diff --git a/src/gateway/__tests__/body-size.test.ts b/src/gateway/__tests__/body-size.test.ts
new file mode 100644
index 0000000..ff73b74
--- /dev/null
+++ b/src/gateway/__tests__/body-size.test.ts
@@ -0,0 +1,193 @@
+import { describe, it, expect, beforeAll, beforeEach, afterAll, vi } from "vitest";
+import http from "node:http";
+import { TdaiGateway } from "../server.js";
+
+interface RequestResult {
+  status: number;
+  body: string;
+  headers: http.IncomingHttpHeaders;
+}
+
+/**
+ * POST a body to the gateway. Two body modes:
+ *  - "trusted" (default): let Node compute Content-Length from the body.
+ *  - "lying-cl": set Content-Length to a small value but stream a larger
+ *    body — emulates a hostile client.
+ *  - "no-cl":  use Transfer-Encoding: chunked, so the server cannot fail
+ *    fast on Content-Length and must rely on running-total.
+ */
+async function postBody(
+  port: number,
+  path: string,
+  body: Buffer,
+  mode: "trusted" | "lying-cl" | "no-cl" = "trusted",
+  fakeCl?: number,
+): Promise<RequestResult> {
+  return new Promise((resolve, reject) => {
+    const headers: Record<string, string> = { "Content-Type": "application/json" };
+    if (mode === "trusted") {
+      headers["Content-Length"] = String(body.length);
+    } else if (mode === "lying-cl") {
+      headers["Content-Length"] = String(fakeCl ?? 10);
+    } else if (mode === "no-cl") {
+      headers["Transfer-Encoding"] = "chunked";
+    }
+    const req = http.request(
+      { host: "127.0.0.1", port, path, method: "POST", headers },
+      (res) => {
+        const chunks: Buffer[] = [];
+        res.on("data", (c) => chunks.push(c));
+        res.on("end", () =>
+          resolve({
+            status: res.statusCode ?? 0,
+            headers: res.headers,
+            body: Buffer.concat(chunks).toString("utf-8"),
+          }),
+        );
+      },
+    );
+    req.on("error", (err) => {
+      // ECONNRESET is expected when the server destroys the socket on
+      // PayloadTooLarge before the body finishes uploading. Treat it as
+      // a successful "rejected" outcome — the test then asserts via
+      // status === 413 on a follow-up request OR via the error here.
+      resolve({ status: 0, headers: {}, body: String(err) });
+    });
+    if (mode === "no-cl") {
+      // Stream chunks to give the server a chance to abort mid-upload.
+      const chunkSize = 4096;
+      let offset = 0;
+      const flush = () => {
+        if (offset >= body.length) {
+          req.end();
+          return;
+        }
+        const ok = req.write(body.subarray(offset, offset + chunkSize));
+        offset += chunkSize;
+        if (ok) setImmediate(flush);
+        else req.once("drain", flush);
+      };
+      flush();
+    } else {
+      req.write(body);
+      req.end();
+    }
+  });
+}
+
+describe("Gateway request body size limit", () => {
+  let gateway: TdaiGateway;
+  const PORT = 18433;
+
+  beforeAll(async () => {
+    // 1 KiB cap — small enough to exercise the limit without producing
+    // megabyte-sized test fixtures, large enough to fit a small valid
+    // JSON body for the happy path.
+    vi.stubEnv("TDAI_GATEWAY_MAX_BODY_BYTES", "1024");
+    gateway = new TdaiGateway({
+      server: { port: PORT, host: "127.0.0.1" },
+    } as never);
+    await gateway.start();
+  });
+
+  // vitest config has `unstubEnvs: true`, which resets stubs before each
+  // test. `parseJsonBody`'s default `maxBytes` arg re-reads the env on
+  // every call, so the stub must be re-applied here.
+  beforeEach(() => {
+    vi.stubEnv("TDAI_GATEWAY_MAX_BODY_BYTES", "1024");
+  });
+
+  afterAll(async () => {
+    await gateway.stop();
+  });
+
+  it("accepts a small JSON body under the limit", async () => {
+    // /recall with missing fields → 400, but that proves the body parsed.
