diff --git a/moli/src/cli.rs b/moli/src/cli.rs
index 56f64415d7..03546187ad 100644
--- a/moli/src/cli.rs
+++ b/moli/src/cli.rs
@@ -470,10 +470,10 @@ pub struct CommonArgs {
     #[arg(long, num_args = 0..=1, default_missing_value = "")]
     pub log_filter_scopes: Option<String>,
 
-    #[arg(short = 'A', long)]
+    #[arg(short = 'A', long, conflicts_with = "user_agent_suffix")]
     pub user_agent: Option<String>,
 
-    #[arg(long)]
+    #[arg(long, conflicts_with = "user_agent")]
     pub user_agent_suffix: Option<String>,
 
     /// Unencrypted PKCS#8 Ed25519 private key used for Web Bot Auth signatures.
diff --git a/moli/tests/cli.rs b/moli/tests/cli.rs
index 8c75c0af95..07b39c8c83 100644
--- a/moli/tests/cli.rs
+++ b/moli/tests/cli.rs
@@ -674,8 +674,6 @@ fn app_config_uses_moli_user_agent_defaults() {
         "fetch",
         "--user-agent",
         "ExampleBrowser/1.0",
-        "--user-agent-suffix",
-        "ignored",
         "https://example.com",
     ]))
     .unwrap();
@@ -683,6 +681,21 @@ fn app_config_uses_moli_user_agent_defaults() {
     assert_eq!(config.browser.fetch().user_agent(), "ExampleBrowser/1.0");
 }
 
+#[test]
+fn user_agent_and_suffix_conflict() {
+    let error = Cli::try_parse_from(normalize_args_for_compat([
+        "moli",
+        "fetch",
+        "-A",
+        "ExampleBrowser/1.0",
+        "--user-agent-suffix",
+        "internal-tester",
+        "https://example.com",
+    ]))
+    .unwrap_err();
+    assert_eq!(error.kind(), clap::error::ErrorKind::ArgumentConflict);
+}
+
 #[test]
 fn app_config_makes_fetch_quiet_unless_log_level_is_set() {
     let quiet_cli = Cli::try_parse_from(normalize_args_for_compat([