diff --git a/mod/content/overtake_probe.gs.lua b/mod/content/overtake_probe.gs.lua
index 1b9d2c5..b040d11 100644
--- a/mod/content/overtake_probe.gs.lua
+++ b/mod/content/overtake_probe.gs.lua
@@ -4,5 +4,8 @@ function data()
 		updateScript = {
 			fileName = "overtake_probe.script@update",
 		},
+		guiUpdateScript = {
+			fileName = "overtake_probe.script@guiUpdate",
+		},
 	}
 end
diff --git a/mod/content/overtake_probe.script.tl b/mod/content/overtake_probe.script.tl
index 455a655..1afc6c9 100644
--- a/mod/content/overtake_probe.script.tl
+++ b/mod/content/overtake_probe.script.tl
@@ -8,7 +8,7 @@
 local TAG = "[overtake_probe] "
 local INTERVAL = 2.0
 local MAX_GAP = 40.0
-local MAX_REBUILDS = 3 -- hand-built proposals were silently rejected; now via api.engine.util.proposal.replaceSegment
+local MAX_REBUILDS = 0 -- off while testing the GUI-thread position flip (replaceSegment works, see vehicle-probe.md)
 local MAX_SETLINES = 0 -- set-line disabled: lanes ahead get re-planned by the engine anyway, no forced lane change
 local MAX_JUMPS = 0 -- dead: sendCommand(makeComponentExchangeCmd(...)) fails in engine state even without a callback
 local JUMP_AHEAD = 15.0
@@ -207,7 +207,94 @@ local function trace(state : GameScriptState<ProbeState>)
 	state:set(st)
 end
 
-local ret : GameScript<ProbeState> = {
+-- EXPERIMENT (GUI thread, where sendCommand callbacks work): flip the positions of a follower and its leader by
+-- exchanging their MovePath components (route + position) with api.cmd.debug.makeComponentExchangeCmd.
+local MAX_FLIPS = 2
+local FLIP_SCAN_FRAMES = 60 -- scan for a pair about once per second
+local FLIP_TRACE_FRAMES = 600 -- then log both vehicles every 10 frames for ~10 s
+
+local record GuiState
+	frames : integer
+	flips : integer
+	traceLeft : integer
+	a : integer
+	b : integer
+end
+
+local function movePathOf(v : integer) : Engine.Component.MovePath
+	return api.engine.getComponent(v, api.type.ComponentType.MOVE_PATH) as Engine.Component.MovePath
+end
+
+local function flipLine(v : integer) : string
+	local mp = movePathOf(v)
+	if mp == nil then return "veh=" .. v .. " no MovePath" end
+	local edge, _dir = edgeAt(mp)
+	return string.format("veh=%d seg=%s lane=%s pos=%.1f v=%.2f", v, tostring(edge and edge.entity), tostring(edge and edge.index), mp.dyn.pathPos.pos, mp.dyn.speed)
+end
+
+local function sendFlip(gs : GuiState, follower : integer, leader : integer)
+	local mpF, mpL = movePathOf(follower), movePathOf(leader)
+	p("FLIP_TEST before " .. flipLine(follower) .. " | " .. flipLine(leader))
+	local function done(label : string) : function(any, boolean)
+		return function(res : any, success : boolean)
+			p(string.format("FLIP_TEST result %s success=%s res=%s", label, tostring(success), tostring(res)))
+		end
+	end
+	api.cmd.sendCommand(api.cmd.debug.makeComponentExchangeCmd(follower, mpL as Engine.Component), done("follower<-leader"))
+	api.cmd.sendCommand(api.cmd.debug.makeComponentExchangeCmd(leader, mpF as Engine.Component), done("leader<-follower"))
+	gs.flips = gs.flips + 1
+	gs.traceLeft = FLIP_TRACE_FRAMES
+	gs.a, gs.b = follower, leader
+end
+
+-- same pair rule as the engine-side probe: same segment + lane, follower < MAX_GAP behind a moving leader
+local function findPair() : integer, integer
+	local player = api.engine.util.getPlayer()
+	local byLane : {string : {Car}} = {}
+	for _, v in ipairs(api.engine.getEntitiesWithComponent(api.type.ComponentType.TRANSPORT_VEHICLE, { requireOwnedByPlayer = player })) do
+		local lv = api.engine.getComponent(v, api.type.ComponentType.LAND_VEHICLE) as Engine.Component.LandVehicle
+		local mp = movePathOf(v)
+		if lv ~= nil and not lv.isTram and mp ~= nil then
+			local edge, dir = edgeAt(mp)
+			if edge ~= nil then
+				local key = edge.entity .. ":" .. edge.index
+				byLane[key] = byLane[key] or {}
+				table.insert(byLane[key], { v = v, pos = mp.dyn.pathPos.pos, pos01 = mp.dyn.pathPos.pos01, speed = mp.dyn.speed, dir = dir })
+			end
+		end
+	end
+	for _, cars in pairs(byLane) do
+		for i = 1, #cars do
+			for j = 1, #cars do
+				local a, b = cars[i], cars[j]
+				local ahead = (a.dir and b.pos > a.pos) or (not a.dir and b.pos < a.pos)
+				if i ~= j and ahead and math.abs(b.pos - a.pos) < MAX_GAP and b.speed > 1.0 then
+					return a.v, b.v
+				end
+			end
+		end
+	end
+	return nil, nil
+end
+
+local function guiStep(gsState : GameScriptState<GuiState>)
+	local gs = gsState:get()
+	gs.frames = (gs.frames or 0) + 1
+	gs.flips = gs.flips or 0
+	gs.traceLeft = gs.traceLeft or 0
+	if gs.traceLeft > 0 then
+		if gs.traceLeft % 10 == 0 then
+			p(string.format("FLIP_TRACE f=%d %s | %s", FLIP_TRACE_FRAMES - gs.traceLeft, flipLine(gs.a), flipLine(gs.b)))
+		end
+		gs.traceLeft = gs.traceLeft - 1
+	elseif gs.flips < MAX_FLIPS and gs.frames % FLIP_SCAN_FRAMES == 0 then
+		local follower, leader = findPair()
+		if follower ~= nil then sendFlip(gs, follower, leader) end
+	end
+	gsState:set(gs)
+end
+
+local ret : GameScriptWithGui<ProbeState, GuiState> = {
 	update = function(_userParams : table, state : GameScriptState<ProbeState>, dt : number) : nil
 		ticks = ticks + 1
 		elapsed = elapsed + dt
@@ -217,6 +304,9 @@ local ret : GameScript<ProbeState> = {
 		elapsed = 0.0
 		xpcall(dump, function(err : any) p("ERROR " .. tostring(err)) end, dt, state)
 	end,
+	guiUpdate = function(_userParams : table, _state : GameScriptStateReadOnly<ProbeState>, guiState : GameScriptState<GuiState>)
+		xpcall(guiStep, function(err : any) p("ERROR gui " .. tostring(err)) end, guiState)
+	end,
 }
 
 return ret