diff --git a/ChangeLog.d/union-initialization.txt b/ChangeLog.d/union-initialization.txt
new file mode 100644
index 0000000..a63e1eb
--- /dev/null
+++ b/ChangeLog.d/union-initialization.txt
@@ -0,0 +1,15 @@
+Bugfix
+   * Fix failures of PSA multipart or interruptible operations when the
+     library or the application is built with a compiler where
+     "union foo x = {0}" does not initialize non-default members of the
+     union, such as GCC 15 and some versions of Clang 18. This affected MAC
+     multipart operations, MAC-based key derivation operations, interruptible
+     signature, interruptible verification, and potentially other operations
+     when using third-party drivers. This also affected one-shot MAC
+     operations using the built-in implementation. Fixes #9814.
+   * On entry to PSA driver entry points that set up a multipart operation
+     ("xxx_setup"), the operation object is supposed to be all-bits-zero.
+     This was sometimes not the case when an operation object is reused,
+     or with compilers where "union foo x = {0}" does not initialize
+     non-default members of the union. The PSA core now ensures that this
+     guarantee is met in all cases. Fixes #9975.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index e7f99ad..9c28609 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -2400,8 +2400,11 @@
         goto exit;
     }
 
-    /* Ensure all of the context is zeroized, since PSA_HASH_OPERATION_INIT only
-     * directly zeroes the int-sized dummy member of the context union. */
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
     memset(&operation->ctx, 0, sizeof(operation->ctx));
 
     status = psa_driver_wrapper_hash_setup(operation, alg);
@@ -2596,6 +2599,13 @@
         return PSA_ERROR_BAD_STATE;
     }
 
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&target_operation->ctx, 0, sizeof(target_operation->ctx));
+
     psa_status_t status = psa_driver_wrapper_hash_clone(source_operation,
                                                         target_operation);
     if (status != PSA_SUCCESS) {
@@ -2693,6 +2703,13 @@
         goto exit;
     }
 
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&operation->ctx, 0, sizeof(operation->ctx));
+
     status = psa_get_and_lock_key_slot_with_policy(
         key,
         &slot,
@@ -3619,6 +3636,13 @@
         return PSA_ERROR_BAD_STATE;
     }
 
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&operation->ctx, 0, sizeof(operation->ctx));
+
     status = psa_sign_verify_check_alg(0, alg);
     if (status != PSA_SUCCESS) {
         operation->error_occurred = 1;
@@ -3779,6 +3803,13 @@
         return PSA_ERROR_BAD_STATE;
     }
 
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&operation->ctx, 0, sizeof(operation->ctx));
+
     status = psa_sign_verify_check_alg(0, alg);
     if (status != PSA_SUCCESS) {
         operation->error_occurred = 1;
@@ -4446,6 +4477,14 @@
     }
     operation->default_iv_length = PSA_CIPHER_IV_LENGTH(slot->attr.type, alg);
 
+
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&operation->ctx, 0, sizeof(operation->ctx));
+
     /* Try doing the operation through a driver before using software fallback. */
     if (cipher_operation == MBEDTLS_ENCRYPT) {
         status = psa_driver_wrapper_cipher_encrypt_setup(operation,
@@ -5079,6 +5118,13 @@
         goto exit;
     }
 
+    /* Make sure the driver-dependent part of the operation is zeroed.
+     * This is a guarantee we make to drivers. Initializing the operation
+     * does not necessarily take care of it, since the context is a
+     * union and initializing a union does not necessarily initialize
+     * all of its members. */
+    memset(&operation->ctx, 0, sizeof(operation->ctx));
+
     if (is_encrypt) {
         key_usage = PSA_KEY_USAGE_ENCRYPT;
     } else {
@@ -5599,6 +5645,17 @@
 #if defined(BUILTIN_ALG_ANY_HKDF) || \
     defined(MBEDTLS_PSA_BUILTIN_ALG_TLS12_PRF) || \
     defined(MBEDTLS_PSA_BUILTIN_ALG_TLS12_PSK_TO_MS)
+
+/** Internal helper to set up an HMAC operation with a key passed directly.
+ *
+ * \param[in,out] operation     A MAC operation object. It does not need to
+ *                              be initialized.
+ * \param hash_alg              The hash algorithm used for HMAC.
+ * \param hmac_key              The HMAC key.
+ * \param hmac_key_length       Length of \p hmac_key in bytes.
+ *
+ * \return A PSA status code.
+ */
 static psa_status_t psa_key_derivation_start_hmac(
     psa_mac_operation_t *operation,
     psa_algorithm_t hash_alg,
@@ -5611,6 +5668,14 @@
     psa_set_key_bits(&attributes, PSA_BYTES_TO_BITS(hmac_key_length));
     psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_SIGN_HASH);
 
