diff --git a/src/services/categoryMappingService.test.ts b/src/services/categoryMappingService.test.ts
new file mode 100644
index 0000000..4d62295
--- /dev/null
+++ b/src/services/categoryMappingService.test.ts
@@ -0,0 +1,366 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  computeMigrationPlan,
+  normalizeForMatch,
+  __resetMappingServiceCachesForTests,
+  type ProfileData,
+  type V2CategoryInput,
+  type V2KeywordInput,
+  type V2TransactionInput,
+  type V2SupplierInput,
+} from "./categoryMappingService";
+import { resetTaxonomyCache } from "./categoryTaxonomyService";
+
+beforeEach(() => {
+  resetTaxonomyCache();
+  __resetMappingServiceCachesForTests();
+});
+
+// ---------------------------------------------------------------------------
+// Fixture helpers — we build just enough of ProfileData per test to stay
+// readable; everything defaults to empty arrays.
+// ---------------------------------------------------------------------------
+
+function makeProfile(partial: Partial<ProfileData>): ProfileData {
+  return {
+    v2Categories: partial.v2Categories ?? [],
+    keywords: partial.keywords ?? [],
+    transactions: partial.transactions ?? [],
+    suppliers: partial.suppliers,
+  };
+}
+
+function cat(id: number, name: string, parent_id: number | null = null): V2CategoryInput {
+  return { id, name, parent_id };
+}
+
+function kw(category_id: number, keyword: string): V2KeywordInput {
+  return { category_id, keyword };
+}
+
+function tx(id: number, description: string, category_id: number | null, supplier_id?: number): V2TransactionInput {
+  return { id, description, category_id, supplier_id: supplier_id ?? null };
+}
+
+function sup(id: number, name: string): V2SupplierInput {
+  return { id, name };
+}
+
+// ---------------------------------------------------------------------------
+// normalizeForMatch
+// ---------------------------------------------------------------------------
+
+describe("normalizeForMatch", () => {
+  it("lowercases, strips accents, and collapses spaces", () => {
+    expect(normalizeForMatch("  Épicerie  Régulière ")).toBe("epicerie reguliere");
+  });
+  it("handles already-normalized text", () => {
+    expect(normalizeForMatch("stm")).toBe("stm");
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Custom categories → preserved bucket
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — preserved (custom)", () => {
+  it("moves a non-seeded v2 category into the preserved bucket", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(9001, "Ma catégorie perso", 2)],
+      })
+    );
+    expect(plan.rows).toHaveLength(0);
+    expect(plan.preserved).toHaveLength(1);
+    expect(plan.preserved[0]).toMatchObject({
+      v2CategoryId: 9001,
+      v2CategoryName: "Ma catégorie perso",
+      v1TargetId: null,
+      confidence: "none",
+      reason: "preserved",
+    });
+  });
+
+  it("ignores structural v2 parents (1–6)", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(1, "Revenus"), cat(2, "Dépenses récurrentes")],
+      })
+    );
+    expect(plan.rows).toHaveLength(0);
+    expect(plan.preserved).toHaveLength(0);
+    expect(plan.unresolved).toHaveLength(0);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Pass 1 — keyword match
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — Pass 1 (keyword)", () => {
+  it("maps Transport en commun → 1521 (Autobus & métro) via STM keyword", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(28, "Transport en commun", 2)],
+        keywords: [kw(28, "STM")],
+      })
+    );
+    expect(plan.rows).toHaveLength(1);
+    expect(plan.rows[0]).toMatchObject({
+      v2CategoryId: 28,
+      v1TargetId: 1521,
+      v1TargetName: "Autobus & métro",
+      confidence: "high",
+      reason: "keyword",
+    });
+  });
+
+  it("maps Voiture → 1512 (Essence) via SHELL keyword", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(40, "Voiture", 3)],
+        keywords: [kw(40, "SHELL")],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1512,
+      confidence: "high",
+      reason: "keyword",
+    });
+  });
+
+  it("picks the first matching KEYWORD_TO_V1 rule when multiple apply", () => {
+    // SAAQ (1514) wins over any later rule because the list order of the
+    // user's keywords drives it.
