diff --git a/browse/src/find-security-sidecar.ts b/browse/src/find-security-sidecar.ts
new file mode 100644
index 000000000..0ba242523
--- /dev/null
+++ b/browse/src/find-security-sidecar.ts
@@ -0,0 +1,78 @@
+/**
+ * find-security-sidecar — resolve the Node entry that runs the L4 ML
+ * classifier sidecar.
+ *
+ * The sidecar can't be bundled into the compiled browse binary because
+ * onnxruntime-node fails to dlopen from Bun's compile extract dir. It runs
+ * as a separate Node subprocess instead. This module resolves the right
+ * path + interpreter on each platform:
+ *
+ *   1. Prefer node on PATH + a bundled JS entry at
+ *      browse/dist/security-sidecar.js (built by package.json's
+ *      build:security-sidecar script).
+ *   2. Dev fallback: node + browse/src/security-sidecar-entry.ts via tsx
+ *      (only available in the source checkout, not the compiled install).
+ *   3. If Node is missing or no entry resolves, return null. The /pty-inject-scan
+ *      endpoint then responds with l4 { available: false } and the extension
+ *      degrades to WARN+confirm (D7).
+ */
+
+import { existsSync } from "fs";
+import { join, dirname } from "path";
+import { execFileSync } from "child_process";
+
+export interface SidecarLocation {
+  node: string;
+  entry: string;
+  /** "compiled" if running from browse/dist/, "dev" if running from src */
+  mode: "compiled" | "dev";
+}
+
+function nodeOnPath(): string | null {
+  try {
+    execFileSync("node", ["--version"], { stdio: "ignore", timeout: 2000 });
+    return "node";
+  } catch {
+    return null;
+  }
+}
+
+function browseRoot(): string {
+  // When running compiled, __dirname (via import.meta.dir) points at the
+  // Bun extract temp. Walk up until we find a directory containing
+  // browse/dist/ or browse/src/.
+  let candidate = dirname(import.meta.path || "");
+  for (let i = 0; i < 6; i += 1) {
+    if (existsSync(join(candidate, "browse", "dist", "security-sidecar.js"))) {
+      return candidate;
+    }
+    if (existsSync(join(candidate, "src", "security-sidecar-entry.ts"))) {
+      return candidate;
+    }
+    const next = dirname(candidate);
+    if (next === candidate) break;
+    candidate = next;
+  }
+  return process.cwd();
+}
+
+export function findSecuritySidecar(): SidecarLocation | null {
+  const node = nodeOnPath();
+  if (!node) return null;
+
+  const root = browseRoot();
+
+  const compiled = join(root, "browse", "dist", "security-sidecar.js");
+  if (existsSync(compiled)) {
+    return { node, entry: compiled, mode: "compiled" };
+  }
+
+  // Dev fallback. Compiled installs won't have src/ on disk so this only
+  // resolves when running from the source checkout.
+  const devEntry = join(root, "src", "security-sidecar-entry.ts");
+  if (existsSync(devEntry)) {
+    return { node, entry: devEntry, mode: "dev" };
+  }
+
+  return null;
+}
diff --git a/browse/src/security-sidecar-entry.ts b/browse/src/security-sidecar-entry.ts
new file mode 100644
index 000000000..bd10285ee
--- /dev/null
+++ b/browse/src/security-sidecar-entry.ts
@@ -0,0 +1,120 @@
+/**
+ * Security sidecar entry — Node script that hosts the L4 ML classifier on
+ * behalf of the compiled browse server.
+ *
+ * Why a sidecar:
+ *   - browse/src/security-classifier.ts depends on @huggingface/transformers
+ *     which loads onnxruntime-node, a native module that fails to `dlopen`
+ *     from Bun's compile-binary temp extraction dir (CLAUDE.md "Sidebar
+ *     security stack" section). Importing the classifier into server.ts
+ *     would brick the compiled binary at startup.
