diff --git a/src/services/balance.service.test.ts b/src/services/balance.service.test.ts
index ecefc19..c9d55d9 100644
--- a/src/services/balance.service.test.ts
+++ b/src/services/balance.service.test.ts
@@ -36,6 +36,17 @@ import {
   saveSnapshotAtomic,
   getPreviousSnapshot,
   validateLineKindInvariants,
+  validateDetailedSnapshot,
+  isDetailedLine,
+  normalizeSecuritySymbol,
+  roundToCent,
+  findOrCreateSecurity,
+  listSecurities,
+  getSecurity,
+  updateSecurity,
+  listHoldingsBySnapshotLine,
+  getHoldingsForLatestSnapshot,
+  computeUnrealizedGain,
   PRICED_VALUE_TOLERANCE,
   BalanceServiceError,
   getSnapshotTotalsByDate,
@@ -609,6 +620,8 @@ describe("balance accounts — vehicle_type (#202)", () => {
         is_active: 1,
         archived_at: null,
         vehicle_type: "tfsa",
+        kind: "simple",
+        detailed_since: null,
         created_at: "",
         updated_at: "",
       },
@@ -633,6 +646,8 @@ describe("balance accounts — vehicle_type (#202)", () => {
         is_active: 1,
         archived_at: null,
         vehicle_type: null,
+        kind: "simple",
+        detailed_since: null,
         created_at: "",
         updated_at: "",
       },
@@ -655,6 +670,8 @@ describe("balance accounts — vehicle_type (#202)", () => {
         is_active: 1,
         archived_at: null,
         vehicle_type: "tfsa",
+        kind: "simple",
+        detailed_since: null,
         created_at: "",
         updated_at: "",
       },
@@ -722,6 +739,8 @@ describe("updateBalanceAccount", () => {
         notes: null,
         is_active: 1,
         archived_at: null,
+        kind: "simple",
+        detailed_since: null,
         created_at: "",
         updated_at: "",
       },
@@ -938,43 +957,52 @@ describe("upsertSnapshotLines (simple kind)", () => {
 
   it("clears existing lines, inserts each line with NULL quantity/unit_price, and bumps updated_at", async () => {
     mockSelect.mockResolvedValueOnce([FAKE_SNAPSHOT]);
+    // #212: the rewrite is now wrapped in BEGIN/COMMIT for holdings atomicity.
     mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
       .mockResolvedValueOnce({ rowsAffected: 1 }) // delete
       .mockResolvedValueOnce({ lastInsertId: 100, rowsAffected: 1 }) // insert 1
       .mockResolvedValueOnce({ lastInsertId: 101, rowsAffected: 1 }) // insert 2
-      .mockResolvedValueOnce({ rowsAffected: 1 }); // update updated_at
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // update updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
     await upsertSnapshotLines(5, [
       { account_id: 1, value: 1234.56 },
       { account_id: 2, value: 0 },
     ]);
-    // 1st call = DELETE
-    expect(mockExecute.mock.calls[0][0]).toContain(
+    // 1st call = BEGIN, 2nd = DELETE
+    expect(mockExecute.mock.calls[0][0]).toBe("BEGIN");
+    expect(mockExecute.mock.calls[1][0]).toContain(
       "DELETE FROM balance_snapshot_lines"
     );
     // Inserts use literal NULL for quantity/unit_price (simple kind invariant)
-    const insertSql = mockExecute.mock.calls[1][0] as string;
+    const insertSql = mockExecute.mock.calls[2][0] as string;
     expect(insertSql).toContain("INSERT INTO balance_snapshot_lines");
     expect(insertSql).toMatch(/VALUES\s*\(\s*\$1,\s*\$2,\s*NULL,\s*NULL,\s*\$3/);
     expect(insertSql).toContain("'manual'");
     // First insert params
-    expect(mockExecute.mock.calls[1][1]).toEqual([5, 1, 1234.56]);
+    expect(mockExecute.mock.calls[2][1]).toEqual([5, 1, 1234.56]);
     // Second insert params (zero is allowed)
-    expect(mockExecute.mock.calls[2][1]).toEqual([5, 2, 0]);
-    // Final call = UPDATE updated_at on parent snapshot
-    expect(mockExecute.mock.calls[3][0]).toContain(
-      "UPDATE balance_snapshots"
+    expect(mockExecute.mock.calls[3][1]).toEqual([5, 2, 0]);
+    // UPDATE updated_at on parent snapshot, then COMMIT last.
+    expect(mockExecute.mock.calls[4][0]).toContain("UPDATE balance_snapshots");
+    expect(mockExecute.mock.calls[4][0]).toContain("updated_at");
+    expect(mockExecute.mock.calls[mockExecute.mock.calls.length - 1][0]).toBe(
+      "COMMIT"
     );
-    expect(mockExecute.mock.calls[3][0]).toContain("updated_at");
   });
 
   it("clears all lines when called with an empty array", async () => {
     mockSelect.mockResolvedValueOnce([FAKE_SNAPSHOT]);
     mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
       .mockResolvedValueOnce({ rowsAffected: 3 }) // delete only
-      .mockResolvedValueOnce({ rowsAffected: 1 }); // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
     await upsertSnapshotLines(5, []);
-    // Only DELETE + UPDATE updated_at — no INSERTs
-    expect(mockExecute).toHaveBeenCalledTimes(2);
+    // BEGIN + DELETE + UPDATE updated_at + COMMIT — no INSERTs
+    expect(mockExecute).toHaveBeenCalledTimes(4);
+    expect(mockExecute.mock.calls[0][0]).toBe("BEGIN");
+    expect(mockExecute.mock.calls[3][0]).toBe("COMMIT");
   });
 });
 
@@ -1207,9 +1235,11 @@ describe("upsertSnapshotLines — priced kind", () => {
   it("inserts a priced line with quantity + unit_price + value", async () => {
     mockSelect.mockResolvedValueOnce([FAKE_SNAPSHOT]);
     mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
       .mockResolvedValueOnce({ rowsAffected: 1 }) // delete
       .mockResolvedValueOnce({ lastInsertId: 100, rowsAffected: 1 }) // insert
-      .mockResolvedValueOnce({ rowsAffected: 1 }); // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
     await upsertSnapshotLines(5, [
       {
         account_id: 7,
@@ -1219,20 +1249,22 @@ describe("upsertSnapshotLines — priced kind", () => {
         value: 255,
       },
     ]);
-    const insertSql = mockExecute.mock.calls[1][0] as string;
+    const insertSql = mockExecute.mock.calls[2][0] as string;
     expect(insertSql).toContain("INSERT INTO balance_snapshot_lines");
     // Priced inserts use parameter placeholders for qty/price (not literal NULLs)
     expect(insertSql).toMatch(/VALUES\s*\(\s*\$1,\s*\$2,\s*\$3,\s*\$4,\s*\$5/);
-    expect(mockExecute.mock.calls[1][1]).toEqual([5, 7, 10, 25.5, 255]);
+    expect(mockExecute.mock.calls[2][1]).toEqual([5, 7, 10, 25.5, 255]);
   });
 
   it("supports a mix of simple + priced lines in the same batch", async () => {
     mockSelect.mockResolvedValueOnce([FAKE_SNAPSHOT]);
     mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
       .mockResolvedValueOnce({ rowsAffected: 1 }) // delete
       .mockResolvedValueOnce({ lastInsertId: 100, rowsAffected: 1 }) // insert simple
       .mockResolvedValueOnce({ lastInsertId: 101, rowsAffected: 1 }) // insert priced
-      .mockResolvedValueOnce({ rowsAffected: 1 }); // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // bump updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
     await upsertSnapshotLines(5, [
       { account_id: 1, value: 1000 },
       {
@@ -1244,15 +1276,15 @@ describe("upsertSnapshotLines — priced kind", () => {
       },
     ]);
     // Simple insert uses literal NULLs for qty/price
-    expect(mockExecute.mock.calls[1][0] as string).toMatch(
+    expect(mockExecute.mock.calls[2][0] as string).toMatch(
       /VALUES\s*\(\s*\$1,\s*\$2,\s*NULL,\s*NULL,\s*\$3/
     );
-    expect(mockExecute.mock.calls[1][1]).toEqual([5, 1, 1000]);
+    expect(mockExecute.mock.calls[2][1]).toEqual([5, 1, 1000]);
     // Priced insert uses placeholders
-    expect(mockExecute.mock.calls[2][0] as string).toMatch(
+    expect(mockExecute.mock.calls[3][0] as string).toMatch(
       /VALUES\s*\(\s*\$1,\s*\$2,\s*\$3,\s*\$4,\s*\$5/
     );
-    expect(mockExecute.mock.calls[2][1]).toEqual([5, 7, 10, 50, 500]);
+    expect(mockExecute.mock.calls[3][1]).toEqual([5, 7, 10, 50, 500]);
   });
 });
 
