diff --git a/src/wp_ecc_kmgmt.c b/src/wp_ecc_kmgmt.c
index 4b5f31e2..5ebb0e1f 100644
--- a/src/wp_ecc_kmgmt.c
+++ b/src/wp_ecc_kmgmt.c
@@ -747,7 +747,7 @@ static int wp_ecc_get_params_enc_pub_key(wp_Ecc* ecc, OSSL_PARAM params[],
     p = OSSL_PARAM_locate(params, key);
     if (p != NULL) {
         int rc;
-        word32 outLen = (word32)p->return_size;
+        word32 outLen = (word32)p->data_size;
 
         if (ecc->hasPub == 0) {
             ok = 0;
diff --git a/src/wp_ecx_kmgmt.c b/src/wp_ecx_kmgmt.c
index b26df4ec..b9291a88 100644
--- a/src/wp_ecx_kmgmt.c
+++ b/src/wp_ecx_kmgmt.c
@@ -547,7 +547,7 @@ static int wp_ecx_get_params_enc_pub_key(wp_Ecx* ecx, OSSL_PARAM params[],
 
     p = OSSL_PARAM_locate(params, key);
     if (p != NULL) {
-        word32 outLen = (word32)p->return_size;
+        word32 outLen = (word32)p->data_size;
 
         if (p->data == NULL) {
             outLen = ecx->data->len;
@@ -584,7 +584,7 @@ static int wp_ecx_get_params_priv_key(wp_Ecx* ecx, OSSL_PARAM params[])
 
     p = OSSL_PARAM_locate(params, OSSL_PKEY_PARAM_PRIV_KEY);
     if (p != NULL) {
-        word32 outLen = (word32)p->return_size;
+        word32 outLen = (word32)p->data_size;
 
         if (p->data == NULL) {
             outLen = ecx->data->len;
diff --git a/test/test_ecc.c b/test/test_ecc.c
index 58d816d9..18d4aa71 100644
--- a/test/test_ecc.c
+++ b/test/test_ecc.c
@@ -519,6 +519,83 @@ int test_eckeygen_p256(void *data)
 
     return err;
 }
+
+/* get_params for the ECC encoded public key must size the export buffer from
+ * data_size, not a stale/zero return_size. Compare a stale-return_size export
+ * against a known-good fresh export of the same key. */
+int test_ecc_p256_get_params_stale_ret(void *data)
+{
+    int err = 0;
+    EVP_PKEY_CTX *ctx = NULL;
+    EVP_PKEY *key = NULL;
+    unsigned char ref[128];
+    unsigned char readback[128];
+    size_t refLen = 0;
+    OSSL_PARAM params[2];
+
+    (void)data;
+
+    PRINT_MSG("P-256 encoded pub key get_params with stale return_size");
+
+    err = (ctx = EVP_PKEY_CTX_new_from_name(wpLibCtx, "EC", NULL)) == NULL;
+    if (err == 0) {
+        err = EVP_PKEY_keygen_init(ctx) != 1;
+    }
+    if (err == 0) {
+        err = EVP_PKEY_CTX_set_ec_paramgen_curve_nid(ctx,
+                                                     NID_X9_62_prime256v1) != 1;
+    }
+    if (err == 0) {
+        err = EVP_PKEY_keygen(ctx, &key) != 1;
+    }
+
+    /* Reference export via a fresh param. Its return_size defaults to
+     * OSSL_PARAM_UNMODIFIED, so this block succeeds even with the bug; the
+     * stale-return_size query below is the actual regression check. */
+    if (err == 0) {
+        params[0] = OSSL_PARAM_construct_octet_string(
+            OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY, ref, sizeof(ref));
+        params[1] = OSSL_PARAM_construct_end();
+        if (EVP_PKEY_get_params(key, params) != 1) {
+            PRINT_ERR_MSG("reference encoded pub key get_params failed");
+            err = 1;
+        }
+        else {
+            refLen = params[0].return_size;
+        }
+    }
+    if (err == 0 && (refLen == 0 || refLen > sizeof(ref))) {
+        PRINT_ERR_MSG("unexpected encoded pub key length: %zu", refLen);
+        err = 1;
+    }
+
+    /* Same query with return_size forced to 0 to mimic a reused param. */
+    if (err == 0) {
+        memset(readback, 0, sizeof(readback));
+        params[0] = OSSL_PARAM_construct_octet_string(
+            OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY, readback, sizeof(readback));
+        params[0].return_size = 0;
+        params[1] = OSSL_PARAM_construct_end();
+        if (EVP_PKEY_get_params(key, params) != 1) {
+            PRINT_ERR_MSG("encoded pub key get_params failed with stale "
+                          "return_size");
+            err = 1;
+        }
+    }
+    if (err == 0 && params[0].return_size != refLen) {
+        PRINT_ERR_MSG("encoded pub key return_size wrong: %zu vs %zu",
+                      params[0].return_size, refLen);
+        err = 1;
+    }
+    if (err == 0 && memcmp(readback, ref, refLen) != 0) {
+        PRINT_ERR_MSG("encoded pub key bytes not returned correctly");
+        err = 1;
+    }
+
+    EVP_PKEY_free(key);
+    EVP_PKEY_CTX_free(ctx);
+    return err;
+}
 #endif /* WP_HAVE_EC_P256 */
 
