diff --git a/apps/web/src/hooks/__tests__/encounter-reducer.test.ts b/apps/web/src/hooks/__tests__/encounter-reducer.test.ts
new file mode 100644
index 0000000..350fb86
--- /dev/null
+++ b/apps/web/src/hooks/__tests__/encounter-reducer.test.ts
@@ -0,0 +1,416 @@
+import type {
+	BestiaryIndexEntry,
+	ConditionId,
+	PlayerCharacter,
+} from "@initiative/domain";
+import {
+	combatantId,
+	createEncounter,
+	EMPTY_UNDO_REDO_STATE,
+	isDomainError,
+	playerCharacterId,
+} from "@initiative/domain";
+import { describe, expect, it, vi } from "vitest";
+import { type EncounterState, encounterReducer } from "../use-encounter.js";
+
+vi.mock("../../persistence/encounter-storage.js", () => ({
+	loadEncounter: vi.fn().mockReturnValue(null),
+	saveEncounter: vi.fn(),
+}));
+
+vi.mock("../../persistence/undo-redo-storage.js", () => ({
+	loadUndoRedoStacks: vi.fn().mockReturnValue(EMPTY_UNDO_REDO_STATE),
+	saveUndoRedoStacks: vi.fn(),
+}));
+
+function emptyState(): EncounterState {
+	return {
+		encounter: {
+			combatants: [],
+			activeIndex: 0,
+			roundNumber: 1,
+		},
+		undoRedoState: EMPTY_UNDO_REDO_STATE,
+		events: [],
+		nextId: 0,
+		lastCreatureId: null,
+	};
+}
+
+function stateWith(...names: string[]): EncounterState {
+	let state = emptyState();
+	for (const name of names) {
+		state = encounterReducer(state, { type: "add-combatant", name });
+	}
+	return state;
+}
+
+function stateWithHp(name: string, maxHp: number): EncounterState {
+	const state = stateWith(name);
+	const id = state.encounter.combatants[0].id;
+	return encounterReducer(state, {
+		type: "set-hp",
+		id,
+		maxHp,
+	});
+}
+
+const BESTIARY_ENTRY: BestiaryIndexEntry = {
+	name: "Goblin",
+	source: "MM",
+	ac: 15,
+	hp: 7,
+	dex: 14,
+	cr: "1/4",
+	initiativeProficiency: 0,
+	size: "Small",
+	type: "humanoid",
+};
+
+describe("encounterReducer", () => {
+	describe("add-combatant", () => {
+		it("adds a combatant and pushes undo", () => {
+			const next = encounterReducer(emptyState(), {
+				type: "add-combatant",
+				name: "Goblin",
+			});
+
+			expect(next.encounter.combatants).toHaveLength(1);
+			expect(next.encounter.combatants[0].name).toBe("Goblin");
+			expect(next.undoRedoState.undoStack).toHaveLength(1);
+			expect(next.nextId).toBe(1);
+		});
+
+		it("applies optional init values", () => {
+			const next = encounterReducer(emptyState(), {
+				type: "add-combatant",
+				name: "Goblin",
+				init: { initiative: 15, ac: 13, maxHp: 7 },
+			});
+
+			const c = next.encounter.combatants[0];
+			expect(c.initiative).toBe(15);
+			expect(c.ac).toBe(13);
+			expect(c.maxHp).toBe(7);
+			expect(c.currentHp).toBe(7);
+		});
+
+		it("increments IDs", () => {
+			const s1 = encounterReducer(emptyState(), {
+				type: "add-combatant",
+				name: "A",
+			});
+			const s2 = encounterReducer(s1, {
+				type: "add-combatant",
+				name: "B",
+			});
+
+			expect(s2.encounter.combatants[0].id).toBe("c-1");
+			expect(s2.encounter.combatants[1].id).toBe("c-2");
+		});
+
+		it("returns unchanged state for invalid name", () => {
+			const state = emptyState();
+			const next = encounterReducer(state, {
+				type: "add-combatant",
+				name: "",
+			});
+
+			expect(next).toBe(state);
+		});
+	});
+
+	describe("remove-combatant", () => {
+		it("removes combatant and pushes undo", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "remove-combatant",
+				id,
+			});
+
+			expect(next.encounter.combatants).toHaveLength(0);
+			expect(next.undoRedoState.undoStack).toHaveLength(2);
+		});
+	});
+
+	describe("edit-combatant", () => {
+		it("renames combatant", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "edit-combatant",
+				id,
+				newName: "Hobgoblin",
+			});
+
+			expect(next.encounter.combatants[0].name).toBe("Hobgoblin");
+		});
+	});
+
+	describe("advance-turn / retreat-turn", () => {
