diff --git a/.github/workflows/nightly-compat.yml b/.github/workflows/nightly-compat.yml
index eabc2073b32..9515d6b2189 100644
--- a/.github/workflows/nightly-compat.yml
+++ b/.github/workflows/nightly-compat.yml
@@ -71,7 +71,20 @@ jobs:
           node-version: "26"
           cache: npm
       - uses: ./.github/actions/npm-ci-retry
+      # #8090 — this is the ONLY Node 26 build in the whole CI matrix (ci.yml pins
+      # CI_NODE_VERSION=24). It failed every nightly with the runner-reclaimed
+      # signature ("The runner has received a shutdown signal" / "The operation was
+      # canceled", no exit code) always at the same Turbopack compile phase — a
+      # classic OOM kill on the memory-constrained ubuntu-latest runner. Turbopack's
+      # native (Rust, off-V8-heap) allocation is NOT bounded by --max-old-space-size
+      # and peaks far higher than webpack on this large module graph (#6409), and is
+      # heavier still under Node 26. Use the documented webpack fallback here: it still
+      # validates that the app *builds* on Node 26 (the point of this compat job) at a
+      # much lower memory peak. Turbopack-on-Node-24 stays covered by ci.yml's build
+      # job. See docs/reference/ENVIRONMENT.md (OMNIROUTE_USE_TURBOPACK) and #6409.
       - run: npm run build
+        env:
+          OMNIROUTE_USE_TURBOPACK: "0"
 
   compat-tests:
     name: Node ${{ matrix.node }} Compat Tests (${{ matrix.shard }}/4)
diff --git a/tests/unit/nightly-compat-node26-webpack-8090.test.ts b/tests/unit/nightly-compat-node26-webpack-8090.test.ts
new file mode 100644
index 00000000000..5ae7d9caa04
--- /dev/null
+++ b/tests/unit/nightly-compat-node26-webpack-8090.test.ts
@@ -0,0 +1,64 @@
+/**
+ * Regression guard for #8090 — "nightly-compat: Node 24/26 build failures".
+ *
+ * Class 2 of the triage (the live item): the `compat-build-26` job in
+ * .github/workflows/nightly-compat.yml is the ONLY place in the entire CI matrix
+ * that runs `npm run build` on Node 26 (ci.yml pins CI_NODE_VERSION=24). Every
+ * nightly it failed with the runner-reclaimed signature — "The runner has received
+ * a shutdown signal" / "The operation was canceled", no exit code, always at the
+ * same Turbopack compile phase — the classic OOM-kill pattern on the
+ * memory-constrained ubuntu-latest runner.
+ *
+ * Turbopack's native (Rust, off-V8-heap) allocation is NOT bounded by
+ * --max-old-space-size and peaks far higher than webpack on OmniRoute's large module
+ * graph (#6409), heavier still under Node 26. The codebase's own documented escape
+ * hatch for RAM-constrained environments is the webpack fallback
+ * (OMNIROUTE_USE_TURBOPACK=0; see docs/reference/ENVIRONMENT.md). The fix wires that
+ * fallback into the Node 26 compat build so it still validates the app *builds* on
+ * Node 26 without OOMing the runner.
+ *
+ * This guard asserts the compat-build-26 job keeps the webpack fallback so a future
+ * edit cannot silently flip it back to the OOM-prone Turbopack path.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "../..");
+
+function extractJobBlock(yaml: string, jobName: string): string {
+  const lines = yaml.split("\n");
+  // Job keys live at 2-space indentation under `jobs:`.
+  const startIdx = lines.findIndex((l) => new RegExp(`^  ${jobName}:\\s*$`).test(l));
+  assert.ok(startIdx !== -1, `nightly-compat.yml must define the ${jobName} job`);
+  const block: string[] = [];
+  for (let i = startIdx + 1; i < lines.length; i++) {
+    const line = lines[i];
+    // Next 2-space-indented key ends the block (but blank lines / deeper indent stay).
+    if (/^  \S/.test(line)) break;
+    block.push(line);
+  }
+  return block.join("\n");
+}
+
+test("#8090 compat-build-26 uses the webpack fallback (OMNIROUTE_USE_TURBOPACK=0) to avoid Node 26 OOM", () => {
+  const wf = fs.readFileSync(
+    path.join(repoRoot, ".github/workflows/nightly-compat.yml"),
+    "utf-8"
+  );
+  const jobBlock = extractJobBlock(wf, "compat-build-26");
+
+  assert.match(
+    jobBlock,
+    /npm run build/,
+    "compat-build-26 must still run the production build on Node 26"
+  );
+  assert.match(
+    jobBlock,
+    /OMNIROUTE_USE_TURBOPACK:\s*["']0["']/,
+    "compat-build-26 must set OMNIROUTE_USE_TURBOPACK=0 (webpack fallback) so the sole " +
+      "Node 26 build does not OOM the runner on Turbopack's unbounded native memory (#8090/#6409)"
+  );
+});