diff --git a/canvas/src/components/tabs/__tests__/BudgetSection.test.tsx b/canvas/src/components/tabs/__tests__/BudgetSection.test.tsx
new file mode 100644
index 00000000..7372ca0d
--- /dev/null
+++ b/canvas/src/components/tabs/__tests__/BudgetSection.test.tsx
@@ -0,0 +1,330 @@
+// @vitest-environment jsdom
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { render, screen, cleanup, fireEvent } from "@testing-library/react";
+import React from "react";
+import { BudgetSection } from "../BudgetSection";
+import { api } from "@/lib/api";
+
+// Queue-based mock for the api module. Each api call shifts from the queue.
+// Tests push with qGet/qPatch and the module-level mockImplementation
+// reads from the queue.
+type QueueEntry = { body?: unknown; err?: Error };
+const apiQueue: QueueEntry[] = [];
+
+vi.mock("@/lib/api", () => ({
+  api: {
+    get: vi.fn(async (path: string) => {
+      const next = apiQueue.shift();
+      if (!next) throw new Error(`api.get queue exhausted at: ${path}`);
+      if (next.err) throw next.err;
+      return next.body;
+    }),
+    patch: vi.fn(async (path: string, _body?: unknown) => {
+      const next = apiQueue.shift();
+      if (!next) throw new Error(`api.patch queue exhausted at: ${path}`);
+      if (next.err) throw next.err;
+      return next.body;
+    }),
+  },
+}));
+
+afterEach(cleanup);
+
+beforeEach(() => {
+  apiQueue.length = 0;
+  vi.clearAllMocks();
+});
+
+const WS_ID = "budget-test-ws";
+
+function qGet(body: unknown) {
+  apiQueue.push({ body });
+}
+
+function qGetErr(status: number, msg: string) {
+  apiQueue.push({ err: new Error(`${msg}: ${status}`) });
+}
+
+function qPatch(body: unknown) {
+  apiQueue.push({ body });
+}
+
+function qPatchErr(status: number, msg: string) {
+  apiQueue.push({ err: new Error(`${msg}: ${status}`) });
+}
+
+function makeBudget(overrides: Partial<{
+  budget_limit: number | null;
+  budget_used: number;
+  budget_remaining: number | null;
+}> = {}) {
+  return {
+    budget_limit: 10_000,
+    budget_used: 3_500,
+    budget_remaining: 6_500,
+    ...overrides,
+  };
+}
+
+describe("BudgetSection", () => {
+  describe("loading state", () => {
+    it("shows loading indicator while fetching", async () => {
+      let resolveGet: (v: unknown) => void;
+      vi.mocked(api.get).mockImplementationOnce(
+        async () => new Promise((r) => { resolveGet = r as (v: unknown) => void; }),
+      );
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      expect(screen.getByTestId("budget-loading")).toBeTruthy();
+
+      // Resolve after render to verify state clears
+      resolveGet!(makeBudget());
+      await vi.waitFor(() => {
+        expect(screen.queryByTestId("budget-loading")).toBeNull();
+      });
+    });
+  });
+
+  describe("fetch error state", () => {
+    it("shows error message on non-402 fetch failure", async () => {
+      qGetErr(500, "Internal Server Error");
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-fetch-error")).toBeTruthy();
+      });
+      expect(screen.getByTestId("budget-fetch-error")!.textContent).toContain("500");
+    });
+
+    it("shows 402 as exceeded banner, not fetch error", async () => {
+      // 402 means the budget limit was hit — different UX from a network/API error.
+      qGetErr(402, "Payment Required");
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-exceeded-banner")).toBeTruthy();
+      });
+      expect(screen.queryByTestId("budget-fetch-error")).toBeNull();
+    });
+  });
+
+  describe("budget loaded — display", () => {
+    it("renders used / limit stats row", async () => {
+      qGet(makeBudget({ budget_limit: 10_000, budget_used: 3_500 }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-used-value")!.textContent).toBe("3,500");
+      });
+      expect(screen.getByTestId("budget-limit-value")!.textContent).toBe("10,000");
+    });
+
+    it("renders 'Unlimited' when budget_limit is null", async () => {
+      qGet(makeBudget({ budget_limit: null, budget_used: 1_000, budget_remaining: null }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-limit-value")!.textContent).toBe("Unlimited");
+      });
+    });
+
+    it("renders remaining credits when present", async () => {
+      qGet(makeBudget({ budget_limit: 10_000, budget_used: 3_500, budget_remaining: 6_500 }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-remaining")!.textContent).toContain("6,500");
+        expect(screen.getByTestId("budget-remaining")!.textContent).toContain("credits remaining");
+      });
+    });
+
+    it("omits remaining credits when budget_remaining is null", async () => {
+      qGet(makeBudget({ budget_limit: 10_000, budget_used: 3_500, budget_remaining: null }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.queryByTestId("budget-remaining")).toBeNull();
+      });
+    });
+
+    it("caps progress bar at 100% when used > limit", async () => {
+      // Over-limit: 12000 used of 10000 limit should show 100%, not 120%.
