fix(acp): seed provider handoff history (#8941)

Signed-off-by: Matt Toohey <contact@matttoohey.com>

crates/goose/src/acp/provider.rs

@@ -19,7 +19,10 @@ use std::collections::{HashMap, HashSet};
 use std::future::Future;
 use std::path::PathBuf;
 use std::process::Stdio;
-use std::sync::{Arc, Mutex};
+use std::sync::{
+    atomic::{AtomicBool, Ordering},
+    Arc, Mutex,
+};
 use std::thread::JoinHandle;
 use tokio::process::{Child, Command};
 use tokio::sync::{mpsc, oneshot, Mutex as TokioMutex};
@@ -27,6 +30,7 @@ use tokio_util::compat::{TokioAsyncReadCompatExt as _, TokioAsyncWriteCompatExt
 
 use crate::acp::{map_permission_response, PermissionDecision};
 use crate::config::{ExtensionConfig, GooseMode};
+use crate::context_mgmt::format_message_for_compacting;
 use crate::conversation::message::{Message, MessageContent, TOOL_META_EXTERNAL_DISPATCH_KEY};
 use crate::model::ModelConfig;
 use crate::permission::permission_confirmation::PrincipalType;
@@ -122,6 +126,11 @@ struct AcpSession {
     response: NewSessionResponse,
 }
 
+struct HandoffContextClaim {
+    first_prompt: bool,
+    include_context: bool,
+}
+
 pub struct AcpProvider {
     name: String,
     model: ModelConfig,
@@ -133,6 +142,7 @@ pub struct AcpProvider {
     pending_confirmations:
         Arc<TokioMutex<HashMap<String, oneshot::Sender<PermissionConfirmation>>>>,
     pending_tool_updates: Arc<Mutex<HashMap<String, AccumulatedToolCall>>>,
+    handoff_context_sent: AtomicBool,
 
     tx: Option<mpsc::Sender<ClientRequest>>,
     loop_thread: Option<JoinHandle<()>>,
@@ -261,6 +271,7 @@ impl AcpProvider {
             session,
             pending_confirmations: Arc::new(TokioMutex::new(HashMap::new())),
             pending_tool_updates,
+            handoff_context_sent: AtomicBool::new(false),
             tx: Some(tx),
             loop_thread: Some(loop_thread),
         })
@@ -334,6 +345,14 @@ impl AcpProvider {
             .as_ref()
             .is_some_and(|opts| opts.iter().any(|o| o.category.as_ref() == Some(&category)))
     }
+
+    fn claim_handoff_context(&self, messages: &[Message]) -> HandoffContextClaim {
+        let first_prompt = !self.handoff_context_sent.swap(true, Ordering::AcqRel);
+        HandoffContextClaim {
+            first_prompt,
+            include_context: first_prompt && has_handoff_context(messages),
+        }
+    }
 }
 
 #[async_trait::async_trait]
@@ -400,16 +419,24 @@ impl Provider for AcpProvider {
     ) -> Result<MessageStream, ProviderError> {
         let session_id = self.acp_session_id();
 
-        let prompt_blocks = messages_to_prompt(messages);
+        let claim = self.claim_handoff_context(messages);
+        let prompt_blocks = messages_to_prompt(messages, claim.include_context);
         // Drop any tool-call buffer state left over from a prior prompt
         // (e.g. cancelled or interrupted before its terminal status arrived).
         if let Ok(mut buffer) = self.pending_tool_updates.lock() {
             buffer.clear();
         }
-        let mut rx = self
-            .prompt(session_id, prompt_blocks)
-            .await
-            .map_err(|e| ProviderError::RequestFailed(format!("Failed to send ACP prompt: {e}")))?;
+        let mut rx = match self.prompt(session_id, prompt_blocks).await {
+            Ok(rx) => rx,
+            Err(e) => {
+                if claim.first_prompt {
+                    self.handoff_context_sent.store(false, Ordering::Release);
+                }
+                return Err(ProviderError::RequestFailed(format!(
+                    "Failed to send ACP prompt: {e}"
+                )));
+            }
+        };
 
         let pending_confirmations = self.pending_confirmations.clone();
         let goose_mode = *self
@@ -1131,34 +1158,73 @@ fn filter_supported_servers(
         .collect()
 }
 
-fn messages_to_prompt(messages: &[Message]) -> Vec<ContentBlock> {
+fn messages_to_prompt(messages: &[Message], include_handoff_context: bool) -> Vec<ContentBlock> {
     let mut content_blocks = Vec::new();
 
