diff --git a/apps/lightcraft-cli/src/main.rs b/apps/lightcraft-cli/src/main.rs
--- a/apps/lightcraft-cli/src/main.rs
+++ b/apps/lightcraft-cli/src/main.rs
@@ -565,7 +565,11 @@ fn run(args: &[String]) -> Result<(), String> {
                 .or(v.get("method"))
                 .and_then(Value::as_str)
                 .ok_or_else(|| format!("{path}:{}: needs `command` or `method`", n + 1))?;
-            let params = v.get("params").cloned().filter(|p| p.is_object()).unwrap_or_else(|| json!({}));
+            let params = match v.get("params") {
+                None | Some(Value::Null) => json!({}),
+                Some(Value::Object(params)) => Value::Object(params.clone()),
+                Some(_) => return Err(format!("{path}:{}: `params` must be a JSON object", n + 1)),
+            };
             steps.push(Step { id: id.to_string(), params });
         }
     }
diff --git a/apps/lightcraft-cli/tests/it/cli.rs b/apps/lightcraft-cli/tests/it/cli.rs
--- a/apps/lightcraft-cli/tests/it/cli.rs
+++ b/apps/lightcraft-cli/tests/it/cli.rs
@@ -513,6 +513,32 @@ fn run_cli(args: &[&str], stdin: Option<&str>) -> (bool, Vec<Value>, String) {
     (o.status.success(), lines, String::from_utf8_lossy(&o.stderr).to_string())
 }
 
+#[test]
+fn run_script_params_validate_before_any_commands() {
+    let path = tmp("run-invalid-params.jsonl");
+    let path_str = path.to_str().unwrap();
+    for params in [json!([]), json!("invalid"), json!(42), json!(true)] {
+        let script = format!("{}\n{}\n", json!({"command": "library.info"}), json!({"command": "develop.reset", "params": params}));
+        std::fs::write(&path, &script).unwrap();
+        for (source, stdin) in [("-", Some(script.as_str())), (path_str, None)] {
+            let (ok, lines, err) = run_cli(&["--script", source], stdin);
+            assert!(!ok, "{source}: {params}: {lines:?}");
+            assert!(lines.is_empty(), "{source}: {params}: {lines:?}");
+            assert!(err.contains(&format!("{source}:2")), "{err}");
+            assert!(err.contains("`params` must be a JSON object"), "{err}");
+        }
+    }
+    for script in
+        ["{\"command\":\"library.info\"}\n", "{\"command\":\"library.info\",\"params\":null}\n", "{\"command\":\"library.info\",\"params\":{}}\n"]
+    {
+        let (ok, lines, err) = run_cli(&["--script", "-"], Some(script));
+        assert!(ok, "{script}: {err}");
+        assert_eq!(lines.len(), 1, "{script}: {lines:?}");
+        assert_eq!(lines[0]["ok"], true, "{script}: {lines:?}");
+    }
+    std::fs::remove_file(path).unwrap();
+}
+
 #[test]
 fn run_subcommand_chains_commands_and_persists_a_library() {
     let input = tmp("run-in.png");