diff --git a/RSYNC_COMPAT.md b/RSYNC_COMPAT.md
index 8f4960c..1370073 100644
--- a/RSYNC_COMPAT.md
+++ b/RSYNC_COMPAT.md
@@ -24,7 +24,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--help` | Show help | ✅ Implemented | Prints usage and exits; `-h` is not accepted |
 | `-V`, `--version` | Print version | ✅ Implemented | |
 | `--info=FLAGS` | Fine-grained info verbosity | ❌ Not Implemented | Removed because it had no effect |
-| `--debug=FLAGS` | Fine-grained debug verbosity | ❌ Not Implemented | Removed because it had no effect |
+| `--debug=FLAGS` | Fine-grained debug verbosity | ⚠️ Partial | `io`, `proto`, `pack`, and `util` are supported; other rsync categories are rejected |
 | `--stderr=MODE` | Change stderr output mode | ⚠️ Partial | `errors` (default) and `all` are supported; `client` is rejected because FastSync has no rsync message channel |
 | `--no-motd` | Suppress daemon MOTD | ❌ Not Implemented | |
 | `--exclude=PATTERN` | Exclude files matching pattern | ✅ Implemented | Glob matching in scanner |
diff --git a/src/client/client_cli.c b/src/client/client_cli.c
index a4a2a57..0df49a8 100644
--- a/src/client/client_cli.c
+++ b/src/client/client_cli.c
@@ -104,6 +104,55 @@ static int set_stderr_mode(const char* value) {
 
 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count);
 
+static int parse_debug_flags(const char* value, Config* config) {
+  if (!value || value[0] == '\0' || value[0] == ',' || value[strlen(value) - 1] == ',' ||
+      strstr(value, ",,")) {
+    log_message(LOG_LEVEL_ERROR, "--debug requires at least one flag");
+    return -1;
+  }
+
+  char* flags = str_dup(value);
+  if (!flags) {
+    log_message(LOG_LEVEL_ERROR, "memory allocation failed for --debug");
+    return -1;
+  }
+  uint32_t parsed = (uint32_t)config->debug_level;
+  char* saveptr = NULL;
+  for (char* token = strtok_r(flags, ",", &saveptr); token != NULL;
+       token = strtok_r(NULL, ",", &saveptr)) {
+    uint32_t flag = 0;
+    if (strcmp(token, "help") == 0) {
+      print_debug_usage();
+      free(flags);
+      return 1;
+    } else if (strcmp(token, "all") == 0) {
+      parsed = LOG_DEBUG_ALL;
+      continue;
+    } else if (strcmp(token, "none") == 0) {
+      parsed = 0;
+      continue;
+    } else if (strcmp(token, "io") == 0) {
+      flag = LOG_DEBUG_IO;
+    } else if (strcmp(token, "proto") == 0) {
+      flag = LOG_DEBUG_PROTO;
+    } else if (strcmp(token, "pack") == 0) {
+      flag = LOG_DEBUG_PACK;
+    } else if (strcmp(token, "util") == 0) {
+      flag = LOG_DEBUG_UTIL;
+    } else {
+      log_message(LOG_LEVEL_ERROR, "unsupported --debug flag: %s", token);
+      free(flags);
+      return -1;
+    }
+    parsed |= flag;
+  }
+  free(flags);
+  config->debug_level = (int)parsed;
+  set_log_debug_flags(parsed);
+  set_log_level(LOG_LEVEL_DEBUG);
+  return 0;
+}
+
 /* Parse a string as an unsigned long long. Returns 0 on success, -1 on error. */
 static int parse_ull_arg(const char* val, unsigned long long* out, const char* optname) {
   char* end;
@@ -389,6 +438,16 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
       verbose = true;
     } else if (opt_is(argv[i], "-q", "--quiet")) {
       config->quiet = true;
+    } else if (strncmp(argv[i], "--debug=", 8) == 0) {
+      int debug_ret = parse_debug_flags(argv[i] + 8, config);
+      if (debug_ret != 0)
+        return debug_ret;
+    } else if (opt_is(argv[i], "--debug", NULL)) {
+      if (i + 1 >= argc)
+        return parse_debug_flags(NULL, config);
+      int debug_ret = parse_debug_flags(argv[++i], config);
+      if (debug_ret != 0)
+        return debug_ret;
     } else if (opt_is(argv[i], "-T", NULL) && i + 1 < argc) {
       if (set_positive_int_option(&config->timeout, argv[++i], "-T") != 0)
         return -1;
diff --git a/src/client/usage.c b/src/client/usage.c
index a4c9034..9201ef5 100644
--- a/src/client/usage.c
+++ b/src/client/usage.c
@@ -40,6 +40,7 @@ void print_usage(void) {
   printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  -q, --quiet         Suppress non-error output\n");
+  printf("  --debug=FLAGS       Fine-grained debug logging (use --debug=help for flags)\n");
   printf("  -M, --preserve      Preserve file metadata\n");
   printf("  --chunk-size <n>    Chunk size in bytes (default: %d)\n", DEFAULT_CHUNK_SIZE);
   printf("  --source-dir <path> Source directory\n");
@@ -78,3 +79,9 @@ void print_usage(void) {
   printf("  --help              Show this help\n");
   printf("  -V, --version       Show version\n");
 }
+
+void print_debug_usage(void) {
+  printf("Supported debug flags: IO,PROTO,PACK,UTIL,ALL,NONE\n");
+  printf("Flags may be comma-separated, for example: --debug=io,proto\n");
+  printf("Other rsync debug flags are unsupported and rejected.\n");
+}
diff --git a/src/client/usage.h b/src/client/usage.h
index ef480c5..ca8d65b 100644
--- a/src/client/usage.h
+++ b/src/client/usage.h
@@ -2,5 +2,6 @@
 #define USAGE_H
 