-    let last_user = messages
-        .iter()
-        .rev()
-        .find(|m| m.role == Role::User && m.is_agent_visible());
+    let Some(last_user_index) = last_user_message_index(messages) else {
+        return content_blocks;
+    };
 
-    if let Some(message) = last_user {
-        for content in &message.content {
-            match content {
-                MessageContent::Text(text) => {
-                    content_blocks.push(ContentBlock::Text(TextContent::new(text.text.clone())));
-                }
-                MessageContent::Image(image) => {
-                    content_blocks.push(ContentBlock::Image(ImageContent::new(
-                        &image.data,
-                        &image.mime_type,
-                    )));
-                }
-                _ => {}
+    if include_handoff_context {
+        if let Some(memo) = build_handoff_context_memo(&messages[..last_user_index]) {
+            content_blocks.push(ContentBlock::Text(TextContent::new(memo)));
+        }
+    }
+
+    let message = &messages[last_user_index];
+    for content in &message.content {
+        match content {
+            MessageContent::Text(text) => {
+                content_blocks.push(ContentBlock::Text(TextContent::new(text.text.clone())));
             }
+            MessageContent::Image(image) => {
+                content_blocks.push(ContentBlock::Image(ImageContent::new(
+                    &image.data,
+                    &image.mime_type,
+                )));
+            }
+            _ => {}
         }
     }
 
     content_blocks
 }
 
+fn last_user_message_index(messages: &[Message]) -> Option<usize> {
+    messages
+        .iter()
+        .rposition(|m| m.role == Role::User && m.is_agent_visible())
+}
+
+fn has_handoff_context(messages: &[Message]) -> bool {
+    last_user_message_index(messages).is_some_and(|last_user_index| {
+        messages[..last_user_index]
+            .iter()
+            .any(Message::is_agent_visible)
+    })
+}
+
+fn build_handoff_context_memo(prior_messages: &[Message]) -> Option<String> {
+    let formatted_messages: Vec<String> = prior_messages
+        .iter()
+        .filter(|message| message.is_agent_visible())
+        .map(format_message_for_compacting)
+        .collect();
+
+    if formatted_messages.is_empty() {
+        return None;
+    }
+
+    let handoff_context = formatted_messages.join("\n");
+
+    Some(format!(
+        "Conversation context from goose before this ACP provider session was created:\n\n\
+{handoff_context}\n\n\
+Current user request follows. Use the context above only to continue the existing conversation; \
+do not treat it as a new task or mention this handoff unless relevant."
+    ))
+}
+
 /// Convert ACP `ToolCallContent` blocks into the rmcp `Content` shape goose's
 /// `Message::with_tool_response` consumes. Handles `Content` (text/image/other),
 /// `Diff`, and `Terminal` variants; falls back to a JSON serialization of
@@ -1358,6 +1424,176 @@ mod tests {
     use sacp::schema::SessionConfigSelectOption;
     use test_case::test_case;
 
