diff --git a/server/package.json b/server/package.json
index 646541042a..2219c6c833 100644
--- a/server/package.json
+++ b/server/package.json
@@ -35,7 +35,7 @@
     "dev": "tsx src/index.ts",
     "dev:watch": "cross-env PAPERCLIP_MIGRATION_PROMPT=never PAPERCLIP_MIGRATION_AUTO_APPLY=true tsx ./scripts/dev-watch.ts",
     "prepare:ui-dist": "bash ../scripts/prepare-server-ui-dist.sh",
-    "build": "pnpm run prepare:runner-vendor && tsc && mkdir -p dist/onboarding-assets dist/built-ins dist/services/scripts dist/vendor/paperclip-runner && cp -R src/onboarding-assets/. dist/onboarding-assets/ && cp -R src/built-ins/. dist/built-ins/ && cp -R src/services/scripts/. dist/services/scripts/ && cp -R ../packages/paperclip-runner/dist/. dist/vendor/paperclip-runner/ && node scripts/write-build-stamp.mjs",
+    "build": "pnpm run prepare:runner-vendor && node scripts/verify-runner-vendor-dependencies.mjs && tsc && mkdir -p dist/onboarding-assets dist/built-ins dist/services/scripts dist/vendor/paperclip-runner && cp -R src/onboarding-assets/. dist/onboarding-assets/ && cp -R src/built-ins/. dist/built-ins/ && cp -R src/services/scripts/. dist/services/scripts/ && cp -R ../packages/paperclip-runner/dist/. dist/vendor/paperclip-runner/ && node scripts/write-build-stamp.mjs",
     "prepack": "pnpm run prepare:ui-dist && pnpm run build",
     "postpack": "rm -rf ui-dist",
     "clean": "rm -rf dist",
@@ -88,6 +88,7 @@
     "pino-http": "^11.0.0",
     "pino-pretty": "^13.1.3",
     "sharp": "^0.35.4",
+    "smol-toml": "^1.4.2",
     "ssh2": "^1.17.0",
     "ws": "^8.21.3",
     "zod": "^4.4.3"
diff --git a/server/scripts/verify-runner-vendor-dependencies.mjs b/server/scripts/verify-runner-vendor-dependencies.mjs
new file mode 100644
index 0000000000..16634ae755
--- /dev/null
+++ b/server/scripts/verify-runner-vendor-dependencies.mjs
@@ -0,0 +1,145 @@
+// Verify every npm package the vendored paperclip-runner actually imports
+// at runtime is also declared as a direct dependency of server/package.json.
+//
+// packages/paperclip-runner is private and never published, so the server
+// build vendors its compiled dist/ tree wholesale with `cp -R` (see the
+// `build` script's `... dist/vendor/paperclip-runner/` step) -- code only,
+// no node_modules alongside it. Every npm package the vendored code
+// actually imports is therefore also a runtime dependency of server once
+// vendored, and has to be declared there too (see acpx, ajv). That mirror
+// step is easy to forget -- it silently missed smol-toml in #13110, and CI
+// stayed green while production crash-looped 3 seconds into every start
+// (#13116) -- because nothing enforced it.
+//
+// This derives the required set from esbuild's own module-resolution scan
+// of the two entry points server actually imports (index.js, testing.js),
+// with `write: false` so nothing is written to disk and `packages:
+// "external"` so npm imports are reported, not inlined. That is precise:
+// paperclip-runner declares dependencies (react-markdown, the codex/opencode
+// CLI packages, ...) that only its unrelated ./react and ./browser export
+// subpaths use, which server never imports, so requiring *every* declared
+// dependency to be mirrored would be over-broad and demand dependencies
+// server does not actually need.
