fix(agent): show the details an agent sends with an error (#2563)

ACP agents report failures as a generic JSON-RPC error and put the cause in data.details (or a string data). Only the message was shown, so such failures read just "Internal error". The details are now appended unless the message already contains them.

crates/ide-agent/src/lib.rs

@@ -1722,7 +1722,7 @@ impl Task {
                     format!("The agent process exited:\n{}", tail.join("\n"))
                 }
             }
-            RpcError::Server { message, .. } => message,
+            RpcError::Server { message, data, .. } => with_details(message, data.as_ref()),
             e => e.to_string(),
         }
     }
@@ -2180,6 +2180,24 @@ fn usage_limit(e: &RpcError) -> Option<Option<f64>> {
         .then_some(None)
 }
 
+/// An error's message with the detail ACP agents put in `data` (`{"details": …}` or a string):
+/// generic codes such as "Internal error" say nothing without it.
+fn with_details(message: String, data: Option<&Value>) -> String {
+    let details = match data {
+        Some(Value::String(text)) => text.as_str(),
+        Some(data) => data.get("details").and_then(Value::as_str).unwrap_or_default(),
+        None => "",
+    }
+    .trim();
+    if details.is_empty() || message.contains(details) {
+        message
+    } else if message.trim().is_empty() {
+        details.to_owned()
+    } else {
+        format!("{message}: {details}")
+    }
+}
+
 fn client_info() -> Value {
     json!({ "name": "OpenIDEA", "version": env!("CARGO_PKG_VERSION") })
 }

crates/ide-agent/src/tests.rs

@@ -2568,3 +2568,17 @@ fn streamed_text_is_sent_as_appends() {
     let rewritten = entry_event("t", &mut streamed, None, 5, &agent(&format!("{}x!??", "é".repeat(4999))));
     assert!(matches!(rewritten, AgentEvent::Entry { .. }), "a rewritten end is sent whole");
 }
+
+#[test]
+fn error_messages_keep_the_agents_details() {
+    let details = json!({ "details": "spawn claude ENOENT" });
+    assert_eq!(with_details("Internal error".into(), Some(&details)), "Internal error: spawn claude ENOENT");
+    assert_eq!(
+        with_details("Internal error".into(), Some(&json!("model not found"))),
+        "Internal error: model not found"
+    );
+    assert_eq!(with_details(String::new(), Some(&details)), "spawn claude ENOENT");
+    assert_eq!(with_details("spawn claude ENOENT".into(), Some(&details)), "spawn claude ENOENT");
+    assert_eq!(with_details("Internal error".into(), Some(&json!({ "other": 1 }))), "Internal error");
+    assert_eq!(with_details("Internal error".into(), None), "Internal error");
+}