diff --git a/src/domain/index.ts b/src/domain/index.ts
index ab0b404..94c7962 100644
--- a/src/domain/index.ts
+++ b/src/domain/index.ts
@@ -9,3 +9,4 @@ export * from "./charges.ts";
 export * from "./fold.ts";
 export * from "./views.ts";
 export * from "./seed.ts";
+export * from "./plans.ts";
diff --git a/src/domain/plans.test.ts b/src/domain/plans.test.ts
new file mode 100644
index 0000000..bcdbbe8
--- /dev/null
+++ b/src/domain/plans.test.ts
@@ -0,0 +1,36 @@
+import { test, expect } from "bun:test";
+import { equalSlices, sliceTotal, slicesBalance } from "./plans.ts";
+
+test("equal slices divide the total across consecutive months", () => {
+  const slices = equalSlices({ amount: 3000, currency: "DKK" }, "2026-10", 6);
+  expect(slices).toHaveLength(6);
+  expect(slices[0]).toEqual({ month: "2026-10", amount: { amount: 500, currency: "DKK" } });
+  expect(slices[5]!.month).toBe("2027-03");
+});
+
+test("uneven totals still sum exactly, the last slice absorbing the drift", () => {
+  const slices = equalSlices({ amount: 100, currency: "DKK" }, "2026-01", 3);
+  expect(slices.map((s) => s.amount.amount)).toEqual([33.33, 33.33, 33.34]);
+  expect(sliceTotal(slices)).toBe(100);
+});
+
+test("a single slice takes the whole total", () => {
+  const slices = equalSlices({ amount: 99.99, currency: "DKK" }, "2026-01", 1);
+  expect(slices).toEqual([
+    { month: "2026-01", amount: { amount: 99.99, currency: "DKK" } },
+  ]);
+});
+
+test("a count below one is rejected", () => {
+  expect(() => equalSlices({ amount: 100, currency: "DKK" }, "2026-01", 0)).toThrow(
+    /at least one month/i,
+  );
+});
+
+test("slicesBalance reports the shortfall the editor must show", () => {
+  const slices = equalSlices({ amount: 100, currency: "DKK" }, "2026-01", 2);
+  expect(slicesBalance({ amount: 100, currency: "DKK" }, slices)).toBe(0);
+
+  slices[0]!.amount.amount = 30;
+  expect(slicesBalance({ amount: 100, currency: "DKK" }, slices)).toBe(20);
+});
diff --git a/src/domain/plans.ts b/src/domain/plans.ts
new file mode 100644
index 0000000..8f481fa
--- /dev/null
+++ b/src/domain/plans.ts
@@ -0,0 +1,34 @@
+import { distributeByWeight, roundMoney } from "./money.ts";
+import { addMonths } from "./months.ts";
+import type { Money, MonthId, ScheduleSlice } from "./types.ts";
+
+/** Splits a total into `count` consecutive monthly slices that sum exactly. */
+export function equalSlices(
+  total: Money,
+  startMonth: MonthId,
+  count: number,
+): ScheduleSlice[] {
+  if (!Number.isInteger(count) || count < 1) {
+    throw new Error("A finance plan needs at least one month");
+  }
+  const weights = new Array(count).fill(1);
+  const amounts = distributeByWeight(
+    total.amount,
+    weights,
+    count - 1,
+    total.currency,
+  );
+  return amounts.map((amount, i) => ({
+    month: addMonths(startMonth, i),
+    amount: { amount, currency: total.currency },
+  }));
+}
+
+export function sliceTotal(slices: ScheduleSlice[]): number {
+  return roundMoney(slices.reduce((sum, s) => sum + s.amount.amount, 0));
+}
+
+/** total - sum(slices). Positive means the plan is short of the total. */
+export function slicesBalance(total: Money, slices: ScheduleSlice[]): number {
+  return roundMoney(total.amount - sliceTotal(slices), total.currency);
+}
diff --git a/src/store/actions.test.ts b/src/store/actions.test.ts
new file mode 100644
index 0000000..5c9b587
--- /dev/null
+++ b/src/store/actions.test.ts
@@ -0,0 +1,315 @@
+import { test, expect, describe } from "bun:test";
+import * as actions from "./actions.ts";
+import { createSeedDataset } from "../domain/seed.ts";
+import type { Dataset } from "../domain/types.ts";
+
+function draft(): Dataset {
