Fix encounter difficulty tier classification for 5.5e and PF2e

The 2024 DMG and PF2e GM Core define XP budgets as ceilings ("spend as
much of your XP budget as you can without going over"), not floors like
the 2014 DMG thresholds. The tier is now the lowest budget the XP does
not exceed: an encounter over the Moderate budget is High even if below
the High budget. PF2e now also uses the Trivial budget (40 or less),
GM Core remaster party-size adjustments (Low +20), and counts creatures
more than 4 levels below the party as 0 XP. 2014 rules are unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/web/src/hooks/__tests__/use-difficulty.test.tsx

@@ -341,10 +341,10 @@ describe("useDifficulty", () => {
 		await waitFor(() => {
 			expect(result.current).not.toBeNull();
 			// Level 3 budget: low=150, mod=225, high=400
-			// CR 1/4 = 50 XP -> trivial
+			// CR 1/4 = 50 XP ≤ 150 Low budget -> Low
 			expect(result.current?.thresholds[0].value).toBe(150);
 			expect(result.current?.totalMonsterXp).toBe(50);
-			expect(result.current?.tier).toBe(0);
+			expect(result.current?.tier).toBe(1);
 		});
 	});
 

packages/domain/src/__tests__/encounter-difficulty.test.ts

@@ -49,9 +49,9 @@ function enemy(cr: string) {
 }
 
 describe("calculateEncounterDifficulty — 5.5e edition", () => {
-	it("returns tier 0 when monster XP is below Low threshold", () => {
+	it("returns tier 0 when monster XP is 0", () => {
 		// 4x level 1: Low = 200, Moderate = 300, High = 400
-		// 1x CR 0 = 0 XP -> tier 0
+		// 1x CR 0 = 0 XP -> tier 0 (5.5e has no Trivial tier; 0 bars only at 0 XP)
 		const result = calculateEncounterDifficulty(
 			[party(1), party(1), party(1), party(1), enemy("0")],
 			"5.5e",
@@ -77,9 +77,29 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 		expect(result.totalMonsterXp).toBe(200);
 	});
 
-	it("returns tier 2 for 5x level 3 vs 1150 XP", () => {
+	it("returns tier 2 when XP spends exactly the Moderate budget", () => {
 		// 5x level 3: Low = 750, Moderate = 1125, High = 2000
-		// CR 3 (700) + CR 2 (450) = 1150 XP >= 1125 Moderate
+		// CR 4 (1100) + CR 1/8 (25) = 1125 XP = Moderate budget
+		const result = calculateEncounterDifficulty(
+			[
+				party(3),
+				party(3),
+				party(3),
+				party(3),
+				party(3),
+				enemy("4"),
+				enemy("1/8"),
+			],
+			"5.5e",
+		);
+		expect(result.tier).toBe(2);
+		expect(result.totalMonsterXp).toBe(1125);
+		expect(result.thresholds[1].value).toBe(1125);
+	});
+
+	it("returns tier 3 when XP exceeds the Moderate budget", () => {
+		// 5x level 3: Low = 750, Moderate = 1125, High = 2000
+		// CR 3 (700) + CR 2 (450) = 1150 XP > 1125 Moderate → High
 		const result = calculateEncounterDifficulty(
 			[
 				party(3),
@@ -92,14 +112,24 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 			],
 			"5.5e",
 		);
-		expect(result.tier).toBe(2);
+		expect(result.tier).toBe(3);
 		expect(result.totalMonsterXp).toBe(1150);
-		expect(result.thresholds[1].value).toBe(1125);
 	});
 
-	it("returns tier 3 when XP meets High threshold", () => {
+	it("returns tier 3 for 5x level 5 vs 5000 XP (over Moderate budget)", () => {
-		// 4x level 1: High = 400
+		// 5x level 5: Low = 2500, Moderate = 3750, High = 5500
-		// 2x CR 1 = 400 XP -> tier 3
+		// CR 9 = 5000 XP > 3750 Moderate → High, even though below the High budget
+		const result = calculateEncounterDifficulty(
+			[party(5), party(5), party(5), party(5), party(5), enemy("9")],
+			"5.5e",
+		);
+		expect(result.tier).toBe(3);
+		expect(result.totalMonsterXp).toBe(5000);
+	});
+
+	it("returns tier 3 when XP exceeds the Moderate budget", () => {
+		// 4x level 1: Moderate = 300
+		// 2x CR 1 = 400 XP > 300 -> tier 3
 		const result = calculateEncounterDifficulty(
 			[party(1), party(1), party(1), party(1), enemy("1"), enemy("1")],
 			"5.5e",
@@ -120,6 +150,7 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 	it("handles mixed party levels", () => {
 		// 3x level 3 + 1x level 2
 		// Total: low=550, mod=825, high=1400
+		// 700 XP > 550 Low, ≤ 825 Moderate → Moderate
 		const result = calculateEncounterDifficulty(
 			[party(3), party(3), party(3), party(2), enemy("3")],
 			"5.5e",
@@ -130,7 +161,7 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 			{ label: "High", value: 1400 },
 		]);
 		expect(result.totalMonsterXp).toBe(700);
-		expect(result.tier).toBe(1);
+		expect(result.tier).toBe(2);
 	});
 
