diff --git a/docs/datamart/operations.md b/docs/datamart/operations.md
index 2894ba9..813f9b6 100644
--- a/docs/datamart/operations.md
+++ b/docs/datamart/operations.md
@@ -15,10 +15,10 @@ or building the separate Next.js web app the PRD describes, is a later step; see
 
 ## Configuration
 
-| Variable | Needed for | If unset |
-| --- | --- | --- |
-| `DATAGOV_API_KEY` | Data.gov tools | They return `configuration_required` (HTTP 503) with a catalog.data.gov handoff. DEMO_KEY is never used as a fallback. |
-| `DATAMART_RPM_LOC`, `DATAMART_RPM_DATAGOV` | Optional per-minute request budgets | Defaults of 15/minute each |
+| Variable                                   | Needed for                          | If unset                                                                                                               |
+| ------------------------------------------ | ----------------------------------- | ---------------------------------------------------------------------------------------------------------------------- |
+| `DATAGOV_API_KEY`                          | Data.gov tools                      | They return `configuration_required` (HTTP 503) with a catalog.data.gov handoff. DEMO_KEY is never used as a fallback. |
+| `DATAMART_RPM_LOC`, `DATAMART_RPM_DATAGOV` | Optional per-minute request budgets | Defaults of 15/minute each                                                                                             |
 
 Put production values in the team vault and the service environment, not a
 committed `.env`.
@@ -29,6 +29,11 @@ committed `.env`.
   stops calling loc.gov for an hour after any 429 or 503, because loc.gov restarts
   its one-hour block on every request made during it. Responses are cached
   (searches 30 min, items 6 h).
+  On 2026-10-08, right after the first deploy, loc.gov answered the production
+  host (152.53.47.37) with a browser challenge, while the same calls worked from
+  a residential IP the day before. Datamart reports `source_unavailable` and
+  pauses for an hour; it does not bypass the challenge. If it persists, ask the
+  Library of Congress to allow the host, or serve LoC from another egress.
 - **Data.gov**: a personal key allows 1,000 requests/hour. Datamart sends at most
   15/minute and pauses for an hour after a 429. Searches are cached 10 minutes.
 
diff --git a/src/datamart/http.js b/src/datamart/http.js
index 0c0535c..d039c6f 100644
--- a/src/datamart/http.js
+++ b/src/datamart/http.js
@@ -14,11 +14,7 @@ import { DatamartError } from './errors.js';
 export const USER_AGENT = 'datamart.help/0.1 (+https://datamart.help)';
 
 /** Hosts adapters may call. Anything else is refused before a socket opens. */
-export const ALLOWED_HOSTS = new Set([
-  'api.gsa.gov',
-  'www.loc.gov',
-  'geocoding.geo.census.gov',
-]);
+export const ALLOWED_HOSTS = new Set(['api.gsa.gov', 'www.loc.gov', 'geocoding.geo.census.gov']);
 
 const cache = new Map(); // key -> { expires, value }
 const MAX_CACHE_ENTRIES = 500;
@@ -72,7 +68,8 @@ function spend(provider) {
  */
 function coolDown(provider, retryAfterSeconds) {
   const floor = limitsFor(provider).cooldownMs;
-  const asked = Number.isFinite(retryAfterSeconds) && retryAfterSeconds > 0 ? retryAfterSeconds * 1000 : 0;
+  const asked =
+    Number.isFinite(retryAfterSeconds) && retryAfterSeconds > 0 ? retryAfterSeconds * 1000 : 0;
   const ms = Math.max(floor, asked);
   cooldowns.set(provider, Date.now() + ms);
   persistCooldowns();
@@ -117,7 +114,9 @@ export function resetHttpState() {
 
 function looksLikeChallenge(contentType, text) {
   if (/json/i.test(contentType)) return false;
-  return /captcha|cf-chl|challenge-platform|perfdrive|Just a moment|Access denied/i.test(text.slice(0, 4000));
+  return /captcha|cf-chl|challenge-platform|perfdrive|Just a moment|Access denied/i.test(
+    text.slice(0, 4000)
+  );
 }
 
