Add reply file for agent chaining

reply holds only the clean assistant text from the most recently
completed turn. It is written atomically when Submit returns and cleared
(as a documented side-effect) when a new prompt is submitted. Stat
reports the real byte length and bumps Qid.Vers on each change, so
polling via stat size > 0 is the intended signal that a reply is ready.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -8,8 +8,9 @@ A 9P server that exposes [ollie](../ollie) agent sessions as a virtual filesyste
 ollie/
   ctl                   write: "new [backend=x] [model=x] [agent=x]" | "kill <session-id>"
   <session-id>/
-    prompt              write: submit a prompt to the agent
+    prompt              write: submit a prompt to the agent (clears reply)
     chat                read:  cumulative conversation history
+    reply               read:  assistant text from the most recent turn only
     state               read:  current agent state (idle, thinking, calling: <tool>)
     ctl                 write: stop | interrupt | /<slash-command>
     backend             r/w:   active backend name
@@ -122,7 +123,7 @@ echo "kill <session-id>" > ~/mnt/ollie/ctl
 
 **Remote access** — 9P is a network protocol. Export the namespace over the network and access agent sessions from another machine using the same file interface, with no additional daemon or API layer.
 
-**Agent chaining** — one agent's `chat` output piped into another session's `prompt`. Model composition at the shell level.
+**Agent chaining** — pipe one agent's `reply` into another session's `prompt`. Poll until `reply` is non-empty (stat size > 0), consume it, submit to the next agent. Writing to `prompt` clears `reply` as a side-effect, so the handoff is clean.
 
 **Hooks and watchers** — poll `state` in a loop to trigger actions when the agent finishes, or use `tail -f chat` to react to new output. Build lightweight event-driven pipelines with standard shell tools.
 

internal/p9/server.go

@@ -58,6 +58,10 @@ type session struct {
 	// state is the current agent state: "idle", "thinking", "calling: <tool>"
 	state     string
 	modelName string
+	// reply holds only the assistant text from the most recently completed
+	// turn. Cleared when a new prompt is submitted. Read-only; 0444.
+	reply     []byte
+	replyVers uint32
 }
 
 func (sess *session) setState(s string) {
@@ -204,7 +208,7 @@ func (s *Server) pathType(path string) string {
 			return ""
 		}
 		switch parts[1] {
-		case "ctl", "prompt", "chat", "backend", "agent", "model", "state":
+		case "ctl", "prompt", "chat", "reply", "backend", "agent", "model", "state":
 			return "file"
 		}
 	}
@@ -327,9 +331,12 @@ func (s *Server) read(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
 	}
 
-	// chat is served directly from the session's append-only log.
+	// chat and reply are served directly from session buffers by offset.
-	if pathBase(path) == "chat" {
+	switch pathBase(path) {
+	case "chat":
 		return s.readChat(fc, path)
+	case "reply":
+		return s.readReply(fc, path)
 	}
 
 	content := s.readFile(path)
@@ -376,6 +383,35 @@ func (s *Server) readChat(fc *plan9.Fcall, path string) *plan9.Fcall {
 	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
 }
 
+// readReply serves bytes from the session's reply buffer at the requested offset.
+func (s *Server) readReply(fc *plan9.Fcall, path string) *plan9.Fcall {
+	parts := strings.SplitN(strings.TrimPrefix(path, "/"), "/", 2)
+	if len(parts) != 2 {
+		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: 0}
+	}
+	s.mu.RLock()
+	sess := s.sessions[parts[0]]
+	s.mu.RUnlock()
+	if sess == nil {
+		return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: 0}
+	}
+
+	sess.mu.RLock()
+	buf := sess.reply
+	sess.mu.RUnlock()
+
+	var data []byte
+	off := int(fc.Offset)
+	if off < len(buf) {
+		end := off + int(fc.Count)
+		if end > len(buf) {
+			end = len(buf)
+		}
+		data = buf[off:end]
+	}
+	return &plan9.Fcall{Type: plan9.Rread, Tag: fc.Tag, Count: uint32(len(data)), Data: data}
+}
+
 // readFile returns the text content for a readable session file.
 func (s *Server) readFile(path string) string {
 	parts := strings.SplitN(strings.TrimPrefix(path, "/"), "/", 2)
@@ -507,8 +543,24 @@ func (s *Server) handleWrite(path, input string) {
 	switch fileName {
 	case "prompt":
 		sess.appendChat([]byte("user: " + input + "\n"))
+		// Clear reply before submission — documented side-effect.
+		sess.mu.Lock()
+		sess.reply = nil
+		sess.replyVers++
+		sess.mu.Unlock()
 		sess.setState("thinking")
-		sess.core.Submit(sess.ctx, input, publish)
+		var replyBuf []byte
+		sess.core.Submit(sess.ctx, input, func(ev agent.Event) {
+			if ev.Role == "assistant" {
+				replyBuf = append(replyBuf, []byte(ev.Content)...)
+			}
+			publish(ev)
+		})
+		// Atomically publish reply on turn completion.
+		sess.mu.Lock()
+		sess.reply = replyBuf
+		sess.replyVers++
+		sess.mu.Unlock()
 		sess.setState("idle")
 
 	case "ctl":
@@ -786,6 +838,7 @@ func (s *Server) readDir(path string, offset uint64, count uint32) []byte {
 			{"ctl", 0200},
 			{"prompt", 0200},
 			{"chat", 0444},
+			{"reply", 0444},
 			{"state", 0444},
 			{"backend", 0666},
 			{"agent", 0666},
@@ -848,7 +901,7 @@ func (s *Server) makeStat(path string) plan9.Dir {
 		switch base {
 		case "ctl", "prompt":
 			mode = 0200
-		case "chat", "state":
+		case "chat", "reply", "state":
 			mode = 0444
 		case "backend", "agent", "model":
 			mode = 0666
@@ -868,7 +921,7 @@ func (s *Server) makeStat(path string) plan9.Dir {
 
 	// For chat files, report the actual log size and Qid version so that
 	// polling tools (tail -f) can detect growth via stat.
-	if base == "chat" {
+	if base == "chat" || base == "reply" {
 		parts := strings.SplitN(strings.TrimPrefix(path, "/"), "/", 2)
 		if len(parts) == 2 {
 			s.mu.RLock()
@@ -876,8 +929,13 @@ func (s *Server) makeStat(path string) plan9.Dir {
 			s.mu.RUnlock()
 			if sess != nil {
 				sess.mu.RLock()
-				dir.Length = uint64(len(sess.chatLog))
+				if base == "chat" {
-				dir.Qid.Vers = sess.chatVers
+					dir.Length = uint64(len(sess.chatLog))
+					dir.Qid.Vers = sess.chatVers
+				} else {
+					dir.Length = uint64(len(sess.reply))
+					dir.Qid.Vers = sess.replyVers
+				}
 				sess.mu.RUnlock()
 			}
 		}