diff --git a/test/ossfuzz/json_load_dump_fuzzer.cc b/test/ossfuzz/json_load_dump_fuzzer.cc
index 09f52d2..bc3844e 100644
--- a/test/ossfuzz/json_load_dump_fuzzer.cc
+++ b/test/ossfuzz/json_load_dump_fuzzer.cc
@@ -1,13 +1,44 @@
 #include <stdint.h>
+#include <stdlib.h>
 #include <sys/types.h>
+#include <inttypes.h>
 
 #include "jansson.h"
 
+static int enable_diags;
+
+#define FUZZ_DEBUG(FMT, ...)                                                  \
+        if (enable_diags)                                                     \
+        {                                                                     \
+          fprintf(stderr, FMT, ##__VA_ARGS__);                                \
+          fprintf(stderr, "\n");                                              \
+        }
+
+
+static int json_dump_counter(const char *buffer, size_t size, void *data)
+{
+  uint64_t *counter = reinterpret_cast<uint64_t *>(data);
+  *counter += size;
+  return 0;
+}
+
+
+#define NUM_COMMAND_BYTES  (sizeof(size_t) + sizeof(size_t) + 1)
+
+#define FUZZ_DUMP_CALLBACK 0x00
+#define FUZZ_DUMP_STRING   0x01
+
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
   json_error_t error;
+  unsigned char dump_mode;
 
-  if (size < sizeof(size_t) + sizeof(size_t))
+  // Enable or disable diagnostics based on the FUZZ_VERBOSE environment flag.
+  enable_diags = (getenv("FUZZ_VERBOSE") != NULL);
+
+  FUZZ_DEBUG("Input data length: %zd", size);
+
+  if (size < NUM_COMMAND_BYTES)
   {
     return 0;
   }
@@ -15,12 +46,53 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
   // Use the first sizeof(size_t) bytes as load flags.
   size_t load_flags = *(const size_t*)data;
   data += sizeof(size_t);
-  size -= sizeof(size_t);
+
+  FUZZ_DEBUG("load_flags: 0x%zx\n"
+             "& JSON_REJECT_DUPLICATES =  0x%zx\n"
+             "& JSON_DECODE_ANY =         0x%zx\n"
+             "& JSON_DISABLE_EOF_CHECK =  0x%zx\n"
+             "& JSON_DECODE_INT_AS_REAL = 0x%zx\n"
+             "& JSON_ALLOW_NUL =          0x%zx\n",
+             load_flags,
+             load_flags & JSON_REJECT_DUPLICATES,
+             load_flags & JSON_DECODE_ANY,
+             load_flags & JSON_DISABLE_EOF_CHECK,
+             load_flags & JSON_DECODE_INT_AS_REAL,
+             load_flags & JSON_ALLOW_NUL);
 
   // Use the next sizeof(size_t) bytes as dump flags.
   size_t dump_flags = *(const size_t*)data;
   data += sizeof(size_t);
-  size -= sizeof(size_t);
+
+  FUZZ_DEBUG("dump_flags: 0x%zx\n"
+             "& JSON_MAX_INDENT =     0x%zx\n"
+             "& JSON_COMPACT =        0x%zx\n"
+             "& JSON_ENSURE_ASCII =   0x%zx\n"
+             "& JSON_SORT_KEYS =      0x%zx\n"
+             "& JSON_PRESERVE_ORDER = 0x%zx\n"
+             "& JSON_ENCODE_ANY =     0x%zx\n"
+             "& JSON_ESCAPE_SLASH =   0x%zx\n"
+             "& JSON_REAL_PRECISION = 0x%zx\n"
+             "& JSON_EMBED =          0x%zx\n",
+             dump_flags,
+             dump_flags & JSON_MAX_INDENT,
+             dump_flags & JSON_COMPACT,
+             dump_flags & JSON_ENSURE_ASCII,
+             dump_flags & JSON_SORT_KEYS,
+             dump_flags & JSON_PRESERVE_ORDER,
+             dump_flags & JSON_ENCODE_ANY,
+             dump_flags & JSON_ESCAPE_SLASH,
+             ((dump_flags >> 11) & 0x1F) << 11,
+             dump_flags & JSON_EMBED);
+
+  // Use the next byte as the dump mode.
+  dump_mode = data[0];
+  data++;
+
+  FUZZ_DEBUG("dump_mode: 0x%x", (unsigned int)dump_mode);
+
+  // Remove the command bytes from the size total.
+  size -= NUM_COMMAND_BYTES;
 
   // Attempt to load the remainder of the data with the given load flags.
   const char* text = reinterpret_cast<const char *>(data);
@@ -31,11 +103,24 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
     return 0;
   }
 
-  // Attempt to dump the loaded json object with the given dump flags.
-  char* out = json_dumps(jobj, dump_flags);
-  if (out)
+  if (dump_mode & FUZZ_DUMP_STRING)
   {
-    free(out);
+    // Dump as a string. Remove indents so that we don't run out of memory.
+    char *out = json_dumps(jobj, dump_flags & ~JSON_MAX_INDENT);
+    if (out != NULL)
+    {
+      free(out);
+    }
+  }
+  else
+  {
+    // Default is callback mode.
+    //
+    // Attempt to dump the loaded json object with the given dump flags.
+    uint64_t counter = 0;
+
+    json_dump_callback(jobj, json_dump_counter, &counter, dump_flags);
+    FUZZ_DEBUG("Counter function counted %" PRIu64 " bytes.", counter);
   }
 
   if (jobj)
@@ -44,4 +129,4 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
   }
 
   return 0;
-}
\ No newline at end of file
+}