+  return createSeedDataset("2026-09");
+}
+
+describe("months", () => {
+  test("ensureMonth creates a missing month with zero income", () => {
+    const data = draft();
+    const month = actions.ensureMonth(data, "2026-10");
+    expect(month.income.amount).toBe(0);
+    expect(data.months.map((m) => m.id)).toEqual(["2026-09", "2026-10"]);
+  });
+
+  test("ensureMonth returns the existing month rather than duplicating it", () => {
+    const data = draft();
+    actions.ensureMonth(data, "2026-09");
+    expect(data.months).toHaveLength(1);
+  });
+
+  test("setIncome creates the month if needed", () => {
+    const data = draft();
+    actions.setIncome(data, "2026-11", { amount: 25000, currency: "DKK" });
+    expect(data.months.find((m) => m.id === "2026-11")!.income.amount).toBe(25000);
+  });
+
+  test("setIncome rounds the amount to the currency's minor unit", () => {
+    const data = draft();
+    actions.setIncome(data, "2026-11", { amount: 25000.005, currency: "DKK" });
+    expect(data.months.find((m) => m.id === "2026-11")!.income.amount).toBe(25000.01);
+    actions.setIncome(data, "2026-12", { amount: 12.345, currency: "DKK" });
+    expect(data.months.find((m) => m.id === "2026-12")!.income.amount).toBe(12.35);
+  });
+
+  test("setRuleOverride sets and clears an override", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleOverride(data, "2026-09", postId, {
+      kind: "percentOfIncome",
+      percent: 15,
+    });
+    expect(data.months[0]!.ruleOverrides[postId]).toEqual({
+      kind: "percentOfIncome",
+      percent: 15,
+    });
+
+    actions.setRuleOverride(data, "2026-09", postId, null);
+    expect(data.months[0]!.ruleOverrides[postId]).toBeUndefined();
+  });
+
+  test("setRuleOverride rounds a fixed rule's amount and a percent rule's percent", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setRuleOverride(data, "2026-09", postId, {
+      kind: "fixed",
+      amount: { amount: 12.345, currency: "DKK" },
+    });
+    expect(data.months[0]!.ruleOverrides[postId]).toEqual({
+      kind: "fixed",
+      amount: { amount: 12.35, currency: "DKK" },
+    });
+
+    actions.setRuleOverride(data, "2026-09", postId, {
+      kind: "percentOfIncome",
+      percent: 12.3456,
+    });
+    expect(data.months[0]!.ruleOverrides[postId]).toEqual({
+      kind: "percentOfIncome",
+      percent: 12.35,
+    });
+  });
+});
+
+describe("posts", () => {
+  test("addPost appends with the next order value", () => {
+    const data = draft();
+    const post = actions.addPost(data, "Travel", "EUR", {
+      kind: "percentOfIncome",
+      percent: 5,
+    });
+    expect(post.order).toBe(3);
+    expect(post.archived).toBe(false);
+    expect(data.posts).toHaveLength(4);
+  });
+
+  test("addPost rounds a fixed standing rule's amount and a percent's percent", () => {
+    const data = draft();
+    const fixedPost = actions.addPost(data, "Rent", "DKK", {
+      kind: "fixed",
+      amount: { amount: 12.345, currency: "DKK" },
+    });
+    expect(fixedPost.standingRule).toEqual({
+      kind: "fixed",
+      amount: { amount: 12.35, currency: "DKK" },
+    });
+
+    const percentPost = actions.addPost(data, "Travel", "EUR", {
+      kind: "percentOfIncome",
+      percent: 12.3456,
+    });
+    expect(percentPost.standingRule).toEqual({
+      kind: "percentOfIncome",
+      percent: 12.35,
+    });
+  });
+
+  test("setPostArchived toggles without deleting", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.setPostArchived(data, postId, true);
+    expect(data.posts.find((p) => p.id === postId)!.archived).toBe(true);
+    expect(data.posts).toHaveLength(3);
+  });
+
+  test("movePost swaps order with its neighbour", () => {
