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

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

crates/bxcore/tests/wire.rs

@@ -94,10 +94,35 @@ fn tool_response_failed() {
 fn tool_response_denied() {
     let body = ToolResponse::Denied {
         reason: DenyReason::TaintTooHigh,
+        detail: None,
     };
     check("tool_response_denied.json", response(9, true, body));
 }
 
+/// A detail is written only when there is one, so the fixture above has no `detail` key.
+#[test]
+fn tool_response_denied_with_a_detail() {
+    let body = ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid argument shape: unknown field `file`, expected `text`".to_string()),
+    };
+    check("tool_response_denied_detail.json", response(9, true, body));
+}
+
+#[test]
+fn a_detail_must_be_a_string() {
+    let good = fixture("tool_response_denied_detail.json");
+    assert!(serde_json::from_str::<Envelope>(&good).is_ok());
+    let at = good.find("\"detail\":").unwrap();
+    for value in ["7", "[\"a\"]", "{}", "true"] {
+        let bad = format!("{}\"detail\":{value}}}}}}}", &good[..at]);
+        assert!(
+            serde_json::from_str::<Envelope>(&bad).is_err(),
+            "accepted {bad}"
+        );
+    }
+}
+
 #[test]
 fn error_message() {
     let body = WireError {