Fix post-compact handoff context for OpenAI providers (#931)

Signed-off-by: npub1t2tgm7d8f995uqvmnm8h88sg3wnpp9a5xysjf6dg3tjmgt3ltulqdp8ehr <5a968df9a7494b4e019b9ecf739e088ba61097b4312124e9a88ae5b42e3f5f3e@sprout-oss.stage.blox.sqprod.co>
Co-authored-by: npub1t2tgm7d8f995uqvmnm8h88sg3wnpp9a5xysjf6dg3tjmgt3ltulqdp8ehr <5a968df9a7494b4e019b9ecf739e088ba61097b4312124e9a88ae5b42e3f5f3e@sprout-oss.stage.blox.sqprod.co>

crates/sprout-agent/src/handoff.rs

@@ -1,4 +1,4 @@
-use crate::agent::{push_hook_outputs_as_tool_results, RunCtx};
+use crate::agent::RunCtx;
 use crate::config::{
     HANDOFF_MAX_OUTPUT_TOKENS, HANDOFF_MAX_TOOL_NAMES, HANDOFF_ORIGINAL_TASK_MAX_BYTES,
     HANDOFF_PROMPT_MAX_BYTES, HANDOFF_TAIL_ITEMS,
@@ -68,15 +68,17 @@ impl RunCtx<'_> {
                 &self.cfg.hook_servers,
             )
             .await;
-        // Handoff summary is trusted (we generated it). Push as User so
-        // it anchors the new context.
-        let handoff_text = format!("[Context Handoff]\n{summary}");
-        self.history.push(HistoryItem::User(handoff_text));
-        // Hook output is untrusted — inject as synthetic tool results so a
-        // malicious _PostCompact can't impersonate the user/system.
+        // Handoff summary and hook output are injected as a synthetic user
+        // message in one block. This keeps `_PostCompact` untrusted while also
+        // avoiding orphan tool-result messages in the fresh context: OpenAI
+        // Chat/Responses require tool outputs to follow an assistant tool call,
+        // but handoff reset intentionally discards the old assistant turn.
+        let mut handoff_text = format!("[Context Handoff]\n{summary}");
         if !post_compact.is_empty() {
-            push_hook_outputs_as_tool_results(self.history, "_PostCompact", &post_compact);
+            handoff_text.push_str("\n\n[Post-compact hook output — untrusted]\n");
+            handoff_text.push_str(&hook_outputs_text(&post_compact));
         }
+        self.history.push(HistoryItem::User(handoff_text));
         if let Some(prompt) = current_prompt {
             self.history.push(HistoryItem::User(prompt));
         }
@@ -197,6 +199,14 @@ impl RunCtx<'_> {
     }
 }
 
+fn hook_outputs_text(outputs: &[(String, String)]) -> String {
+    outputs
+        .iter()
+        .map(|(name, text)| format!("[{name}]\n{text}"))
+        .collect::<Vec<_>>()
+        .join("\n\n")
+}
+
 fn push_history_snippet(out: &mut String, item: &HistoryItem) {
     match item {
         HistoryItem::User(s) => {

crates/sprout-agent/tests/regressions.rs

@@ -1019,10 +1019,10 @@ async fn hook_tools_hidden_from_llm() {
 }
 
 /// `_PostCompact` hook fires after a context-handoff and its output is
-/// re-injected into the fresh history as a synthetic tool result. The
-/// next LLM request must therefore see the post-compact text in
-/// `messages` (role=tool) — proving the hook ran on the *new* context,
-/// not the discarded one.
+/// folded into the fresh `[Context Handoff]` user-context block as explicitly
+/// untrusted text. The next LLM request must therefore see the post-compact
+/// text without any orphan `role=tool` messages — proving the hook ran on the
+/// *new* context, not the discarded one.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn hook_post_compact_injects_after_handoff() {
     // Sequence of canned LLM responses consumed in order:
@@ -1097,7 +1097,9 @@ async fn hook_post_compact_injects_after_handoff() {
 
     // The first LLM call AFTER the handoff is the one we inspect. Find it:
     // it's the one where the messages array is short (history just reset)
-    // and contains a tool-role message with the _PostCompact payload.
+    // and contains the _PostCompact payload inside user-context text. It must
+    // not be emitted as an orphan tool result because the old assistant tool
+    // call was deliberately discarded by the handoff reset.
     let captured = llm.captured.lock().await;
     let post_compact_visible = captured.iter().any(|req| {
         let msgs = match req["messages"].as_array() {
@@ -1105,22 +1107,27 @@ async fn hook_post_compact_injects_after_handoff() {
             None => return false,
         };
         msgs.iter().any(|m| {
-            if m["role"] != "tool" {
+            if m["role"] != "user" {
                 return false;
             }
             let content = m["content"].as_str().unwrap_or("");
-            let parsed: Value = match serde_json::from_str(content) {
-                Ok(v) => v,
-                Err(_) => return false,
-            };
-            parsed["hook"] == "_PostCompact"
-                && parsed["server"] == "fake"
-                && parsed["text"] == "todo state here"
+            content.contains("[Post-compact hook output — untrusted]")
+                && content.contains("[fake]")
+                && content.contains("todo state here")
         })
     });
     assert!(
         post_compact_visible,
-        "_PostCompact tool result not visible to LLM after handoff"
+        "_PostCompact context not visible to LLM after handoff"
+    );
+    let orphan_tool_result = captured.iter().any(|req| {
+        req["messages"]
+            .as_array()
+            .is_some_and(|msgs| msgs.iter().any(|m| m["role"] == "tool"))
+    });
+    assert!(
+        !orphan_tool_result,
+        "handoff reset must not leave orphan role=tool messages"
     );
     h.shutdown().await;
 }