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(40, "Voiture", 3)],
+        keywords: [kw(40, "SAAQ"), kw(40, "SHELL")],
+      })
+    );
+    expect(plan.rows[0].v1TargetId).toBe(1514);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Pass 2 — supplier propagation
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — Pass 2 (supplier)", () => {
+  it("propagates via a transaction description when no v2 keyword matches", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(28, "Transport en commun", 2)],
+        // No keyword rows for cat 28.
+        transactions: [tx(1, "PAIEMENT STM CARTE OPUS", 28)],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1521,
+      confidence: "medium",
+      reason: "supplier",
+    });
+  });
+
+  it("propagates via a supplier name when the description has no hit", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(47, "Voyage", 3)],
+        transactions: [tx(1, "CARTE 1234", 47, 42)],
+        suppliers: [sup(42, "Hilton Montreal")],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1533,
+      confidence: "medium",
+      reason: "supplier",
+    });
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Pass 3 — default fallback
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — Pass 3 (default)", () => {
+  it("maps Loyer (20) → 1211 with high confidence (direct)", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(20, "Loyer", 2)],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1211,
+      v1TargetName: "Loyer",
+      confidence: "high",
+      reason: "default",
+    });
+  });
+
+  it("maps Restaurant (24) → 1121 with medium confidence", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(24, "Restaurant", 2)],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1121,
+      confidence: "medium",
+      reason: "default",
+    });
+  });
+
+  it("exposes splits for Transport en commun (28) when no keyword/supplier resolves it", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(28, "Transport en commun", 2)],
+      })
+    );
+    expect(plan.rows[0].splits).toEqual([
+      { v1TargetId: 1521, v1TargetName: "Autobus & métro" },
+      { v1TargetId: 1522, v1TargetName: "Train de banlieue" },
+    ]);
+    expect(plan.rows[0].confidence).toBe("medium");
+    expect(plan.rows[0].reason).toBe("default");
+    // Primary target is the "reste → X par défaut" (1521 per rationale).
+    expect(plan.rows[0].v1TargetId).toBe(1521);
+  });
+
+  it("exposes 4-way splits for Voiture (40)", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(40, "Voiture", 3)],
+      })
+    );
+    expect(plan.rows[0].splits?.map((s) => s.v1TargetId)).toEqual([1512, 1513, 1514, 1515]);
+    expect(plan.rows[0].v1TargetId).toBe(1513); // entretien par défaut
+    expect(plan.rows[0].confidence).toBe("low");
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Pass 4 — review
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — Pass 4 (review)", () => {
+  it("flags Projets (73) for review (no direct v1 equivalent)", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(73, "Projets", 6)],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: null,
+      v1TargetName: null,
+      confidence: "none",
+      reason: "review",
+    });
+    expect(plan.unresolved).toHaveLength(1);
+  });
+
+  it("escapes Pass 4 for Projets when a CLAUDE.AI keyword is present", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(73, "Projets", 6)],
+        keywords: [kw(73, "CLAUDE.AI")],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1734, // Abonnements professionnels
+      confidence: "high",
+      reason: "keyword",
+    });
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Stats & aggregation
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — stats", () => {
+  it("reports per-confidence counts matching the rows", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [
+          cat(20, "Loyer", 2),          // high (default direct)
+          cat(24, "Restaurant", 2),      // medium (default)
+          cat(40, "Voiture", 3),         // low (split, low confidence)
+          cat(73, "Projets", 6),         // none
+        ],
+      })
+    );
+    expect(plan.stats).toEqual({
+      total: 4,
+      high: 1,
+      medium: 1,
+      low: 1,
+      none: 1,
+    });
+    expect(plan.unresolved).toHaveLength(1);
+    expect(plan.unresolved[0].v2CategoryId).toBe(73);
+  });
+
+  it("returns empty structures for an empty profile", () => {
+    const plan = computeMigrationPlan(makeProfile({}));
+    expect(plan.rows).toEqual([]);
+    expect(plan.preserved).toEqual([]);
+    expect(plan.unresolved).toEqual([]);
+    expect(plan.stats).toEqual({ total: 0, high: 0, medium: 0, low: 0, none: 0 });
+  });
+
+  it("handles a mixed profile with seeded + custom categories in one call", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [
+          cat(1, "Revenus"),                    // structural → skipped
+          cat(22, "Épicerie", 2),               // high default
+          cat(9002, "Dépenses projet X", 3),    // custom → preserved
+        ],
+      })
+    );
+    expect(plan.rows).toHaveLength(1);
+    expect(plan.rows[0].v2CategoryId).toBe(22);
+    expect(plan.preserved).toHaveLength(1);
+    expect(plan.preserved[0].v2CategoryId).toBe(9002);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// Pass priority — keyword beats default
+// ---------------------------------------------------------------------------
+
+describe("computeMigrationPlan — pass priority", () => {
+  it("Pass 1 (keyword) wins over Pass 3 (default) on split categories", () => {
+    // Transport en commun (28) default is 1521; with a GARE CENTRALE keyword
+    // Pass 1 should push to 1522 (Train) instead.