 	it("returns tier 0 with no enemies", () => {
@@ -164,7 +195,7 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 			"5.5e",
 		);
 		expect(result.totalMonsterXp).toBe(175); // 25 + 50 + 100
-		expect(result.tier).toBe(0); // 175 < 200 Low
+		expect(result.tier).toBe(1); // 175 ≤ 200 Low budget → Low
 	});
 
 	it("ignores unknown CRs (0 XP)", () => {
@@ -191,7 +222,7 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 			"5.5e",
 		);
 		expect(result.totalMonsterXp).toBe(250);
-		expect(result.tier).toBe(1); // 250 >= 200 Low, < 300 Moderate
+		expect(result.tier).toBe(2); // 250 > 200 Low budget, ≤ 300 Moderate → Moderate
 	});
 
 	it("floors net monster XP at 0", () => {
@@ -254,7 +285,7 @@ describe("calculateEncounterDifficulty — 5.5e edition", () => {
 			{ label: "High", value: 800 },
 		]);
 		expect(result.totalMonsterXp).toBe(700);
-		expect(result.tier).toBe(2); // 700 >= 450 Moderate, < 800 High
+		expect(result.tier).toBe(3); // 700 > 450 Moderate budget → High
 	});
 });
 
@@ -448,8 +479,8 @@ describe("pf2eCreatureXp", () => {
 		expect(pf2eCreatureXp(5 + diff, 5)).toBe(expectedXp);
 	});
 
-	it("clamps level diff below −4 to −4 (10 XP)", () => {
+	it("returns 0 XP for creatures more than 4 levels below the party", () => {
-		expect(pf2eCreatureXp(0, 10)).toBe(10);
+		expect(pf2eCreatureXp(0, 10)).toBe(0);
 	});
 
 	it("clamps level diff above +4 to +4 (160 XP)", () => {
@@ -459,7 +490,7 @@ describe("pf2eCreatureXp", () => {
 
 describe("calculateEncounterDifficulty — pf2e edition", () => {
 	it("returns Trivial (tier 0) for 40 XP with party of 4", () => {
-		// 1 creature at party level = 40 XP, below Low (60)
+		// 1 creature at party level = 40 XP, within the Trivial band (40 or less)
 		const result = calculateEncounterDifficulty(
 			[party(5), party(5), party(5), party(5), pf2eEnemy(5)],
 			"pf2e",
@@ -507,7 +538,7 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 	});
 
 	it("returns Extreme (tier 4) for 160 XP", () => {
-		// 1 creature at +4 = 160 XP
+		// 1 creature at +4 = 160 XP > Severe budget (120)
 		const result = calculateEncounterDifficulty(
 			[party(5), party(5), party(5), party(5), pf2eEnemy(9)],
 			"pf2e",
@@ -516,8 +547,28 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 		expect(result.totalMonsterXp).toBe(160);
 	});
 
-	it("returns tier 0 when XP is below Low threshold", () => {
+	it("returns Moderate (tier 2) for XP between the Low and Moderate budgets", () => {
-		// 1 creature at −4 = 10 XP, Low = 60
+		// +1 (60) + −4 (10) = 70 XP > Low (60), ≤ Moderate (80)
+		const result = calculateEncounterDifficulty(
+			[party(5), party(5), party(5), party(5), pf2eEnemy(6), pf2eEnemy(1)],
+			"pf2e",
+		);
+		expect(result.tier).toBe(2);
+		expect(result.totalMonsterXp).toBe(70);
+	});
+
+	it("returns Extreme (tier 4) for XP just over the Severe budget", () => {
+		// +3 (120) + −4 (10) = 130 XP > Severe (120)
+		const result = calculateEncounterDifficulty(
+			[party(5), party(5), party(5), party(5), pf2eEnemy(8), pf2eEnemy(1)],
+			"pf2e",
+		);
+		expect(result.tier).toBe(4);
+		expect(result.totalMonsterXp).toBe(130);
+	});
+
+	it("returns tier 0 when XP is within the Trivial band", () => {
+		// 1 creature at −4 = 10 XP ≤ Trivial (40)
 		const result = calculateEncounterDifficulty(
 			[party(5), party(5), party(5), party(5), pf2eEnemy(1)],
 			"pf2e",
@@ -526,6 +577,15 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 		expect(result.totalMonsterXp).toBe(10);
 	});
 
