diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b3fc00bf..13513662 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -57,6 +57,13 @@ jobs:
       - name: Verify the quoted adapter count and license wording
         run: node scripts/adapter-count.mjs --check
 
+      # The Docker image ships only hoisted dependencies; a nested one passes
+      # every test here and then crashes the container (see the script).
+      - name: Verify no backend dependency is nested out of the image
+        run: |
+          node --test scripts/check-runtime-deps.test.mjs
+          node scripts/check-runtime-deps.mjs
+
       - name: Verify catalog.ts is up-to-date with adapter JSONs
         run: |
           node scripts/regenerate-catalog.mjs
@@ -150,12 +157,36 @@ jobs:
     name: Docker build
     runs-on: ubuntu-latest
     needs: [backend, frontend]
-    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    # Every push to main, and the pull requests that can break the image
+    # without breaking a unit test (decided in the first step). js-yaml 5
+    # (#809) passed every PR check and only failed here, after the merge.
+    if: (github.event_name == 'push' && github.ref == 'refs/heads/main') || github.event_name == 'pull_request'
 
     steps:
       - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Decide whether this change can break the image
+        id: scope
+        env:
+          EVENT: ${{ github.event_name }}
+          BASE_REF: ${{ github.base_ref }}
+        run: |
+          if [ "$EVENT" != "pull_request" ]; then
+            echo "run=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          changed=$(git diff --name-only "origin/$BASE_REF...HEAD")
+          if echo "$changed" | grep -qE '(^|/)package(-lock)?\.json$|^Dockerfile$|^\.dockerignore$|^start\.sh$|^packages/backend/prisma/|^packages/backend/prisma\.config\.ts$'; then
+            echo "run=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "No dependency, Dockerfile or startup change: skipping the image boot."
+            echo "run=false" >> "$GITHUB_OUTPUT"
+          fi
 
       - name: Build unified image
+        if: steps.scope.outputs.run == 'true'
         run: docker build -t anythingmcp:ci .
 
       # Building proves the image assembles, not that it runs. v0.8.0 shipped a
@@ -163,6 +194,7 @@ jobs:
       # require resolved differently under dist/ than under src/: the image
       # built clean, every test passed, and the container crash-looped. Boot it.
       - name: Boot the image and wait for /health
+        if: steps.scope.outputs.run == 'true'
         run: |
           set -euo pipefail
           docker network create amcp-ci
diff --git a/Dockerfile b/Dockerfile
index 56779ee2..e9231607 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -47,6 +47,19 @@ RUN mkdir -p packages/frontend && \
 RUN npm install --omit=dev --network-timeout=600000 && \
     rm -rf node_modules/typescript node_modules/react-dom node_modules/react
 
+# The runner stage ships only this stage's hoisted node_modules. A package npm
+# had to nest under packages/backend/node_modules (a second version of
+# something the root also needs) would be missing there, and the backend would
+# load the root's version at runtime: js-yaml 5 crash-looped the image that way
+# (#809). Fail the build here instead. scripts/check-runtime-deps.mjs runs the
+# same check on the lockfile in CI.
+RUN nested=$(ls -A packages/backend/node_modules 2>/dev/null | grep -v '^\.bin$' || true); \
+    if [ -n "$nested" ]; then \
+      echo "Nested backend dependencies would be missing from the image:" >&2; \
+      echo "$nested" >&2; \
+      exit 1; \
+    fi
+
 # ── Stage 2: Build Backend ──────────────────────────────────────────────────
 FROM node:${NODE_VERSION} AS backend-builder
 WORKDIR /app
diff --git a/packages/backend/src/auth/server-picker.spec.ts b/packages/backend/src/auth/server-picker.spec.ts
index d2f02862..ec31d029 100644
--- a/packages/backend/src/auth/server-picker.spec.ts
+++ b/packages/backend/src/auth/server-picker.spec.ts
@@ -167,8 +167,11 @@ describe('choosing what a client may reach', () => {
     expect(pending).toBeDefined();
     expect(pending[1]).toBe('u1');
     expect(pending[2]).toMatchObject({ httpOnly: true, signed: true });
-    // The rendered form must not carry the user id anywhere.
-    expect(res.send.mock.calls[0][0]).not.toContain('u1');
+    // The rendered form must not carry the user id anywhere. The CSRF token
+    // is random base64url and contains "u1" in about 1 run out of 100, so
+    // leave it out of the search.
+    const html: string = res.send.mock.calls[0][0];
+    expect(html.replace(/name="csrf" value="[^"]*"/g, '')).not.toContain('u1');
   });
 