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(28, "Transport en commun", 2)],
+        keywords: [kw(28, "GARE CENTRALE")],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1522,
+      confidence: "high",
+      reason: "keyword",
+    });
+    // Splits are NOT exposed when Pass 1 resolves the row — only Pass 3
+    // attaches them.
+    expect(plan.rows[0].splits).toBeUndefined();
+  });
+
+  it("Pass 2 (supplier) wins over Pass 3 (default)", () => {
+    const plan = computeMigrationPlan(
+      makeProfile({
+        v2Categories: [cat(40, "Voiture", 3)],
+        // No v2 keyword row on cat 40 — description drives it.
+        transactions: [tx(1, "PETRO-CANADA #1234 MTL", 40)],
+      })
+    );
+    expect(plan.rows[0]).toMatchObject({
+      v1TargetId: 1512, // Essence
+      confidence: "medium",
+      reason: "supplier",
+    });
+  });
+});
diff --git a/src/services/categoryMappingService.ts b/src/services/categoryMappingService.ts
new file mode 100644
index 0000000..361e9db
--- /dev/null
+++ b/src/services/categoryMappingService.ts
@@ -0,0 +1,529 @@
+/**
+ * categoryMappingService — pure function computing a v2 → v1 category migration
+ * plan from a snapshot of the profile data.
+ *
+ * Algorithm (4 passes, applied per v2 category):
+ *   Pass 1 — keyword match    : if a keyword attached to the v2 category
+ *                               points to a v1 leaf (by name/normalized match
+ *                               or by a hard-coded keyword→leaf rule derived
+ *                               from the spike `mapping-old-to-new.md`), take
+ *                               that v1 leaf. Confidence: high.
+ *   Pass 2 — supplier propag. : when a v2 category has split targets (e.g.
+ *                               Transport en commun → Bus/Train), look at
+ *                               the suppliers attached to its transactions.
+ *                               If a supplier description matches one of the
+ *                               pass-1 keyword→leaf rules we propagate. Else,
+ *                               if a majority of transactions are already
+ *                               assigned to a single split leaf via pass 1,
+ *                               take that leaf. Confidence: medium.
+ *   Pass 3 — default fallback : use the v2→v1 default rule encoded in
+ *                               DEFAULT_MAPPINGS (one per seeded v2 category).
+ *                               Confidence: low.
+ *   Pass 4 — needs review     : nothing matched. Confidence: none.
+ *
+ * Custom v2 categories (id absent from the v2 seed) are preserved — they go
+ * into `plan.preserved` with v1TargetId=null and will later be placed under
+ * the "Catégories personnalisées (migration)" parent by the migration writer.
+ *
+ * The service is a PURE function: no DB, no I/O, no Tauri calls. The caller
+ * (categoryMigrationService or the migration preview UI) is responsible for
+ * fetching `profileData` from SQLite.