+    /* Make sure the whole the operation is zeroed.
+     * It isn't enough to require the caller to initialize operation to
+     * PSA_MAC_OPERATION_INIT, since one field is a union and initializing
+     * a union does not necessarily initialize all of its members.
+     * psa_mac_setup() would handle PSA_MAC_OPERATION_INIT, but here we
+     * bypass it and call lower-level functions directly. */
+    memset(operation, 0, sizeof(*operation));
+
     operation->is_sign = 1;
     operation->mac_size = PSA_HASH_LENGTH(hash_alg);
 
@@ -5835,7 +5900,7 @@
 {
     psa_algorithm_t hash_alg = PSA_ALG_HKDF_GET_HASH(alg);
     uint8_t hash_length = PSA_HASH_LENGTH(hash_alg);
-    psa_mac_operation_t hmac = PSA_MAC_OPERATION_INIT;
+    psa_mac_operation_t hmac;
     size_t hmac_output_length;
     psa_status_t status, cleanup_status;
 
@@ -6036,7 +6101,14 @@
     psa_key_attributes_t *attributes)
 {
     psa_status_t status;
-    psa_mac_operation_t mac_operation = PSA_MAC_OPERATION_INIT;
+    psa_mac_operation_t mac_operation;
+    /* Make sure the whole the operation is zeroed.
+     * PSA_MAC_OPERATION_INIT does not necessarily do it fully,
+     * since one field is a union and initializing a union does not
+     * necessarily initialize all of its members.
+     * psa_mac_setup() would do it, but here we bypass it and call
+     * lower-level functions directly. */
+    memset(&mac_operation, 0, sizeof(mac_operation));
     size_t mac_output_length;
     uint8_t U_i[PSA_MAC_MAX_SIZE];
     uint8_t *U_accumulator = pbkdf2->output_block;
@@ -8629,7 +8701,11 @@
         goto exit;
     }
 
-    memset(&operation->data.inputs, 0, sizeof(operation->data.inputs));
+    /* Make sure the variable-purpose part of the operation is zeroed.
+     * Initializing the operation does not necessarily take care of it,
+     * since the context is a union and initializing a union does not
+     * necessarily initialize all of its members. */
+    memset(&operation->data, 0, sizeof(operation->data));
 
     operation->alg = cipher_suite->algorithm;
     operation->primitive = PSA_PAKE_PRIMITIVE(cipher_suite->type,
diff --git a/library/psa_crypto_mac.c b/library/psa_crypto_mac.c
index 8fe6218..4464158 100644
--- a/library/psa_crypto_mac.c
+++ b/library/psa_crypto_mac.c
@@ -465,6 +465,15 @@
 {
     psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
     mbedtls_psa_mac_operation_t operation = MBEDTLS_PSA_MAC_OPERATION_INIT;
+    /* Make sure the whole operation is zeroed.
+     * PSA_MAC_OPERATION_INIT does not necessarily do it fully,
+     * since one field is a union and initializing a union does not
+     * necessarily initialize all of its members.
+     * In multipart operations, this is done in the API functions,
+     * before driver dispatch, since it needs to be done before calling
+     * the driver entry point. Here, we bypass the multipart API,
+     * so it's our job. */
+    memset(&operation, 0, sizeof(operation));
 
     status = psa_mac_setup(&operation,
                            attributes, key_buffer, key_buffer_size,
diff --git a/tests/scripts/components-compiler.sh b/tests/scripts/components-compiler.sh
index d69d529..2864087 100644
--- a/tests/scripts/components-compiler.sh
+++ b/tests/scripts/components-compiler.sh
@@ -94,10 +94,7 @@
     loc_cflags="$ASAN_CFLAGS -DPSA_CRYPTO_DRIVER_TEST_ALL"
     loc_cflags="${loc_cflags} '-DMBEDTLS_USER_CONFIG_FILE=\"../tests/configs/user-config-for-test.h\"'"
     loc_cflags="${loc_cflags} -I../framework/tests/include -O2"
-    # Until https://github.com/Mbed-TLS/mbedtls/issues/9814 is fixed,
-    # disable the new problematic optimization.
-    loc_cflags="${loc_cflags} -fzero-init-padding-bits=unions"
-    # Also allow a warning that we don't yet comply to.
+    # Allow a warning that we don't yet comply to.
     # https://github.com/Mbed-TLS/mbedtls/issues/9944
     loc_cflags="${loc_cflags} -Wno-error=unterminated-string-initialization"