diff --git a/packages/web/src/api/activities.test.ts b/packages/web/src/api/activities.test.ts
index 161aaca3..ddc2e04d 100644
--- a/packages/web/src/api/activities.test.ts
+++ b/packages/web/src/api/activities.test.ts
@@ -1,5 +1,10 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
-import { fetchActivities, fetchActivitySummary } from "./activities";
+import {
+  fetchActivities,
+  fetchActivitySummary,
+  fetchMultiSportDailySummary,
+  fetchMultiSportMetrics,
+} from "./activities";
 
 // Stub the axios client so the tests see exactly the URL fetchActivities builds.
 const { get } = vi.hoisted(() => ({ get: vi.fn() }));
@@ -69,3 +74,34 @@ describe("fetchActivitySummary query serialization", () => {
     expect(await fetchActivitySummary({ sports: [] })).toEqual([]);
   });
 });
+
+describe("multi-sport fetchers with an empty sports selection", () => {
+  beforeEach(() => {
+    get.mockReset();
+    get.mockResolvedValue({ data: {} });
+  });
+
+  // Regression: both fetchers built params as
+  //   new URLSearchParams({ sports: options.sports.join(",") })
+  // and `[].join(",")` is "", which URLSearchParams still emits as `?sports=`.
+  // The handler rejects a blank value with 400, so hiding every sport in
+  // Settings and then clicking the dashboard heatmap's "Visible" chip produced
+  // a generic "Failed to load calendar data" error where an empty state belonged.
+  it.each([
+    ["fetchMultiSportDailySummary", fetchMultiSportDailySummary],
+    ["fetchMultiSportMetrics", fetchMultiSportMetrics],
+  ])("%s makes no request and resolves empty", async (_name, fn) => {
+    const result = await fn({ year: 2026, sports: [] });
+
+    expect(get).not.toHaveBeenCalled();
+    expect(result).toEqual({});
+  });
+
+  it("still sends sports when the selection is non-empty", async () => {
+    get.mockResolvedValue({ data: { bySport: {} } });
+    await fetchMultiSportDailySummary({ year: 2026, sports: ["cycling"] });
+
+    const params = new URLSearchParams(requestedUrl().split("?")[1]);
+    expect(params.get("sports")).toBe("cycling");
+  });
+});
diff --git a/packages/web/src/api/activities.ts b/packages/web/src/api/activities.ts
index 8c2508d3..f9271506 100644
--- a/packages/web/src/api/activities.ts
+++ b/packages/web/src/api/activities.ts
@@ -217,13 +217,19 @@ export interface FetchMultiSportOptions {
 }
 
 /**
- * Fetch daily summaries for multiple sports in a single request.
- * Returns a map of sport → daily data (Record<string, DailyActivity>).
+ * Build the shared query string for the multi-sport endpoints.
+ *
+ * Extracted so both callers inherit the same empty-array contract: `[].join(",")`
+ * is `""`, and URLSearchParams still emits the key, so `?sports=` reaches a
+ * backend that rejects a blank value with 400. The single-sport fetchers already
+ * follow the omit-if-empty discipline; this brings the batch ones in line.
+ *
+ * Callers must short-circuit on an empty `sports` array rather than rely on this —
+ * an empty selection has no meaningful request to make.
  */
-export const fetchMultiSportDailySummary = async (
-  options: FetchMultiSportOptions
-): Promise<Record<string, Record<string, DailyActivity>>> => {
-  const params = new URLSearchParams({ sports: options.sports.join(",") });
+function buildMultiSportParams(options: FetchMultiSportOptions): URLSearchParams {
+  const params = new URLSearchParams();
+  if (options.sports.length) params.set("sports", options.sports.join(","));
   if (options.from && options.to) {
     params.set("from", options.from);
     params.set("to", options.to);
@@ -231,6 +237,22 @@ export const fetchMultiSportDailySummary = async (
   if (options.tz) {
     params.set("tz", options.tz);
   }
+  return params;
+}
+
+/**
+ * Fetch daily summaries for multiple sports in a single request.
+ * Returns a map of sport → daily data (Record<string, DailyActivity>).
+ */
+export const fetchMultiSportDailySummary = async (
+  options: FetchMultiSportOptions
+): Promise<Record<string, Record<string, DailyActivity>>> => {
+  // No sports selected is a legitimate UI state (every sport hidden in Settings,
+  // then the "Visible" chip clicked). It is not a request: the backend 400s on a
+  // blank `sports` value, which surfaced as a generic "Failed to load calendar
+  // data" error where an empty state belonged.
+  if (options.sports.length === 0) return {};
+  const params = buildMultiSportParams(options);
   const url = `activities/${options.year}/source?${params.toString()}`;
 
