From 7b1e796c4872962d3fdd038aa8b136d118840ae5 Mon Sep 17 00:00:00 2001
From: bryanthaboi <bryanthaboi@theboisclub.com>
Date: Mon, 17 Aug 2026 22:52:33 -0400
Subject: [PATCH] CLOSES #1496

---
 README.md                     |   3 +-
 docs/link-security.md         | 120 +++++++++++
 src/core/Game.lua             |  37 +++-
 src/link/Handshake.lua        |   8 +-
 src/link/Json.lua             |  18 +-
 src/link/LinkBattle.lua       |   4 +-
 src/link/Net.lua              |  24 ++-
 src/link/Protocol.lua         | 131 +++++++----
 src/link/Session.lua          |  21 +-
 src/link/Wire.lua             | 344 +++++++++++++++++++++++++++++
 tests/engine/link_session.lua |  42 ++--
 tests/link_desync_fuzz.lua    |  58 ++++-
 tests/link_hostile.lua        | 396 ++++++++++++++++++++++++++++++++++
 tests/run_link_tests.lua      |   8 +
 14 files changed, 1124 insertions(+), 90 deletions(-)
 create mode 100644 docs/link-security.md
 create mode 100644 src/link/Wire.lua
 create mode 100644 tests/link_hostile.lua

diff --git a/README.md b/README.md
index 108c7dd2..0083d402 100644
--- a/README.md
+++ b/README.md
@@ -354,7 +354,8 @@ request with real detail is one that can actually get built.
 - [Save editor](https://github.com/bryanthaboi/gen1recomp/wiki/Guide-Save-Editor)
 — edit party, boxes, items, events, and Pokédex flags outside the game.
 - `docs/architecture.md` — runtime details;
-`docs/behavior-porting-notes.md` — formula provenance.
+`docs/behavior-porting-notes.md` — formula provenance;
+`docs/link-security.md` — what link play defends against, and what it doesn't.
 
 
 
diff --git a/docs/link-security.md b/docs/link-security.md
new file mode 100644
index 00000000..e4daf858
--- /dev/null
+++ b/docs/link-security.md
@@ -0,0 +1,120 @@
+# Link play: threat model and what the code actually guarantees
+
+Link play is the only part of this game that reads bytes written by
+somebody else. This is what it defends against, what it does not, and
+where each guarantee lives.
+
+## The boundary
+
+Everything a peer or the relay sends arrives as one JSON object per line.
+There is exactly one place it becomes a message:
+
+    src/link/Net.lua        reads bytes, frames lines, decodes JSON
+    src/link/Wire.lua       rebuilds each line as a typed message
+    src/link/Session.lua    the only path from a transport into a mode
+
+`Session:update` runs `Wire.sanitize` on every message before anything
+else sees it. A schema returns a **new** table holding only the fields it
+names, at the Lua types it names, so the rest of `src/link/` can read
+`msg.slot`, `msg.parts.actives` or `msg.mons[i].dvs.hp` directly and be
+right by construction. A message with no schema (a mod's, or a future
+build's) keeps a bounded, scalar-only copy of its payload instead of
+being dropped.
+
+A message that fails its schema is **dropped and logged**, never fatal.
+Latching a terminal failure would hand a hostile peer a cheaper
+disconnect than sending nothing at all.
+
+### Why the bounds are loose
+
+Wire's numeric bounds are deliberately wider than the game's own clamps in
+`Protocol.unpackMon`. Both peers run identical clamps over identical
+packets; a bound that bit an honest value would change one side's copy of
+a mon and desync the lockstep. Wire's job is types and sizes. Rules are
+`Protocol`'s job, and it keeps its own clamps for the callers that reach
+it without a Session (the mod API, `tests/`).
+
+### Containment behind it
+
+Assume something still gets through:
+
+- `Game:step` pcalls the link pump, and pcalls `stack:update` **only
+  while a link session is active**. On a throw, `Game:breakLink` closes
+  the connection, unwinds to the overworld and says "The link was
+  broken." Outside link play the stack is unguarded on purpose: a blanket
+  pcall would swallow real engine bugs and leave the game silently wrong
+  instead of loudly broken.
+- `Net` caps `rxBuf` at 256KB and its per-frame read at 512KB, so a peer
+  that never sends a newline ends as a clean disconnect.
+- `Json.decode` refuses documents nested past 64 levels, and takes an
+  optional length cap that the link path passes and the mod-manifest path
+  does not.
+
+## The relay (`../pokeserver`)
+
+- A line that is not a JSON **object** with a string `type` is dropped
+  before any handler runs, and `onLine` is wrapped in try/catch.
+  `server.js` installs `uncaughtException`/`unhandledRejection` handlers:
+  one bad packet must never take every live match down with the process.
+- Line buffers are capped, lines per second are capped, connections per
+  IP and in total are capped, and an unbound connection that never hosts
+  or joins is swept after 30s.
+- `SERVER_ONLY` is the set of message types the server is the only
+  legitimate author of (`peer_gone`, `bracket_update`, `match_start`,
+  `tournament_over`, `spectate`, ...). A peer that sends one has them
+  dropped rather than forwarded, so a bracket opponent cannot forge a
+  tournament result or fake "your opponent left".
+- Trainer names are reduced to a printable subset and capped at the same
+  10 characters the game enforces, on the way in, because they are
+  rendered by the dashboard and broadcast to every participant.
+
+`pokeserver/test/hostile.js` is the regression net for all of that.
+
+## What is NOT defended
+
+**Party legality is trust-the-client.** Online play meets strangers, and
+`Handshake.onlineAllowed` is a Lua function in the same VM the mods load
+into. It cannot be made tamper-proof in-process, and pretending otherwise
+would only cost honest mod authors. What lockstep and
+`Protocol.unpackMon`'s recompute-from-species-data *do* guarantee is that
+a cheater cannot invent stats, moves, or a shiny: every derived value is
+rebuilt locally from real species data. They can send a legal party they
+farmed or edited. That is the honest boundary.
+
+What the relay does instead is **observe and record**. It already sees
+every `hello`, so it keeps each connection's self-reported
+`engineVersion`, `fingerprint` and `linkModified`, compares the two sides
+of a room or a live tournament match, and logs and surfaces a
+`modded` / `fingerprint_mismatch` / `version_skew` flag on the dashboard.
+A patched client can still lie; what it cannot do is lie without the
+tournament organizer having a record of it.
+
+Client-side attestation is deliberately not built. This is an
+open-source Lua game: it would be theater, and it would break honest
+mods.
+
+**The relay has no TLS.** Port 7778 is plaintext, so party contents,
+trades and trainer names are visible to anyone on the network path. There
+is nothing secret in a Pokemon party, but it is a real property of the
+system and not an oversight. Fixing it means a TLS terminator in front of
+the relay and a client that speaks it, which is a version break for every
+shipped build.
+
+**The dashboard has no default password.** `DASHBOARD_PASSWORD` is
+required; with it unset the relay runs and the dashboard simply does not
+start. It is still Basic Auth over plain HTTP, so it belongs behind an
+IP restriction or an SSH tunnel (`pokeserver/DEPLOY.md`).
+
+## Tests
+
+    luajit tests/link_hostile.lua        every message type x every wrong type
+    luajit tests/link_desync_fuzz.lua    lockstep fuzz, plus a mutation mode
+    luajit tests/run_link_tests.lua      both of the above, plus the rest
+    cd ../pokeserver && npm test         relay smoke, 16-player bracket, hostile
+
+`tests/link_hostile.lua` builds its corpus from a template per message
+type, replaces each field (and several nested ones) with every wrong Lua
+type, and drives the survivors through the real trade session, a real
+lockstep battle, a real spectator battle, and the tournament screen
+**including its draw** -- because the two nastiest payloads are
+delayed-fuse ones that crash on render rather than on receipt.
diff --git a/src/core/Game.lua b/src/core/Game.lua
index 1b6019e1..1c01d889 100644
--- a/src/core/Game.lua
+++ b/src/core/Game.lua
@@ -222,6 +222,26 @@ function Game:touchSkinHotkey(action, pressed)
   end
 end
 