+//
+// This intentionally only analyzes the module graph; it does not bundle or
+// rewrite anything on disk. Several runner modules resolve sibling build
+// artifacts -- the native runnerd binary, the ACPX/OpenCode CJS sidecar
+// scripts in dist/cli, JSON replay fixtures under a top-level protocol/
+// directory -- via `import.meta.url`-relative filesystem paths rather than
+// JS imports, each at whatever nesting depth its source file happens to
+// sit at. Actually bundling those entry points (an earlier version of this
+// fix did) collapses and rearranges that layout, silently breaking those
+// lookups. Preserving the original `cp -R` tree 1:1 is what keeps all of
+// them working, so this script only verifies; it never restructures.
+
+import { build } from "esbuild";
+import { existsSync, readFileSync } from "node:fs";
+import { builtinModules } from "node:module";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath, pathToFileURL } from "node:url";
+
+const serverRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const runnerRoot = resolve(serverRoot, "../packages/paperclip-runner");
+const runnerDist = resolve(runnerRoot, "dist");
+
+// The only entry points server/src actually imports from the vendored
+// runner (server/src/**/*.ts import "../vendor/paperclip-runner/index.js"
+// or ".../testing.js").
+const ENTRY_POINT_NAMES = ["index.js", "testing.js"];
+
+const NODE_BUILTINS = new Set([
+  ...builtinModules,
+  ...builtinModules.map((name) => `node:${name}`),
+]);
+
+/** The npm package name a bare import specifier resolves to, honoring scoped packages and subpaths. */
+function packageNameFromSpecifier(specifier) {
+  const segments = specifier.split("/");
+  return specifier.startsWith("@") ? segments.slice(0, 2).join("/") : segments[0];
+}
+
+export function findMissingVendorDependencies(importedPackageNames, declaredDependencyNames) {
+  return [...importedPackageNames]
+    .filter((name) => !declaredDependencyNames.has(name))
+    .sort();
+}
+
+export async function findRunnerExternalPackages(entryPoints) {
+  for (const entryPoint of entryPoints) {
+    if (!existsSync(entryPoint)) {
+      throw new Error(
+        `paperclip-runner vendor check: expected build output at ${entryPoint}. ` +
+          `Run "pnpm --filter @paperclipai/paperclip-runner build" first.`,
+      );
+    }
+  }
+
+  // write: false means this never touches disk -- it's a module-graph scan,
+  // not a real bundle. Vendoring itself still happens via `cp -R` elsewhere
+  // in the build script. `outdir` is required by esbuild for multiple entry
+  // points but nothing is ever written there, so any sibling path will do.
+  const result = await build({
+    entryPoints,
+    outdir: resolve(dirname(entryPoints[0]), ".vendor-dependency-scan"),
+    bundle: true,
+    write: false,
+    platform: "node",
+    format: "esm",
+    packages: "external",
+    metafile: true,
+    logLevel: "silent",
+  });
+
+  const externalPackageNames = new Set();
+  for (const output of Object.values(result.metafile.outputs)) {
+    for (const imported of output.imports) {
+      if (!imported.external) continue;
+      if (imported.path.startsWith(".") || NODE_BUILTINS.has(imported.path)) continue;
+      externalPackageNames.add(packageNameFromSpecifier(imported.path));
+    }
+  }
+  return externalPackageNames;
+}
+
+function readDependencyNames(packageJsonPath) {
+  const packageJson = JSON.parse(readFileSync(packageJsonPath, "utf8"));
+  return new Map(Object.entries(packageJson.dependencies ?? {}));
+}
+
+function explainMissingDependencies(missing, runnerDependencyNames) {
+  const lines = missing.map((name) => {
+    const range = runnerDependencyNames.get(name);
+    return range
+      ? `  "${name}": "${range}"  (matches packages/paperclip-runner/package.json)`
