diff --git a/lib/crates/arc-agent/src/compaction.rs b/lib/crates/arc-agent/src/compaction.rs
index 7c59493da..ed7170e12 100644
--- a/lib/crates/arc-agent/src/compaction.rs
+++ b/lib/crates/arc-agent/src/compaction.rs
@@ -80,8 +80,9 @@ pub async fn compact_context(
     };
 
     let summarization_prompt = format!(
-        "You are summarizing a coding assistant conversation to provide continuity. A new context \
-window will continue this work with only your summary and the most recent messages.\n\n\
+        "You are creating a handoff document for a different coding assistant that will take over \
+this task. That assistant will only see your summary and the most recent messages — nothing else \
+from the conversation so far.\n\n\
 Write a summary using EXACTLY these sections:\n\n\
 ## Goal\nWhat the user asked for and any constraints or preferences stated.\n\n\
 ## Progress\nWhat was accomplished, with file paths and key decisions.\n\n\
@@ -89,8 +90,9 @@ Write a summary using EXACTLY these sections:\n\n\
 ## Failed Approaches\nWhat was tried and didn't work, and why.\n\n\
 ## Open Issues\nBugs, edge cases, or TODOs that remain.\n\n\
 ## Next Steps\nWhat should happen next to make progress.\n\n\
-Be specific — include file paths, function names, and error messages. Omit pleasantries \
-and conversational filler.{file_ops_section}"
+Be thorough and specific — the assistant taking over has no prior context. Include file paths, \
+function names, error messages, and exact values. Omit pleasantries and conversational filler.\
+{file_ops_section}"
     );
 
     let summary_request = Request {
@@ -124,7 +126,10 @@ and conversational filler.{file_ops_section}"
         summary_len = summary_text.len(),
         "Compaction summary generated"
     );
-    let summary_content = format!("[Context Summary]\n{summary_text}");
+    let summary_content = format!(
+        "A different assistant began this task and produced the following summary. \
+Build on their progress — do not repeat completed steps.\n\n{summary_text}"
+    );
     let summary_token_estimate = summary_content.len() / 4;
 
     history.compact(preserve_count, summary_content);
diff --git a/lib/crates/arc-agent/src/history.rs b/lib/crates/arc-agent/src/history.rs
index 693a15687..d42575d4d 100644
--- a/lib/crates/arc-agent/src/history.rs
+++ b/lib/crates/arc-agent/src/history.rs
@@ -21,11 +21,13 @@ impl History {
             return;
         }
         let preserved = self.turns.split_off(self.turns.len() - preserve_count);
+        let extracted_user_messages = extract_recent_user_messages(&self.turns, 20_000);
         self.turns.clear();
         self.turns.push(Turn::System {
             content: summary,
             timestamp: std::time::SystemTime::now(),
         });
+        self.turns.extend(extracted_user_messages);
         self.turns.extend(preserved);
         self.strip_opaque_provider_items();
     }
@@ -99,6 +101,29 @@ impl History {
     }
 }
 
+/// Walk discarded turns in reverse, collecting `Turn::User` variants up to
+/// a token budget (estimated at ~4 chars per token). Returns them in
+/// chronological order so they can be inserted between the summary and the
+/// preserved tail.
+fn extract_recent_user_messages(discarded: &[Turn], token_budget: usize) -> Vec<Turn> {
+    let char_budget = token_budget * 4;
+    let mut total_chars = 0;
+    let mut collected: Vec<Turn> = Vec::new();
+
+    for turn in discarded.iter().rev() {
+        if let Turn::User { content, .. } = turn {
+            if total_chars + content.len() > char_budget {
+                break;
+            }
+            total_chars += content.len();
+            collected.push(turn.clone());
+        }
+    }
+
+    collected.reverse();
+    collected
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -115,7 +140,8 @@ mod tests {
             });
         }
         history.compact(4, "Summary of old conversation".into());
-        assert_eq!(history.turns().len(), 5); // 1 summary + 4 preserved
+        // 1 summary + 4 extracted user messages + 4 preserved = 9
+        assert_eq!(history.turns().len(), 9);
     }
 
     #[test]
@@ -142,11 +168,16 @@ mod tests {
         }
         history.compact(4, "Summary".into());
         let turns = history.turns();
-        // Last 4 turns should be msg 4..7
-        assert!(matches!(&turns[1], Turn::User { content, .. } if content == "msg 4"));
-        assert!(matches!(&turns[2], Turn::User { content, .. } if content == "msg 5"));
-        assert!(matches!(&turns[3], Turn::User { content, .. } if content == "msg 6"));
-        assert!(matches!(&turns[4], Turn::User { content, .. } if content == "msg 7"));
+        // Layout: summary, extracted user msgs (0..3), preserved (4..7)
+        assert!(matches!(&turns[0], Turn::System { .. }));
+        assert!(matches!(&turns[1], Turn::User { content, .. } if content == "msg 0"));
+        assert!(matches!(&turns[2], Turn::User { content, .. } if content == "msg 1"));
+        assert!(matches!(&turns[3], Turn::User { content, .. } if content == "msg 2"));
+        assert!(matches!(&turns[4], Turn::User { content, .. } if content == "msg 3"));
+        assert!(matches!(&turns[5], Turn::User { content, .. } if content == "msg 4"));
+        assert!(matches!(&turns[6], Turn::User { content, .. } if content == "msg 5"));
+        assert!(matches!(&turns[7], Turn::User { content, .. } if content == "msg 6"));
+        assert!(matches!(&turns[8], Turn::User { content, .. } if content == "msg 7"));
     }
 