@@ -2206,3 +2238,556 @@ describe("balance.service.prices", () => {
     expect(invoke).not.toHaveBeenCalled();
   });
 });
+
+// =============================================================================
+// Détail par titre (Issue #212) — securities CRUD, detailed save, holdings,
+// unrealized gain, detailed→simple guard.
+// =============================================================================
+
+describe("normalizeSecuritySymbol", () => {
+  it("uppercases and trims", () => {
+    expect(normalizeSecuritySymbol("  aapl  ")).toBe("AAPL");
+    expect(normalizeSecuritySymbol("btc")).toBe("BTC");
+    expect(normalizeSecuritySymbol("VeQt.TO")).toBe("VEQT.TO");
+  });
+});
+
+describe("roundToCent", () => {
+  it("rounds to two decimals (half-up at the cent)", () => {
+    expect(roundToCent(13.2038)).toBe(13.2);
+    expect(roundToCent(0.005)).toBe(0.01);
+    expect(roundToCent(100)).toBe(100);
+    expect(roundToCent(2.675)).toBeCloseTo(2.68, 5);
+  });
+});
+
+describe("findOrCreateSecurity", () => {
+  it("rejects an empty/whitespace symbol", async () => {
+    await expect(
+      findOrCreateSecurity({ symbol: "   ", asset_type: "stock" })
+    ).rejects.toMatchObject({ code: "security_symbol_required" });
+    expect(mockExecute).not.toHaveBeenCalled();
+  });
+
+  it("rejects an invalid asset_type", async () => {
+    await expect(
+      // @ts-expect-error runtime guard
+      findOrCreateSecurity({ symbol: "AAPL", asset_type: "bond" })
+    ).rejects.toMatchObject({ code: "security_asset_type_invalid" });
+    expect(mockExecute).not.toHaveBeenCalled();
+  });
+
+  it("UPSERTs on the NORMALIZED symbol and returns the row (casing-dedup)", async () => {
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 }); // upsert
+    mockSelect.mockResolvedValueOnce([
+      {
+        id: 3,
+        symbol: "AAPL",
+        name: null,
+        currency: "CAD",
+        asset_type: "stock",
+        created_at: "",
+        updated_at: "",
+      },
+    ]);
+    const sec = await findOrCreateSecurity({
+      symbol: "  aapl ",
+      asset_type: "stock",
+    });
+    expect(sec.id).toBe(3);
+    // The INSERT and the lookup both use the normalized symbol.
+    expect(mockExecute.mock.calls[0][1]?.[0]).toBe("AAPL");
+    expect(mockExecute.mock.calls[0][0]).toContain("ON CONFLICT(symbol) DO UPDATE");
+    expect(mockSelect.mock.calls[0][1]).toEqual(["AAPL"]);
+  });
+
+  it("a second call with different casing resolves the SAME row (dedup)", async () => {
+    // First create.
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 });
+    mockSelect.mockResolvedValueOnce([
+      { id: 9, symbol: "BTC", name: null, currency: "CAD", asset_type: "crypto", created_at: "", updated_at: "" },
+    ]);
+    const a = await findOrCreateSecurity({ symbol: "btc", asset_type: "crypto" });
+    // Second call, different casing — the normalized conflict target returns id 9.
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 });
+    mockSelect.mockResolvedValueOnce([
+      { id: 9, symbol: "BTC", name: null, currency: "CAD", asset_type: "crypto", created_at: "", updated_at: "" },
+    ]);
+    const b = await findOrCreateSecurity({ symbol: "  BtC  ", asset_type: "crypto" });
+    expect(a.id).toBe(b.id);
+    expect(b.id).toBe(9);
+  });
+
+  it("threads an executor when given (in-txn create)", async () => {
+    const execSelect = vi.fn().mockResolvedValueOnce([
+      { id: 1, symbol: "VEQT.TO", name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+    ]);
+    const execExecute = vi.fn().mockResolvedValueOnce({ rowsAffected: 1 });
+    const exec = { select: execSelect, execute: execExecute };
+    await findOrCreateSecurity({ symbol: "veqt.to", asset_type: "stock" }, exec);
+    // The provided executor was used, NOT the top-level db handle.
+    expect(execExecute).toHaveBeenCalledTimes(1);
+    expect(mockExecute).not.toHaveBeenCalled();
+    expect(mockSelect).not.toHaveBeenCalled();
+  });
+});
+
+describe("listSecurities / getSecurity / updateSecurity", () => {
+  it("listSecurities orders by symbol", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+    await listSecurities();
+    expect(mockSelect.mock.calls[0][0]).toContain("FROM balance_securities");
+    expect(mockSelect.mock.calls[0][0]).toContain("ORDER BY symbol ASC");
+  });
+
+  it("getSecurity returns null when absent", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+    expect(await getSecurity(404)).toBeNull();
+  });
+
+  it("updateSecurity rejects on missing id", async () => {
+    mockSelect.mockResolvedValueOnce([]); // getSecurity → none
+    await expect(updateSecurity(7, { name: "x" })).rejects.toMatchObject({
+      code: "security_not_found",
+    });
+    expect(mockExecute).not.toHaveBeenCalled();
+  });
+
+  it("updateSecurity re-normalizes symbol and validates asset_type", async () => {
+    mockSelect.mockResolvedValueOnce([
+      { id: 7, symbol: "AAPL", name: "Apple", currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+    ]);
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 });
+    await updateSecurity(7, { symbol: " msft ", asset_type: "stock" });
+    const params = mockExecute.mock.calls[0][1] as unknown[];
+    expect(params[0]).toBe("MSFT"); // normalized
+  });
+
+  it("updateSecurity rejects an invalid asset_type", async () => {
+    mockSelect.mockResolvedValueOnce([
+      { id: 7, symbol: "AAPL", name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+    ]);
+    await expect(
+      // @ts-expect-error runtime guard
+      updateSecurity(7, { asset_type: "etf" })
+    ).rejects.toMatchObject({ code: "security_asset_type_invalid" });
+    expect(mockExecute).not.toHaveBeenCalled();
+  });
+});
+
+describe("isDetailedLine", () => {
+  it("is true iff a holdings array is present (even empty)", () => {
+    expect(isDetailedLine({ account_id: 1, value: 0, holdings: [] })).toBe(true);
+    expect(
+      isDetailedLine({
+        account_id: 1,
+        value: 10,
+        holdings: [
+          { symbol: "AAPL", asset_type: "stock", quantity: 1, unit_price: 10, value: 10 },
+        ],
+      })
+    ).toBe(true);
+    expect(isDetailedLine({ account_id: 1, value: 10 })).toBe(false);
+  });
+});
+
+describe("validateDetailedSnapshot", () => {
+  const h = (value: number, over: Partial<{ symbol: string; quantity: number; unit_price: number }> = {}) => ({
+    symbol: over.symbol ?? "AAPL",
+    asset_type: "stock" as const,
+    quantity: over.quantity ?? 1,
+    unit_price: over.unit_price ?? value,
+    value,
+  });
+
+  it("accepts a detailed line whose value = rounded-cent SUM of holdings", () => {
+    expect(() =>
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 300,
+        holdings: [h(100, { symbol: "AAPL" }), h(200, { symbol: "MSFT" })],
+      })
+    ).not.toThrow();
+  });
+
+  it("tolerates a detailed account with NO holdings (pre-pivot aggregated)", () => {
+    expect(() =>
+      validateDetailedSnapshot({ account_id: 7, value: 5000, holdings: [] })
+    ).not.toThrow();
+  });
+
+  it("rejects a detailed line that carries a scalar quantity/unit_price", () => {
+    expect(() =>
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 100,
+        quantity: 1,
+        holdings: [h(100)],
+      })
+    ).toThrow(BalanceServiceError);
+    let scalarErr: unknown;
+    try {
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 100,
+        unit_price: 100,
+        holdings: [h(100)],
+      });
+    } catch (e) {
+      scalarErr = e;
+    }
+    expect((scalarErr as BalanceServiceError).code).toBe(
+      "snapshot_detailed_must_be_aggregate"
+    );
+  });
+
+  it("rejects when line value ≠ rounded-cent SUM", () => {
+    let caught: unknown;
+    try {
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 305, // should be 300
+        holdings: [h(100), h(200, { symbol: "MSFT" })],
+      });
+    } catch (e) {
+      caught = e;
+    }
+    expect((caught as BalanceServiceError).code).toBe(
+      "snapshot_detailed_value_mismatch"
+    );
+  });
+
+  it("rejects a holding with an empty symbol or bad asset_type or non-finite numbers", () => {
+    expect(() =>
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 10,
+        holdings: [{ symbol: "  ", asset_type: "stock", quantity: 1, unit_price: 10, value: 10 }],
+      })
+    ).toThrowError(/snapshot_holding_invalid|required/);
+    expect(() =>
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 10,
+        // @ts-expect-error runtime guard
+        holdings: [{ symbol: "AAPL", asset_type: "bond", quantity: 1, unit_price: 10, value: 10 }],
+      })
+    ).toThrow(BalanceServiceError);
+    expect(() =>
+      validateDetailedSnapshot({
+        account_id: 7,
+        value: 10,
+        holdings: [{ symbol: "AAPL", asset_type: "stock", quantity: NaN, unit_price: 10, value: 10 }],
+      })
+    ).toThrow(BalanceServiceError);
+  });
+
+  it("accepts N≥20 holdings — per-holding rounding never accumulates past a cent", () => {
+    // 21 positions each at 0.005 → roundToCent ⇒ 0.01 each ⇒ exact SUM 0.21.
+    // A naive Σ(raw)=0.105 with an absolute ε=0.01 would mis-handle this;
+    // rounding each first makes the comparison exact on whole cents.
+    const holdings = Array.from({ length: 21 }, (_, i) => ({
+      symbol: `S${i}`,
+      asset_type: "stock" as const,
+      quantity: 1,
+      unit_price: 0.005,
+      value: 0.005,
+    }));
+    expect(() =>
+      validateDetailedSnapshot({ account_id: 7, value: 0.21, holdings })
+    ).not.toThrow();
+    // Off-by-a-cent must still be rejected exactly.
+    let offByCent: unknown;
+    try {
+      validateDetailedSnapshot({ account_id: 7, value: 0.22, holdings });
+    } catch (e) {
+      offByCent = e;
+    }
+    expect((offByCent as BalanceServiceError).code).toBe(
+      "snapshot_detailed_value_mismatch"
+    );
+  });
+});
+
+describe("saveSnapshotAtomic — detailed account (holdings)", () => {
+  it("writes the aggregated line + securities + holdings inside ONE BEGIN/COMMIT", async () => {
+    mockSelect
+      .mockResolvedValueOnce([]) // dup-check: free date
+      // findOrCreateSecurity AAPL lookup, then MSFT lookup
+      .mockResolvedValueOnce([
+        { id: 11, symbol: "AAPL", name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+      ])
+      .mockResolvedValueOnce([
+        { id: 12, symbol: "MSFT", name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+      ]);
+    mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
+      .mockResolvedValueOnce({ lastInsertId: 42, rowsAffected: 1 }) // INSERT snapshot
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // DELETE lines
+      .mockResolvedValueOnce({ lastInsertId: 500, rowsAffected: 1 }) // INSERT aggregated line
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // DELETE holdings for line 500
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // UPSERT security AAPL
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // INSERT holding AAPL
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // UPSERT security MSFT
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // INSERT holding MSFT
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // UPDATE updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
+
+    const res = await saveSnapshotAtomic({
+      existingSnapshotId: null,
+      snapshot_date: "2026-05-30",
+      lines: [
+        {
+          account_id: 7,
+          value: 300,
+          holdings: [
+            { symbol: "aapl", asset_type: "stock", quantity: 2, unit_price: 50, value: 100, book_cost: 80 },
+            { symbol: "msft", asset_type: "stock", quantity: 1, unit_price: 200, value: 200, book_cost: 150 },
+          ],
+        },
+      ],
+    });
+
+    expect(res.snapshotId).toBe(42);
+    const calls = mockExecute.mock.calls.map((c: unknown[]) => String(c[0]));
+    // The aggregated line is inserted with NULL qty/price and the SUM value.
+    const aggInsertIdx = calls.findIndex((s: string) => s.includes("INSERT INTO balance_snapshot_lines"));
+    expect(mockExecute.mock.calls[aggInsertIdx][1]).toEqual([42, 7, 300]);
+    // Holdings are deleted then inserted referencing the captured line id (500).
+    expect(calls).toContain("DELETE FROM balance_snapshot_holdings WHERE snapshot_line_id = $1");
+    const holdingInsert = mockExecute.mock.calls.find((c: unknown[]) =>
+      String(c[0]).includes("INSERT INTO balance_snapshot_holdings")
+    );
+    expect(holdingInsert?.[1]?.[0]).toBe(500); // snapshot_line_id
+    // Begins once, commits last, never rolls back.
+    expect(mockExecute.mock.calls[0][0]).toBe("BEGIN");
