fix compaction to restore system message

crates/g3-core/src/lib.rs

@@ -421,6 +421,7 @@ Format this as a detailed but concise summary that can be used to resume the con
     }
 
     /// Reset the context window with a summary
+    /// Preserves the original system prompt as the first message
     pub fn reset_with_summary(
         &mut self,
         summary: String,
@@ -433,10 +434,31 @@ Format this as a detailed but concise summary that can be used to resume the con
             .map(|m| m.content.len())
             .sum();
 
+        // Preserve the original system prompt (first message) and optionally the README (second message)
+        let original_system_prompt = self.conversation_history.first().cloned();
+        let readme_message = self.conversation_history.get(1).and_then(|msg| {
+            if matches!(msg.role, MessageRole::System) && 
+               (msg.content.contains("Project README") || msg.content.contains("Agent Configuration")) {
+                Some(msg.clone())
+            } else {
+                None
+            }
+        });
+
         // Clear the conversation history
         self.conversation_history.clear();
         self.used_tokens = 0;
 
+        // Re-add the original system prompt first (critical invariant)
+        if let Some(system_prompt) = original_system_prompt {
+            self.add_message(system_prompt);
+        }
+
+        // Re-add the README message if it existed
+        if let Some(readme) = readme_message {
+            self.add_message(readme);
+        }
+
         // Add the summary as a system message
         let summary_message = Message::new(
             MessageRole::System,

crates/g3-core/tests/test_reset_with_summary.rs

@@ -0,0 +1,159 @@
+//! Tests for reset_with_summary to ensure system prompt is preserved after compaction
+
+use g3_core::ContextWindow;
+use g3_providers::{Message, MessageRole};
+
+/// Test that reset_with_summary preserves the original system prompt
+#[test]
+fn test_reset_with_summary_preserves_system_prompt() {
+    let mut context = ContextWindow::new(10000);
+
+    // Add the system prompt as the first message (simulating agent initialization)
+    let system_prompt = "You are G3, an AI programming agent...";
+    context.add_message(Message::new(MessageRole::System, system_prompt.to_string()));
+
+    // Add some conversation history
+    context.add_message(Message::new(MessageRole::User, "Task: Write a function".to_string()));
+    context.add_message(Message::new(MessageRole::Assistant, "I'll help you write that function.".to_string()));
+    context.add_message(Message::new(MessageRole::User, "Thanks, now add tests".to_string()));
+    context.add_message(Message::new(MessageRole::Assistant, "Here are the tests.".to_string()));
+
+    // Verify we have 5 messages before reset
+    assert_eq!(context.conversation_history.len(), 5);
+
+    // Reset with summary
+    let summary = "We discussed writing a function and adding tests.".to_string();
+    let latest_user_msg = Some("Continue with the implementation".to_string());
+    context.reset_with_summary(summary, latest_user_msg);
+
+    // Verify the first message is still the system prompt
+    assert!(!context.conversation_history.is_empty(), "Conversation history should not be empty");
+    
+    let first_message = &context.conversation_history[0];
+    assert!(
+        matches!(first_message.role, MessageRole::System),
+        "First message should be a System message, got {:?}",
+        first_message.role
+    );
+    assert!(
+        first_message.content.contains("You are G3"),
+        "First message should contain the system prompt 'You are G3', got: {}",
+        &first_message.content[..first_message.content.len().min(100)]
+    );
+
+    // Verify the summary was added as a separate system message
+    let has_summary = context.conversation_history.iter().any(|m| {
+        matches!(m.role, MessageRole::System) && m.content.contains("Previous conversation summary")
+    });
+    assert!(has_summary, "Should have a summary message");
+
+    // Verify the latest user message was added
+    let has_user_msg = context.conversation_history.iter().any(|m| {
+        matches!(m.role, MessageRole::User) && m.content.contains("Continue with the implementation")
+    });
+    assert!(has_user_msg, "Should have the latest user message");
+}
+
+/// Test that reset_with_summary preserves README message if present
+#[test]
+fn test_reset_with_summary_preserves_readme() {
+    let mut context = ContextWindow::new(10000);
+
+    // Add the system prompt as the first message
+    let system_prompt = "You are G3, an AI programming agent...";
+    context.add_message(Message::new(MessageRole::System, system_prompt.to_string()));
+
+    // Add README as second system message
+    let readme_content = "# Project README\n\nThis is a test project.";
+    context.add_message(Message::new(MessageRole::System, readme_content.to_string()));
+
+    // Add some conversation history
+    context.add_message(Message::new(MessageRole::User, "Task: Write a function".to_string()));
+    context.add_message(Message::new(MessageRole::Assistant, "Done.".to_string()));
+
+    // Verify we have 4 messages before reset
+    assert_eq!(context.conversation_history.len(), 4);
+
+    // Reset with summary
+    let summary = "We wrote a function.".to_string();
+    context.reset_with_summary(summary, None);
+
+    // Verify the first message is still the system prompt
+    let first_message = &context.conversation_history[0];
+    assert!(
+        first_message.content.contains("You are G3"),
+        "First message should be the system prompt"
+    );
+
+    // Verify the README was preserved as the second message
+    let second_message = &context.conversation_history[1];
+    assert!(
+        matches!(second_message.role, MessageRole::System),
+        "Second message should be a System message"
+    );
+    assert!(
+        second_message.content.contains("Project README"),
+        "Second message should be the README"
+    );
+}
+
+/// Test that reset_with_summary works when there's no README
+#[test]
+fn test_reset_with_summary_without_readme() {
+    let mut context = ContextWindow::new(10000);
+
+    // Add only the system prompt (no README)
+    let system_prompt = "You are G3, an AI programming agent...";
+    context.add_message(Message::new(MessageRole::System, system_prompt.to_string()));
+
+    // Add conversation without README
+    context.add_message(Message::new(MessageRole::User, "Hello".to_string()));
+    context.add_message(Message::new(MessageRole::Assistant, "Hi there!".to_string()));
+
+    // Reset with summary
+    let summary = "Greeted the user.".to_string();
+    context.reset_with_summary(summary, None);
+
+    // Verify the first message is still the system prompt
+    let first_message = &context.conversation_history[0];
+    assert!(
+        first_message.content.contains("You are G3"),
+        "First message should be the system prompt"
+    );
+
+    // Verify we have system prompt + summary (no README)
+    // The second message should be the summary, not a README
+    let second_message = &context.conversation_history[1];
+    assert!(
+        second_message.content.contains("Previous conversation summary"),
+        "Second message should be the summary when no README exists"
+    );
+}
+
+/// Test that reset_with_summary handles Agent Configuration in addition to README
+#[test]
+fn test_reset_with_summary_preserves_agent_configuration() {
+    let mut context = ContextWindow::new(10000);
+
+    // Add the system prompt as the first message
+    let system_prompt = "You are G3, an AI programming agent...";
+    context.add_message(Message::new(MessageRole::System, system_prompt.to_string()));
+
+    // Add Agent Configuration as second system message
+    let agents_content = "# Agent Configuration\n\nSpecial instructions for this project.";
+    context.add_message(Message::new(MessageRole::System, agents_content.to_string()));
+
+    // Add some conversation history
+    context.add_message(Message::new(MessageRole::User, "Task: Do something".to_string()));
+
+    // Reset with summary
+    let summary = "Did something.".to_string();
+    context.reset_with_summary(summary, None);
+
+    // Verify the Agent Configuration was preserved
+    let second_message = &context.conversation_history[1];
+    assert!(
+        second_message.content.contains("Agent Configuration"),
+        "Second message should be the Agent Configuration"
+    );
+}