diff --git a/src/client.ts b/src/client.ts
index 2f235a1..7f70ad6 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -12,6 +12,37 @@ export function buildEndpointUrl(base: string, siteUuid?: string | null): string
     : trimmed;
 }
 
+/**
+ * What a refusal means, in terms someone can act on.
+ *
+ * Every Pulse route that addresses an existing site requires a credential, so a rejection is now the
+ * most likely failure a misconfigured project hits — and the three causes have three different fixes.
+ * Reporting them as `Patchstack returned 401` names none of them, and this output is frequently read by
+ * an AI agent that has no other way to find out what to do next.
+ *
+ * The distinction 401 cannot make on its own is whether we HELD a credential. Having none is a setup
+ * step that was never run; having one rejected is a credential that has expired, been revoked, or whose
+ * site no longer exists. A 403 is different again: the credential is valid and simply is not for this
+ * site, which usually means a `.patchstackrc.json` carrying someone else's UUID.
+ *
+ * @returns the message, or null when the status is not an authentication failure
+ */
+export function authFailureMessage(status: number, config: Config): string | null {
+  const hasCredential = typeof config.pulseAuth === 'string' && config.pulseAuth.length > 0;
+
+  if (status === 401 && !hasCredential) {
+    return 'Patchstack requires an API credential for this site and none is configured. Run `npx patchstack-connect login`, or set PATCHSTACK_API_KEY.';
+  }
+  if (status === 401) {
+    return 'Patchstack rejected this API credential. It may have expired, been revoked, or the site may no longer exist. Run `npx patchstack-connect login` to issue a new one.';
+  }
+  if (status === 403) {
+    return 'This API credential is not permitted to act on this site. Check that siteUuid in .patchstackrc.json matches the credential (a credential is issued for one site).';
+  }
+
+  return null;
+}
+
 /** Build the live Pulse rules URL corresponding to a manifest endpoint override. */
 export function buildRulesUrl(manifestEndpoint: string, siteUuid: string): string {
   const url = new URL(manifestEndpoint);
@@ -114,6 +145,10 @@ export async function postInputMap(
       // one side is out of date, and guessing at compatibility is how a consumer misreads a document.
       return { result: 'failed', message: `Patchstack does not accept this map schema (version ${map.version}). Update @patchstack/connect.` };
     }
+    const refused = authFailureMessage(response.status, config);
+    if (refused !== null) {
+      return { result: 'failed', message: refused };
+    }
     if (!response.ok) {
       return { result: 'failed', message: `Patchstack returned ${response.status}.` };
     }
@@ -167,6 +202,18 @@ export async function postPackageRemoved(config: Config): Promise<PackageRemoved
     if (response.status === 404) {
       return { result: 'gone', message: null };
     }
+    // A site that has been deleted answers 401 here, not 404: this route resolves the site FROM the
+    // credential, so once the site record is gone the credential resolves to nothing. Reporting that as
+    // an auth failure would tell someone to re-run `login` over a site that no longer exists, so the
+    // question is put to the endpoint that can still answer it — widget settings, which needs no
+    // credential and 404s for a removed site.
+    if (response.status === 401 && (await fetchSiteStatus(config)) === 'removed') {
+      return { result: 'gone', message: null };
+    }
+    const refused = authFailureMessage(response.status, config);
+    if (refused !== null) {
+      return { result: 'failed', message: refused };
+    }
     if (!response.ok) {
       return { result: 'failed', message: `Patchstack returned ${response.status}.` };
     }
@@ -277,6 +324,11 @@ export async function postManifest(
     );
   }
 
+  const refused = authFailureMessage(response.status, config);
+  if (refused !== null) {
+    throw new PatchstackError(refused, 'UNAUTHORIZED');
+  }
+
   if (response.status < 200 || response.status >= 300) {
     throw new PatchstackError(
       `Patchstack returned ${response.status}: ${text.slice(0, 200)}`,
diff --git a/src/types.ts b/src/types.ts
index 4a76649..21d6ca3 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -85,6 +85,7 @@ export class PatchstackError extends Error {
       | 'NETWORK_ERROR'
       | 'NETWORK_TIMEOUT'
       | 'SITE_NOT_FOUND'
+      | 'UNAUTHORIZED'
       | 'VALIDATION_ERROR'
       | 'SERVER_ERROR',
     public readonly cause?: unknown,
diff --git a/tests/auth-failure-message.test.ts b/tests/auth-failure-message.test.ts
new file mode 100644
index 0000000..acf41cd
--- /dev/null
+++ b/tests/auth-failure-message.test.ts
@@ -0,0 +1,130 @@
+import { describe, expect, it, vi } from 'vitest';
+import { authFailureMessage, postInputMap, postPackageRemoved, postManifest } from '../src/client.js';
+import { PatchstackError, type Config } from '../src/types.js';
+
+/**
+ * A refusal has to say which of three things went wrong, because each has a different fix.
+ *
+ * Every Pulse route addressing an existing site requires a credential, so a rejection is the likeliest
+ * failure a misconfigured project meets — and `Patchstack returned 401` is the one report that helps
+ * nobody. This output is often read by an AI agent with no other source of the answer, so "which setup
+ * step did I miss" has to be answerable from the text alone.