+    expect(mockExecute.mock.calls[mockExecute.mock.calls.length - 1][0]).toBe("COMMIT");
+    expect(calls).not.toContain("ROLLBACK");
+  });
+
+  it("ROLLS BACK the whole save when a holding INSERT fails (no partial line/holdings)", async () => {
+    mockSelect
+      .mockResolvedValueOnce([]) // dup-check
+      .mockResolvedValueOnce([
+        { id: 11, symbol: "AAPL", name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+      ]); // security AAPL lookup
+    mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
+      .mockResolvedValueOnce({ lastInsertId: 42, rowsAffected: 1 }) // INSERT snapshot
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // DELETE lines
+      .mockResolvedValueOnce({ lastInsertId: 500, rowsAffected: 1 }) // INSERT aggregated line
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // DELETE holdings
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // UPSERT security AAPL
+      .mockRejectedValueOnce(new Error("holding FK violation")) // INSERT holding FAILS
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // ROLLBACK
+
+    await expect(
+      saveSnapshotAtomic({
+        existingSnapshotId: null,
+        snapshot_date: "2026-05-30",
+        lines: [
+          {
+            account_id: 7,
+            value: 100,
+            holdings: [
+              { symbol: "AAPL", asset_type: "stock", quantity: 2, unit_price: 50, value: 100 },
+            ],
+          },
+        ],
+      })
+    ).rejects.toThrow("holding FK violation");
+
+    // ROLLBACK was the last call, no COMMIT happened.
+    expect(mockExecute.mock.calls[mockExecute.mock.calls.length - 1][0]).toBe("ROLLBACK");
+    expect(
+      mockExecute.mock.calls.some((c: unknown[]) => c[0] === "COMMIT")
+    ).toBe(false);
+  });
+
+  it("recomputes the aggregated value server-side from the rounded-cent SUM", async () => {
+    // Caller passes value=0.21 with 21×0.005 holdings; the line must store 0.21.
+    mockSelect.mockResolvedValueOnce([]); // dup-check
+    for (let i = 0; i < 21; i++) {
+      mockSelect.mockResolvedValueOnce([
+        { id: 100 + i, symbol: `S${i}`, name: null, currency: "CAD", asset_type: "stock", created_at: "", updated_at: "" },
+      ]);
+    }
+    // BEGIN, INSERT snapshot, DELETE lines, INSERT line, DELETE holdings,
+    // then 21×(upsert security + insert holding), UPDATE, COMMIT.
+    mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // BEGIN
+      .mockResolvedValueOnce({ lastInsertId: 42, rowsAffected: 1 }) // INSERT snapshot
+      .mockResolvedValueOnce({ rowsAffected: 0 }) // DELETE lines
+      .mockResolvedValueOnce({ lastInsertId: 500, rowsAffected: 1 }) // INSERT line
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // DELETE holdings
+    for (let i = 0; i < 21; i++) {
+      mockExecute
+        .mockResolvedValueOnce({ rowsAffected: 1 }) // upsert security
+        .mockResolvedValueOnce({ rowsAffected: 1 }); // insert holding
+    }
+    mockExecute
+      .mockResolvedValueOnce({ rowsAffected: 1 }) // UPDATE updated_at
+      .mockResolvedValueOnce({ rowsAffected: 0 }); // COMMIT
+
+    const holdings = Array.from({ length: 21 }, (_, i) => ({
+      symbol: `S${i}`,
+      asset_type: "stock" as const,
+      quantity: 1,
+      unit_price: 0.005,
+      value: 0.005,
+    }));
+    await saveSnapshotAtomic({
+      existingSnapshotId: null,
+      snapshot_date: "2026-05-30",
+      lines: [{ account_id: 7, value: 0.21, holdings }],
+    });
+    const lineInsert = mockExecute.mock.calls.find((c: unknown[]) =>
+      String(c[0]).includes("INSERT INTO balance_snapshot_lines")
+    );
+    // 21 × roundToCent(0.005)=0.01 ⇒ 0.21 stored.
+    expect(lineInsert?.[1]).toEqual([42, 7, 0.21]);
+  });
+});
+
+describe("updateBalanceAccount — detailed→simple guard (#212)", () => {
+  const detailedAccount = {
+    id: 7,
+    balance_category_id: 1,
+    name: "Courtage",
+    symbol: null,
+    currency: "CAD",
+    notes: null,
+    is_active: 1,
+    archived_at: null,
+    vehicle_type: null,
+    kind: "detailed",
+    detailed_since: "2026-05-01",
+    created_at: "",
+    updated_at: "",
+  };
+
+  it("rejects detailed→simple when holdings exist (typed error, no UPDATE)", async () => {
+    mockSelect
+      .mockResolvedValueOnce([detailedAccount]) // getBalanceAccount
+      .mockResolvedValueOnce([{ n: 3 }]); // holdings count > 0
+    await expect(
+      updateBalanceAccount(7, { kind: "simple" })
+    ).rejects.toMatchObject({ code: "account_kind_detailed_has_holdings" });
+    // Guard fires before the UPDATE.
+    expect(
+      mockExecute.mock.calls.some((c: unknown[]) =>
+        String(c[0]).includes("UPDATE balance_accounts")
+      )
+    ).toBe(false);
+  });
+
+  it("ALLOWS detailed→simple when no holdings exist yet", async () => {
+    mockSelect
+      .mockResolvedValueOnce([detailedAccount]) // getBalanceAccount
+      .mockResolvedValueOnce([{ n: 0 }]); // no holdings
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 }); // UPDATE
+    await updateBalanceAccount(7, { kind: "simple" });
+    const sql = mockExecute.mock.calls[0][0] as string;
+    expect(sql).toContain("kind = $7");
+    const params = mockExecute.mock.calls[0][1] as unknown[];
+    expect(params[6]).toBe("simple"); // kind
+  });
+
+  it("does NOT run the holdings check on simple→detailed (only the dangerous direction is gated)", async () => {
+    const simpleAccount = { ...detailedAccount, kind: "simple", detailed_since: null };
+    mockSelect.mockResolvedValueOnce([simpleAccount]); // getBalanceAccount only
+    mockExecute.mockResolvedValueOnce({ rowsAffected: 1 }); // UPDATE
+    await updateBalanceAccount(7, { kind: "detailed", detailed_since: "2026-06-01" });
+    // Exactly one SELECT (the account read) — no holdings count query.
+    expect(mockSelect).toHaveBeenCalledTimes(1);
+    const params = mockExecute.mock.calls[0][1] as unknown[];
+    expect(params[6]).toBe("detailed");
+    expect(params[7]).toBe("2026-06-01"); // detailed_since normalized
+  });
+
+  it("rejects an invalid kind value", async () => {
+    mockSelect.mockResolvedValueOnce([{ ...detailedAccount, kind: "simple" }]);
+    await expect(
+      // @ts-expect-error runtime guard
+      updateBalanceAccount(7, { kind: "weird" })
+    ).rejects.toMatchObject({ code: "account_kind_invalid" });
+  });
+});
+
+describe("listHoldingsBySnapshotLine / getHoldingsForLatestSnapshot", () => {
+  it("listHoldingsBySnapshotLine joins the security and filters by line", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+    await listHoldingsBySnapshotLine(500);
+    const sql = mockSelect.mock.calls[0][0] as string;
+    expect(sql).toContain("FROM balance_snapshot_holdings h");
+    expect(sql).toContain("JOIN balance_securities s");
+    expect(sql).toContain("WHERE h.snapshot_line_id = $1");
+    expect(mockSelect.mock.calls[0][1]).toEqual([500]);
+  });
+
+  it("getHoldingsForLatestSnapshot EXCLUDES quantity-0 positions", async () => {
+    mockSelect.mockResolvedValueOnce([]);
+    await getHoldingsForLatestSnapshot(7);
+    const sql = mockSelect.mock.calls[0][0] as string;
+    // Latest-snapshot subquery + the qty<>0 filter for the prefill.
+    expect(sql).toContain("h.quantity <> 0");
+    expect(sql).toContain("ORDER BY s2.snapshot_date DESC");
+    expect(mockSelect.mock.calls[0][1]).toEqual([7]);
+  });
+});
+
+describe("computeUnrealizedGain", () => {
+  it("computes per-holding gain and % from book_cost", () => {
+    const r = computeUnrealizedGain([
+      { security_id: 1, value: 120, book_cost: 100, symbol: "AAPL" },
+      { security_id: 2, value: 80, book_cost: 100, symbol: "MSFT" },
+    ]);
+    expect(r.total_value).toBe(200);
+    expect(r.total_book_cost).toBe(200);
+    expect(r.total_gain).toBe(0);
+    expect(r.total_gain_pct).toBe(0);
+    expect(r.holdings[0].gain).toBe(20);
+    expect(r.holdings[0].gain_pct).toBeCloseTo(0.2, 5);
+    expect(r.holdings[1].gain).toBe(-20);
+    expect(r.holdings[1].gain_pct).toBeCloseTo(-0.2, 5);
+    expect(r.has_unknown_book_cost).toBe(false);
+  });
+
+  it("book_cost = 0 ⇒ gain = value but gain_pct = null (N/A, no divide-by-zero)", () => {
+    const r = computeUnrealizedGain([
+      { security_id: 1, value: 50, book_cost: 0 },
+    ]);
+    expect(r.holdings[0].gain).toBe(50);
+    expect(r.holdings[0].gain_pct).toBeNull();
+    // book_cost 0 contributes to the (zero) aggregate denominator ⇒ total % null.
+    expect(r.total_book_cost).toBe(0);
+    expect(r.total_gain_pct).toBeNull();
+    expect(r.has_unknown_book_cost).toBe(false);
+  });
+
+  it("NULL book_cost ⇒ gain & % null, EXCLUDED from aggregate, flagged", () => {
+    const r = computeUnrealizedGain([
+      { security_id: 1, value: 120, book_cost: 100 },
+      { security_id: 2, value: 300, book_cost: null },
+    ]);
+    // The NULL-book_cost holding's value still counts toward total_value…
+    expect(r.total_value).toBe(420);
+    // …but NOT toward book_cost / gain aggregates.
+    expect(r.total_book_cost).toBe(100);
+    expect(r.total_gain).toBe(20);
+    expect(r.total_gain_pct).toBeCloseTo(0.2, 5);
+    expect(r.holdings[1].gain).toBeNull();
+    expect(r.holdings[1].gain_pct).toBeNull();
+    expect(r.has_unknown_book_cost).toBe(true);
+  });
+
+  it("empty holdings ⇒ zeros and null %", () => {
+    const r = computeUnrealizedGain([]);
+    expect(r.total_value).toBe(0);
+    expect(r.total_book_cost).toBe(0);
+    expect(r.total_gain).toBe(0);
+    expect(r.total_gain_pct).toBeNull();
+    expect(r.has_unknown_book_cost).toBe(false);
+    expect(r.holdings).toEqual([]);
+  });
+});
diff --git a/src/services/balance.service.ts b/src/services/balance.service.ts
index 6688254..acfacb0 100644
--- a/src/services/balance.service.ts
+++ b/src/services/balance.service.ts
@@ -15,12 +15,16 @@ import { loadProfiles } from "./profileService";
 import type {
   AccountReturn,
   BalanceAccount,
+  BalanceAccountKind,
   BalanceAccountTransferWithTransaction,
   BalanceAccountWithCategory,
   BalanceAssetType,
   BalanceCategory,
   BalanceCategoryKind,
+  BalanceSecurity,
   BalanceSnapshot,
+  BalanceSnapshotHolding,
+  BalanceSnapshotHoldingWithSecurity,
   BalanceSnapshotLine,
   BalanceTransferDirection,
   BalanceVehicleType,
@@ -52,6 +56,15 @@ export type BalanceErrorCode =
   | "snapshot_priced_unit_price_required"
   | "snapshot_priced_value_mismatch"
   | "snapshot_simple_must_be_scalar"
+  // Issue #212 — securities + detailed snapshot (holdings)
+  | "security_symbol_required"
+  | "security_asset_type_invalid"
+  | "security_not_found"
+  | "snapshot_detailed_must_be_aggregate"
+  | "snapshot_detailed_value_mismatch"
+  | "snapshot_holding_invalid"
+  | "account_kind_invalid"
+  | "account_kind_detailed_has_holdings"
   // Issue #142 — transfers + returns
   | "transfer_direction_invalid"
   | "transfer_already_linked"
@@ -305,6 +318,7 @@ export async function listBalanceAccounts(options?: {
   return db.select<BalanceAccountWithCategory[]>(
     `SELECT a.id, a.balance_category_id, a.name, a.symbol, a.currency,
             a.notes, a.is_active, a.archived_at, a.vehicle_type,
+            a.kind, a.detailed_since,
             a.created_at, a.updated_at,
             c.key AS category_key, c.i18n_key AS category_i18n_key,
             c.kind AS category_kind, c.asset_type AS category_asset_type,
@@ -322,7 +336,8 @@ export async function getBalanceAccount(
   const db = await getDb();
   const rows = await db.select<BalanceAccount[]>(
     `SELECT id, balance_category_id, name, symbol, currency, notes,
-            is_active, archived_at, vehicle_type, created_at, updated_at
+            is_active, archived_at, vehicle_type, kind, detailed_since,
+            created_at, updated_at
      FROM balance_accounts
      WHERE id = $1`,
     [id]
@@ -427,6 +442,20 @@ export interface UpdateBalanceAccountInput {
    * automobile type.
    */
   vehicle_type?: BalanceVehicleType | null;
+  /**
+   * Entry mode (migration v15). 'simple' = one denormalized value, 'detailed' =
+   * per-security holdings. Pass to change, omit to leave unchanged. Switching
+   * `detailed → simple` is rejected with `account_kind_detailed_has_holdings`
+   * when the account already has holdings recorded — UI gating alone is
+   * insufficient (Issue #212 service backstop).
+   */
+  kind?: BalanceAccountKind;
+  /**
+   * Authoritative pivot date (ISO YYYY-MM-DD) from which detailed entry is
+   * expected (migration v15). Pass a value to set, `null` to clear, omit to
+   * leave unchanged.
+   */
+  detailed_since?: string | null;
 }
 
