feat(session): implement streaming state with text delta and tool‑call diff handling

- Introduce `StreamingMessageState` to track full text, last tool calls, and completion.
- Add `StreamDiff`, `TextDelta`, and `TextDeltaKind` for describing incremental changes.
- SessionController now maintains a `stream_states` map keyed by response IDs.
- `apply_stream_chunk` uses the new state to emit append/replace text deltas and tool‑call updates, handling final chunks and cleanup.
- `Conversation` gains `set_stream_content` to replace streaming content and manage metadata.
- Ensure stream state is cleared on cancel, conversation reset, and controller clear.

crates/owlen-core/src/conversation.rs

@@ -190,6 +190,46 @@ impl ConversationManager {
         Ok(())
     }
 
+    /// Replace the current streaming content for a message.
+    pub fn set_stream_content(
+        &mut self,
+        message_id: Uuid,
+        content: impl Into<String>,
+        is_final: bool,
+    ) -> Result<()> {
+        let index = self
+            .message_index
+            .get(&message_id)
+            .copied()
+            .ok_or_else(|| crate::Error::Unknown(format!("Unknown message id: {message_id}")))?;
+
+        let conversation = self.active_mut();
+        if let Some(message) = conversation.messages.get_mut(index) {
+            message.content = content.into();
+            message.metadata.remove(PLACEHOLDER_FLAG);
+            message.timestamp = std::time::SystemTime::now();
+            let millis = std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .unwrap_or_default()
+                .as_millis() as u64;
+            message.metadata.insert(
+                LAST_CHUNK_TS.to_string(),
+                Value::Number(Number::from(millis)),
+            );
+
+            if is_final {
+                message
+                    .metadata
+                    .insert(STREAMING_FLAG.to_string(), Value::Bool(false));
+                self.streaming.remove(&message_id);
+            } else if let Some(info) = self.streaming.get_mut(&message_id) {
+                info.last_update = Instant::now();
+            }
+        }
+
+        Ok(())
+    }
+
     /// Set placeholder text for a streaming message
     pub fn set_stream_placeholder(
         &mut self,
@@ -254,8 +294,12 @@ impl ConversationManager {
             .ok_or_else(|| crate::Error::Unknown(format!("Unknown message id: {message_id}")))?;
 
         if let Some(message) = self.active_mut().messages.get_mut(index) {
+            if tool_calls.is_empty() {
+                message.tool_calls = None;
+            } else {
                 message.tool_calls = Some(tool_calls);
             }
+        }
 
         Ok(())
     }

crates/owlen-core/src/session.rs

@@ -66,6 +66,101 @@ struct PendingToolRequest {
     tool_calls: Vec<ToolCall>,
 }
 
+#[derive(Debug, Default)]
+struct StreamingMessageState {
+    full_text: String,
+    last_tool_calls: Option<Vec<ToolCall>>,
+    finished: bool,
+}
+
+#[derive(Debug)]
+struct StreamDiff {
+    text: Option<TextDelta>,
+    tool_calls: Option<Vec<ToolCall>>,
+}
+
+#[derive(Debug)]
+struct TextDelta {
+    content: String,
+    mode: TextDeltaKind,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum TextDeltaKind {
+    Append,
+    Replace,
+}
+
+impl StreamingMessageState {
+    fn new() -> Self {
+        Self::default()
+    }
+
+    fn ingest(&mut self, chunk: &ChatResponse) -> StreamDiff {
+        if self.finished {
+            return StreamDiff {
+                text: None,
+                tool_calls: None,
+            };
+        }
+
+        let mut text_delta = None;
+        let incoming = chunk.message.content.clone();
+
+        if incoming != self.full_text {
+            if incoming.starts_with(&self.full_text) {
+                let delta = incoming[self.full_text.len()..].to_string();
+                if !delta.is_empty() {
+                    text_delta = Some(TextDelta {
+                        content: delta,
+                        mode: TextDeltaKind::Append,
+                    });
+                }
+            } else {
+                text_delta = Some(TextDelta {
+                    content: incoming.clone(),
+                    mode: TextDeltaKind::Replace,
+                });
+            }
+            self.full_text = incoming;
+        }
+
+        let mut tool_delta = None;
+        if let Some(tool_calls) = chunk.message.tool_calls.clone() {
+            if tool_calls.is_empty() {
+                let previously_had_calls = self
+                    .last_tool_calls
+                    .as_ref()
+                    .map(|prev| !prev.is_empty())
+                    .unwrap_or(false);
+                if previously_had_calls {
+                    tool_delta = Some(Vec::new());
+                }
+                self.last_tool_calls = None;
+            } else {
+                let is_new = self
+                    .last_tool_calls
+                    .as_ref()
+                    .map(|prev| prev != &tool_calls)
+                    .unwrap_or(true);
+                if is_new {
+                    tool_delta = Some(tool_calls.clone());
+                }
+                self.last_tool_calls = Some(tool_calls);
+            }
+        }
+
+        StreamDiff {
+            text: text_delta,
+            tool_calls: tool_delta,
+        }
+    }
+
+    fn mark_finished(&mut self) {
+        self.finished = true;
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct ToolConsentResolution {
     pub request_id: Uuid,
@@ -144,6 +239,7 @@ pub struct SessionController {
     missing_oauth_servers: Vec<String>,
     event_tx: Option<UnboundedSender<ControllerEvent>>,
     pending_tool_requests: HashMap<Uuid, PendingToolRequest>,
+    stream_states: HashMap<Uuid, StreamingMessageState>,
 }
 
 async fn build_tools(
@@ -471,6 +567,7 @@ impl SessionController {
             missing_oauth_servers,
             event_tx,
             pending_tool_requests: HashMap::new(),
+            stream_states: HashMap::new(),
         })
     }
 
