@@ -0,0 +1,102 @@
+import assert from "node:assert/strict";
+import { readdirSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+
+// The commands the docs tell people to run must exist: every `npm run <x>`
+// is a package.json script, and every documented `--flag` is one the engine
+// still knows. Renaming or removing a script or a flag fails here until the
+// docs follow.
+
+const REPO = fileURLToPath(new URL("..", import.meta.url));
+
+// improvements.md is a historical log: its entries describe commands and
+// flags as they were when each problem was found, and are never rewritten.
+const HISTORICAL_DOCS = new Set(["improvements.md"]);
+
+// Flags in the docs that belong to other tools (git, npm, node).
+const EXTERNAL_FLAGS = new Set(["--allow-unrelated-histories"]);
+
+function docFiles(): string[] {
+  const files = readdirSync(REPO).filter(
+    (name) => name.endsWith(".md") && !HISTORICAL_DOCS.has(name),
+  );
+  const walk = (dir: string) => {
+    for (const entry of readdirSync(join(REPO, dir), { withFileTypes: true })) {
+      const path = `${dir}/${entry.name}`;
+      if (entry.isDirectory()) walk(path);
+      else if (entry.name.endsWith(".md")) files.push(path);
+    }
+  };
+  walk("docs");
+  walk("examples");
+  return files.sort();
+}
+
+interface Mention {
+  file: string;
+  value: string;
+}
+
+function mentions(): { scripts: Mention[]; flags: Mention[] } {
+  const scripts: Mention[] = [];
+  const flags: Mention[] = [];
+  for (const file of docFiles()) {
+    const text = readFileSync(join(REPO, file), "utf8");
+    for (const line of text.split("\n")) {
+      for (const match of line.matchAll(/npm run ([a-z][a-z:-]*)/g))
+        scripts.push({ file, value: match[1]! });
+      // Flags written after `npm run …` on the same line.
+      const start = line.indexOf("npm run ");
+      if (start >= 0)
+        for (const match of line
+          .slice(start)
+          .matchAll(/(?<![\w-])(--[a-z][a-z-]*[a-z])/g))
+          flags.push({ file, value: match[1]! });
+    }
+    // Flags mentioned on their own in prose: `--overwrite`, `--resolve=ours`.
+    for (const match of text.matchAll(/`(--[a-z][a-z-]*[a-z])(?:[= ][^`]*)?`/g))
+      flags.push({ file, value: match[1]! });
+  }
+  return { scripts, flags };
+}
+
+function sourceText(): string {
+  return readdirSync(join(REPO, "src"))
+    .filter((name) => name.endsWith(".ts"))
+    .map((name) => readFileSync(join(REPO, "src", name), "utf8"))
+    .join("\n");
+}
+
+const unique = (list: Mention[]) =>
+  [...new Map(list.map((m) => [`${m.value} (${m.file})`, m])).keys()].sort();
+
+test("every `npm run <script>` in the docs is a package.json script", () => {
+  const scripts = Object.keys(
+    JSON.parse(readFileSync(join(REPO, "package.json"), "utf8")).scripts,
+  );
+  const { scripts: used } = mentions();
+  assert.ok(
+    used.length > 50,
+    `expected many npm run mentions, found ${used.length}`,
+  );
+  assert.deepEqual(unique(used.filter((m) => !scripts.includes(m.value))), []);
+});
+
+test("every documented --flag is one the engine knows", () => {
+  const source = sourceText();
+  const { flags } = mentions();
+  assert.ok(
+    flags.length > 20,
+    `expected many flag mentions, found ${flags.length}`,
+  );
+  assert.deepEqual(
+    unique(
+      flags.filter(
+        (m) => !EXTERNAL_FLAGS.has(m.value) && !source.includes(m.value),
+      ),
+    ),
+    [],
+  );
+});
@@ -0,0 +1,75 @@
+import assert from "node:assert/strict";
+import { existsSync, readdirSync, readFileSync, statSync } from "node:fs";
+import { dirname, join, normalize, relative } from "node:path";
+import test from "node:test";
+import { fileURLToPath } from "node:url";
+
+// Every relative link and #anchor in the docs must resolve. The README links
+// into a set of guides that link to each other, so renaming a file or a
+// heading would otherwise break links silently.
+
+const REPO = fileURLToPath(new URL("..", import.meta.url));
+
+function markdownFiles(): string[] {
+  const files = readdirSync(REPO).filter((name) => name.endsWith(".md"));
+  const walk = (dir: string) => {
+    for (const entry of readdirSync(join(REPO, dir), { withFileTypes: true })) {
+      const path = `${dir}/${entry.name}`;
+      if (entry.isDirectory()) walk(path);
+      else if (entry.name.endsWith(".md")) files.push(path);
+    }
+  };
+  walk("docs");
+  walk("examples");
+  return files.sort();
+}
+
+const withoutCode = (text: string) => text.replace(/```[\s\S]*?```/g, "");
+
+// GitHub's heading anchor: lowercase, punctuation dropped, spaces to dashes.
+function anchorsOf(file: string): Set<string> {
+  const text = withoutCode(readFileSync(file, "utf8"));
+  return new Set(
+    [...text.matchAll(/^#{1,6} (.*)$/gm)].map((match) =>
+      match[1]!
+        .toLowerCase()
+        .trim()
+        .replace(/[^\w\- ]+/g, "")
+        .replace(/ /g, "-"),
+    ),
+  );
+}
+
+test("every relative link and anchor in the docs resolves", () => {
+  const files = markdownFiles();
+  const broken: string[] = [];
+  for (const file of files) {
+    const text = withoutCode(readFileSync(join(REPO, file), "utf8"));
+    for (const match of text.matchAll(/\]\(([^)\s]+)\)/g)) {
+      const link = match[1]!;
+      if (/^(https?:|mailto:)/.test(link)) continue;
+      const [path, anchor] = link.split("#");
+      const target = path
+        ? normalize(join(REPO, dirname(file), decodeURIComponent(path)))
+        : join(REPO, file);
+      if (!existsSync(target)) {
+        broken.push(`${file}: ${link} (no such file)`);
+        continue;
+      }
+      if (
+        anchor &&
+        statSync(target).isFile() &&
+        target.endsWith(".md") &&
+        !anchorsOf(target).has(anchor)
+      )
+        broken.push(
+          `${file}: ${link} (no heading #${anchor} in ${relative(REPO, target)})`,
+        );
+    }
+  }
+  assert.ok(
+    files.length > 20,
+    `expected the docs tree, found ${files.length} files`,
+  );
+  assert.deepEqual(broken, []);
+});