diff --git a/src/BPSecLib_Private.h b/src/BPSecLib_Private.h
index b0f97db..3b59d40 100644
--- a/src/BPSecLib_Private.h
+++ b/src/BPSecLib_Private.h
@@ -514,15 +514,31 @@ int BSL_BundleCtx_DeleteBundle(BSL_BundleRef_t *bundle, BSL_ReasonCode_t reason_
  * @note Uses semantics similar to @c memcpy().
  *
  * @param[in] bundle Context bundle
- * @param[in] block_num Number of block requesting re-allocated of BTSD
- * @param[in] bytesize Size of new BTSD
+ * @param[in] block_num The unique block number for which BTSD will be resized.
+ * @param[in] btsd_size Size of new BTSD content.
  * @return 0 on success, negative on failure.
  */
-int BSL_BundleCtx_ReallocBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t bytesize);
+int BSL_BundleCtx_ReallocBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_size);
 
+/** Construct a new sequential reader for BTSD content.
+ *
+ * @param[in] bundle Context bundle
+ * @param[in] block_num The unique block number for which BTSD will be read from.
+ * @return Pointer to the new reader or NULL if some failure occurs.
+ */
 BSL_SeqReader_t *BSL_BundleCtx_ReadBTSD(const BSL_BundleRef_t *bundle, uint64_t block_num);
 
-BSL_SeqWriter_t *BSL_BundleCtx_WriteBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_len);
+/** Construct a new sequential writer for BTSD content.
+ *
+ * @param[in] bundle Context bundle
+ * @param[in] block_num The unique block number for which BTSD will be overwritten.
+ * @param btsd_size The total total size of BTSD content that will be written.
+ * The actual sequence of writes must not exceed this total size or it will be considered an error.
+ * If the actual sequence of writes does not reach this size it should be zero-padded and logged
+ * as an anomaly.
+ * @return Pointer to the new writer or NULL if some failure occurs.
+ */
+BSL_SeqWriter_t *BSL_BundleCtx_WriteBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_size);
 
 /** @brief Security role of an operation
  */
diff --git a/src/BPSecLib_Public.h b/src/BPSecLib_Public.h
index bdf12da..a757e64 100644
--- a/src/BPSecLib_Public.h
+++ b/src/BPSecLib_Public.h
@@ -330,9 +330,11 @@ typedef struct
     /// @brief Host BPA function to remove a given canonical block from the bundle
     int (*block_remove_fn)(BSL_BundleRef_t *bundle_ref, uint64_t block_num);
 
-    /// @brief Host BPA function to reallocate a canonical block's BTSD, keeping existing data in-place.
-    /// @deprecated use sequential writer to do this
-    int (*block_realloc_btsd_fn)(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t bytesize);
+    /** Host BPA function to reallocate a canonical block's BTSD content, keeping existing data in-place.
+     *
+     * @deprecated use sequential writer to do replace BTSD content
+     */
+    int (*block_realloc_btsd_fn)(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t btsd_size);
 
     /** Host BPA function do create a new sequential reader on a single block-type-specific data.
      *
@@ -350,11 +352,14 @@ typedef struct
      *
      * @param[in] bundle_ref The bundle to read data from.
      * @param block_num The specific block number to write BTSD into.
-     * @param total_size A hint as to the total size that will be written.
+     * @param btsd_size The total total size of BTSD content that will be written.
+     * The actual sequence of writes must not exceed this total size or it will be considered an error.
+     * If the actual sequence of writes does not reach this size it should be zero-padded and logged
+     * as an anomaly.
      * @return A pointer to a reader struct or NULL if the reader cannot
      * be configured for any reason.
      */
-    struct BSL_SeqWriter_s *(*block_write_btsd_fn)(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t total_size);
+    struct BSL_SeqWriter_s *(*block_write_btsd_fn)(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t btsd_size);
 
     /// @brief Host BPA function to delete Bundle with a reason code. This can be called multiple times per-bundle with
     /// different reason codes
diff --git a/src/backend/HostInterface.c b/src/backend/HostInterface.c
index b55306f..06afa12 100644
--- a/src/backend/HostInterface.c
+++ b/src/backend/HostInterface.c
@@ -127,12 +127,12 @@ int BSL_BundleCtx_DeleteBundle(BSL_BundleRef_t *bundle, BSL_ReasonCode_t reason_
     return (result == 0) ? BSL_SUCCESS : BSL_ERR_HOST_CALLBACK_FAILED;
 }
 
-int BSL_BundleCtx_ReallocBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t bytesize)
+int BSL_BundleCtx_ReallocBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_size)
 {
     CHK_ARG_NONNULL(bundle);
     CHK_ARG_EXPR(block_num > 0);
     CHK_PRECONDITION(HostDescriptorTable.block_remove_fn != NULL);
-    int result = HostDescriptorTable.block_realloc_btsd_fn(bundle, block_num, bytesize);
+    int result = HostDescriptorTable.block_realloc_btsd_fn(bundle, block_num, btsd_size);
     return (result == 0) ? BSL_SUCCESS : BSL_ERR_HOST_CALLBACK_FAILED;
 }
 
