diff --git a/unit_test/test_spdm_responder/CMakeLists.txt b/unit_test/test_spdm_responder/CMakeLists.txt
index 9337c519838..3973e98b27d 100644
--- a/unit_test/test_spdm_responder/CMakeLists.txt
+++ b/unit_test/test_spdm_responder/CMakeLists.txt
@@ -45,6 +45,7 @@ target_sources(test_spdm_responder
         supported_event_types.c
         subscribe_event_types_ack.c
         error_test/key_pair_info_err.c
+        error_test/set_key_pair_info_ack_err.c
         error_test/supported_event_types_err.c
         error_test/subscribe_event_types_ack_err.c
         error_test/vendor_response_err.c
diff --git a/unit_test/test_spdm_responder/error_test/set_key_pair_info_ack_err.c b/unit_test/test_spdm_responder/error_test/set_key_pair_info_ack_err.c
new file mode 100644
index 00000000000..4a753930e15
--- /dev/null
+++ b/unit_test/test_spdm_responder/error_test/set_key_pair_info_ack_err.c
@@ -0,0 +1,354 @@
+/**
+ *  Copyright Notice:
+ *  Copyright 2026 DMTF. All rights reserved.
+ *  License: BSD 3-Clause License. For full text see link: https://github.com/DMTF/libspdm/blob/main/LICENSE.md
+ **/
+
+#include "spdm_unit_test.h"
+#include "internal/libspdm_responder_lib.h"
+
+#if LIBSPDM_ENABLE_CAPABILITY_SET_KEY_PAIR_INFO_CAP
+
+static uint8_t m_spdm_request_buffer[0x1000];
+static uint8_t m_spdm_response_buffer[0x1000];
+
+static void set_standard_state(libspdm_context_t *spdm_context)
+{
+    spdm_context->connection_info.version = SPDM_MESSAGE_VERSION_13 <<
+                                            SPDM_VERSION_NUMBER_SHIFT_BIT;
+    spdm_context->connection_info.connection_state = LIBSPDM_CONNECTION_STATE_NEGOTIATED;
+    spdm_context->response_state = LIBSPDM_RESPONSE_STATE_NORMAL;
+    spdm_context->local_context.capability.flags |=
+        SPDM_GET_CAPABILITIES_RESPONSE_FLAGS_SET_KEY_PAIR_INFO_CAP;
+}
+
+/**
+ * Test 1: Negotiated SPDM version is less than 1.3, which does not support SET_KEY_PAIR_INFO.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_UNSUPPORTED_REQUEST.
+ **/
+static void rsp_set_key_pair_info_ack_err_case1(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x01;
+
+    set_standard_state(spdm_context);
+
+    /* SPDM 1.2 does not support SET_KEY_PAIR_INFO. */
+    spdm_context->connection_info.version = SPDM_MESSAGE_VERSION_12 <<
+                                            SPDM_VERSION_NUMBER_SHIFT_BIT;
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_12;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_ERASE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    request_size = sizeof(spdm_set_key_pair_info_request_t);
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_12);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_UNSUPPORTED_REQUEST);
+    assert_int_equal(spdm_response->header.param2, SPDM_SET_KEY_PAIR_INFO);
+}
+
+/**
+ * Test 2: SPDM version field in SET_KEY_PAIR_INFO request does not match the connection's
+ *         negotiated version.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_VERSION_MISMATCH.
+ **/
+static void rsp_set_key_pair_info_ack_err_case2(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x02;
+
+    set_standard_state(spdm_context);
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    /* Version in message header does not match the negotiated version (1.3). */
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_12;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_ERASE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    request_size = sizeof(spdm_set_key_pair_info_request_t);
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_13);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_VERSION_MISMATCH);
+    assert_int_equal(spdm_response->header.param2, 0);
+}
+
+/**
+ * Test 3: request_size is smaller than the minimum size of a SET_KEY_PAIR_INFO request.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_INVALID_REQUEST.
