diff --git a/src/core/sse_adapter.ts b/src/core/sse_adapter.ts
index b10b8a9..490e729 100644
--- a/src/core/sse_adapter.ts
+++ b/src/core/sse_adapter.ts
@@ -17,6 +17,10 @@ export function streamDatastar(
     aborted: boolean;
   }) => Promise<void>,
 ) {
+  // Set anti-buffering headers for Nginx / Traefik reverse proxies
+  c.header("X-Accel-Buffering", "no");
+  c.header("Cache-Control", "no-cache, no-transform");
+
   return streamSSE(c, async (stream) => {
     // Add custom datastar helper methods
     const adapter = {
diff --git a/src/main.ts b/src/main.ts
index 7670316..9ceaa1f 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -3,6 +3,7 @@ import { serveStatic } from "jsr:@hono/hono@4/deno";
 import { initDb } from "./core/db.ts";
 import { pingValkey } from "./core/valkey.ts";
 import { contentNegotiation } from "./core/content_negotiation.ts";
+import { payloadCapGuard } from "./core/auth_guards.ts";
 
 import { authRoutes } from "./features/auth/routes.tsx";
 import { adminRoutes } from "./features/admin/routes.tsx";
@@ -11,6 +12,7 @@ import { sessionRoutes } from "./features/sessions/routes.tsx";
 
 const app: Hono = new Hono();
 
+app.use("*", payloadCapGuard);
 app.use("*", contentNegotiation());
 
 // Serve static assets (specifically Datastar and client scripts)
diff --git a/src/tests/arch/content_negotiation.test.ts b/src/tests/arch/content_negotiation.test.ts
new file mode 100644
index 0000000..09192fa
--- /dev/null
+++ b/src/tests/arch/content_negotiation.test.ts
@@ -0,0 +1,43 @@
+import { assertEquals } from "jsr:@std/assert@1";
+import { determineClientType } from "../../core/content_negotiation.ts";
+import { Hono } from "jsr:@hono/hono@4";
+
+Deno.test("[Arch] Content Negotiation: Correctly resolves datastar, browser, shell, and cli clients", async () => {
+  const testApp = new Hono();
+  testApp.get("/test-negotiate", (c) => {
+    const clientType = determineClientType(c);
+    return c.json({ clientType });
+  });
+
+  // Test 1: Datastar request
+  const reqDatastar = new Request("http://localhost/test-negotiate", {
+    headers: { "Datastar-Request": "true" },
+  });
+  const resDatastar = await testApp.fetch(reqDatastar);
+  const dataDatastar = await resDatastar.json();
+  assertEquals(dataDatastar.clientType, "datastar");
+
+  // Test 2: Standard Browser HTML request
+  const reqBrowser = new Request("http://localhost/test-negotiate", {
+    headers: { "Accept": "text/html" },
+  });
+  const resBrowser = await testApp.fetch(reqBrowser);
+  const dataBrowser = await resBrowser.json();
+  assertEquals(dataBrowser.clientType, "browser");
+
+  // Test 3: Curl CLI request for shell script
+  const reqShell = new Request("http://localhost/test-negotiate", {
+    headers: { "User-Agent": "curl/7.88.1" },
+  });
+  const resShell = await testApp.fetch(reqShell);
+  const dataShell = await resShell.json();
+  assertEquals(dataShell.clientType, "shell");
+
+  // Test 4: Explicit JSON REST client
+  const reqJson = new Request("http://localhost/test-negotiate", {
+    headers: { "Accept": "application/json" },
+  });
+  const resJson = await testApp.fetch(reqJson);
+  const dataJson = await resJson.json();
+  assertEquals(dataJson.clientType, "cli");
+});
diff --git a/src/tests/arch/error_fragment.test.ts b/src/tests/arch/error_fragment.test.ts
new file mode 100644
index 0000000..379290f
--- /dev/null
+++ b/src/tests/arch/error_fragment.test.ts
@@ -0,0 +1,28 @@
+import { assertEquals, assertStringIncludes } from "jsr:@std/assert@1";
+import { renderErrorToastFragment } from "../../core/error_fragments.tsx";
+import app from "../../main.ts";
+
+Deno.test("[Arch] Error Fragment: renderErrorToastFragment outputs Datastar morph target #status-banner", async () => {
+  const errorMessage = "Invalid PIN code or workshop pass expired.";
+  const fragment = renderErrorToastFragment(errorMessage);
+  const html = String(await fragment);
+
+  assertStringIncludes(html, 'id="status-banner"');
+  assertStringIncludes(html, errorMessage);
+  assertStringIncludes(html, "display: block;");
+});
+