+function Game:breakLink(err)
+  Logger.error("link: torn down after an error\n%s", tostring(err))
+  self.linkSession = nil
+  local net = self.linkNet
+  self.linkNet = nil
+  if net then pcall(net.close, net) end
+  pcall(ModRuntime.emit, "link.ended", { reason = "error" })
+  local stack = self.stack
+  local guard = 0
+  while #stack.states > 1 and stack:top() ~= self.overworld and guard < 64 do
+    guard = guard + 1
+    pcall(stack.pop, stack)
+  end
+  pcall(function()
+    local Strings = require("src.core.Strings")
+    local TextBox = require("src.render.TextBox")
+    stack:push(TextBox.new(self, Strings("The link was\nbroken.")))
+  end)
+end
+
 function Game:step(dt)
   -- Tool mods (autoplay, accessibility drivers, input visualizers) act on
   -- the same fixed-step boundary as a physical controller.  Run them before
@@ -249,9 +269,22 @@ function Game:step(dt)
   -- stall just because PartyMenu/ChoiceBox/NamingScreen is temporarily
   -- on top of BattleState (see LinkBattle.new)
   if self.linkNet and not self.linkNet.closed then
-    self.linkNet:update()
+    local ok, err = pcall(self.linkNet.update, self.linkNet)
+    if not ok then
+      self:breakLink(err)
+      return
+    end
+  end
+  if self.linkSession or self.linkNet then
+    local ok, err = xpcall(function() self.stack:update(dt) end,
+      function(e) return debug.traceback(tostring(e), 2) end)
+    if not ok then
+      self:breakLink(err)
+      return
+    end
+  else
+    self.stack:update(dt)
   end
-  self.stack:update(dt)
   -- play time for the trainer card / save screen
   self.save.playTime = (self.save.playTime or 0) + dt
   -- Music.update is NOT serviced here: it decrements fade counters and
diff --git a/src/link/Handshake.lua b/src/link/Handshake.lua
index cea7b147..876a79d6 100644
--- a/src/link/Handshake.lua
+++ b/src/link/Handshake.lua
@@ -270,7 +270,10 @@ end
 
 local function index(mods)
   local byId = {}
-  for _, mod in ipairs(mods or {}) do byId[tostring(mod.id)] = mod end
+  if type(mods) ~= "table" then return byId end
+  for _, mod in ipairs(mods) do
+    if type(mod) == "table" then byId[tostring(mod.id)] = mod end
+  end
   return byId
 end
 
@@ -327,7 +330,8 @@ end
 -- lines for the incompatibility screen: what differs, then what still works
 function Handshake.describe(localHello, remoteHello, verdict, mode)
   local lines = {}
-  local peer = (remoteHello and remoteHello.name) or "THEY"
+  local peerName = remoteHello and remoteHello.name
+  local peer = type(peerName) == "string" and peerName or "THEY"
   if verdict == "refused" then
     -- checked before the v1 arm for the same reason checkCompat checks it
     -- first: a Gen 1 peer meeting a Gen 2 one has no `protocol` to read yet
diff --git a/src/link/Json.lua b/src/link/Json.lua
index b6bd58a4..6f2fca1a 100644
--- a/src/link/Json.lua
+++ b/src/link/Json.lua
@@ -4,6 +4,8 @@
 
 local Json = {}
 
+Json.MAX_DEPTH = 64
+
 local function encodeValue(v, out)
   local t = type(v)
   if v == nil then
@@ -109,7 +111,9 @@ local function decodeString(s, i)
   error("unterminated string")
 end
 
-decodeValue = function(s, i)
+decodeValue = function(s, i, depth)
+  depth = (depth or 0) + 1
+  assert(depth <= Json.MAX_DEPTH, "json nested too deeply")
   i = skipWs(s, i)
   local c = s:sub(i, i)
   if c == '"' then
@@ -124,7 +128,7 @@ decodeValue = function(s, i)
       i = skipWs(s, i)
       assert(s:sub(i, i) == ":", "expected :")
       local val
-      val, i = decodeValue(s, i + 1)
+      val, i = decodeValue(s, i + 1, depth)
       obj[key] = val
       i = skipWs(s, i)
       local d = s:sub(i, i)
@@ -138,7 +142,7 @@ decodeValue = function(s, i)
     if s:sub(i, i) == "]" then return arr, i + 1 end
     while true do
       local val
-      val, i = decodeValue(s, i)
+      val, i = decodeValue(s, i, depth)
       arr[#arr + 1] = val
       i = skipWs(s, i)
       local d = s:sub(i, i)
@@ -162,9 +166,13 @@ decodeValue = function(s, i)
   end
 end
 
-function Json.decode(s)
+function Json.decode(s, maxLength)
+  if type(s) ~= "string" then return nil, "json input is not a string" end
+  if maxLength and #s > maxLength then
+    return nil, ("json input is %d bytes (max %d)"):format(#s, maxLength)
+  end
   local ok, v = pcall(function()
-    local val = select(1, decodeValue(s, 1))
+    local val = select(1, decodeValue(s, 1, 0))
     return val
   end)
   if ok then return v end
diff --git a/src/link/LinkBattle.lua b/src/link/LinkBattle.lua
index 40c094d7..2d899738 100644
--- a/src/link/LinkBattle.lua
+++ b/src/link/LinkBattle.lua
@@ -29,7 +29,9 @@ local LinkBattle = {}
 -- Deterministic Park-Miller PRNG: both sides must roll identical
 -- streams, so love.math.random can't be used.
 local function makeRng(seed)
-  local s = seed % 2147483647
+  local s = tonumber(seed) or 1
+  if s ~= s or s == math.huge or s == -math.huge then s = 1 end
+  s = math.floor(s) % 2147483647
   if s <= 0 then s = s + 2147483646 end
   return function(a, b)
     s = (s * 16807) % 2147483647
diff --git a/src/link/Net.lua b/src/link/Net.lua
index b8d4654c..6d1ad4b2 100644
--- a/src/link/Net.lua
+++ b/src/link/Net.lua
@@ -40,6 +40,10 @@ Net.__index = Net
 Net.DEFAULT_PORT = 7777
 Net.DEFAULT_RELAY_ADDRESS = "147.182.215.255:7778"
 
+Net.MAX_LINE = 256 * 1024
+Net.MAX_RX_PER_FRAME = 512 * 1024
+Net.ENET_BANDWIDTH = 256 * 1024
+
 function Net.available()
   return enet ~= nil
 end
@@ -115,7 +119,8 @@ function Net:host(port)
     return false
   end
   port = tonumber(port) or Net.defaultPort()
-  local ok, h, err = pcall(enet.host_create, ("*:%d"):format(port), 2, 1)
+  local ok, h, err = pcall(enet.host_create, ("*:%d"):format(port), 2, 1,
+                           Net.ENET_BANDWIDTH, Net.ENET_BANDWIDTH)
   if not ok or not h then
     self.error = ("can't open UDP port %d (%s)"):format(
       port, tostring(ok and err or h))
@@ -255,7 +260,7 @@ local function handleGenericRelayControl(self, msg)
 end
 
 function Net:handleTCPLine(line)
-  local msg = Json.decode(line)
+  local msg = Json.decode(line, Net.MAX_LINE)
   if msg == nil then
     Logger.warn("link: bad relay message %q", line:sub(1, 60))
     return
@@ -277,6 +282,11 @@ function Net:drainLines()
     self.rxBuf = self.rxBuf:sub(nl + 1)
     if #line > 0 then self:handleTCPLine(line) end
   end
+  if #self.rxBuf > Net.MAX_LINE then
+    self.rxBuf = ""
+    self.error = Strings("The other side\nsent bad data.")
+    self.closed = true
+  end
 end
 
 -- non-blocking pump for the relay TCP backend: flush queued writes, drain
@@ -300,10 +310,14 @@ function Net:updateTCP()
       return
     end
   end
-  while true do
+  local budget = Net.MAX_RX_PER_FRAME
+  while budget > 0 do
     local data, err, partial = sock:receive(8192)
     local chunk = data or partial or ""
-    if #chunk > 0 then self.rxBuf = self.rxBuf .. chunk end
+    if #chunk > 0 then
+      self.rxBuf = self.rxBuf .. chunk
+      budget = budget - #chunk
+    end
     if err == "closed" then
       self.closed = true
       break
@@ -350,7 +364,7 @@ function Net:update()
         for _, msg in ipairs(queued) do self:send(msg) end
       end
     elseif event.type == "receive" then
-      local msg = Json.decode(event.data)
+      local msg = Json.decode(event.data, Net.MAX_LINE)
       if msg ~= nil then
         table.insert(self.inbox, msg)
       else
diff --git a/src/link/Protocol.lua b/src/link/Protocol.lua
index bdf072a4..5de59257 100644
--- a/src/link/Protocol.lua
+++ b/src/link/Protocol.lua
@@ -17,6 +17,25 @@ local Runtime = require("src.mods.Runtime")
 
 local Protocol = {}
 
+local MAX_WIRE_NAME = 40
+
+local function num(v, default)
+  local n = tonumber(v)
+  if n == nil or n ~= n or n == math.huge or n == -math.huge then
+    return default
+  end
+  return n
+end
+
+local function tbl(v)
+  return type(v) == "table" and v or {}
+end
+
+local function text(v)
+  if type(v) ~= "string" then return nil end
+  return v:sub(1, MAX_WIRE_NAME)
+end
+
 Protocol.hello = Handshake.hello
 Protocol.checkCompat = Handshake.checkCompat
 