@@ -145,13 +145,13 @@ BSL_SeqReader_t *BSL_BundleCtx_ReadBTSD(const BSL_BundleRef_t *bundle, uint64_t
     return HostDescriptorTable.block_read_btsd_fn(bundle, block_num);
 }
 
-BSL_SeqWriter_t *BSL_BundleCtx_WriteBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_len)
+BSL_SeqWriter_t *BSL_BundleCtx_WriteBTSD(BSL_BundleRef_t *bundle, uint64_t block_num, size_t btsd_size)
 {
     if (!bundle || !HostDescriptorTable.block_write_btsd_fn)
     {
         return NULL;
     }
-    return HostDescriptorTable.block_write_btsd_fn(bundle, block_num, btsd_len);
+    return HostDescriptorTable.block_write_btsd_fn(bundle, block_num, btsd_size);
 }
 
 void BSL_HostDescriptors_Get(BSL_HostDescriptors_t *desc)
diff --git a/src/mock_bpa/agent.c b/src/mock_bpa/agent.c
index 0a8bb44..594442a 100644
--- a/src/mock_bpa/agent.c
+++ b/src/mock_bpa/agent.c
@@ -119,9 +119,9 @@ int MockBPA_GetBlockMetadata(const BSL_BundleRef_t *bundle_ref, uint64_t block_n
     return 0;
 }
 
-int MockBPA_ReallocBTSD(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t bytesize)
+int MockBPA_ReallocBTSD(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t btsd_size)
 {
-    if (!bundle_ref || !bundle_ref->data || block_num == 0 || bytesize == 0)
+    if (!bundle_ref || !bundle_ref->data || block_num == 0 || btsd_size == 0)
     {
         return -1;
     }
@@ -137,13 +137,13 @@ int MockBPA_ReallocBTSD(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t
 
     if (found_block->btsd == NULL)
     {
-        found_block->btsd     = BSL_calloc(1, bytesize);
-        found_block->btsd_len = bytesize;
+        found_block->btsd     = BSL_calloc(1, btsd_size);
+        found_block->btsd_len = btsd_size;
     }
     else
     {
-        found_block->btsd     = BSL_realloc(found_block->btsd, bytesize);
-        found_block->btsd_len = bytesize;
+        found_block->btsd     = BSL_realloc(found_block->btsd, btsd_size);
+        found_block->btsd_len = btsd_size;
     }
 
     // Return -9 if malloc/realloc faile. Return 0 for success.
@@ -162,6 +162,8 @@ struct MockBPA_BTSD_Data_s
     size_t size;
     /// File opened for the buffer
     FILE *file;
+    /// Local dead-reckoned cursor into #file
+    size_t curs;
 };
 
 static int MockBPA_ReadBTSD_Read(void *user_data, void *buf, size_t *bufsize)
@@ -173,6 +175,7 @@ static int MockBPA_ReadBTSD_Read(void *user_data, void *buf, size_t *bufsize)
     ASSERT_PRECONDITION(obj->file);
 
     const size_t got = fread(buf, 1, *bufsize, obj->file);
+    obj->curs += got;
     BSL_LOG_DEBUG("reading up to %zd bytes, got %zd", *bufsize, got);
     *bufsize = got;
     return 0;
@@ -208,6 +211,7 @@ static struct BSL_SeqReader_s *MockBPA_ReadBTSD(const BSL_BundleRef_t *bundle_re
     obj->ptr   = found_block->btsd;
     obj->size  = found_block->btsd_len;
     obj->file  = fmemopen(obj->ptr, obj->size, "rb");
+    obj->curs  = 0;
 
     BSL_SeqReader_t *reader = BSL_calloc(1, sizeof(BSL_SeqReader_t));
     if (!reader)
@@ -229,7 +233,15 @@ static int MockBPA_WriteBTSD_Write(void *user_data, const void *buf, size_t size
     CHK_ARG_NONNULL(buf);
     ASSERT_PRECONDITION(obj->file);
 
+    const size_t excess = (obj->curs + size) - obj->size;
+    if (excess > 0)
+    {
+        BSL_LOG_ERR("write too large for buffer of %zu by %zu bytes", obj->size, excess);
+        return BSL_ERR_FAILURE;
+    }
+
     const size_t got = fwrite(buf, 1, size, obj->file);
+    obj->curs += got;
     BSL_LOG_DEBUG("writing up to %zd bytes, got %zd", size, got);
     if (got < size)
     {
@@ -245,7 +257,11 @@ static void MockBPA_WriteBTSD_Deinit(void *user_data)
     ASSERT_PRECONDITION(obj->file);
 
     fclose(obj->file);
-    BSL_LOG_DEBUG("closed block %p with size %zu", obj->block, obj->size);
+    BSL_LOG_DEBUG("closed block %p with size %zu and cursor %zu", obj->block, obj->size, obj->curs);
+    if (obj->curs < obj->size)
+    {
+        BSL_LOG_ERR("closed block %p for writing with only %zu of %zu written", obj->block, obj->curs, obj->size);
+    }
 