+	it("counts creatures more than 4 levels below the party as 0 XP", () => {
+		// Party level 6: a level-1 creature is 5 below → 0 XP
+		const result = calculateEncounterDifficulty(
+			[party(6), party(6), party(6), party(6), pf2eEnemy(1), pf2eEnemy(6)],
+			"pf2e",
+		);
+		expect(result.totalMonsterXp).toBe(40); // only the at-level creature counts
+	});
+
 	it("adjusts thresholds for 5 PCs (increases by adjustment)", () => {
 		const result = calculateEncounterDifficulty(
 			[party(5), party(5), party(5), party(5), party(5), pf2eEnemy(5)],
@@ -533,7 +593,7 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 		);
 		expect(result.thresholds).toEqual([
 			{ label: "Trivial", value: 50 },
-			{ label: "Low", value: 75 },
+			{ label: "Low", value: 80 },
 			{ label: "Moderate", value: 100 },
 			{ label: "Severe", value: 150 },
 			{ label: "Extreme", value: 200 },
@@ -547,7 +607,7 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 		);
 		expect(result.thresholds).toEqual([
 			{ label: "Trivial", value: 30 },
-			{ label: "Low", value: 45 },
+			{ label: "Low", value: 40 },
 			{ label: "Moderate", value: 60 },
 			{ label: "Severe", value: 90 },
 			{ label: "Extreme", value: 120 },
@@ -561,9 +621,9 @@ describe("calculateEncounterDifficulty — pf2e edition", () => {
 		);
 		// 1 PC: adjustment = −3
 		// Trivial: 40 + (−3 * 10) = 10
-		// Low: 60 + (−3 * 15) = 15
+		// Low: 60 + (−3 * 20) = 0
 		expect(result.thresholds[0].value).toBe(10);
-		expect(result.thresholds[1].value).toBe(15);
+		expect(result.thresholds[1].value).toBe(0);
 		expect(result.thresholds[2].value).toBe(20); // 80 − 60
 		expect(result.thresholds[3].value).toBe(30); // 120 − 90
 		expect(result.thresholds[4].value).toBe(40); // 160 − 120

packages/domain/src/encounter-difficulty.ts

@@ -187,10 +187,14 @@ const PF2E_THRESHOLDS_BASE = {
 	extreme: 160,
 } as const;
 
-/** PF2e per-PC adjustment to each threshold (added per PC beyond 4, subtracted per PC fewer). */
+/**
+ * PF2e per-PC adjustment to each threshold (added per PC beyond 4, subtracted
+ * per PC fewer). GM Core remaster values — Low is 20 there, not the
+ * pre-remaster CRB's 15.
+ */
 const PF2E_THRESHOLD_ADJUSTMENTS = {
 	trivial: 10,
-	low: 15,
+	low: 20,
 	moderate: 20,
 	severe: 30,
 	extreme: 40,
@@ -230,13 +234,19 @@ export function derivePartyLevel(levels: readonly number[]): number {
 	return Math.round(sum / levels.length);
 }
 
-/** Returns PF2e XP for a creature given its level and the party level. */
+/**
+ * Returns PF2e XP for a creature given its level and the party level.
+ * Creatures more than 4 levels below the party have no row in the GM Core
+ * table and pose no meaningful threat — they are worth 0 XP. Creatures more
+ * than 4 levels above are counted as the +4 row (160 XP).
+ */
 export function pf2eCreatureXp(
 	creatureLevel: number,
 	partyLevel: number,
 ): number {
-	const diff = Math.max(-4, Math.min(4, creatureLevel - partyLevel));
+	const diff = creatureLevel - partyLevel;
-	return PF2E_LEVEL_DIFF_XP[diff] ?? 0;
+	if (diff < -4) return 0;
+	return PF2E_LEVEL_DIFF_XP[Math.min(4, diff)] ?? 0;
 }
 
 function calculatePf2eBudget(partySize: number) {
@@ -304,7 +314,11 @@ export interface CombatantDescriptor {
 	readonly side: "party" | "enemy";
 }
 
-function determineTier(
+/**
+ * 2014 DMG: thresholds are floors — "the closest threshold that is lower
+ * than the adjusted XP value determines the encounter's difficulty".
+ */
+function tierFromThresholds(
 	xp: number,
 	tierThresholds: readonly number[],
 ): DifficultyTier {
@@ -314,6 +328,23 @@ function determineTier(
 	return 0;
 }
 
+/**
+ * 2024 DMG / PF2e: budgets are ceilings — an encounter belongs to the lowest
+ * tier whose budget it does not exceed ("spend as much of your XP budget as
+ * you can without going over"; PF2e Trivial is "40 XP or less"). XP above the
+ * last budget falls into the tier beyond it (5.5e High / PF2e Extreme have no
+ * upper bound).
+ */
+function tierFromBudgets(
+	xp: number,
+	budgets: readonly number[],
+): DifficultyTier {
+	for (let i = 0; i < budgets.length; i++) {
+		if (xp <= budgets[i]) return i as DifficultyTier;
+	}
+	return budgets.length as DifficultyTier;
+}
+
 function accumulateBudget5_5e(levels: readonly number[]) {
 	const budget = { low: 0, moderate: 0, high: 0 };
 	for (const level of levels) {
@@ -397,11 +428,11 @@ export function calculateEncounterDifficulty(
 		];
 
