diff --git a/packages/chess/src/modifiers/schema.ts b/packages/chess/src/modifiers/schema.ts
index 5ace534..8c8ad94 100644
--- a/packages/chess/src/modifiers/schema.ts
+++ b/packages/chess/src/modifiers/schema.ts
@@ -1,10 +1,14 @@
 /**
  * Zod schemas for ModifierProfile serialization and validation.
  *
- * The `value` field on TypeModifier and InstanceModifier is intentionally
- * `z.unknown()` — per-kind value validation happens at descriptor
- * registration time, not at profile-parse time. This keeps the schema
- * forwards-compatible with future modifier kinds.
+ * The `value` field on TypeModifier and InstanceModifier uses `z.any()`
+ * rather than `z.unknown()` — Zod v4 infers `z.unknown()` object fields as
+ * optional (`value?: unknown`) which conflicts with the required `value:
+ * unknown` in the TypeModifier/InstanceModifier interfaces. `z.any()` infers
+ * as a required `any` field while still accepting all values.
+ *
+ * Per-kind value validation happens at descriptor registration time, not at
+ * profile-parse time. This keeps the schema forwards-compatible.
  */
 import { z } from "zod";
 import type { ModifierProfile } from "./types.js";
@@ -47,7 +51,7 @@ export const TypeModifierSchema = z.object({
   kind: ModifierKindIdSchema,
   pieceType: PieceTypeSchema,
   color: PieceColorExtSchema,
-  value: z.unknown(),
+  value: z.any() as z.ZodType<unknown>,
 });
 
 // ---------------------------------------------------------------------------
@@ -57,7 +61,7 @@ export const TypeModifierSchema = z.object({
 export const InstanceModifierSchema = z.object({
   kind: ModifierKindIdSchema,
   square: SquareStringSchema,
-  value: z.unknown(),
+  value: z.any() as z.ZodType<unknown>,
 });
 
 // ---------------------------------------------------------------------------
diff --git a/packages/chess/src/ui/Lobby.tsx b/packages/chess/src/ui/Lobby.tsx
index 09fb469..3be840c 100644
--- a/packages/chess/src/ui/Lobby.tsx
+++ b/packages/chess/src/ui/Lobby.tsx
@@ -15,6 +15,8 @@ import {
 import type { LayoutRequest } from '../net/types';
 import { LayoutPicker } from './LayoutPicker';
 import { LayoutEditor } from './LayoutEditor';
+import { parseModifierProfile } from '../modifiers/schema';
+import type { ModifierProfile } from '../modifiers/types';
 
 interface LobbyProps {
   /** Optional — when provided, create/join/solo flows reset the local
@@ -38,6 +40,10 @@ export function Lobby({ chessState }: LobbyProps = {}) {
   const [selectedLayout, setSelectedLayout] =
     useState<StartingLayout>(CLASSIC_LAYOUT);
   const [editorOpen, setEditorOpen] = useState(false);
+  // Modifier profile pre-selected from the ?modifierProfile URL param.
+  // Stored here for future integration with the modifier profile picker.
+  const [_preselectedProfile, setPreselectedProfile] =
+    useState<ModifierProfile | null>(null);
 
   useEffect(() => {
     const layoutId = searchParams.get('layoutId');
@@ -69,6 +75,22 @@ export function Lobby({ chessState }: LobbyProps = {}) {
         }
       }
     }
+
+    // Pre-select a modifier profile from the ?modifierProfile param.
+    // The profile is base64-encoded JSON. Silently ignore on parse
+    // failure — a bad link shouldn't break the lobby.
+    const modifierProfileParam = searchParams.get('modifierProfile');
+    if (modifierProfileParam !== null) {
+      try {
+        const profile = parseModifierProfile(
+          JSON.parse(atob(modifierProfileParam)),
+        );
+        setPreselectedProfile(profile);
+      } catch {
+        /* invalid param — ignore */
+      }
+    }
+
     // Only depends on searchParams: we pre-select from query params
     // on first mount or when the URL changes. selectedLayout is
     // intentionally omitted so user picks after mount don't re-trigger
diff --git a/packages/chess/src/ui/PerTypePanel.tsx b/packages/chess/src/ui/PerTypePanel.tsx
new file mode 100644
index 0000000..a16c0f7
--- /dev/null
+++ b/packages/chess/src/ui/PerTypePanel.tsx
@@ -0,0 +1,330 @@
+/**
+ * PerTypePanel — left panel of the Modifier Profile Editor.
+ *
+ * Lists existing per-type modifiers and provides an inline form to add new ones.
+ * Each modifier applies to ALL pieces of a given type+color combination.
