diff --git a/jsconfig.json b/jsconfig.json
new file mode 100644
index 00000000..a8f02f2c
--- /dev/null
+++ b/jsconfig.json
@@ -0,0 +1,29 @@
+{
+  // First typecheck slice (npm run typecheck). strict stays off until a folder is clean.
+  // tsc does not load this file unless invoked with -p jsconfig.json.
+  // Widen one path at a time: see types/README.md.
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "checkJs": true,
+    "allowJs": true,
+    "noEmit": true,
+    "strict": false,
+    "skipLibCheck": true,
+    "types": [],
+    "lib": ["ES2022"],
+    "maxNodeModuleJsDepth": 0
+  },
+  "include": [
+    "shared/**/*.js",
+    "server/sim/rng.js",
+    "server/sim/dir.js",
+    "server/sim/constants.js",
+    "types/**/*.js"
+  ],
+  "exclude": [
+    "shared/protocol.js",
+    "shared/bandBonds.js"
+  ]
+}
diff --git a/package-lock.json b/package-lock.json
index a5ef6302..78d2a72b 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -21,7 +21,8 @@
         "@eslint/js": "^10.0.1",
         "eslint": "^10.12.0",
         "globals": "^17.13.0",
-        "puppeteer-core": "^25.12.0"
+        "puppeteer-core": "^25.12.0",
+        "typescript": "^5.9.2"
       },
       "engines": {
         "node": ">=22"
@@ -2334,6 +2335,20 @@
       "engines": {
         "node": ">= 18"
       }
+    },
+    "node_modules/typescript": {
+      "version": "5.9.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",
+      "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
     }
   }
 }
diff --git a/package.json b/package.json
index 3a40f32e..cf814805 100644
--- a/package.json
+++ b/package.json
@@ -35,7 +35,8 @@
     "setup": "node tools/setup.mjs",
     "doctor": "node tools/doctor.mjs",
     "launch": "node scripts/launch.mjs",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit --checkJs -p jsconfig.json"
   },
   "engines": {
     "node": ">=22"
@@ -52,6 +53,7 @@
     "@eslint/js": "^10.0.1",
     "eslint": "^10.12.0",
     "globals": "^17.13.0",
-    "puppeteer-core": "^25.12.0"
+    "puppeteer-core": "^25.12.0",
+    "typescript": "^5.9.2"
   }
 }
diff --git a/types/README.md b/types/README.md
new file mode 100644
index 00000000..3e409922
--- /dev/null
+++ b/types/README.md
@@ -0,0 +1,25 @@
+# Typecheck
+
+`npm run typecheck` runs `tsc --noEmit --checkJs -p jsconfig.json`.
+
+The first slice is the closed set that already typechecks:
+
+- `shared/` except `shared/protocol.js` and `shared/bandBonds.js` (their JSDoc does not match the code yet; see below)
+- `server/sim/rng.js`, `server/sim/dir.js`, `server/sim/constants.js` (the last imports `GEO` from shared)
+- `types/` — JSDoc for `Unit`, `DamageInfo`, the snapshot tuples, and the protocol message envelope
+
+`strict` is off. `tsc` does not read `jsconfig.json` on its own; the script passes `-p`.
+
+## Widen it
+
+Add one file or one folder to `jsconfig.json` `include`, then run `npm run typecheck`. Fix JSDoc that disagrees with the code in that folder. Leave `strict` off until the folder reports nothing. Do not add `@ts-nocheck`.
+
+Suggested order once the sim split settles: `server/sim/grid.js`, `server/sim/body.js`, `server/sim/buffs.js`, then `server/sim/damage.js` against the typedefs in `types/core.js`. The sim does not import `types/`. The typedefs are a reference, not a runtime dependency.
+
+`types/core.js` follows docs/DESIGN.md §5.2, §5.4, §8 and docs/SIM.md §4, §9. Where those two disagree on `DamageInfo.type`, the typedef uses the union `makeDamageInfo` actually builds (`'elemental'` as well as `'element'`).
+
+## Not in the first slice
+
+`shared/protocol.js` — the catalogue JSDoc is ahead of a few object literals (`$optional` sits on a `Record` of checkers; `unitStatsEntry` does not list `side` or `silenced`, and the code reads `side`). Fix the comments, then remove it from `exclude`.
+
+`shared/bandBonds.js` — the data-shape JSDoc rejects an empty fallback object and a tuple callback. Same treatment.
diff --git a/types/core.js b/types/core.js
new file mode 100644
index 00000000..effe5f3a
--- /dev/null
+++ b/types/core.js
@@ -0,0 +1,119 @@
+/**
+ * Core shapes for the sim and the wire protocol.
+ *
+ * Taken from docs/DESIGN.md §5.2 (Unit), §5.4 (DamageInfo), §8 (envelope, b.snap)
+ * and docs/SIM.md §4 (DamageInfo as makeDamageInfo builds it) and §9 (snapshot tuples).
+ * The sim does not import this file. It is here so the shapes have one JSDoc home
+ * while server/sim is still being split.
