diff --git a/src/ipc/CMakeLists.txt b/src/ipc/CMakeLists.txt
index 427cd6ea3fe..904d72f56e1 100644
--- a/src/ipc/CMakeLists.txt
+++ b/src/ipc/CMakeLists.txt
@@ -3,6 +3,7 @@
 # file COPYING or https://opensource.org/license/mit/.
 
 add_library(bitcoin_ipc STATIC EXCLUDE_FROM_ALL
+  capnp/mining.cpp
   capnp/protocol.cpp
   interfaces.cpp
   process.cpp
diff --git a/src/ipc/capnp/mining-types.h b/src/ipc/capnp/mining-types.h
index 3204a0b6617..2e60b43fcf3 100644
--- a/src/ipc/capnp/mining-types.h
+++ b/src/ipc/capnp/mining-types.h
@@ -13,4 +13,14 @@
 #include <node/types.h>
 #include <validation.h>
 
+namespace mp {
+// Custom serialization for BlockValidationState.
+void CustomBuildMessage(InvokeContext& invoke_context,
+                        const BlockValidationState& src,
+                        ipc::capnp::messages::BlockValidationState::Builder&& builder);
+void CustomReadMessage(InvokeContext& invoke_context,
+                       const ipc::capnp::messages::BlockValidationState::Reader& reader,
+                       BlockValidationState& dest);
+} // namespace mp
+
 #endif // BITCOIN_IPC_CAPNP_MINING_TYPES_H
diff --git a/src/ipc/capnp/mining.capnp b/src/ipc/capnp/mining.capnp
index 13d9e2ffb03..4ea69d16c98 100644
--- a/src/ipc/capnp/mining.capnp
+++ b/src/ipc/capnp/mining.capnp
@@ -39,6 +39,12 @@ struct BlockCreateOptions $Proxy.wrap("node::BlockCreateOptions") {
     coinbaseOutputMaxAdditionalSigops @2 :UInt64 $Proxy.name("coinbase_output_max_additional_sigops");
 }
 
-# TODO add fields to this struct
-struct BlockValidationState $Proxy.wrap("BlockValidationState") {
+# Note: serialization of the BlockValidationState C++ type is somewhat fragile
+# and using the struct can be awkward. It would be good if testBlockValidity
+# method were changed to return validity information in a simpler format.
+struct BlockValidationState {
+    mode @0 :Int32;
+    result @1 :Int32;
+    rejectReason @2 :Text;
+    debugMessage @3 :Text;
 }
diff --git a/src/ipc/capnp/mining.cpp b/src/ipc/capnp/mining.cpp
new file mode 100644
index 00000000000..0f9533c1c73
--- /dev/null
+++ b/src/ipc/capnp/mining.cpp
@@ -0,0 +1,47 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <ipc/capnp/mining-types.h>
+#include <ipc/capnp/mining.capnp.proxy-types.h>
+
+#include <mp/proxy-types.h>
+
+namespace mp {
+void CustomBuildMessage(InvokeContext& invoke_context,
+                        const BlockValidationState& src,
+                        ipc::capnp::messages::BlockValidationState::Builder&& builder)
+{
+    if (src.IsValid()) {
+        builder.setMode(0);
+    } else if (src.IsInvalid()) {
+        builder.setMode(1);
+    } else if (src.IsError()) {
+        builder.setMode(2);
+    } else {
+        assert(false);
+    }
+    builder.setResult(static_cast<int>(src.GetResult()));
+    builder.setRejectReason(src.GetRejectReason());
+    builder.setDebugMessage(src.GetDebugMessage());
+}
+
+void CustomReadMessage(InvokeContext& invoke_context,
+                       const ipc::capnp::messages::BlockValidationState::Reader& reader,
+                       BlockValidationState& dest)
+{
+    if (reader.getMode() == 0) {
+        assert(reader.getResult() == 0);
+        assert(reader.getRejectReason().size() == 0);
+        assert(reader.getDebugMessage().size() == 0);
+    } else if (reader.getMode() == 1) {
+        dest.Invalid(static_cast<BlockValidationResult>(reader.getResult()), reader.getRejectReason(), reader.getDebugMessage());
+    } else if (reader.getMode() == 2) {
+        assert(reader.getResult() == 0);
+        dest.Error(reader.getRejectReason());
+        assert(reader.getDebugMessage().size() == 0);
+    } else {
+        assert(false);
+    }
+}
+} // namespace mp
diff --git a/src/test/ipc_test.capnp b/src/test/ipc_test.capnp
index 32c01dba463..46cd08b94a2 100644
--- a/src/test/ipc_test.capnp
+++ b/src/test/ipc_test.capnp
@@ -9,7 +9,9 @@ $Cxx.namespace("gen");
 
