diff --git a/server/src/engine/answers-edge.test.ts b/server/src/engine/answers-edge.test.ts
index 7b120ad..2c87634 100644
--- a/server/src/engine/answers-edge.test.ts
+++ b/server/src/engine/answers-edge.test.ts
@@ -95,3 +95,90 @@ describe('checkAnswer edge cases', () => {
     expect(checkAnswer('FALSE', ['false'])).toBeGreaterThanOrEqual(0.9)
   })
 })
+
+describe('checkAnswer adversarial profiling — target <1ms per check', () => {
+  const TARGET_MS = 1
+
+  it('2000-char response', () => {
+    const longAnswer = 'x'.repeat(2000)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(longAnswer, ['Bitcoin', '42', 'Satoshi Nakamoto'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('2000-char response containing the answer buried deep', () => {
+    const longAnswer = 'z'.repeat(1900) + ' Bitcoin ' + 'z'.repeat(91)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(longAnswer, ['Bitcoin'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('regex metacharacters in response — no backtracking', () => {
+    const regexBomb = '(a+)+$'.repeat(200) + '.*?.*?.*?' + '['.repeat(100)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(regexBomb, ['42', 'Bitcoin'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('regex metacharacters in accepted answer — escaped safely', () => {
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer('42', ['(a+)+$', '.*+', '[test]'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('unicode heavy response — CJK, emoji, combining chars', () => {
+    const unicode = '比特币₿🚀'.repeat(300) + ' Bitcoin ' + '漢字'.repeat(100)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(unicode, ['Bitcoin', '比特币'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('unicode combining characters and diacritics', () => {
+    // Zalgo text: base char + many combining marks
+    const zalgo = 'B' + '\u0300\u0301\u0302\u0303\u0304'.repeat(50) + 'itcoin'
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(zalgo, ['Bitcoin'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('many accepted answers (20) with long response', () => {
+    const answers = Array.from({ length: 20 }, (_, i) => `answer_variant_${i}_satoshi`)
+    const response = 'a'.repeat(1000) + ' answer_variant_19_satoshi ' + 'b'.repeat(1000)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(response, answers)
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('pathological whitespace — tabs, newlines, mixed', () => {
+    const ws = '\t\n\r '.repeat(500) + 'Bitcoin' + ' \t\n'.repeat(500)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(ws, ['Bitcoin'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('numeric answer in huge response — boundary regex safe', () => {
+    // Tests the dynamic RegExp(numStr) path with number buried in text
+    const response = 'word '.repeat(400) + '21000000' + ' word'.repeat(400)
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(response, ['21000000'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+
+  it('contraction-heavy 2000-char response', () => {
+    const contractions = "can't don't won't doesn't isn't aren't wasn't weren't hasn't haven't hadn't couldn't shouldn't wouldn't it's that's they're we're you're "
+    const response = contractions.repeat(15) // ~2000 chars
+    const start = performance.now()
+    for (let i = 0; i < 100; i++) checkAnswer(response, ['cannot', 'will not', 'they are'])
+    const avg = (performance.now() - start) / 100
+    expect(avg).toBeLessThan(TARGET_MS)
+  })
+})
diff --git a/server/src/lib/validators.test.ts b/server/src/lib/validators.test.ts
index ab249d6..faaae3b 100644
--- a/server/src/lib/validators.test.ts
+++ b/server/src/lib/validators.test.ts
@@ -24,6 +24,7 @@ import {
   startTournamentSchema,
   joinRankedSchema,
   testWebhookSchema,
+  sanitizeError,
 } from './validators.js'
 
 // --- Primitive schemas ---
@@ -267,6 +268,68 @@ describe('joinRankedSchema', () => {
   })
 })
 