   try {
@@ -258,14 +280,9 @@ export const fetchMultiSportDailySummary = async (
 export const fetchMultiSportMetrics = async (
   options: FetchMultiSportOptions
 ): Promise<Record<string, SportMetrics>> => {
-  const params = new URLSearchParams({ sports: options.sports.join(",") });
-  if (options.from && options.to) {
-    params.set("from", options.from);
-    params.set("to", options.to);
-  }
-  if (options.tz) {
-    params.set("tz", options.tz);
-  }
+  // See fetchMultiSportDailySummary: an empty selection has no request to make.
+  if (options.sports.length === 0) return {};
+  const params = buildMultiSportParams(options);
   const url = `activities/${options.year}/metrics?${params.toString()}`;
 
   try {
diff --git a/packages/web/src/api/client.test.ts b/packages/web/src/api/client.test.ts
index 8bc5c5da..9b12bc4f 100644
--- a/packages/web/src/api/client.test.ts
+++ b/packages/web/src/api/client.test.ts
@@ -268,3 +268,72 @@ function createAxiosError(status: number, config: InternalAxiosRequestConfig): A
     toJSON: () => ({}),
   }) as AxiosError;
 }
+
+describe("auth-init re-arming (request interceptor)", () => {
+  beforeEach(() => {
+    resetClient();
+    client = getClient();
+    configureClientAuth(mockAuthService);
+    vi.mocked(mockAuthService.waitForAuthReady).mockReset();
+    vi.mocked(mockAuthService.getIdToken).mockReset().mockResolvedValue("original-token");
+    mockLogger.error.mockClear();
+  });
+
+  function captureAdapter() {
+    const seen: InternalAxiosRequestConfig[] = [];
+    client.defaults.adapter = vi.fn().mockImplementation((config: InternalAxiosRequestConfig) => {
+      seen.push(config);
+      return Promise.resolve({ status: 200, statusText: "OK", headers: {}, config, data: {} });
+    });
+    return seen;
+  }
+
+  const authHeaderOf = (config: InternalAxiosRequestConfig | undefined) =>
+    (config?.headers as unknown as Record<string, unknown> | undefined)?.Authorization as
+      string | undefined;
+
+  // Regression: authInitPromise used to cache the *result* of the readiness race,
+  // not just the in-flight wait. One throw (or one lost race against the 5s timer)
+  // left a resolved-false promise in the closure for the lifetime of the tab, so
+  // every later request skipped token injection, took a 401, and paid a
+  // refresh+retry round trip. The 401 interceptor kept it correct, which is why it
+  // stayed invisible. Found independently by two audits a week apart
+  // (2026-08-17-web:M1 and 2026-08-24-web:M1).
+  it("recovers on a later request after the first auth-init throws", async () => {
+    vi.mocked(mockAuthService.waitForAuthReady)
+      .mockRejectedValueOnce(new Error("firebase hiccup"))
+      .mockResolvedValue(undefined);
+
+    const seen = captureAdapter();
+
+    await client.get("activities");
+    expect(authHeaderOf(seen[0])).toBeUndefined(); // first request is the casualty
+
+    await client.get("activities");
+    expect(authHeaderOf(seen[1])).toBe("Bearer original-token"); // recovered
+  });
+
+  it("caches a successful init and does not re-wait on later requests", async () => {
+    vi.mocked(mockAuthService.waitForAuthReady).mockResolvedValue(undefined);
+    const seen = captureAdapter();
+
+    await client.get("activities");
+    await client.get("activities/1");
+
+    expect(authHeaderOf(seen[0])).toBe("Bearer original-token");
+    expect(authHeaderOf(seen[1])).toBe("Bearer original-token");
+    // The success path is still coalesced — one wait, not one per request.
+    expect(mockAuthService.waitForAuthReady).toHaveBeenCalledTimes(1);
+  });
+
+  it("logs the error cause rather than reporting a timeout that did not happen", async () => {
+    vi.mocked(mockAuthService.waitForAuthReady).mockRejectedValueOnce(new Error("boom"));
+    captureAdapter();
+
+    await client.get("activities");
+
+    const messages = mockLogger.error.mock.calls.map((c) => String(c[0]));
+    expect(messages.some((m) => m.includes("Auth initialization errored"))).toBe(true);
+    expect(messages.some((m) => m.includes("timed out"))).toBe(false);
+  });
+});
diff --git a/packages/web/src/api/client.ts b/packages/web/src/api/client.ts
index 7906d717..141373ba 100644
--- a/packages/web/src/api/client.ts
+++ b/packages/web/src/api/client.ts
@@ -100,6 +100,12 @@ function getClient() {
  * - **Test isolation**: prevents interceptor state from leaking between tests
  * - **HMR**: allows re-configuration when modules are hot-replaced in development
  *
+ * Note on `authInitPromise`: it lives inside `configureClientAuth`'s closure and is
+ * deliberately not reset here. Clearing `configured` means the next
+ * `configureClientAuth()` call builds a fresh closure with its own null handle, and
+ * the interceptor no longer caches a failed outcome — so there is no stale negative
+ * for this function to clear.
+ *
  * @internal — intended for tests and HMR; not for production application code.
  */
 export function resetClient(): void {
@@ -109,6 +115,15 @@ export function resetClient(): void {
 