+		it("advances and retreats turn", () => {
+			const state = stateWith("A", "B");
+			const advanced = encounterReducer(state, {
+				type: "advance-turn",
+			});
+			expect(advanced.encounter.activeIndex).toBe(1);
+
+			const retreated = encounterReducer(advanced, {
+				type: "retreat-turn",
+			});
+			expect(retreated.encounter.activeIndex).toBe(0);
+		});
+
+		it("returns unchanged state on empty encounter", () => {
+			const state = emptyState();
+			const next = encounterReducer(state, { type: "advance-turn" });
+			expect(next).toBe(state);
+		});
+	});
+
+	describe("set-hp / adjust-hp / set-temp-hp", () => {
+		it("sets max HP", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "set-hp",
+				id,
+				maxHp: 20,
+			});
+
+			expect(next.encounter.combatants[0].maxHp).toBe(20);
+			expect(next.encounter.combatants[0].currentHp).toBe(20);
+		});
+
+		it("adjusts HP", () => {
+			const state = stateWithHp("Goblin", 20);
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "adjust-hp",
+				id,
+				delta: -5,
+			});
+
+			expect(next.encounter.combatants[0].currentHp).toBe(15);
+		});
+
+		it("sets temp HP", () => {
+			const state = stateWithHp("Goblin", 20);
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "set-temp-hp",
+				id,
+				tempHp: 5,
+			});
+
+			expect(next.encounter.combatants[0].tempHp).toBe(5);
+		});
+	});
+
+	describe("set-ac", () => {
+		it("sets AC", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "set-ac",
+				id,
+				value: 15,
+			});
+
+			expect(next.encounter.combatants[0].ac).toBe(15);
+		});
+	});
+
+	describe("set-initiative", () => {
+		it("sets initiative", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "set-initiative",
+				id,
+				value: 18,
+			});
+
+			expect(next.encounter.combatants[0].initiative).toBe(18);
+		});
+	});
+
+	describe("toggle-condition / toggle-concentration", () => {
+		it("toggles condition", () => {
+			const state = stateWith("Goblin");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "toggle-condition",
+				id,
+				conditionId: "blinded" as ConditionId,
+			});
+
+			expect(next.encounter.combatants[0].conditions).toContain("blinded");
+		});
+
+		it("toggles concentration", () => {
+			const state = stateWith("Wizard");
+			const id = state.encounter.combatants[0].id;
+			const next = encounterReducer(state, {
+				type: "toggle-concentration",
+				id,
+			});
+
+			expect(next.encounter.combatants[0].isConcentrating).toBe(true);
+		});
+	});
+
+	describe("clear-encounter", () => {
+		it("clears combatants, resets history and nextId", () => {
+			const state = stateWith("A", "B");
+			const next = encounterReducer(state, {
+				type: "clear-encounter",
+			});
+
+			expect(next.encounter.combatants).toHaveLength(0);
+			expect(next.undoRedoState.undoStack).toHaveLength(0);
+			expect(next.undoRedoState.redoStack).toHaveLength(0);
+			expect(next.nextId).toBe(0);
+		});
+	});
+
+	describe("undo / redo", () => {
+		it("undo restores previous state", () => {
+			const state = stateWith("Goblin");
+			const next = encounterReducer(state, { type: "undo" });
+
+			expect(next.encounter.combatants).toHaveLength(0);
+			expect(next.undoRedoState.undoStack).toHaveLength(0);
+			expect(next.undoRedoState.redoStack).toHaveLength(1);
+		});
+
+		it("redo restores undone state", () => {
+			const state = stateWith("Goblin");
+			const undone = encounterReducer(state, { type: "undo" });
+			const redone = encounterReducer(undone, { type: "redo" });
+
+			expect(redone.encounter.combatants).toHaveLength(1);
+			expect(redone.encounter.combatants[0].name).toBe("Goblin");
+		});
+
+		it("undo returns unchanged state when stack is empty", () => {
+			const state = emptyState();
+			const next = encounterReducer(state, { type: "undo" });
+			expect(next).toBe(state);
+		});
+
+		it("redo returns unchanged state when stack is empty", () => {