+// --- sanitizeError ---
+
+describe('sanitizeError', () => {
+  it('returns fallback for non-Error values', () => {
+    expect(sanitizeError('string error', 'fallback')).toBe('fallback')
+    expect(sanitizeError(null, 'fallback')).toBe('fallback')
+    expect(sanitizeError(undefined, 'fallback')).toBe('fallback')
+    expect(sanitizeError(42, 'fallback')).toBe('fallback')
+  })
+
+  it('passes through safe error messages', () => {
+    expect(sanitizeError(new Error('Bot not found'), 'fallback')).toBe('Bot not found')
+    expect(sanitizeError(new Error('Payment already confirmed'), 'fallback')).toBe('Payment already confirmed')
+    expect(sanitizeError(new Error('already in a fight'), 'fallback')).toBe('already in a fight')
+    expect(sanitizeError(new Error('Invoice creation failed'), 'fallback')).toBe('Invoice creation failed')
+  })
+
+  it('strips messages with TypeScript file paths', () => {
+    expect(sanitizeError(new Error('TypeError at /src/engine/payments.ts:42'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('Cannot read property of null at file.ts:10'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with JavaScript file paths', () => {
+    expect(sanitizeError(new Error('ReferenceError in module.js:5'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('Error in handler.mjs '), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with /src/ paths', () => {
+    expect(sanitizeError(new Error('Failed to load /src/config/keys'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with node_modules paths', () => {
+    expect(sanitizeError(new Error('Error in /node_modules/drizzle-orm/dist/index.js'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with stack trace fragments', () => {
+    expect(sanitizeError(new Error('at Object.runInContext (vm.js:130)'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('at Module._compile (internal/modules)'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('at async Router.handle'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with SQLite errors', () => {
+    expect(sanitizeError(new Error('SQLITE_CONSTRAINT: UNIQUE constraint failed'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('SQLITE_ERROR: no such table: users'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with system errors', () => {
+    expect(sanitizeError(new Error('ENOENT: no such file or directory'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('ECONNREFUSED 127.0.0.1:5432'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('EACCES: permission denied'), 'fallback')).toBe('fallback')
+  })
+
+  it('strips messages with absolute paths', () => {
+    expect(sanitizeError(new Error('Cannot open /Users/deploy/app/db.sqlite'), 'fallback')).toBe('fallback')
+    expect(sanitizeError(new Error('File not found: /home/app/config.json'), 'fallback')).toBe('fallback')
+  })
+
+  it('returns fallback for empty message', () => {
+    expect(sanitizeError(new Error(''), 'fallback')).toBe('fallback')
+  })
+})
+
 // --- Attack inputs ---
 
 describe('attack inputs', () => {
diff --git a/server/src/lib/validators.ts b/server/src/lib/validators.ts
index b633f1b..d53a703 100644
--- a/server/src/lib/validators.ts
+++ b/server/src/lib/validators.ts
@@ -143,6 +143,21 @@ export const testWebhookSchema = z.object({
 
 // --- Error formatting helper ---
 
+/** Patterns that indicate internal details that should not be exposed to clients */
+const UNSAFE_PATTERNS = /\.(ts|js|mjs)(:|$|\s|\))|\/src\/|\/node_modules\/|at\s+(Object|Module|Function|async)[\s.]|SQLITE_|ENOENT|ECONNREFUSED|EACCES|EPERM|errno|\/Users\/|\/home\/|\\x[0-9a-f]/i
+
+/**
+ * Sanitize an error message before returning it to the client.
+ * Strips internal details like file paths, stack fragments, and system errors.
+ * Returns the original message if it appears safe, or the fallback otherwise.
+ */
+export function sanitizeError(err: unknown, fallback: string): string {
+  if (!(err instanceof Error)) return fallback
+  const msg = err.message
+  if (!msg || UNSAFE_PATTERNS.test(msg)) return fallback
+  return msg
+}
+
 /** Map Zod validation errors to user-friendly messages by field name */
 export function formatZodError(
   error: z.ZodError,
diff --git a/server/src/routes/admin.ts b/server/src/routes/admin.ts
index c7315a2..4b3beac 100644
--- a/server/src/routes/admin.ts
+++ b/server/src/routes/admin.ts
@@ -4,6 +4,7 @@ import { eq, desc, sql, count } from 'drizzle-orm'
 import { getActiveSSECount } from './fights.js'
 import { createBackup } from '../engine/backup.js'
 import { isCreatorPubkey } from '../lib/constants.js'
+import { sanitizeError } from '../lib/validators.js'
 
 const startTime = Date.now()
 
