agent/new: fork_at= parameter for historical context forking

Sub-agents can now inherit parent context truncated at a specific
user turn: fork_at=5 means 'fork from the state after the 5th
user message.' Enables backtracking without losing unsummarized
context from before a failed approach.

Without fork_at (or fork_at=0), full history is inherited as before.

cmd/olliesrv/internal/fs/spec.go

@@ -345,13 +345,17 @@ func buildSessionChildren(
 			virtfs.FileNode("new", 0666,
 				virtfs.Doc("Create agent. With prompt=, runs as sub-agent: blocks until done, returns reply."),
 				virtfs.Rdwr(func(ctx context.Context, data []byte) ([]byte, error) {
-					params, prompt, parentID := parseAgentNewRequest(data)
+					params, prompt, parentID, forkAt := parseAgentNewRequest(data)
 					params.ParentID = parentID
 
-					// Sub-agent inherits parent's context.
+					// Sub-agent inherits parent's context (optionally truncated at fork_at).
 					if prompt != "" && parentID != "" {
 						if parent := s.FindAgent(parentID); parent != nil {
-							params.History = agent.RestoreHistoryFromMessages(parent.Messages())
+							msgs := parent.Messages()
+							if forkAt > 0 {
+								msgs = messagesUpToTurn(msgs, forkAt)
+							}
+							params.History = agent.RestoreHistoryFromMessages(msgs)
 						}
 					}
 

cmd/olliesrv/internal/fs/support.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 
 	"ollie/cmd/olliesrv/internal/agent"
+	"ollie/cmd/olliesrv/internal/backend"
 	"ollie/cmd/olliesrv/internal/session"
 	"ollie/toolsrv/protocol"
 	"ollie/virtfs"
@@ -90,11 +91,11 @@ var (
 //   - Space-separated key=value (legacy format from GUI)
 //
 // A bare word (no =) is treated as the profile name (legacy positional arg).
-// Returns AgentParams, the prompt (empty if not a sub-agent request), and the parent agent ID.
-func parseAgentNewRequest(data []byte) (session.AgentParams, string, string) {
+// Returns AgentParams, prompt, parent agent ID, and fork_at (user-turn index, 0=all).
+func parseAgentNewRequest(data []byte) (session.AgentParams, string, string, int) {
 	s := strings.TrimSpace(string(data))
 	if s == "" {
-		return session.AgentParams{Profile: "default"}, "", ""
+		return session.AgentParams{Profile: "default"}, "", "", 0
 	}
 
 	var tokens []string
@@ -106,6 +107,7 @@ func parseAgentNewRequest(data []byte) (session.AgentParams, string, string) {
 
 	p := session.AgentParams{Profile: "default"}
 	var prompt, parent string
+	var forkAt int
 	for _, tok := range tokens {
 		tok = strings.TrimSpace(tok)
 		if tok == "" {
@@ -130,10 +132,27 @@ func parseAgentNewRequest(data []byte) (session.AgentParams, string, string) {
 				prompt = v
 			case "parent":
 				parent = v
+			case "fork_at":
+				fmt.Sscanf(v, "%d", &forkAt)
 			}
 		} else if p.Profile == "default" {
 			p.Profile = tok
 		}
 	}
-	return p, prompt, parent
+	return p, prompt, parent, forkAt
+}
+
+// messagesUpToTurn returns messages up to (and including) the Nth user turn.
+// Turn 1 = first user message and everything before the second user message.
+func messagesUpToTurn(msgs []backend.Message, n int) []backend.Message {
+	userCount := 0
+	for i, m := range msgs {
+		if m.Role == "user" {
+			userCount++
+			if userCount > n {
+				return msgs[:i]
+			}
+		}
+	}
+	return msgs
 }