& reason);
 /**
-* Check for standard transaction types
-* @param[in] mapInputs       Map of previous transactions that have outputs we're spending
-* @return True if all inputs (scriptSigs) use only standard transaction forms
-*/
-bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs);
+ * Check for standard transaction types
+ * @param[in] mapInputs       Map of previous transactions that have outputs we're spending
+ * @returns valid TxValidationState if all inputs (scriptSigs) use only standard transaction forms else returns
+ * invalid TxValidationState which states why the first invalid input is not standard
+ */
+TxValidationState ValidateInputsStandardness(const CTransaction& tx, const CCoinsViewCache& mapInputs);
 /**
 * Check if the transaction is over standard P2WSH resources limit:
 * 3600bytes witnessScript size, 80bytes per witness stack element, 100 witness stack elements
diff --git a/src/test/fuzz/coins_view.cpp b/src/test/fuzz/coins_view.cpp
index dacef9a7..d283224a 100644
--- a/src/test/fuzz/coins_view.cpp
+++ b/src/test/fuzz/coins_view.cpp
@@ -252,7 +252,7 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                 assert(expected_code_path);
             },
             [&] {
-                (void)AreInputsStandard(CTransaction{random_mutable_transaction}, coins_view_cache);
+                (void)ValidateInputsStandardness(CTransaction{random_mutable_transaction}, coins_view_cache);
             },
             [&] {
                 TxValidationState state;
diff --git a/src/test/fuzz/transaction.cpp b/src/test/fuzz/transaction.cpp
index c9eb1122..78629c45 100644
--- a/src/test/fuzz/transaction.cpp
+++ b/src/test/fuzz/transaction.cpp
@@ -89,7 +89,7 @@ FUZZ_TARGET(transaction, .init = initialize_transaction)
 
     CCoinsView coins_view;
     const CCoinsViewCache coins_view_cache(&coins_view);
-    (void)AreInputsStandard(tx, coins_view_cache);
+    (void)ValidateInputsStandardness(tx, coins_view_cache);
     (void)IsWitnessStandard(tx, coins_view_cache);
 
     if (tx.GetTotalSize() < 250'000) { // Avoid high memory usage (with msan) due to json encoding
diff --git a/src/test/script_p2sh_tests.cpp b/src/test/script_p2sh_tests.cpp
index b4641118..858723d4 100644
--- a/src/test/script_p2sh_tests.cpp
+++ b/src/test/script_p2sh_tests.cpp
@@ -275,7 +275,7 @@ BOOST_AUTO_TEST_CASE(switchover)
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_EQUALVERIFY, ScriptErrorString(err));
 }
 
-BOOST_AUTO_TEST_CASE(AreInputsStandard)
+BOOST_AUTO_TEST_CASE(ValidateInputsStandardness)
 {
     CCoinsView coinsDummy;
     CCoinsViewCache coins(&coinsDummy);
@@ -360,7 +360,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
     txTo.vin[3].scriptSig << OP_11 << OP_11 << std::vector<unsigned char>(oneAndTwo.begin(), oneAndTwo.end());
     txTo.vin[4].scriptSig << std::vector<unsigned char>(fifteenSigops.begin(), fifteenSigops.end());
 
-    BOOST_CHECK(::AreInputsStandard(CTransaction(txTo), coins));
+    BOOST_CHECK(::ValidateInputsStandardness(CTransaction(txTo), coins).IsValid());
     // 22 P2SH sigops for all inputs (1 for vin[0], 6 for vin[3], 15 for vin[4]
     BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txTo), coins), 22U);
 
@@ -379,7 +379,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
     txToNonStd1.vin[0].prevout.hash = txFrom.GetHash();
     txToNonStd1.vin[0].scriptSig << std::vector<unsigned char>(sixteenSigops.begin(), sixteenSigops.end());
 
-    BOOST_CHECK(!::AreInputsStandard(CTransaction(txToNonStd1), coins));
+    BOOST_CHECK(::ValidateInputsStandardness(CTransaction(txToNonStd1), coins).IsInvalid());
     BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd1), coins), 16U);
 
