diff --git a/src/__tests__/single-pdfjs-guard.test.ts b/src/__tests__/single-pdfjs-guard.test.ts
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+++ b/src/__tests__/single-pdfjs-guard.test.ts
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+/**
+ * Structural guard: the server runs one pdf.js, and only one module opens it.
+ *
+ * ## The failure this freezes (GitHub #1277)
+ *
+ * Under Node, pdf.js keeps its in-process worker on `globalThis.pdfjsWorker`,
+ * and whichever copy opens a document first owns it until the process exits.
+ * The local text read ran a library that pinned its own, older pdfjs-dist,
+ * while the rasterizer ran the app's. Whichever ran first broke the other
+ * with "The API version … does not match the Worker version …": after a text
+ * read every thumbnail and every vision read of a scan failed, after a render
+ * every local text read failed without a word, until a restart. Nothing in
+ * the gate noticed, because each path passed on its own and the image tracing
+ * shipped every `pdfjs-dist@*` the store held.
+ *
+ * ## What it proves and what it does not
+ *
+ *   - The lockfile resolves exactly one pdfjs-dist, the version package.json
+ *     pins, and no package in it depends on pdfjs-dist except the app itself.
+ *     A new dependency that brings its own pdf.js, even the same version,
+ *     fails here and has to be looked at; every line of the lockfile that
+ *     names pdfjs-dist is either understood or reported.
+ *   - The checkout resolves that same version from the working directory,
+ *     which is how the server finds it.
+ *   - No production module under `src/` imports pdfjs-dist except
+ *     `src/lib/documents/pdfjs.ts`, so both paths open documents with the same
+ *     options, and nothing imports pdf-parse.
+ *
+ * It does not prove the image carries one copy; the Dockerfile hoists the
+ * pinned version by name and the tracing glob can only find what the lockfile
+ * installed. The behaviour itself is pinned by the two
+ * `pdfjs-one-runtime-*` tests under `src/lib/documents/__tests__/`.
+ */
+import { createRequire } from "node:module";
+import { join } from "node:path";
+
+import { describe, expect, it } from "vitest";
+
+import { readSourceFolder, readSourceText } from "./helpers/source-files";
+
+const ROOT = process.cwd();
+/** Relative to `src/`. */
+const LOADER = "lib/documents/pdfjs.ts";
+
+const pkg = JSON.parse(readSourceText(join(ROOT, "package.json"))) as {
+  dependencies?: Record<string, string>;
+  devDependencies?: Record<string, string>;
+};
+const pinned =
+  pkg.dependencies?.["pdfjs-dist"] ?? pkg.devDependencies?.["pdfjs-dist"];
+
+/** Every lockfile line naming pdfjs-dist, sorted into what it means. */
+function readLockfile(lockfile: string): {
+  versions: Set<string>;
+  rootImporters: number;
+  unexplained: string[];
+} {
+  const lines = lockfile.split("\n");
+  const versions = new Set<string>();
+  let rootImporters = 0;
+  const unexplained: string[] = [];
+  lines.forEach((line, i) => {
+    if (!line.includes("pdfjs-dist")) return;
+    // A resolved copy: `  pdfjs-dist@6.4.299:` in `packages:` and again in
+    // `snapshots:` (where a peer suffix in parentheses may follow).
+    const resolved =
+      /^ {2}'?pdfjs-dist@(\d+\.\d+\.\d+[\w.+-]*)(?:\(.*\))?'?:/.exec(line);
+    if (resolved) {
+      versions.add(resolved[1]!);
+      return;
+    }
+    // The app's own entry under `importers:`, followed by its specifier.
+    if (
+      /^ {6}pdfjs-dist:\s*$/.test(line) &&
+      /^\s+specifier:/.test(lines[i + 1] ?? "")
+    ) {
+      rootImporters++;
+      return;
+    }
+    // Anything else is another package depending on pdfjs-dist, an
+    // override, or a spelling this reader does not know: all reported.