+ *   - sidebar-agent.ts (the previous host of the classifier) was removed
+ *     when the PTY proved out. The classifier file still ships but had no
+ *     caller — exactly the gap codex flagged in #1370.
+ *
+ * This entry runs under plain Node (resolved by find-security-sidecar.ts).
+ * It reads NDJSON requests from stdin and writes NDJSON responses to stdout.
+ *
+ * Protocol (one JSON object per line, both directions):
+ *   request:  { id: string, op: "scan-page-content" | "ping", text?: string }
+ *   response: { id: string, ok: true, verdict: LayerSignal } |
+ *             { id: string, ok: false, error: string }
+ *
+ * Lifecycle:
+ *   - Spawned lazily by security-sidecar-client.ts on first /pty-inject-scan
+ *   - Exits when stdin closes (parent gone) — standard Node behavior
+ *   - Exits on SIGTERM cleanly
+ *
+ * Failure modes:
+ *   - Model download fails → reply { ok: false, error: "model-load" } and
+ *     keep the loop alive for the next request (caller decides whether to
+ *     retry or fail-safe to L1-L3-only)
+ */
+
+import * as readline from "readline";
+import { scanPageContent, getClassifierStatus, loadTestsavant } from "./security-classifier";
+
+interface Request {
+  id: string;
+  op: "scan-page-content" | "ping" | "status";
+  text?: string;
+}
+
+interface OkResponse {
+  id: string;
+  ok: true;
+  verdict?: unknown;
+  status?: unknown;
+}
+
+interface ErrResponse {
+  id: string;
+  ok: false;
+  error: string;
+}
+
+function write(obj: OkResponse | ErrResponse): void {
+  process.stdout.write(JSON.stringify(obj) + "\n");
+}
+
+async function handle(req: Request): Promise<void> {
+  if (!req || typeof req.id !== "string") {
+    // Drop unidentifiable requests silently — protocol invariant.
+    return;
+  }
+  try {
+    if (req.op === "ping") {
+      write({ id: req.id, ok: true, verdict: { layer: "ping", verdict: "alive", score: 0 } });
+      return;
+    }
+    if (req.op === "status") {
+      write({ id: req.id, ok: true, status: getClassifierStatus() });
+      return;
+    }
+    if (req.op === "scan-page-content") {
+      if (typeof req.text !== "string") {
+        write({ id: req.id, ok: false, error: "missing-text" });
+        return;
+      }
+      // Warm the classifier once per process; subsequent scans are fast.
+      await loadTestsavant().catch(() => {
+        // loadTestsavant degrades gracefully; scanPageContent below will
+        // return a fail-open verdict if the model never loaded.
+      });
+      const verdict = await scanPageContent(req.text);
+      write({ id: req.id, ok: true, verdict });
+      return;
+    }
+    write({ id: req.id, ok: false, error: `unknown-op:${(req as { op?: unknown }).op}` });
+  } catch (err) {
+    const msg = err instanceof Error ? err.message : String(err);
+    write({ id: req.id, ok: false, error: msg });
+  }
+}
+
+function main(): void {
+  // readline buffers stdin into one-line chunks. Stay alive until stdin
+  // closes (parent gone) — Node exits naturally then.
+  const rl = readline.createInterface({ input: process.stdin });
+  rl.on("line", (line) => {
+    if (!line.trim()) return;
+    let req: Request;
+    try {
+      req = JSON.parse(line) as Request;
+    } catch {
+      // Malformed line — write a generic error without an id, callers can
+      // detect via missing id and trip the circuit breaker.
+      write({ id: "<malformed>", ok: false, error: "malformed-json" });
+      return;
+    }
+    // Fire-and-forget; concurrent requests get id-correlated responses.
+    void handle(req);
+  });
+  rl.on("close", () => {
+    process.exit(0);
+  });
+  process.on("SIGTERM", () => process.exit(0));
+  process.on("SIGINT", () => process.exit(0));
+}
+
+main();