 export async function updateBalanceAccount(
@@ -474,13 +503,51 @@ export async function updateBalanceAccount(
     input.vehicle_type !== undefined
       ? normalizeVehicleType(input.vehicle_type)
       : existing.vehicle_type ?? null;
+  // Entry mode (migration v15). Validate against the enum; preserve when
+  // omitted. The detailed → simple downgrade is the dangerous transition —
+  // gated below.
+  const kind: BalanceAccountKind =
+    input.kind !== undefined ? input.kind : existing.kind;
+  if (kind !== "simple" && kind !== "detailed") {
+    throw new BalanceServiceError(
+      "account_kind_invalid",
+      `account kind must be 'simple' or 'detailed', got ${kind}`
+    );
+  }
+  const detailedSince =
+    input.detailed_since !== undefined
+      ? input.detailed_since === null
+        ? null
+        : normalizeSnapshotDate(input.detailed_since)
+      : existing.detailed_since ?? null;
+  // Service backstop (spec finding 🟢 TECHNIQUE): block a detailed → simple
+  // flip when holdings exist, otherwise they'd be orphaned (the line keeps a
+  // total value while the per-title rows dangle). The UI disables this too,
+  // but a direct service call must not bypass the invariant.
+  if (existing.kind === "detailed" && kind === "simple") {
+    const db0 = await getDb();
+    const held = await db0.select<Array<{ n: number }>>(
+      `SELECT COUNT(*) AS n
+         FROM balance_snapshot_holdings h
+         JOIN balance_snapshot_lines l ON l.id = h.snapshot_line_id
+        WHERE l.account_id = $1`,
+      [id]
+    );
+    if ((held[0]?.n ?? 0) > 0) {
+      throw new BalanceServiceError(
+        "account_kind_detailed_has_holdings",
+        `Account ${id} has detailed holdings and cannot be switched back to simple`
+      );
+    }
+  }
   const db = await getDb();
   await db.execute(
     `UPDATE balance_accounts
        SET balance_category_id = $1, name = $2, symbol = $3, notes = $4,
-           is_active = $5, vehicle_type = $6, updated_at = CURRENT_TIMESTAMP
-     WHERE id = $7`,
-    [categoryId, name, symbol, notes, isActive, vehicleType, id]
+           is_active = $5, vehicle_type = $6, kind = $7, detailed_since = $8,
+           updated_at = CURRENT_TIMESTAMP
+     WHERE id = $9`,
+    [categoryId, name, symbol, notes, isActive, vehicleType, kind, detailedSince, id]
   );
 }
 