+    unexplained.push(`${i + 1}: ${line.trim()}`);
+  });
+  return { versions, rootImporters, unexplained };
+}
+
+/** Production modules under `src/` that import a pdf.js package. */
+function pdfjsImporters(files: Map<string, string>): string[] {
+  const specifier =
+    /(?:\bfrom\s*|\bimport\s*\(\s*|\brequire\s*\(\s*|^\s*import\s*)["'](?:pdfjs-dist|pdf-parse)(?:\/[^"']*)?["']/m;
+  return [...files]
+    .filter(([, source]) => specifier.test(source))
+    .map(([path]) => path)
+    .sort();
+}
+
+/** Production modules under `src/`, keyed by their path relative to it. */
+function productionSources(): Map<string, string> {
+  return new Map(
+    readSourceFolder("src", {
+      floor: 1000,
+      extensions: [".ts", ".tsx", ".mts", ".js", ".mjs"],
+      include: (rel) =>
+        !rel.startsWith("generated/") &&
+        !rel.includes("__tests__/") &&
+        !/\.(?:test|spec)\.[mc]?[tj]sx?$/.test(rel),
+    }),
+  );
+}
+
+describe("one pdf.js (#1277)", () => {
+  it("package.json pins pdfjs-dist exactly and lists no pdf-parse", () => {
+    expect(pinned, "pdfjs-dist is a direct dependency").toMatch(
+      /^\d+\.\d+\.\d+$/,
+    );
+    expect(pkg.dependencies?.["pdf-parse"]).toBeUndefined();
+    expect(pkg.devDependencies?.["pdf-parse"]).toBeUndefined();
+  });
+
+  it("the lockfile resolves one pdfjs-dist, and only the app depends on it", () => {
+    const lock = readLockfile(readSourceText(join(ROOT, "pnpm-lock.yaml")));
+    expect(lock.unexplained, "lockfile lines naming pdfjs-dist").toEqual([]);
+    expect([...lock.versions]).toEqual([pinned]);
+    expect(lock.rootImporters).toBe(1);
+  });
+
+  it("the checkout resolves the pinned copy", () => {
+    const anchor = createRequire(join(ROOT, "package.json"));
+    const resolved = JSON.parse(
+      readSourceText(anchor.resolve("pdfjs-dist/package.json")),
+    ) as { version: string };
+    expect(resolved.version).toBe(pinned);
+  });
+
+  it("only the shared loader imports pdf.js", () => {
+    const sources = productionSources();
+    expect(sources.has(LOADER), `${LOADER} exists`).toBe(true);
+    expect(pdfjsImporters(sources)).toEqual([LOADER]);
+  });
+
+  it("the readers catch what they exist to catch", () => {
+    const twoCopies = [
+      "importers:",
+      "  .:",
+      "    devDependencies:",
+      "      pdfjs-dist:",
+      "        specifier: 6.4.299",
+      "        version: 6.4.299",
+      "packages:",
+      "  pdfjs-dist@5.4.296:",
+      "  pdfjs-dist@6.4.299:",
+      "snapshots:",
+      "  some-parser@2.4.5:",
+      "    dependencies:",
+      "      pdfjs-dist: 5.4.296",
+    ].join("\n");
+    const lock = readLockfile(twoCopies);
+    expect([...lock.versions].sort()).toEqual(["5.4.296", "6.4.299"]);
+    expect(lock.unexplained).toEqual(["13: pdfjs-dist: 5.4.296"]);
+
+    const importers = pdfjsImporters(
+      new Map([
+        [LOADER, 'await import("pdfjs-dist/legacy/build/pdf.mjs")'],
+        ["a", 'import { PDFParse } from "pdf-parse";'],
+        ["b", "const m = await import(\n  'pdfjs-dist'\n);"],
+        ["c", 'import "pdfjs-dist/legacy/build/pdf.worker.mjs";'],
+        ["d", '// mentions "pdfjs-dist" in prose only'],
+      ]),
+    );
+    expect(importers).toEqual(["a", "b", "c", LOADER].sort());
+  });
+});