     CMutableTransaction txToNonStd2;
@@ -391,7 +391,7 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
     txToNonStd2.vin[0].prevout.hash = txFrom.GetHash();
     txToNonStd2.vin[0].scriptSig << std::vector<unsigned char>(twentySigops.begin(), twentySigops.end());
 
-    BOOST_CHECK(!::AreInputsStandard(CTransaction(txToNonStd2), coins));
+    BOOST_CHECK(::ValidateInputsStandardness(CTransaction(txToNonStd2), coins).IsInvalid());
     BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd2), coins), 20U);
 }
 
diff --git a/src/test/transaction_tests.cpp b/src/test/transaction_tests.cpp
index b2a70057..5ff39fdc 100644
--- a/src/test/transaction_tests.cpp
+++ b/src/test/transaction_tests.cpp
@@ -411,7 +411,7 @@ BOOST_AUTO_TEST_CASE(test_Get)
     t1.vout[0].nValue = 90*CENT;
     t1.vout[0].scriptPubKey << OP_1;
 
-    BOOST_CHECK(AreInputsStandard(CTransaction(t1), coins));
+    BOOST_CHECK(ValidateInputsStandardness(CTransaction(t1), coins).IsValid());
 }
 
 static void CreateCreditAndSpend(const FillableSigningProvider& keystore, const CScript& outscript, CTransactionRef& output, CMutableTransaction& input, bool success = true)
@@ -1052,7 +1052,7 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
 
     // 2490 sigops is below the limit.
     BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(tx_max_sigops), coins), 2490);
-    BOOST_CHECK(::AreInputsStandard(CTransaction(tx_max_sigops), coins));
+    BOOST_CHECK(::ValidateInputsStandardness(CTransaction(tx_max_sigops), coins).IsValid());
 
     // Adding one more input will bump this to 2505, hitting the limit.
     tx_create.vout.emplace_back(424242, max_sigops_p2sh);
@@ -1063,8 +1063,17 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
     tx_max_sigops.vin.emplace_back(prev_txid, p2sh_inputs_count, CScript() << ToByteVector(max_sigops_redeem_script));
     AddCoins(coins, CTransaction(tx_create), 0, false);
     BOOST_CHECK_GT((p2sh_inputs_count + 1) * MAX_P2SH_SIGOPS, MAX_TX_LEGACY_SIGOPS);
-    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(tx_max_sigops), coins), 2505);
-    BOOST_CHECK(!::AreInputsStandard(CTransaction(tx_max_sigops), coins));
+    auto legacy_sigops_count = GetP2SHSigOpCount(CTransaction(tx_max_sigops), coins);
+    BOOST_CHECK_EQUAL(legacy_sigops_count, 2505);
+    std::string reject_reason("bad-txns-nonstandard-inputs");
+    std::string sigop_limit_reject_debug_message("non-witness sigops exceed bip54 limit");
+    {
+        auto validation_state = ValidateInputsStandardness(CTransaction(tx_max_sigops), coins);
+        BOOST_CHECK(validation_state.IsInvalid());
+        BOOST_CHECK_EQUAL(validation_state.GetRejectReason(), reject_reason);
+        BOOST_CHECK_EQUAL(validation_state.GetDebugMessage(), sigop_limit_reject_debug_message);
+    }
+
 
     // Now, check the limit can be reached with regular P2PK outputs too. Use a separate
     // preparation transaction, to demonstrate spending coins from a single tx is irrelevant.
@@ -1082,8 +1091,7 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
     AddCoins(coins, CTransaction(tx_create_p2pk), 0, false);
 
     // The transaction now contains exactly 2500 sigops, the check should pass.
-    BOOST_CHECK_EQUAL(p2sh_inputs_count * MAX_P2SH_SIGOPS + p2pk_inputs_count * 1, MAX_TX_LEGACY_SIGOPS);
-    BOOST_CHECK(::AreInputsStandard(CTransaction(tx_max_sigops), coins));
+    BOOST_CHECK(::ValidateInputsStandardness(CTransaction(tx_max_sigops), coins).IsValid());
 
