diff --git a/main.go b/main.go
index da9ab7cedfaea036da8c486e0ee88879f51eef29..00e67aa8ec3773fe5ece3008153bfe69a8c891bb 100644
--- a/main.go
+++ b/main.go
@@ -67,11 +67,12 @@ formatted-commit -t refactor -s "web/git-bug" -m "fancy shmancy" \
 		}
 
 		if len(trailers) > 0 {
-			commitMsg.WriteString("\n\n")
-			for _, trailer := range trailers {
-				commitMsg.WriteString(trailer)
-				commitMsg.WriteString("\n")
+			trailersBlock, err := buildTrailersBlock(trailers)
+			if err != nil {
+				return fmt.Errorf("failed to build trailers: %w", err)
 			}
+			commitMsg.WriteString("\n\n")
+			commitMsg.WriteString(trailersBlock)
 		}
 
 		fmt.Print(commitMsg.String())
@@ -82,7 +83,7 @@ formatted-commit -t refactor -s "web/git-bug" -m "fancy shmancy" \
 func init() {
 	rootCmd.Flags().StringVarP(&commitType, "type", "t", "", "commit type (required)")
 	rootCmd.Flags().StringVarP(&message, "message", "m", "", "commit message (required)")
-	rootCmd.Flags().StringSliceVarP(&trailers, "trailer", "T", []string{}, "trailer in 'Sentence-case-key: value' format (optional, repeatable)")
+	rootCmd.Flags().StringArrayVarP(&trailers, "trailer", "T", []string{}, "trailer in 'Sentence-case-key: value' format (optional, repeatable)")
 	rootCmd.Flags().StringVarP(&body, "body", "b", "", "commit body (optional)")
 	rootCmd.Flags().StringVarP(&scope, "scope", "s", "", "commit scope (optional)")
 	rootCmd.Flags().BoolVarP(&breakingChange, "breaking", "B", false, "mark as breaking change (optional)")
diff --git a/trailers.go b/trailers.go
new file mode 100644
index 0000000000000000000000000000000000000000..82b6b7299866e223d3a43fa3dca4334dbd2dabde
--- /dev/null
+++ b/trailers.go
@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: Amolith <amolith@secluded.site>
+//
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package main
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+func validateTrailer(trailer string) error {
+	if trailer == "" {
+		return fmt.Errorf("trailer cannot be empty")
+	}
+
+	lines := strings.Split(trailer, "\n")
+	firstLine := lines[0]
+
+	if len(firstLine) > 0 && unicode.IsSpace(rune(firstLine[0])) {
+		return fmt.Errorf("trailer key cannot start with whitespace: %q", trailer)
+	}
+
+	colonIdx := strings.Index(firstLine, ":")
+	if colonIdx == -1 {
+		return fmt.Errorf("trailer must contain ':' separator: %q", trailer)
+	}
+
+	key := firstLine[:colonIdx]
+
+	if strings.TrimSpace(key) != key {
+		return fmt.Errorf("trailer key cannot have leading or trailing whitespace: %q", key)
+	}
+
+	if strings.ContainsAny(key, " \t") {
+		return fmt.Errorf("trailer key cannot contain whitespace: %q", key)
+	}
+
+	if key == "" {
+		return fmt.Errorf("trailer key cannot be empty: %q", trailer)
+	}
+
+	for i, line := range lines[1:] {
+		if line == "" {
+			continue
+		}
+		if len(line) > 0 && !unicode.IsSpace(rune(line[0])) {
+			return fmt.Errorf("trailer continuation line %d must start with whitespace: %q", i+2, line)
+		}
+	}
+
+	return nil
+}
+
+func buildTrailersBlock(trailers []string) (string, error) {
+	if len(trailers) == 0 {
+		return "", nil
+	}
+
+	for _, trailer := range trailers {
+		if err := validateTrailer(trailer); err != nil {
+			return "", err
+		}
+	}
+
+	return strings.Join(trailers, "\n"), nil
+}