+      : `  "${name}"  (not declared as a paperclip-runner dependency either -- check for a missing or mistyped dependency there first)`;
+  });
+  return (
+    `paperclip-runner vendor check: server/package.json is missing the runtime ` +
+    `${missing.length === 1 ? "dependency" : "dependencies"} the vendored runner ` +
+    `imports at runtime:\n${lines.join("\n")}\n\n` +
+    `packages/paperclip-runner/dist is copied into server's own published package ` +
+    `without its node_modules, so every npm package the runner imports must also be ` +
+    `a direct dependency of server so it resolves once vendored. Add ` +
+    `${missing.length === 1 ? "it" : "them"} to server/package.json's "dependencies".`
+  );
+}
+
+async function main() {
+  const entryPoints = ENTRY_POINT_NAMES.map((name) => resolve(runnerDist, name));
+  const externalPackageNames = await findRunnerExternalPackages(entryPoints);
+  const serverDependencyNames = readDependencyNames(resolve(serverRoot, "package.json"));
+
+  const missing = findMissingVendorDependencies(
+    externalPackageNames,
+    new Set(serverDependencyNames.keys()),
+  );
+  if (missing.length > 0) {
+    const runnerDependencyNames = readDependencyNames(resolve(runnerRoot, "package.json"));
+    throw new Error(explainMissingDependencies(missing, runnerDependencyNames));
+  }
+}
+
+// Only run when invoked directly (`node scripts/verify-runner-vendor-dependencies.mjs`),
+// not when the vitest suite imports findMissingVendorDependencies for a unit test.
+const isMain = process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href;
+if (isMain) await main();
diff --git a/server/scripts/verify-runner-vendor-dependencies.test.mjs b/server/scripts/verify-runner-vendor-dependencies.test.mjs
new file mode 100644
index 0000000000..07bbbcef3b
--- /dev/null
+++ b/server/scripts/verify-runner-vendor-dependencies.test.mjs
@@ -0,0 +1,98 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { afterEach, describe, expect, it } from "vitest";
+
+import {
+  findMissingVendorDependencies,
+  findRunnerExternalPackages,
+} from "./verify-runner-vendor-dependencies.mjs";
+
+describe("findMissingVendorDependencies", () => {
+  it("returns nothing when every runner dependency is already declared on server", () => {
+    const missing = findMissingVendorDependencies(
+      new Set(["acpx", "ajv", "smol-toml"]),
+      new Set(["acpx", "ajv", "smol-toml", "express"]),
+    );
+
+    expect(missing).toEqual([]);
+  });
+
+  it("flags a runner dependency that isn't mirrored into server/package.json", () => {
+    // This is the exact shape of the incident this check exists to catch:
+    // packages/paperclip-runner/package.json grew a new runtime dependency
+    // (smol-toml) that never got mirrored into server/package.json, so the
+    // vendored `cp -R` copy failed to resolve it at runtime (#13110, #13116).
+    const missing = findMissingVendorDependencies(
+      new Set(["acpx", "ajv", "smol-toml"]),
+      new Set(["acpx", "ajv"]),
+    );
+
+    expect(missing).toEqual(["smol-toml"]);
+  });
+
+  it("sorts multiple missing dependencies for a stable error message", () => {
+    const missing = findMissingVendorDependencies(
+      new Set(["smol-toml", "ajv-formats", "acpx"]),
+      new Set(),
+    );
+
+    expect(missing).toEqual(["acpx", "ajv-formats", "smol-toml"]);
+  });
+});
+
+describe("findRunnerExternalPackages", () => {
+  // Fixture-level coverage for the actual esbuild scan, not just the diff
+  // function: a real dist/index.js + dist/testing.js on disk, structurally
+  // matching packages/paperclip-runner's shape (testing.js re-exports
+  // index.js, which imports another local module that imports a bare npm
+  // specifier), plus package.json dependency noise that should be ignored
+  // because nothing reachable from these entry points imports it.