+ **/
+static void rsp_set_key_pair_info_ack_err_case3(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x03;
+
+    set_standard_state(spdm_context);
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_13;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_ERASE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    /* request_size is one byte too small. */
+    request_size = sizeof(spdm_set_key_pair_info_request_t) - 1;
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_13);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_INVALID_REQUEST);
+    assert_int_equal(spdm_response->header.param2, 0);
+}
+
+/**
+ * Test 4: Connection has not yet reached the NEGOTIATED state when SET_KEY_PAIR_INFO is received.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_UNEXPECTED_REQUEST.
+ **/
+static void rsp_set_key_pair_info_ack_err_case4(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x04;
+
+    set_standard_state(spdm_context);
+
+    /* Connection has not been negotiated yet. */
+    spdm_context->connection_info.connection_state = LIBSPDM_CONNECTION_STATE_AFTER_VERSION;
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_13;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_ERASE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    request_size = sizeof(spdm_set_key_pair_info_request_t);
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_13);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_UNEXPECTED_REQUEST);
+    assert_int_equal(spdm_response->header.param2, 0);
+}
+
+/**
+ * Test 5: Responder does not advertise SET_KEY_PAIR_INFO_CAP.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_UNSUPPORTED_REQUEST.
+ **/
+static void rsp_set_key_pair_info_ack_err_case5(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x05;
+
+    set_standard_state(spdm_context);
+
+    /* Clear SET_KEY_PAIR_INFO_CAP so the capability check fails. */
+    spdm_context->local_context.capability.flags &=
+        ~SPDM_GET_CAPABILITIES_RESPONSE_FLAGS_SET_KEY_PAIR_INFO_CAP;
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_13;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_ERASE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    request_size = sizeof(spdm_set_key_pair_info_request_t);
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_13);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_UNSUPPORTED_REQUEST);
+    assert_int_equal(spdm_response->header.param2, SPDM_SET_KEY_PAIR_INFO);
+}
+
+/**
+ * Test 6: Request specifies both a non-zero desired_asym_algo and a non-zero
+ *         desired_pqc_asym_algo, which is not allowed. Requires SPDM 1.4 or above so
+ *         that the desired_pqc_asym_algo field is parsed from the request.
+ * Expected Behavior: Responder returns SPDM_ERROR_CODE_INVALID_REQUEST.
+ **/
+static void rsp_set_key_pair_info_ack_err_case6(void **state)
+{
+    libspdm_return_t status;
+    libspdm_test_context_t *spdm_test_context;
+    libspdm_context_t *spdm_context;
+    spdm_set_key_pair_info_request_t *spdm_request;
+    size_t request_size;
+    spdm_error_response_t *spdm_response;
+    size_t response_size = sizeof(m_spdm_response_buffer);
+    uint8_t *ptr;
+    uint8_t desired_pqc_asym_algo_len;
+
+    spdm_test_context = *state;
+    spdm_context = spdm_test_context->spdm_context;
+    spdm_test_context->case_id = 0x06;
+
+    set_standard_state(spdm_context);
+
+    /* SPDM 1.4 is required to include the desired_pqc_asym_algo field. */
+    spdm_context->connection_info.version = SPDM_MESSAGE_VERSION_14 <<
+                                            SPDM_VERSION_NUMBER_SHIFT_BIT;
+
+    desired_pqc_asym_algo_len = sizeof(uint32_t);
+
+    request_size = sizeof(spdm_set_key_pair_info_request_t) +
