diff --git a/.markdown-link-check.json b/.markdown-link-check.json
index 28893e3f..fcd24c93 100644
--- a/.markdown-link-check.json
+++ b/.markdown-link-check.json
@@ -22,6 +22,5 @@
   "timeout": "20s",
   "retryOn429": true,
   "retryCount": 3,
-  "fallbackHttpStatus": [200, 206, 429],
   "aliveStatusCodes": [200, 206]
 }
diff --git a/tests/markdown-link-check-config.test.mjs b/tests/markdown-link-check-config.test.mjs
new file mode 100644
index 00000000..c38b465b
--- /dev/null
+++ b/tests/markdown-link-check-config.test.mjs
@@ -0,0 +1,162 @@
+// Guards .markdown-link-check.json, the configuration `npm run check:links`
+// hands to markdown-link-check. Nothing else in the suite reads that file.
+//
+// It fails silently by construction. markdown-link-check does not pass the
+// parsed config object through to the checker: its CLI copies a fixed list of
+// keys onto `opts` and drops everything else, with no warning and no non-zero
+// exit. A key that is misspelled, renamed by a dependency bump, or invented
+// outright is therefore indistinguishable from one that works — the run stays
+// green and the setting simply does not apply.
+//
+// That is how `fallbackHttpStatus` survived in this file: it sat next to
+// `retryOn429`/`retryCount` reading as the clause that tolerates a rate-limited
+// host, while liveness was decided solely by `aliveStatusCodes`. The cost of
+// the mistake is not a crash, it is a config file that documents a guarantee
+// the tool is not providing.
+//
+// The accepted key set below is derived from the installed CLI source rather
+// than hard-coded, so bumping the markdown-link-check devDependency past a
+// rename or removal fails here instead of quietly disabling a setting.
+
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import test from 'node:test';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+
+const read = (relativePath) =>
+  readFileSync(join(repoRoot, relativePath), 'utf8');
+
+const CONFIG_PATH = '.markdown-link-check.json';
+const CLI_PATH = 'node_modules/markdown-link-check/markdown-link-check';
+
+// Every config property the CLI assigns onto its options object. That is the
+// shape every honoured setting is read through (`opts.x = config.x`), so the
+// resulting set is exactly the keys the tool consults; anything else in the
+// JSON is inert.
+//
+// Method calls are excluded: the CLI also does `config.trim()` on the
+// `--config` argument itself, which is a string, not a setting.
+export function acceptedKeys(cliSource) {
+  const keys = new Set();
+  for (const match of cliSource.matchAll(/=\s*config\.([A-Za-z_$][\w$]*)/g)) {
+    const next = cliSource
+      .slice(match.index + match[0].length)
+      .match(/^\s*(.)/);
+    if (next?.[1] === '(') continue;
+    keys.add(match[1]);
+  }
+  return keys;
+}
+
+const accepted = acceptedKeys(read(CLI_PATH));
+const config = JSON.parse(read(CONFIG_PATH));
+
+test('the installed CLI exposes a recognisable config key set', () => {
+  // Fails if markdown-link-check stops reading keys off a `config` object —
+  // at which point the derivation above is measuring nothing and every other
+  // assertion in this file is vacuously true.