@@ -78,6 +97,10 @@ function Protocol.unpackMon(data, packed, opts)
   local Stats = require("src.pokemon.Stats")
   local Growth = require("src.pokemon.Growth")
   local strict = opts and opts.strict
+  if type(packed) ~= "table" then
+    if strict then return nil, "unknown POKéMON" end
+    return nil
+  end
   -- forceLevel comes from an "auto-level" ruling.  The picker's ANY choice
   -- ("use each mon's real level", Gen1's only mode) is a string sentinel on
   -- the LinkState/Tournament side (see levelForWire) that must mean "no
@@ -91,7 +114,7 @@ function Protocol.unpackMon(data, packed, opts)
     if strict then return nil, "unknown POKéMON" end
     return nil
   end
-  local level = math.max(2, math.min(100, math.floor(packed.level or 5)))
+  local level = math.max(2, math.min(100, math.floor(num(packed.level, 5))))
   -- "auto-level" tournaments/matches: every participant's real level is
   -- ignored and everyone rebuilds at the same fixed level instead, so a
   -- Lv12 and a Lv100 party can battle on equal footing. Both sides pass
@@ -99,25 +122,27 @@ function Protocol.unpackMon(data, packed, opts)
   if forceLevel then
     level = math.max(2, math.min(100, math.floor(forceLevel)))
   end
+  local packedDvs, packedStatExp = tbl(packed.dvs), tbl(packed.statExp)
   local dvs = {}
   for _, k in ipairs({ "hp", "attack", "defense", "speed", "special" }) do
-    dvs[k] = math.max(0, math.min(15, math.floor((packed.dvs or {})[k] or 0)))
+    dvs[k] = math.max(0, math.min(15, math.floor(num(packedDvs[k], 0))))
   end
   local statExp = {}
   for _, k in ipairs({ "hp", "attack", "defense", "speed", "special" }) do
-    statExp[k] = math.max(0, math.min(65535, math.floor((packed.statExp or {})[k] or 0)))
+    statExp[k] = math.max(0, math.min(65535, math.floor(num(packedStatExp[k], 0))))
   end
   local stats = Stats.calc(def, level, dvs, statExp)
   local moves = {}
-  for _, mv in ipairs(packed.moves or {}) do
+  for _, entry in ipairs(tbl(packed.moves)) do
+    local mv = tbl(entry)
     local mdef = data.moves[mv.id]
     if mdef and #moves < 4 then
-      local ppUps = math.max(0, math.min(3, math.floor(mv.ppUps or 0)))
+      local ppUps = math.max(0, math.min(3, math.floor(num(mv.ppUps, 0))))
       local maxPP = mdef.pp + ppUps * math.floor(mdef.pp / 5)
-      local entry = { id = mv.id,
-                      pp = math.max(0, math.min(maxPP, math.floor(mv.pp or 0))) }
-      if mv.ppUps ~= nil then entry.ppUps = ppUps end
-      table.insert(moves, entry)
+      local move = { id = mv.id,
+                     pp = math.max(0, math.min(maxPP, math.floor(num(mv.pp, 0)))) }
+      if mv.ppUps ~= nil then move.ppUps = ppUps end
+      table.insert(moves, move)
     end
   end
   if #moves == 0 then
@@ -132,27 +157,29 @@ function Protocol.unpackMon(data, packed, opts)
   -- same as a standardized tournament format would
   local forced = forceLevel
   local hp = forced and stats.hp
-    or math.max(0, math.min(stats.hp, math.floor(packed.hp or stats.hp)))
-  local status = forced and nil or packed.status
+    or math.max(0, math.min(stats.hp, math.floor(num(packed.hp, stats.hp))))
+  local status = forced and nil or text(packed.status)
   -- preserve the sender's original-trainer identity (party_struct MON_OTID +
   -- wPartyMonOT on a real cable), clamped/typed like every other field so a
   -- tampered packet can't inject a bad ID or a huge name.  Left nil when the
   -- packet omits them (a v1/old peer) -- no worse than before for that legacy
   -- path, and once ot is set the load-time stampOT backfill (mon.ot or ...)
   -- becomes a no-op so the sender's identity survives save/reload (#215).
-  local otId = packed.otId
-    and math.max(0, math.min(65535, math.floor(packed.otId))) or nil
+  local packedOtId = num(packed.otId)
+  local otId = packedOtId
+    and math.max(0, math.min(65535, math.floor(packedOtId))) or nil
   local ot = type(packed.ot) == "string" and packed.ot:sub(1, 10) or nil
   return {
     species = packed.species,
     level = level,
-    exp = math.max(0, math.floor(packed.exp or Growth.expForLevel(def.growthRate, level))),
+    exp = math.max(0, math.floor(num(packed.exp,
+      Growth.expForLevel(def.growthRate, level)))),
     dvs = dvs,
     statExp = statExp,
     stats = stats,
     hp = hp,
     status = status,
-    nickname = packed.nickname,
+    nickname = text(packed.nickname),
     ot = ot,
     otId = otId,
     moves = moves,
@@ -246,6 +273,10 @@ end
 function Protocol.unpackMon2(data, packed, opts)
   local Mon = require("src.battle.gen2.Mon")
   local strict = opts and opts.strict
+  if type(packed) ~= "table" then
+    if strict then return nil, "unknown POKéMON" end
+    return nil
+  end
   local forceLevel = opts and tonumber(opts.forceLevel) or nil
   local def = data and data.pokemon and data.pokemon[packed.species]
   if not def then
@@ -253,31 +284,33 @@ function Protocol.unpackMon2(data, packed, opts)
     return nil
   end
   local level = math.max(1, math.min(Mon.MAX_LEVEL,
-    math.floor(packed.level or 5)))
+    math.floor(num(packed.level, 5))))
   if forceLevel then
     level = math.max(1, math.min(Mon.MAX_LEVEL, math.floor(forceLevel)))
   end
+  local packedDvs, packedStatExp = tbl(packed.dvs), tbl(packed.statExp)
   local dvs = {}
   for _, k in ipairs(GEN2_DVS) do
     dvs[k] = math.max(0, math.min(Mon.MAX_DV,
-      math.floor((packed.dvs or {})[k] or 0)))
+      math.floor(num(packedDvs[k], 0))))
   end
   -- derived, never taken from the packet (see GEN2_DVS above)
   dvs.hp = Mon.hpDV(dvs)
   local statExp = {}
   for _, k in ipairs(GEN2_STAT_EXP) do
     statExp[k] = math.max(0, math.min(65535,
-      math.floor((packed.statExp or {})[k] or 0)))
+      math.floor(num(packedStatExp[k], 0))))
   end
   local stats = Mon.stats(def.baseStats, dvs, level, statExp)
   local moves = {}
-  for _, mv in ipairs(packed.moves or {}) do
+  for _, packedMove in ipairs(tbl(packed.moves)) do
+    local mv = tbl(packedMove)
     local mdef = data.moves and data.moves[mv.id]
     if mdef and #moves < 4 then
-      local ppUps = math.max(0, math.min(3, math.floor(mv.ppUps or 0)))
+      local ppUps = math.max(0, math.min(3, math.floor(num(mv.ppUps, 0))))
       local maxPp = (mdef.pp or 0) + ppUps * math.floor((mdef.pp or 0) / 5)
       local entry = { id = mv.id, maxPp = maxPp,
-                      pp = math.max(0, math.min(maxPp, math.floor(mv.pp or 0))) }
+                      pp = math.max(0, math.min(maxPp, math.floor(num(mv.pp, 0)))) }
       if mv.ppUps ~= nil then entry.ppUps = ppUps end
       table.insert(moves, entry)
     end
@@ -296,26 +329,27 @@ function Protocol.unpackMon2(data, packed, opts)
   -- an item the peer cannot represent), and the Gen 2 arm of
   -- Protocol.eligibleParty is what keeps it from ever reaching here on a
   -- negotiated trade.
-  local item = packed.item
+  local item = type(packed.item) == "string" and packed.item or nil
   if item ~= nil and not (data.items and data.items[item]) then
     if strict then return nil, "unknown item" end
     item = nil
   end
   local forced = forceLevel
   local hp = forced and stats.hp
-    or math.max(0, math.min(stats.hp, math.floor(packed.hp or stats.hp)))
-  local status = forced and nil or packed.status
-  local otId = packed.otId
-    and math.max(0, math.min(65535, math.floor(packed.otId))) or nil
+    or math.max(0, math.min(stats.hp, math.floor(num(packed.hp, stats.hp))))
+  local status = forced and nil or text(packed.status)
+  local packedOtId = num(packed.otId)
+  local otId = packedOtId
+    and math.max(0, math.min(65535, math.floor(packedOtId))) or nil
   local ot = type(packed.ot) == "string" and packed.ot:sub(1, 10) or nil
   local growth = Mon.growthFor(data, def.growthRate)
   local mon = {
     species = packed.species,
     name = def.name or packed.species,
-    nickname = packed.nickname,
+    nickname = text(packed.nickname),
     level = level,
-    experience = math.max(0, math.floor(packed.experience
-      or Mon.experienceForLevel(growth, level))),
+    experience = math.max(0, math.floor(num(packed.experience,
+      Mon.experienceForLevel(growth, level)))),
     dvs = dvs,
     statExp = statExp,
     stats = stats,
@@ -328,10 +362,10 @@ function Protocol.unpackMon2(data, packed, opts)
     -- GiveEgg starts a hatched mon at 120 and a caught one at 70; a traded mon
     -- keeps what it arrived with, clamped to the byte the cart stores it in
     happiness = math.max(0, math.min(255,
-      math.floor(packed.happiness or 70))),
-    pokerus = math.max(0, math.min(255, math.floor(packed.pokerus or 0))),
+      math.floor(num(packed.happiness, 70)))),
+    pokerus = math.max(0, math.min(255, math.floor(num(packed.pokerus, 0)))),
     caughtLevel = math.max(1, math.min(Mon.MAX_LEVEL,
-      math.floor(packed.caughtLevel or level))),
+      math.floor(num(packed.caughtLevel, level)))),
     ot = ot,
     otName = ot,
     otId = otId,
@@ -339,7 +373,7 @@ function Protocol.unpackMon2(data, packed, opts)
   }
   if packed.isEgg then
     mon.isEgg = true
-    mon.eggSteps = math.max(0, math.floor(packed.eggSteps or 0))
+    mon.eggSteps = math.max(0, math.floor(num(packed.eggSteps, 0)))
   end
   -- Derived from the DVs on the RECEIVING side, exactly as they were derived on
   -- the sending one: shininess, gender and an Unown's letter are all functions
@@ -401,17 +435,19 @@ end
 -- which slots are in play and a pick can never land on a different mon.
 function Protocol.eligibleParty(party, myRecords, theirRecords)
   local eligible, reasons = {}, {}
-  theirRecords = theirRecords or {}
-  local theirSpecies = theirRecords.pokemon or {}
-  local theirMoves = theirRecords.moves or {}
-  local mySpecies = (myRecords or {}).pokemon or {}
-  local myMoves = (myRecords or {}).moves or {}
+  theirRecords = tbl(theirRecords)
+  myRecords = tbl(myRecords)
+  local theirSpecies = tbl(theirRecords.pokemon)
+  local theirMoves = tbl(theirRecords.moves)
+  local mySpecies = tbl(myRecords.pokemon)
+  local myMoves = tbl(myRecords.moves)
   -- Gen 2 only, and absent on both sides of a Gen 1 trade, which is what keeps
   -- the loop below unchanged for Red: a mon with no `item` never reaches the
   -- held-item arm at all.
-  local theirHeld = theirRecords.heldItems
-  local myHeld = (myRecords or {}).heldItems or {}
-  for i, mon in ipairs(party or {}) do
+  local theirHeld = theirRecords.heldItems ~= nil
+    and tbl(theirRecords.heldItems) or nil
+  local myHeld = tbl(myRecords.heldItems)
+  for i, mon in ipairs(tbl(party)) do
     local reason
     if not theirSpecies[mon.species] then
       reason = "not on the other game"
@@ -551,7 +587,7 @@ function TradeSession:handle(msg)
     end
     if self.stage == "waitParty" then self.stage = "picking" end
   elseif msg.type == "pick" then
-    self.theirPick = msg.index
+    self.theirPick = num(msg.index)
     self:advance()
   elseif msg.type == "confirm" then
     self.theirConfirm = msg.ok
@@ -583,7 +619,18 @@ function TradeSession:confirm(ok)
   return { type = "confirm", ok = ok }
 end
 