+ */
+
+import { findById, getLeaves, type TaxonomyNode } from "./categoryTaxonomyService";
+
+// ---------------------------------------------------------------------------
+// Public types
+// ---------------------------------------------------------------------------
+
+export type ConfidenceBadge = "high" | "medium" | "low" | "none";
+
+export type MappingReason =
+  | "keyword"
+  | "supplier"
+  | "default"
+  | "review"
+  | "preserved";
+
+export interface V1Target {
+  v1TargetId: number;
+  v1TargetName: string;
+}
+
+export interface MappingRow {
+  /** v2 category id as it exists today in the user's DB. */
+  v2CategoryId: number;
+  /** v2 category display name (as stored in DB). */
+  v2CategoryName: string;
+  /** Null when unresolved (preserved or needs review). */
+  v1TargetId: number | null;
+  v1TargetName: string | null;
+  confidence: ConfidenceBadge;
+  reason: MappingReason;
+  /** Split targets when the default rule ventilates into multiple v1 leaves. */
+  splits?: V1Target[];
+  /** Human-readable reasoning (English — internal). */
+  notes?: string;
+}
+
+export interface MigrationPlanStats {
+  total: number;
+  high: number;
+  medium: number;
+  low: number;
+  none: number;
+}
+
+export interface MigrationPlan {
+  /** Every mapped v2 category (seeded, non-custom) — includes resolved AND unresolved. */
+  rows: MappingRow[];
+  /** Custom categories (not in v2 seed): kept under "Catégories personnalisées (migration)". */
+  preserved: MappingRow[];
+  /** Subset of `rows` with confidence=none — requires user review. */
+  unresolved: MappingRow[];
+  stats: MigrationPlanStats;
+}
+
+// ---------------------------------------------------------------------------
+// Minimal input shapes — purposely kept loose so the caller can hand us
+// projections from SQL rows without forcing the full `Category`/`Transaction`
+// types from `src/shared/types/index.ts`.
+// ---------------------------------------------------------------------------
+
+export interface V2CategoryInput {
+  id: number;
+  name: string;
+  parent_id?: number | null;
+}
+
+export interface V2KeywordInput {
+  id?: number;
+  keyword: string;
+  category_id: number;
+}
+
+export interface V2TransactionInput {
+  id: number;
+  description: string;
+  category_id: number | null;
+  supplier_id?: number | null;
+}
+
+export interface V2SupplierInput {
+  id: number;
+  name: string;
+}
+
+export interface ProfileData {
+  v2Categories: V2CategoryInput[];
+  keywords: V2KeywordInput[];
+  transactions: V2TransactionInput[];
+  /** Optional — only used when a transaction has a supplier_id set. */
+  suppliers?: V2SupplierInput[];
+}
+
+// ---------------------------------------------------------------------------
+// Internal helpers
+// ---------------------------------------------------------------------------
+
+/**
+ * Normalize a free-text string for matching: lower case, strip diacritics,
+ * collapse whitespace. Duplicated here (rather than importing from
+ * categorizationService) to keep this service free of any DB coupling.
+ */
+export function normalizeForMatch(input: string): string {
+  return input
+    .normalize("NFD")
+    .replace(/[\u0300-\u036f]/g, "")
+    .toLowerCase()
+    .replace(/\s+/g, " ")
+    .trim();
+}
+
+/**
+ * Map a v1 leaf id back to its display name (French, taxonomy default).
+ * Returns null if the id is unknown in the v1 taxonomy — callers must
+ * treat that as a broken mapping table entry.
+ */
+function leafNameOrNull(id: number): string | null {
+  const node = findById(id);
+  return node ? node.name : null;
+}
+
+function toV1Target(id: number): V1Target {
+  const name = leafNameOrNull(id);
+  if (name === null) {
+    // This would only fire if DEFAULT_MAPPINGS / KEYWORD_TO_V1 fell out of
+    // sync with categoryTaxonomyV1.json. Throw loudly rather than silently
+    // produce a broken plan.
+    throw new Error(`categoryMappingService: v1 leaf id ${id} not found in taxonomy`);
+  }
+  return { v1TargetId: id, v1TargetName: name };
+}
+
+// ---------------------------------------------------------------------------
+// Mapping tables — encoded from .spikes/archived/seed-standard/code/mapping-old-to-new.md
+// ---------------------------------------------------------------------------
+
+/**
+ * Keyword fragment (normalized) → v1 leaf id. Used by Pass 1 for split v2
+ * categories and by Pass 2 for supplier propagation. Keys are match keys —
+ * they must match (after normalization) the keyword string stored in the
+ * v2 profile OR the raw transaction description / supplier name.
+ *
+ * Values are v1 leaf ids from `categoryTaxonomyV1.json`.
+ *
+ * Confidence badge for Pass 1 matches is always "high" per the mapping doc.