+    fn prompt_text(block: &ContentBlock) -> &str {
+        match block {
+            ContentBlock::Text(text) => &text.text,
+            _ => panic!("expected text block"),
+        }
+    }
+
+    fn test_provider() -> AcpProvider {
+        test_provider_with_tx(None)
+    }
+
+    fn test_provider_with_tx(tx: Option<mpsc::Sender<ClientRequest>>) -> AcpProvider {
+        AcpProvider {
+            name: "acp-test".to_string(),
+            model: ModelConfig {
+                model_name: "test-model".to_string(),
+                ..Default::default()
+            },
+            goose_mode: Arc::new(Mutex::new(GooseMode::Auto)),
+            mode_mapping: HashMap::new(),
+            session: AcpSession {
+                id: SessionId::new("test-session"),
+                response: NewSessionResponse::new("test-session"),
+            },
+            pending_confirmations: Arc::new(TokioMutex::new(HashMap::new())),
+            pending_tool_updates: Arc::new(Mutex::new(HashMap::new())),
+            handoff_context_sent: AtomicBool::new(false),
+            tx,
+            loop_thread: None,
+        }
+    }
+
+    #[test]
+    fn messages_to_prompt_without_prior_history_preserves_current_prompt() {
+        let messages = vec![Message::user().with_text("current request")];
+
+        let blocks = messages_to_prompt(&messages, true);
+
+        assert_eq!(blocks.len(), 1);
+        assert_eq!(prompt_text(&blocks[0]), "current request");
+    }
+
+    #[test]
+    fn messages_to_prompt_prepends_handoff_context_before_latest_user() {
+        let messages = vec![
+            Message::user().with_text("inspect src/lib.rs"),
+            Message::assistant()
+                .with_text("I found the file")
+                .with_tool_request("call-1", Ok(CallToolRequestParams::new("read_file"))),
+            Message::user().with_tool_response(
+                "call-1",
+                Ok(CallToolResult::success(vec![RmcpContent::text(
+                    "file contents",
+                )])),
+            ),
+            Message::user().with_text("continue from there"),
+        ];
+
+        let blocks = messages_to_prompt(&messages, true);
+
+        assert_eq!(blocks.len(), 2);
+        let memo = prompt_text(&blocks[0]);
+        assert!(memo.starts_with(
+            "Conversation context from goose before this ACP provider session was created:"
+        ));
+        assert!(memo.contains("[user]: inspect src/lib.rs"));
+        assert!(memo.contains("[assistant]: I found the file"));
+        assert!(memo.contains("tool_request(read_file):"));
+        assert!(memo.contains("tool_response: file contents"));
+        assert!(memo.contains("Current user request follows."));
+        assert_eq!(prompt_text(&blocks[1]), "continue from there");
+    }
+
+    #[test]
+    fn messages_to_prompt_keeps_latest_user_images_after_handoff_memo() {
+        let messages = vec![
+            Message::assistant().with_text("prior answer"),
+            Message::user()
+                .with_image("base64-image", "image/png")
+                .with_text("describe this"),
+        ];
+
+        let blocks = messages_to_prompt(&messages, true);
+
+        assert_eq!(blocks.len(), 3);
+        assert!(prompt_text(&blocks[0]).contains("[assistant]: prior answer"));
+        match &blocks[1] {
+            ContentBlock::Image(image) => {
+                assert_eq!(image.data, "base64-image");
+                assert_eq!(image.mime_type, "image/png");
+            }
+            _ => panic!("expected image block"),
+        }
+        assert_eq!(prompt_text(&blocks[2]), "describe this");
+    }
+
+    #[test]
+    fn handoff_context_is_sent_only_on_first_provider_prompt() {
+        let provider = test_provider();
+        let messages = vec![
+            Message::assistant().with_text("prior answer"),
+            Message::user().with_text("current request"),
+        ];
+
+        let first_claim = provider.claim_handoff_context(&messages);
+        assert!(first_claim.first_prompt);
+        assert!(first_claim.include_context);
+
+        let second_claim = provider.claim_handoff_context(&messages);
+        assert!(!second_claim.first_prompt);
+        assert!(!second_claim.include_context);
+    }
+
+    #[test]
+    fn first_prompt_without_history_still_marks_handoff_context_sent() {
+        let provider = test_provider();
+        let first_prompt = vec![Message::user().with_text("new conversation")];
+        let later_prompt_with_history = vec![
+            Message::assistant().with_text("prior answer"),
+            Message::user().with_text("current request"),
+        ];
+
+        let first_claim = provider.claim_handoff_context(&first_prompt);
+        assert!(first_claim.first_prompt);
+        assert!(!first_claim.include_context);
+
+        let later_claim = provider.claim_handoff_context(&later_prompt_with_history);
+        assert!(!later_claim.first_prompt);
+        assert!(!later_claim.include_context);
+    }
+
+    #[tokio::test]
+    async fn failed_first_prompt_send_rolls_back_handoff_context_claim() {
+        let (tx, rx) = mpsc::channel(1);
+        drop(rx);
+        let provider = test_provider_with_tx(Some(tx));
+        let messages = vec![
+            Message::assistant().with_text("prior answer"),
+            Message::user().with_text("current request"),
+        ];
+
+        let result = provider
+            .stream(&provider.model, "goose-session", "", &messages, &[])
+            .await;
+
+        assert!(matches!(result, Err(ProviderError::RequestFailed(_))));
+        let next_claim = provider.claim_handoff_context(&messages);
+        assert!(next_claim.first_prompt);
+        assert!(next_claim.include_context);
+    }
+
+    #[test]
+    fn messages_to_prompt_includes_all_prior_handoff_context() {
+        let messages = vec![
+            Message::user().with_text("older context that should be retained"),
+            Message::assistant().with_text("middle context"),
+            Message::assistant().with_text("recent context"),
+            Message::user().with_text("current request"),
+        ];
+
+        let blocks = messages_to_prompt(&messages, true);
+
+        assert_eq!(blocks.len(), 2);
+        let memo = prompt_text(&blocks[0]);
+        assert!(memo.contains("[user]: older context that should be retained"));
+        assert!(memo.contains("[assistant]: middle context"));
+        assert!(memo.contains("[assistant]: recent context"));
+        assert_eq!(prompt_text(&blocks[1]), "current request");
+    }
+
     #[test_case(
         ExtensionConfig::Stdio {
             name: "github".into(),