 		return {
-			tier: determineTier(totalCreatureXp, [
+			tier: tierFromBudgets(totalCreatureXp, [
+				budget.trivial,
 				budget.low,
 				budget.moderate,
 				budget.severe,
-				budget.extreme,
 			]),
 			totalMonsterXp: totalCreatureXp,
 			thresholds,
@@ -423,11 +454,9 @@ export function calculateEncounterDifficulty(
 			{ label: "High", value: budget.high },
 		];
 		return {
-			tier: determineTier(totalMonsterXp, [
+			// 5.5e has no tier below Low, so tier 0 (empty bars) only at 0 XP.
-				budget.low,
+			// The High budget is display-only: anything over Moderate is High.
-				budget.moderate,
+			tier: tierFromBudgets(totalMonsterXp, [0, budget.low, budget.moderate]),
-				budget.high,
-			]),
 			totalMonsterXp,
 			thresholds,
 			encounterMultiplier: undefined,
@@ -450,7 +479,7 @@ export function calculateEncounterDifficulty(
 	];
 
 	return {
-		tier: determineTier(adjustedXp, [
+		tier: tierFromThresholds(adjustedXp, [
 			budget.medium,
 			budget.hard,
 			budget.deadly,

specs/008-encounter-difficulty/spec.md

@@ -3,7 +3,7 @@
 **Feature Branch**: `008-encounter-difficulty`
 **Created**: 2026-03-27
 **Status**: Draft
-**Input**: Gitea issue #18 — "Encounter difficulty indicator (5.5e XP budget)", Gitea issue #22 — "Combatant side assignment for encounter difficulty", Gitea issue #23 — "2014 DMG encounter difficulty calculation"
+**Input**: Gitea issue #18 — "Encounter difficulty indicator (5.5e XP budget)", Gitea issue #22 — "Combatant side assignment for encounter difficulty", Gitea issue #23 — "2014 DMG encounter difficulty calculation", Gitea issue #28 — "PF2e encounter difficulty calculation"
 
 ## User Scenarios & Testing *(mandatory)*
 
@@ -19,15 +19,15 @@ A game master is building an encounter by adding monsters and player characters.
 
 **Acceptance Scenarios**:
 
-1. **Given** an encounter with at least one PC combatant whose player character has a level and at least one bestiary-linked combatant, **When** the total monster XP is below the Low threshold, **Then** the indicator shows three empty bars (trivial difficulty).
+1. **Given** an encounter with at least one PC combatant whose player character has a level and at least one bestiary-linked combatant, **When** the total monster XP is 0, **Then** the indicator shows three empty bars (the 2024 rules define no tier below Low, so empty bars appear only at 0 XP).
 
-2. **Given** an encounter where total monster XP meets or exceeds the Low threshold but is below Moderate, **When** the indicator renders, **Then** it shows one filled green bar and the tooltip reads "Low encounter difficulty".
+2. **Given** an encounter where total monster XP is greater than 0 and within the Low budget, **When** the indicator renders, **Then** it shows one filled green bar and the tooltip reads "Low encounter difficulty".
 
-3. **Given** an encounter where total monster XP meets or exceeds the Moderate threshold but is below High, **When** the indicator renders, **Then** it shows two filled yellow bars and the tooltip reads "Moderate encounter difficulty".
+3. **Given** an encounter where total monster XP exceeds the Low budget but is within the Moderate budget, **When** the indicator renders, **Then** it shows two filled yellow bars and the tooltip reads "Moderate encounter difficulty".
 
-4. **Given** an encounter where total monster XP meets or exceeds the High threshold, **When** the indicator renders, **Then** it shows three filled red bars and the tooltip reads "High encounter difficulty".
+4. **Given** an encounter where total monster XP exceeds the Moderate budget, **When** the indicator renders, **Then** it shows three filled red bars and the tooltip reads "High encounter difficulty" (per the 2024 DMG, budgets are maximums — an encounter over the Moderate budget is a High encounter even if it is below the High budget).
 
-5. **Given** an encounter where total monster XP exceeds the High threshold by a large margin, **When** the indicator renders, **Then** it still shows three filled red bars (High is the cap — there is no "above High" tier).
+5. **Given** an encounter where total monster XP exceeds the High budget by a large margin, **When** the indicator renders, **Then** it still shows three filled red bars (High is the cap — there is no "above High" tier).
 
 6. **Given** the difficulty indicator is visible, **When** a bestiary-linked combatant is added or removed, **Then** the indicator updates immediately to reflect the new difficulty tier.
 
@@ -39,6 +39,12 @@ A game master is building an encounter by adding monsters and player characters.
 
 10. **Given** the user switches the rules edition in settings, **When** returning to the encounter, **Then** the indicator tooltip reflects the new edition's labels immediately.
 
+11. **Given** the rules edition is set to PF2e, **When** the indicator renders, **Then** it displays four bars instead of three, supporting five visual states: zero filled bars for Trivial, one green bar for Low, two yellow bars for Moderate, three orange bars for Severe, four red bars for Extreme.
+
+12. **Given** the rules edition is set to PF2e, **When** the indicator tooltip renders, **Then** it uses PF2e tier labels (e.g., "Severe encounter difficulty").
+
+13. **Given** the user switches the rules edition to or from PF2e, **When** returning to the encounter, **Then** the indicator bar count (3 vs 4) and tier labels update immediately.
+
 ---
 
 ### Indicator Visibility
@@ -63,6 +69,10 @@ The difficulty indicator only appears when meaningful calculation is possible. I
 
 5. **Given** an encounter with one leveled PC combatant and one bestiary-linked monster, **When** the last bestiary-linked monster is removed and the remaining custom combatants have no `cr` assigned, **Then** the indicator disappears.
 
