diff --git a/docs/web-ui/agent.md b/docs/web-ui/agent.md
index cafc87e9..a6e3d07d 100644
--- a/docs/web-ui/agent.md
+++ b/docs/web-ui/agent.md
@@ -162,6 +162,9 @@ select-all / select-none / `✓ mark done` bulk row above the list POSTs
 the checked ids to `POST /api/todos/mark-done`, which dismisses them
 from the harness-local store (same effect as `cancel_loose_end(kind:
 "todo")`, just from the web UI instead of the agent's own tool calls).
+If the todo store errors or doesn't answer for any id, the endpoint returns
+500 naming how many ids failed, and the panel shows `failed: …` instead of
+`✓ marked done`.
 
 ## Live view
 
diff --git a/hive-agent/src/web_ui/actions.rs b/hive-agent/src/web_ui/actions.rs
index ff512163..42932535 100644
--- a/hive-agent/src/web_ui/actions.rs
+++ b/hive-agent/src/web_ui/actions.rs
@@ -8,6 +8,7 @@ use axum::{
     response::{IntoResponse, Response},
 };
 use serde::{Deserialize, Serialize};
+use std::future::Future;
 
 use super::{AppState, SigintOutcome, error_response};
 
@@ -143,7 +144,9 @@ pub(super) struct MarkTodosDoneForm {
 /// MCP tool — unchanged. Unknown/already-acked ids just don't add to the
 /// `acked` count (same "acking twice is not a new action" semantics as the
 /// single-id path); a request with no ids or where every id fails to parse
-/// is rejected as a client error rather than silently acking nothing.
+/// is rejected as a client error rather than silently acking nothing. An id
+/// whose store call errors or gets no answer fails the request with a 500
+/// naming how many failed.
 pub(super) async fn post_mark_todos_done(Form(form): Form<MarkTodosDoneForm>) -> Response {
     let ids: Vec<i64> = form
         .ids
@@ -153,18 +156,144 @@ pub(super) async fn post_mark_todos_done(Form(form): Form<MarkTodosDoneForm>) ->
     if ids.is_empty() {
         return error_response(StatusCode::BAD_REQUEST, "mark-done: no todo ids selected");
     }
-    let mut acked = 0u64;
+    mark_ids_done(ids, |id| async move {
+        crate::todo_server::dial(&hive_agent_sock::Request::MarkTodoDone { id }).await
+    })
+    .await
+}
+
+/// The handler body behind `dial`, which answers one id's `MarkTodoDone`
+/// (`None` when the store doesn't answer). Tests inject it so they never
+/// reach the live in-agent socket.
+async fn mark_ids_done<F, Fut>(ids: Vec<i64>, dial: F) -> Response
+where
+    F: Fn(i64) -> Fut,
+    Fut: Future<Output = Option<hive_agent_sock::Response>>,
+{
+    let mut answers = Vec::with_capacity(ids.len());
     for id in ids {
-        if let Some(hive_agent_sock::Response::Acked { count }) =
-            crate::todo_server::dial(&hive_agent_sock::Request::MarkTodoDone { id }).await
-        {
-            acked += count;
+        answers.push(dial(id).await);
+    }
+    match settle_mark_done(answers) {
+        Ok(acked) => axum::Json(MarkTodosDoneBody { acked }).into_response(),
+        Err(message) => error_response(StatusCode::INTERNAL_SERVER_ERROR, &message),
+    }
+}
+
+/// Folds the per-id answers into the acked total, or the operator-facing
+/// failure. `None` is a dial that got no answer; `Response::Err` and any
+/// other variant are failures. `Acked { count: 0 }` is a success.
+fn settle_mark_done(answers: Vec<Option<hive_agent_sock::Response>>) -> Result<u64, String> {
+    let total = answers.len();
+    let mut acked = 0u64;
+    let mut failed = 0usize;
+    let mut first_error = None;
+    for answer in answers {
+        match answer {
+            Some(hive_agent_sock::Response::Acked { count }) => acked += count,
+            Some(hive_agent_sock::Response::Err { message }) => {
+                failed += 1;
+                first_error.get_or_insert(message);
+            }
+            Some(other) => {
+                failed += 1;
+                first_error.get_or_insert(format!("unexpected response: {other:?}"));
+            }
+            None => {
+                failed += 1;
+                first_error.get_or_insert_with(|| "the todo store did not answer".to_owned());
+            }
         }
     }
-    axum::Json(MarkTodosDoneBody { acked }).into_response()
+    match first_error {
+        None => Ok(acked),
+        Some(error) => Err(format!(
+            "mark-done: {failed} of {total} failed ({acked} marked done): {error}"
+        )),
+    }
 }
 