   it('escapes a workspace name instead of interpolating it raw', async () => {
diff --git a/scripts/check-runtime-deps.mjs b/scripts/check-runtime-deps.mjs
new file mode 100644
index 00000000..682400f8
--- /dev/null
+++ b/scripts/check-runtime-deps.mjs
@@ -0,0 +1,46 @@
+#!/usr/bin/env node
+/**
+ * Fails when a production dependency of the backend is nested under
+ * packages/backend/node_modules in package-lock.json.
+ *
+ * The Docker runtime stage copies only the hoisted /app/node_modules of the
+ * prod-deps stage into backend/node_modules. npm nests a package under the
+ * workspace when the backend needs a different version of something the root
+ * also needs. That copy never reaches the image: the backend then loads the
+ * root's version and can crash at boot, while unit tests (which resolve the
+ * nested copy) stay green. js-yaml 5 did exactly that on 2026-10-02 (#809).
+ *
+ * Fix it by aligning versions so npm can hoist one copy, or change the
+ * Dockerfile to ship nested packages (and keep the root's copy for the
+ * packages that need it) before allowing an exception here.
+ *
+ *   node scripts/check-runtime-deps.mjs            # checks ./package-lock.json
+ *   node scripts/check-runtime-deps.mjs <lockfile>
+ */
+import { readFileSync } from 'node:fs';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const NESTED = 'packages/backend/node_modules/';
+
+/** Production packages the lockfile nests under the backend workspace. */
+export function nestedRuntimeDeps(lock) {
+  return Object.entries(lock.packages ?? {})
+    .filter(([path, meta]) => path.startsWith(NESTED) && !meta.dev)
+    .map(([path, meta]) => ({ name: path.slice(NESTED.length), version: meta.version }));
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) {
+  const root = join(dirname(fileURLToPath(import.meta.url)), '..');
+  const file = process.argv[2] ?? join(root, 'package-lock.json');
+  const nested = nestedRuntimeDeps(JSON.parse(readFileSync(file, 'utf8')));
+  if (nested.length === 0) {
+    console.log('No backend production dependency is nested: the runtime image gets them all.');
+    process.exit(0);
+  }
+  console.error('::error::These backend production dependencies are nested under packages/backend/node_modules');
+  console.error('and would be missing from the Docker image (it ships only the hoisted node_modules):');
+  for (const d of nested) console.error(`  - ${d.name}@${d.version}`);
+  console.error('Align the versions so npm hoists a single copy. See scripts/check-runtime-deps.mjs.');
+  process.exit(1);
+}
diff --git a/scripts/check-runtime-deps.test.mjs b/scripts/check-runtime-deps.test.mjs
new file mode 100644
index 00000000..a2a1b7e6
--- /dev/null
+++ b/scripts/check-runtime-deps.test.mjs
@@ -0,0 +1,29 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { nestedRuntimeDeps } from './check-runtime-deps.mjs';
+
+test('flags a production package nested under the backend workspace', () => {
+  const lock = {
+    packages: {
+      '': {},
+      'node_modules/js-yaml': { version: '4.3.2' },
+      'packages/backend/node_modules/js-yaml': { version: '5.4.1' },
+      'packages/backend/node_modules/js-yaml/node_modules/argparse': { version: '2.0.1' },
+    },
+  };
+  assert.deepEqual(nestedRuntimeDeps(lock).map((d) => d.name), [
+    'js-yaml',
+    'js-yaml/node_modules/argparse',
+  ]);
+});
+
+test('ignores dev-only nested packages and hoisted ones', () => {
+  const lock = {
+    packages: {
+      'node_modules/js-yaml': { version: '4.3.2' },
+      'packages/backend/node_modules/typescript': { version: '6.0.0', dev: true },
+      'packages/frontend/node_modules/react': { version: '19.0.0' },
+    },
+  };
+  assert.deepEqual(nestedRuntimeDeps(lock), []);
+});