diff --git a/src/workspace.test.ts b/src/workspace.test.ts
new file mode 100644
index 0000000..8b4255a
--- /dev/null
+++ b/src/workspace.test.ts
@@ -0,0 +1,128 @@
+import { describe, it, expect, vi } from "vitest";
+import { SumEngine } from "./engine";
+import { Workspace, SheetSource } from "./workspace";
+
+function makeWorkspace(initial: Record<string, { title: string; text: string }>) {
+  const engine = new SumEngine();
+  const store = new Map(Object.entries(initial).map(([id, v]) => [id, { id, ...v }]));
+  const sheets = (): SheetSource[] => [...store.values()];
+  const ws = new Workspace(engine, sheets);
+  return { engine, store, ws };
+}
+
+describe("Workspace exports", () => {
+  it("exposes assigned variables, total and last through @Sheet.key", () => {
+    const { ws } = makeWorkspace({
+      budget: { title: "Budget", text: "rent = $500\nfood = $200" },
+    });
+    const r = ws.evaluateSheet("dash", "@Budget.rent\n@Budget.total\n@Budget.last");
+    expect(r[0].text).toBe("$500");
+    expect(r[1].text).toBe("$700");
+    expect(r[2].text).toBe("$200");
+  });
+
+  it("caches a sheet's exports until invalidated", () => {
+    const { ws, store } = makeWorkspace({
+      budget: { title: "Budget", text: "rent = $500" },
+    });
+    expect(ws.evaluateSheet("dash", "@Budget.rent")[0].text).toBe("$500");
+    store.get("budget")!.text = "rent = $900";
+    expect(ws.evaluateSheet("dash", "@Budget.rent")[0].text).toBe("$500");
+    ws.invalidate("budget");
+    expect(ws.evaluateSheet("dash", "@Budget.rent")[0].text).toBe("$900");
+  });
+
+  it("evaluates a sheet lazily — only when it's actually referenced", () => {
+    const { engine, ws } = makeWorkspace({
+      never: { title: "Never", text: "1 + 1" },
+    });
+    const spy = vi.spyOn(engine, "evaluateDocument");
+    ws.evaluateSheet("dash", "hello");
+    expect(spy.mock.calls.some(([text]) => text === "1 + 1")).toBe(false);
+    ws.evaluateSheet("dash", "@Never.total");
+    expect(spy.mock.calls.some(([text]) => text === "1 + 1")).toBe(true);
+  });
+});
+
+describe("Workspace invalidation", () => {
+  it("invalidating a sheet also invalidates its dependents, not unrelated sheets", () => {
+    const { ws, store } = makeWorkspace({
+      a: { title: "A", text: "x = 10" },
+      b: { title: "B", text: "z = @A.x" },
+      c: { title: "C", text: "y = 5" },
+    });
+    expect(ws.evaluateSheet("dash", "@B.z")[0].text).toBe("10");
+    expect(ws.evaluateSheet("dash", "@C.y")[0].text).toBe("5");
+
+    store.get("a")!.text = "x = 99";
+    expect(ws.evaluateSheet("dash", "@B.z")[0].text).toBe("10");
+
+    ws.invalidate("a");
+    expect(ws.evaluateSheet("dash", "@B.z")[0].text).toBe("99");
+    expect(ws.evaluateSheet("dash", "@C.y")[0].text).toBe("5");
+  });
+});
+
+describe("Workspace cycle detection", () => {
+  it("a true two-sheet cycle terminates instead of looping forever", () => {
+    const { ws } = makeWorkspace({
+      a: { title: "A", text: "z = @B.total" },
+      b: { title: "B", text: "z = @A.total" },
+    });
+    const r = ws.evaluateSheet("dash", "@A.z");
+    expect(r[0].value).toBeNull();
+    expect(r[0].error).toBeDefined();
+  });
+
+  it("a sheet referencing itself surfaces a circular reference immediately", () => {
+    const { ws } = makeWorkspace({ self: { title: "Self", text: "" } });
+    const text = "a = 5\nb = @Self.a";
+    expect(() => ws.evaluateSheet("self", text)).not.toThrow();
+    const r = ws.evaluateSheet("self", text);
+    expect(r[1].value).toBeNull();
+    expect(r[1].error).toBe("circular reference");
+  });
+});
+
+describe("Workspace.renameSheet", () => {
+  it("rewrites bare-form references to the new title", () => {
+    const { ws } = makeWorkspace({
+      budget: { title: "Budget", text: "rent = $500" },