     // Now, add some Segwit inputs. We add one for each defined Segwit output type. The limit
     // is exclusively on non-witness sigops and therefore those should not be counted.
@@ -1099,7 +1107,7 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
 
     // The transaction now still contains exactly 2500 sigops, the check should pass.
     AddCoins(coins, CTransaction(tx_create_segwit), 0, false);
-    BOOST_REQUIRE(::AreInputsStandard(CTransaction(tx_max_sigops), coins));
+    BOOST_REQUIRE(::ValidateInputsStandardness(CTransaction(tx_max_sigops), coins).IsValid());
 
     // Add one more P2PK input. We'll reach the limit.
     tx_create_p2pk.vout.emplace_back(212121, p2pk_script);
@@ -1110,8 +1118,14 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
         tx_max_sigops.vin.emplace_back(prev_txid, i);
     }
     AddCoins(coins, CTransaction(tx_create_p2pk), 0, false);
-    BOOST_CHECK_GT(p2sh_inputs_count * MAX_P2SH_SIGOPS + p2pk_inputs_count * 1, MAX_TX_LEGACY_SIGOPS);
-    BOOST_CHECK(!::AreInputsStandard(CTransaction(tx_max_sigops), coins));
+    auto legacy_sigop_count_p2pk = p2sh_inputs_count * MAX_P2SH_SIGOPS + p2pk_inputs_count * 1;
+    BOOST_CHECK_GT(legacy_sigop_count_p2pk, MAX_TX_LEGACY_SIGOPS);
+    {
+        auto validation_state = ValidateInputsStandardness(CTransaction(tx_max_sigops), coins);
+        BOOST_CHECK(validation_state.IsInvalid());
+        BOOST_CHECK_EQUAL(validation_state.GetRejectReason(), reject_reason);
+        BOOST_CHECK_EQUAL(validation_state.GetDebugMessage(), sigop_limit_reject_debug_message);
+    }
 }
 
 /** Sanity check the return value of SpendsNonAnchorWitnessProg for various output types. */
diff --git a/src/validation.cpp b/src/validation.cpp
index 7f97f99b..433abd45 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -894,8 +894,11 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return false; // state filled in by CheckTxInputs
     }
 
-    if (m_pool.m_opts.require_standard && !AreInputsStandard(tx, m_view)) {
-        return state.Invalid(TxValidationResult::TX_INPUTS_NOT_STANDARD, "bad-txns-nonstandard-inputs");
+    if (m_pool.m_opts.require_standard) {
+        state = ValidateInputsStandardness(tx, m_view);
+        if (state.IsInvalid()) {
+            return false;
+        }
     }
 
     // Check for non-standard witnesses.
diff --git a/test/functional/mempool_sigoplimit.py b/test/functional/mempool_sigoplimit.py
index 6b2a0268..f07f7756 100755
--- a/test/functional/mempool_sigoplimit.py
+++ b/test/functional/mempool_sigoplimit.py
@@ -208,7 +208,8 @@ class BytesPerSigOpTest(BitcoinTestFramework):
         nonstd_tx = CTransaction()
         nonstd_tx.vin = [CTxIn(op, CScript([b"", packed_redeem_script])) for op in outpoints]
         nonstd_tx.vout = [CTxOut(0, CScript([OP_RETURN, b""]))]
-        assert_raises_rpc_error(-26, "bad-txns-nonstandard-inputs", self.nodes[0].sendrawtransaction, nonstd_tx.serialize().hex())
+        assert_raises_rpc_error(-26, "bad-txns-nonstandard-inputs, non-witness sigops exceed bip54 limit",
+                                        self.nodes[0].sendrawtransaction, nonstd_tx.serialize().hex())
 
         # Spending one less accounts for 2490 legacy sigops and is standard.
         std_tx = deepcopy(nonstd_tx)