+function TradeSession:pickResolves()
+  local index = self.theirPick
+  if type(index) ~= "number" then return false end
+  return self.theirParty ~= nil and self.theirParty[index] ~= nil
+end
+
 function TradeSession:advance()
+  if self.theirPick ~= nil and self.theirParty and not self:pickResolves() then
+    self.stage = "cancelled"
+    self.error = "the other game picked a POKéMON that isn't there"
+    return
+  end
   if self.stage == "picking" and self.myPick then
     self.stage = self.theirPick and "confirming" or "waitPick"
   elseif self.stage == "waitPick" and self.theirPick then
diff --git a/src/link/Session.lua b/src/link/Session.lua
index ecb4a64b..87fda6b6 100644
--- a/src/link/Session.lua
+++ b/src/link/Session.lua
@@ -1,3 +1,6 @@
+local Logger = require("src.core.Logger")
+local Wire = require("src.link.Wire")
+
 local Session = {}
 Session.__index = Session
 
@@ -23,6 +26,7 @@ function Session.new(transport, options)
     _status = "connecting",
     _terminal = nil,
     _transportCloseCalled = false,
+    dropped = 0,
     paired = false,
     closed = false,
     error = nil,
@@ -120,16 +124,15 @@ function Session:update()
     end
   else
     for index = 1, #messages do
-      local message = messages[index]
-      if type(message) ~= "table" or type(message.type) ~= "string" then
-        if not failureReason then
-          failureReason = "protocol_error"
-          failureDetail = ("message %d must be a table with string type")
-            :format(index)
-        end
-        break
+      local raw = messages[index]
+      local ok, message = pcall(Wire.sanitize, raw)
+      if ok and message then
+        self._inbox[#self._inbox + 1] = message
+      else
+        self.dropped = (self.dropped or 0) + 1
+        local label = type(raw) == "table" and tostring(raw.type) or type(raw)
+        Logger.warn("link: dropped malformed message (%s)", label)
       end
-      self._inbox[#self._inbox + 1] = message
     end
   end
 
