diff --git a/scripts/build/assembleStandalone.mjs b/scripts/build/assembleStandalone.mjs
index 464d712045..7df83d1444 100644
--- a/scripts/build/assembleStandalone.mjs
+++ b/scripts/build/assembleStandalone.mjs
@@ -83,6 +83,15 @@ const NATIVE_ASSET_ENTRIES = [
     src: ["node_modules", "better-sqlite3", "build"],
     dest: ["node_modules", "better-sqlite3", "build"],
   },
+  {
+    // TPROXY IP_TRANSPARENT addon (Fase 3 / Epic A). Built by build-tproxy-native
+    // before assembly; Linux-only + opt-in, so the source is absent on non-Linux
+    // builds → syncNativeAssetsToDir skips it gracefully. The runtime loader
+    // (transparentSocket.ts) resolves it cwd-relative to this same dest.
+    label: "TPROXY transparent-socket addon (Linux-only, opt-in)",
+    src: ["src", "mitm", "tproxy", "native", "build", "Release", "transparent.node"],
+    dest: ["src", "mitm", "tproxy", "native", "build", "Release", "transparent.node"],
+  },
 ];
 
 /** @type {{label:string, src:string[], dest:string[]}[]} */
diff --git a/scripts/build/build-next-isolated.mjs b/scripts/build/build-next-isolated.mjs
index 071517d76f..323b428409 100644
--- a/scripts/build/build-next-isolated.mjs
+++ b/scripts/build/build-next-isolated.mjs
@@ -209,6 +209,25 @@ export async function main() {
         );
       }
 
+      // Best-effort: build the TPROXY native addon (Linux-only, opt-in) BEFORE
+      // assembling, so its transparent.node is present for assembleStandalone's
+      // NATIVE_ASSET_ENTRIES copy. Non-Linux / no-toolchain is non-fatal — the
+      // capture mode degrades gracefully when the addon is absent.
+      try {
+        const { buildTproxyNative } = await import("./build-tproxy-native.mjs");
+        const res = buildTproxyNative(projectRoot);
+        console.log(
+          res.built
+            ? "[build-next-isolated] Built TPROXY native addon (transparent.node)"
+            : `[build-next-isolated] TPROXY native addon skipped: ${res.reason}`
+        );
+      } catch (nativeErr) {
+        console.warn(
+          "[build-next-isolated] Non-fatal error building TPROXY native addon:",
+          nativeErr?.message
+        );
+      }
+
       try {
         console.log(
           "[build-next-isolated] Assembling standalone bundle (static + public + natives + extras)..."
diff --git a/scripts/build/build-tproxy-native.mjs b/scripts/build/build-tproxy-native.mjs
new file mode 100644
index 0000000000..ab7f082f4a
--- /dev/null
+++ b/scripts/build/build-tproxy-native.mjs
@@ -0,0 +1,54 @@
+/**
+ * Best-effort build of the TPROXY IP_TRANSPARENT native addon so the production
+ * build can copy `build/Release/transparent.node` into the standalone bundle
+ * (assembleStandalone's NATIVE_ASSET_ENTRIES). Called from build-next-isolated.mjs
+ * before the standalone is assembled.
+ *
+ * IP_TRANSPARENT is Linux-only, so this is a no-op everywhere else. A missing C
+ * toolchain is NOT fatal — the TPROXY capture mode degrades gracefully when the
+ * addon is absent (transparentSocket.ts returns "unavailable"). Every effectful
+ * seam (platform/run/exists) is injectable so the decision logic is unit-testable.
+ *
+ * Hard Rule #13: the command + args are a fixed allowlist (no interpolation of
+ * external/runtime values); `cwd` is derived from `projectRoot`, never user input.
+ */
+import { execFileSync } from "node:child_process";
+import { existsSync } from "node:fs";
+import path from "node:path";
+
+/**
+ * @param {string} projectRoot
+ * @param {{ platform?: string, run?: (cmd:string, args:string[], cwd:string) => void,
+ *           exists?: (p:string) => boolean }} [opts]
+ * @returns {{ built: boolean, reason?: string }}
+ */
+export function buildTproxyNative(projectRoot, opts = {}) {
+  const platform = opts.platform ?? process.platform;
+  const run = opts.run ?? defaultRun;
+  const exists = opts.exists ?? existsSync;
+
+  if (platform !== "linux") {
+    return { built: false, reason: "non-linux host (IP_TRANSPARENT is Linux-only)" };
+  }
+
+  const nativeDir = path.join(projectRoot, "src", "mitm", "tproxy", "native");
+  if (!exists(path.join(nativeDir, "binding.gyp"))) {
+    return { built: false, reason: "native sources absent (binding.gyp not found)" };
+  }
+
+  const out = path.join(nativeDir, "build", "Release", "transparent.node");
+  try {
+    run("npx", ["--yes", "node-gyp", "rebuild"], nativeDir);
+  } catch (err) {
+    return { built: false, reason: `toolchain/build failed: ${err?.message ?? String(err)}` };
+  }
+  if (!exists(out)) {
+    return { built: false, reason: "node-gyp produced no transparent.node" };
+  }
+  return { built: true };
+}
+
+/** @type {(cmd: string, args: string[], cwd: string) => void} */
+function defaultRun(cmd, args, cwd) {
+  execFileSync(cmd, args, { cwd, stdio: "inherit" });
+}
diff --git a/src/mitm/tproxy/native/README.md b/src/mitm/tproxy/native/README.md
index cbf3f034f8..34ee35fed2 100644
--- a/src/mitm/tproxy/native/README.md
+++ b/src/mitm/tproxy/native/README.md
@@ -26,16 +26,30 @@ validated against the same kernel (see PR #4139).
