diff --git a/AGENTS.md b/AGENTS.md
index e4923f2..12c071a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -181,6 +181,7 @@ Config is JSON (HJSON with comments supported). Default path depends on OS:
 | backup_timeout | `60` | Stream timeout in minutes (1-1440) |
 | compression_level | `9` | Gzip level (1-9) |
 | exclude_patterns | `[]` | Regex patterns to exclude from backup |
+| filename_prefix | `backup` | Prefix for backup filename in Content-Disposition header |
 
 ## Architecture & Key Patterns
 
@@ -278,4 +279,3 @@ Init scripts provided in `contrib/`:
 
 See `TODO.org` for current backlog:
 - Consider alternative compression algorithms (bz2, xz, lzo, lz4)
-- Configurable filename prefix for `Content-Disposition` header
diff --git a/README.md b/README.md
index dd46ef7..3746f51 100644
--- a/README.md
+++ b/README.md
@@ -56,6 +56,7 @@ AI suggests to fill this section with useful information, despite that all optio
 | backup_timeout | 60 | Timeout in minutes for backup streaming operations. | If a backup takes longer than this, the connection will be terminated. Useful for large backups. Range: 1-1440 minutes (1 day max). |
 | compression_level | 9 | Gzip compression level for tar.gz archives. | 1 (fastest) to 9 (best compression). Higher values produce smaller files but take longer to compress. |
 | exclude_patterns | [] | Regex patterns to exclude from backup. | E.g., `[".*\\.tmp$", "/node_modules/"]`. Files matching any pattern are skipped. |
+| filename_prefix | "backup" | Prefix for backup filename in Content-Disposition header. | E.g., "mybackup" produces `mybackup-20260325-092341.tar.gz`. |
 
 ## Best practices
 
diff --git a/TODO.org b/TODO.org
index 11966d2..94b6422 100644
--- a/TODO.org
+++ b/TODO.org
@@ -9,6 +9,6 @@
    - [ ] lz4 for speed, xz for better compression
    - [ ] Add compression_algorithm config option
 
-** TODO Configurable filename prefix
-   - [ ] Move "backup" prefix to config (currently hardcoded in newServer.go)
-   - [ ] Default: "backup", e.g., "backup-20260325-092341.tar.gz"
+** DONE Configurable filename prefix
+   - [X] Move "backup" prefix to config (currently hardcoded in newServer.go)
+   - [X] Default: "backup", e.g., "backup-20260325-092341.tar.gz"
diff --git a/data/config_example.json b/data/config_example.json
index 6386e15..c8cdb8d 100644
--- a/data/config_example.json
+++ b/data/config_example.json
@@ -33,5 +33,17 @@
     "directories": [
         "/path/to/directory1",
         "/path/to/directory2"
-    ]
+    ],
+
+    /* Backup streaming timeout in minutes.                                                                           */
+    "backup_timeout": 60,
+
+    /* Gzip compression level (1-9, where 9 is best compression).                                                    */
+    "compression_level": 9,
+
+    /* Regex patterns to exclude from backup.                                                                         */
+    "exclude_patterns": [],
+
+    /* Prefix for backup filename in Content-Disposition header.                                                      */
+    "filename_prefix": "backup"
 }
diff --git a/internal/backer/loadConfig.go b/internal/backer/loadConfig.go
index 9dcebd6..5dbbe5c 100644
--- a/internal/backer/loadConfig.go
+++ b/internal/backer/loadConfig.go
@@ -128,6 +128,13 @@ func LoadConfig(path string) error {
 		return errors.New("Option password is not set")
 	}
 
+	// FilenamePrefix: default.
+	if C.FilenamePrefix == "" {
+		C.FilenamePrefix = "backup"
+
+		log.Warnf("Config option filename_prefix is not set, fallback to %s", C.FilenamePrefix)
+	}
+
 	// Compile exclude patterns.
 	excludePatterns = make([]*regexp.Regexp, 0, len(C.ExcludePatterns))
 
diff --git a/internal/backer/loadConfig_test.go b/internal/backer/loadConfig_test.go
index 39ea0bb..8058435 100644
--- a/internal/backer/loadConfig_test.go
+++ b/internal/backer/loadConfig_test.go
@@ -170,6 +170,27 @@ func TestLoadConfigNoHTTPS(t *testing.T) {
 	}
 }
 
+// TestLoadConfigFilenamePrefixCustom tests custom filename prefix.
+func TestLoadConfigFilenamePrefixCustom(t *testing.T) {
+	config := `{
+		"user": "test",
+		"password": "test",
+		"directories": ["../../test_data/test1/foo"],
+		"nohttps": true,
+		"filename_prefix": "mybackup"
+	}`
+	tmpFile := writeTempConfig(t, config)
+
+	err := LoadConfig(tmpFile)
+	if err != nil {
+		t.Errorf("Expected no error, got %v", err)
+	}
+
+	if C.FilenamePrefix != "mybackup" {
+		t.Errorf("C.FilenamePrefix should be mybackup, got %s", C.FilenamePrefix)
+	}
+}
+
 // TestLoadConfigDefaultValues tests default values are set correctly.
 func TestLoadConfigDefaultValues(t *testing.T) {
 	config := `{
@@ -201,6 +222,10 @@ func TestLoadConfigDefaultValues(t *testing.T) {
 	if C.LogLevel != "info" {
 		t.Errorf("C.LogLevel should default to info, got %s", C.LogLevel)
 	}
+
+	if C.FilenamePrefix != "backup" {
+		t.Errorf("C.FilenamePrefix should default to backup, got %s", C.FilenamePrefix)
+	}
 }
 
 // writeTempConfig is a helper to write a temp config file for testing.
diff --git a/internal/backer/newServer.go b/internal/backer/newServer.go
index efea9fe..92c8fe0 100644
--- a/internal/backer/newServer.go
+++ b/internal/backer/newServer.go
@@ -82,7 +82,7 @@ func NewServer(configPath string) (*http.Server, error) {
 		}()
 
 		timestamp := time.Now().Format("20060102-150405")
-		filename := fmt.Sprintf("backup-%s.tar.gz", timestamp)
+		filename := fmt.Sprintf("%s-%s.tar.gz", C.FilenamePrefix, timestamp)
 
 		w.Header().Set("Content-Disposition", "attachment; filename="+filename)
 		w.Header().Set("Content-Type", "application/octet-stream")
diff --git a/internal/backer/types.go b/internal/backer/types.go
index 0b8cf1d..5f7390d 100644
--- a/internal/backer/types.go
+++ b/internal/backer/types.go
@@ -17,6 +17,7 @@ type (
 		BackupTimeout    int      `json:"backup_timeout"`    // Timeout in minutes for backup streaming.
 		CompressionLevel int      `json:"compression_level"` // Gzip compression level (1-9, default 9).
 		ExcludePatterns  []string `json:"exclude_patterns"`  // Regex patterns to exclude from backup.
+		FilenamePrefix   string   `json:"filename_prefix"`   // Prefix for backup filename (default: "backup").
 	}
 )
 
diff --git a/llms.txt b/llms.txt
index 4c6faba..214b199 100644
--- a/llms.txt
+++ b/llms.txt
@@ -61,6 +61,7 @@ curl -u user:pass https://server:8086/archive -o backup.tar.gz
 | backup_timeout | 60 | Stream timeout in minutes (1-1440) |
 | compression_level | 9 | Gzip level (1-9) |
 | exclude_patterns | [] | Regex patterns to exclude from backup |
+| filename_prefix | backup | Prefix for backup filename in Content-Disposition header |
 
 ## Key Patterns