+    const res = await postBody(
+      PORT,
+      "/recall",
+      Buffer.from(JSON.stringify({ query: "hi", session_key: "k" })),
+    );
+    // Body parsed successfully → status is whatever the handler returns,
+    // NOT 413.
+    expect(res.status).not.toBe(413);
+    expect(res.status).not.toBe(500);
+  });
+
+  it("rejects with 413 when Content-Length declares a body over the limit", async () => {
+    // 2 KiB body, honest Content-Length — fail-fast path.
+    const big = Buffer.alloc(2 * 1024, "x");
+    const res = await postBody(PORT, "/recall", big, "trusted");
+    expect(res.status).toBe(413);
+    expect(res.body).toMatch(/exceeds 1024 bytes/);
+  });
+
+  it("rejects with 413 when a lying Content-Length is small but actual body exceeds the limit", async () => {
+    // Streamed mode (no Content-Length) — server tracks running total.
+    // Body is 4 KiB but server cap is 1 KiB.
+    const big = Buffer.alloc(4 * 1024, "x");
+    const res = await postBody(PORT, "/recall", big, "no-cl");
+    // Either the server replied 413 cleanly, or it tore the socket down
+    // mid-upload (ECONNRESET) — both are acceptable signals that the
+    // running-total guard fired. What is NOT acceptable: a 2xx/4xx that
+    // implies the full body was buffered.
+    if (res.status === 413) {
+      expect(res.body).toMatch(/exceeds 1024 bytes/);
+    } else {
+      expect(res.status).toBe(0); // ECONNRESET / socket hangup
+    }
+  });
+
+  it("returns 413 from the dispatcher, NOT 500", async () => {
+    // Regression guard: a stray `catch` somewhere upstream wrapping
+    // PayloadTooLargeError into a generic 500 would silently break the
+    // contract for clients that retry on 5xx but not on 4xx.
+    const big = Buffer.alloc(5 * 1024, "x");
+    const res = await postBody(PORT, "/capture", big, "trusted");
+    expect(res.status).toBe(413);
+  });
+
+  it("includes a descriptive error body with the declared limit", async () => {
+    const big = Buffer.alloc(2 * 1024, "x");
+    const res = await postBody(PORT, "/seed", big, "trusted");
+    expect(res.status).toBe(413);
+    // JSON envelope { "error": "..." } from sendError().
+    const parsed = JSON.parse(res.body) as { error: string };
+    expect(parsed.error).toMatch(/1024 bytes/);
+  });
+});
+
+describe("Gateway body-size limit env override", () => {
+  it("respects TDAI_GATEWAY_MAX_BODY_BYTES at gateway construction time", async () => {
+    // Tiny cap: 50 bytes — even a minimal valid /recall JSON exceeds it.
+    vi.stubEnv("TDAI_GATEWAY_MAX_BODY_BYTES", "50");
+    const PORT = 18434;
+    const gw = new TdaiGateway({ server: { port: PORT, host: "127.0.0.1" } } as never);
+    await gw.start();
+    try {
+      const body = Buffer.from(JSON.stringify({ query: "x".repeat(80), session_key: "k" }));
+      const res = await postBody(PORT, "/recall", body, "trusted");
+      expect(res.status).toBe(413);
+      expect(res.body).toMatch(/exceeds 50 bytes/);
+    } finally {
+      await gw.stop();
+    }
+  });
+
+  it("falls back to the default cap when TDAI_GATEWAY_MAX_BODY_BYTES is malformed", async () => {
+    // Garbage env should NOT cause the daemon to start with an undefined
+    // / NaN cap — that would either reject every request or cap nothing.
+    vi.stubEnv("TDAI_GATEWAY_MAX_BODY_BYTES", "not-a-number");
+    const PORT = 18435;
+    const gw = new TdaiGateway({ server: { port: PORT, host: "127.0.0.1" } } as never);
+    await gw.start();
+    try {
+      // Default is 8 MiB — a small valid body must succeed.