 npm run build:native:tproxy      # -> build/Release/transparent.node
 ```
 
-`build/` and `prebuilds/` are git-ignored. Distribution via per-platform
-prebuilds (linux-x64 / linux-arm64) is a follow-up — IP_TRANSPARENT is
-Linux-only, so only Linux prebuilds are needed; everywhere else the loader
-returns "unavailable".
+`build/` and `prebuilds/` are git-ignored — the binary is **built, never
+committed**.
 
-## Remaining for the full Epic A (gated follow-ups)
+## Distribution (wired into the production build)
 
-- The TPROXY listener that adopts the fd, terminates TLS, and feeds the capture
-  buffer (reusing the MITM path).
-- `repairMitm()` calling `revertTproxy()` so a crash flushes the mangle rules.
-- The capture-mode route + Traffic Inspector UI tab.
-- A live end-to-end intercept (TPROXY → listener) in a dedicated test
-  environment (needs a routing scenario; risky on a production proxy).
+1. **`scripts/build/build-tproxy-native.mjs`** runs `node-gyp rebuild` during
+   `npm run build` (called from `build-next-isolated.mjs` before the standalone is
+   assembled). Linux-only; a missing toolchain is **non-fatal** (the capture mode
+   degrades gracefully). The Docker builder already ships `python3 make g++`.
+2. **`assembleStandalone.mjs`** (`NATIVE_ASSET_ENTRIES`) copies
+   `build/Release/transparent.node` into the standalone bundle at the same
+   relative path. The source is absent on non-Linux builds → the copy skips it.
+3. **`transparentSocket.ts`** resolves the addon both module-relative (dev/source)
+   and **cwd-relative** (`<cwd>/src/mitm/tproxy/native/...`, where the standalone
+   server runs and step 2 placed it).
+
+IP_TRANSPARENT is Linux-only, so only the linux-x64 path is built; everywhere else
+the loader returns "unavailable" and TPROXY capture mode stays disabled. (Linux
+ARM64 / other arches: build on the target, or add to CI when needed.)
+
+## Epic A status: COMPLETE (validated e2e on the VPS)
+
+The full intercept → TLS-terminate → decrypt → capture (`source:"tproxy"`) →
+re-encrypted forward → upstream round-trip was validated end-to-end on the VPS
+(kernel 6.8.0), including the anti-loop fix (PR #4229). Wired via the capture
+manager (#4208), the local-only route + CA installer (#4211), and the Traffic
+Inspector toggle (#4216).
diff --git a/src/mitm/tproxy/transparentSocket.ts b/src/mitm/tproxy/transparentSocket.ts
index 2ed3d33448..46a9d232f1 100644
--- a/src/mitm/tproxy/transparentSocket.ts
+++ b/src/mitm/tproxy/transparentSocket.ts
@@ -18,6 +18,7 @@
  */
 import { createRequire } from "node:module";
 import { platform } from "node:os";
+import path from "node:path";
 
 export interface TransparentAddon {
   /** socket()+SO_REUSEADDR+IP_TRANSPARENT+bind()+listen(); returns the raw fd. */
@@ -28,24 +29,42 @@ export interface TransparentAddon {
   connectMarked(ip: string, port: number, mark: number): number;
 }
 
-/** Candidate locations for the built/prebuilt addon, relative to this module. */
-const ADDON_PATHS = [
-  "./native/build/Release/transparent.node",
-  "./native/prebuilds/transparent.node",
+/** Path of the built/prebuilt addon, relative to `src/mitm/tproxy/`. */
+const ADDON_REL_PATHS = [
+  "native/build/Release/transparent.node",
+  "native/prebuilds/transparent.node",
 ];
 
+/**
+ * Candidate require() specifiers, in priority order:
+ *  - module-relative (`./native/...`) — dev / source runs where this file sits
+ *    next to `native/`;
+ *  - cwd-absolute (`<cwd>/src/mitm/tproxy/native/...`) — the standalone/Docker
+ *    bundle, where this module is compiled into `.next/server/...` (so the
+ *    module-relative path misses) but `assembleStandalone` copies the addon to
+ *    `<standalone-root>/src/mitm/tproxy/native/...` and the server runs with
+ *    cwd = the standalone root.