+ */
+const config = (over: Partial<Config> = {}): Config =>
+  ({
+    endpoint: 'https://api.test/monitor/pulse/manifest',
+    siteUuid: '11111111-1111-4111-8111-111111111111',
+    apiKey: null,
+    pulseAuth: null,
+    timeoutMs: 5_000,
+    environment: 'production',
+    ...over,
+  }) as Config;
+
+const WITH_CREDENTIAL = config({ pulseAuth: 'a-secret-40-chars-long-enough-for-this-1' });
+
+const respond = (status: number, body = '{}') =>
+  vi.fn(async () => new Response(body, { status, headers: { 'Content-Type': 'application/json' } }));
+
+describe('authFailureMessage', () => {
+  it('tells an unconfigured project to obtain a credential', () => {
+    const said = authFailureMessage(401, config());
+
+    expect(said).toContain('login');
+    expect(said).toContain('PATCHSTACK_API_KEY');
+  });
+
+  it('distinguishes a rejected credential from a missing one', () => {
+    // The distinction 401 cannot make by itself, and the one that decides the remedy: nothing to set up
+    // versus something to renew. Asserted as a difference so neither message can drift into the other.
+    const missing = authFailureMessage(401, config());
+    const rejected = authFailureMessage(401, WITH_CREDENTIAL);
+
+    expect(rejected).not.toBe(missing);
+    expect(rejected).toMatch(/expired|revoked/);
+    // A removed site also answers 401 on the authenticated routes, so a message that named only expiry
+    // would send someone to re-run `login` against a site that no longer exists.
+    expect(rejected).toMatch(/no longer exist/);
+  });
+
+  it('reads 403 as the wrong site rather than a missing credential', () => {
+    const said = authFailureMessage(403, WITH_CREDENTIAL);
+
+    expect(said).toContain('.patchstackrc.json');
+    expect(said).not.toMatch(/expired|revoked/);
+  });
+
+  it('says nothing about statuses that are not authentication failures', () => {
+    // The control. A helper that answered for every status would relabel a 500 or a 422 as an auth
+    // problem, which is worse than the bare status code it replaced.
+    for (const status of [200, 404, 422, 429, 500, 503]) {
+      expect(authFailureMessage(status, WITH_CREDENTIAL), `status ${status}`).toBeNull();
+    }
+  });
+});
+
+describe('the write paths report a refusal in those terms', () => {
+  it('map upload fails with the remedy, not the number', async () => {
+    vi.stubGlobal('fetch', respond(401));
+
+    const outcome = await postInputMap(config(), { version: 3, endpoints: [] });
+
+    expect(outcome.result).toBe('failed');
+    expect(outcome).toHaveProperty('message');
+    const message = (outcome as { message: string }).message;
+    expect(message).not.toMatch(/^Patchstack returned/);
+    expect(message).toContain('login');
+  });
+
+  it('manifest push throws UNAUTHORIZED rather than SERVER_ERROR', async () => {
+    vi.stubGlobal('fetch', respond(403));
+
+    await expect(postManifest(WITH_CREDENTIAL, { packages: [] } as never)).rejects.toMatchObject({
+      code: 'UNAUTHORIZED',
+    });
+  });
+
+  it('still reports a genuine server error as one', async () => {
+    // The other half of the control: the auth handling must not have swallowed every failure branch.
+    vi.stubGlobal('fetch', respond(500, 'upstream exploded'));
+
+    await expect(postManifest(WITH_CREDENTIAL, { packages: [] } as never)).rejects.toBeInstanceOf(
+      PatchstackError,
+    );
+    await expect(postManifest(WITH_CREDENTIAL, { packages: [] } as never)).rejects.toMatchObject({
+      code: 'SERVER_ERROR',
+    });
+  });
+});
+
+describe('a removed site is still reported as removed', () => {
+  it('reads 401 on package-removed as "gone" when the site really is gone', async () => {
+    // The regression this guards. That route resolves the site FROM the credential, so a deleted site
+    // answers 401 rather than 404 — verified against the server. Left unhandled, uninstalling a site
+    // that was already deleted in the dashboard would advise re-running `login`.
+    const fetchMock = vi.fn(async (url: string) => {
+      if (url.includes('/widget/settings/')) return new Response('{}', { status: 404 });
+      return new Response('{}', { status: 401 });
+    });
+    vi.stubGlobal('fetch', fetchMock);
+
+    const outcome = await postPackageRemoved(WITH_CREDENTIAL);
+
+    expect(outcome.result).toBe('gone');
+  });
+
+  it('reports an auth failure when the site is still there', async () => {
+    // The control that keeps the branch above honest: 401 must not become a blanket "gone", or a genuine
+    // credential problem would read as a site that no longer exists and be silently ignored.
+    const fetchMock = vi.fn(async (url: string) => {
+      if (url.includes('/widget/settings/')) return new Response('{}', { status: 200 });
+      return new Response('{}', { status: 401 });
+    });
+    vi.stubGlobal('fetch', fetchMock);
+
+    const outcome = await postPackageRemoved(WITH_CREDENTIAL);
+
+    expect(outcome.result).toBe('failed');
+    expect((outcome as { message: string }).message).toMatch(/credential/i);
+  });
+});