 #ifdef WP_HAVE_EC_P384
diff --git a/test/test_ecx.c b/test/test_ecx.c
index d316446e..c8a4d689 100644
--- a/test/test_ecx.c
+++ b/test/test_ecx.c
@@ -758,6 +758,109 @@ int test_ecx_x25519_raw_priv_roundtrip(void *data)
     return err;
 }
 
+/* get_params must size the export buffer from the caller's data_size, not
+ * return_size. A reused param can carry a stale/zero return_size from a prior
+ * size query; using it would truncate or fail the export. */
+int test_ecx_x25519_get_params_stale_ret(void *data)
+{
+    int err = 0;
+    EVP_PKEY *pkey = NULL;
+    static const unsigned char privKey[] = {
+        0x77, 0x07, 0x6d, 0x0a, 0x73, 0x18, 0xa5, 0x7d,
+        0x3c, 0x16, 0xc1, 0x72, 0x51, 0xb2, 0x66, 0x45,
+        0xdf, 0x4c, 0x2f, 0x87, 0xeb, 0xc0, 0x99, 0x2a,
+        0xb1, 0x77, 0xfb, 0xa5, 0x1d, 0xb9, 0x2c, 0x2a
+    };
+    /* RFC 7748 Section 6.1 Alice public key, derived from privKey above. */
+    static const unsigned char pubKey[] = {
+        0x85, 0x20, 0xf0, 0x09, 0x89, 0x30, 0xa7, 0x54,
+        0x74, 0x8b, 0x7d, 0xdc, 0xb4, 0x3e, 0xf7, 0x5a,
+        0x0d, 0xbf, 0x3a, 0x0d, 0x26, 0x38, 0x1a, 0xf4,
+        0xeb, 0xa4, 0xa9, 0x8e, 0xaa, 0x9b, 0x4e, 0x6a
+    };
+    unsigned char readback[32];
+    OSSL_PARAM params[2];
+
+    (void)data;
+
+    PRINT_MSG("X25519 get_params with stale (zero) return_size");
+
+    pkey = EVP_PKEY_new_raw_private_key_ex(wpLibCtx, "X25519", NULL,
+        privKey, sizeof(privKey));
+    if (pkey == NULL) {
+        PRINT_ERR_MSG("Failed to import X25519 private key");
+        err = 1;
+    }
+
+    /* Private key: full 32-byte buffer but return_size forced to 0. */
+    if (err == 0) {
+        memset(readback, 0, sizeof(readback));
+        params[0] = OSSL_PARAM_construct_octet_string(
+            OSSL_PKEY_PARAM_PRIV_KEY, readback, sizeof(readback));
+        params[0].return_size = 0;
+        params[1] = OSSL_PARAM_construct_end();
+        if (EVP_PKEY_get_params(pkey, params) != 1) {
+            PRINT_ERR_MSG("priv key get_params failed with stale return_size");
+            err = 1;
+        }
+    }
+    if (err == 0 && params[0].return_size != sizeof(privKey)) {
+        PRINT_ERR_MSG("priv key return_size wrong: %zu", params[0].return_size);
+        err = 1;
+    }
+    if (err == 0 && memcmp(readback, privKey, sizeof(privKey)) != 0) {
+        PRINT_ERR_MSG("priv key bytes not returned correctly");
+        err = 1;
+    }
+
+    /* Public key: same stale-return_size condition on the export path. */
+    if (err == 0) {
+        memset(readback, 0, sizeof(readback));
+        params[0] = OSSL_PARAM_construct_octet_string(
+            OSSL_PKEY_PARAM_PUB_KEY, readback, sizeof(readback));
+        params[0].return_size = 0;
+        params[1] = OSSL_PARAM_construct_end();
+        if (EVP_PKEY_get_params(pkey, params) != 1) {
+            PRINT_ERR_MSG("pub key get_params failed with stale return_size");