+ *
+ * Value editing is uiForm-driven:
+ *   "number"           → plain number input (validated by Zod schema)
+ *   "percentage"       → 0–100 input, mapped internally to 0–1
+ *   "promotion-target" → select: queen / rook / bishop / knight / disabled
+ *   "direction-set"    → placeholder (T21+)
+ *   "capture-flags"    → placeholder (T21+)
+ */
+import { useState } from 'react';
+import { MODIFIER_REGISTRY } from '../modifiers/index.js';
+import type { TypeModifier, ModifierDescriptor } from '../modifiers/types.js';
+import type { PieceType, PieceColor } from '../schema.js';
+
+const PIECE_TYPES: PieceType[] = [
+  'pawn', 'knight', 'bishop', 'rook', 'queen', 'king',
+];
+const COLORS: (PieceColor | 'both')[] = ['white', 'black', 'both'];
+
+interface Props {
+  modifiers: readonly TypeModifier[];
+  onAdd: (modifier: TypeModifier) => void;
+  onDelete: (index: number) => void;
+}
+
+/**
+ * Parse the raw string input into the typed value expected by the
+ * descriptor's Zod schema. Returns `undefined` when the input is
+ * empty or unconvertible (blocks the Save button).
+ */
+function parseFormValue(descriptor: ModifierDescriptor, rawValue: string): unknown {
+  switch (descriptor.uiForm) {
+    case 'number': {
+      if (rawValue === '') return undefined;
+      const n = Number(rawValue);
+      return isNaN(n) ? undefined : n;
+    }
+    case 'percentage': {
+      if (rawValue === '') return undefined;
+      const n = Number(rawValue);
+      return isNaN(n) ? undefined : n / 100;
+    }
+    case 'promotion-target':
+      return rawValue || undefined;
+    case 'direction-set':
+    case 'capture-flags':
+      // Placeholder forms cannot produce a valid value yet.
+      return undefined;
+  }
+}
+
+export function PerTypePanel({ modifiers, onAdd, onDelete }: Props) {
+  const [formOpen, setFormOpen] = useState(false);
+  const [pieceType, setPieceType] = useState<PieceType>('pawn');
+  const [color, setColor] = useState<PieceColor | 'both'>('both');
+
+  const descriptors = MODIFIER_REGISTRY.list();
+  const [kindId, setKindId] = useState<string>(descriptors[0]?.id ?? '');
+  const [rawValue, setRawValue] = useState('');
+
+  const descriptor = descriptors.find((d) => d.id === kindId);
+  const parsedValue = descriptor ? parseFormValue(descriptor, rawValue) : undefined;
+  const isValid =
+    descriptor !== undefined &&
+    parsedValue !== undefined &&
+    descriptor.valueSchema.safeParse(parsedValue).success;
+
+  function resetForm() {
+    setPieceType('pawn');
+    setColor('both');
+    setKindId(descriptors[0]?.id ?? '');
+    setRawValue('');
+  }
+
+  function handleSave() {
+    if (!isValid || !descriptor) return;
+    onAdd({
+      kind: descriptor.id,
+      pieceType,
+      color,
+      value: parsedValue,
+    });
+    setFormOpen(false);
+    resetForm();
+  }
+
+  return (
+    <div className="flex flex-col h-full overflow-hidden">
+      {/* Panel header */}
+      <div className="px-4 pt-4 pb-2 border-b border-neutral-200 bg-neutral-50">
+        <p className="text-xs font-bold text-neutral-400 uppercase tracking-widest">
+          Per-Type Modifiers
+        </p>
+      </div>
+
+      {/* Scrollable body */}
+      <div className="flex-1 overflow-y-auto p-4 space-y-2 bg-neutral-50">
+        {/* Empty state */}
+        {modifiers.length === 0 && !formOpen && (
+          <p className="text-sm text-neutral-400 italic">No type modifiers yet.</p>
+        )}
+
+        {/* Existing modifier rows */}
+        {modifiers.map((mod, i) => {
+          const desc = MODIFIER_REGISTRY.get(mod.kind);
+          return (
+            <div
+              key={i}
+              data-testid="type-modifier-row"
+              className="flex items-center justify-between bg-white border border-neutral-200 rounded-lg px-3 py-2 text-sm"
+            >
+              <span>
+                <span className="font-semibold capitalize">{mod.pieceType}</span>
+                {' '}
+                <span className="text-neutral-500">({mod.color})</span>
+                {': '}
+                <span className="text-neutral-700">
+                  {desc ? desc.describe(mod.value) : String(mod.value)}
+                </span>
+              </span>
+              <button
+                type="button"
+                onClick={() => onDelete(i)}
+                aria-label="Delete modifier"
+                className="ml-2 text-neutral-400 hover:text-red-600 transition-colors"
+              >
+                ×
+              </button>
+            </div>
+          );
+        })}
+
+        {/* Add button — hidden while form is open */}
+        {!formOpen && (
+          <button
+            type="button"
+            data-testid="add-type-modifier"
+            onClick={() => setFormOpen(true)}
+            className="w-full px-3 py-2 text-sm font-semibold text-neutral-700 bg-white border border-dashed border-neutral-300 rounded-lg hover:bg-neutral-50 transition-colors"
+          >
+            + Add Type Modifier
+          </button>
+        )}
+
+        {/* Inline add form */}
+        {formOpen && (
+          <div className="border border-neutral-200 rounded-lg bg-white p-3 space-y-3">
+            {/* Piece type */}
+            <div>
+              <label className="block text-xs font-semibold text-neutral-500 uppercase tracking-wide mb-1">
+                Piece Type
+              </label>
+              <select
+                data-testid="piece-type-select"
+                value={pieceType}