+                   sizeof(uint8_t) + sizeof(uint16_t) + sizeof(uint32_t) + sizeof(uint8_t) +
+                   sizeof(uint8_t) + desired_pqc_asym_algo_len;
+
+    libspdm_zero_mem(m_spdm_request_buffer, request_size);
+
+    spdm_request = (spdm_set_key_pair_info_request_t *)m_spdm_request_buffer;
+    spdm_request->header.spdm_version = SPDM_MESSAGE_VERSION_14;
+    spdm_request->header.request_response_code = SPDM_SET_KEY_PAIR_INFO;
+    spdm_request->header.param1 = SPDM_SET_KEY_PAIR_INFO_CHANGE_OPERATION;
+    spdm_request->header.param2 = 0;
+    spdm_request->key_pair_id = 1;
+
+    ptr = (uint8_t *)(spdm_request + 1);
+    ptr += sizeof(uint8_t); /* reserved byte */
+
+    libspdm_write_uint16(ptr, 0); /* desired_key_usage = 0 */
+    ptr += sizeof(uint16_t);
+
+    /* Set desired_asym_algo to a non-zero one-hot value. */
+    libspdm_write_uint32(ptr, SPDM_KEY_PAIR_ASYM_ALGO_CAP_ECC256);
+    ptr += sizeof(uint32_t);
+
+    *ptr = 0; /* desired_assoc_cert_slot_mask = 0 */
+    ptr += sizeof(uint8_t);
+
+    *ptr = desired_pqc_asym_algo_len;
+    ptr += sizeof(uint8_t);
+
+    /* Set desired_pqc_asym_algo to a non-zero one-hot value. Both asym_algo and
+     * pqc_asym_algo being non-zero is invalid. */
+    libspdm_write_uint32(ptr, SPDM_KEY_PAIR_PQC_ASYM_ALGO_CAP_ML_DSA_44);
+
+    status = libspdm_get_response_set_key_pair_info_ack(
+        spdm_context, request_size, m_spdm_request_buffer,
+        &response_size, m_spdm_response_buffer);
+
+    spdm_response = (spdm_error_response_t *)m_spdm_response_buffer;
+
+    assert_int_equal(status, LIBSPDM_STATUS_SUCCESS);
+    assert_int_equal(response_size, sizeof(spdm_error_response_t));
+    assert_int_equal(spdm_response->header.spdm_version, SPDM_MESSAGE_VERSION_14);
+    assert_int_equal(spdm_response->header.request_response_code, SPDM_ERROR);
+    assert_int_equal(spdm_response->header.param1, SPDM_ERROR_CODE_INVALID_REQUEST);
+    assert_int_equal(spdm_response->header.param2, 0);
+}
+
+int libspdm_rsp_set_key_pair_info_ack_error_test(void)
+{
+    const struct CMUnitTest test_cases[] = {
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case1),
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case2),
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case3),
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case4),
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case5),
+        cmocka_unit_test(rsp_set_key_pair_info_ack_err_case6),
+    };
+
+    libspdm_test_context_t test_context = {
+        LIBSPDM_TEST_CONTEXT_VERSION,
+        false,
+    };
+
+    libspdm_setup_test_context(&test_context);
+
+    return cmocka_run_group_tests(test_cases,
+                                  libspdm_unit_test_group_setup,
+                                  libspdm_unit_test_group_teardown);
+}
+
+#endif /* LIBSPDM_ENABLE_CAPABILITY_SET_KEY_PAIR_INFO_CAP */
diff --git a/unit_test/test_spdm_responder/test_spdm_responder.c b/unit_test/test_spdm_responder/test_spdm_responder.c
index 7957e667dc7..568123d4014 100644
--- a/unit_test/test_spdm_responder/test_spdm_responder.c
+++ b/unit_test/test_spdm_responder/test_spdm_responder.c
@@ -40,6 +40,7 @@ int libspdm_rsp_key_pair_info_error_test(void);
 
 #if LIBSPDM_ENABLE_CAPABILITY_SET_KEY_PAIR_INFO_CAP
 int libspdm_rsp_set_key_pair_info_ack_test(void);
+int libspdm_rsp_set_key_pair_info_ack_error_test(void);
 #endif /* LIBSPDM_ENABLE_CAPABILITY_SET_KEY_PAIR_INFO_CAP*/
 
 #if LIBSPDM_RESPOND_IF_READY_SUPPORT
@@ -180,6 +181,9 @@ int main(void)
     if (libspdm_rsp_set_key_pair_info_ack_test() != 0) {
         return_value = 1;
     }
+    if (libspdm_rsp_set_key_pair_info_ack_error_test() != 0) {
+        return_value = 1;
+    }
     #endif /* LIBSPDM_ENABLE_CAPABILITY_SET_KEY_PAIR_INFO_CAP*/
 
     #if LIBSPDM_RESPOND_IF_READY_SUPPORT