@@ -115,7 +116,7 @@ adminRouter.get('/backup', (c) => {
     const path = createBackup()
     return c.json({ ok: true, path })
   } catch (err: unknown) {
-    const msg = err instanceof Error ? err.message : 'Backup failed'
+    const msg = sanitizeError(err, 'Backup failed')
     return c.json({ error: msg }, 500)
   }
 })
diff --git a/server/src/routes/error-handling.test.ts b/server/src/routes/error-handling.test.ts
index d31100f..5a859dd 100644
--- a/server/src/routes/error-handling.test.ts
+++ b/server/src/routes/error-handling.test.ts
@@ -94,4 +94,22 @@ describe('global error handler', () => {
       expect(content).not.toMatch(/c\.json\([^)]*error\.stack/)
     }
   })
+
+  it('route catch blocks use sanitizeError instead of raw err.message', async () => {
+    const fs = await import('fs')
+    const path = await import('path')
+    const routesDir = path.resolve(import.meta.dirname || '.', '.')
+    // Files that handle errors returned to clients (excluding webhook test tools: bots.ts, docs.ts)
+    const checkedFiles = ['payments.ts', 'queue.ts', 'fights.ts', 'admin.ts', 'tournaments.ts']
+
+    for (const file of checkedFiles) {
+      const filePath = path.join(routesDir, file)
+      if (!fs.existsSync(filePath)) continue
+      const content = fs.readFileSync(filePath, 'utf-8')
+      // Should not have the old pattern: err instanceof Error ? err.message : '...'
+      // in the context of returning to c.json (route responses)
+      const unsafePattern = /c\.json\(\{[^}]*err\s*(instanceof\s+Error\s*\?\s*err\.message|\.message)/
+      expect(content, `${file} should use sanitizeError, not raw err.message in responses`).not.toMatch(unsafePattern)
+    }
+  })
 })
diff --git a/server/src/routes/fights.ts b/server/src/routes/fights.ts
index 30111bb..24ee767 100644
--- a/server/src/routes/fights.ts
+++ b/server/src/routes/fights.ts
@@ -13,7 +13,7 @@ import { getPendingChallenge, getPendingAnswers, submitHumanResponse } from '../
 import { getPendingPollChallenge, submitPollResponse, isPollingBot } from '../engine/poll-responses.js'
 import { authenticateBot } from '../middleware/bot-auth.js'
 import { checkAnswer } from '../engine/answers.js'
-import { respondSchema, reactSchema } from '../lib/validators.js'
+import { respondSchema, reactSchema, sanitizeError } from '../lib/validators.js'
 
 const isValidId = (id: string) => /^[a-zA-Z0-9_-]{1,64}$/.test(id)
 
@@ -250,7 +250,7 @@ fightsRouter.post('/matchmake/:botId', botRateLimit(10_000), async (c) => {
   try {
     fightId = await runFightAsync(botId, opponent.id)
   } catch (err) {
-    const msg = err instanceof Error ? err.message : 'Fight failed to start'
+    const msg = sanitizeError(err, 'Fight failed to start')
     return c.json({ error: msg }, 400)
   }
 
@@ -304,7 +304,7 @@ fightsRouter.post('/practice/:botId', botRateLimit(10_000), async (c) => {
     const overrides = isPollingBot(bot.webhookUrl) ? { botAWebhookUrl: 'http://human.local/' } : undefined
     fightId = await runFightAsync(botId, opponent.id, 'free', overrides)
   } catch (err) {
-    const msg = err instanceof Error ? err.message : 'Fight failed to start'
+    const msg = sanitizeError(err, 'Fight failed to start')
     return c.json({ error: msg }, 400)
   }
 