+6. **Given** the rules edition is set to PF2e and an encounter has leveled PC combatants and creatures with a creature level (from bestiary data), **When** the top bar renders, **Then** the difficulty indicator is shown. CR is not required — PF2e uses creature level, not CR.
+
+7. **Given** the rules edition is set to PF2e and an encounter has bestiary-linked PF2e creatures but no PC combatants with levels, **When** the top bar renders, **Then** the difficulty indicator is hidden.
+
 ---
 
 ### Player Character Level
@@ -93,7 +103,7 @@ The game master can set an optional level (1-20) when creating or editing a play
 
 **Story ED-4 — Correct XP budget from 5.5e rules (Priority: P1)**
 
-The difficulty calculation uses the 2024 5.5e XP Budget per Character table and a standard CR-to-XP mapping. The party's XP budget is the sum of per-character budgets for each PC combatant that has a level. The total monster XP is the sum of XP values for each bestiary-linked combatant's CR. The difficulty tier is determined by comparing total monster XP against the Low, Moderate, and High budget thresholds.
+The difficulty calculation uses the 2024 5.5e XP Budget per Character table and a standard CR-to-XP mapping. The party's XP budget is the sum of per-character budgets for each PC combatant that has a level. The total monster XP is the sum of XP values for each bestiary-linked combatant's CR. The difficulty tier is determined by comparing total monster XP against the Low, Moderate, and High budgets, where each budget is the tier's maximum ("spend as much of your XP budget as you can without going over" — 2024 DMG).
 
 **Why this priority**: Incorrect calculation would make the feature misleading — the math must match the published rules.
 
@@ -101,9 +111,9 @@ The difficulty calculation uses the 2024 5.5e XP Budget per Character table and
 
 **Acceptance Scenarios**:
 
-1. **Given** a party of four level 1 PCs (Low budget: 50 each = 200 total), **When** facing a single Bugbear (CR 1, 200 XP), **Then** the difficulty is Low (200 XP meets the Low threshold of 200 but is below Moderate at 300).
+1. **Given** a party of four level 1 PCs (Low budget: 50 each = 200 total), **When** facing a single Bugbear (CR 1, 200 XP), **Then** the difficulty is Low (200 XP spends exactly the Low budget without going over).
 
-2. **Given** a party of five level 3 PCs (Moderate budget: 225 each = 1,125 total), **When** facing monsters totaling 1,125 XP, **Then** the difficulty is Moderate.
+2. **Given** a party of five level 3 PCs (Moderate budget: 225 each = 1,125 total), **When** facing monsters totaling 1,125 XP, **Then** the difficulty is Moderate. **When** facing monsters totaling 1,150 XP (over the Moderate budget), **Then** the difficulty is High.
 
 3. **Given** a party with PCs at different levels (e.g., three level 3 and one level 2), **When** the budget is calculated, **Then** each PC's budget is looked up individually by level and summed (not averaged).
 
@@ -151,6 +161,12 @@ The game master taps the difficulty indicator to open a breakdown panel. The pan
 
 9. **Given** the rules edition is set to 5e (2014) and the breakdown panel is open, **When** viewing the party budget section, **Then** the panel shows four threshold columns (Easy, Medium, Hard, Deadly) instead of three (Low, Moderate, High).
 
+10. **Given** the rules edition is set to PF2e and the breakdown panel is open, **When** viewing the creature list, **Then** each creature shows its level, level difference from party level, and XP contribution — not CR.
+
+11. **Given** the rules edition is set to PF2e and the breakdown panel is open, **When** viewing the party budget section, **Then** the panel shows five threshold columns (Trivial/Low/Moderate/Severe/Extreme) and indicates the derived party level.
+
+12. **Given** the rules edition is set to PF2e and the party size differs from 4, **When** the breakdown panel is open, **Then** the panel shows the adjusted thresholds and a brief explanation of the party size adjustment.
+
 ---
 
 ### Manual CR Assignment
@@ -253,10 +269,46 @@ A game master who runs games using the 2014 (original 5e) rules selects "5e (201
 