 void print_usage(void);
+void print_debug_usage(void);
 
 #endif
diff --git a/src/server/server.c b/src/server/server.c
index 8aa02e9..f8a2fcc 100644
--- a/src/server/server.c
+++ b/src/server/server.c
@@ -381,6 +381,7 @@ int main(int argc, char* argv[]) {
       stdio_mode = true;
     } else if (strcmp(argv[i], "-v") == 0 || strcmp(argv[i], "--verbose") == 0) {
       set_log_level(LOG_LEVEL_DEBUG);
+      set_log_debug_flags(LOG_DEBUG_ALL);
     } else if (strcmp(argv[i], "--tls") == 0) {
       use_tls = true;
     } else if (strcmp(argv[i], "--cert") == 0 && i + 1 < argc) {
diff --git a/src/shared/chunk.c b/src/shared/chunk.c
index 3af1c63..fb6fd72 100644
--- a/src/shared/chunk.c
+++ b/src/shared/chunk.c
@@ -290,7 +290,7 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
 }
 
 Data* chunk_compress(Chunk* chunk, int compression_level, bool use_metadata) {
-  log_message(LOG_LEVEL_DEBUG, "Starting to compress chunk");
+  log_debug_message(LOG_DEBUG_PACK, "Starting to compress chunk");
   Data* serialized = chunk_serialize(chunk, use_metadata);
   if (serialized == NULL)
     return NULL;
@@ -298,7 +298,7 @@ Data* chunk_compress(Chunk* chunk, int compression_level, bool use_metadata) {
   data_destroy(serialized);
   if (compressed == NULL)
     return NULL;
-  log_message(LOG_LEVEL_DEBUG, "Chunk successfully compressed");
+  log_debug_message(LOG_DEBUG_PACK, "Chunk successfully compressed");
   return compressed;
 }
 
diff --git a/src/shared/compression.c b/src/shared/compression.c
index 1a42ba5..30033e3 100644
--- a/src/shared/compression.c
+++ b/src/shared/compression.c
@@ -28,7 +28,7 @@ bool compression_should_skip(const char* path) {
 }
 