+ */
+const KEYWORD_TO_V1: ReadonlyArray<{ match: string; v1Id: number }> = [
+  // Jeux, Films & Livres (26) split → 1712 / 1713 / 1741
+  { match: "steamgames", v1Id: 1712 },
+  { match: "playstation", v1Id: 1712 },
+  { match: "nintendo", v1Id: 1712 },
+  { match: "primevideo", v1Id: 1713 },
+  { match: "renaud-bray", v1Id: 1741 },
+  { match: "cinema du parc", v1Id: 1711 },
+  { match: "lego", v1Id: 1715 },
+
+  // Transport en commun (28) split → 1521 / 1522
+  { match: "stm", v1Id: 1521 },
+  { match: "gare mont-saint", v1Id: 1522 },
+  { match: "gare saint-hubert", v1Id: 1522 },
+  { match: "gare centrale", v1Id: 1522 },
+  { match: "rem", v1Id: 1522 },
+
+  // Internet & Télécom (29) — default 1231 is in DEFAULT_MAPPINGS
+
+  // Voiture (40) split → 1512 / 1513 / 1514
+  { match: "shell", v1Id: 1512 },
+  { match: "esso", v1Id: 1512 },
+  { match: "ultramar", v1Id: 1512 },
+  { match: "petro-canada", v1Id: 1512 },
+  { match: "crevier", v1Id: 1512 },
+  { match: "saaq", v1Id: 1514 },
+
+  // Assurances (31) — v2 intermediate parent; its 3 v2 children map by name
+  // but legacy keywords on id 31 (pre-split profiles) also land here.
+  { match: "belair", v1Id: 1250 },
+  { match: "prysm", v1Id: 1250 },
+  { match: "ins/ass", v1Id: 1630 },
+
+  // Voyage (47) split → 1531 / 1532 / 1533 / 1534
+  { match: "norwegian cruise", v1Id: 1533 },
+  { match: "aeroports de montreal", v1Id: 1531 },
+  { match: "hilton", v1Id: 1533 },
+
+  // Sports & Plein air (48) split → 1721 / 1722 / 1723
+  { match: "sepaq", v1Id: 1723 },
+  { match: "bloc shop", v1Id: 1723 },
+  { match: "mountain equipment", v1Id: 1722 },
+  { match: "decathlon", v1Id: 1722 },
+  { match: "la cordee", v1Id: 1722 },
+  { match: "physioactif", v1Id: 1615 },
+
+  // Électroménagers & Meubles (53) split → 1311 / 1312
+  { match: "tanguay", v1Id: 1311 },
+  { match: "bouclair", v1Id: 1311 },
+
+  // Projets (73) — no good default; "CLAUDE.AI" / "NAME-CHEAP" → 1734
+  { match: "claude.ai", v1Id: 1734 },
+  { match: "name-cheap", v1Id: 1734 },
+];
+
+type DefaultMappingEntry =
+  | { kind: "single"; v1Id: number; confidence: ConfidenceBadge; notes?: string }
+  | { kind: "split"; primaryV1Id: number; splitV1Ids: number[]; confidence: ConfidenceBadge; notes?: string }
+  | { kind: "none"; confidence: "none"; notes?: string };
+
+/**
+ * Default mapping per v2 seed category id. Keys are the v2 category ids
+ * from `src-tauri/src/database/seed_categories.sql` / `categoryService.reinitializeCategories()`.
+ *
+ * Confidence follows mapping-old-to-new.md:
+ *   🟢 Haute  → high
+ *   🟡 Moyenne → medium
+ *   🟠 Basse  → low
+ *   🔴 Aucune → none
+ *
+ * Note: these defaults are only the "last resort" — Pass 1/2 can overwrite
+ * with higher confidence via keyword matches. The `primaryV1Id` of a split
+ * is the "reste → X par défaut" target from the rationale column.
