diff --git a/.github/actions/npm-ci-retry/action.yml b/.github/actions/npm-ci-retry/action.yml
index ba27eb694d..73766e7098 100644
--- a/.github/actions/npm-ci-retry/action.yml
+++ b/.github/actions/npm-ci-retry/action.yml
@@ -1,9 +1,45 @@
 name: npm ci with retry
-description: Run npm ci with retries for transient registry/network failures.
+description: >-
+  Install dependencies. Restores node_modules from the Actions cache when the exact
+  lockfile / runner / Node version / postinstall inputs match; otherwise runs npm ci
+  with retries for transient registry/network failures and saves the tree for the
+  next run.
+inputs:
+  cache:
+    description: Set to "false" to skip the node_modules cache and always run npm ci.
+    required: false
+    default: "true"
 runs:
   using: composite
   steps:
-    - shell: bash
+    - name: Resolve Node version for the cache key
+      id: node
+      shell: bash
+      run: echo "version=$(node --version)" >> "$GITHUB_OUTPUT"
+
+    # #8084 D3 (plan 3.8.51 task 5): every job used to pay ~80-90 s of `npm ci` even
+    # with setup-node's npm tarball cache warm — 36 jobs per ci.yml run, ~55 min of
+    # runner time per run just installing. A node_modules cache keyed on EVERYTHING
+    # that shapes the tree lets a hit skip the install entirely.
+    #
+    # No restore-keys on purpose (same rule as the ESLint cache, #11600): a partial
+    # tree from another lockfile / Node / postinstall script is exactly the kind of
+    # silent drift a lockfile-pinned CI must never inherit. Exact key or a full npm ci.
+    #
+    # postinstall (scripts/build/postinstall.mjs + helpers) only mutates node_modules
+    # on a plain install — its dist/ branch is gated on dist/ existing, which never
+    # holds at install time in CI — so the cached tree already carries its effects.
+    - name: Restore node_modules
+      id: node-modules
+      if: inputs.cache == 'true'
+      uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+      with:
+        path: node_modules
+        key: node-modules-${{ runner.os }}-${{ runner.arch }}-${{ steps.node.outputs.version }}-${{ hashFiles('package-lock.json', '.npmrc', 'scripts/build/postinstall.mjs', 'scripts/build/postinstallSupport.mjs', 'scripts/build/colocateOptionals.mjs', 'scripts/build/fixTlsClientNodeBinary.mjs', 'scripts/build/fixPlaywrightAndroid.mjs', 'scripts/build/native-binary-compat.mjs') }}
+
+    - name: npm ci (with retry)
+      if: steps.node-modules.outputs.cache-hit != 'true'
+      shell: bash
       run: |
         set -euo pipefail
 
@@ -15,7 +51,8 @@ runs:
             echo "npm ci attempt $attempt/$max_attempts after transient failure"
           fi
 
-          if npm ci; then
+          # --no-audit: `audit:deps` is its own gate; the inline audit only adds latency.
+          if npm ci --no-audit --no-fund; then
             exit 0
           fi
 
@@ -27,3 +64,8 @@ runs:
           sleep "$delay_seconds"
           delay_seconds=$((delay_seconds * 2))
         done
+
+    - name: node_modules restored from cache
+      if: steps.node-modules.outputs.cache-hit == 'true'
+      shell: bash
+      run: echo "node_modules restored from cache (key hit) — npm ci skipped"
diff --git a/tests/unit/build/npm-ci-retry-composite.test.ts b/tests/unit/build/npm-ci-retry-composite.test.ts
new file mode 100644
index 0000000000..4823eac1bb
--- /dev/null
+++ b/tests/unit/build/npm-ci-retry-composite.test.ts
@@ -0,0 +1,90 @@
+/**
+ * .github/actions/npm-ci-retry — node_modules cache contract (#8084 D3, plan 3.8.51 task 5).
+ *
+ * Every CI job installs through this composite (36× per ci.yml run, ~80-90 s each with only
+ * the npm tarball cache). The node_modules cache must (a) key on everything that shapes the
+ * tree, (b) never fall back to a partial tree from another key (#11600 rule), and (c) keep
+ * the retry loop as the miss path. Pin those so a later "simplification" cannot reopen it.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+import { parse } from "yaml";
+
+const ACTION = path.resolve(
+  import.meta.dirname,
+  "../../../.github/actions/npm-ci-retry/action.yml"
+);
+const raw = fs.readFileSync(ACTION, "utf8");
+const action = parse(raw) as {
+  runs: { using: string; steps: Array<Record<string, unknown>> };
+  inputs?: Record<string, { default?: string }>;
+};
+
+const step = (id: string) =>
+  action.runs.steps.find((s) => s.id === id) as Record<string, unknown> | undefined;
+
+test("composite restores node_modules via actions/cache with an exact, fully-qualified key", () => {
+  const cache = step("node-modules");
+  assert.ok(cache, "missing restore step with id node-modules");
+  assert.match(String(cache!.uses), /^actions\/cache@/);
+  const w = cache!.with as Record<string, string>;
+  assert.equal(w.path, "node_modules");
+  for (const input of [
+    "runner.os",
+    "runner.arch",
+    "steps.node.outputs.version",
+    "package-lock.json",
+    ".npmrc",
+  ]) {
+    assert.ok(w.key.includes(input), `cache key must include ${input}`);
+  }
+  // Every postinstall script that mutates node_modules must be part of the key.
+  for (const script of [
+    "scripts/build/postinstall.mjs",
+    "scripts/build/postinstallSupport.mjs",
+    "scripts/build/colocateOptionals.mjs",
+    "scripts/build/fixTlsClientNodeBinary.mjs",
+    "scripts/build/fixPlaywrightAndroid.mjs",
+    "scripts/build/native-binary-compat.mjs",
+  ]) {
+    assert.ok(w.key.includes(script), `cache key must include ${script}`);
+    assert.ok(
+      fs.existsSync(path.resolve(import.meta.dirname, "../../..", script)),
+      `${script} vanished — update the key`
+    );
+  }
+  assert.equal(
+    w["restore-keys"],
+    undefined,
+    "no restore-keys: exact key or a full npm ci (#11600)"
+  );
+});
+
+test("npm ci is the cache-miss path and still retries", () => {
+  const install = action.runs.steps.find((s) => String(s.name).startsWith("npm ci"));
+  assert.ok(install);
+  assert.equal(install!.if, "steps.node-modules.outputs.cache-hit != 'true'");
+  assert.match(String(install!.run), /max_attempts=3/);
+  assert.match(String(install!.run), /npm ci --no-audit --no-fund/);
+});
+
+test("cache can be disabled per caller and defaults on", () => {
+  assert.equal(action.inputs?.cache?.default, "true");
+  assert.equal(step("node-modules")!.if, "inputs.cache == 'true'");
+});
+
+test("every postinstall helper imported by postinstall.mjs is in the cache key", () => {
+  const post = fs.readFileSync(
+    path.resolve(import.meta.dirname, "../../../scripts/build/postinstall.mjs"),
+    "utf8"
+  );
+  const imports = [...post.matchAll(/from "\.\/([a-zA-Z-]+\.mjs)"/g)].map(
+    (m) => `scripts/build/${m[1]}`
+  );
+  assert.ok(imports.length >= 4, "expected postinstall.mjs to import its helpers");
+  const key = (step("node-modules")!.with as Record<string, string>).key;
+  for (const imp of imports)
+    assert.ok(key.includes(imp), `postinstall imports ${imp} but the cache key omits it`);
+});