 const AUTH_READY_TIMEOUT_MS = 5000;
 
+/**
+ * Outcome of the one-time auth-readiness race.
+ *
+ * A discriminated result rather than a boolean so the request interceptor can
+ * report *which* failure happened — the two paths (timer won the race vs.
+ * waitForAuthReady threw) previously shared one "timed out" log line.
+ */
+type AuthInitOutcome = "ready" | "timeout" | "error";
+
 /**
  * Configure the API client with an auth service.
  * Registers a request interceptor that waits for auth readiness and injects tokens.
@@ -122,7 +137,7 @@ export function configureClientAuth(authService: AuthService): void {
   configured = true;
 
   const instance = getClient();
-  let authInitPromise: Promise<boolean> | null = null;
+  let authInitPromise: Promise<AuthInitOutcome> | null = null;
 
   instance.interceptors.request.use(async (config) => {
     // Only our own API gateway gets auth tokens and trace propagation;
@@ -139,29 +154,52 @@ export function configureClientAuth(authService: AuthService): void {
 
     // Wait for initial auth state with timeout (only on first request).
     // Uses a shared promise so concurrent requests coalesce into one wait.
+    //
+    // Only a *successful* outcome is cached. Caching a failed race would poison
+    // the whole tab: every later request would await the same resolved-false
+    // promise, skip token injection via the early return below, take a 401, and
+    // pay a refresh+retry round trip — for the rest of the session, with no
+    // recovery short of a reload. The 401 response interceptor keeps that
+    // correct, which is exactly why it went unnoticed; the cost is silent
+    // latency plus one logger.error per request in production.
+    //
+    // The `authInitPromise === attempt` check means concurrent requests that are
+    // all awaiting this same attempt share one retry rather than stampeding N.
     if (!authInitPromise) {
-      authInitPromise = (async () => {
+      const attempt = (async (): Promise<AuthInitOutcome> => {
         try {
-          const timeoutPromise = new Promise<false>((resolve) => {
-            setTimeout(() => resolve(false), AUTH_READY_TIMEOUT_MS);
+          const timeoutPromise = new Promise<AuthInitOutcome>((resolve) => {
+            setTimeout(() => resolve("timeout"), AUTH_READY_TIMEOUT_MS);
           });
-          const authPromise = authService.waitForAuthReady().then(() => true as const);
+          const authPromise = authService.waitForAuthReady().then(() => "ready" as const);
           return await Promise.race([authPromise, timeoutPromise]);
         } catch (e) {
           logger.error(
             "Auth initialization failed:",
             e instanceof Error ? e.message : "unknown error"
           );
-          return false;
+          return "error";
         }
       })();
+      authInitPromise = attempt;
+      void attempt.then((outcome) => {
+        if (outcome !== "ready" && authInitPromise === attempt) {
+          authInitPromise = null;
+        }
+      });
     }
 