+      qGet(makeBudget({ budget_limit: 10_000, budget_used: 12_000, budget_remaining: null }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        const fill = screen.getByTestId("budget-progress-fill");
+        expect(fill.getAttribute("style")).toContain("100%");
+      });
+    });
+
+    it("omits progress bar when budget_limit is null (unlimited)", async () => {
+      qGet(makeBudget({ budget_limit: null, budget_used: 5_000, budget_remaining: null }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.queryByTestId("budget-progress-fill")).toBeNull();
+      });
+    });
+  });
+
+  describe("budget exceeded (402)", () => {
+    it("shows exceeded banner when load returns 402", async () => {
+      qGetErr(402, "Payment Required");
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-exceeded-banner")).toBeTruthy();
+        expect(screen.getByTestId("budget-exceeded-banner")!.textContent).toContain("Budget exceeded");
+      });
+    });
+
+    it("clears exceeded banner after successful save", async () => {
+      qGetErr(402, "Payment Required");
+      qPatch(makeBudget({ budget_limit: 50_000, budget_used: 0, budget_remaining: 50_000 }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-exceeded-banner")).toBeTruthy();
+      });
+
+      const input = screen.getByTestId("budget-limit-input");
+      fireEvent.change(input, { target: { value: "50000" } });
+
+      const saveBtn = screen.getByTestId("budget-save-btn");
+      fireEvent.click(saveBtn);
+
+      await vi.waitFor(() => {
+        expect(screen.queryByTestId("budget-exceeded-banner")).toBeNull();
+      });
+    });
+  });
+
+  describe("save flow", () => {
+    it("shows save error on non-402 patch failure", async () => {
+      qGet(makeBudget());
+      qPatchErr(500, "Internal Server Error");
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-limit-input")).toBeTruthy();
+      });
+
+      const saveBtn = screen.getByTestId("budget-save-btn");
+      fireEvent.click(saveBtn);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-save-error")).toBeTruthy();
+        expect(screen.getByTestId("budget-save-error")!.textContent).toContain("500");
+      });
+    });
+
+    it("updates input to new limit value after successful save", async () => {
+      qGet(makeBudget({ budget_limit: 10_000 }));
+      qPatch(makeBudget({ budget_limit: 20_000 }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      // Wait for the input to appear (loading → loaded)
+      await vi.waitFor(() => {
+        expect(screen.queryByTestId("budget-loading")).toBeNull();
+      });
+
+      const input = screen.getByTestId("budget-limit-input") as HTMLInputElement;
+      // Debug: check what values are rendered
+      const limitValue = screen.getByTestId("budget-limit-value")?.textContent;
+      expect(input.value).toBe("10000"); // initial value from API
+      expect(limitValue).toBe("10,000");
+
+      fireEvent.change(input, { target: { value: "20000" } });
+      expect(input.value).toBe("20000");
+
+      fireEvent.click(screen.getByTestId("budget-save-btn"));
+
+      await vi.waitFor(() => {
+        expect((screen.getByTestId("budget-limit-input") as HTMLInputElement).value).toBe("20000");
+      });
+    });
+
+    it("sends null when input is cleared (unlimited)", async () => {
+      qGet(makeBudget({ budget_limit: 10_000 }));
+      qPatch(makeBudget({ budget_limit: null }));
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-limit-input")).toBeTruthy();
+      });
+
+      const input = screen.getByTestId("budget-limit-input") as HTMLInputElement;
+      fireEvent.change(input, { target: { value: "" } });
+      fireEvent.click(screen.getByTestId("budget-save-btn"));
+
+      await vi.waitFor(() => {
+        // After save with null limit, input should show empty (unlimited)
+        expect(input.value).toBe("");
+      });
+    });
+
+    it("shows saving state on button while patch is in flight", async () => {
+      qGet(makeBudget());
+      let resolvePatch: (v: unknown) => void;
+      vi.mocked(api.patch).mockImplementationOnce(
+        async () => new Promise((r) => { resolvePatch = r as (v: unknown) => void; }),
+      );
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-limit-input")).toBeTruthy();
+      });
+
+      fireEvent.change(screen.getByTestId("budget-limit-input"), { target: { value: "50000" } });
+      fireEvent.click(screen.getByTestId("budget-save-btn"));
+
+      const btn = screen.getByTestId("budget-save-btn");
+      expect(btn.textContent).toContain("Saving");
+
+      resolvePatch!(makeBudget({ budget_limit: 50_000 }));
+      await vi.waitFor(() => {
+        expect(btn.textContent).toContain("Save");
+      });
+    });
+  });
+
+  describe("isApiError402 — regression coverage", () => {
+    it("classifies ': 402' with space as 402", async () => {
+      qGetErr(402, "Payment Required");
+      qPatch(makeBudget());
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-exceeded-banner")).toBeTruthy();
+      });
+    });
+
+    it("classifies non-402 error messages as regular fetch errors", async () => {
+      qGetErr(503, "Service Unavailable");
+
+      render(<BudgetSection workspaceId={WS_ID} />);
+
+      await vi.waitFor(() => {
+        expect(screen.getByTestId("budget-fetch-error")).toBeTruthy();
+      });
+      expect(screen.queryByTestId("budget-exceeded-banner")).toBeNull();
+    });
+  });
+});