diff --git a/prod-compose.yml b/prod-compose.yml
index 1bd73b1..6444cd5 100644
--- a/prod-compose.yml
+++ b/prod-compose.yml
@@ -16,6 +16,10 @@ services:
       PRICE_UPDATE_INTERVAL_MINUTES: "${PRICE_UPDATE_INTERVAL_MINUTES:-15}"
       ZOMBIE_ZERO_COUNT: "${ZOMBIE_ZERO_COUNT:-1000}"
       HASHTAG_ACTIVE_HOURS: "${HASHTAG_ACTIVE_HOURS:-24}"
+      MAX_POSITION_SHARES: "${MAX_POSITION_SHARES:-100}"
+      MAX_POSITION_VALUE: "${MAX_POSITION_VALUE:-1000}"
+      FUND_MAX_POSITION_SHARES: "${FUND_MAX_POSITION_SHARES:-1000}"
+      FUND_MAX_POSITION_VALUE: "${FUND_MAX_POSITION_VALUE:-10000}"
     depends_on:
       postgres:
         condition: service_healthy
diff --git a/src/app/api/trade/route.ts b/src/app/api/trade/route.ts
index c0ef58b..7ef0cf9 100644
--- a/src/app/api/trade/route.ts
+++ b/src/app/api/trade/route.ts
@@ -3,8 +3,14 @@ import { getServerSession } from 'next-auth'
 import { authOptions } from '@/lib/auth'
 import { prisma } from '@/lib/prisma'
 import { calcTrade } from '@/lib/pricing'
+import { formatCurrency } from '@/lib/utils'
 import { z } from 'zod'
 
+const MAX_POSITION_SHARES       = parseInt(process.env.MAX_POSITION_SHARES        ?? '100',   10)
+const MAX_POSITION_VALUE        = parseInt(process.env.MAX_POSITION_VALUE         ?? '1000',  10)
+const FUND_MAX_POSITION_SHARES  = parseInt(process.env.FUND_MAX_POSITION_SHARES   ?? '1000',  10)
+const FUND_MAX_POSITION_VALUE   = parseInt(process.env.FUND_MAX_POSITION_VALUE    ?? '10000', 10)
+
 const tradeSchema = z.object({
   hashtagId: z.string().min(1),
   type: z.enum(['BUY_LONG', 'SELL_LONG', 'BUY_SHORT', 'SELL_SHORT']),
@@ -63,6 +69,25 @@ export async function POST(req: NextRequest) {
     return NextResponse.json({ error: 'Insufficient balance.' }, { status: 400 })
   }
 
+  if (type === 'BUY_LONG' || type === 'BUY_SHORT') {
+    const maxShares = fundId ? FUND_MAX_POSITION_SHARES : MAX_POSITION_SHARES
+    const maxValue  = fundId ? FUND_MAX_POSITION_VALUE  : MAX_POSITION_VALUE
+    const newTotalShares = (existingPosition?.shares ?? 0) + shares
+    const newTotalValue  = newTotalShares * hashtag.currentPrice
+    if (newTotalShares > maxShares) {
+      return NextResponse.json(
+        { error: `Position limit: max ${maxShares.toLocaleString()} shares per hashtag.` },
+        { status: 400 },
+      )
+    }
+    if (newTotalValue > maxValue) {
+      return NextResponse.json(
+        { error: `Position limit: max ${formatCurrency(maxValue)} position value per hashtag.` },
+        { status: 400 },
+      )
+    }
+  }
+
   if (type === 'SELL_LONG') {
     if (!existingPosition || existingPosition.shares < shares) {
       return NextResponse.json({ error: 'Insufficient shares to sell.' }, { status: 400 })
diff --git a/src/lib/utils.ts b/src/lib/utils.ts
index 7cf2948..a0d1fd1 100644
--- a/src/lib/utils.ts
+++ b/src/lib/utils.ts
@@ -6,6 +6,15 @@ export function cn(...inputs: ClassValue[]) {
 }
 
 export function formatCurrency(value: number): string {
+  const abs = Math.abs(value)
+  if (abs >= 10_000) {
+    return new Intl.NumberFormat('en-US', {
+      style: 'currency',
+      currency: 'USD',
+      notation: 'compact',
+      maximumFractionDigits: 2,
+    }).format(value)
+  }
   return new Intl.NumberFormat('en-US', {
     style: 'currency',
     currency: 'USD',