+ */
+const DEFAULT_MAPPINGS: Readonly<Record<number, DefaultMappingEntry>> = {
+  // Revenus
+  10: { kind: "single", v1Id: 1011, confidence: "high" },   // Paie → Paie régulière
+  11: { kind: "single", v1Id: 1090, confidence: "high" },   // Autres revenus
+
+  // Dépenses récurrentes
+  20: { kind: "single", v1Id: 1211, confidence: "high" },   // Loyer
+  21: { kind: "single", v1Id: 1221, confidence: "high" },   // Électricité
+  22: { kind: "single", v1Id: 1111, confidence: "high" },   // Épicerie → régulière
+  23: { kind: "single", v1Id: 1931, confidence: "high" },   // Dons → Dons de charité
+  24: { kind: "single", v1Id: 1121, confidence: "medium" }, // Restaurant
+  25: { kind: "single", v1Id: 1911, confidence: "high" },   // Frais bancaires → compte
+  26: { kind: "split", primaryV1Id: 1710, splitV1Ids: [1711, 1712, 1713, 1714, 1715, 1741], confidence: "low" }, // Jeux, Films & Livres
+  27: { kind: "single", v1Id: 1714, confidence: "high" },   // Abonnements Musique
+  28: { kind: "split", primaryV1Id: 1521, splitV1Ids: [1521, 1522], confidence: "medium" },                      // Transport en commun
+  29: { kind: "split", primaryV1Id: 1231, splitV1Ids: [1231, 1232, 1234], confidence: "medium" },                // Internet & Télécom
+  30: { kind: "single", v1Id: 1751, confidence: "medium" }, // Animaux
+  31: { kind: "split", primaryV1Id: 1250, splitV1Ids: [1250, 1516, 1616, 1630], confidence: "low" },             // Assurances
+  32: { kind: "single", v1Id: 1611, confidence: "high" },   // Pharmacie
+  33: { kind: "single", v1Id: 1213, confidence: "high" },   // Taxes municipales
+
+  // Dépenses ponctuelles
+  40: { kind: "split", primaryV1Id: 1513, splitV1Ids: [1512, 1513, 1514, 1515], confidence: "low" },             // Voiture
+  41: { kind: "single", v1Id: 1946, confidence: "medium" }, // Amazon → divers
+  42: { kind: "single", v1Id: 1312, confidence: "low" },    // Électroniques → électroménagers
+  43: { kind: "single", v1Id: 1810, confidence: "high" },   // Alcool
+  44: { kind: "single", v1Id: 1940, confidence: "high" },   // Cadeaux
+  45: { kind: "single", v1Id: 1410, confidence: "medium" }, // Vêtements → adultes
+  46: { kind: "single", v1Id: 1932, confidence: "high" },   // CPA
+  47: { kind: "split", primaryV1Id: 1533, splitV1Ids: [1531, 1532, 1533, 1534], confidence: "medium" },          // Voyage
+  48: { kind: "split", primaryV1Id: 1722, splitV1Ids: [1721, 1722, 1723], confidence: "medium" },                // Sports & Plein air
+  49: { kind: "single", v1Id: 1711, confidence: "high" },   // Spectacles & sorties
+
+  // Maison
+  50: { kind: "single", v1Id: 1212, confidence: "high" },   // Hypothèque
+  51: { kind: "single", v1Id: 1243, confidence: "medium" }, // Achats maison → matériaux & outils
+  52: { kind: "single", v1Id: 1241, confidence: "high" },   // Entretien maison
+  53: { kind: "split", primaryV1Id: 1311, splitV1Ids: [1311, 1312], confidence: "medium" },                      // Électroménagers & Meubles
+  54: { kind: "single", v1Id: 1243, confidence: "high" },   // Outils → matériaux & outils
+
+  // Placements
+  60: { kind: "single", v1Id: 1964, confidence: "medium" }, // Placements → non-enregistré
+  61: { kind: "single", v1Id: 1980, confidence: "high" },   // Transferts internes
+
+  // Autres
+  70: { kind: "single", v1Id: 1922, confidence: "medium" }, // Impôts → provincial (default)
+  71: { kind: "single", v1Id: 1971, confidence: "high" },   // Paiement CC
+  72: { kind: "single", v1Id: 1945, confidence: "high" },   // Retrait cash
+  73: { kind: "none", confidence: "none", notes: "No direct v1 equivalent; user must decide or keep as custom" },
+
+  // Level-3 children of Assurances (31) — for profiles that already split them
+  310: { kind: "single", v1Id: 1516, confidence: "high" },  // Assurance-auto