+      const body = Buffer.from(JSON.stringify({ query: "hi", session_key: "k" }));
+      const res = await postBody(PORT, "/recall", body, "trusted");
+      expect(res.status).not.toBe(413);
+    } finally {
+      await gw.stop();
+    }
+  });
+});
diff --git a/src/gateway/server.ts b/src/gateway/server.ts
index bd7d0a0..5ffdb0d 100644
--- a/src/gateway/server.ts
+++ b/src/gateway/server.ts
@@ -63,11 +63,72 @@ function createConsoleLogger(): Logger {
 // Request body parser
 // ============================
 
-async function parseJsonBody<T>(req: http.IncomingMessage): Promise<T> {
+/** Default cap on request body size (bytes). Large enough for /seed payloads
+ *  with hundreds of historical sessions; small enough that a single malicious
+ *  /capture cannot OOM the daemon. Override at runtime via the
+ *  TDAI_GATEWAY_MAX_BODY_BYTES env var. */
+const DEFAULT_MAX_BODY_BYTES = 8 * 1024 * 1024; // 8 MiB
+
+function resolveMaxBodyBytes(): number {
+  const raw = process.env.TDAI_GATEWAY_MAX_BODY_BYTES;
+  if (!raw) return DEFAULT_MAX_BODY_BYTES;
+  const n = Number.parseInt(raw, 10);
+  return Number.isFinite(n) && n > 0 ? n : DEFAULT_MAX_BODY_BYTES;
+}
+
+/** Thrown when an incoming request body exceeds the size cap. The dispatcher
+ *  catches this and replies with HTTP 413, NOT 500. */
+class PayloadTooLargeError extends Error {
+  constructor(public readonly limitBytes: number) {
+    super(`Request body exceeds ${limitBytes} bytes`);
+    this.name = "PayloadTooLargeError";
+  }
+}
+
+async function parseJsonBody<T>(
+  req: http.IncomingMessage,
+  maxBytes: number = resolveMaxBodyBytes(),
+): Promise<T> {
   return new Promise((resolve, reject) => {
+    // Fast path: trust a present Content-Length header to fail before we
+    // buffer anything. A lying client (CL smaller than actual body) is still
+    // caught by the running-total check below.
+    const cl = req.headers["content-length"];
+    if (cl !== undefined) {
+      const declared = Number.parseInt(cl, 10);
+      if (Number.isFinite(declared) && declared > maxBytes) {
+        // Pause the request stream instead of destroying it: the
+        // dispatcher needs the response side of this socket to be writable
+        // to send the 413. The handler returns immediately after writing
+        // the response, after which Node will close the keep-alive socket
+        // and the lying client's residual upload bytes are discarded.
+        req.pause();
+        reject(new PayloadTooLargeError(maxBytes));
+        return;
+      }
+    }
+
     const chunks: Buffer[] = [];
-    req.on("data", (chunk: Buffer) => chunks.push(chunk));
+    let received = 0;
+    let rejected = false;
+    req.on("data", (chunk: Buffer) => {
+      if (rejected) return;
+      received += chunk.length;
+      if (received > maxBytes) {
+        rejected = true;
+        // Pause the request stream instead of destroying it: the
+        // dispatcher needs the response side of this socket to be writable
+        // to send the 413. The handler returns immediately after writing
+        // the response, after which Node will close the keep-alive socket
+        // and the lying client's residual upload bytes are discarded.
+        req.pause();
+        reject(new PayloadTooLargeError(maxBytes));
+        return;
+      }
+      chunks.push(chunk);
+    });
     req.on("end", () => {
+      if (rejected) return;
       try {
         const body = Buffer.concat(chunks).toString("utf-8");
         resolve(JSON.parse(body) as T);
@@ -204,6 +265,11 @@ export class TdaiGateway {
           sendError(res, 404, `Not found: ${method} ${pathname}`);
       }
     } catch (err) {
+      if (err instanceof PayloadTooLargeError) {
+        this.logger.warn(`Request body too large [${method} ${pathname}]: ${err.message}`);
+        sendError(res, 413, err.message);
+        return;
+      }
       const msg = err instanceof Error ? err.message : String(err);
       this.logger.error(`Request error [${method} ${pathname}]: ${msg}`);
       sendError(res, 500, msg);