 Data* data_compress(Data* data_to_compress, int compression_level) {
-  log_message(LOG_LEVEL_DEBUG, "Starting to compress data");
+  log_debug_message(LOG_DEBUG_UTIL, "Starting to compress data");
   size_t dst_size = ZSTD_compressBound(data_to_compress->size);
   Data* compressed_data = data_create_empty(dst_size);
   if (compressed_data == NULL)
@@ -66,8 +66,8 @@ Data* data_compress(Data* data_to_compress, int compression_level) {
   compressed_data->size = output.pos;
   ZSTD_freeCCtx(cctx);
 
-  log_message(LOG_LEVEL_DEBUG, "Data succesfully compressed from %zu to %zu",
-              data_to_compress->size, compressed_data->size);
+  log_debug_message(LOG_DEBUG_UTIL, "Data succesfully compressed from %zu to %zu",
+                    data_to_compress->size, compressed_data->size);
   return compressed_data;
 }
 
@@ -75,7 +75,7 @@ Data* data_decompress_limited(Data* compressed_data, size_t maximum_size) {
   if (!compressed_data || (!compressed_data->data && compressed_data->size != 0) ||
       maximum_size == 0)
     return NULL;
-  log_message(LOG_LEVEL_DEBUG, "Start to decompress data");
+  log_debug_message(LOG_DEBUG_UTIL, "Start to decompress data");
   unsigned long long dst_size =
       ZSTD_getFrameContentSize(compressed_data->data, compressed_data->size);
   if (ZSTD_isError(dst_size)) {
@@ -155,7 +155,7 @@ Data* data_decompress_limited(Data* compressed_data, size_t maximum_size) {
   uncompressed_data->size = output.pos;
   ZSTD_freeDCtx(dctx);
 
-  log_message(LOG_LEVEL_DEBUG, "Decompressed data successfully");
+  log_debug_message(LOG_DEBUG_UTIL, "Decompressed data successfully");
   return uncompressed_data;
 }
 
diff --git a/src/shared/log.c b/src/shared/log.c
index 83db013..0f4dec8 100644
--- a/src/shared/log.c
+++ b/src/shared/log.c
@@ -7,6 +7,7 @@
 
 static const char* log_level_strings[] = {"DEBUG", "INFO", "WARN", "ERROR"};
 static LogLevel current_log_level = LOG_LEVEL_WARNING;
+static uint32_t current_debug_flags = 0;
 static FILE* log_fp = NULL;
 static _Thread_local bool eight_bit_output;
 static LogStderrMode stderr_mode = LOG_STDERR_ERRORS;
@@ -15,6 +16,14 @@ void set_log_level(LogLevel level) {
   current_log_level = level;
 }
 
+void set_log_debug_flags(uint32_t flags) {
+  current_debug_flags = flags;
+}
+
+uint32_t get_log_debug_flags(void) {
+  return current_debug_flags;
+}
+
 void log_set_file(FILE* fp) {
   log_fp = fp;
 }
@@ -71,6 +80,27 @@ void log_message(LogLevel log_level, const char* format, ...) {
   }
 }
 
+void log_debug_message(LogDebugFlag flag, const char* format, ...) {
+  if (current_log_level > LOG_LEVEL_DEBUG || !(current_debug_flags & flag))
+    return;
+
+  time_t now = time(NULL);
+  struct tm t;
+  if (!localtime_r(&now, &t))
+    return;
+
+  va_list args;
+  va_start(args, format);
+  write_message(stdout, LOG_LEVEL_DEBUG, t, format, args);
+  va_end(args);
+
+  if (log_fp) {
+    va_start(args, format);
+    write_message(log_fp, LOG_LEVEL_DEBUG, t, format, args);
+    va_end(args);
+  }
+}
+
 void log_perror(const char* context) {
   log_message(LOG_LEVEL_ERROR, "%s: %s", context, strerror(errno));
 }
diff --git a/src/shared/log.h b/src/shared/log.h
index 0aa2943..849199f 100644
--- a/src/shared/log.h
+++ b/src/shared/log.h
@@ -3,13 +3,25 @@
 