 #[derive(Serialize)]
 struct MarkTodosDoneBody {
     acked: u64,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{mark_ids_done, settle_mark_done};
+
+    async fn body_text(response: axum::response::Response) -> String {
+        let bytes = axum::body::to_bytes(response.into_body(), usize::MAX)
+            .await
+            .unwrap();
+        String::from_utf8(bytes.to_vec()).unwrap()
+    }
+
+    #[tokio::test]
+    async fn mark_done_handler_answers_500_with_the_message_when_the_store_does_not_answer() {
+        let response = mark_ids_done(vec![1, 2], |_| async { None }).await;
+        assert_eq!(
+            response.status(),
+            axum::http::StatusCode::INTERNAL_SERVER_ERROR
+        );
+        let body = body_text(response).await;
+        assert!(body.contains("2 of 2 failed"), "{body}");
+        assert!(body.contains("did not answer"), "{body}");
+    }
+
+    #[tokio::test]
+    async fn mark_done_handler_answers_500_with_the_store_error() {
+        let response = mark_ids_done(vec![7], |_| async {
+            Some(hive_agent_sock::Response::Err {
+                message: "database is locked".to_owned(),
+            })
+        })
+        .await;
+        assert_eq!(
+            response.status(),
+            axum::http::StatusCode::INTERNAL_SERVER_ERROR
+        );
+        assert!(body_text(response).await.contains("database is locked"));
+    }
+
+    #[tokio::test]
+    async fn mark_done_handler_answers_200_with_the_acked_total() {
+        let response = mark_ids_done(vec![1, 2], |_| async {
+            Some(hive_agent_sock::Response::Acked { count: 1 })
+        })
+        .await;
+        assert_eq!(response.status(), axum::http::StatusCode::OK);
+        assert_eq!(body_text(response).await, r#"{"acked":2}"#);
+    }
+
+    #[test]
+    fn mark_done_with_no_answer_is_an_error() {
+        let err = settle_mark_done(vec![None]).unwrap_err();
+        assert!(err.contains("1 of 1 failed"), "{err}");
+    }
+
+    #[test]
+    fn mark_done_surfaces_the_store_error() {
+        let answers = vec![Some(hive_agent_sock::Response::Err {
+            message: "database is locked".to_owned(),
+        })];
+        let err = settle_mark_done(answers).unwrap_err();
+        assert!(err.contains("database is locked"), "{err}");
+    }
+
+    #[test]
+    fn mark_done_partial_failure_reports_both_counts() {
+        let answers = vec![Some(hive_agent_sock::Response::Acked { count: 1 }), None];
+        let err = settle_mark_done(answers).unwrap_err();
+        assert!(err.contains("1 of 2 failed (1 marked done)"), "{err}");
+    }
+
+    #[test]
+    fn mark_done_acking_nothing_new_is_still_success() {
+        let answers = vec![
+            Some(hive_agent_sock::Response::Acked { count: 0 }),
+            Some(hive_agent_sock::Response::Acked { count: 2 }),
+        ];
+        assert_eq!(settle_mark_done(answers).unwrap(), 2);
+    }
+}