feat: add difficulty field to prompts and wire into fight pacing

Round 1-2 picks easy prompts, 3-4 medium, 5+ hard (when tagged).
Graceful fallback to any prompt when no difficulty-tagged prompts exist.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/challenge-data.ts

@@ -1,12 +1,14 @@
 // Challenge prompt data — separated from challenge logic
 
 export type PromptTheme = 'bitcoin' | 'conspiracy' | 'pc_culture' | 'bot_coding'
+export type PromptDifficulty = 'easy' | 'medium' | 'hard'
 
 export interface PromptEntry {
   prompt: string
   answers?: string[]
   choices?: string[]
   theme?: PromptTheme
+  difficulty?: PromptDifficulty
 }
 
 export interface ChallengeTemplate {

server/src/engine/challenges.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool } from './challenges.js'
+import { pickChallenge, pickRankedChallenge, getAllChallengeTypes, getAnswerPool, roundToDifficulty } from './challenges.js'
 
 describe('pickChallenge', () => {
   it('returns a valid challenge', () => {
@@ -172,6 +172,45 @@ describe('getAnswerPool', () => {
   })
 })
 
+describe('roundToDifficulty', () => {
+  it('rounds 1-2 are easy', () => {
+    expect(roundToDifficulty(1)).toBe('easy')
+    expect(roundToDifficulty(2)).toBe('easy')
+  })
+
+  it('rounds 3-4 are medium', () => {
+    expect(roundToDifficulty(3)).toBe('medium')
+    expect(roundToDifficulty(4)).toBe('medium')
+  })
+
+  it('rounds 5+ are hard', () => {
+    expect(roundToDifficulty(5)).toBe('hard')
+    expect(roundToDifficulty(6)).toBe('hard')
+    expect(roundToDifficulty(10)).toBe('hard')
+  })
+})
+
+describe('pickChallenge with roundNumber', () => {
+  it('accepts roundNumber parameter without error', () => {
+    const c = pickChallenge(new Set(), null, undefined, 1)
+    expect(c).toBeTruthy()
+    expect(c.prompt).toBeTruthy()
+  })
+
+  it('works for all round numbers 1-10', () => {
+    for (let r = 1; r <= 10; r++) {
+      const c = pickChallenge(new Set(), null, undefined, r)
+      expect(c.type).toBeTruthy()
+      expect(c.prompt).toBeTruthy()
+    }
+  })
+
+  it('still works without roundNumber (backward compatible)', () => {
+    const c = pickChallenge(new Set(), null)
+    expect(c).toBeTruthy()
+  })
+})
+
 describe('prompt data integrity', () => {
   it('all 800 prompts are accessible via pickChallenge', () => {
     // Run enough picks to verify the system works

server/src/engine/challenges.ts

@@ -1,4 +1,4 @@
-import { TEMPLATES, type ChallengeTemplate, type PromptEntry, type PromptTheme } from './challenge-data.js'
+import { TEMPLATES, type ChallengeTemplate, type PromptEntry, type PromptTheme, type PromptDifficulty } from './challenge-data.js'
 import { EXTRA_PROMPTS } from './challenges-extra.js'
 import { pick } from '../lib/utils.js'
 
@@ -14,7 +14,7 @@ export interface Challenge {
   displayPrompt?: string
 }
 
-export type { ChallengeTemplate, PromptEntry, PromptTheme }
+export type { ChallengeTemplate, PromptEntry, PromptTheme, PromptDifficulty }
 
 
 // Merge extra prompts into templates
@@ -54,7 +54,14 @@ function pickTheme(): PromptTheme | undefined {
   return undefined
 }
 
-export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | null, themeBias?: PromptTheme): Challenge {
+/** Map round number to target difficulty: Round 1-2 easy, 3-4 medium, 5+ hard */
+export function roundToDifficulty(round: number): PromptDifficulty {
+  if (round <= 2) return 'easy'
+  if (round <= 4) return 'medium'
+  return 'hard'
+}
+
+export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | null, themeBias?: PromptTheme, roundNumber?: number): Challenge {
   let available = TEMPLATES.filter(t => !usedTypes.has(t.type))
   if (available.length === 0) available = TEMPLATES
 
@@ -71,7 +78,8 @@ export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | n
 
   const template = pick(pool)
   const targetTheme = themeBias || pickTheme()
-  return templateToChallenge(template, targetTheme)
+  const targetDifficulty = roundNumber ? roundToDifficulty(roundNumber) : undefined
+  return templateToChallenge(template, targetTheme, targetDifficulty)
 }
 
 /** Ranked challenge: no multiple choice, only harder creative/open-ended prompts */
@@ -102,13 +110,18 @@ export function pickRankedChallenge(usedTypes: Set<string>): Challenge {
   }
 }
 
-function templateToChallenge(template: ChallengeTemplate, targetTheme?: PromptTheme): Challenge {
+function templateToChallenge(template: ChallengeTemplate, targetTheme?: PromptTheme, targetDifficulty?: PromptDifficulty): Challenge {
   // Prefer prompts matching target theme if any are tagged
   let prompts = template.prompts
   if (targetTheme) {
-    const themed = template.prompts.filter(p => p.theme === targetTheme)
+    const themed = prompts.filter(p => p.theme === targetTheme)
     if (themed.length > 0) prompts = themed
   }
+  // Prefer prompts matching target difficulty if any are tagged
+  if (targetDifficulty) {
+    const byDifficulty = prompts.filter(p => p.difficulty === targetDifficulty)
+    if (byDifficulty.length > 0) prompts = byDifficulty
+  }
   const entry = pick(prompts)
 
   // Determine choices

server/src/engine/orchestrator.ts

@@ -360,7 +360,7 @@ async function executeFightRounds(fightId: string, botA: BotRecord, botB: BotRec
     lastRound = round
     const challenge = round === retroRound
       ? generateRetroChallenge()
-      : pickChallenge(usedTypes, arena.modifier)
+      : pickChallenge(usedTypes, arena.modifier, undefined, round)
     usedTypes.add(challenge.type)
 
     emit(fightId, 'round_start', {