+ */
+function addonCandidates(cwd: string): string[] {
+  return [
+    ...ADDON_REL_PATHS.map((p) => `./${p}`),
+    ...ADDON_REL_PATHS.map((p) => path.join(cwd, "src", "mitm", "tproxy", p)),
+  ];
+}
+
 /**
  * Attempt to load the native addon. Returns null (never throws) when the host is
- * non-Linux or the addon hasn't been built. `req`/`os` are injectable for tests.
+ * non-Linux or the addon hasn't been built. `req`/`os`/`cwd` are injectable for tests.
  */
 export function loadTransparentAddon(
   req: (path: string) => unknown = createRequire(import.meta.url),
-  os: () => string = platform
+  os: () => string = platform,
+  cwd: () => string = () => process.cwd()
 ): TransparentAddon | null {
   if (os() !== "linux") return null; // IP_TRANSPARENT is a Linux-only socket option
-  for (const path of ADDON_PATHS) {
+  for (const candidate of addonCandidates(cwd())) {
     try {
-      const mod = req(path) as Partial<TransparentAddon> | undefined;
+      const mod = req(candidate) as Partial<TransparentAddon> | undefined;
       if (
         mod &&
         typeof mod.createTransparentListener === "function" &&
diff --git a/tests/unit/build/assemble-standalone.test.ts b/tests/unit/build/assemble-standalone.test.ts
index c955c115f3..edb0eed4f5 100644
--- a/tests/unit/build/assemble-standalone.test.ts
+++ b/tests/unit/build/assemble-standalone.test.ts
@@ -33,6 +33,7 @@ function seedSidecarSources(root: string) {
   const files = [
     "node_modules/wreq-js/rust/lib.so",
     "node_modules/better-sqlite3/build/Release/better_sqlite3.node",
+    "src/mitm/tproxy/native/build/Release/transparent.node",
     "node_modules/@swc/helpers/package.json",
     "node_modules/pino-abstract-transport/index.js",
     "node_modules/pino-pretty/index.js",
@@ -128,5 +129,27 @@ test("async and sync sidecar copy paths produce identical bundle trees", async (
     asyncTree,
     "sync (assembleStandalone) and async (sync*ToDir) must copy the same sidecar tree"
   );
+  // The TPROXY native addon must land at the cwd-relative path the runtime loader
+  // (transparentSocket.ts) resolves in the standalone bundle.
+  assert.ok(
+    asyncTree.includes("src/mitm/tproxy/native/build/Release/transparent.node"),
+    "TPROXY transparent.node copied into the standalone bundle"
+  );
+  fs.rmSync(tmp, { recursive: true, force: true });
+});
+
+test("the TPROXY addon source is skipped gracefully when it was not built (non-Linux)", async () => {
+  const tmp = fs.mkdtempSync(path.join(os.tmpdir(), "assemble-skip-"));
+  const projectRoot = path.join(tmp, "src-root");
+  // Seed everything EXCEPT the tproxy addon (simulating a non-Linux build).
+  fs.mkdirSync(projectRoot, { recursive: true });
+  const out = path.join(tmp, "out");
+  fs.mkdirSync(out, { recursive: true });
+  // No throw even though src/mitm/tproxy/native/... is absent.
+  await syncStandaloneNativeAssets(projectRoot, fs.promises, { log() {} }, out);
+  assert.ok(
+    !fs.existsSync(path.join(out, "src/mitm/tproxy/native/build/Release/transparent.node")),
+    "absent addon is simply not copied (graceful skip)"
+  );
   fs.rmSync(tmp, { recursive: true, force: true });
 });
diff --git a/tests/unit/tproxy-build-native.test.ts b/tests/unit/tproxy-build-native.test.ts
new file mode 100644
index 0000000000..ec7f9fa6ec
--- /dev/null
+++ b/tests/unit/tproxy-build-native.test.ts
@@ -0,0 +1,78 @@
+/**
+ * Best-effort build of the TPROXY IP_TRANSPARENT native addon (build 4e/N). It
+ * runs at production-build time so assembleStandalone can copy transparent.node
+ * into the standalone bundle. IP_TRANSPARENT is Linux-only and a missing toolchain
+ * is NOT fatal (the capture mode degrades gracefully), so the decision logic must
+ * skip cleanly off-Linux / when sources are absent / when node-gyp fails. All
+ * effectful seams (platform/run/exists) are injected.
