versionbits: Limit live activation params and activation warnings per BIP323

diff --git a/src/test/fuzz/versionbits.cpp b/src/test/fuzz/versionbits.cpp
index a2085e6a..0ee86e6e 100644
--- a/src/test/fuzz/versionbits.cpp
+++ b/src/test/fuzz/versionbits.cpp
@@ -14,6 +14,7 @@
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
 #include <test/fuzz/util.h>
+#include <test/util/versionbits.h>
 
 #include <cstdint>
 #include <limits>
@@ -31,7 +32,7 @@ public:
     {
         assert(dep.period > 0);
         assert(dep.threshold <= dep.period);
-        assert(0 <= dep.bit && dep.bit < 32 && dep.bit < VERSIONBITS_NUM_BITS);
+        assert(0 <= dep.bit && dep.bit < 32 && dep.bit < VERSIONBITS_MAX_NUM_BITS);
         assert(0 <= dep.min_activation_height);
     }
 
@@ -126,7 +127,7 @@ FUZZ_TARGET(versionbits, .init = initialize)
         assert(0 < dep.threshold && dep.threshold <= dep.period); // must be able to both pass and fail threshold!
 
         // select deployment parameters: bit, start time, timeout
-        dep.bit = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, VERSIONBITS_NUM_BITS - 1);
+        dep.bit = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, VERSIONBITS_MAX_NUM_BITS - 1);
 
         if (always_active_test) {
             dep.nStartTime = Consensus::BIP9Deployment::ALWAYS_ACTIVE;
diff --git a/src/test/util/versionbits.h b/src/test/util/versionbits.h
new file mode 100644
index 00000000..478b7882
--- /dev/null
+++ b/src/test/util/versionbits.h
@@ -0,0 +1,13 @@
+// Copyright (c) 2026-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_TEST_UTIL_VERSIONBITS_H
+#define BITCOIN_TEST_UTIL_VERSIONBITS_H
+
+#include <versionbits.h>
+
+/** Total possible bits available for versionbits per original BIP 9 specification */
+static constexpr int VERSIONBITS_MAX_NUM_BITS{29};
+
+#endif // BITCOIN_TEST_UTIL_VERSIONBITS_H
diff --git a/src/test/versionbits_tests.cpp b/src/test/versionbits_tests.cpp
index ad792053..77384c5f 100644
--- a/src/test/versionbits_tests.cpp
+++ b/src/test/versionbits_tests.cpp
@@ -8,6 +8,7 @@
 #include <test/util/random.h>
 #include <test/util/common.h>
 #include <test/util/setup_common.h>
+#include <test/util/versionbits.h>
 #include <util/chaintype.h>
 #include <versionbits.h>
 #include <versionbits_impl.h>
@@ -454,9 +455,19 @@ BOOST_FIXTURE_TEST_CASE(versionbits_computeblockversion, BlockVersionTest)
             // not take precedence over STARTED/LOCKED_IN. So all softforks on
             // the same bit might overlap, even when non-overlapping start-end
             // times are picked.
-            const uint32_t dep_mask{uint32_t{1} << chainParams->GetConsensus().vDeployments[dep].bit};
+            const auto& dep_info = chainParams->GetConsensus().vDeployments[dep];
+            const uint32_t dep_mask{uint32_t{1} << dep_info.bit};
             BOOST_CHECK(!(chain_all_vbits & dep_mask));
             chain_all_vbits |= dep_mask;
+            BOOST_CHECK(0 <= dep_info.bit && dep_info.bit < VERSIONBITS_MAX_NUM_BITS);
+            if (chain_type != ChainType::REGTEST) {
+                if (dep == Consensus::DEPLOYMENT_TESTDUMMY) {
+                    BOOST_CHECK_EQUAL(dep_info.nStartTime, Consensus::BIP9Deployment::NEVER_ACTIVE);
+                    BOOST_CHECK_EQUAL(dep_info.nTimeout, Consensus::BIP9Deployment::NO_TIMEOUT);
+                } else {
+                    BOOST_CHECK(dep_info.bit < VERSIONBITS_NUM_BITS);
+                }
+            }
             check_computeblockversion(vbcache, chainParams->GetConsensus(), dep);
         }
     }
diff --git a/src/versionbits.h b/src/versionbits.h
index 59b0cbee..0ac17287 100644
--- a/src/versionbits.h
+++ b/src/versionbits.h
@@ -21,8 +21,8 @@ static const int32_t VERSIONBITS_LAST_OLD_BLOCK_VERSION = 4;
 static const int32_t VERSIONBITS_TOP_BITS = 0x20000000UL;
 /** What bitmask determines whether versionbits is in use */
 static const int32_t VERSIONBITS_TOP_MASK = 0xE0000000UL;
-/** Total bits available for versionbits */
-static const int32_t VERSIONBITS_NUM_BITS = 29;
+/** Total bits available for versionbits (BIP 323) */
+static const int32_t VERSIONBITS_NUM_BITS = 5;
 
 /** Opaque type for BIP9 state. See versionbits_impl.h for details. */
 enum class ThresholdState : uint8_t;
diff --git a/test/functional/feature_versionbits_warning.py b/test/functional/feature_versionbits_warning.py
index 79f512ad..1f858dc7 100755
--- a/test/functional/feature_versionbits_warning.py
+++ b/test/functional/feature_versionbits_warning.py
@@ -18,7 +18,7 @@ from test_framework.test_framework import BitcoinTestFramework
 VB_PERIOD = 144           # versionbits period length for regtest
 VB_THRESHOLD = 108        # versionbits activation threshold for regtest
 VB_TOP_BITS = 0x20000000
-VB_UNKNOWN_BIT = 27       # Choose a bit unassigned to any deployment
+VB_UNKNOWN_BIT = 3        # Choose a bit unassigned to any deployment
 VB_UNKNOWN_VERSION = VB_TOP_BITS | (1 << VB_UNKNOWN_BIT)
 
 WARN_UNKNOWN_RULES_ACTIVE = f"Unknown new rules activated (versionbit {VB_UNKNOWN_BIT})"