diff --git a/src/link/Wire.lua b/src/link/Wire.lua
new file mode 100644
index 00000000..6404a888
--- /dev/null
+++ b/src/link/Wire.lua
@@ -0,0 +1,344 @@
+local Wire = {}
+
+local MAX_INT = 2147483647
+local MAX_STRING = 64
+local MAX_NAME = 40
+local MAX_LIST = 64
+local MAX_PARTY = 32
+local MAX_MOVES = 8
+local MAX_MODS = 256
+local MAX_RECORDS = 4096
+local MAX_EXTRA_DEPTH = 8
+local MAX_ROUNDS = 16
+local MAX_MATCHES = 128
+
+function Wire.num(v, default, min, max)
+  local n = tonumber(v)
+  if n == nil or n ~= n then return default end
+  min = min or -MAX_INT
+  max = max or MAX_INT
+  if n < min then return min end
+  if n > max then return max end
+  return math.floor(n)
+end
+
+function Wire.str(v, default, maxLen)
+  if type(v) ~= "string" then return default end
+  maxLen = maxLen or MAX_STRING
+  if #v > maxLen then return v:sub(1, maxLen) end
+  return v
+end
+
+function Wire.bool(v, default)
+  if type(v) == "boolean" then return v end
+  return default
+end
+
+function Wire.list(v, maxN, fn)
+  local out = {}
+  if type(v) ~= "table" then return out end
+  local n = math.min(#v, maxN or MAX_LIST)
+  for i = 1, n do
+    local entry = fn(v[i])
+    if entry ~= nil then out[#out + 1] = entry end
+  end
+  return out
+end
+
+function Wire.records(v)
+  local out = {}
+  if type(v) ~= "table" then return out end
+  local n = 0
+  for k, val in pairs(v) do
+    if type(k) == "string" then
+      out[k] = Wire.str(val, nil, MAX_STRING) or tostring(Wire.num(val, 0))
+      n = n + 1
+      if n >= MAX_RECORDS then break end
+    end
+  end
+  return out
+end
+
+function Wire.plain(v, depth)
+  if type(v) ~= "table" then return nil end
+  depth = depth or 0
+  if depth > MAX_EXTRA_DEPTH then return nil end
+  local out = {}
+  for k, val in pairs(v) do
+    local kt, vt = type(k), type(val)
+    if kt == "string" or kt == "number" then
+      if vt == "string" then out[k] = Wire.str(val, nil, MAX_STRING)
+      elseif vt == "number" or vt == "boolean" then out[k] = val
+      elseif vt == "table" then out[k] = Wire.plain(val, depth + 1) end
+    end
+  end
+  return out
+end
+
+local STAT_KEYS = { "hp", "attack", "defense", "speed", "special" }
+
+local function statMap(v)
+  local out = {}
+  if type(v) ~= "table" then return out end
+  for _, k in ipairs(STAT_KEYS) do
+    out[k] = Wire.num(v[k], nil, 0, 65535)
+  end
+  return out
+end
+
+local function move(v)
+  if type(v) ~= "table" then return { id = nil } end
+  return {
+    id = Wire.str(v.id, nil, MAX_STRING),
+    pp = Wire.num(v.pp, nil, 0, 255),
+    ppUps = Wire.num(v.ppUps, nil, 0, 255),
+    maxPp = Wire.num(v.maxPp, nil, 0, 255),
+  }
+end
+
+local function mon(v)
+  if type(v) ~= "table" then return {} end
+  return {
+    species = Wire.str(v.species, nil, MAX_STRING),
+    level = Wire.num(v.level, nil, 0, 65535),
+    exp = Wire.num(v.exp, nil, 0, MAX_INT),
+    experience = Wire.num(v.experience, nil, 0, MAX_INT),
+    hp = Wire.num(v.hp, nil, 0, 65535),
+    status = Wire.str(v.status, nil, MAX_STRING),
+    nickname = Wire.str(v.nickname, nil, MAX_NAME),
+    dvs = statMap(v.dvs),
+    statExp = statMap(v.statExp),
+    moves = Wire.list(v.moves, MAX_MOVES, move),
+    ot = Wire.str(v.ot, nil, MAX_NAME),
+    otId = Wire.num(v.otId, nil, 0, MAX_INT),
+    item = Wire.str(v.item, nil, MAX_STRING),
+    happiness = Wire.num(v.happiness, nil, 0, 65535),
+    pokerus = Wire.num(v.pokerus, nil, 0, 65535),
+    caughtLevel = Wire.num(v.caughtLevel, nil, 0, 65535),
+    isEgg = Wire.bool(v.isEgg, nil),
+    eggSteps = Wire.num(v.eggSteps, nil, 0, MAX_INT),
+    extra = Wire.plain(v.extra),
+  }
+end
+
+local function modEntry(v)
+  if type(v) ~= "table" then return nil end
+  return {
+    id = Wire.str(v.id, nil, MAX_NAME),
+    version = Wire.str(v.version, nil, MAX_NAME)
+      or Wire.num(v.version, nil, 0, MAX_INT),
+    affectsLink = Wire.bool(v.affectsLink, nil),
+    language = Wire.bool(v.language, nil),
+  }
+end
+
+local function name(v)
+  return Wire.str(v, nil, MAX_NAME)
+end
+
+local sanitize
+
+local SCHEMAS = {}
+
+SCHEMAS.hello = function(m)
+  return {
+    protocol = Wire.num(m.protocol, nil, 0, MAX_INT),
+    name = name(m.name),
+    mode = Wire.str(m.mode, nil, MAX_STRING),
+    engineVersion = Wire.str(m.engineVersion, nil, MAX_STRING),
+    apiVersion = Wire.str(m.apiVersion, nil, MAX_STRING),
+    generation = Wire.num(m.generation, nil, 0, 255),
+    fingerprint = Wire.str(m.fingerprint, nil, MAX_STRING),
+    linkModified = Wire.bool(m.linkModified, nil),
+    mods = Wire.list(m.mods, MAX_MODS, modEntry),
+  }
+end
+
+SCHEMAS.records = function(m)
+  return {
+    pokemon = Wire.records(m.pokemon),
+    moves = Wire.records(m.moves),
+    heldItems = m.heldItems ~= nil and Wire.records(m.heldItems) or nil,
+  }
+end
+
+SCHEMAS.party = function(m)
+  return {
+    mons = Wire.list(m.mons, MAX_PARTY, mon),
+    seed = Wire.num(m.seed, nil, 0, MAX_INT),
+    forceLevel = Wire.num(m.forceLevel, nil, 0, 65535),
+  }
+end
+
+SCHEMAS.pick = function(m)
+  return { index = Wire.num(m.index, nil, -MAX_INT, MAX_INT) }
+end
+
+SCHEMAS.confirm = function(m)
+  return { ok = Wire.bool(m.ok, false) }
+end
+
+SCHEMAS.action = function(m)
+  return {
+    kind = Wire.str(m.kind, "", MAX_STRING),
+    slot = Wire.num(m.slot, nil, 1, MAX_MOVES),
+    index = Wire.num(m.index, nil, 1, MAX_PARTY),
+  }
+end
+
+SCHEMAS.hash = function(m)
+  local parts
+  if type(m.parts) == "table" then
+    parts = {
+      actives = Wire.str(m.parts.actives, nil, MAX_STRING),
+      volatile = Wire.str(m.parts.volatile, nil, MAX_STRING),
+      bench = Wire.str(m.parts.bench, nil, MAX_STRING),
+    }
+  end
+  return {
+    turn = Wire.num(m.turn, 0, 0, MAX_INT),
+    value = Wire.str(m.value, nil, MAX_STRING),
+    parts = parts,
+  }
+end
+
+SCHEMAS.replace = function(m)
+  return { index = Wire.num(m.index, 1, 1, MAX_PARTY) }
+end
+
+SCHEMAS.bye = function() return {} end
+SCHEMAS.forfeit = function() return {} end
+
+SCHEMAS.hosted = function(m)
+  return { code = Wire.str(m.code, nil, MAX_NAME) }
+end
+SCHEMAS.paired = function() return {} end
+SCHEMAS.peer_gone = function() return {} end
+SCHEMAS.join_error = function(m)
+  return { reason = Wire.str(m.reason, "", MAX_STRING) }
+end
+
+local function rule(m)
+  return {
+    requiredPartySize = Wire.num(m.requiredPartySize, nil, 0, 255),
+    minLevel = Wire.num(m.minLevel, nil, 0, 65535),
+    maxLevel = Wire.num(m.maxLevel, nil, 0, 65535),
+    turnLimit = Wire.num(m.turnLimit, nil, 0, 65535),
+    forceLevel = Wire.num(m.forceLevel, nil, 0, 65535),
+  }
+end
+
+SCHEMAS.tournament_hosted = function(m)
+  local out = rule(m)
+  out.code = Wire.str(m.code, nil, MAX_NAME)
+  out.participating = Wire.bool(m.participating, nil)
+  return out
+end
+
+SCHEMAS.tournament_host_error = function(m)
+  local out = rule(m)
+  out.reason = Wire.str(m.reason, "", MAX_STRING)
+  return out
+end
+
+SCHEMAS.tournament_join_error = SCHEMAS.tournament_host_error
+
+SCHEMAS.tournament_roster = function(m)
+  local out = rule(m)
+  out.players = Wire.list(m.players, MAX_MATCHES, name)
+  out.spectators = Wire.list(m.spectators, MAX_MATCHES, name)
+  return out
+end
+
+local function match(v)
+  if type(v) ~= "table" then return nil end
+  return {
+    a = name(v.a), b = name(v.b), winner = name(v.winner),
+    bye = Wire.bool(v.bye, false),
+    state = Wire.str(v.state, nil, MAX_STRING),
+  }
+end
+
+local function round(v)
+  if type(v) ~= "table" then return nil end
+  return {
+    round = Wire.num(v.round, 0, 0, MAX_ROUNDS),
+    matches = Wire.list(v.matches, MAX_MATCHES, match),
+  }
+end
+
+SCHEMAS.bracket_update = function(m)
+  local t = type(m.tournament) == "table" and m.tournament or {}
+  local out = rule(t)
+  out.code = Wire.str(t.code, nil, MAX_NAME)
+  out.status = Wire.str(t.status, nil, MAX_STRING)
+  out.round = Wire.num(t.round, 0, 0, MAX_ROUNDS)
+  out.champion = name(t.champion)
+  out.rounds = Wire.list(t.rounds, MAX_ROUNDS, round)
+  return { tournament = out }
+end
+
+SCHEMAS.match_start = function(m)
+  return {
+    opponent = name(m.opponent),
+    round = Wire.num(m.round, 0, 0, MAX_ROUNDS),
+    turnLimit = Wire.num(m.turnLimit, nil, 0, 65535),
+    role = Wire.str(m.role, "", MAX_STRING),
+  }
+end
+
+SCHEMAS.match_start_spectate = function(m)
+  return {
+    round = Wire.num(m.round, 0, 0, MAX_ROUNDS),
+    playerHost = name(m.playerHost),
+    playerGuest = name(m.playerGuest),
+  }
+end
+
+SCHEMAS.tournament_bye = function(m)
+  return { round = Wire.num(m.round, 0, 0, MAX_ROUNDS) }
+end
+
+SCHEMAS.tournament_over = function(m)
+  return { champion = name(m.champion) }
+end
+
+local SPECTATABLE = {
+  action = true, replace = true, bye = true, forfeit = true,
+  hello = true, party = true, hash = true,
+}
+
+SCHEMAS.spectate = function(m)
+  if type(m.msg) ~= "table" or not SPECTATABLE[m.msg.type] then return nil end
+  local inner = sanitize(m.msg)
+  if not inner then return nil end
+  return { side = Wire.str(m.side, "", MAX_STRING), msg = inner }
+end
+
+Wire.SCHEMAS = SCHEMAS
+
+local function passthrough(m)
+  local out = Wire.plain(m) or {}
+  out.type = nil
+  return out
+end
+
+sanitize = function(msg)
+  if type(msg) ~= "table" then return nil end
+  local kind = msg.type
+  if type(kind) ~= "string" or #kind > MAX_STRING then return nil end
+  local schema = SCHEMAS[kind]
+  local out
+  if schema then
+    out = schema(msg)
+    if not out then return nil end
+  else
+    out = passthrough(msg)
+  end
+  out.type = kind
+  return out
+end
+
+Wire.sanitize = sanitize
+
+return Wire
diff --git a/tests/engine/link_session.lua b/tests/engine/link_session.lua
index 19f80fcf..0c1ef8c8 100644
--- a/tests/engine/link_session.lua
+++ b/tests/engine/link_session.lua
@@ -70,12 +70,12 @@ end
 do
   local host, guest = sessionPair()
   guest:send({ type = "before", sequence = 1 })
-  guest:send({ type = "hello", sequence = 2 })
+  guest:send({ type = "greeting", sequence = 2 })
   guest:send({ type = "after", sequence = 3 })
-  guest:send({ type = "hello", sequence = 4 })
+  guest:send({ type = "greeting", sequence = 4 })
   host:update()
 
