diff --git a/runtime/CMakeLists.txt b/runtime/CMakeLists.txt
index 2fd3f634..49905e35 100644
--- a/runtime/CMakeLists.txt
+++ b/runtime/CMakeLists.txt
@@ -812,6 +812,17 @@ if(BUILD_TESTING)
                 "${PSXRECOMP_ROOT}/runtime/src/netplay_bios_settle.c")
             target_link_libraries(lobby_chat_ring_test PRIVATE recomp_net)
             add_test(NAME lobby_chat_ring_test COMMAND lobby_chat_ring_test)
+            add_executable(netplay_bios_wire_test
+                tests/test_netplay_bios_wire.c
+                src/netplay_bios_settle.c
+                src/lobby_ws/rnet_ws.c
+                src/lobby_ws/rnet_sha1.c)
+            target_include_directories(netplay_bios_wire_test PRIVATE
+                include src src/lobby_ws ${_RNET_ROOT}/include)
+            target_compile_definitions(netplay_bios_wire_test PRIVATE
+                PSX_HAS_LOBBY_CLIENT=1 PSX_HAS_RECOMP_NET=1)
+            target_link_libraries(netplay_bios_wire_test PRIVATE recomp_net)
+            add_test(NAME netplay_bios_wire_test COMMAND netplay_bios_wire_test)
         endif()
 
         add_executable(rb_post_tip_filter_test tests/test_rb_post_tip_filter.c)
diff --git a/runtime/tests/test_netplay_bios_wire.c b/runtime/tests/test_netplay_bios_wire.c
new file mode 100644
index 00000000..8154d5c5
--- /dev/null
+++ b/runtime/tests/test_netplay_bios_wire.c
@@ -0,0 +1,86 @@
+/* Exercise the actual lobby parser/serializer and seat reconciliation.
+ * No server or sockets: link the same real dependencies as chat-ring tests. */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+#include <stdio.h>
+#include <string.h>
+#include "../src/psx_lobby_client.c"
+
+uint64_t psx_host_mono_ms(void) { return 1; }
+static int failures;
+#define CHECK(c) do { if (!(c)) { fprintf(stderr, "%d: %s\n", __LINE__, #c); ++failures; } } while (0)
+
+static void wire_roundtrip(void) {
+    PsxLobbyMatchCaps before, after;
+    char wire[2048], object[1024];
+    memset(&before, 0, sizeof(before));
+    before.valid = 1;
+    before.aspect_num = 16;
+    before.aspect_den = 9;
+    before.rollback = 1;
+    strcpy(before.language, "it");
+    strcpy(before.session_bios, "scph1001");
+    before.session_bios_crc = 0xD786F0B9u;
+    CHECK(append_match_caps_json(wire, sizeof(wire), &before) > 0);
+    CHECK(json_extract_object(wire, "match_caps", object, sizeof(object)));
+    parse_match_caps_object(object, &after);
+    CHECK(after.valid && after.session_bios_crc == before.session_bios_crc);
+    CHECK(after.rollback == 1 && after.aspect_num == 16 && after.aspect_den == 9);
+    CHECK(strcmp(after.language, "it") == 0);
+    parse_match_caps_object("{\"session_bios\":\"scph1001\"}", &after);
+    CHECK(after.valid && after.session_bios_crc == 0); /* legacy host */
+    parse_match_caps_object("{\"session_bios\":\"openbios\",\"session_bios_crc\":\"d786f0b9\"}", &after);
+    CHECK(after.session_bios_crc == 0); /* irrelevant retail CRC ignored */
+    parse_match_caps_object("{\"session_bios\":\"scph1001\",\"session_bios_crc\":\"invalid\"}", &after);
+    CHECK(after.session_bios_crc == 0);
+}
+
+static void local_offer_wins(void) {
+    char token[16], why[192];
+    uint32_t crc;
+    memset(&g_lc, 0, sizeof(g_lc));
+    strcpy(g_lc.player_id, "me");
+    g_lc.is_host = 1;
+    g_lc.member_count = 2;
+    strcpy(g_lc.members[0].player_id, "me");
+    strcpy(g_lc.members[1].player_id, "peer");
+    for (int i = 0; i < 2; ++i) {
+        g_lc.members[i].bios_offer_valid = 1;
+        g_lc.members[i].bios_can_scph1001 = 1;
+        g_lc.members[i].bios_retail_crc = 0x37157331u;
+    }
+    /* The local file changed after the stale server echo, now matching peer. */
+    g_lc.members[0].bios_retail_crc = 0xD786F0B9u;
+    g_lc.bios_offer.valid = 1;
+    g_lc.bios_offer.can_scph1001 = 1;
+    g_lc.bios_offer.retail_crc = 0x37157331u;
+    CHECK(psx_lobby_settle_session_bios(token, sizeof(token), &crc, why, sizeof(why)) == 0);
+    CHECK(strcmp(token, "scph1001") == 0 && crc == 0x37157331u && !why[0]);
+    g_lc.members[1].bios_retail_crc = 0xD786F0B9u;
+    CHECK(psx_lobby_settle_session_bios(token, sizeof(token), &crc, why, sizeof(why)) == 1);
+    CHECK(!token[0] && crc == 0 && strstr(why, "SCPH-1001") && strstr(why, "SCPH-5552"));
+    g_lc.bios_offer.can_openbios = 1;
+    g_lc.members[1].bios_can_openbios = 1;
+    CHECK(psx_lobby_settle_session_bios(token, sizeof(token), &crc, why, sizeof(why)) == 0);
+    CHECK(strcmp(token, "openbios") == 0 && crc == 0);
+}
+
+static void exit_message_bounds(void) {
+    char long_message[512];
+    memset(long_message, 'x', sizeof(long_message));
+    long_message[sizeof(long_message)-1] = 0;
+    psx_lobby_set_last_error(long_message);
+    CHECK(strlen(g_lc.join.last_error) == sizeof(g_lc.join.last_error)-1);
+    psx_lobby_set_last_error(NULL);
+    CHECK(!g_lc.join.last_error[0]);
+}
+
+int main(void) {
+    wire_roundtrip();
+    local_offer_wins();
+    exit_message_bounds();
+    if (failures) return 1;
+    puts("netplay BIOS wire/local-offer regression: PASS");
+    return 0;
+}