 ---
 
+### PF2e Rules Edition
+
+**Story ED-10 — PF2e encounter difficulty calculation (Priority: P2)**
+
+A game master running Pathfinder 2e selects "Pathfinder 2e" in the Rules Edition setting. The difficulty indicator uses PF2e's level-based calculation: each creature's XP contribution is determined by its level relative to the party level using a standard XP table, and the total is compared against Trivial/Low/Moderate/Severe/Extreme budget thresholds. Party level is auto-derived from the PCs' levels. No encounter multiplier is used — creature XP is summed directly.
+
+**Why this priority**: The core indicator (ED-1) and 5.5e calculation (ED-4) must work first. PF2e support extends the existing system with a fundamentally different calculation paradigm (level-based instead of CR-based).
+
+**Independent Test**: Can be tested by setting rules edition to PF2e, creating an encounter with leveled PCs and PF2e bestiary creatures, and verifying the indicator uses PF2e thresholds, XP table, and labels.
+
+**Acceptance Scenarios**:
+
+1. **Given** the rules edition is set to PF2e and an encounter has leveled PCs and PF2e bestiary creatures, **When** the difficulty is calculated, **Then** the system derives each creature's XP from the PF2e creature level vs party level XP table and compares the total against Trivial/Low/Moderate/Severe/Extreme thresholds.
+
+2. **Given** a party of four level 3 PCs and a creature at level 3 (level difference 0), **When** the XP is calculated, **Then** the creature contributes 40 XP. The total (40) is within the Trivial band ("40 XP or less") — difficulty is Trivial.
+
+3. **Given** a party of four level 5 PCs and a creature at level 9 (level difference +4), **When** the XP is calculated, **Then** the creature contributes 160 XP. The total (160) exceeds the Severe budget (120) — difficulty is Extreme.
+
+4. **Given** PCs at levels 3, 3, 3, and 5, **When** the party level is derived, **Then** it is 3 (the most common level among PCs).
+
+5. **Given** PCs at levels 2, 4, 6, and 8 (all different), **When** the party level is derived, **Then** it is 5 (the average, rounded to the nearest integer).
+
+6. **Given** a party of 5 PCs (one more than the base of 4) at the Moderate tier, **When** the budget thresholds are calculated, **Then** each threshold is increased by its per-tier adjustment: Trivial +10, Low +20, Moderate +20, Severe +30, Extreme +40.
+
+7. **Given** a party of 3 PCs (one fewer than the base of 4) at the Moderate tier, **When** the budget thresholds are calculated, **Then** each threshold is decreased by its per-tier adjustment: Trivial −10, Low −20, Moderate −20, Severe −30, Extreme −40.
+
+8. **Given** the rules edition is set to PF2e, **When** the indicator visual states are rendered, **Then** 0 bars = Trivial, 1 green bar = Low, 2 yellow bars = Moderate, 3 orange bars = Severe, 4 red bars = Extreme.
+
+9. **Given** the user changes the rules edition from 5.5e to PF2e while an encounter is open, **When** the setting is saved, **Then** the difficulty indicator updates immediately to reflect the PF2e calculation, labels, and 4-bar layout.
+
+10. **Given** the rules edition is set to PF2e and a creature has a level difference below −4, **When** XP is calculated, **Then** the creature contributes 0 XP (the GM Core table has no row below −4; such creatures pose no meaningful threat).
+
+11. **Given** the rules edition is set to PF2e and a creature has a level difference above +4, **When** XP is calculated, **Then** the creature contributes 160 XP (the maximum in the table — level difference +4).
+
+---
+
 ### Edge Cases
 
-- **All bars empty (trivial)**: When total monster XP is greater than 0 but below the Low threshold, the indicator shows three empty bars. This communicates "we can calculate, but it's trivial."
+- **All bars empty (5.5e)**: Only when total monster XP is exactly 0 — the 2024 rules define no tier below Low, so any nonzero XP within the Low budget is a Low encounter. (For 2014, adjusted XP below the Easy threshold shows empty bars.)
-- **Zero monster XP**: If all combatants with `creatureId` have CR 0 (0 XP), the indicator shows three empty bars (trivial).
+- **Zero monster XP**: If all combatants with `creatureId` have CR 0 (0 XP), the indicator shows three empty bars.
 - **Mixed party levels**: PCs at different levels each contribute their own budget — the system handles heterogeneous parties correctly.
 - **Duplicate PC combatants**: If the same player character is added to the encounter multiple times, each copy contributes to the party budget independently (each counts as a party member).
 - **CR fractions**: Bestiary creatures can have fractional CRs (e.g., "1/4", "1/2"). The CR-to-XP lookup must handle these string formats.
@@ -274,6 +326,14 @@ A game master who runs games using the 2014 (original 5e) rules selects "5e (201
 - **2014 multiplier floor (×0.5)**: A single monster with 6+ PCs uses ×0.5 per the 2014 DMG party size adjustment rule.
 - **2014 multiplier ceiling (×5)**: 15+ monsters with fewer than 3 PCs shifts ×4 upward to ×5 per the 2014 DMG party size adjustment rule.
 - **Edition switch with breakdown panel open**: If the breakdown panel is open when the user switches editions in settings, the panel content updates to reflect the new edition's labels, thresholds, and (for 2014) the encounter multiplier.
+- **PF2e creature level outside table range**: A creature more than 4 levels below the party contributes 0 XP (no row in the GM Core table — no meaningful threat). A creature more than 4 levels above the party contributes 160 XP (the +4 row).
+- **PF2e party level derivation — tie in mode**: If two levels are equally common among PCs (e.g., two level 3 and two level 5), the party level is the average of all PC levels, rounded to the nearest integer.
+- **PF2e party level — single PC**: With one PC, the party level equals that PC's level.
+- **PF2e budget thresholds floored at 0**: If party size adjustment would reduce a threshold below 0, it is floored at 0.
+- **PF2e no encounter multiplier**: Unlike D&D 5e (2014), PF2e never applies an encounter multiplier. Total creature XP is used directly.
+- **PF2e creatures without level**: Custom combatants without `creatureId` (and thus no creature level) are excluded from PF2e XP calculation. Only bestiary-linked PF2e creatures with a level contribute.
+- **PF2e 4-bar indicator in edition switch**: When switching from a D&D edition (3 bars) to PF2e (4 bars) or vice versa, the indicator bar count updates immediately along with the tier labels.
+- **PF2e side assignment**: Side assignment (party/enemy) works identically to D&D — enemy-side creatures add XP, party-side creatures subtract XP. Net XP is floored at 0.
 