 #include <stdio.h>
 #include <stdbool.h>
+#include <stdint.h>
 
 typedef enum { LOG_LEVEL_DEBUG, LOG_LEVEL_INFO, LOG_LEVEL_WARNING, LOG_LEVEL_ERROR } LogLevel;
 typedef enum { LOG_STDERR_ERRORS, LOG_STDERR_ALL } LogStderrMode;
 
+typedef enum {
+  LOG_DEBUG_IO = 1u << 0,
+  LOG_DEBUG_PROTO = 1u << 1,
+  LOG_DEBUG_PACK = 1u << 2,
+  LOG_DEBUG_UTIL = 1u << 3,
+  LOG_DEBUG_ALL = (1u << 4) - 1,
+} LogDebugFlag;
+
 void log_message(LogLevel log_level, const char* message, ...);
 void log_perror(const char* context);
 void set_log_level(LogLevel level);
+void set_log_debug_flags(uint32_t flags);
+uint32_t get_log_debug_flags(void);
+void log_debug_message(LogDebugFlag flag, const char* message, ...);
 void log_set_file(FILE* fp);
 void log_set_8_bit_output(bool enabled);
 bool log_get_8_bit_output(void);
diff --git a/src/shared/protocol.c b/src/shared/protocol.c
index ae37a95..aa79fa4 100644
--- a/src/shared/protocol.c
+++ b/src/shared/protocol.c
@@ -200,7 +200,7 @@ static int deadline_remaining_ms(const struct timespec* deadline) {
 bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t data_size) {
   if (!data && data_size != 0)
     return false;
-  log_message(LOG_LEVEL_DEBUG, "    Sending n Data: %zu", data_size);
+  log_debug_message(LOG_DEBUG_IO, "    Sending n Data: %zu", data_size);
   if (!session)
     return false;
   int fd = session->write_fd;
@@ -244,12 +244,12 @@ bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t dat
     if (session->ssl)
       wait_events = POLLOUT;
   }
-  log_message(LOG_LEVEL_DEBUG, "    Send n Data: %zu", total_bytes_send);
+  log_debug_message(LOG_DEBUG_IO, "    Send n Data: %zu", total_bytes_send);
   return true;
 }
 
 bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_size) {
-  log_message(LOG_LEVEL_DEBUG, "    Receiving n Data: %zu", data_size);
+  log_debug_message(LOG_DEBUG_IO, "    Receiving n Data: %zu", data_size);
   if (!session)
     return false;
   int fd = session->read_fd;
@@ -303,7 +303,7 @@ bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_s
     if (session->ssl)
       wait_events = POLLIN;
   }
-  log_message(LOG_LEVEL_DEBUG, "    Received n Data: %zu", total_bytes_received);
+  log_debug_message(LOG_DEBUG_IO, "    Received n Data: %zu", total_bytes_received);
   return true;
 }
 
@@ -344,9 +344,7 @@ bool protocol_send_str(ProtocolSession* session, const char* data) {
     return false;
   if (!protocol_send_n_data(session, data, size))
     return false;
-  char* escaped = output_escape(data, session->eight_bit_output);
-  log_message(LOG_LEVEL_DEBUG, "Send String: %s", escaped ? escaped : "<allocation failed>");
-  free(escaped);
+  log_debug_message(LOG_DEBUG_PROTO, "Send String: %s", data);
   return true;
 }
 
@@ -374,9 +372,7 @@ char* protocol_receive_str(ProtocolSession* session) {
   }
   data[size] = '\0';
   session->total_allocated_bytes += size + 1;
-  char* escaped = output_escape(data, session->eight_bit_output);
-  log_message(LOG_LEVEL_DEBUG, "Received String: %s", escaped ? escaped : "<allocation failed>");
-  free(escaped);
+  log_debug_message(LOG_DEBUG_PROTO, "Received String: %s", data);
   return data;
 }
 
@@ -390,7 +386,7 @@ bool protocol_send_data(ProtocolSession* session, const Data* data) {
     return false;
   if (!protocol_send_n_data(session, data->data, data_size))
     return false;
-  log_message(LOG_LEVEL_DEBUG, "Send %lld data", data_size);
+  log_debug_message(LOG_DEBUG_PROTO, "Send %lld data", data_size);
   return true;
 }
 