+            err = 1;
+        }
+    }
+    if (err == 0 && params[0].return_size != sizeof(pubKey)) {
+        PRINT_ERR_MSG("pub key return_size wrong: %zu", params[0].return_size);
+        err = 1;
+    }
+    if (err == 0 && memcmp(readback, pubKey, sizeof(pubKey)) != 0) {
+        PRINT_ERR_MSG("pub key bytes not returned correctly");
+        err = 1;
+    }
+
+    /* Encoded public key: same helper, exercised via its own key string. */
+    if (err == 0) {
+        memset(readback, 0, sizeof(readback));
+        params[0] = OSSL_PARAM_construct_octet_string(
+            OSSL_PKEY_PARAM_ENCODED_PUBLIC_KEY, readback, sizeof(readback));
+        params[0].return_size = 0;
+        params[1] = OSSL_PARAM_construct_end();
+        if (EVP_PKEY_get_params(pkey, params) != 1) {
+            PRINT_ERR_MSG("enc pub key get_params failed with stale "
+                          "return_size");
+            err = 1;
+        }
+    }
+    if (err == 0 && params[0].return_size != sizeof(pubKey)) {
+        PRINT_ERR_MSG("enc pub key return_size wrong: %zu",
+                      params[0].return_size);
+        err = 1;
+    }
+    if (err == 0 && memcmp(readback, pubKey, sizeof(pubKey)) != 0) {
+        PRINT_ERR_MSG("enc pub key bytes not returned correctly");
+        err = 1;
+    }
+
+    EVP_PKEY_free(pkey);
+    return err;
+}
+
 /* A zero-length private key octet string must be rejected without an
  * out-of-bounds read (privData[len-1] with len==0). */
 int test_ecx_import_zero_priv(void *data)
diff --git a/test/unit.c b/test/unit.c
index c802390c..c8afafb0 100644
--- a/test/unit.c
+++ b/test/unit.c
@@ -407,6 +407,7 @@ TEST_CASE test_case[] = {
 #ifdef WP_HAVE_EC_P256
     #ifdef WP_HAVE_ECKEYGEN
         TEST_DECL(test_eckeygen_p256, NULL),
+        TEST_DECL(test_ecc_p256_get_params_stale_ret, NULL),
     #endif
     #ifdef WP_HAVE_ECDH
     #ifdef WP_HAVE_ECKEYGEN
@@ -515,6 +516,7 @@ TEST_CASE test_case[] = {
     TEST_DECL(test_ecx_null_init, NULL),
 #ifdef WP_HAVE_X25519
     TEST_DECL(test_ecx_x25519_raw_priv_roundtrip, NULL),
+    TEST_DECL(test_ecx_x25519_get_params_stale_ret, NULL),
     TEST_DECL(test_ecx_import_zero_priv, NULL),
 #endif
     TEST_DECL(test_ecx_dup, NULL),
diff --git a/test/unit.h b/test/unit.h
index 6b617944..e003fadd 100644
--- a/test/unit.h
+++ b/test/unit.h
@@ -380,6 +380,7 @@ int test_eckeygen_p224(void *data);
 
 #ifdef WP_HAVE_EC_P256
 int test_eckeygen_p256(void *data);
+int test_ecc_p256_get_params_stale_ret(void *data);
 #endif /* WP_HAVE_EC_P256 */
 
 #ifdef WP_HAVE_EC_P384
@@ -522,6 +523,7 @@ int test_ecx_misc(void *data);
 int test_ecx_null_init(void *data);
 #ifdef WP_HAVE_X25519
 int test_ecx_x25519_raw_priv_roundtrip(void *data);
+int test_ecx_x25519_get_params_stale_ret(void *data);
 int test_ecx_import_zero_priv(void *data);
 #endif /* WP_HAVE_X25519 */
 int test_ecx_dup(void *data);