diff --git a/core/libs/prx/libSceAgc/CMakeLists.txt b/core/libs/prx/libSceAgc/CMakeLists.txt
index f1822c8ce..8553cfd66 100644
--- a/core/libs/prx/libSceAgc/CMakeLists.txt
+++ b/core/libs/prx/libSceAgc/CMakeLists.txt
@@ -83,6 +83,19 @@ if(BUILD_TESTING)
     endif()
 endif()
 
+add_test_executable(agc_packet_info_tests EXCLUDE_FROM_ALL tests/PacketInfo.cpp)
+target_include_directories(agc_packet_info_tests PRIVATE ${LIBS_INCLUDE_DIR})
+target_link_libraries(agc_packet_info_tests PRIVATE libSceAgc libc)
+configure_windows_unwind(agc_packet_info_tests)
+if(BUILD_TESTING)
+    set_target_properties(agc_packet_info_tests PROPERTIES EXCLUDE_FROM_ALL FALSE)
+    add_test(NAME agc_packet_info_tests COMMAND agc_packet_info_tests)
+    if(WIN32)
+        set_tests_properties(agc_packet_info_tests PROPERTIES ENVIRONMENT_MODIFICATION
+            "PATH=path_list_prepend:$<TARGET_FILE_DIR:libc>")
+    endif()
+endif()
+
 add_test_executable(agc_register_command_tests EXCLUDE_FROM_ALL
         tests/RegisterCommands.cpp
         Command/src/Packet.cpp
diff --git a/core/libs/prx/libSceAgc/Misc/src/PacketInfo.cpp b/core/libs/prx/libSceAgc/Misc/src/PacketInfo.cpp
index 12f74258a..a0d16bb57 100644
--- a/core/libs/prx/libSceAgc/Misc/src/PacketInfo.cpp
+++ b/core/libs/prx/libSceAgc/Misc/src/PacketInfo.cpp
@@ -9,9 +9,12 @@
 extern "C" {
 
 uint32_t APS5_VABI sceAgcGetPacketSize(uint32_t* packet) {
- (void)packet;
- NotImplemented_nid_no_patch(__func__);
- return 0;
+    Agc::Command::CheckAddress(reinterpret_cast<std::uintptr_t>(packet), alignof(uint32_t), __func__);
+    Agc::Command::Require((packet[0] & 0xc0000000u) == 0xc0000000u, __func__, "source pointer is not a PM4 packet");
+    if ((packet[0] & 0x3fffff00u) == 0x3fff1000u) {
+        return 1;
+    }
+    return ((packet[0] >> 16u) & 0x3fffu) + 2u;
 }
 
 APS5_EXPORT("V++UgBtQhn0", sceAgcGetDataPacketPayloadAddressUnk);
diff --git a/core/libs/prx/libSceAgc/tests/PacketInfo.cpp b/core/libs/prx/libSceAgc/tests/PacketInfo.cpp
new file mode 100644
index 000000000..5e95906b5
--- /dev/null
+++ b/core/libs/prx/libSceAgc/tests/PacketInfo.cpp
@@ -0,0 +1,66 @@
+#include "prx/libc/include/general/VabiMacros.hpp"
+#include "prx/libc/include/Shutdown.hpp"
+
+#include <array>
+#include <cstdint>
+#include <cstdio>
+#include <stdexcept>
+#include <utility>
+
+extern "C" std::uint32_t APS5_VABI sceAgcGetPacketSize(std::uint32_t* packet);
+
+namespace {
+
+void check(bool condition, const char* message) {
+    if (!condition) throw std::runtime_error(message);
+}
+
+template <typename TAction>
+void expectFailure(TAction action) {
+    try {
+        action();
+    } catch (const std::runtime_error& error) {
+        check(error.what()[0] != '\0', "empty exception message");
+        return;
+    }
+    throw std::runtime_error("expected an exception");
+}
+
+void testPacketSize() {
+    const std::array<std::pair<std::uint32_t, std::uint32_t>, 7> sizes{{
+        {0xc0047600u, 6}, {0xc0001000u, 2}, {0xfffe1000u, 0x4000}, {0xffff7600u, 0x4001},
+        {0xffff1000u, 1}, {0xffff1001u, 1}, {0xffff10fcu, 1}}};
+    for (const auto& [header, size] : sizes) {
+        std::array<std::uint32_t, 2> words{header, 0xffff1000u};
+        check(sceAgcGetPacketSize(words.data()) == size, "packet size mismatch");
+        check(words[0] == header && words[1] == 0xffff1000u, "packet size query modified the packet");
+    }
+}
+
+void testRejections() {
+    for (const auto header : {0x80000000u, 0x3fff1000u, 0x40001000u, 0x00047600u}) {
+        std::uint32_t word = header;
+        expectFailure([&] { sceAgcGetPacketSize(&word); });
+    }
+    expectFailure([] { sceAgcGetPacketSize(nullptr); });
+    std::array<std::uint32_t, 2> words{0x00100000u, 0x000000c0u};
+    auto* misaligned = reinterpret_cast<std::uint32_t*>(reinterpret_cast<unsigned char*>(words.data()) + 1);
+    expectFailure([&] { sceAgcGetPacketSize(misaligned); });
+}
+
+}
+
+int main() {
+    try {
+        testPacketSize();
+        testRejections();
+        LibcRunShutdown_nid_postfix();
+        std::puts("AGC packet info tests passed");
+        return 0;
+    } catch (const std::exception& error) {
+        std::fprintf(stderr, "%s\n", error.what());
+        try { LibcRunShutdown_nid_postfix(); }
+        catch (const std::exception& shutdown) { std::fprintf(stderr, "shutdown: %s\n", shutdown.what()); }
+        return 1;
+    }
+}