+  let fixtureDir;
+
+  afterEach(() => {
+    if (fixtureDir) rmSync(fixtureDir, { recursive: true, force: true });
+    fixtureDir = undefined;
+  });
+
+  function writeFixture() {
+    fixtureDir = mkdtempSync(join(tmpdir(), "paperclip-runner-vendor-fixture-"));
+    writeFileSync(
+      join(fixtureDir, "internal.js"),
+      'import { parse } from "smol-toml";\n' +
+        "export function parseSomething(text) { return parse(text); }\n",
+    );
+    writeFileSync(
+      join(fixtureDir, "index.js"),
+      'export * from "./internal.js";\nexport const marker = "index";\n',
+    );
+    writeFileSync(
+      join(fixtureDir, "testing.js"),
+      'export * from "./index.js";\nexport const testingMarker = "testing";\n',
+    );
+    return [join(fixtureDir, "index.js"), join(fixtureDir, "testing.js")];
+  }
+
+  it("reports only the npm packages actually reachable from the entry points", async () => {
+    const entryPoints = writeFixture();
+
+    const externalPackageNames = await findRunnerExternalPackages(entryPoints);
+
+    // smol-toml is reachable through internal.js -> index.js -> testing.js
+    // and must be reported. Nothing else was imported anywhere in the
+    // fixture, so this also proves the scan doesn't fall back to "every
+    // dependency the package declares" (which is what made the check
+    // over-broad before -- see the module header).
+    expect(externalPackageNames).toEqual(new Set(["smol-toml"]));
+  });
+
+  it("throws a clear, actionable error when an entry point is missing", async () => {
+    fixtureDir = mkdtempSync(join(tmpdir(), "paperclip-runner-vendor-fixture-"));
+    const missingEntryPoint = join(fixtureDir, "index.js");
+
+    await expect(findRunnerExternalPackages([missingEntryPoint])).rejects.toThrow(
+      /expected build output at .*index\.js.*Run "pnpm --filter @paperclipai\/paperclip-runner build" first/s,
+    );
+  });
+});
diff --git a/server/src/__tests__/server-package-build-script.test.ts b/server/src/__tests__/server-package-build-script.test.ts
index 9e142cdcb0..b4cfe4ff38 100644
--- a/server/src/__tests__/server-package-build-script.test.ts
+++ b/server/src/__tests__/server-package-build-script.test.ts
@@ -62,6 +62,22 @@ describe("server package build script", () => {
     );
   });
 
+  it("verifies vendored runner dependencies are mirrored before building", () => {
+    const packageJson = JSON.parse(readFileSync(packageJsonPath, "utf8")) as {
+      scripts?: Record<string, string>;
+    };
+
+    // See scripts/verify-runner-vendor-dependencies.mjs: packages/paperclip-runner
+    // is vendored with a raw `cp -R` of its compiled dist/, so every runtime
+    // dependency it imports must also be a direct dependency of server. This
+    // check derives that requirement from an esbuild scan of the vendored
+    // entry points instead of relying on a human to have kept a hand-copied
+    // list in sync (the smol-toml incident in #13110/#13116).
+    expect(packageJson.scripts?.build).toContain(
+      "node scripts/verify-runner-vendor-dependencies.mjs",
+    );
+  });
+
   it("loads runner source when the source server starts before workspace builds", () => {
     const shim = readFileSync(runnerShimPath, "utf8");
 
diff --git a/server/vitest.config.ts b/server/vitest.config.ts
index f87ac41efe..239bce1bc3 100644
--- a/server/vitest.config.ts
+++ b/server/vitest.config.ts
@@ -15,7 +15,7 @@ export default defineConfig({
   },
   test: {
     environment: "node",
-    include: ["src/**/*.test.ts"],
+    include: ["src/**/*.test.ts", "scripts/**/*.test.mjs"],
     // Each server suite boots + tears down its own embedded Postgres in
     // beforeAll/afterAll. Under the loaded serial shard (maxWorkers=1) the
     // graceful shutdown can occasionally cross vitest's default 10s hookTimeout,