+    const data = draft();
+    actions.movePost(data, data.posts[1]!.id, -1);
+    expect([...data.posts].sort((a, b) => a.order - b.order).map((p) => p.name)).toEqual([
+      "Food",
+      "Video Games",
+      "Events and Social",
+    ]);
+  });
+
+  test("movePost at the boundary is a no-op", () => {
+    const data = draft();
+    actions.movePost(data, data.posts[0]!.id, -1);
+    expect(data.posts[0]!.order).toBe(0);
+  });
+
+  test("updatePost renames without touching other fields", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.updatePost(data, postId, { name: "Games" });
+    const post = data.posts.find((p) => p.id === postId)!;
+    expect(post.name).toBe("Games");
+    expect(post.currency).toBe("DKK");
+  });
+
+  test("updatePost rounds a fixed standing rule's amount and a percent's percent", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    actions.updatePost(data, postId, {
+      standingRule: { kind: "fixed", amount: { amount: 12.345, currency: "DKK" } },
+    });
+    expect(data.posts.find((p) => p.id === postId)!.standingRule).toEqual({
+      kind: "fixed",
+      amount: { amount: 12.35, currency: "DKK" },
+    });
+
+    actions.updatePost(data, postId, {
+      standingRule: { kind: "percentOfIncome", percent: 12.3456 },
+    });
+    expect(data.posts.find((p) => p.id === postId)!.standingRule).toEqual({
+      kind: "percentOfIncome",
+      percent: 12.35,
+    });
+  });
+
+  test("an unknown post id throws", () => {
+    expect(() => actions.updatePost(draft(), "ghost", { name: "x" })).toThrow(/ghost/);
+  });
+});
+
+describe("purchases", () => {
+  const newPurchase = (postId: string) => ({
+    date: "2026-09-14",
+    description: "Groceries",
+    total: { amount: 200, currency: "DKK" as const },
+    splitMode: "percent" as const,
+    splits: [{ postId, value: 100, absorbsRemainder: true }],
+    schedule: null,
+  });
+
+  test("addPurchase assigns an id and ensures the month exists", () => {
+    const data = draft();
+    const purchase = actions.addPurchase(data, {
+      ...newPurchase(data.posts[0]!.id),
+      date: "2026-12-01",
+    });
+    expect(purchase.id).toBeTruthy();
+    expect(data.months.map((m) => m.id)).toContain("2026-12");
+  });
+
+  test("addPurchase rounds the total amount, split values, and schedule slice amounts", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    const purchase = actions.addPurchase(data, {
+      date: "2026-09-14",
+      description: "Sofa",
+      total: { amount: 199.995, currency: "DKK" },
+      splitMode: "percent",
+      splits: [{ postId, value: 33.3333, absorbsRemainder: true }],
+      schedule: {
+        slices: [
+          { month: "2026-09", amount: { amount: 66.665, currency: "DKK" } },
+          { month: "2026-10", amount: { amount: 66.665, currency: "DKK" } },
+          { month: "2026-11", amount: { amount: 66.665, currency: "DKK" } },
+        ],
+      },
+    });
+    expect(purchase.total.amount).toBe(200);
+    expect(purchase.splits[0]!.value).toBe(33.33);
+    expect(purchase.schedule!.slices.map((s) => s.amount.amount)).toEqual([
+      66.67, 66.67, 66.67,
+    ]);
+  });
+
+  test("updatePurchase replaces the given fields", () => {
+    const data = draft();
+    const purchase = actions.addPurchase(data, newPurchase(data.posts[0]!.id));
+    actions.updatePurchase(data, purchase.id, { description: "Dinner" });
+    expect(data.purchases[0]!.description).toBe("Dinner");
+  });
+
+  test("updatePurchase rounds the total amount, split values, and schedule slice amounts", () => {
+    const data = draft();
+    const postId = data.posts[0]!.id;
+    const purchase = actions.addPurchase(data, newPurchase(postId));
