diff --git a/packages/server/src/protocol.test.ts b/packages/server/src/protocol.test.ts
index 695f1d6..e372372 100644
--- a/packages/server/src/protocol.test.ts
+++ b/packages/server/src/protocol.test.ts
@@ -681,12 +681,31 @@ describe("ModifierProfileSchema", () => {
     expect(r.success).toBe(false);
   });
 
-  it("rejects a profile with unknown modifier kind", () => {
+  it("accepts a profile with arbitrary modifier kind (T3 widening for custom ids)", () => {
+    // Pre-T3 the wire enum rejected unknown kinds. The widening was
+    // forced by user-authored CustomModifierIds (arbitrary strings like
+    // "custom:shield"); kind validity is now enforced by the engine's
+    // registry-dispatch fallback at apply time, not at the wire.
     const r = ModifierProfileSchema.safeParse({
       ...validProfile,
       perType: [
         {
-          kind: "teleport",
+          kind: "custom:teleport",
+          pieceType: "pawn",
+          color: "white",
+          value: 1,
+        },
+      ],
+    });
+    expect(r.success).toBe(true);
+  });
+
+  it("rejects a profile with empty-string modifier kind", () => {
+    const r = ModifierProfileSchema.safeParse({
+      ...validProfile,
+      perType: [
+        {
+          kind: "",
           pieceType: "pawn",
           color: "white",
           value: 1,
diff --git a/packages/server/src/protocol.ts b/packages/server/src/protocol.ts
index d9b97f6..9f431d0 100644
--- a/packages/server/src/protocol.ts
+++ b/packages/server/src/protocol.ts
@@ -195,14 +195,32 @@ export type ResolvedLayout = z.infer<typeof ResolvedLayoutSchema>;
 // assertion below.
 // ---------------------------------------------------------------------------
 
-const ModifierKindIdSchema = z.enum([
+/**
+ * Built-in modifier ids — documented here for reference; the actual
+ * parse schema below accepts ANY non-empty string.
+ *
+ * T3 widens `kind` away from a literal enum because user-authored
+ * custom modifier descriptors use arbitrary ids (e.g. "custom:shield").
+ * Rejecting unknown kinds server-side would drop every profile that
+ * references a custom descriptor, breaking multiplayer end-to-end.
+ *
+ * Validity of the kind is enforced ELSEWHERE: the chess engine's
+ * apply pipeline looks the id up in MODIFIER_REGISTRY (built-ins)
+ * then engine.customModifiers (user-authored). Unknown kinds are
+ * warned and skipped — forwards-compat with a new built-in by
+ * design.
+ */
+const BUILTIN_MODIFIER_KINDS = [
   "hp-bonus",
   "range-bonus",
   "direction-additions",
   "capture-flags",
   "promotion-override",
   "damage-resistance",
-]);
+] as const;
+void BUILTIN_MODIFIER_KINDS; // documentation anchor; server parse accepts any string
+
+const ModifierKindIdSchema = z.string().min(1);
 
 const ModifierColorExtSchema = z.enum(["white", "black", "both"]);