@@ -857,6 +924,189 @@ export async function listLinesBySnapshot(
   );
 }
 
+// -----------------------------------------------------------------------------
+// Securities catalogue (Issue #212 / Bilan détail par titre — #3)
+// -----------------------------------------------------------------------------
+//
+// `balance_securities` is a shared catalogue keyed by a NORMALIZED symbol
+// (UPPER(TRIM(...))). The SQL column is already `UNIQUE COLLATE NOCASE` (v14),
+// but we normalize in TS too so the stored value is canonical and the dedup is
+// deterministic regardless of the caller's casing/whitespace.
+
+/** Minimal executor surface shared by the top-level db handle and a txn. */
+interface SqlExecutor {
+  select<T>(query: string, bindValues?: unknown[]): Promise<T>;
+  execute(
+    query: string,
+    bindValues?: unknown[]
+  ): Promise<{ lastInsertId?: number; rowsAffected?: number }>;
+}
+
+/**
+ * Canonical symbol form: UPPER(TRIM(...)). Matches the v14/v16 SQL
+ * (`UPPER(TRIM(a.symbol))`) so a TS-created security and a migration-created
+ * one collapse to the same `balance_securities` row.
+ */
+export function normalizeSecuritySymbol(symbol: string): string {
+  return symbol.trim().toUpperCase();
+}
+
+export interface FindOrCreateSecurityInput {
+  symbol: string;
+  /** Defaults to 'CAD'. */
+  currency?: string;
+  /** 'stock' | 'crypto' — required; routes the price-fetch flow. */
+  asset_type: BalanceAssetType;
+  /** Optional human-readable name. */
+  name?: string | null;
+}
+
+const ASSET_TYPES: readonly BalanceAssetType[] = ["stock", "crypto"];
+
+/**
+ * UPSERT a security by NORMALIZED symbol, returning its row. Idempotent: a
+ * second call with the same symbol (any casing) returns the existing row,
+ * updating `name`/`asset_type`/`currency` only when the caller passes new
+ * values. Callable inside an open transaction by threading `exec` (so a
+ * brand-new symbol can create its security then its holding atomically inside
+ * `saveSnapshotAtomic`); omit `exec` to use the top-level db handle.
+ *
+ * @throws `security_symbol_required` on empty symbol,
+ *         `security_asset_type_invalid` on a non-enum asset_type.
+ */
+export async function findOrCreateSecurity(
+  input: FindOrCreateSecurityInput,
+  exec?: SqlExecutor
+): Promise<BalanceSecurity> {
+  const symbol = normalizeSecuritySymbol(input.symbol ?? "");
+  if (symbol.length === 0) {
+    throw new BalanceServiceError(
+      "security_symbol_required",
+      "Security symbol is required"
+    );
+  }
+  if (!ASSET_TYPES.includes(input.asset_type)) {
+    throw new BalanceServiceError(
+      "security_asset_type_invalid",
+      `asset_type must be one of ${ASSET_TYPES.join(", ")}`
+    );
+  }
+  const currency = input.currency ?? BALANCE_CURRENCY_CAD;
+  const name = input.name ? input.name.trim() || null : null;
+  const db: SqlExecutor = exec ?? (await getDb());
+  // ON CONFLICT(symbol) DO UPDATE keeps the row's metadata fresh (e.g. a name
+  // arriving on a later save) while preserving its id. COALESCE keeps an
+  // existing name when the new payload omits one. The column is UNIQUE COLLATE
+  // NOCASE so the conflict target is the normalized symbol.
+  await db.execute(
+    `INSERT INTO balance_securities (symbol, name, currency, asset_type)
+     VALUES ($1, $2, $3, $4)
+     ON CONFLICT(symbol) DO UPDATE SET
+       name = COALESCE($2, balance_securities.name),
+       currency = $3,
+       asset_type = $4,
+       updated_at = CURRENT_TIMESTAMP`,
+    [symbol, name, currency, input.asset_type]
+  );
+  const rows = await db.select<BalanceSecurity[]>(
+    `SELECT id, symbol, name, currency, asset_type, created_at, updated_at
+       FROM balance_securities
+      WHERE symbol = $1`,
+    [symbol]
+  );
+  if (rows.length === 0) {
+    // Should be unreachable — the UPSERT just guaranteed the row exists.
+    throw new BalanceServiceError(
+      "security_not_found",
+      `Security ${symbol} not found after upsert`
+    );
+  }
+  return rows[0];
+}
+
+/** List every security in the catalogue, ordered by symbol. */
+export async function listSecurities(): Promise<BalanceSecurity[]> {
+  const db = await getDb();
+  return db.select<BalanceSecurity[]>(
+    `SELECT id, symbol, name, currency, asset_type, created_at, updated_at
+       FROM balance_securities
+      ORDER BY symbol ASC`
+  );
+}
+
+/** Fetch one security by id, or `null` when absent. */
+export async function getSecurity(id: number): Promise<BalanceSecurity | null> {
+  const db = await getDb();
+  const rows = await db.select<BalanceSecurity[]>(
+    `SELECT id, symbol, name, currency, asset_type, created_at, updated_at
+       FROM balance_securities
+      WHERE id = $1`,
+    [id]
+  );
+  return rows[0] ?? null;
+}
+
+export interface UpdateSecurityInput {
+  /** New normalized symbol; omit to leave unchanged. */
+  symbol?: string;
+  name?: string | null;
+  currency?: string;
+  asset_type?: BalanceAssetType;
+}
+
+/**
+ * Update a security's metadata. Symbol is re-normalized when provided.
+ * @throws `security_not_found`, `security_symbol_required`,
+ *         `security_asset_type_invalid`.
+ */
+export async function updateSecurity(
+  id: number,
+  input: UpdateSecurityInput
+): Promise<void> {
+  const existing = await getSecurity(id);
+  if (!existing) {
+    throw new BalanceServiceError(
+      "security_not_found",
+      `Security ${id} not found`
+    );
+  }
+  let symbol = existing.symbol;
+  if (input.symbol !== undefined) {
+    symbol = normalizeSecuritySymbol(input.symbol);
+    if (symbol.length === 0) {
+      throw new BalanceServiceError(
+        "security_symbol_required",
+        "Security symbol is required"
+      );
+    }
+  }
+  let assetType = existing.asset_type;
+  if (input.asset_type !== undefined) {
+    if (!ASSET_TYPES.includes(input.asset_type)) {
+      throw new BalanceServiceError(
+        "security_asset_type_invalid",
+        `asset_type must be one of ${ASSET_TYPES.join(", ")}`
+      );
+    }
+    assetType = input.asset_type;
+  }
+  const name =
+    input.name !== undefined
+      ? input.name === null
+        ? null
+        : input.name.trim() || null
+      : existing.name;
+  const currency = input.currency ?? existing.currency;
+  const db = await getDb();
+  await db.execute(
+    `UPDATE balance_securities
+       SET symbol = $1, name = $2, currency = $3, asset_type = $4,
+           updated_at = CURRENT_TIMESTAMP
+     WHERE id = $5`,
+    [symbol, name, currency, assetType, id]
+  );
+}
+
 /**
  * Tolerance ε used by the priced-kind invariant `value === quantity * unit_price`.
  *
@@ -869,6 +1119,17 @@ export async function listLinesBySnapshot(
  */
 export const PRICED_VALUE_TOLERANCE = 0.01;
 