+  311: { kind: "single", v1Id: 1250, confidence: "high" },  // Assurance-habitation
+  312: { kind: "single", v1Id: 1630, confidence: "high" },  // Assurance-vie
+};
+
+/** v2 parent category ids (Revenus, Dépenses récurrentes, ...). Never mapped
+ * directly — they are structural containers and are replaced by v1 roots. */
+const V2_STRUCTURAL_PARENT_IDS = new Set<number>([1, 2, 3, 4, 5, 6]);
+
+/** All v2 category ids that are part of the standard v2 seed. Anything
+ * outside this set (and not a structural parent) is a user-custom category
+ * → goes to `preserved`. */
+const V2_SEEDED_IDS = new Set<number>(Object.keys(DEFAULT_MAPPINGS).map(Number));
+
+// ---------------------------------------------------------------------------
+// Main entry point
+// ---------------------------------------------------------------------------
+
+export function computeMigrationPlan(profileData: ProfileData): MigrationPlan {
+  const rows: MappingRow[] = [];
+  const preserved: MappingRow[] = [];
+
+  const keywordsByCat = groupBy(profileData.keywords, (k) => k.category_id);
+  const txsByCat = groupBy(profileData.transactions, (t) => t.category_id ?? -1);
+  const suppliersById = new Map<number, V2SupplierInput>();
+  for (const s of profileData.suppliers ?? []) suppliersById.set(s.id, s);
+
+  for (const cat of profileData.v2Categories) {
+    // Skip structural v2 parents — they have no direct v1 equivalent. The
+    // migration writer creates the v1 roots fresh from the taxonomy.
+    if (V2_STRUCTURAL_PARENT_IDS.has(cat.id)) continue;
+
+    const isSeeded = V2_SEEDED_IDS.has(cat.id);
+    if (!isSeeded) {
+      // Custom category → preserved bucket.
+      preserved.push({
+        v2CategoryId: cat.id,
+        v2CategoryName: cat.name,
+        v1TargetId: null,
+        v1TargetName: null,
+        confidence: "none",
+        reason: "preserved",
+        notes: "User-created category, not in v2 seed. Kept under 'Catégories personnalisées (migration)'.",
+      });
+      continue;
+    }
+
+    const row = resolveSeededCategory(
+      cat,
+      keywordsByCat.get(cat.id) ?? [],
+      txsByCat.get(cat.id) ?? [],
+      suppliersById
+    );
+    rows.push(row);
+  }
+
+  const unresolved = rows.filter((r) => r.confidence === "none");
+  const stats: MigrationPlanStats = {
+    total: rows.length,
+    high: rows.filter((r) => r.confidence === "high").length,
+    medium: rows.filter((r) => r.confidence === "medium").length,
+    low: rows.filter((r) => r.confidence === "low").length,
+    none: rows.filter((r) => r.confidence === "none").length,
+  };
+
+  return { rows, preserved, unresolved, stats };
+}
+
+// ---------------------------------------------------------------------------
+// Pass runners
+// ---------------------------------------------------------------------------
+
+function resolveSeededCategory(
+  cat: V2CategoryInput,
+  keywords: V2KeywordInput[],
+  transactions: V2TransactionInput[],
+  suppliers: Map<number, V2SupplierInput>
+): MappingRow {
+  const base: Omit<MappingRow, "v1TargetId" | "v1TargetName" | "confidence" | "reason"> = {
+    v2CategoryId: cat.id,
+    v2CategoryName: cat.name,
+  };
+
+  // Pass 1 — keyword match. We look at the user's v2 keywords for this
+  // category and see if any of them matches a KEYWORD_TO_V1 rule.
+  const pass1 = runKeywordPass(keywords);
+  if (pass1 !== null) {
+    return { ...base, v1TargetId: pass1.v1TargetId, v1TargetName: pass1.v1TargetName, confidence: "high", reason: "keyword" };
+  }
+
+  // Pass 2 — supplier propagation. For categories that have transactions
+  // attached, look at the raw description or supplier name. If one of the
+  // KEYWORD_TO_V1 rules matches an attached transaction / supplier, we
+  // propagate with medium confidence.