+                onChange={(e) => setPieceType(e.target.value as PieceType)}
+                className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+              >
+                {PIECE_TYPES.map((t) => (
+                  <option key={t} value={t}>
+                    {t.charAt(0).toUpperCase() + t.slice(1)}
+                  </option>
+                ))}
+              </select>
+            </div>
+
+            {/* Color */}
+            <div>
+              <label className="block text-xs font-semibold text-neutral-500 uppercase tracking-wide mb-1">
+                Color
+              </label>
+              <select
+                data-testid="color-select"
+                value={color}
+                onChange={(e) => setColor(e.target.value as PieceColor | 'both')}
+                className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+              >
+                {COLORS.map((c) => (
+                  <option key={c} value={c}>
+                    {c.charAt(0).toUpperCase() + c.slice(1)}
+                  </option>
+                ))}
+              </select>
+            </div>
+
+            {/* Modifier kind */}
+            <div>
+              <label className="block text-xs font-semibold text-neutral-500 uppercase tracking-wide mb-1">
+                Modifier
+              </label>
+              <select
+                data-testid="kind-select"
+                value={kindId}
+                onChange={(e) => {
+                  setKindId(e.target.value);
+                  setRawValue('');
+                }}
+                className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+              >
+                {descriptors.map((d) => (
+                  <option key={d.id} value={d.id}>
+                    {d.label}
+                  </option>
+                ))}
+              </select>
+            </div>
+
+            {/* Value input — rendered by uiForm type */}
+            {descriptor !== undefined && (
+              <div>
+                <label className="block text-xs font-semibold text-neutral-500 uppercase tracking-wide mb-1">
+                  Value
+                </label>
+                <ValueInput
+                  descriptor={descriptor}
+                  rawValue={rawValue}
+                  onChange={setRawValue}
+                />
+              </div>
+            )}
+
+            {/* Actions */}
+            <div className="flex gap-2 pt-1">
+              <button
+                type="button"
+                data-testid="save-type-modifier"
+                onClick={handleSave}
+                disabled={!isValid}
+                className="flex-1 px-3 py-1.5 text-xs font-semibold text-white bg-neutral-900 rounded hover:bg-neutral-800 disabled:opacity-40 disabled:cursor-not-allowed transition-colors"
+              >
+                Save
+              </button>
+              <button
+                type="button"
+                onClick={() => {
+                  setFormOpen(false);
+                  resetForm();
+                }}
+                className="px-3 py-1.5 text-xs font-semibold text-neutral-700 bg-neutral-100 rounded hover:bg-neutral-200 transition-colors"
+              >
+                Cancel
+              </button>
+            </div>
+          </div>
+        )}
+      </div>
+    </div>
+  );
+}
+
+// ── Value input subcomponent ────────────────────────────────────────────
+
+function ValueInput({
+  descriptor,
+  rawValue,
+  onChange,
+}: {
+  descriptor: ModifierDescriptor;
+  rawValue: string;
+  onChange: (v: string) => void;
+}) {
+  switch (descriptor.uiForm) {
+    case 'number':
+      return (
+        <input
+          type="number"
+          data-testid="value-input"
+          value={rawValue}
+          onChange={(e) => onChange(e.target.value)}
+          placeholder="Enter a number"
+          className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+        />
+      );
+
+    case 'percentage':
+      return (
+        <div className="flex items-center gap-2">
+          <input
+            type="number"
+            data-testid="value-input"
+            min={0}
+            max={100}
+            step={1}
+            value={rawValue}
+            onChange={(e) => onChange(e.target.value)}
+            placeholder="0–100"
+            className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+          />
+          <span className="text-sm text-neutral-500 flex-shrink-0">%</span>
+        </div>
+      );
+
+    case 'promotion-target':
+      return (
+        <select
+          data-testid="value-input"
+          value={rawValue}
+          onChange={(e) => onChange(e.target.value)}
+          className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
+        >
+          <option value="">— select —</option>
+          {(['queen', 'rook', 'bishop', 'knight', 'disabled'] as const).map(
+            (v) => (
+              <option key={v} value={v}>
+                {v.charAt(0).toUpperCase() + v.slice(1)}
+              </option>
+            ),
+          )}
+        </select>
+      );
+
+    case 'direction-set':
+      return (
+        <p className="text-xs text-neutral-400 italic py-1">
+          Direction set editor coming soon
+        </p>
+      );
+
+    case 'capture-flags':
+      return (
+        <p className="text-xs text-neutral-400 italic py-1">
+          Capture flags editor coming soon
+        </p>
+      );
+  }
+}