diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 025c03173..b3e92144e 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -2511,10 +2511,11 @@ func (al *AgentLoop) executeToolCalls(
 			})
 
 		// Heartbeat lazy worktree: create worktree on first write-tool call
+		// Always use ai.Workspace (not GetPlanWorkDir) to avoid creating worktrees
+		// against stale project paths from previous plans.
 		if opts.Background && isWriteTool(tc.Name) && !agent.IsInWorktree(opts.SessionKey) {
 			taskName := "heartbeat-" + time.Now().Format("20060102")
-			hbDir := agent.ContextBuilder.GetPlanWorkDir()
-			if wt, wtErr := agent.ActivateWorktree(opts.SessionKey, taskName, hbDir); wtErr == nil {
+			if wt, wtErr := agent.ActivateWorktree(opts.SessionKey, taskName, agent.Workspace); wtErr == nil {
 				logger.InfoCF("agent", "Heartbeat worktree created", map[string]any{"branch": wt.Branch})
 			}
 		}
diff --git a/pkg/tools/workspace_ctx.go b/pkg/tools/workspace_ctx.go
index 9460431a9..b8657647f 100644
--- a/pkg/tools/workspace_ctx.go
+++ b/pkg/tools/workspace_ctx.go
@@ -61,6 +61,16 @@ func resolveFS(ctx context.Context, fs fileSystem, path string) fileSystem {
 		return fs
 	}
 
+	// Absolute paths under the original workspace use original fs
+	// (e.g. heartbeat reading workspace files while in a worktree)
+	if filepath.IsAbs(path) {
+		if sfs, ok := fs.(*sandboxFs); ok {
+			if strings.HasPrefix(path, sfs.workspace+"/") || path == sfs.workspace {
+				return fs
+			}
+		}
+	}
+
 	// Only sandboxFs supports workspace override
 
 	if sfs, ok := fs.(*sandboxFs); ok {