 ---
 
@@ -294,7 +354,12 @@ The system MUST calculate the party's XP budget by summing the per-character bud
 The system MUST calculate the net monster XP by summing the XP value (derived from CR) for each enemy-side combatant that has a CR and subtracting the XP value for each party-side combatant that has a CR. For bestiary-linked combatants, CR is derived from the creature data via `creatureId`. For custom combatants, CR comes from the optional `cr` field. Combatants with neither `creatureId` nor `cr` are excluded. The net monster XP MUST be floored at 0.
 
 #### FR-005 — Difficulty tier determination
-The system MUST determine the encounter difficulty tier by comparing total monster XP (adjusted XP for 2014) against the party's thresholds. For 5.5e: Low, Moderate, and High (3 tiers). For 2014: Easy, Medium, Hard, and Deadly (4 tiers). The tier is the highest threshold that the total XP meets or exceeds. If below the lowest threshold, the encounter is trivial (5.5e) or easy (2014). The visual indicator maps identically across editions: 0 bars = Trivial/Easy, 1 green = Low/Medium, 2 yellow = Moderate/Hard, 3 red = High/Deadly.
+The system MUST determine the encounter difficulty tier by comparing total monster XP (adjusted XP for 2014) against the party's thresholds. The comparison direction differs by edition, matching each DMG's rules:
+
+- **2014 (Easy/Medium/Hard/Deadly)**: thresholds are floors — "the closest threshold that is lower than the adjusted XP value determines the encounter's difficulty" (2014 DMG). The tier is the highest threshold the adjusted XP meets or exceeds; below the Easy threshold the encounter is effectively trivial (0 bars).
+- **5.5e (Low/Moderate/High)**: budgets are ceilings — "spend as much of your XP budget as you can without going over" (2024 DMG). The tier is the lowest budget the total XP does not exceed: XP within the Low budget is Low, over Low but within Moderate is Moderate, over Moderate is High. The High budget is display-only guidance; it does not gate the High tier. The 2024 rules define no tier below Low, so 0 bars appear only at 0 net XP.
+
+The visual indicator maps identically across editions: 0 bars = 0 XP/Easy, 1 green = Low/Medium, 2 yellow = Moderate/Hard, 3 red = High/Deadly.
 
 #### FR-006 — Difficulty indicator in top bar
 The system MUST display a 3-bar difficulty indicator in the top bar, positioned to the right of the active combatant name.
@@ -321,7 +386,7 @@ The player character create and edit forms MUST include an optional level field
 The player character level MUST be persisted and restored across sessions, consistent with existing player character persistence behavior.
 
 #### FR-014 — High is the cap
-When total monster XP exceeds the High threshold, the indicator MUST display the High state (three red bars). There is no tier above High.
+When total monster XP exceeds the High budget, the indicator MUST display the High state (three red bars). There is no tier above High.
 
 #### FR-015 — Optional CR and side fields on Combatant
 The `Combatant` entity MUST support an optional `cr` field accepting standard 5e challenge rating strings ("0", "1/8", "1/4", "1/2", "1"–"30") and an optional `side` field accepting `"party"` or `"enemy"`.
@@ -377,6 +442,33 @@ Switching the rules edition in settings MUST immediately update the difficulty i
 #### FR-032 — Settings label reflects broader scope
 The settings modal section currently labeled "Conditions" MUST be relabeled to "Rules Edition" to reflect that the edition toggle controls both condition descriptions and difficulty calculation. This supersedes spec 003 FR-096 which scoped the label to conditions only.
 