+/**
+ * Round a monetary amount to the cent. The detailed-snapshot invariant rounds
+ * every holding's value to the cent and compares the SUM **exactly** to the
+ * stored aggregated line value — no float tolerance (decision 2026-06-03). This
+ * sidesteps the rounding-accumulation problem that a single absolute ε would
+ * hit once N holdings are summed (see Issue #212 spec finding 🟡 SECURITE).
+ */
+export function roundToCent(value: number): number {
+  return Math.round(value * 100) / 100;
+}
+
 export interface SnapshotLineInput {
   account_id: number;
   /**
@@ -888,6 +1149,44 @@ export interface SnapshotLineInput {
   quantity?: number | null;
   /** Required for priced lines, must be NULL for simple. */
   unit_price?: number | null;
+  /**
+   * Per-security breakdown for a `detailed` account (Issue #212). When this
+   * field is **present** (a defined array, even empty), the line is treated as
+   * detailed: the aggregated row stores the rounded-cent SUM of the holdings
+   * with `quantity`/`unit_price` NULL, and each holding is written to
+   * `balance_snapshot_holdings` in the same transaction. An EMPTY array models a
+   * detailed account at/just after its pivot with no positions yet entered
+   * (pre-pivot aggregated rows are produced WITHOUT this field — they take the
+   * simple path). `undefined` ⇒ not a detailed line (simple or legacy priced
+   * scalar path, untouched).
+   */
+  holdings?: SnapshotHoldingInput[];
+}
+
+/**
+ * One position inside a `detailed` account's snapshot line (Issue #212).
+ * References its security by NORMALIZED symbol (+ asset_type/currency) so
+ * `findOrCreateSecurity` can resolve-or-create it inside the save transaction.
+ * Downstream #213/#214 (editor reducer + multi-title UI) produce this shape.
+ */
+export interface SnapshotHoldingInput {
+  /** Security symbol; normalized (UPPER/TRIM) by the service. */
+  symbol: string;
+  /** Asset class — required for find-or-create. */
+  asset_type: BalanceAssetType;
+  /** Defaults to 'CAD'. */
+  currency?: string;
+  /** Optional human-readable security name. */
+  security_name?: string | null;
+  quantity: number;
+  unit_price: number;
+  /** Position value (= quantity × unit_price); re-rounded to the cent server-side. */
+  value: number;
+  /** Acquisition cost basis for the unrealized-gain column; optional. */
+  book_cost?: number | null;
+  /** 'manual' | 'maximus-api'; defaults to 'manual'. */
+  price_source?: string | null;
+  price_fetched_at?: string | null;
 }
 
