feat(context-window): show current/max context usage per tab/model

Add a 'Context Window' sidebar view showing the live context occupancy
(latest request's input+output) against the model's maximum context
window, resolved dynamically from the models.dev catalog.

- core: models.dev catalog module (resolveContextLimit) with disk cache,
  TTL, stale-fallback + offline penalty memo; null for unknown models.
- api: GET /models/context-limit?provider=&modelId=.
- frontend: ContextWindowPanel + computeContextUsage helper; App resolves
  + caches the active model's max (anthropic/opencode-anthropic only);
  percent shown to 2 decimals; degrades to bare token count when max
  unknown.
- tests: core catalog (13), api route (3), frontend helper (6).

1 files changed, 158 insertions, 0 deletions

diff --git a/packages/core/tests/models/catalog.test.ts b/packages/core/tests/models/catalog.test.ts
new file mode 100644
index 0000000..51043e6
--- /dev/null
+++ b/packages/core/tests/models/catalog.test.ts
@@ -0,0 +1,158 @@
+import { existsSync, rmSync, utimesSync, writeFileSync } from "node:fs";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import {
+	__resetCatalogCacheForTests,
+	getModelsCatalog,
+	resolveContextLimit,
+} from "../../src/models/catalog.js";
+
+const CACHE_PATH = "/tmp/dispatch/models-dev.json";
+
+// A trimmed models.dev-shaped catalog covering the providers we support.
+const CATALOG = {
+	anthropic: {
+		id: "anthropic",
+		models: {
+			"claude-sonnet-4-5": { limit: { context: 200000, output: 64000 } },
+			"claude-sonnet-4-6": { limit: { context: 1000000, output: 64000 } },
+		},
+	},
+	opencode: {
+		id: "opencode",
+		models: {
+			"glm-4-6": { limit: { context: 131072, output: 8192 } },
+		},
+	},
+};
+
+function mockFetchOnce(catalog: unknown, ok = true, status = 200) {
+	const fn = vi.fn(() =>
+		Promise.resolve({
+			ok,
+			status,
+			text: () => Promise.resolve(JSON.stringify(catalog)),
+		} as Response),
+	);
+	vi.stubGlobal("fetch", fn);
+	return fn;
+}
+
+beforeEach(() => {
+	__resetCatalogCacheForTests();
+	if (existsSync(CACHE_PATH)) rmSync(CACHE_PATH);
+	delete process.env.DISPATCH_DISABLE_MODELS_FETCH;
+});
+
+afterEach(() => {
+	vi.unstubAllGlobals();
+	if (existsSync(CACHE_PATH)) rmSync(CACHE_PATH);
+});
+
+describe("resolveContextLimit", () => {
+	it("resolves a known anthropic model to its context window", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBe(200000);
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-6")).toBe(1000000);
+	});
+
+	it("maps opencode-anthropic to the anthropic catalog, then opencode fallback", async () => {
+		mockFetchOnce(CATALOG);
+		// Present in the anthropic catalog.
+		expect(await resolveContextLimit("opencode-anthropic", "claude-sonnet-4-5")).toBe(200000);
+		// Absent in anthropic, found in the opencode gateway catalog.
+		expect(await resolveContextLimit("opencode-anthropic", "glm-4-6")).toBe(131072);
+	});
+
+	it("returns null for an unknown model id", async () => {
+		mockFetchOnce(CATALOG);
+		expect(await resolveContextLimit("anthropic", "no-such-model")).toBeNull();
+	});
+
+	it("returns null for an unsupported provider (no network needed)", async () => {
+		const fetchFn = mockFetchOnce(CATALOG);
+		expect(await resolveContextLimit("google", "gemini-2.5-pro")).toBeNull();
+		expect(await resolveContextLimit("anthropic", "")).toBeNull();
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it("returns null when the model has no positive context limit", async () => {
+		mockFetchOnce({
+			anthropic: { id: "anthropic", models: { broken: { limit: { context: 0 } } } },
+		});
+		expect(await resolveContextLimit("anthropic", "broken")).toBeNull();
+	});
+
+	it("does not throw on a malformed provider entry missing `models`", async () => {
+		// A provider object without a `models` map must degrade to null, not crash.
+		mockFetchOnce({ anthropic: { id: "anthropic" } });
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBeNull();
+	});
+
+	it("does not throw when limit/context fields are absent", async () => {
+		mockFetchOnce({ anthropic: { id: "anthropic", models: { m: {} } } });
+		expect(await resolveContextLimit("anthropic", "m")).toBeNull();
+	});
+});
+
+describe("getModelsCatalog caching", () => {
+	it("fetches once and serves the in-process memo on subsequent calls", async () => {
+		const fetchFn = mockFetchOnce(CATALOG);
+		await resolveContextLimit("anthropic", "claude-sonnet-4-5");
+		await resolveContextLimit("anthropic", "claude-sonnet-4-6");
+		await getModelsCatalog();
+		expect(fetchFn).toHaveBeenCalledTimes(1);
+	});
+
+	it("reuses a fresh disk cache without re-fetching across processes", async () => {
+		// Simulate another process having written a fresh cache.
+		writeFileSync(CACHE_PATH, JSON.stringify(CATALOG), "utf-8");
+		const fetchFn = vi.fn(() => Promise.reject(new Error("network should not be hit")));
+		vi.stubGlobal("fetch", fetchFn);
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBe(200000);
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it("falls back to a STALE disk cache when the network fails", async () => {
+		writeFileSync(CACHE_PATH, JSON.stringify(CATALOG), "utf-8");
+		// Age the cache well past the TTL so the fetch path is taken.
+		const old = Date.now() / 1000 - 3600;
+		utimesSync(CACHE_PATH, old, old);
+		const fetchFn = vi.fn(() => Promise.reject(new Error("offline")));
+		vi.stubGlobal("fetch", fetchFn);
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBe(200000);
+		expect(fetchFn).toHaveBeenCalledTimes(1);
+		warn.mockRestore();
+	});
+
+	it("returns null when fetch fails and no cache exists", async () => {
+		const fetchFn = vi.fn(() => Promise.reject(new Error("offline")));
+		vi.stubGlobal("fetch", fetchFn);
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBeNull();
+		warn.mockRestore();
+	});
+
+	it("does not hit the network when DISPATCH_DISABLE_MODELS_FETCH is set", async () => {
+		process.env.DISPATCH_DISABLE_MODELS_FETCH = "1";
+		const fetchFn = vi.fn(() => Promise.reject(new Error("should not fetch")));
+		vi.stubGlobal("fetch", fetchFn);
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBeNull();
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it("memoizes the fallback after a failed fetch so it does not re-hit the network", async () => {
+		const fetchFn = vi.fn(() => Promise.reject(new Error("offline")));
+		vi.stubGlobal("fetch", fetchFn);
+		const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+		// First lookup triggers the (failing) fetch.
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-5")).toBeNull();
+		// Subsequent lookups within the penalty window must NOT re-fetch.
+		expect(await resolveContextLimit("anthropic", "claude-sonnet-4-6")).toBeNull();
+		await getModelsCatalog();
+		expect(fetchFn).toHaveBeenCalledTimes(1);
+		warn.mockRestore();
+	});
+});