diff --git a/openfut-adapter-fifa17/src/fut/owned_query.rs b/openfut-adapter-fifa17/src/fut/owned_query.rs
index 3cc33a1..5f7699e 100644
--- a/openfut-adapter-fifa17/src/fut/owned_query.rs
+++ b/openfut-adapter-fifa17/src/fut/owned_query.rs
@@ -69,6 +69,18 @@ pub struct Fifa17ClubQuery {
     pub start: Option<u32>,
     /// Pagination page size.
     pub count: Option<u32>,
+    /// `defId=` — a comma-joined list of DEFINITION ids. The documented club
+    /// grammar says the client sends EITHER the filter block above OR this list,
+    /// never both.
+    ///
+    /// CAPTURED BUT NOT APPLIED, deliberately. The grammar is single-source (one
+    /// decompile plus one live log line, and that log line carried no `defId`),
+    /// so the exact semantics — definition id as `resourceId`, presumably — are
+    /// not confirmed against an observed request. Filtering on a wrong reading
+    /// would turn "too many items" into "zero items", which is the worse failure.
+    /// The host logs it instead, so the first real occurrence is visible and the
+    /// filter can be written against evidence rather than a guess.
+    pub def_ids: Vec<i64>,
 }
 
 /// Minimal percent/`+` decoding, dependency-free. FIFA sends bare tokens and
@@ -128,6 +140,14 @@ pub fn parse_club_query(query: &str) -> Fifa17ClubQuery {
             "sort" => out.sort = Some(v),
             "start" => out.start = v.parse().ok(),
             "count" => out.count = v.parse().ok(),
+            // Comma-joined, digits only — the same reading the item-definition
+            // routes already use for this parameter.
+            "defId" => {
+                out.def_ids = v
+                    .split(',')
+                    .filter_map(|d| d.trim().parse::<i64>().ok())
+                    .collect()
+            }
             _ => {}
         }
     }
@@ -339,6 +359,7 @@ mod tests {
                 sort: Some("desc".into()),
                 start: Some(10),
                 count: Some(11),
+                def_ids: Vec::new(),
             }
         );
     }
@@ -528,3 +549,25 @@ mod tests {
         );
     }
 }
+
+#[cfg(test)]
+mod def_id_tests {
+    use super::*;
+
+    /// `defId=` is CAPTURED so the host can report it, and deliberately does NOT
+    /// participate in the Core query — narrowing on an unconfirmed reading of the
+    /// parameter would answer "zero items" where today we answer "too many".
+    #[test]
+    fn def_id_list_is_captured_but_never_narrows_the_core_query() {
+        let q = parse_club_query("?year=2017&type=player&defId=20801,117617092,84044103");
+        assert_eq!(q.def_ids, vec![20801, 117_617_092, 84_044_103]);
+        assert_eq!(q.item_type.as_deref(), Some("player"));
+
+        // Non-numeric entries are dropped rather than poisoning the list.
+        let q = parse_club_query("?defId=20801,,notanid,42");
+        assert_eq!(q.def_ids, vec![20801, 42]);
+
+        // Absent means empty, never a phantom filter.
+        assert!(parse_club_query("?type=player").def_ids.is_empty());
+    }
+}
diff --git a/openfut-utas-host/src/lib.rs b/openfut-utas-host/src/lib.rs
index acf9e10..6ad85d9 100644
--- a/openfut-utas-host/src/lib.rs
+++ b/openfut-utas-host/src/lib.rs
@@ -2206,6 +2206,22 @@ pub fn handle_club(query: &str, deps: &ClubDeps<'_>) -> (WireResponse, ClubLog)
     if !deps.hidden.is_empty() {
         filter.push_str(&format!(",hidden={}", deps.hidden.len()));
     }
+    // The documented club grammar allows a comma-joined `defId=` list INSTEAD of
+    // the filter block, and this host does not narrow on it. That grammar is
+    // single-source and no observed request has ever carried one, so guessing the
+    // semantics could turn "too many items" into "zero items". Make the first
+    // real occurrence impossible to miss instead of silently answering wrong.
+    if !raw.def_ids.is_empty() {
+        filter.push_str(&format!(",defId={}", raw.def_ids.len()));
+        eprintln!(
+            "utas-host owner=RUST route=club NOTICE unhandled defId list ({} id(s): {:?}) \
+             — the response is NOT narrowed to them. This is the first observation of a \
+             parameter only ever seen in a decompile; capture the full request and \
+             implement the filter against it.",
+            raw.def_ids.len(),
+            &raw.def_ids[..raw.def_ids.len().min(8)]
+        );
+    }
 
     match deps.core.query_owned(&base.to_query_pairs()) {
         Ok(page) => {