diff --git a/server/src/routes/payments.ts b/server/src/routes/payments.ts
index 3db6f12..8e44e76 100644
--- a/server/src/routes/payments.ts
+++ b/server/src/routes/payments.ts
@@ -6,7 +6,7 @@ import { eq } from 'drizzle-orm'
 import { createEntryInvoice, checkPaymentStatus, redeemCashuToken } from '../engine/payments.js'
 import { encrypt, decrypt } from '../engine/crypto.js'
 import { rateLimit } from '../middleware/rate-limit.js'
-import { connectWalletSchema, createInvoiceSchema, submitCashuSchema, disconnectWalletSchema, zapSchema, formatZodError } from '../lib/validators.js'
+import { connectWalletSchema, createInvoiceSchema, submitCashuSchema, disconnectWalletSchema, zapSchema, formatZodError, sanitizeError } from '../lib/validators.js'
 
 export const paymentsRouter = new Hono()
 
@@ -103,7 +103,7 @@ paymentsRouter.post('/create-invoice', rateLimit(60_000, 10), async (c) => {
     const result = await createEntryInvoice(botId)
     return c.json(result)
   } catch (err) {
-    const message = err instanceof Error ? err.message : 'Invoice creation failed'
+    const message = sanitizeError(err, 'Invoice creation failed')
     return c.json({ error: message }, 500)
   }
 })
@@ -118,7 +118,7 @@ paymentsRouter.get('/check/:paymentId', rateLimit(60_000, 30), async (c) => {
     const status = await checkPaymentStatus(paymentId)
     return c.json({ status })
   } catch (err) {
-    const message = err instanceof Error ? err.message : 'Status check failed'
+    const message = sanitizeError(err, 'Status check failed')
     return c.json({ error: message }, 500)
   }
 })
@@ -194,7 +194,7 @@ paymentsRouter.post('/submit-cashu', async (c) => {
       status: result.valid ? 'confirmed' : 'failed',
     })
   } catch (err) {
-    const message = err instanceof Error ? err.message : 'Cashu redemption failed'
+    const message = sanitizeError(err, 'Cashu redemption failed')
     return c.json({ error: message }, 500)
   }
 })
diff --git a/server/src/routes/queue.ts b/server/src/routes/queue.ts
index 73a71b6..82da57f 100644
--- a/server/src/routes/queue.ts
+++ b/server/src/routes/queue.ts
@@ -4,7 +4,7 @@ import { eq } from 'drizzle-orm'
 import { joinQueue, leaveQueue, getQueueSize, getQueueSnapshot } from '../engine/queue.js'
 import { joinRankedQueue, getRankedQueueStatus } from '../engine/ranked-queue.js'
 import { rateLimit } from '../middleware/rate-limit.js'
-import { joinRankedSchema } from '../lib/validators.js'
+import { joinRankedSchema, sanitizeError } from '../lib/validators.js'
 
 export const queueRouter = new Hono()
 
@@ -33,8 +33,10 @@ queueRouter.post('/join/:botId', async (c) => {
     const fightId = await joinQueue(botId)
     return c.json({ fightId, message: 'Matched! Fight starting.' })
   } catch (err: any) {
-    const message = err instanceof Error ? err.message : 'Queue error'
-    const status = message.includes('already in a fight') ? 409 : 500
+    const raw = err instanceof Error ? err.message : ''
+    const isConflict = raw.includes('already in a fight')
+    const message = isConflict ? raw : sanitizeError(err, 'Queue error')
+    const status = isConflict ? 409 : 500
     return c.json({ error: message, fightId: err?.fightId || undefined }, status)
   }
 })
@@ -76,7 +78,7 @@ queueRouter.post('/join-ranked/:botId', async (c) => {
     const fightId = await joinRankedQueue(botId, paymentId)
     return c.json({ fightId, message: 'Ranked match found! Fight starting.' })
   } catch (err) {
-    const message = err instanceof Error ? err.message : 'Ranked queue error'
+    const message = sanitizeError(err, 'Ranked queue error')
     return c.json({ error: message }, 500)
   }
 })