-    const ready = await authInitPromise;
-    if (!ready) {
+    const outcome = await authInitPromise;
+    if (outcome !== "ready") {
+      // Report the cause that actually occurred. Reporting "timed out" for the
+      // catch path sent readers looking for a slow network when the real signal
+      // was a thrown error.
       logger.error(
-        `Auth initialization timed out after ${AUTH_READY_TIMEOUT_MS}ms. ` +
-          "Request will proceed without auth token and likely receive 401."
+        outcome === "timeout"
+          ? `Auth initialization timed out after ${AUTH_READY_TIMEOUT_MS}ms. ` +
+              "Proceeding without an auth token; the 401 interceptor will refresh and retry."
+          : "Auth initialization errored. " +
+              "Proceeding without an auth token; the 401 interceptor will refresh and retry."
       );
       return config;
     }
diff --git a/packages/web/src/api/errors.test.ts b/packages/web/src/api/errors.test.ts
index 7ceb42d4..3c5b7436 100644
--- a/packages/web/src/api/errors.test.ts
+++ b/packages/web/src/api/errors.test.ts
@@ -162,6 +162,26 @@ describe("API Error Utilities", () => {
       consoleErrorSpy.mockRestore();
     });
 
+    // Regression: a native AbortError is a DOMException, which is NOT
+    // `instanceof Error` in browsers, so it fell through to the
+    // `new Error(String(err))` wrap and came back as a plain Error named
+    // "Error" — failing every branch of isCancellationError() and silently
+    // reclassifying a cancellation as a real failure.
+    it("re-throws a native AbortError unwrapped so it stays a cancellation", () => {
+      const abort = new DOMException("The operation was aborted.", "AbortError");
+      expect(abort instanceof Error).toBe(false); // the premise of the bug
+
+      let thrown: unknown;
+      try {
+        throwApiError(abort, "test");
+      } catch (e) {
+        thrown = e;
+      }
+
+      expect(thrown).toBe(abort);
+      expect(isCancellationError(thrown)).toBe(true);
+    });
+
     it("should throw auth error for 401", () => {
       const error = createAxiosError(401);
       expect(() => throwApiError(error, "testFunc")).toThrow(
diff --git a/packages/web/src/api/errors.ts b/packages/web/src/api/errors.ts
index f9439737..9bda5e6f 100644
--- a/packages/web/src/api/errors.ts
+++ b/packages/web/src/api/errors.ts
@@ -150,6 +150,16 @@ export function throwApiError(err: unknown, context: string): never {
   if (err instanceof Error) {
     throw err;
   }
+  // Re-throw cancellations unwrapped. A native AbortError is a DOMException,
+  // which is NOT `instanceof Error` in browsers, so it would otherwise fall to
+  // the wrap below and come back as a plain Error named "Error" — failing every
+  // branch of isCancellationError() above. That silently reclassifies a
+  // cancellation as a real failure for any caller following the documented
+  // isCancellationError pattern, contradicting the contract stated in the
+  // activities.ts header.
+  if (isCancellationError(err)) {
+    throw err;
+  }
   // Wrap non-Error values, preserving original as cause for debugging
   const wrapped = new Error(String(err));
   (wrapped as Error & { cause?: unknown }).cause = err;