diff --git a/server/src/engine/challenges.test.ts b/server/src/engine/challenges.test.ts
index 915a388..adcfd80 100644
--- a/server/src/engine/challenges.test.ts
+++ b/server/src/engine/challenges.test.ts
@@ -1,5 +1,7 @@
 import { describe, it, expect } from 'vitest'
 import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool, roundToDifficulty } from './challenges.js'
+import { TEMPLATES } from './challenge-data.js'
+import { EXTRA_PROMPTS } from './challenges-extra.js'
 
 describe('pickChallenge', () => {
   it('returns a valid challenge', () => {
@@ -211,6 +213,38 @@ describe('pickChallenge with roundNumber', () => {
   })
 })
 
+describe('theme selection', () => {
+  it('theme bias parameter increases themed prompt selection', () => {
+    let bitcoinWithBias = 0
+    let bitcoinWithout = 0
+    const runs = 500
+    for (let i = 0; i < runs; i++) {
+      const biased = pickChallenge(new Set(), null, 'bitcoin')
+      const unbiased = pickChallenge(new Set(), null)
+      // Check if prompt text contains bitcoin-related content
+      // Theme-biased should prefer bitcoin prompts when available
+      if (biased.prompt.toLowerCase().includes('bitcoin') || biased.prompt.toLowerCase().includes('blockchain')) bitcoinWithBias++
+      if (unbiased.prompt.toLowerCase().includes('bitcoin') || unbiased.prompt.toLowerCase().includes('blockchain')) bitcoinWithout++
+    }
+    // With bias, should get more bitcoin prompts
+    expect(bitcoinWithBias).toBeGreaterThanOrEqual(bitcoinWithout)
+  })
+
+  it('all prompts have theme tags after tagging', () => {
+    let tagged = 0
+    let total = 0
+    for (const t of TEMPLATES) {
+      const extras = EXTRA_PROMPTS[t.type] || []
+      for (const p of [...t.prompts, ...extras]) {
+        total++
+        if (p.theme) tagged++
+      }
+    }
+    // All prompts should have themes
+    expect(tagged).toBe(total)
+  })
+})
+
 describe('prompt data integrity', () => {
   it('all 800 prompts are accessible via pickChallenge', () => {
     // Run enough picks to verify the system works