 /**
@@ -962,6 +1261,209 @@ export function validateLineKindInvariants(
   }
 }
 
+/**
+ * A line is "detailed" when it carries a `holdings` array (even empty). The
+ * presence of the field — not the account's stored kind — drives the save
+ * path, so a detailed account can still write a pre-pivot aggregated row by
+ * simply omitting `holdings`.
+ */
+export function isDetailedLine(line: SnapshotLineInput): boolean {
+  return line.holdings !== undefined;
+}
+
+/**
+ * Validate a `detailed` line and its holdings (Issue #212). Companion to
+ * `validateLineKindInvariants` (which stays UNCHANGED for the simple/priced
+ * scalar path). Called ahead of any DB mutation.
+ *
+ * Rules (detail by security):
+ *  - WITH holdings ⇒ the aggregated line MUST carry `quantity`/`unit_price`
+ *    NULL (the total has no single qty/price) AND `line.value` MUST equal the
+ *    rounded-cent SUM of the holdings' rounded-cent values, compared EXACTLY
+ *    (no float tolerance — decision 2026-06-03). Each holding must have finite
+ *    quantity/unit_price/value, a normalizable symbol and a valid asset_type.
+ *  - WITHOUT holdings (empty array) ⇒ pre-pivot / no-position-yet aggregated
+ *    row is tolerated: `value` only needs to be finite (validated by the
+ *    caller's `validateLineKindInvariants` simple pass; here it's a no-op).
+ *
+ * @throws `BalanceServiceError` with a typed code on the first failure.
+ */
+export function validateDetailedSnapshot(line: SnapshotLineInput): void {
+  const holdings = line.holdings ?? [];
+  if (typeof line.value !== "number" || !Number.isFinite(line.value)) {
+    throw new BalanceServiceError(
+      "snapshot_value_invalid",
+      `Line for account ${line.account_id}: value must be a finite number`
+    );
+  }
+  // Pre-pivot / empty: an aggregated total with no per-title breakdown. The
+  // line value stands on its own; nothing further to assert.
+  if (holdings.length === 0) {
+    return;
+  }
+  // The aggregated row must not carry a scalar qty/price — those live per
+  // holding. Allow NULL/undefined only.
+  if (line.quantity !== undefined && line.quantity !== null) {
+    throw new BalanceServiceError(
+      "snapshot_detailed_must_be_aggregate",
+      `Line for account ${line.account_id}: detailed line must not carry a scalar quantity`
+    );
+  }
+  if (line.unit_price !== undefined && line.unit_price !== null) {
+    throw new BalanceServiceError(
+      "snapshot_detailed_must_be_aggregate",
+      `Line for account ${line.account_id}: detailed line must not carry a scalar unit_price`
+    );
+  }
+  let centSum = 0;
+  for (const h of holdings) {
+    if (typeof h.symbol !== "string" || normalizeSecuritySymbol(h.symbol) === "") {
+      throw new BalanceServiceError(
+        "snapshot_holding_invalid",
+        `Line for account ${line.account_id}: holding symbol is required`
+      );
+    }
+    if (!ASSET_TYPES.includes(h.asset_type)) {
+      throw new BalanceServiceError(
+        "snapshot_holding_invalid",
+        `Line for account ${line.account_id}: holding ${h.symbol} has invalid asset_type`
+      );
+    }
+    if (typeof h.quantity !== "number" || !Number.isFinite(h.quantity)) {
+      throw new BalanceServiceError(
+        "snapshot_holding_invalid",
+        `Line for account ${line.account_id}: holding ${h.symbol} quantity must be finite`
+      );
+    }
+    if (typeof h.unit_price !== "number" || !Number.isFinite(h.unit_price)) {
+      throw new BalanceServiceError(
+        "snapshot_holding_invalid",
+        `Line for account ${line.account_id}: holding ${h.symbol} unit_price must be finite`
+      );
+    }
+    if (typeof h.value !== "number" || !Number.isFinite(h.value)) {
+      throw new BalanceServiceError(
+        "snapshot_holding_invalid",
+        `Line for account ${line.account_id}: holding ${h.symbol} value must be finite`
+      );
+    }
+    centSum += roundToCent(h.value);
+  }
+  // Compare the rounded-cent SUM exactly against the rounded-cent line total.
+  // Both sides are rounded so the equality is on whole cents — no ε.
+  const expected = roundToCent(centSum);
+  if (roundToCent(line.value) !== expected) {
+    throw new BalanceServiceError(
+      "snapshot_detailed_value_mismatch",
+      `Line for account ${line.account_id}: value ${line.value} does not match rounded-cent SUM of holdings (${expected})`
+    );
+  }
+}
+
+/**
+ * Validate every input line ahead of any DB mutation. Detailed lines (those
+ * carrying a `holdings` field) go through `validateDetailedSnapshot`; all
+ * others keep the unchanged `validateLineKindInvariants` scalar pass. Shared by
+ * `upsertSnapshotLines` and `saveSnapshotAtomic` so the two save entry points
+ * never diverge.
+ */
+function validateAllLines(lines: SnapshotLineInput[]): void {
+  for (const line of lines) {
+    if (isDetailedLine(line)) {
+      validateDetailedSnapshot(line);
+    } else {
+      validateLineKindInvariants(line);
+    }
+  }
+}
+
+/**
+ * Insert one snapshot line and, for a detailed line, rewrite its holdings in
+ * the SAME executor (transaction). Returns the inserted line id.
+ *
+ * Detailed path: the aggregated row stores the rounded-cent SUM of the
+ * holdings (qty/price NULL); after capturing its `lastInsertId`, existing
+ * holdings for that line are DELETEd (defensive — a fresh line has none), then
+ * each holding's security is found-or-created and the holding INSERTed. The
+ * server-side total is recomputed from the rounded-cent SUM so the line's
+ * `value` is authoritative regardless of the caller's arithmetic.
+ *
+ * Simple / legacy-priced scalar path: unchanged — one INSERT, no holdings.
+ */
+async function insertSnapshotLineWithHoldings(
+  exec: SqlExecutor,
+  snapshotId: number,
+  line: SnapshotLineInput
+): Promise<number> {
+  if (isDetailedLine(line)) {
+    const holdings = line.holdings ?? [];
+    // Recompute the aggregated total server-side from the rounded-cent SUM —
+    // the stored line value is the source of truth for the aggregators.
+    const total = roundToCent(
+      holdings.reduce((acc, h) => acc + roundToCent(h.value), 0)
+    );
+    const lineRes = await exec.execute(
+      `INSERT INTO balance_snapshot_lines
+         (snapshot_id, account_id, quantity, unit_price, value, price_source)
+       VALUES ($1, $2, NULL, NULL, $3, 'manual')`,
+      [snapshotId, line.account_id, total]
+    );
+    const lineId = lineRes.lastInsertId as number;
+    // Defensive clear — a freshly inserted line can't have holdings yet, but
+    // this keeps the helper safe if it's ever reused on an existing line.
+    await exec.execute(
+      `DELETE FROM balance_snapshot_holdings WHERE snapshot_line_id = $1`,
+      [lineId]
+    );
+    for (const h of holdings) {
+      const security = await findOrCreateSecurity(
+        {
+          symbol: h.symbol,
+          asset_type: h.asset_type,
+          currency: h.currency,
+          name: h.security_name,
+        },
+        exec
+      );
+      await exec.execute(
+        `INSERT INTO balance_snapshot_holdings
+           (snapshot_line_id, security_id, quantity, unit_price, value,
+            book_cost, price_source, price_fetched_at)
+         VALUES ($1, $2, $3, $4, $5, $6, $7, $8)`,
+        [
+          lineId,
+          security.id,
+          h.quantity,
+          h.unit_price,
+          roundToCent(h.value),
+          h.book_cost ?? null,
+          h.price_source ?? "manual",
+          h.price_fetched_at ?? null,
+        ]
+      );
+    }
+    return lineId;
+  }
+  // Simple / legacy priced scalar line — unchanged behaviour.
+  const kind = line.account_kind ?? "simple";
+  if (kind === "simple") {
+    const res = await exec.execute(
+      `INSERT INTO balance_snapshot_lines
+         (snapshot_id, account_id, quantity, unit_price, value, price_source)
+       VALUES ($1, $2, NULL, NULL, $3, 'manual')`,
+      [snapshotId, line.account_id, line.value]
+    );
+    return res.lastInsertId as number;
+  }
+  const res = await exec.execute(
+    `INSERT INTO balance_snapshot_lines
+       (snapshot_id, account_id, quantity, unit_price, value, price_source)
+     VALUES ($1, $2, $3, $4, $5, 'manual')`,
+    [snapshotId, line.account_id, line.quantity, line.unit_price, line.value]
+  );
+  return res.lastInsertId as number;
+}
+
 /**
  * Upsert a batch of snapshot lines. Each input row is inserted or
  * replaced atomically per account; lines for accounts not present in
@@ -990,50 +1492,46 @@ export async function upsertSnapshotLines(
       `Snapshot ${snapshotId} not found`
     );
   }
-  // Validate every input up-front before mutating anything.
-  for (const line of lines) {
-    validateLineKindInvariants(line);
-  }
+  // Validate every input up-front before mutating anything (detailed lines go
+  // through validateDetailedSnapshot, scalar lines through the unchanged pass).
+  validateAllLines(lines);
 
   const db = await getDb();
-  // Strategy: clear and rewrite. Snapshot lines are small (one per active
-  // account, typically < 20) so the simplicity outweighs the diff-tracking
-  // savings. CASCADE guarantees consistency on partial failures.
-  await db.execute(
-    "DELETE FROM balance_snapshot_lines WHERE snapshot_id = $1",
-    [snapshotId]
-  );
-  for (const line of lines) {
-    const kind = line.account_kind ?? "simple";
-    if (kind === "simple") {
-      await db.execute(
-        `INSERT INTO balance_snapshot_lines
-           (snapshot_id, account_id, quantity, unit_price, value, price_source)
-         VALUES ($1, $2, NULL, NULL, $3, 'manual')`,
-        [snapshotId, line.account_id, line.value]
-      );
-    } else {
-      await db.execute(
-        `INSERT INTO balance_snapshot_lines
-           (snapshot_id, account_id, quantity, unit_price, value, price_source)
-         VALUES ($1, $2, $3, $4, $5, 'manual')`,
-        [
-          snapshotId,
-          line.account_id,
-          line.quantity,
-          line.unit_price,
-          line.value,
-        ]
-      );
+  // Strategy: clear and rewrite, wrapped in an explicit transaction so the
+  // line + holdings writes commit together. Snapshot lines are small (one per
+  // active account, typically < 20). CASCADE on snapshot_line_id wipes the old
+  // holdings when their parent line is DELETEd here, so a detailed account's
+  // per-title rows never outlive their line.
+  let inTxn = false;
+  try {
+    await db.execute("BEGIN");
+    inTxn = true;
+    await db.execute(
+      "DELETE FROM balance_snapshot_lines WHERE snapshot_id = $1",
+      [snapshotId]
+    );
+    for (const line of lines) {
+      await insertSnapshotLineWithHoldings(db, snapshotId, line);
     }
+    // Bump the parent snapshot's updated_at so list views can sort by recency.
+    await db.execute(
+      `UPDATE balance_snapshots
+         SET updated_at = CURRENT_TIMESTAMP
+       WHERE id = $1`,
+      [snapshotId]
+    );
+    await db.execute("COMMIT");
+    inTxn = false;
+  } catch (e) {
+    if (inTxn) {
+      try {
+        await db.execute("ROLLBACK");
+      } catch {
+        // Defensive: preserve the original error over a rollback failure.
+      }
+    }
+    throw e;
   }
-  // Bump the parent snapshot's updated_at so list views can sort by recency.
-  await db.execute(
-    `UPDATE balance_snapshots
-       SET updated_at = CURRENT_TIMESTAMP
-     WHERE id = $1`,
-    [snapshotId]
-  );
 }
 
 /**
@@ -1073,10 +1571,9 @@ export async function saveSnapshotAtomic(input: {
   moveToDate?: string | null;
 }): Promise<{ snapshotId: number }> {
   // Validate every line ahead of time so the transaction never opens for
-  // a doomed save. Mirrors `upsertSnapshotLines` invariants.
-  for (const line of input.lines) {
-    validateLineKindInvariants(line);
-  }
+  // a doomed save. Detailed lines go through validateDetailedSnapshot; scalar
+  // lines keep the unchanged validateLineKindInvariants pass.
+  validateAllLines(input.lines);
 
   const db = await getDb();
   let inTxn = false;
@@ -1133,35 +1630,16 @@ export async function saveSnapshotAtomic(input: {
     }
 