 using Proxy = import "/mp/proxy.capnp";
 $Proxy.include("test/ipc_test.h");
-$Proxy.includeTypes("ipc/capnp/common-types.h");
+$Proxy.includeTypes("test/ipc_test_types.h");
+
+using Mining = import "../ipc/capnp/mining.capnp";
 
 interface FooInterface $Proxy.wrap("FooImplementation") {
     add @0 (a :Int32, b :Int32) -> (result :Int32);
@@ -17,4 +19,5 @@ interface FooInterface $Proxy.wrap("FooImplementation") {
     passUniValue @2 (arg :Text) -> (result :Text);
     passTransaction @3 (arg :Data) -> (result :Data);
     passVectorChar @4 (arg :Data) -> (result :Data);
+    passBlockState @5 (arg :Mining.BlockValidationState) -> (result :Mining.BlockValidationState);
 }
diff --git a/src/test/ipc_test.cpp b/src/test/ipc_test.cpp
index 91afce77786..91eba9214f0 100644
--- a/src/test/ipc_test.cpp
+++ b/src/test/ipc_test.cpp
@@ -12,6 +12,7 @@
 #include <test/ipc_test.capnp.proxy.h>
 #include <test/ipc_test.h>
 #include <tinyformat.h>
+#include <validation.h>
 
 #include <future>
 #include <thread>
@@ -101,6 +102,25 @@ void IpcPipeTest()
     std::vector<char> vec2{foo->passVectorChar(vec1)};
     BOOST_CHECK_EQUAL(std::string_view(vec1.begin(), vec1.end()), std::string_view(vec2.begin(), vec2.end()));
 
+    BlockValidationState bs1;
+    bs1.Invalid(BlockValidationResult::BLOCK_CHECKPOINT, "reject reason", "debug message");
+    BlockValidationState bs2{foo->passBlockState(bs1)};
+    BOOST_CHECK_EQUAL(bs1.IsValid(), bs2.IsValid());
+    BOOST_CHECK_EQUAL(bs1.IsError(), bs2.IsError());
+    BOOST_CHECK_EQUAL(bs1.IsInvalid(), bs2.IsInvalid());
+    BOOST_CHECK_EQUAL(static_cast<int>(bs1.GetResult()), static_cast<int>(bs2.GetResult()));
+    BOOST_CHECK_EQUAL(bs1.GetRejectReason(), bs2.GetRejectReason());
+    BOOST_CHECK_EQUAL(bs1.GetDebugMessage(), bs2.GetDebugMessage());
+
+    BlockValidationState bs3;
+    BlockValidationState bs4{foo->passBlockState(bs3)};
+    BOOST_CHECK_EQUAL(bs3.IsValid(), bs4.IsValid());
+    BOOST_CHECK_EQUAL(bs3.IsError(), bs4.IsError());
+    BOOST_CHECK_EQUAL(bs3.IsInvalid(), bs4.IsInvalid());
+    BOOST_CHECK_EQUAL(static_cast<int>(bs3.GetResult()), static_cast<int>(bs4.GetResult()));
+    BOOST_CHECK_EQUAL(bs3.GetRejectReason(), bs4.GetRejectReason());
+    BOOST_CHECK_EQUAL(bs3.GetDebugMessage(), bs4.GetDebugMessage());
+
     // Test cleanup: disconnect pipe and join thread
     disconnect_client();
     thread.join();
diff --git a/src/test/ipc_test.h b/src/test/ipc_test.h
index ca8a5f61b51..2d215a20f1b 100644
--- a/src/test/ipc_test.h
+++ b/src/test/ipc_test.h
@@ -8,6 +8,7 @@
 #include <primitives/transaction.h>
 #include <univalue.h>
 #include <util/fs.h>
+#include <validation.h>
 
 class FooImplementation
 {
@@ -17,6 +18,7 @@ public:
     UniValue passUniValue(UniValue v) { return v; }
     CTransactionRef passTransaction(CTransactionRef t) { return t; }
     std::vector<char> passVectorChar(std::vector<char> v) { return v; }
+    BlockValidationState passBlockState(BlockValidationState s) { return s; }
 };
 
 void IpcPipeTest();
diff --git a/src/test/ipc_test_types.h b/src/test/ipc_test_types.h
new file mode 100644
index 00000000000..b1d4829aa71
--- /dev/null
+++ b/src/test/ipc_test_types.h
@@ -0,0 +1,12 @@
+// Copyright (c) 2024 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_IPC_TEST_TYPES_H
+#define BITCOIN_TEST_IPC_TEST_TYPES_H
+
+#include <ipc/capnp/common-types.h>
+#include <ipc/capnp/mining-types.h>
+#include <test/ipc_test.capnp.h>
+
+#endif // BITCOIN_TEST_IPC_TEST_TYPES_H