+      dash: { title: "Dashboard", text: "@Budget.rent + 1" },
+    });
+    expect(ws.renameSheet("budget", "Budget", "Money")).toEqual([{ id: "dash", text: "@Money.rent + 1" }]);
+  });
+
+  it("switches to bracket form when the new title needs it", () => {
+    const { ws } = makeWorkspace({
+      budget: { title: "Budget", text: "rent = $500" },
+      dash: { title: "Dashboard", text: "@Budget.rent" },
+    });
+    expect(ws.renameSheet("budget", "Budget", "Monthly Budget")).toEqual([
+      { id: "dash", text: "@[Monthly Budget].rent" },
+    ]);
+  });
+
+  it("rewrites existing bracket-form references too", () => {
+    const { ws } = makeWorkspace({
+      trip: { title: "Trip to Lisbon", text: "food = 25" },
+      dash: { title: "Dashboard", text: "@[Trip to Lisbon].food" },
+    });
+    expect(ws.renameSheet("trip", "Trip to Lisbon", "Lisbon")).toEqual([{ id: "dash", text: "@Lisbon.food" }]);
+  });
+
+  it("never rewrites the renamed sheet's own text", () => {
+    const { ws } = makeWorkspace({
+      budget: { title: "Budget", text: "a = 1\nb = @Budget.a" },
+    });
+    expect(ws.renameSheet("budget", "Budget", "Money")).toEqual([]);
+  });
+
+  it("leaves unrelated sheets untouched", () => {
+    const { ws } = makeWorkspace({
+      budget: { title: "Budget", text: "rent = $500" },
+      other: { title: "Other", text: "just text, no refs" },
+    });
+    expect(ws.renameSheet("budget", "Budget", "Money")).toEqual([]);
+  });
+});
diff --git a/src/workspace.ts b/src/workspace.ts
new file mode 100644
index 0000000..45c726e
--- /dev/null
+++ b/src/workspace.ts
@@ -0,0 +1,144 @@
+// Owns the cross-sheet dependency graph: which sheets a `@Sheet.key`
+// reference can see, cached exports, cycle detection, and rename rewrites.
+// The engine has no concept of "other sheets" — that lives entirely here.
+import type { SumEngine, LineResult } from "./engine";
+import type { Value } from "./engine/types";
+import type { XRefResolution } from "./engine/evaluator";
+
+export interface SheetSource {
+  id: string;
+  title: string;
+  text: string;
+}
+
+interface SheetExports {
+  vars: Map<string, Value>;
+  total: Value | null;
+  last: Value | null;
+}
+
+const XREF_SCAN_RE = /@(?:\[([^\]]+)\]|([\p{L}_][\p{L}\d_]*))\./gu;
+
+function escapeRegExp(s: string): string {
+  return s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+export class Workspace {
+  private cache = new Map<string, SheetExports>();
+  private resolving: string[] = [];
+
+  constructor(private engine: SumEngine, private sheets: () => SheetSource[]) {}
+
+  /** Evaluate a sheet as the user's active/open document — refs resolve live. */
+  evaluateSheet(sheetId: string, text: string): LineResult[] {
+    this.resolving.push(sheetId);
+    try {
+      return this.engine.evaluateDocument(text, (sheet, key) => this.resolveXRef(sheet, key));
+    } finally {
+      this.resolving.pop();
+    }
+  }
+
+  /** Drop cached exports for a sheet and everything that transitively depends on it. */
+  invalidate(sheetId: string): void {
+    const dirty = new Set<string>([sheetId]);
+    let changed = true;
+    while (changed) {
+      changed = false;
+      for (const s of this.sheets()) {
+        if (dirty.has(s.id)) continue;
+        for (const title of this.referencedTitles(s.text)) {
+          const target = this.findSheetByTitle(title);
+          if (target && dirty.has(target.id)) {
+            dirty.add(s.id);
+            changed = true;
+            break;
+          }
+        }
+      }
+    }
+    for (const id of dirty) this.cache.delete(id);
+  }
+
+  invalidateAll(): void {
+    this.cache.clear();
+  }
+
+  /**
+   * Explicit rename: rewrite `@OldTitle.` / `@[Old Title].` to the new name
+   * in every *other* sheet. Returns only the sheets whose text changed.