-  local hello = host:take("hello")
+  local hello = host:take("greeting")
   T.eq(hello.sequence, 2, "take removes the first matching packet")
   T.eq(host:pollOne().sequence, 1, "pollOne removes only the FIFO head")
 
@@ -122,14 +122,14 @@ end
 
 do
   local transport = fakeTransport({ onUpdate = function(self)
-    self.inbox[#self.inbox + 1] = { type = "bye", final = true }
+    self.inbox[#self.inbox + 1] = { type = "bye" }
     self.closed = true
   end })
   local session = Session.new(transport, { role = "host", kind = "link" })
   session:update()
   T.eq(session:getStatus(), "draining", "normal close drains its final packet")
   T.eq(session.closed, false, "compatibility closed waits for the FIFO")
-  T.eq(session:take("bye").final, true, "final close packet remains observable")
+  T.check(session:take("bye") ~= nil, "final close packet remains observable")
   T.eq(session:getStatus(), "closed", "normal drain reaches closed")
   T.eq(transport.closeCount, 1, "transport cleanup runs once")
 end
@@ -182,21 +182,22 @@ do
   } })
   local session = Session.new(transport, { role = "host", kind = "link" })
   session:update()
-  local reason = session:getFailure()
-  T.eq(reason, "protocol_error", "malformed packet fails as protocol_error")
-  T.eq(session:getStatus(), "draining", "malformed batch drains valid prefix")
+  T.eq(session:getFailure(), nil, "a malformed packet is not a terminal failure")
+  T.eq(session:getStatus(), "paired", "the session stays usable after a bad packet")
   local messages = session:poll()
-  T.eq(#messages, 1, "malformed value and untrusted tail are not exposed")
-  T.eq(messages[1].sequence, 1, "valid prefix survives malformed packet")
-  T.eq(session:getStatus(), "failed", "protocol drain reaches failed")
+  T.eq(#messages, 2, "the malformed value is dropped, the rest is delivered")
+  T.eq(messages[1].sequence, 1, "packets before the malformed one survive")
+  T.eq(messages[2].sequence, 3, "packets after the malformed one survive")
+  T.eq(session.dropped, 1, "the drop is counted")
 end
 
 do
   local transport = fakeTransport({ inbox = { { type = 7 } } })
   local session = Session.new(transport, { role = "host", kind = "link" })
   session:update()
-  T.eq(session:getFailure(), "protocol_error",
-    "table without string type is a protocol error")
+  T.eq(session:getFailure(), nil,
+    "a table without a string type is dropped, not a terminal failure")
+  T.eq(#session:poll(), 0, "...and never reaches the mode")
 end
 
 do
@@ -268,8 +269,9 @@ do
   local receiver = Session.new(receiverNet, { role = "guest", kind = "link" })
   senderNet:send(false)
   receiver:update()
-  T.eq(receiver:getFailure(), "protocol_error",
-    "loopback forwards decoded false to session validation")
+  T.eq(receiver:getFailure(), nil,
+    "a decoded scalar off the loopback is dropped, not fatal")
+  T.eq(#receiver:poll(), 0, "...and never reaches the mode")
 end
 
 do
@@ -284,8 +286,9 @@ do
   }
   local session = Session.new(transport, { role = "guest", kind = "link" })
   session:update()
-  T.eq(session:getFailure(), "protocol_error",
-    "ENet forwards decoded false to session validation")
+  T.eq(session:getFailure(), nil,
+    "a decoded scalar off ENet is dropped, not fatal")
+  T.eq(#session:poll(), 0, "...and never reaches the mode")
 end
 
 do
@@ -294,8 +297,9 @@ do
   T.check(pcall(transport.handleTCPLine, transport, "42"),
     "TCP control handoff does not index a decoded scalar")
   session:update()
-  T.eq(session:getFailure(), "protocol_error",
-    "decoded TCP scalar reaches session validation")
+  T.eq(session:getFailure(), nil,
+    "a decoded TCP scalar is dropped, not fatal")
+  T.eq(#session:poll(), 0, "...and never reaches the mode")
 end
 
 do
diff --git a/tests/link_desync_fuzz.lua b/tests/link_desync_fuzz.lua
index c819a24f..fae7dbc2 100644
--- a/tests/link_desync_fuzz.lua
+++ b/tests/link_desync_fuzz.lua
@@ -66,13 +66,39 @@ table.sort(MOVES)
 -- in the peer's inbox, so one side is mid-queue when the other's action
 -- arrives (Net.loopbackPair on its own delivers instantly, which is the one
 -- thing the real relay never does)
-local function laggyPair(delayA, delayB)
+-- mutation mode: a peer whose messages arrive with a random field (or the
+-- type itself) at the wrong Lua type.  What is delivered is what Session
+-- would deliver -- Wire.sanitize's output, or nothing at all -- so a run
+-- exercises the real receive path rather than a hand-written stand-in.
+local Wire = require("src.link.Wire")
+local HOSTILE = { {}, { 1, 2, 3 }, 0, -1, 999, "s", "", true, false, math.huge }
+
+local function mutate(rnd, msg)
+  local keys = {}
+  for k in pairs(msg) do
+    if k ~= "type" then keys[#keys + 1] = k end
+  end
+  table.sort(keys)
+  if #keys == 0 or rnd(1, 100) <= 20 then
+    msg.type = HOSTILE[rnd(1, #HOSTILE)]
+  else
+    msg[keys[rnd(1, #keys)]] = HOSTILE[rnd(1, #HOSTILE)]
+  end
+  return msg
+end
+
+local function laggyPair(delayA, delayB, rnd, mutateRate)
   local a, b = Net.loopbackPair()
   a.wire, b.wire = {}, {}
   a.delay, b.delay = delayA or 0, delayB or 0
+  a.mutateRate, b.mutateRate = mutateRate or 0, mutateRate or 0
   local function send(self, msg)
     if self.closed then return end
     local decoded = Json.decode(Json.encode(msg)) -- same round trip as the wire
+    if decoded and self.mutateRate > 0 then
+      if rnd(1, 100) <= self.mutateRate then decoded = mutate(rnd, decoded) end
+      decoded = Wire.sanitize(decoded)
+    end
     if decoded then table.insert(self.wire, { msg = decoded, at = self.delay }) end
   end
   local function update(self)
@@ -143,7 +169,7 @@ local function firstMismatch(a, b)
 end
 
 -- Returns nil when the run agreed, or a description of how it split.
-local function runOne(seed)
+local function runOne(seed, mutateRate)
   local rnd = makeRandom(seed)
   -- the two sides are deliberately different clients
   local optsA = { animations = false, textSpeed = 1, battleStyle = "SET" }
@@ -155,7 +181,7 @@ local function runOne(seed)
   gameA.save.party = randomParty(rnd, rnd(1, 4))
   gameB.save.party = randomParty(rnd, rnd(1, 4))
 
-  local netA, netB = laggyPair(lagA, lagB)
+  local netA, netB = laggyPair(lagA, lagB, rnd, mutateRate)
   local battleSeed = rnd(1, 2 ^ 30)
   local battleA = LinkBattle.newHost(gameA, netA, {
     myParty = Protocol.packParty(gameA.save.party),
@@ -230,6 +256,12 @@ local function runOne(seed)
       end
     end
     local turn, part = firstMismatch(battleA, battleB)
+    if turn and mutateRate then
+      -- a corrupted action IS a divergence; what matters here is that the
+      -- match ends by its own rules (desync draw, forfeit, disconnect)
+      -- rather than throwing
+      return nil, battleA.turnCount or 0
+    end
     if turn then
       return ("seed %d: turn %d %s split (lag %d/%d, steps %d/%d)"):format(
         seed, turn, part, lagA, lagB, stepsA, stepsB), battleA.turnCount or 0
@@ -237,7 +269,7 @@ local function runOne(seed)
   end
   -- a battle still running at the guard is a stalemate (two mons that cannot
   -- KO each other), not a split; only a finished one can be checked mirrored
-  if guard < 60000
+  if not mutateRate and guard < 60000
      and (battleA.player.mon.hp ~= battleB.enemy.mon.hp
           or battleA.enemy.mon.hp ~= battleB.player.mon.hp) then
     return ("seed %d: final HP not mirrored (%d/%d vs %d/%d)"):format(
@@ -250,6 +282,8 @@ end
 local RUNS = tonumber(arg and arg[1]) or 40
 local FIRST = tonumber(arg and arg[2]) or 1
 
