diff --git a/packages/pokemon/src/__tests__/egg.test.ts b/packages/pokemon/src/__tests__/egg.test.ts
index 723fefe02..6cc8b3305 100644
--- a/packages/pokemon/src/__tests__/egg.test.ts
+++ b/packages/pokemon/src/__tests__/egg.test.ts
@@ -1,5 +1,5 @@
 import { describe, test, expect } from 'bun:test'
-import { checkEggEligibility, generateEgg, advanceEggSteps, isEggReadyToHatch } from '../core/egg'
+import { checkEggEligibility, generateEgg, advanceEggSteps, isEggReadyToHatch, hatchEgg } from '../core/egg'
 import type { BuddyData } from '../types'
 import { generateCreature } from '../core/creature'
 
@@ -113,3 +113,48 @@ describe('isEggReadyToHatch', () => {
 		expect(isEggReadyToHatch(egg)).toBe(false)
 	})
 })
+
+describe('hatchEgg', () => {
+	test('creates a creature and removes egg', async () => {
+		const data = makeBuddyData()
+		const egg = { id: 'egg-1', obtainedAt: Date.now(), stepsRemaining: 0, totalSteps: 2000, speciesId: 'charmander' as const }
+		const result = await hatchEgg(data, egg)
+
+		expect(result.creature.speciesId).toBe('charmander')
+		expect(result.buddyData.creatures.length).toBe(data.creatures.length + 1)
+		expect(result.buddyData.eggs.length).toBe(0)
+	})
+
+	test('adds creature to party when slot available', async () => {
+		const data = makeBuddyData()
+		const egg = { id: 'egg-1', obtainedAt: Date.now(), stepsRemaining: 0, totalSteps: 2000, speciesId: 'pikachu' as const }
+		const result = await hatchEgg(data, egg)
+		const newCreature = result.creature
+		const inParty = result.buddyData.party.includes(newCreature.id)
+		expect(inParty).toBe(true)
+	})
+
+	test('increments totalEggsObtained', async () => {
+		const data = makeBuddyData()
+		const egg = { id: 'egg-1', obtainedAt: Date.now(), stepsRemaining: 0, totalSteps: 2000, speciesId: 'squirtle' as const }
+		const result = await hatchEgg(data, egg)
+		expect(result.buddyData.stats.totalEggsObtained).toBe(1)
+	})
+
+	test('updates dex entry with new species', async () => {
+		const data = makeBuddyData()
+		const egg = { id: 'egg-1', obtainedAt: Date.now(), stepsRemaining: 0, totalSteps: 2000, speciesId: 'charmander' as const }
+		const result = await hatchEgg(data, egg)
+		const entry = result.buddyData.dex.find(d => d.speciesId === 'charmander')
+		expect(entry).toBeDefined()
+		expect(entry!.caughtCount).toBe(1)
+	})
+
+	test('increments caughtCount for existing dex entry', async () => {
+		const data = makeBuddyData()
+		const egg = { id: 'egg-1', obtainedAt: Date.now(), stepsRemaining: 0, totalSteps: 2000, speciesId: 'bulbasaur' as const }
+		const result = await hatchEgg(data, egg)
+		const entry = result.buddyData.dex.find(d => d.speciesId === 'bulbasaur')
+		expect(entry!.caughtCount).toBe(2)
+	})
+})
diff --git a/packages/pokemon/src/__tests__/species.test.ts b/packages/pokemon/src/__tests__/species.test.ts
index e6ea5db6e..c12eef47a 100644
--- a/packages/pokemon/src/__tests__/species.test.ts
+++ b/packages/pokemon/src/__tests__/species.test.ts
@@ -1,5 +1,5 @@
 import { describe, test, expect } from 'bun:test'
-import { getSpeciesData, getAllSpeciesData, DEX_TO_SPECIES } from '../data/species'
+import { getSpeciesData, getAllSpeciesData, DEX_TO_SPECIES, ensureSpeciesData } from '../data/species'
 import { ALL_SPECIES_IDS } from '../types'
 import type { SpeciesId } from '../types'
 
@@ -67,3 +67,29 @@ describe('DEX_TO_SPECIES', () => {
 		expect(DEX_TO_SPECIES[25]).toBe('pikachu')
 	})
 })
+
+describe('ensureSpeciesData', () => {
+	test('resolves without error', async () => {
+		await expect(ensureSpeciesData()).resolves.toBeUndefined()
+	})
+})
+
+describe('getSpeciesData - supplementary fields', () => {
+	test('has baseHappiness', () => {
+		expect(getSpeciesData('bulbasaur').baseHappiness).toBe(70)
+	})
+
+	test('pikachu has higher captureRate', () => {
+		expect(getSpeciesData('pikachu').captureRate).toBeGreaterThan(getSpeciesData('charmander').captureRate)
+	})
+
+	test('has names with en key', () => {
+		const data = getSpeciesData('charmander')
+		expect(data.names).toBeDefined()
+		expect(data.names.en).toBe('Charmander')
+	})
+
+	test('shinyChance is 1/4096', () => {
+		expect(getSpeciesData('bulbasaur').shinyChance).toBe(1 / 4096)
+	})
+})