+   */
+  renameSheet(renamedId: string, oldTitle: string, newTitle: string): { id: string; text: string }[] {
+    const oldEsc = escapeRegExp(oldTitle.trim());
+    const bareRe = new RegExp(`@${oldEsc}\\.`, "gu");
+    const bracketRe = new RegExp(`@\\[\\s*${oldEsc}\\s*\\]\\.`, "gu");
+    const trimmedNew = newTitle.trim();
+    const replacement = /^[\p{L}_][\p{L}\d_]*$/u.test(trimmedNew) ? `@${trimmedNew}.` : `@[${trimmedNew}].`;
+    const out: { id: string; text: string }[] = [];
+    for (const s of this.sheets()) {
+      if (s.id === renamedId) continue;
+      const rewritten = s.text.replace(bareRe, replacement).replace(bracketRe, replacement);
+      if (rewritten !== s.text) out.push({ id: s.id, text: rewritten });
+    }
+    return out;
+  }
+
+  private referencedTitles(text: string): string[] {
+    const out: string[] = [];
+    for (const m of text.matchAll(XREF_SCAN_RE)) out.push((m[1] ?? m[2]).trim());
+    return out;
+  }
+
+  private findSheetByTitle(title: string): SheetSource | undefined {
+    const needle = title.trim().toLowerCase();
+    return this.sheets().find((s) => s.title.trim().toLowerCase() === needle);
+  }
+
+  private resolveXRef(sheetTitle: string, key: string): XRefResolution {
+    const target = this.findSheetByTitle(sheetTitle);
+    if (!target) return { ok: false, reason: `sheet "${sheetTitle}" not found` };
+    if (this.resolving.includes(target.id)) {
+      return { ok: false, reason: "circular reference" };
+    }
+    const exports = this.exportsFor(target);
+    if (key === "total") {
+      return exports.total
+        ? { ok: true, value: exports.total }
+        : { ok: false, reason: `sheet "${sheetTitle}" has no total` };
+    }
+    if (key === "last") {
+      return exports.last
+        ? { ok: true, value: exports.last }
+        : { ok: false, reason: `sheet "${sheetTitle}" has no result` };
+    }
+    const v = exports.vars.get(key);
+    if (!v) return { ok: false, reason: `no variable "${key}" in "${sheetTitle}"` };
+    return { ok: true, value: v };
+  }
+
+  private exportsFor(sheet: SheetSource): SheetExports {
+    const cached = this.cache.get(sheet.id);
+    if (cached) return cached;
+    this.resolving.push(sheet.id);
+    let results: LineResult[];
+    try {
+      results = this.engine.evaluateDocument(sheet.text, (s, k) => this.resolveXRef(s, k));
+    } finally {
+      this.resolving.pop();
+    }
+    const vars = new Map<string, Value>();
+    for (const r of results) {
+      if (r.assign && r.value) vars.set(r.assign, r.value);
+    }
+    let last: Value | null = null;
+    for (let i = results.length - 1; i >= 0; i--) {
+      if (results[i].kind === "normal" && results[i].value) {
+        last = results[i].value;
+        break;
+      }
+    }
+    const exports: SheetExports = { vars, total: this.engine.totalValueOf(results), last };
+    this.cache.set(sheet.id, exports);
+    return exports;
+  }
+}