+  assert.ok(
+    accepted.size >= 5,
+    `expected to recover the config keys markdown-link-check reads from ${CLI_PATH}, found ${accepted.size}: ${[...accepted].join(', ') || '(none)'}. If the CLI was restructured, update acceptedKeys() to match.`,
+  );
+  for (const key of ['ignorePatterns', 'aliveStatusCodes', 'retryOn429']) {
+    assert.ok(
+      accepted.has(key),
+      `${CLI_PATH} no longer reads config.${key}; the option was renamed or removed upstream`,
+    );
+  }
+});
+
+test('every key in .markdown-link-check.json is one the installed CLI reads', () => {
+  const ignored = Object.keys(config).filter((key) => !accepted.has(key));
+  assert.deepEqual(
+    ignored,
+    [],
+    `${CONFIG_PATH} declares keys markdown-link-check never reads, so they have no effect on 'npm run check:links': ${ignored.join(', ')}. Remove them, or correct them to one of: ${[...accepted].sort().join(', ')}.`,
+  );
+});
+
+test('acceptedKeys recovers assignments and ignores unrelated text', () => {
+  const recovered = acceptedKeys(
+    [
+      'opts.timeout = config.timeout;',
+      'opts.reporters = config.reporters ?? opts.reporters;',
+      'input.opts.config = config.trim();',
+      'if (config.notAssigned) { ... }',
+      'const x = configuration.spelled;',
+    ].join('\n'),
+  );
+  assert.ok(recovered.has('timeout'));
+  assert.ok(recovered.has('reporters'));
+  // A method call on the config argument is not a setting.
+  assert.ok(!recovered.has('trim'));
+  // A bare read that never reaches opts does not make the key honoured.
+  assert.ok(!recovered.has('notAssigned'));
+  // `configuration.spelled` must not be mistaken for `config.spelled`.
+  assert.ok(!recovered.has('spelled'));
+});
+
+test('every ignorePatterns entry carries a pattern that compiles as a RegExp', () => {
+  const patterns = config.ignorePatterns ?? [];
+  assert.ok(
+    Array.isArray(patterns) && patterns.length > 0,
+    `${CONFIG_PATH}: ignorePatterns must be a non-empty array`,
+  );
+  for (const entry of patterns) {
+    assert.equal(
+      typeof entry?.pattern,
+      'string',
+      `${CONFIG_PATH}: each ignorePatterns entry needs a string 'pattern', got ${JSON.stringify(entry)}`,
+    );
+    // markdown-link-check builds `new RegExp(entry.pattern)` per link; an
+    // uncompilable pattern throws mid-run rather than failing a link.
+    assert.doesNotThrow(
+      () => new RegExp(entry.pattern),
+      `${CONFIG_PATH}: ignorePatterns entry ${JSON.stringify(entry.pattern)} is not a valid regular expression`,
+    );
+  }
+});
+
+test('aliveStatusCodes is a non-empty list of HTTP status codes', () => {
+  const codes = config.aliveStatusCodes;
+  assert.ok(
+    Array.isArray(codes) && codes.length > 0,
+    `${CONFIG_PATH}: aliveStatusCodes decides which responses count as a working link; an empty or missing list marks every link dead`,
+  );
+  for (const code of codes) {
+    assert.ok(
+      Number.isInteger(code) && code >= 100 && code <= 599,
+      `${CONFIG_PATH}: aliveStatusCodes entry ${JSON.stringify(code)} is not an HTTP status code`,
+    );
+  }
+});
+
+test('retryOn429 is paired with a retryCount that permits a retry', () => {
+  if (config.retryOn429 !== true) return;
+  assert.ok(
+    Number.isInteger(config.retryCount) && config.retryCount >= 1,
+    `${CONFIG_PATH}: retryOn429 is on but retryCount is ${JSON.stringify(config.retryCount)}; link-check only retries while attempts < retryCount, so the retry never happens`,
+  );
+});
+
+test('timeout is a duration string link-check can parse', () => {
+  if (config.timeout === undefined) return;
+  assert.match(
+    config.timeout,
+    /^\d+(?:\.\d+)?\s*(?:ms|s|m|h)$/,
+    `${CONFIG_PATH}: timeout is passed to ms(); ${JSON.stringify(config.timeout)} is not a duration it parses`,
+  );
+});
+
+test('the config is pinned to the repository root', () => {
+  // The package script passes `--config .markdown-link-check.json` relative to
+  // the repo root; a moved file would be silently ignored by the CLI.
+  const { scripts } = JSON.parse(read('package.json'));
+  assert.ok(
+    Object.values(scripts).some((script) => script.includes(CONFIG_PATH)),
+    `no package script references ${CONFIG_PATH}; if check:links stopped passing --config, the settings in this file no longer apply`,
+  );
+});