+    actions.updatePurchase(data, purchase.id, {
+      total: { amount: 199.995, currency: "DKK" },
+      splits: [{ postId, value: 33.3333, absorbsRemainder: true }],
+      schedule: {
+        slices: [
+          { month: "2026-09", amount: { amount: 66.665, currency: "DKK" } },
+        ],
+      },
+    });
+    const updated = data.purchases.find((p) => p.id === purchase.id)!;
+    expect(updated.total.amount).toBe(200);
+    expect(updated.splits[0]!.value).toBe(33.33);
+    expect(updated.schedule!.slices[0]!.amount.amount).toBe(66.67);
+  });
+
+  test("deletePurchase removes it", () => {
+    const data = draft();
+    const purchase = actions.addPurchase(data, newPurchase(data.posts[0]!.id));
+    actions.deletePurchase(data, purchase.id);
+    expect(data.purchases).toHaveLength(0);
+  });
+
+  test("cancelScheduleFrom records the cancellation month", () => {
+    const data = draft();
+    const purchase = actions.addPurchase(data, {
+      ...newPurchase(data.posts[0]!.id),
+      schedule: {
+        slices: [
+          { month: "2026-09", amount: { amount: 100, currency: "DKK" } },
+          { month: "2026-10", amount: { amount: 100, currency: "DKK" } },
+        ],
+      },
+    });
+    actions.cancelScheduleFrom(data, purchase.id, "2026-10");
+    expect(data.purchases[0]!.schedule!.cancelledFromMonth).toBe("2026-10");
+  });
+
+  test("cancelling an unscheduled purchase throws", () => {
+    const data = draft();
+    const purchase = actions.addPurchase(data, newPurchase(data.posts[0]!.id));
+    expect(() => actions.cancelScheduleFrom(data, purchase.id, "2026-10")).toThrow(
+      /no finance plan/i,
+    );
+  });
+});
+
+describe("settings", () => {
+  test("setFxRate inserts then updates in place", () => {
+    const data = draft();
+    actions.setFxRate(data, {
+      currency: "EUR",
+      baseUnitsPerOne: 7.46,
+      updatedAt: "2026-09-01",
+      source: "manual",
+    });
+    expect(data.fxRates).toHaveLength(1);
+
+    actions.setFxRate(data, {
+      currency: "EUR",
+      baseUnitsPerOne: 7.5,
+      updatedAt: "2026-09-02",
+      source: "api",
+    });
+    expect(data.fxRates).toHaveLength(1);
+    expect(data.fxRates[0]!.baseUnitsPerOne).toBe(7.5);
+  });
+
+  test("setFxRate preserves a 6-decimal rate unrounded", () => {
+    const data = draft();
+    actions.setFxRate(data, {
+      currency: "USD",
+      baseUnitsPerOne: 7.123456,
+      updatedAt: "2026-09-01",
+      source: "api",
+    });
+    expect(data.fxRates.find((r) => r.currency === "USD")!.baseUnitsPerOne).toBe(
+      7.123456,
+    );
+  });
+
+  test("setBaseCurrency drops the new base's own rate row", () => {
+    const data = draft();
+    actions.setFxRate(data, {
+      currency: "EUR",
+      baseUnitsPerOne: 7.46,
+      updatedAt: "2026-09-01",
+      source: "manual",
+    });
+    actions.setBaseCurrency(data, "EUR");
+    expect(data.settings.baseCurrency).toBe("EUR");
+    expect(data.fxRates.find((r) => r.currency === "EUR")).toBeUndefined();
+  });
+});
diff --git a/src/store/actions.ts b/src/store/actions.ts
new file mode 100644
index 0000000..7c311d7
--- /dev/null
+++ b/src/store/actions.ts
@@ -0,0 +1,226 @@
+import { newId } from "../domain/seed.ts";
+import { monthOf } from "../domain/months.ts";
+import { roundMoney } from "../domain/money.ts";
+import type {
+  Currency,
+  Dataset,
+  FxRate,
+  Money,
+  Month,
+  MonthId,
+  Post,
+  PostId,
+  Purchase,
+  PurchaseId,
+  Rule,
+  Schedule,
+  Split,
+} from "../domain/types.ts";
+
+/**
+ * Every function here mutates a draft Dataset in place. They are called from
+ * store.mutate(), which clones before applying and writes through afterwards.
