diff --git a/crates/engine/src/cmd/edit.rs b/crates/engine/src/cmd/edit.rs
index bb54fe0..a812c65 100644
--- a/crates/engine/src/cmd/edit.rs
+++ b/crates/engine/src/cmd/edit.rs
@@ -196,8 +196,11 @@ fn cell_set(s: &mut Session, p: &Json) -> Result<Json> {
         Some(c) => c,
         None => s.doc()?.selection.active,
     };
-    let input =
-        str_param(p, "input").or_else(|| str_param(p, "value")).map(str::to_string).or_else(|| p.get("value").map(json_to_input)).unwrap_or_default();
+    let input = str_param(p, "input")
+        .or_else(|| str_param(p, "value"))
+        .map(str::to_string)
+        .or_else(|| p.get("value").map(json_to_input))
+        .ok_or_else(|| bad("cell.set", "missing `input` (or `value`)"))?;
     let array = bool_param(p, "array").unwrap_or(false);
     let protected =
         s.doc()?.wb.sheet(sheet).is_some_and(|sh| sh.is_protected() && s.doc().is_ok_and(|d| d.wb.styles.get(sh.style_id(at)).protection.locked));
diff --git a/crates/mcp/src/tests.rs b/crates/mcp/src/tests.rs
index cf590be..1d4abb4 100644
--- a/crates/mcp/src/tests.rs
+++ b/crates/mcp/src/tests.rs
@@ -339,6 +339,18 @@ fn serve_loop_over_buffers() {
     assert!(lines[2]["result"]["content"][0]["text"].as_str().unwrap().contains("42"));
 }
 
+#[test]
+fn cell_set_without_input_is_error() {
+    let mut s = server();
+    ok(&mut s, "set_cell", json!({"cell": "B2", "input": "keep"}));
+    fails(&mut s, "execute_command", json!({"command": "cell.set", "params": {"cell": "B2"}}));
+    fails(&mut s, "execute_command", json!({"command": "cell.set", "params": {}}));
+    assert_eq!(ok(&mut s, "get_cell", json!({"cell": "B2"}))["text"], "keep");
+    // An explicit empty string still clears.
+    ok(&mut s, "execute_command", json!({"command": "cell.set", "params": {"cell": "B2", "input": ""}}));
+    assert_eq!(ok(&mut s, "get_cell", json!({"cell": "B2"}))["text"], "");
+}
+
 #[test]
 fn remote_backend_over_tcp() {
     // A fake app: answers engine.execute by running a headless session; closes after the first