+			const state = emptyState();
+			const next = encounterReducer(state, { type: "redo" });
+			expect(next).toBe(state);
+		});
+	});
+
+	describe("add-from-bestiary", () => {
+		it("adds creature with HP, AC, and creatureId", () => {
+			const next = encounterReducer(emptyState(), {
+				type: "add-from-bestiary",
+				entry: BESTIARY_ENTRY,
+			});
+
+			const c = next.encounter.combatants[0];
+			expect(c.name).toBe("Goblin");
+			expect(c.maxHp).toBe(7);
+			expect(c.ac).toBe(15);
+			expect(c.creatureId).toBe("mm:goblin");
+			expect(next.lastCreatureId).toBe("mm:goblin");
+			expect(next.undoRedoState.undoStack).toHaveLength(1);
+		});
+
+		it("auto-numbers duplicate names", () => {
+			const s1 = encounterReducer(emptyState(), {
+				type: "add-from-bestiary",
+				entry: BESTIARY_ENTRY,
+			});
+			const s2 = encounterReducer(s1, {
+				type: "add-from-bestiary",
+				entry: BESTIARY_ENTRY,
+			});
+
+			const names = s2.encounter.combatants.map((c) => c.name);
+			expect(names).toContain("Goblin 1");
+			expect(names).toContain("Goblin 2");
+		});
+	});
+
+	describe("add-multiple-from-bestiary", () => {
+		it("adds multiple creatures in one action", () => {
+			const next = encounterReducer(emptyState(), {
+				type: "add-multiple-from-bestiary",
+				entry: BESTIARY_ENTRY,
+				count: 3,
+			});
+
+			expect(next.encounter.combatants).toHaveLength(3);
+			expect(next.undoRedoState.undoStack).toHaveLength(1);
+			expect(next.lastCreatureId).toBe("mm:goblin");
+		});
+	});
+
+	describe("add-from-player-character", () => {
+		it("adds combatant with PC attributes", () => {
+			const pc: PlayerCharacter = {
+				id: playerCharacterId("pc-1"),
+				name: "Aria",
+				ac: 16,
+				maxHp: 30,
+				color: "blue",
+				icon: "sword",
+			};
+			const next = encounterReducer(emptyState(), {
+				type: "add-from-player-character",
+				pc,
+			});
+
+			const c = next.encounter.combatants[0];
+			expect(c.name).toBe("Aria");
+			expect(c.maxHp).toBe(30);
+			expect(c.ac).toBe(16);
+			expect(c.color).toBe("blue");
+			expect(c.icon).toBe("sword");
+			expect(c.playerCharacterId).toBe("pc-1");
+			expect(next.lastCreatureId).toBeNull();
+		});
+	});
+
+	describe("import", () => {
+		it("replaces encounter and undo/redo state", () => {
+			const state = stateWith("A", "B");
+			const enc = createEncounter([
+				{ id: combatantId("c-5"), name: "Imported" },
+			]);
+			if (isDomainError(enc)) throw new Error("Setup failed");
+
+			const next = encounterReducer(state, {
+				type: "import",
+				encounter: enc,
+				undoRedoState: EMPTY_UNDO_REDO_STATE,
+			});
+
+			expect(next.encounter.combatants).toHaveLength(1);
+			expect(next.encounter.combatants[0].name).toBe("Imported");
+			expect(next.nextId).toBe(5);
+		});
+	});
+
+	describe("events accumulation", () => {
+		it("accumulates events across actions", () => {
+			const s1 = encounterReducer(emptyState(), {
+				type: "add-combatant",
+				name: "A",
+			});
+			const s2 = encounterReducer(s1, {
+				type: "add-combatant",
+				name: "B",
+			});
+
+			expect(s2.events.length).toBeGreaterThanOrEqual(2);
+		});
+	});
+});
diff --git a/apps/web/src/hooks/use-encounter.ts b/apps/web/src/hooks/use-encounter.ts
index db5db1f..34602ef 100644
--- a/apps/web/src/hooks/use-encounter.ts
+++ b/apps/web/src/hooks/use-encounter.ts
@@ -81,7 +81,7 @@ type EncounterAction =
 			undoRedoState: UndoRedoState;
 	  };
 
-interface EncounterState {
+export interface EncounterState {
 	readonly encounter: Encounter;
 	readonly undoRedoState: UndoRedoState;
 	readonly events: readonly DomainEvent[];
@@ -268,7 +268,7 @@ function handleAddFromPlayerCharacter(
 
 // -- Reducer --
 
-function encounterReducer(
+export function encounterReducer(
 	state: EncounterState,
 	action: EncounterAction,
 ): EncounterState {