+ *
+ * Every MONEY amount that flows through here is rounded to its currency's
+ * minor unit (via roundMoney) before it lands in the draft, per CLAUDE.md's
+ * "round before persisting" rule. FxRate.baseUnitsPerOne is deliberately
+ * exempted in setFxRate below — an exchange rate is not money, and rounding
+ * it to cents would be wrong in kind (see the comment there).
+ */
+
+function requirePost(draft: Dataset, postId: PostId): Post {
+  const post = draft.posts.find((p) => p.id === postId);
+  if (!post) throw new Error(`Unknown post: ${postId}`);
+  return post;
+}
+
+function requirePurchase(draft: Dataset, purchaseId: PurchaseId): Purchase {
+  const purchase = draft.purchases.find((p) => p.id === purchaseId);
+  if (!purchase) throw new Error(`Unknown purchase: ${purchaseId}`);
+  return purchase;
+}
+
+/** Rounds a non-money percentage (e.g. Rule.percent, Split.value) to 2 decimals. */
+function round2(value: number): number {
+  return Number(value.toFixed(2));
+}
+
+function roundMoneyValue(money: Money): Money {
+  return { amount: roundMoney(money.amount, money.currency), currency: money.currency };
+}
+
+function roundRule(rule: Rule): Rule {
+  if (rule.kind === "fixed") {
+    return { kind: "fixed", amount: roundMoneyValue(rule.amount) };
+  }
+  return { kind: "percentOfIncome", percent: round2(rule.percent) };
+}
+
+function roundSplits(splits: Split[]): Split[] {
+  return splits.map((split) => ({ ...split, value: round2(split.value) }));
+}
+
+function roundSchedule(schedule: Schedule | null): Schedule | null {
+  if (!schedule) return null;
+  return {
+    ...schedule,
+    slices: schedule.slices.map((slice) => ({
+      month: slice.month,
+      amount: roundMoneyValue(slice.amount),
+    })),
+  };
+}
+
+export function ensureMonth(draft: Dataset, monthId: MonthId): Month {
+  const existing = draft.months.find((m) => m.id === monthId);
+  if (existing) return existing;
+  const created: Month = {
+    id: monthId,
+    income: { amount: 0, currency: draft.settings.baseCurrency },
+    ruleOverrides: {},
+  };
+  draft.months.push(created);
+  draft.months.sort((a, b) => a.id.localeCompare(b.id));
+  return created;
+}
+
+export function setIncome(draft: Dataset, monthId: MonthId, income: Money): void {
+  ensureMonth(draft, monthId).income = roundMoneyValue(income);
+}
+
+export function setRuleOverride(
+  draft: Dataset,
+  monthId: MonthId,
+  postId: PostId,
+  rule: Rule | null,
+): void {
+  const month = ensureMonth(draft, monthId);
+  if (rule === null) {
+    delete month.ruleOverrides[postId];
+    return;
+  }
+  month.ruleOverrides[postId] = roundRule(rule);
+}
+
+export function addPost(
+  draft: Dataset,
+  name: string,
+  currency: Currency,
+  standingRule: Rule,
+): Post {
+  const post: Post = {
+    id: newId(),
+    name,
+    order: draft.posts.length,
+    archived: false,
+    currency,
+    standingRule: roundRule(standingRule),
+  };
+  draft.posts.push(post);
+  return post;
+}
+
+export function updatePost(
+  draft: Dataset,
+  postId: PostId,
+  changes: Partial<Omit<Post, "id">>,
+): void {
+  const post = requirePost(draft, postId);
+  const resolved: Partial<Omit<Post, "id">> = { ...changes };
+  if (changes.standingRule) {