+Deno.test("[Arch] Error Fragment: Validation errors on action routes return HTML fragments", async () => {
+  const req = new Request("http://localhost/api/join", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({}),
+  });
+
+  const res = await app.fetch(req);
+  assertEquals(res.status, 400);
+
+  const html = await res.text();
+  assertStringIncludes(html, 'id="status-banner"');
+  assertStringIncludes(html, "Event code or PIN is required");
+});
diff --git a/src/tests/arch/proxy_buffering.test.ts b/src/tests/arch/proxy_buffering.test.ts
new file mode 100644
index 0000000..8d6bc1d
--- /dev/null
+++ b/src/tests/arch/proxy_buffering.test.ts
@@ -0,0 +1,29 @@
+import { assertEquals } from "jsr:@std/assert@1";
+import { Hono } from "jsr:@hono/hono@4";
+import { streamDatastar } from "../../core/sse_adapter.ts";
+
+Deno.test("[Arch] Proxy Buffering: streamDatastar emits anti-buffering headers", async () => {
+  const testApp = new Hono();
+  testApp.get("/api/test-sse", (c) => {
+    return streamDatastar(c, async (stream) => {
+      await stream.write({
+        event: "datastar-fragment",
+        data: '<span id="status">Active</span>',
+      });
+    });
+  });
+
+  const res = await testApp.fetch(new Request("http://localhost/api/test-sse"));
+  assertEquals(res.status, 200);
+
+  // Assert required proxy headers to bypass Nginx / Traefik buffer accumulation
+  assertEquals(res.headers.get("x-accel-buffering"), "no");
+  assertEquals(
+    res.headers.get("content-type")?.includes("text/event-stream"),
+    true,
+  );
+  assertEquals(
+    res.headers.get("cache-control")?.includes("no-cache"),
+    true,
+  );
+});
diff --git a/src/tests/arch/sse_lifecycle.test.ts b/src/tests/arch/sse_lifecycle.test.ts
new file mode 100644
index 0000000..e67fd79
--- /dev/null
+++ b/src/tests/arch/sse_lifecycle.test.ts
@@ -0,0 +1,54 @@
+import { assertEquals } from "jsr:@std/assert@1";
+import { Hono } from "jsr:@hono/hono@4";
+import { streamDatastar } from "../../core/sse_adapter.ts";
+
+Deno.test("[Arch] SSE Lifecycle: streamDatastar executes clean teardown on client abort", async () => {
+  let setupExecuted = false;
+  let teardownExecuted = false;
+
+  const testApp = new Hono();
+  testApp.get("/test/stream", (c) => {
+    return streamDatastar(c, async (stream) => {
+      setupExecuted = true;
+      try {
+        await stream.write({
+          event: "datastar-fragment",
+          data: "<div>Connected</div>",
+        });
+
+        // Simulate keepalive loop until abort
+        while (!stream.aborted) {
+          await stream.sleep(10);
+        }
+      } finally {
+        teardownExecuted = true;
+      }
+    });
+  });
+
+  const abortController = new AbortController();
+  const req = new Request("http://localhost/test/stream", {
+    signal: abortController.signal,
+  });
+
+  const res = await testApp.fetch(req);
+  assertEquals(res.status, 200);
+  assertEquals(res.headers.get("content-type"), "text/event-stream");
+
+  // Read first chunk to ensure stream has started
+  const reader = res.body?.getReader();
+  if (reader) {
+    const { value } = await reader.read();
+    const text = new TextDecoder().decode(value);
+    assertEquals(text.includes("<div>Connected</div>"), true);
+    assertEquals(setupExecuted, true);
+
+    // Abort connection
+    abortController.abort();
+    await reader.cancel();
+  }
+
+  // Allow microtasks to complete teardown
+  await new Promise((resolve) => setTimeout(resolve, 50));
+  assertEquals(teardownExecuted, true);
+});
diff --git a/src/tests/arch/transport_efficiency.test.ts b/src/tests/arch/transport_efficiency.test.ts
new file mode 100644
index 0000000..5e083d4
--- /dev/null
+++ b/src/tests/arch/transport_efficiency.test.ts
@@ -0,0 +1,54 @@
+import { assertEquals, assertNotEquals } from "jsr:@std/assert@1";
+import app from "../../main.ts";
+
+Deno.test("[Arch] Transport Efficiency: Point-to-point routes return text/html or application/json without SSE overhead", async () => {
+  // Test 1: Public Join Page
+  const startJoin = performance.now();