+ */
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import path from "node:path";
+import { buildTproxyNative } from "../../scripts/build/build-tproxy-native.mjs";
+
+const ROOT = "/repo";
+const NATIVE = path.join(ROOT, "src", "mitm", "tproxy", "native");
+const GYP = path.join(NATIVE, "binding.gyp");
+const OUT = path.join(NATIVE, "build", "Release", "transparent.node");
+
+test("skips (no-op) on non-Linux hosts and runs nothing", () => {
+  let ran = false;
+  const res = buildTproxyNative(ROOT, {
+    platform: "darwin",
+    run: () => {
+      ran = true;
+    },
+    exists: () => true,
+  });
+  assert.equal(res.built, false);
+  assert.match(res.reason ?? "", /linux/i);
+  assert.equal(ran, false);
+});
+
+test("skips when the native sources are absent (binding.gyp missing)", () => {
+  let ran = false;
+  const res = buildTproxyNative(ROOT, {
+    platform: "linux",
+    run: () => {
+      ran = true;
+    },
+    exists: (p) => p !== GYP,
+  });
+  assert.equal(res.built, false);
+  assert.match(res.reason ?? "", /sources absent|binding\.gyp/i);
+  assert.equal(ran, false);
+});
+
+test("builds via node-gyp rebuild in the native dir and reports success", () => {
+  const calls = [];
+  const res = buildTproxyNative(ROOT, {
+    platform: "linux",
+    run: (cmd, args, cwd) => calls.push({ cmd, args, cwd }),
+    exists: (p) => p === GYP || p === OUT,
+  });
+  assert.equal(res.built, true);
+  assert.deepEqual(calls, [{ cmd: "npx", args: ["--yes", "node-gyp", "rebuild"], cwd: NATIVE }]);
+});
+
+test("a node-gyp / toolchain failure is non-fatal (built:false with a reason)", () => {
+  const res = buildTproxyNative(ROOT, {
+    platform: "linux",
+    run: () => {
+      throw new Error("gyp ERR! not found: make");
+    },
+    exists: (p) => p === GYP,
+  });
+  assert.equal(res.built, false);
+  assert.match(res.reason ?? "", /toolchain|build failed|make/i);
+});
+
+test("reports failure when node-gyp produced no transparent.node", () => {
+  const res = buildTproxyNative(ROOT, {
+    platform: "linux",
+    run: () => {},
+    exists: (p) => p === GYP, // OUT never appears
+  });
+  assert.equal(res.built, false);
+  assert.match(res.reason ?? "", /no transparent\.node|produced/i);
+});
diff --git a/tests/unit/tproxy-transparent-socket.test.ts b/tests/unit/tproxy-transparent-socket.test.ts
index 128f3c3f10..230fa94029 100644
--- a/tests/unit/tproxy-transparent-socket.test.ts
+++ b/tests/unit/tproxy-transparent-socket.test.ts
@@ -62,6 +62,43 @@ test("loadTransparentAddon rejects a module missing connectMarked (forward anti-
   assert.equal(addon, null);
 });
 
+test("loadTransparentAddon also tries the cwd-relative standalone path", () => {
+  // In the standalone/Docker bundle this module is compiled into .next/server/...
+  // so the module-relative `./native/...` misses; the addon is copied to
+  // <cwd>/src/mitm/tproxy/native/... and the loader must try that absolute path.
+  const tried: string[] = [];
+  const addon = loadTransparentAddon(
+    (p) => {
+      tried.push(p);
+      throw new Error("not here");
+    },
+    () => "linux",
+    () => "/app"
+  );
+  assert.equal(addon, null);
+  assert.ok(
+    tried.some((p) => p === "/app/src/mitm/tproxy/native/build/Release/transparent.node"),
+    `expected a cwd-absolute candidate, got: ${tried.join(", ")}`
+  );
+});
+
+test("loadTransparentAddon loads the addon from the cwd-relative standalone path", () => {
+  const fake = {
+    createTransparentListener: () => 1,
+    setSocketMark: () => {},
+    connectMarked: () => 2,
+  };
+  const addon = loadTransparentAddon(
+    (p) => {
+      if (p.startsWith("/app/")) return fake; // standalone dest; module-relative misses
+      throw new Error("not here");
+    },
+    () => "linux",
+    () => "/app"
+  );
+  assert.equal(addon, fake);
+});
+
 test("isTransparentSocketAvailable returns a boolean (false in CI — addon not built)", () => {
   assert.equal(typeof isTransparentSocketAvailable(), "boolean");
   assert.equal(isTransparentSocketAvailable(), false);