diff --git a/PROOF.md b/PROOF.md
index b47bb4d..30e9a7e 100644
--- a/PROOF.md
+++ b/PROOF.md
@@ -131,7 +131,7 @@ to the trailing window; (4) leaked snapshot timers on unmount → removed. All p
 ## 5. M2 — drill-down (click a comet → its real span, verified against `/traces/:id`)
 
 Clicking a comet resolves it to the correct span and shows that span's **real attributes**. The pick is
-pure CPU math (`app/src/gpu/motion.ts`) that mirrors the shader's closed-form motion (`shaders/river.wgsl`)
+pure CPU math (`app/src/gpu/motion.ts`) that mirrors the shader's closed-form motion (`shaders/river-sim.wgsl`; M4 moved the motion authority here from `river.wgsl`, which is now the draw shader)
 and inverts a screen click back to the nearest comet head; the server now retains each span's raw
 attributes and serves them at `GET /traces/:id` (the SSE stream stays lean — attributes are fetched on
 demand, not streamed).
diff --git a/README.md b/README.md
index 27d9ce1..d6453d0 100644
--- a/README.md
+++ b/README.md
@@ -135,4 +135,4 @@ develop the renderer.
 - ffmpeg isn't required; without it you get `.webm` (plays everywhere). With it, `record.mjs` also emits `.mp4` + `.gif`.
 
 ## Requirements
-node ≥ 20 · python 3 (for `npm run serve`) · a WebGPU-capable Chrome/Edge. See [`CLAUDE.md`](./CLAUDE.md).
+node ≥ 20 · python 3 (for `npm run serve:spike`) · a WebGPU-capable Chrome/Edge. See [`CLAUDE.md`](./CLAUDE.md).
diff --git a/ingest.mjs b/ingest.mjs
index 39a0179..a92419d 100644
--- a/ingest.mjs
+++ b/ingest.mjs
@@ -33,7 +33,7 @@ const events = sorted.map((r) => {
     cacheHit: r.cacheHit === true,
     fallbackUsed: r.fallbackUsed === true,
     guardrail: r.guardrailStatus || null,        // 'pass' | 'flag' | 'block'
-    pii: piiCategories.length > 0,
+    pii: piiCategories.length > 0 || r.guardrailStatus === 'block',  // match server mappers (otlp.ts/poller.ts)
     piiCategories,
   };
 });
diff --git a/server/src/index.ts b/server/src/index.ts
index dbb42a2..7c1a07f 100644
--- a/server/src/index.ts
+++ b/server/src/index.ts
@@ -17,10 +17,18 @@ import { startPoller } from './poller';
 const PORT = Number(process.env.PORT ?? 4319);
 const hub = new Hub(Number(process.env.BUFFER ?? 2000));
 
+const MAX_BODY = 8 * 1024 * 1024; // 8 MB cap on the OTLP POST body — the only network-facing write path
+class BodyTooLarge extends Error {}
+
 function readBody(req: IncomingMessage): Promise<string> {
   return new Promise((resolve, reject) => {
     let b = '';
-    req.on('data', (c) => (b += c));
+    let size = 0;
+    req.on('data', (c) => {
+      size += c.length;
+      if (size > MAX_BODY) { req.destroy(); reject(new BodyTooLarge('request body too large')); return; }
+      b += c;
+    });
     req.on('end', () => resolve(b));
     req.on('error', reject);
   });
@@ -47,7 +55,7 @@ const server = createServer(async (req, res) => {
       res.end('{}');
       if (events.length) console.log(`[otlp] +${events.length} span(s)  buffer=${hub.size} clients=${hub.clientCount}`);
     } catch (e) {
-      res.writeHead(400, { 'content-type': 'application/json' });
+      res.writeHead(e instanceof BodyTooLarge ? 413 : 400, { 'content-type': 'application/json' });
       res.end(JSON.stringify({ error: String(e) }));
     }
     return;
diff --git a/shared/schema.ts b/shared/schema.ts
index 21de098..79c03b8 100644
--- a/shared/schema.ts
+++ b/shared/schema.ts
@@ -10,7 +10,11 @@ export type Guardrail = 'pass' | 'flag' | 'block' | null;
 export interface TraceEvent {
   /** stable id (span id) — optional until the OTLP/poller path fills it (M1/M2 drill-down). */
   id?: string;
-  /** ms from the first event in the trace. */
+  /**
+   * File/replay path: ms from the first event in the trace (`ingest.mjs` relativizes to t0).
+   * Live server path (OTLP/poller): absolute epoch ms — there is no single trace t0 in a
+   * streaming ring buffer; live consumers position comets by arrival time, not `t`.
+   */
   t: number;
   model: string | null;
   provider: string | null;