     // now write-back the BTSD
     BSL_free(obj->block->btsd);
@@ -255,7 +271,7 @@ static void MockBPA_WriteBTSD_Deinit(void *user_data)
     BSL_free(obj);
 }
 
-static struct BSL_SeqWriter_s *MockBPA_WriteBTSD(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t total_size)
+static struct BSL_SeqWriter_s *MockBPA_WriteBTSD(BSL_BundleRef_t *bundle_ref, uint64_t block_num, size_t btsd_size)
 {
     MockBPA_Bundle_t          *bundle    = bundle_ref->data;
     MockBPA_CanonicalBlock_t **found_ptr = MockBPA_BlockByNum_get(bundle->blocks_num, block_num);
@@ -264,7 +280,7 @@ static struct BSL_SeqWriter_s *MockBPA_WriteBTSD(BSL_BundleRef_t *bundle_ref, ui
         return NULL;
     }
     MockBPA_CanonicalBlock_t *found_block = *found_ptr;
-    BSL_LOG_DEBUG("opened block %p for size %zu", found_block, total_size);
+    BSL_LOG_DEBUG("opened block %p for size %zu, previous size %zu", found_block, btsd_size, found_block->btsd_len);
 
     struct MockBPA_BTSD_Data_s *obj = BSL_calloc(1, sizeof(struct MockBPA_BTSD_Data_s));
     if (!obj)
@@ -276,6 +292,15 @@ static struct BSL_SeqWriter_s *MockBPA_WriteBTSD(BSL_BundleRef_t *bundle_ref, ui
     obj->ptr   = NULL;
     obj->size  = 0;
     obj->file  = open_memstream(&obj->ptr, &obj->size);
+    obj->curs  = 0;
+
+    if (btsd_size)
+    {
+        // pre-allocate BTSD size
+        fseeko(obj->file, btsd_size, SEEK_SET);
+        fflush(obj->file);
+        fseeko(obj->file, 0UL, SEEK_SET);
+    }
 
     BSL_SeqWriter_t *writer = BSL_calloc(1, sizeof(BSL_SeqWriter_t));
     if (!writer)
diff --git a/src/mock_bpa/ctr.c b/src/mock_bpa/ctr.c
index 550dd1c..3f352a1 100644
--- a/src/mock_bpa/ctr.c
+++ b/src/mock_bpa/ctr.c
@@ -19,6 +19,10 @@
  * the prime contract 80NM0018D0004 between the Caltech and NASA under
  * subcontract 1700763.
  */
+/** @file
+ * @ingroup mock_bpa
+ * Container structs for BPv7 data.
+ */
 #include <BPSecLib_Private.h>
 #include <m-algo.h>
 
diff --git a/src/mock_bpa/ctr.h b/src/mock_bpa/ctr.h
index ef89a0a..ebdd8aa 100644
--- a/src/mock_bpa/ctr.h
+++ b/src/mock_bpa/ctr.h
@@ -19,6 +19,10 @@
  * the prime contract 80NM0018D0004 between the Caltech and NASA under
  * subcontract 1700763.
  */
+/** @file
+ * @ingroup mock_bpa
+ * Container structs for BPv7 data.
+ */
 #ifndef BSL_MOCK_BPA_CTR_H_
 #define BSL_MOCK_BPA_CTR_H_
 
diff --git a/src/mock_bpa/mock_bpa.c b/src/mock_bpa/mock_bpa.c
index b9b5eb8..721326b 100644
--- a/src/mock_bpa/mock_bpa.c
+++ b/src/mock_bpa/mock_bpa.c
@@ -20,6 +20,7 @@
  * subcontract 1700763.
  */
 /** @file
+ * @ingroup mock_bpa
  * This is the main entry for a mock BPA daemon that communicates through
  * unix domain sockets.
  */
diff --git a/src/mock_bpa/policy_config.c b/src/mock_bpa/policy_config.c
index 1c8abd7..c018540 100644
--- a/src/mock_bpa/policy_config.c
+++ b/src/mock_bpa/policy_config.c
@@ -21,8 +21,8 @@
  */
 
 /** @file
- * Implementations for permutations of policy configurations.
  * @ingroup mock_bpa
+ * Implementations for permutations of policy configurations.
  */
 
 #include "policy_config.h"
diff --git a/src/mock_bpa/policy_config.h b/src/mock_bpa/policy_config.h
index 0e2d5ca..b8ac521 100644
--- a/src/mock_bpa/policy_config.h
+++ b/src/mock_bpa/policy_config.h
@@ -21,8 +21,8 @@
  */
 
 /** @file
- * Definitions for permutations of policy configurations.
  * @ingroup mock_bpa
+ * Definitions for permutations of policy configurations.
  */
 
 #ifndef BSL_MOCK_BPA_POLICY_CONFIG_H_