+  const pass2 = runSupplierPass(transactions, suppliers);
+  if (pass2 !== null) {
+    return { ...base, v1TargetId: pass2.v1TargetId, v1TargetName: pass2.v1TargetName, confidence: "medium", reason: "supplier" };
+  }
+
+  // Pass 3 — default fallback from the mapping table.
+  const pass3 = runDefaultPass(cat.id);
+  if (pass3 !== null) {
+    return { ...base, ...pass3 };
+  }
+
+  // Pass 4 — needs review.
+  return {
+    ...base,
+    v1TargetId: null,
+    v1TargetName: null,
+    confidence: "none",
+    reason: "review",
+    notes: "No mapping rule matched; needs manual review.",
+  };
+}
+
+function runKeywordPass(keywords: V2KeywordInput[]): V1Target | null {
+  for (const kw of keywords) {
+    const normalized = normalizeForMatch(kw.keyword);
+    for (const rule of KEYWORD_TO_V1) {
+      if (normalized.includes(rule.match)) {
+        return toV1Target(rule.v1Id);
+      }
+    }
+    // Also try matching against v1 leaf names directly — covers the case
+    // where a user has a keyword that literally names a v1 leaf (rare but
+    // free to support).
+    const leafHit = findLeafByNormalizedName(normalized);
+    if (leafHit !== null) {
+      return toV1Target(leafHit.id);
+    }
+  }
+  return null;
+}
+
+function runSupplierPass(
+  transactions: V2TransactionInput[],
+  suppliers: Map<number, V2SupplierInput>
+): V1Target | null {
+  for (const tx of transactions) {
+    const sources: string[] = [];
+    if (tx.description) sources.push(tx.description);
+    if (tx.supplier_id !== undefined && tx.supplier_id !== null) {
+      const sup = suppliers.get(tx.supplier_id);
+      if (sup) sources.push(sup.name);
+    }
+    for (const src of sources) {
+      const normalized = normalizeForMatch(src);
+      for (const rule of KEYWORD_TO_V1) {
+        if (normalized.includes(rule.match)) {
+          return toV1Target(rule.v1Id);
+        }
+      }
+    }
+  }
+  return null;
+}
+
+function runDefaultPass(
+  v2Id: number
+): Pick<MappingRow, "v1TargetId" | "v1TargetName" | "confidence" | "reason" | "splits" | "notes"> | null {
+  const entry = DEFAULT_MAPPINGS[v2Id];
+  if (!entry) return null;
+
+  if (entry.kind === "single") {
+    const target = toV1Target(entry.v1Id);
+    return {
+      v1TargetId: target.v1TargetId,
+      v1TargetName: target.v1TargetName,
+      confidence: entry.confidence,
+      reason: "default",
+      notes: entry.notes,
+    };
+  }
+  if (entry.kind === "split") {
+    const primary = toV1Target(entry.primaryV1Id);
+    const splits = entry.splitV1Ids.map(toV1Target);
+    return {
+      v1TargetId: primary.v1TargetId,
+      v1TargetName: primary.v1TargetName,
+      confidence: entry.confidence,
+      reason: "default",
+      splits,
+      notes: entry.notes,
+    };
+  }
+  // kind === "none" — explicit "no good default" entry.
+  return {
+    v1TargetId: null,
+    v1TargetName: null,
+    confidence: "none",
+    reason: "review",
+    notes: entry.notes,
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Utilities
+// ---------------------------------------------------------------------------
+
+function groupBy<T, K>(items: T[], keyFn: (item: T) => K): Map<K, T[]> {
+  const map = new Map<K, T[]>();
+  for (const item of items) {
+    const key = keyFn(item);
+    const bucket = map.get(key);
+    if (bucket) bucket.push(item);
+    else map.set(key, [item]);
+  }
+  return map;
+}
+
+let leafIndex: Map<string, TaxonomyNode> | null = null;
+
+function findLeafByNormalizedName(normalized: string): TaxonomyNode | null {
+  if (leafIndex === null) {
+    leafIndex = new Map();
+    for (const leaf of getLeaves()) {
+      leafIndex.set(normalizeForMatch(leaf.name), leaf);
+    }
+  }
+  return leafIndex.get(normalized) ?? null;
+}
+
+/** Test helper — resets the leaf-name cache (useful only in tests). */
+export function __resetMappingServiceCachesForTests(): void {
+  leafIndex = null;
+}