+#### FR-033 — PF2e creature level-to-XP table
+The system MUST contain the PF2e creature level vs party level XP lookup table: level difference −4=10, −3=15, −2=20, −1=30, 0=40, +1=60, +2=80, +3=120, +4=160. Creatures more than 4 levels below the party have no row in the GM Core table and contribute 0 XP (no meaningful threat). Creatures more than 4 levels above the party are counted as the +4 row (160 XP).
+
+#### FR-034 — PF2e budget thresholds
+The system MUST contain the PF2e encounter budget thresholds for a party of 4: Trivial=40, Low=60, Moderate=80, Severe=120, Extreme=160.
+
+#### FR-035 — PF2e party size budget adjustment
+The system MUST adjust PF2e budget thresholds for party size using the GM Core (remaster) Character Adjustment values. For each PC beyond 4, add per-tier adjustments: Trivial +10, Low +20, Moderate +20, Severe +30, Extreme +40. For each PC fewer than 4, subtract the same amounts. Thresholds are floored at 0. (The pre-remaster Core Rulebook listed Low as 15; this spec follows the remaster.)
+
+#### FR-036 — PF2e party level derivation
+The system MUST derive the party level from PC combatant levels. The party level is the most common (mode) level among PCs with levels. If there is no unique mode (tie or all different), the party level is the average of all PC levels, rounded to the nearest integer.
+
+#### FR-037 — PF2e XP calculation
+The system MUST calculate PF2e encounter XP by summing per-creature XP for all enemy-side bestiary-linked creatures with a level, subtracting per-creature XP for party-side creatures with a level, and flooring the net at 0. No encounter multiplier is applied.
+
+#### FR-038 — PF2e difficulty tier determination
+The system MUST determine the PF2e difficulty tier by comparing total creature XP against the adjusted budget thresholds. The five tiers are Trivial, Low, Moderate, Severe, and Extreme. Budgets are ceilings (GM Core defines Trivial as "40 XP or less"): the tier is the lowest budget the total XP does not exceed — XP within the Trivial budget is Trivial, over Trivial but within Low is Low, and so on; XP over the Severe budget is Extreme. The Extreme budget is display-only guidance; it does not gate the Extreme tier.
+
+#### FR-039 — PF2e 4-bar indicator
+When the rules edition is PF2e, the difficulty indicator MUST display four bars instead of three. Visual states: 0 filled = Trivial, 1 green = Low, 2 yellow = Moderate, 3 orange = Severe, 4 red = Extreme.
+
+#### FR-040 — PF2e breakdown panel content
+When the rules edition is PF2e, the breakdown panel MUST show creature levels and level differences (not CR), per-creature XP based on level difference, the derived party level, and five threshold columns (Trivial/Low/Moderate/Severe/Extreme). Party size adjustments MUST be explained when active.
+
+#### FR-041 — PF2e indicator visibility
+When the rules edition is PF2e, the difficulty indicator MUST be shown when the encounter has at least one PC combatant with a level and at least one bestiary-linked creature with a level. CR is not relevant for PF2e visibility.
+
 ### Key Entities
 
 - **XP Budget Table**: A lookup mapping character level (1-20) to three XP thresholds (Low, Moderate, High), sourced from the 2024 5.5e DMG.
@@ -388,6 +480,9 @@ The settings modal section currently labeled "Conditions" MUST be relabeled to "
 - **Combatant.side**: An optional string field (`"party"` | `"enemy"`) on the existing `Combatant` entity. When undefined, defaults are resolved by the hook layer: PC combatants default to `"party"`, all others to `"enemy"`.
 - **2014 XP Thresholds Table**: A lookup mapping character level (1-20) to four XP thresholds (Easy, Medium, Hard, Deadly), sourced from the 2014 DMG.
 - **EncounterMultiplier**: A lookup mapping monster count ranges to base multiplier values (×1 through ×4), with party size adjustment shifting the multiplier up or down one step (full range ×0.5 through ×5).
+- **PF2e Level Difference XP Table**: A lookup mapping creature level difference from party level (−4 to +4) to XP values (10–160).
+- **PF2e Budget Thresholds**: Base thresholds for a party of 4: Trivial=40, Low=60, Moderate=80, Severe=120, Extreme=160, with per-tier adjustments for party size.
+- **Party Level**: Derived value — the mode of PC levels, or the rounded average when no unique mode exists.
 
 ---
 
@@ -407,6 +502,7 @@ The settings modal section currently labeled "Conditions" MUST be relabeled to "
 - **SC-010**: Party-side combatants with CR correctly subtract their XP from the monster total, and the net XP is never negative.
 - **SC-011**: The 2014 difficulty calculation correctly applies encounter multipliers and party size adjustments per the 2014 DMG rules.
 - **SC-012**: Switching rules edition immediately updates the indicator with no page reload required.
+- **SC-013**: The PF2e difficulty calculation correctly derives party level, computes per-creature XP from level differences, adjusts budgets for party size, and assigns the correct tier for all combinations.
 
 ---
 
@@ -424,3 +520,8 @@ The settings modal section currently labeled "Conditions" MUST be relabeled to "
 - The CR-to-XP lookup table is shared between both editions — only the budget thresholds and multiplier logic differ.
 - MVP baseline does not include the 2014 Adventuring Day XP budget or multipart encounter rules.
 - MVP baseline does not include per-combatant level overrides — level is always derived from the player character template.
+- The PF2e level difference XP table and budget thresholds are static data that do not change at runtime.
+- PF2e creatures always have a `level` field in their bestiary data (already present on `Pf2eCreature`).
+- Custom combatants without `creatureId` cannot contribute to PF2e XP calculation — there is no manual "creature level" assignment equivalent to the manual CR picker. This may be addressed in a future issue.
+- Level differences below −4 contribute 0 XP (strict reading of the GM Core table); above +4 they are counted as the +4 row (160 XP).
+- PF2e does not use CR at all — the CR field on combatants is irrelevant when in PF2e mode.