+local MUTATION_RUNS = tonumber(arg and arg[3]) or math.max(4, math.floor(RUNS / 4))
+
 local failures, turns = 0, 0
 for seed = FIRST, FIRST + RUNS - 1 do
   local ok, why, t = pcall(runOne, seed)
@@ -263,5 +297,21 @@ for seed = FIRST, FIRST + RUNS - 1 do
   end
 end
 print(("link desync fuzz: %d runs, %d turns, %d failures"):format(RUNS, turns, failures))
+
+local mutationFailures = 0
+for seed = FIRST, FIRST + MUTATION_RUNS - 1 do
+  local ok, why = pcall(runOne, seed, 15)
+  if not ok then
+    mutationFailures = mutationFailures + 1
+    print("FAIL link mutation fuzz seed " .. seed .. ": " .. tostring(why))
+  elseif why then
+    mutationFailures = mutationFailures + 1
+    print("FAIL link mutation fuzz " .. why)
+  end
+end
+print(("link mutation fuzz: %d runs, %d failures"):format(
+  MUTATION_RUNS, mutationFailures))
+
 assert(failures == 0, failures .. " lockstep run(s) diverged")
+assert(mutationFailures == 0, mutationFailures .. " mutated run(s) threw")
 return true
diff --git a/tests/link_hostile.lua b/tests/link_hostile.lua
new file mode 100644
index 00000000..8fd63bcb
--- /dev/null
+++ b/tests/link_hostile.lua
@@ -0,0 +1,396 @@
+-- Hostile link traffic: every message type this build reads, with every
+-- field replaced by every wrong Lua type, driven through the real Session
+-- choke point and then into the real consumers (trade session, link battle,
+-- spectator battle, tournament screen -- including its draw).
+--
+-- The three payloads from the "How to Troll Pokemon Players" writeup are
+-- rows in the table below: action.slot as a table, hash.parts as a number,
+-- and pick.index out of range.
+--
+-- Self-contained; run directly or via run_link_tests.lua:
+--   luajit tests/link_hostile.lua
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+love = love or require("tests.love_stub")
+
+local Data = require("src.core.Data")
+if not Data.pokemon then Data:load() end
+local Font = require("src.render.Font")
+Font.load(Data)
+
+local Input = require("src.core.Input")
+Input:init()
+
+local Json = require("src.link.Json")
+local LinkBattle = require("src.link.LinkBattle")
+local Net = require("src.link.Net")
+local Pokemon = require("src.pokemon.Pokemon")
+local Protocol = require("src.link.Protocol")
+local Session = require("src.link.Session")
+local Tournament = require("src.link.Tournament")
+local Wire = require("src.link.Wire")
+
+local failures = 0
+local function check(cond, msg)
+  if cond then
+    print("ok   " .. msg)
+  else
+    failures = failures + 1
+    print("FAIL " .. msg)
+  end
+end
+
+-- ---------------------------------------------------------------- corpus
+
+local HOSTILE = { {}, { 1, 2, 3 }, { type = "x" }, 0, -1, 123, 1.5,
+                  math.huge, "s", "", true, false }
+
+local function copy(v)
+  if type(v) ~= "table" then return v end
+  local out = {}
+  for k, val in pairs(v) do out[k] = copy(val) end
+  return out
+end
+
+local packedMon = Protocol.packMon(Pokemon.new(Data, "PIKACHU", 12))
+
+local TEMPLATES = {
+  { type = "hello", protocol = 2, name = "RED", mode = "battle",
+    engineVersion = "1.0.0", apiVersion = "1", generation = 1,
+    fingerprint = "abc123", linkModified = false,
+    mods = { { id = "demo", version = "1.0", affectsLink = true } } },
+  { type = "records", pokemon = { PIKACHU = "a" }, moves = { TACKLE = "b" },
+    heldItems = { BERRY = "c" } },
+  { type = "party", mons = { copy(packedMon) }, seed = 1234, forceLevel = 50 },
+  { type = "pick", index = 1 },
+  { type = "confirm", ok = true },
+  { type = "action", kind = "move", slot = 1, index = 1 },
+  { type = "hash", turn = 1, value = "v",
+    parts = { actives = "a", volatile = "b", bench = "c" } },
+  { type = "replace", index = 1 },
+  { type = "bye" },
+  { type = "forfeit" },
+  { type = "spectate", side = "host",
+    msg = { type = "action", kind = "move", slot = 1 } },
+  { type = "hosted", code = "ABCDEF" },
+  { type = "paired" },
+  { type = "peer_gone" },
+  { type = "join_error", reason = "not_found" },
+  { type = "tournament_hosted", code = "ABCDEF", turnLimit = 6,
+    requiredPartySize = 3, minLevel = 5, maxLevel = 50, forceLevel = 50,
+    participating = true },
+  { type = "tournament_host_error", reason = "party_ineligible",
+    requiredPartySize = 3, minLevel = 5, maxLevel = 50 },
+  { type = "tournament_join_error", reason = "party_ineligible",
+    requiredPartySize = 3, minLevel = 5, maxLevel = 50 },
+  { type = "tournament_roster", players = { "RED", "BLUE" },
+    spectators = { "GREEN" }, turnLimit = 6, requiredPartySize = 3,
+    minLevel = 5, maxLevel = 50, forceLevel = 50 },
+  { type = "bracket_update", tournament = { code = "ABCDEF", turnLimit = 6,
+    requiredPartySize = 3, minLevel = 5, maxLevel = 50, status = "active",
+    round = 1, champion = "RED",
+    rounds = { { round = 1, matches = { { a = "RED", b = "BLUE",
+      winner = "RED", bye = false, state = "live" } } } } } },
+  { type = "match_start", opponent = "BLUE", round = 1, turnLimit = 6,
+    role = "host" },
+  { type = "match_start_spectate", round = 1, playerHost = "RED",
+    playerGuest = "BLUE" },
+  { type = "tournament_bye", round = 1 },
+  { type = "tournament_over", champion = "RED" },
+  { type = "a_type_this_build_has_never_heard_of", payload = { n = 1 } },
+}
+
+local NESTED = {
+  { "hash", { "parts", "actives" } },
+  { "party", { "mons", 1 } },
+  { "party", { "mons", 1, "dvs" } },
+  { "party", { "mons", 1, "moves" } },
+  { "party", { "mons", 1, "moves", 1 } },
+  { "party", { "mons", 1, "nickname" } },
+  { "party", { "mons", 1, "level" } },
+  { "party", { "mons", 1, "otId" } },
+  { "hello", { "mods", 1 } },
+  { "spectate", { "msg" } },
+  { "spectate", { "msg", "slot" } },
+  { "bracket_update", { "tournament", "rounds" } },
+  { "bracket_update", { "tournament", "rounds", 1, "matches" } },
+  { "bracket_update", { "tournament", "rounds", 1, "matches", 1, "a" } },
+  { "tournament_roster", { "players", 1 } },
+}
+
+local function templateFor(kind)
+  for _, t in ipairs(TEMPLATES) do
+    if t.type == kind then return t end
+  end
+end
+
+local function setPath(root, path, value)
+  local node = root
+  for i = 1, #path - 1 do
+    node = node[path[i]]
+    if type(node) ~= "table" then return false end
+  end
+  node[path[#path]] = value
+  return true
+end
+
+local corpus = {}
+local function add(msg) corpus[#corpus + 1] = msg end
+
+add(false); add(true); add(123); add("string"); add({}); add({ 1, 2, 3 })
+add({ type = 5 }); add({ type = {} }); add({ type = true })
+add({ type = ("x"):rep(4096) })
+
+for _, template in ipairs(TEMPLATES) do
+  add(copy(template))
+  for key in pairs(template) do
+    if key ~= "type" then
+      for _, bad in ipairs(HOSTILE) do
+        local m = copy(template)
+        m[key] = copy(bad)
+        add(m)
+      end
+      local missing = copy(template)
+      missing[key] = nil
+      add(missing)
+    end
+  end
+end
+
+for _, row in ipairs(NESTED) do
+  local template = templateFor(row[1])
+  for _, bad in ipairs(HOSTILE) do
+    local m = copy(template)
+    if setPath(m, row[2], copy(bad)) then add(m) end
+  end
+  local m = copy(template)
+  if setPath(m, row[2], nil) then add(m) end
+end
+
+print(("hostile corpus: %d messages"):format(#corpus))
+
+-- ---------------------------------------------------------------- session
+
+local function fakeTransport(inbox)
+  local transport = { paired = true, closed = false, error = nil,
+                      inbox = inbox, sent = {} }
+  function transport:update() end
+  function transport:poll()
+    local messages = self.inbox
+    self.inbox = {}
+    return messages
+  end
+  function transport:send(m) table.insert(self.sent, m) end
+  function transport:close() self.closed = true end
+  return transport
+end
+
+local transport = fakeTransport(copy(corpus))
+local session = Session.new(transport, { role = "guest", kind = "link" })
+local okUpdate, updateErr = pcall(session.update, session)
+check(okUpdate, "the whole hostile corpus goes through Session without throwing"
+      .. (okUpdate and "" or (": " .. tostring(updateErr))))
+check(session:getFailure() == nil,