     // Rewrite-all strategy (matches `upsertSnapshotLines`): clear
-    // existing lines, then re-insert every line. Cheap because snapshot
-    // line counts are small.
+    // existing lines, then re-insert every line + its holdings. Cheap
+    // because snapshot line counts are small. A detailed line writes its
+    // aggregated row and its holdings via the shared helper, all inside this
+    // same BEGIN/COMMIT — a holding INSERT failure rolls the whole save back.
     await db.execute(
       "DELETE FROM balance_snapshot_lines WHERE snapshot_id = $1",
       [snapshotId]
     );
     for (const line of input.lines) {
-      const kind = line.account_kind ?? "simple";
-      if (kind === "simple") {
-        await db.execute(
-          `INSERT INTO balance_snapshot_lines
-             (snapshot_id, account_id, quantity, unit_price, value, price_source)
-           VALUES ($1, $2, NULL, NULL, $3, 'manual')`,
-          [snapshotId, line.account_id, line.value]
-        );
-      } else {
-        await db.execute(
-          `INSERT INTO balance_snapshot_lines
-             (snapshot_id, account_id, quantity, unit_price, value, price_source)
-           VALUES ($1, $2, $3, $4, $5, 'manual')`,
-          [
-            snapshotId,
-            line.account_id,
-            line.quantity,
-            line.unit_price,
-            line.value,
-          ]
-        );
-      }
+      await insertSnapshotLineWithHoldings(db, snapshotId, line);
     }
     await db.execute(
       `UPDATE balance_snapshots
@@ -1509,6 +1987,174 @@ export async function getAccountsPeriodAnchor(
   );
 }
 
+// -----------------------------------------------------------------------------
+// Holdings reads + unrealized gain (Issue #212 / Bilan détail par titre — #3)
+// -----------------------------------------------------------------------------
+
+/**
+ * Drill-down read: every holding of a single snapshot line, joined with its
+ * security for display (symbol / name / asset_type). Ordered by symbol.
+ */
+export async function listHoldingsBySnapshotLine(
+  lineId: number
+): Promise<BalanceSnapshotHoldingWithSecurity[]> {
+  const db = await getDb();
+  return db.select<BalanceSnapshotHoldingWithSecurity[]>(
+    `SELECT h.id, h.snapshot_line_id, h.security_id, h.quantity, h.unit_price,
+            h.value, h.book_cost, h.price_source, h.price_fetched_at,
+            h.created_at, h.updated_at,
+            s.symbol AS security_symbol, s.name AS security_name,
+            s.asset_type AS security_asset_type
+       FROM balance_snapshot_holdings h
+       JOIN balance_securities s ON s.id = h.security_id
+      WHERE h.snapshot_line_id = $1
+      ORDER BY s.symbol ASC`,
+    [lineId]
+  );
+}
+
+/**
+ * Holdings of an account's LATEST snapshot, one per security, joined with the
+ * security for display. Used to PREFILL the next snapshot's editor (carry the
+ * titles + quantities + book_cost forward; the price gets re-fetched).
+ *
+ * Titles with quantity 0 are EXCLUDED — a position fully sold at the last
+ * snapshot shouldn't be re-offered (Issue #212 acceptance). A title sold then
+ * re-bought reappears because its latest non-zero holding wins.
+ *
+ * "Latest" is resolved per account by the max `snapshot_date` among that
+ * account's lines that actually carry holdings.
+ */
+export async function getHoldingsForLatestSnapshot(
+  accountId: number
+): Promise<BalanceSnapshotHoldingWithSecurity[]> {
+  const db = await getDb();
+  return db.select<BalanceSnapshotHoldingWithSecurity[]>(
+    `SELECT h.id, h.snapshot_line_id, h.security_id, h.quantity, h.unit_price,
+            h.value, h.book_cost, h.price_source, h.price_fetched_at,
+            h.created_at, h.updated_at,
+            s.symbol AS security_symbol, s.name AS security_name,
+            s.asset_type AS security_asset_type
+       FROM balance_snapshot_holdings h
+       JOIN balance_securities s ON s.id = h.security_id
+       JOIN balance_snapshot_lines l ON l.id = h.snapshot_line_id
+      WHERE l.account_id = $1
+        AND l.snapshot_id = (
+          SELECT l2.snapshot_id
+            FROM balance_snapshot_lines l2
+            JOIN balance_snapshots s2 ON s2.id = l2.snapshot_id
+            JOIN balance_snapshot_holdings h2 ON h2.snapshot_line_id = l2.id
+           WHERE l2.account_id = $1
+           ORDER BY s2.snapshot_date DESC
+           LIMIT 1
+        )
+        AND h.quantity <> 0
+      ORDER BY s.symbol ASC`,
+    [accountId]
+  );
+}
+
+/** Per-holding unrealized gain row. */
+export interface HoldingUnrealizedGain {
+  security_id: number;
+  symbol: string;
+  value: number;
+  book_cost: number | null;
+  /** `value - book_cost`, or null when book_cost is unknown (NULL). */
+  gain: number | null;
+  /**
+   * `(value - book_cost) / book_cost`, or null ("N/A") when book_cost is NULL
+   * or 0 — never a divide-by-zero. The UI renders null as the i18n "N/A".
+   */
+  gain_pct: number | null;
+}
+
+/** Account-level aggregated unrealized gain across its latest holdings. */
+export interface AccountUnrealizedGain {
+  /** SUM(value) across the holdings considered. */
+  total_value: number;
+  /** SUM(book_cost) across holdings WITH a known book_cost (NULLs excluded). */
+  total_book_cost: number;
+  /** total_value − total_book_cost across the known-book_cost holdings only. */
+  total_gain: number;
+  /** total_gain / total_book_cost, or null when total_book_cost is 0. */
+  total_gain_pct: number | null;
+  /**
+   * True when at least one holding has a NULL book_cost (excluded from the
+   * aggregate) — the UI flags the % as partial so it isn't read as exhaustive.
+   */
+  has_unknown_book_cost: boolean;
+  /** The per-holding breakdown the aggregate was computed from. */
+  holdings: HoldingUnrealizedGain[];
+}
+
+/**
+ * Compute the unrealized gain (`value − book_cost`) per holding and aggregated,
+ * in value and percent (Issue #212). Pure over the holdings it's given, so it's
+ * trivially unit-testable; pass holdings from `getHoldingsForLatestSnapshot`
+ * (latest snapshot) or `listHoldingsBySnapshotLine` (a specific snapshot).
+ *
+ * Guards:
+ *  - per holding: `book_cost = 0` OR `book_cost = NULL` ⇒ `gain_pct = null`
+ *    ("N/A") so we never divide by zero. A NULL book_cost also yields
+ *    `gain = null` (we don't know the cost basis), whereas `book_cost = 0`
+ *    yields `gain = value` (cost basis is genuinely zero).
+ *  - aggregate: NULL-book_cost holdings are EXCLUDED from `total_book_cost`
+ *    and `total_gain`, and flagged via `has_unknown_book_cost` so the % isn't
+ *    silently understated. `total_gain_pct` is null when no known book_cost
+ *    contributes (sum is 0).
+ */
+export function computeUnrealizedGain(
+  holdings: Array<
+    Pick<BalanceSnapshotHolding, "security_id" | "value" | "book_cost"> & {
+      symbol?: string;
+    }
+  >
+): AccountUnrealizedGain {
+  const rows: HoldingUnrealizedGain[] = [];
+  let totalValue = 0;
+  let totalBookCost = 0;
+  let totalGainKnown = 0;
+  let hasUnknown = false;
+
+  for (const h of holdings) {
+    const value = h.value;
+    totalValue += value;
+    const bc = h.book_cost;
+    let gain: number | null;
+    let gainPct: number | null;
+    if (bc === null || bc === undefined) {
+      // Unknown cost basis — no gain figure, excluded from the aggregate.
+      gain = null;
+      gainPct = null;
+      hasUnknown = true;
+    } else {
+      gain = value - bc;
+      totalBookCost += bc;
+      totalGainKnown += gain;
+      // book_cost = 0 is a real basis (gain = value) but % is undefined.
+      gainPct = bc === 0 ? null : (value - bc) / bc;
+    }
+    rows.push({
+      security_id: h.security_id,
+      symbol: (h as { symbol?: string }).symbol ?? "",
+      value,
+      book_cost: bc ?? null,
+      gain,
+      gain_pct: gainPct,
+    });
+  }
+
+  return {
+    total_value: totalValue,
+    total_book_cost: totalBookCost,
+    total_gain: totalGainKnown,
+    total_gain_pct: totalBookCost === 0 ? null : totalGainKnown / totalBookCost,
+    has_unknown_book_cost: hasUnknown,
+    holdings: rows,
+  };
+}
+
 // -----------------------------------------------------------------------------
 // Returns + transfers (Issue #142 / Bilan #4)
 // -----------------------------------------------------------------------------