diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
index 5871cb1..26bc260 100644
--- a/internal/cli/cli_test.go
+++ b/internal/cli/cli_test.go
@@ -921,6 +921,7 @@ func TestUsageErrors(t *testing.T) {
 		{"malformed agent-card url", []string{"card", "get", "-a", "http://exa mple.com"}},
 		{"missing agent-card file", []string{"card", "get", "-a", "/no/such/card.json"}},
 		{"missing --file-part file", []string{"send", "-a", "http://x", "--file-part", "/no/such/file.txt"}},
+		{"invalid --data-part json", []string{"send", "-a", "http://x", "--data-part", "{bad"}},
 	}
 
 	for _, tt := range tests {
diff --git a/internal/flagparse/parts.go b/internal/flagparse/parts.go
index 229e109..7ab3f26 100644
--- a/internal/flagparse/parts.go
+++ b/internal/flagparse/parts.go
@@ -137,7 +137,7 @@ func buildFilePart(ref, mediaType string) (*a2a.Part, error) {
 func buildDataPart(ref, mediaType string) (*a2a.Part, error) {
 	var data any
 	if err := json.Unmarshal(RawOrInline(ref), &data); err != nil {
-		return nil, fmt.Errorf("--data-part %q is not a readable file or valid JSON: %w", ref, err)
+		return nil, clierr.Usage(fmt.Sprintf("--data-part %q is not a readable file or valid JSON: %v", ref, err))
 	}
 	p := a2a.NewDataPart(data)
 	if mediaType != "" {