feat: fork session on --continue to prevent context contamination (#244)

When cc-connect starts up and picks up the most recent CLI session via
--continue, it now adds --fork-session to create an independent context
branch. This prevents history interleaving when the user is concurrently
working in the same workspace from both the CLI and a messaging platform.

Without this, resuming into an active CLI session corrupts both context
windows — the model sees interleaved tool calls and responses from two
different conversations, leading to hallucinations about files and state.

The fork only applies to --continue (first connection after startup,
where we grab "whatever is latest" which may be an active CLI session).
Plain --resume of cc-connect's own stored session ID is unaffected,
since that session belongs to cc-connect and has no interleaving risk.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/claudecode/session.go

@@ -52,7 +52,17 @@ func newClaudeSession(ctx context.Context, workDir, model, sessionID, mode strin
 	if mode != "" && mode != "default" {
 		args = append(args, "--permission-mode", mode)
 	}
-	if sessionID != "" {
+	switch sessionID {
+	case "":
+		// Truly fresh session — no resume, no continue.
+	case core.ContinueSession:
+		// --continue grabs the most recent session in the workspace, which
+		// may belong to an active CLI terminal. Fork it so the platform
+		// conversation gets its own independent context branch.
+		args = append(args, "--continue", "--fork-session")
+	default:
+		// Resuming a known session ID — this is cc-connect's own session
+		// from a previous connection, safe to resume directly.
 		args = append(args, "--resume", sessionID)
 	}
 	if model != "" {

core/engine.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 	"unicode/utf8"
 )
@@ -192,6 +193,8 @@ type Engine struct {
 
 	quietMu sync.RWMutex
 	quiet   bool // when true, suppress thinking and tool progress messages globally
+
+	hasConnectedOnce atomic.Bool // first connection uses --continue to bridge CLI usage
 }
 
 // workspaceInitFlow tracks a channel that is being onboarded to a workspace.
@@ -1739,9 +1742,17 @@ func (e *Engine) getOrCreateInteractiveStateWith(sessionKey string, p Platform,
 		return state
 	}
 
-	agentSID := session.GetAgentSessionID()
+	// On first connection after engine startup, use --continue to pick up
+	// the most recent CLI session (bridges direct CLI and cc-connect usage).
+	// Subsequent connections respect the stored session ID (or "" for fresh).
+	startSessionID := session.GetAgentSessionID()
+	if !e.hasConnectedOnce.Swap(true) {
+		startSessionID = ContinueSession
+	}
+
+	isResume := startSessionID != ""
 	startAt := time.Now()
-	agentSession, err := agent.StartSession(e.ctx, agentSID)
+	agentSession, err := agent.StartSession(e.ctx, startSessionID)
 	startElapsed := time.Since(startAt)
 	if err != nil {
 		slog.Error("failed to start interactive session", "error", err, "elapsed", startElapsed)
@@ -1750,7 +1761,7 @@ func (e *Engine) getOrCreateInteractiveStateWith(sessionKey string, p Platform,
 		return state
 	}
 	if startElapsed >= slowAgentStart {
-		slog.Warn("slow agent session start", "elapsed", startElapsed, "agent", agent.Name(), "session_id", agentSID)
+		slog.Warn("slow agent session start", "elapsed", startElapsed, "agent", agent.Name(), "session_id", startSessionID)
 	}
 
 	if newID := agentSession.CurrentSessionID(); newID != "" {
@@ -1767,7 +1778,7 @@ func (e *Engine) getOrCreateInteractiveStateWith(sessionKey string, p Platform,
 	}
 	e.interactiveStates[sessionKey] = state
 
-	slog.Info("interactive session started", "session_key", sessionKey, "agent_session", session.GetAgentSessionID(), "elapsed", startElapsed)
+	slog.Info("session spawned", "session_key", sessionKey, "agent_session", session.GetAgentSessionID(), "is_resume", isResume, "elapsed", startElapsed)
 	return state
 }
 

core/message.go

@@ -155,6 +155,11 @@ const (
 	EventThinking          EventType = "thinking"           // thinking/processing status
 )
 
+// ContinueSession is a sentinel value passed to Agent.StartSession to indicate
+// that the agent should pick up the most recent session in the workspace
+// (equivalent to `claude --continue`), rather than resuming a specific session ID.
+const ContinueSession = "__continue__"
+
 // UserQuestion represents a structured question from AskUserQuestion.
 type UserQuestion struct {
 	Question    string             `json:"question"`