+ */
+
+/**
+ * A unit inside one battle. Minimum fields from DESIGN §5.2.
+ * Stats (`maxHp`, `atk`, …) are recomputed by the sim and are not all listed here.
+ *
+ * @typedef {Object} Unit
+ * @property {number} id unique per battle
+ * @property {'ally'|'enemy'} side
+ * @property {'op'|'token'|'enemy'|'device'} kind
+ * @property {string} defId
+ * @property {string|number} ownerId player id; enemies use the field owner or the source player
+ * @property {number} x
+ * @property {number} y
+ * @property {number} [tileR] operators
+ * @property {number} [tileC] operators
+ * @property {'UP'|'RIGHT'|'DOWN'|'LEFT'} [dir] allies: deploy direction. Enemies are not meaningful here.
+ * @property {number} facing horizontal sign, +1 or −1 (LEFT is −1)
+ * @property {number} hp
+ * @property {boolean} alive
+ * @property {number} sp
+ * @property {Object[]} [buffs]
+ * @property {number} [statusFlags]
+ * @property {number[]} [blocking] operators: ids of enemies they block
+ * @property {number|null} [blockedBy] enemies: blocker id
+ * @property {number[][]} [liveRangeGrid] the attack range the detail card shows
+ * @property {string} [form] model form (enemy mode, or a doll's `'doll'`)
+ */
+
+/**
+ * One damage instance, after `makeDamageInfo` (server/sim/damage.js).
+ * DESIGN §5.4 lists `type` as `'phys'|'arts'|'true'|'element'`.
+ * SIM §4 and the normaliser also use `'elemental'` (HP damage from an element burst)
+ * and keep `'element'` for gauge fill, which removes no HP.
+ *
+ * @typedef {Object} DamageInfo
+ * @property {number} amount pre-mitigation
+ * @property {'phys'|'arts'|'true'|'elemental'|'element'} type
+ * @property {'burn'|'necrosis'|'neural'|'apoptosis'|string|null} [element]
+ * @property {number} [atkScale]
+ * @property {number} [defIgnoreFlat]
+ * @property {number} [defIgnorePct]
+ * @property {number} [resIgnoreFlat]
+ * @property {number} [resIgnorePct]
+ * @property {number} [mul]
+ * @property {boolean} [canDodge]
+ * @property {boolean} [isSkill]
+ * @property {boolean} [isSplash]
+ * @property {boolean} [isAttack]
+ * @property {boolean} [isProjectile]
+ * @property {string[]} [tags]
+ * @property {boolean} [cancel] set by a `hit` hook to drop the instance
+ * @property {boolean} [noSp]
+ * @property {boolean} [ignoreSleep]
+ * @property {boolean} [ignoreSelect] reaches an airborne ally; the hit selected nobody
+ * @property {boolean} [sourceless] 无来源: source stats add nothing
+ * @property {number} [attackId] shared by every instance of one normal attack; 0 otherwise
+ * @property {Unit|null} [traitAlly]
+ */
+
+/**
+ * One living unit in a snapshot.
+ * `[id, x, y, hp, maxHp, sp, spMax, flags, anim]` (SIM §9, DESIGN §8.2).
+ * `flags` is the UF bitmask. `anim` is the ANIM code.
+ *
+ * @typedef {[number, number, number, number, number, number, number, number, number]} UnitSnap
+ */
+
+/**
+ * A knocked-out operator waiting to redeploy.
+ * `[id, respawnAt, respawnTime, state, row, col]`.
+ * `state`: 0 counting, 1 timer done and DP short, 2 timer done and the tile is taken.
+ *
+ * @typedef {[number, number, number, number, number, number]} DownSnap
+ */
+
+/**
+ * Element gauge or burst cooldown on one unit.
+ * `[id, element, fill, cooldownEnd, cooldown]`.
+ *
+ * @typedef {[number, string, number, number, number]} ElemSnap
+ */
+
+/**
+ * `snapshot()` (SIM §9). On the wire this object is the payload of `b.snap`,
+ * and `t` is sent as `gt` because the frame's `t` is the message type (DESIGN §8.2).
+ * `dp` is one number. `down` and `elem` are omitted when empty.
+ *
+ * @typedef {Object} FieldSnapshot
+ * @property {string} fieldId
+ * @property {number} t game seconds
+ * @property {UnitSnap[]} units
+ * @property {number} dp
+ * @property {number} killed
+ * @property {number} total
+ * @property {Object<string, number>} [dps]
+ * @property {Object} [boss]
+ * @property {DownSnap[]} [down]
+ * @property {ElemSnap[]} [elem]
+ */
+
+/**
+ * One WebSocket JSON frame (DESIGN §8). Client requests may carry `rid`.
+ * The server's `ok` or `error` echoes it. Server pushes omit it.
+ * Other own fields are the payload. Inbound frames are capped at 64 KB.
+ *
+ * @typedef {Object} ProtocolMessage
+ * @property {string} t message type
+ * @property {number} [rid] request id
+ */
+
+export {};