 /**
@@ -133,10 +132,19 @@ function looksLikeChallenge(contentType, text) {
  * @param {typeof fetch} [opts.fetch]
  */
 export async function fetchJson(url, opts) {
-  const { provider, headers = {}, timeoutMs = 20000, cacheTtlMs = 0, fetch: f = globalThis.fetch } = opts;
+  const {
+    provider,
+    headers = {},
+    timeoutMs = 20000,
+    cacheTtlMs = 0,
+    fetch: f = globalThis.fetch,
+  } = opts;
   const parsed = new URL(url);
   if (parsed.protocol !== 'https:' || !ALLOWED_HOSTS.has(parsed.hostname)) {
-    throw new DatamartError('source_unavailable', `Refusing to fetch non-allowlisted host ${parsed.hostname}`);
+    throw new DatamartError(
+      'source_unavailable',
+      `Refusing to fetch non-allowlisted host ${parsed.hostname}`
+    );
   }
   const key = opts.cacheKey || url;
   if (cacheTtlMs > 0) {
@@ -156,15 +164,21 @@ export async function fetchJson(url, opts) {
     const timedOut = err?.name === 'TimeoutError' || err?.name === 'AbortError';
     throw new DatamartError(
       'source_unavailable',
-      timedOut ? `${provider} did not answer within ${timeoutMs / 1000}s` : `${provider} request failed`,
+      timedOut
+        ? `${provider} did not answer within ${timeoutMs / 1000}s`
+        : `${provider} request failed`,
       { provider }
     );
   }
 
   if (res.status >= 300 && res.status < 400) {
-    throw new DatamartError('source_unavailable', `${provider} redirected (${res.status}); not followed`, {
-      provider,
-    });
+    throw new DatamartError(
+      'source_unavailable',
+      `${provider} redirected (${res.status}); not followed`,
+      {
+        provider,
+      }
+    );
   }
   if ((COOLDOWN_STATUSES[provider] || [429]).includes(res.status)) {
     const wait = coolDown(provider, Number(res.headers.get('retry-after')));
@@ -177,28 +191,38 @@ export async function fetchJson(url, opts) {
   const contentType = res.headers.get('content-type') || '';
   const text = await res.text();
   if (looksLikeChallenge(contentType, text)) {
+    // A challenge means the provider is blocking us; keep calling and we prolong it.
+    const wait = coolDown(provider);
     throw new DatamartError(
       'source_unavailable',
-      `${provider} answered with a browser challenge instead of data; Datamart does not bypass these`,
-      { provider }
+      `${provider} answered with a browser challenge instead of data; Datamart does not bypass these and paused calls to it`,
+      { provider, retry_after_seconds: wait }
     );
   }
   if (res.status === 404) {
     throw new DatamartError('not_found', `${provider} has no such record`, { provider });
   }
   if (res.status === 401 || res.status === 403) {
-    throw new DatamartError('configuration_required', `${provider} rejected Datamart's credentials`, {
-      provider,
-    });
+    throw new DatamartError(
+      'configuration_required',
+      `${provider} rejected Datamart's credentials`,
+      {
+        provider,
+      }
+    );
   }
   if (!res.ok) {
-    throw new DatamartError('source_unavailable', `${provider} returned HTTP ${res.status}`, { provider });
+    throw new DatamartError('source_unavailable', `${provider} returned HTTP ${res.status}`, {
+      provider,
+    });
   }
   let value;
   try {
     value = JSON.parse(text);
   } catch {
-    throw new DatamartError('source_unavailable', `${provider} returned non-JSON content`, { provider });
+    throw new DatamartError('source_unavailable', `${provider} returned non-JSON content`, {
+      provider,
+    });
   }
 
   if (cacheTtlMs > 0) {
diff --git a/test/datamart/providers.test.js b/test/datamart/providers.test.js
index bd0d291..be4f0bc 100644
--- a/test/datamart/providers.test.js
+++ b/test/datamart/providers.test.js
@@ -41,12 +41,21 @@ describe('datamart http', () => {
     expect(called).to.equal(false);
   });
 
-  it('treats a challenge page as an error, not data', async () => {
-    const fetch = async () => html('<title>Radware Captcha Page</title>');
+  it('treats a challenge page as an error, not data, and pauses the provider', async () => {
+    let calls = 0;
+    const fetch = async () => {
+      calls++;
+      return html('<title>Radware Captcha Page</title>');
+    };
     await rejects(
       fetchJson('https://www.loc.gov/search/?fo=json', { provider: 'loc', fetch }),
       'source_unavailable'
     );
+    await rejects(
+      fetchJson('https://www.loc.gov/search/?fo=json&q=2', { provider: 'loc', fetch }),
+      'source_rate_limited'
+    );
+    expect(calls).to.equal(1);
   });
 
   it('does not follow redirects', async () => {