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

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

crates/loopd/tests/broker_port.rs

@@ -48,8 +48,15 @@ fn every_denial_and_a_failure_come_back_as_they_were_sent() {
         DenyReason::StateUnreadable,
     ]
     .into_iter()
-    .map(|reason| ToolResponse::Denied { reason })
+    .map(|reason| ToolResponse::Denied {
+        reason,
+        detail: None,
+    })
     .collect();
+    answers.push(ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid path: n/a".to_string()),
+    });
     answers.push(ToolResponse::Failed {
         message: "the runner arrives in M3b".to_string(),
     });

crates/loopd/tests/tools.rs

@@ -5,7 +5,8 @@ mod support;
 
 use bxcore::{CallId, DenyReason, SessionId, ToolRequest, ToolResponse};
 use loopd::tools::{
-    Dispatch, FakeTools, Pending, Registry, ToolPort, cap_result, denial_text, dispatch,
+    Dispatch, FakeTools, Pending, Registry, ToolPort, cap_result, denial_message, denial_text,
+    dispatch,
 };
 
 fn req(tool: &str, arguments: &str) -> ToolRequest {
@@ -252,9 +253,35 @@ fn every_deny_reason_has_its_sentence() {
     ];
     for (reason, want) in table {
         assert_eq!(denial_text(reason), want, "{reason:?}");
+        assert_eq!(
+            denial_message(reason, None),
+            want,
+            "{reason:?}: without a detail, the sentence alone"
+        );
     }
 }
 
+/// The broker's detail goes after the sentence, before its full stop.
+#[test]
+fn a_denial_with_a_detail_appends_it_to_the_sentence() {
+    assert_eq!(
+        denial_message(
+            DenyReason::InvalidArguments,
+            Some("invalid argument shape: unknown field `file`, expected `text`")
+        ),
+        "Denied: the arguments are not valid for this tool: invalid argument shape: unknown field `file`, expected `text`."
+    );
+    assert_eq!(
+        denial_message(DenyReason::InvalidArguments, Some("invalid path: n/a")),
+        "Denied: the arguments are not valid for this tool: invalid path: n/a."
+    );
+    assert_eq!(
+        denial_message(DenyReason::NoGrant, Some("x")),
+        "Denied: no grant allows this call: x.",
+        "any reason's detail is shown the same way"
+    );
+}
+
 #[test]
 fn results_are_cut_on_a_character_boundary_and_marked() {
     assert_eq!(cap_result("short", 100), ("short".to_string(), false));
@@ -296,7 +323,8 @@ fn fake_tools_answer_echo_deny_the_rest_and_record_calls() {
         assert_eq!(
             fake.call(&req(tool, "{}"), &mut on_pending),
             ToolResponse::Denied {
-                reason: DenyReason::NoGrant
+                reason: DenyReason::NoGrant,
+                detail: None,
             },
             "{tool}: the clock is loopd's own now, not the port's"
         );

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(