bxcore, brokerd, loopd: an invalid-arguments denial says why

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/loopd/tests/turn_broker.rs

@@ -31,6 +31,7 @@ fn tool_events(events: &[TurnEvent]) -> Vec<String> {
 fn a_denial_is_a_fixed_sentence_for_the_model_and_an_event_for_the_owner() {
     let s = setup(vec![ToolResponse::Denied {
         reason: DenyReason::TaintTooHigh,
+        detail: None,
     }]);
     s.server.route(
         CHAT,
@@ -78,10 +79,55 @@ fn a_denial_is_a_fixed_sentence_for_the_model_and_an_event_for_the_owner() {
     );
 }
 
+/// The broker's detail reaches the model, in the log and in the next request; the owner's event
+/// is the same as for any denial.
+#[test]
+fn an_invalid_call_tells_the_model_why() {
+    let s = setup(vec![ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid path: n/a".to_string()),
+    }]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "x");
+    assert!(result.is_ok(), "{result:?}");
+    let want = "Denied: the arguments are not valid for this tool: invalid path: n/a.";
+    match &session.records()[4] {
+        LogRecord::ToolResult {
+            content,
+            class,
+            untrusted,
+            truncated,
+            ..
+        } => {
+            assert_eq!(content, want);
+            assert_eq!(
+                (*class, *untrusted, *truncated),
+                (DataClass::Public, false, false)
+            );
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started read_file",
+            "denied read_file InvalidArguments",
+            "result read_file"
+        ]
+    );
+    let m2 = s.server.requests_to(CHAT)[1].json();
+    assert_eq!(m2["messages"][3]["content"], want);
+}
+
 #[test]
 fn a_denied_call_tool_names_the_target_tool_in_the_denial() {
     let s = setup(vec![ToolResponse::Denied {
         reason: DenyReason::NoGrant,
+        detail: None,
     }]);
     s.server.route(
         CHAT,
@@ -158,6 +204,7 @@ fn a_pending_approval_that_ends_in_a_refusal() {
         pending(7, "2026-09-18T12:15:00.000Z"),
         ToolResponse::Denied {
             reason: DenyReason::ApprovalRefused,
+            detail: None,
         },
     ]);
     s.server.route(