+    resolved.standingRule = roundRule(changes.standingRule);
+  }
+  Object.assign(post, resolved);
+}
+
+/** Archives rather than deletes, because purchases reference posts. */
+export function setPostArchived(
+  draft: Dataset,
+  postId: PostId,
+  archived: boolean,
+): void {
+  requirePost(draft, postId).archived = archived;
+}
+
+export function movePost(draft: Dataset, postId: PostId, direction: -1 | 1): void {
+  const ordered = [...draft.posts].sort((a, b) => a.order - b.order);
+  const index = ordered.findIndex((p) => p.id === postId);
+  const target = index + direction;
+  if (index === -1 || target < 0 || target >= ordered.length) return;
+  const a = ordered[index]!;
+  const b = ordered[target]!;
+  [a.order, b.order] = [b.order, a.order];
+}
+
+export function addPurchase(
+  draft: Dataset,
+  purchase: Omit<Purchase, "id">,
+): Purchase {
+  const created: Purchase = {
+    ...purchase,
+    id: newId(),
+    total: roundMoneyValue(purchase.total),
+    splits: roundSplits(purchase.splits),
+    schedule: roundSchedule(purchase.schedule),
+  };
+  draft.purchases.push(created);
+  // Make sure every month the purchase touches exists, so income can be
+  // entered against it later.
+  const months = created.schedule
+    ? created.schedule.slices.map((s) => s.month)
+    : [monthOf(created.date)];
+  for (const monthId of months) ensureMonth(draft, monthId);
+  return created;
+}
+
+export function updatePurchase(
+  draft: Dataset,
+  purchaseId: PurchaseId,
+  changes: Partial<Omit<Purchase, "id">>,
+): void {
+  const purchase = requirePurchase(draft, purchaseId);
+  const resolved: Partial<Omit<Purchase, "id">> = { ...changes };
+  if (changes.total) {
+    resolved.total = roundMoneyValue(changes.total);
+  }
+  if (changes.splits) {
+    resolved.splits = roundSplits(changes.splits);
+  }
+  if (changes.schedule !== undefined) {
+    resolved.schedule = roundSchedule(changes.schedule);
+  }
+  Object.assign(purchase, resolved);
+}
+
+export function deletePurchase(draft: Dataset, purchaseId: PurchaseId): void {
+  draft.purchases = draft.purchases.filter((p) => p.id !== purchaseId);
+}
+
+export function cancelScheduleFrom(
+  draft: Dataset,
+  purchaseId: PurchaseId,
+  monthId: MonthId,
+): void {
+  const purchase = requirePurchase(draft, purchaseId);
+  if (!purchase.schedule) {
+    throw new Error(`Purchase ${purchaseId} has no finance plan to cancel`);
+  }
+  purchase.schedule.cancelledFromMonth = monthId;
+}
+
+/**
+ * Stores the rate exactly as passed. Exchange rates are not money: they are
+ * kept at six decimal places (see fxApi.ts's toFixed(6)), and rounding to a
+ * currency's minor unit would quantize a rate below 0.01 straight to zero.
+ */
+export function setFxRate(draft: Dataset, rate: FxRate): void {
+  const index = draft.fxRates.findIndex((r) => r.currency === rate.currency);
+  if (index === -1) draft.fxRates.push(rate);
+  else draft.fxRates[index] = rate;
+}
+
+/** The base currency never has a rate row of its own. */
+export function setBaseCurrency(draft: Dataset, currency: Currency): void {
+  draft.settings.baseCurrency = currency;
+  draft.fxRates = draft.fxRates.filter((r) => r.currency !== currency);
+}