@@ -1229,6 +1326,8 @@ impl SessionController {
         match self.provider.stream_prompt(request).await {
             Ok(stream) => {
                 let response_id = self.conversation.start_streaming_response();
+                self.stream_states
+                    .insert(response_id, StreamingMessageState::new());
                 Ok(SessionOutcome::Streaming {
                     response_id,
                     stream,
@@ -1248,14 +1347,43 @@ impl SessionController {
     }
 
     pub fn apply_stream_chunk(&mut self, message_id: Uuid, chunk: &ChatResponse) -> Result<()> {
-        if chunk.message.has_tool_calls() {
-            self.conversation.set_tool_calls_on_message(
+        let state = self.stream_states.entry(message_id).or_default();
+
+        let diff = state.ingest(chunk);
+
+        if let Some(text_delta) = diff.text {
+            match text_delta.mode {
+                TextDeltaKind::Append => {
+                    self.conversation.append_stream_chunk(
                         message_id,
-                chunk.message.tool_calls.clone().unwrap_or_default(),
+                        &text_delta.content,
+                        chunk.is_final,
                     )?;
                 }
+                TextDeltaKind::Replace => {
+                    self.conversation.set_stream_content(
+                        message_id,
+                        text_delta.content,
+                        chunk.is_final,
+                    )?;
+                }
+            }
+        } else if chunk.is_final {
             self.conversation
-            .append_stream_chunk(message_id, &chunk.message.content, chunk.is_final)
+                .append_stream_chunk(message_id, "", true)?;
+        }
+
+        if let Some(tool_calls) = diff.tool_calls {
+            self.conversation
+                .set_tool_calls_on_message(message_id, tool_calls)?;
+        }
+
+        if chunk.is_final {
+            state.mark_finished();
+            self.stream_states.remove(&message_id);
+        }
+
+        Ok(())
     }
 
     pub fn check_streaming_tool_calls(&mut self, message_id: Uuid) -> Option<Vec<ToolCall>> {
@@ -1339,6 +1467,7 @@ impl SessionController {
     }
 
     pub fn cancel_stream(&mut self, message_id: Uuid, notice: &str) -> Result<()> {
+        self.stream_states.remove(&message_id);
         self.conversation
             .cancel_stream(message_id, notice.to_string())
     }
@@ -1376,10 +1505,12 @@ impl SessionController {
 
     pub fn start_new_conversation(&mut self, model: Option<String>, name: Option<String>) {
         self.conversation.start_new(model, name);
+        self.stream_states.clear();
     }
 
     pub fn clear(&mut self) {
         self.conversation.clear();
+        self.stream_states.clear();
     }
 
     pub async fn generate_conversation_description(&self) -> Result<String> {
@@ -1403,6 +1534,29 @@ mod tests {
     use std::sync::Arc;
     use tempfile::tempdir;
 
+    fn make_response(
+        text: &str,
+        tool_calls: Option<Vec<ToolCall>>,
+        is_final: bool,
+    ) -> ChatResponse {
+        let mut message = Message::assistant(text.to_string());
+        message.tool_calls = tool_calls;
+        ChatResponse {
+            message,
+            usage: None,
+            is_streaming: true,
+            is_final,
+        }
+    }
+
+    fn make_tool_call(id: &str, name: &str) -> ToolCall {
+        ToolCall {
+            id: id.to_string(),
+            name: name.to_string(),
+            arguments: serde_json::json!({}),
+        }
+    }
+
     const SERVER_NAME: &str = "oauth-test";
 
     fn build_oauth_config(server: &MockServer) -> McpOAuthConfig {
@@ -1441,6 +1595,47 @@ mod tests {
         config
     }
 
+    #[test]
+    fn streaming_state_tracks_text_deltas() {
+        let mut state = StreamingMessageState::new();
+
+        let diff = state.ingest(&make_response("Hello", None, false));
+        let first = diff.text.expect("text diff");
+        assert_eq!(first.content, "Hello");
+        assert_eq!(first.mode, TextDeltaKind::Append);
+
+        let diff = state.ingest(&make_response("Hello world", None, false));
+        let second = diff.text.expect("second diff");
+        assert_eq!(second.content, " world");
+        assert_eq!(second.mode, TextDeltaKind::Append);
+
+        let diff = state.ingest(&make_response("Hi", None, false));
+        let third = diff.text.expect("third diff");
+        assert_eq!(third.content, "Hi");
+        assert_eq!(third.mode, TextDeltaKind::Replace);
+    }
+
+    #[test]
+    fn streaming_state_detects_tool_call_changes() {
+        let mut state = StreamingMessageState::new();
+        let tool = make_tool_call("call-1", "web.search");
+
+        let diff = state.ingest(&make_response("", Some(vec![tool.clone()]), false));
+        let calls = diff.tool_calls.expect("initial tool call");
+        assert_eq!(calls.len(), 1);
+        assert_eq!(calls[0].name, "web.search");
+
+        let diff = state.ingest(&make_response("", Some(vec![tool.clone()]), false));
+        assert!(
+            diff.tool_calls.is_none(),
+            "duplicate tool call should not emit"
+        );
+
+        let diff = state.ingest(&make_response("", Some(vec![]), false));
+        let cleared = diff.tool_calls.expect("clearing tool calls");
+        assert!(cleared.is_empty());
+    }
+
     async fn build_session(server: &MockServer) -> (SessionController, tempfile::TempDir) {
         unsafe {
             std::env::set_var("OWLEN_MASTER_PASSWORD", "test-password");