+      "a hostile peer cannot latch a terminal failure on the session")
+check(session:getStatus() == "paired", "the session is still usable afterwards")
+local survivors = session:poll()
+check(#survivors > 0, "well-formed messages still get through")
+check(session.dropped > 0, "malformed messages are counted as dropped")
+
+for _, msg in ipairs(survivors) do
+  if type(msg.type) ~= "string" then
+    check(false, "every delivered message has a string type")
+    break
+  end
+end
+check(true, "every delivered message has a string type")
+
+for _, msg in ipairs(survivors) do
+  local ok = pcall(Json.encode, msg)
+  if not ok then
+    check(false, "every delivered message is still encodable (" .. msg.type .. ")")
+    break
+  end
+end
+check(true, "every delivered message is still encodable")
+
+-- ---------------------------------------------------------------- consumers
+
+local function makeFakeGame(species, name)
+  local save = require("src.core.SaveData").newGame()
+  save.player.name = name or "RED"
+  table.insert(save.party, Pokemon.new(Data, species, 20))
+  local stack = { list = {} }
+  function stack:push(s, ...)
+    table.insert(self.list, s)
+    if s.enter then s:enter(...) end
+  end
+  function stack:pop() return table.remove(self.list) end
+  function stack:top() return self.list[#self.list] end
+  function stack:update(dt)
+    local t = self:top()
+    if t and t.update then t:update(dt) end
+  end
+  return { data = Data, input = Input, stack = stack, save = save }
+end
+
+do
+  local crashed
+  for _, msg in ipairs(survivors) do
+    local party = { Pokemon.new(Data, "KADABRA", 30) }
+    local t = Protocol.TradeSession.new(Data, party)
+    local ok, err = pcall(function()
+      t:handle({ type = "party", mons = Protocol.packParty({
+        Pokemon.new(Data, "MACHOKE", 32) }) })
+      t:handle(msg)
+      t:pick(1)
+      t:handle(msg)
+      t:confirm(true)
+      t:handle(msg)
+      if t.stage == "done" then t:apply(nil) end
+    end)
+    if not ok then crashed = ("%s: %s"):format(tostring(msg.type), tostring(err)) end
+    if crashed then break end
+  end
+  check(not crashed, "the trade session survives every hostile message"
+        .. (crashed and (": " .. crashed) or ""))
+end
+
+do
+  local party = { Pokemon.new(Data, "KADABRA", 30) }
+  local t = Protocol.TradeSession.new(Data, party)
+  t:handle({ type = "party",
+             mons = Protocol.packParty({ Pokemon.new(Data, "MACHOKE", 32) }) })
+  t:pick(1)
+  t:handle(Wire.sanitize({ type = "pick", index = 0 }))
+  t:confirm(true)
+  t:handle(Wire.sanitize({ type = "confirm", ok = true }))
+  check(t.stage ~= "done", "an out-of-range pick never reaches a committed trade")
+  check(t.stage == "cancelled", "...it cancels the trade instead")
+end
+
+do
+  local gameA = makeFakeGame("CHARIZARD", "RED")
+  local gameB = makeFakeGame("BLASTOISE", "BLUE")
+  local netA, netB = Net.loopbackPair()
+  local battleA = LinkBattle.newHost(gameA, netA, {
+    myParty = Protocol.packParty(gameA.save.party),
+    theirParty = Protocol.packParty(gameB.save.party),
+    theirName = "BLUE", seed = 4242 })
+  gameA.stack:push(battleA)
+  local crashed
+  for _, msg in ipairs(survivors) do
+    table.insert(netA.inbox, copy(msg))
+    local ok, err = pcall(function()
+      Input.pressed = {}
+      gameA.stack:update(1 / 60)
+    end)
+    if not ok then
+      crashed = ("%s: %s"):format(tostring(msg.type), tostring(err))
+      break
+    end
+  end
+  check(not crashed, "a link battle survives every hostile message"
+        .. (crashed and (": " .. crashed) or ""))
+end
+
+do
+  local gameSpec = makeFakeGame("RATTATA", "WATCHER")
+  local specInbox = {}
+  local specNet = {
+    closed = false,
+    update = function() end,
+    poll = function()
+      local msgs = specInbox
+      specInbox = {}
+      return msgs
+    end,
+    send = function() end,
+    close = function() end,
+  }
+  local battle = LinkBattle.newSpectator(gameSpec, specNet, {
+    hostParty = Protocol.packParty(makeFakeGame("CHARIZARD").save.party),
+    guestParty = Protocol.packParty(makeFakeGame("BLASTOISE").save.party),
+    hostName = "RED", guestName = "BLUE", seed = 99 })
+  gameSpec.stack:push(battle)
+  local crashed
+  for _, msg in ipairs(corpus) do
+    for _, side in ipairs({ "host", "guest", 5, {} }) do
+      local wrapped = Wire.sanitize({ type = "spectate", side = side,
+                                      msg = copy(msg) })
+      if wrapped then table.insert(specInbox, wrapped) end
+    end
+    local ok, err = pcall(function()
+      Input.pressed = {}
+      gameSpec.stack:update(1 / 60)
+    end)
+    if not ok then
+      crashed = tostring(err)
+      break
+    end
+  end
+  check(not crashed, "a spectator battle survives every hostile envelope"
+        .. (crashed and (": " .. crashed) or ""))
+end
+
+do
+  local game = makeFakeGame("PIKACHU", "RED")
+  local exits = 0
+  local t = setmetatable({
+    game = game,
+    stage = "bracket",
+    index = 1,
+    settingsIndex = 1,
+    settings = { turnLimit = 6, requiredPartySize = 3, minLevel = "ANY",
+                 maxLevel = "ANY", forceLevel = "ANY", participating = true },
+    roster = {},
+    spectatorRoster = {},
+    isCreator = false,
+    net = { send = function() end, close = function() end,
+            take = function() return nil end,
+            poll = function() return {} end,
+            hasPending = function() return false end },
+  }, Tournament)
+  t.exitWith = function(self) exits = exits + 1 end
+  local crashed
+  for _, msg in ipairs(survivors) do
+    local ok, err = pcall(function()
+      t:handleMessage(msg)
+      t:draw()
+    end)
+    if not ok then
+      crashed = ("%s: %s"):format(tostring(msg.type), tostring(err))
+      break
+    end
+    if type(t.roster) ~= "table" or type(t.spectatorRoster) ~= "table" then
+      crashed = ("%s left a non-table roster"):format(tostring(msg.type))
+      break
+    end
+  end
+  check(not crashed, "the tournament screen survives every hostile message"
+        .. (crashed and (": " .. crashed) or ""))
+end
+
+-- ---------------------------------------------------------------- json
+do
+  local deep = ("["):rep(4096) .. ("]"):rep(4096)
+  local value, err = Json.decode(deep)
+  check(value == nil and err ~= nil, "a deeply nested document is refused")
+  local long = '{"type":"hello","name":"' .. ("x"):rep(1024) .. '"}'
+  check(Json.decode(long, 256) == nil, "a document past the caller's cap is refused")
+  check(Json.decode(long) ~= nil, "...and the cap is opt-in for other callers")
+end
+
+-- ---------------------------------------------------------------- net caps
+do
+  local n = Net.new()
+  n.rxBuf = ("x"):rep(Net.MAX_LINE + 1)
+  n:drainLines()
+  check(n.closed and n.error ~= nil,
+        "a peer that never sends a newline closes the connection")
+  check(#n.rxBuf == 0, "...and the buffer is released")
+end
+
+print(("\nlink hostile: %d messages, %d failures"):format(#corpus, failures))
+assert(failures == 0, failures .. " hostile-input failure(s)")
+return true
diff --git a/tests/run_link_tests.lua b/tests/run_link_tests.lua
index 2b1921b3..cf751311 100644
--- a/tests/run_link_tests.lua
+++ b/tests/run_link_tests.lua
@@ -645,6 +645,14 @@ check(fxAdvances("messages", "linkNext"),
 local fuzzOk, fuzzErr = pcall(dofile, "tests/link_desync_fuzz.lua")
 check(fuzzOk, "lockstep desync fuzz" .. (fuzzOk and "" or (": " .. tostring(fuzzErr))))
 
+-- ---------------------------------------------------------------- hostile wire
+-- Every message type crossed with every wrong Lua type, through the real
+-- Session choke point and into the real consumers.  The regression net for
+-- the remote-crash payloads; self-contained like the fuzz above.
+local hostileOk, hostileErr = pcall(dofile, "tests/link_hostile.lua")
+check(hostileOk, "hostile wire suite"
+      .. (hostileOk and "" or (": " .. tostring(hostileErr))))
+
 -- ---------------------------------------------------------------- mod link compat
 -- Self-contained like the tests/mod_*.lua suites: own bootstrap and
 -- assert-based checks, so it lands here as a single pass/fail line.