     #[test]
@@ -434,7 +465,8 @@ mod tests {
 
         history.compact(2, "Summary".into());
 
-        let assistant_turn = &history.turns()[2];
+        // Layout: summary, extracted User("old msg"), preserved User("recent msg"), preserved Assistant
+        let assistant_turn = &history.turns()[3];
         if let Turn::Assistant {
             provider_parts,
             tool_calls,
@@ -480,7 +512,8 @@ mod tests {
 
         history.compact(2, "Summary".into());
 
-        let assistant_turn = &history.turns()[2];
+        // Layout: summary, extracted User("old msg"), preserved User("recent msg"), preserved Assistant
+        let assistant_turn = &history.turns()[3];
         if let Turn::Assistant { provider_parts, .. } = assistant_turn {
             assert_eq!(
                 provider_parts.len(),
@@ -526,4 +559,79 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn extract_recent_user_messages_collects_in_chronological_order() {
+        let turns = vec![
+            Turn::User {
+                content: "first".into(),
+                timestamp: SystemTime::now(),
+            },
+            Turn::Assistant {
+                content: "reply".into(),
+                tool_calls: vec![],
+                provider_parts: vec![],
+                usage: Usage::default(),
+                response_id: "r1".into(),
+                timestamp: SystemTime::now(),
+            },
+            Turn::User {
+                content: "second".into(),
+                timestamp: SystemTime::now(),
+            },
+        ];
+        let extracted = extract_recent_user_messages(&turns, 20_000);
+        assert_eq!(extracted.len(), 2);
+        assert!(matches!(&extracted[0], Turn::User { content, .. } if content == "first"));
+        assert!(matches!(&extracted[1], Turn::User { content, .. } if content == "second"));
+    }
+
+    #[test]
+    fn extract_recent_user_messages_respects_token_budget() {
+        let turns = vec![
+            Turn::User {
+                content: "a".repeat(100),
+                timestamp: SystemTime::now(),
+            },
+            Turn::User {
+                content: "b".repeat(100),
+                timestamp: SystemTime::now(),
+            },
+        ];
+        // Budget of 30 tokens = 120 chars; second message (100 chars) fits, first would exceed
+        let extracted = extract_recent_user_messages(&turns, 30);
+        assert_eq!(extracted.len(), 1);
+        assert!(matches!(&extracted[0], Turn::User { content, .. } if content.starts_with('b')));
+    }
+
+    #[test]
+    fn compact_extracts_only_user_turns_from_discarded() {
+        let mut history = History::default();
+        history.push(Turn::User {
+            content: "user msg".into(),
+            timestamp: SystemTime::now(),
+        });
+        history.push(Turn::Assistant {
+            content: "assistant msg".into(),
+            tool_calls: vec![],
+            provider_parts: vec![],
+            usage: Usage::default(),
+            response_id: "r1".into(),
+            timestamp: SystemTime::now(),
+        });
+        history.push(Turn::User {
+            content: "preserved".into(),
+            timestamp: SystemTime::now(),
+        });
+
+        history.compact(1, "Summary".into());
+
+        // Layout: summary, extracted User("user msg"), preserved User("preserved")
+        assert_eq!(history.turns().len(), 3);
+        assert!(matches!(&history.turns()[0], Turn::System { .. }));
+        assert!(matches!(&history.turns()[1], Turn::User { content, .. } if content == "user msg"));
+        assert!(
+            matches!(&history.turns()[2], Turn::User { content, .. } if content == "preserved")
+        );
+    }
 }
diff --git a/lib/crates/arc-agent/src/session.rs b/lib/crates/arc-agent/src/session.rs
index c4d1de3f5..55922357b 100644
--- a/lib/crates/arc-agent/src/session.rs
+++ b/lib/crates/arc-agent/src/session.rs
@@ -564,6 +564,40 @@ impl Session {
                 return Err(self.aborted_error());
             }
 
+            // Pre-turn compaction check: compact before building the request so the
+            // LLM call uses a trimmed context when we're already over threshold.
+            let over_threshold = crate::compaction::check_context_usage(
+                &self.system_prompt,
+                &self.history,
+                self.provider_profile.as_ref(),
+                self.config.compaction_threshold_percent,
+                &self.event_emitter,
+                &self.id,
+            );
+            if over_threshold && self.config.enable_context_compaction {
+                if let Err(e) = crate::compaction::compact_context(
+                    &mut self.history,
+                    &self.llm_client,
+                    self.provider_profile.as_ref(),
+                    &self.system_prompt,
+                    &self.file_tracker,
+                    self.config.compaction_preserve_turns,
+                    &self.event_emitter,
+                    &self.id,
+                )
+                .await
+                {
+                    self.event_emitter.emit(
+                        self.id.clone(),
+                        AgentEvent::Error {
+                            error: AgentError::InvalidState(format!(
+                                "Context compaction failed: {e}"
+                            )),
+                        },
+                    );
+                }
+            }
+
             // Build request
             let request = self.build_request();
 
@@ -1887,7 +1921,7 @@ mod tests {
         // History should have been compacted: summary turn + preserved turns
         let turns = session.history().turns();
         assert!(
-            turns.iter().any(|t| matches!(t, Turn::System { content, .. } if content.contains("[Context Summary]"))),
+            turns.iter().any(|t| matches!(t, Turn::System { content, .. } if content.contains("A different assistant began this task"))),
             "Should contain a summary system turn"
         );
     }