@@ -420,7 +416,7 @@ Data* protocol_receive_data_limited(ProtocolSession* session, unsigned long long
     return NULL;
   }
   session->total_allocated_bytes += allocation_size;
-  log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
+  log_debug_message(LOG_DEBUG_PROTO, "Received %lld data", size);
   Data* result = data_create(data, (size_t)size);
   if (!result) {
     session->total_allocated_bytes -= allocation_size;
@@ -437,28 +433,28 @@ Data* protocol_receive_data(ProtocolSession* session) {
 bool protocol_send_int(ProtocolSession* session, int data) {
   if (!protocol_send_n_data(session, &data, sizeof(int)))
     return false;
-  log_message(LOG_LEVEL_DEBUG, "Send Int: %d", data);
+  log_debug_message(LOG_DEBUG_PROTO, "Send Int: %d", data);
   return true;
 }
 
 bool protocol_receive_int(ProtocolSession* session, int* data) {
   if (!protocol_receive_n_data(session, data, sizeof(int)))
     return false;
-  log_message(LOG_LEVEL_DEBUG, "Received Int: %d", *data);
+  log_debug_message(LOG_DEBUG_PROTO, "Received Int: %d", *data);
   return true;
 }
 
 bool protocol_send_status(ProtocolSession* session, Status status) {
   if (!protocol_send_n_data(session, &status, sizeof(Status)))
     return false;
-  log_message(LOG_LEVEL_DEBUG, "Send Status: %s", status_to_string(status));
+  log_debug_message(LOG_DEBUG_PROTO, "Send Status: %s", status_to_string(status));
   return true;
 }
 
 bool protocol_receive_status(ProtocolSession* session, Status* status) {
   if (!protocol_receive_n_data(session, status, sizeof(Status)))
     return false;
-  log_message(LOG_LEVEL_DEBUG, "Received Status: %s", status_to_string(*status));
+  log_debug_message(LOG_DEBUG_PROTO, "Received Status: %s", status_to_string(*status));
   return true;
 }
 
diff --git a/tests/test_client_cli.c b/tests/test_client_cli.c
index a433e60..d490239 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -269,6 +269,43 @@ static void test_parse_args_valid_compression_level() {
   config_delete(cfg);
 }
 
+static void test_parse_args_debug_flags() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--debug=io,proto,pack,util", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->debug_level, LOG_DEBUG_ALL);
+  EXPECT_EQ_INT(get_log_debug_flags(), LOG_DEBUG_ALL);
+  config_delete(cfg);
+}
+
+static void test_parse_args_debug_help() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--debug=help"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 2, argv, positional_args, &positional_count), 1);
+  config_delete(cfg);
+}
+
+static void test_parse_args_debug_flags_validation() {
+  static const char* const values[] = {"", "io,", ",io", "io,,proto", "acl", "tls", "unknown"};
+  for (size_t i = 0; i < sizeof(values) / sizeof(values[0]); i++) {
+    Config* cfg = config_create();
+    char option[64];
+    snprintf(option, sizeof(option), "--debug=%s", values[i]);
+    char* argv[] = {"fastsync", option, "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+
+    EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), -1);
+    config_delete(cfg);
+  }
+}
+
 /* Test parse_args unknown option returns error */
 static void test_parse_args_unknown_option() {
   Config* cfg = config_create();
@@ -298,7 +335,6 @@ static void test_parse_args_rejects_unimplemented_options() {
                                         "--itemize-changes",
                                         "--out-format",
                                         "--info",
-                                        "--debug",
                                         "--list-only",
                                         "-u",
                                         "--update",
@@ -469,6 +505,9 @@ void test_client_cli() {
   test_parse_args_invalid_server_port();
   test_parse_args_invalid_compression_level();
   test_parse_args_valid_compression_level();
+  test_parse_args_debug_flags();
+  test_parse_args_debug_help();
+  test_parse_args_debug_flags_validation();
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_quiet();