+  const resJoin = await app.fetch(new Request("http://localhost/join"));
+  const durationJoin = performance.now() - startJoin;
+
+  assertEquals(resJoin.status, 200);
+  assertEquals(
+    resJoin.headers.get("content-type")?.includes("text/html"),
+    true,
+  );
+  assertNotEquals(
+    resJoin.headers.get("content-type"),
+    "text/event-stream",
+  );
+  // Ensure synchronous rendering execution is rapid (<50ms under test runtime)
+  assertEquals(durationJoin < 100, true);
+
+  // Test 2: Login Route
+  const startLogin = performance.now();
+  const resLogin = await app.fetch(new Request("http://localhost/login"));
+  const durationLogin = performance.now() - startLogin;
+
+  assertEquals(resLogin.status, 200);
+  assertEquals(
+    resLogin.headers.get("content-type")?.includes("text/html"),
+    true,
+  );
+  assertNotEquals(
+    resLogin.headers.get("content-type"),
+    "text/event-stream",
+  );
+  assertEquals(durationLogin < 100, true);
+});
+
+Deno.test("[Arch] Transport Efficiency: Invariant payload cap guard prevents oversized requests (>16KB)", async () => {
+  // Create an oversized body (20KB)
+  const largeBody = "A".repeat(20 * 1024);
+  const req = new Request("http://localhost/api/join", {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json",
+      "Content-Length": String(largeBody.length),
+    },
+    body: JSON.stringify({ code: largeBody }),
+  });
+
+  const res = await app.fetch(req);
+  // Max 16KB payload cap guard triggers 413 Payload Too Large or 400 validation
+  assertEquals(res.status === 413 || res.status === 400, true);
+});
diff --git a/src/tests/arch/xss_fuzzing.test.tsx b/src/tests/arch/xss_fuzzing.test.tsx
new file mode 100644
index 0000000..502a792
--- /dev/null
+++ b/src/tests/arch/xss_fuzzing.test.tsx
@@ -0,0 +1,76 @@
+import { assertNotEquals, assertStringIncludes } from "jsr:@std/assert@1";
+import { EventCockpitDeckFragment } from "../../features/events/fragments.tsx";
+import { SessionTableFragment } from "../../features/sessions/fragments.tsx";
+import { AdminUsersPageFragment } from "../../features/admin/fragments.tsx";
+
+Deno.test("[Arch] XSS Fuzzing: EventCockpitDeckFragment escapes malicious event names and payloads", () => {
+  const xssPayload =
+    '<script>alert("xss")</script><img src=x onerror=alert(1)>';
+  const mockEvents = [
+    {
+      id: "event-1",
+      name: xssPayload,
+      slug: "xss-test",
+      pin_code: "123-456",
+      seats_claimed: 5,
+      max_seats: 50,
+      expires_at: new Date().toISOString(),
+    },
+  ];
+
+  const html = String(<EventCockpitDeckFragment eventPasses={mockEvents} />);
+
+  // Assert raw script tag is NOT rendered
+  assertNotEquals(html.includes('<script>alert("xss")</script>'), true);
+  // Assert escaped HTML entity is present
+  assertStringIncludes(html, "&lt;script&gt;");
+});
+
+Deno.test("[Arch] XSS Fuzzing: SessionTableFragment escapes malicious session labels", () => {
+  const xssLabel = '"><script>alert(document.cookie)</script>';
+  const mockSessions = [
+    {
+      id: "sess-xss",
+      label: xssLabel,
+      is_agent: true,
+      custom_scopes: ["read:events"],
+      created_at: new Date().toISOString(),
+      expires_at: new Date().toISOString(),
+    },
+  ];
+
+  const html = String(
+    <SessionTableFragment
+      sessions={mockSessions}
+      currentSessionId="sess-xyz"
+    />,
+  );
+
+  assertNotEquals(
+    html.includes('"><script>alert(document.cookie)</script>'),
+    true,
+  );
+  assertStringIncludes(html, "&lt;script&gt;");
+});
+
+Deno.test("[Arch] XSS Fuzzing: AdminUsersPageFragment escapes malicious usernames and emails", () => {
+  const xssUsername = '<img src=x onerror=fetch("evil.com")>';
+  const mockUsers = [
+    {
+      id: "user-1",
+      username: xssUsername,
+      role: "viewer",
+      created_at: new Date().toISOString(),
+    },
+  ];
+
+  const html = String(
+    <AdminUsersPageFragment users={mockUsers} />,
+  );
+
+  assertNotEquals(
+    html.includes('<img src=x onerror=fetch("evil.com")>'),
+    true,
+  );
+  assertStringIncludes(html, "&lt;img");
+});