diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
index 9af68ba..da2e582 100644
--- a/internal/cli/cli_test.go
+++ b/internal/cli/cli_test.go
@@ -981,6 +981,7 @@ func TestUsageErrors(t *testing.T) {
 		{"--endpoint without --transport", []string{"send", "-e", "http://x", "hi"}},
 		{"--endpoint with two --transport", []string{"send", "-e", "http://x", "--transport", "rest", "--transport", "grpc", "hi"}},
 		{"empty --transport", []string{"send", "-a", "http://x", "--transport", "", "hi"}},
+		{"unknown --a2a-version", []string{"send", "-a", "http://x", "--a2a-version", "1", "hi"}},
 		{"unknown server --protocol", []string{"server", "--echo", "--port", "0", "--protocol", "bogus"}},
 		{"unknown server --serve-transport", []string{"server", "--echo", "--port", "0", "--serve-transport", "bogus"}},
 		{"missing server --card file", []string{"server", "--echo", "--port", "0", "--card", "/no/such/card.json"}},
diff --git a/internal/cli/client.go b/internal/cli/client.go
index 801dd64..311f73c 100644
--- a/internal/cli/client.go
+++ b/internal/cli/client.go
@@ -53,6 +53,11 @@ func newAgentClient(ctx context.Context, cfg *globalConfig, extraOpts ...a2aclie
 	if err := cfg.initTLS(); err != nil {
 		return nil, err
 	}
+	switch cfg.a2aVersion {
+	case "", "1.0", "0.3":
+	default:
+		return nil, clierr.Usage(fmt.Sprintf("invalid --a2a-version %q (want 1.0 or 0.3)", cfg.a2aVersion))
+	}
 	switch {
 	case cfg.url != "" && cfg.agentCard.IsSet():
 		return nil, clierr.Usage("--endpoint and --agent-card are mutually exclusive")