diff --git a/desktop/src-tauri/src/commands/mod.rs b/desktop/src-tauri/src/commands/mod.rs
index 761bee9cd..c4ac66121 100644
--- a/desktop/src-tauri/src/commands/mod.rs
+++ b/desktop/src-tauri/src/commands/mod.rs
@@ -46,6 +46,7 @@ pub mod pairing;
 mod personas;
 mod prevent_sleep;
 mod profile;
+mod profiling_log;
 mod project_git;
 mod project_git_branches;
 mod project_git_diff;
@@ -104,6 +105,7 @@ pub use pairing::*;
 pub use personas::*;
 pub use prevent_sleep::*;
 pub use profile::*;
+pub use profiling_log::*;
 pub use project_git::*;
 pub use project_git_branches::*;
 pub use project_git_diff::*;
diff --git a/desktop/src-tauri/src/commands/profiling_log.rs b/desktop/src-tauri/src/commands/profiling_log.rs
new file mode 100644
index 000000000..3141bcb76
--- /dev/null
+++ b/desktop/src-tauri/src/commands/profiling_log.rs
@@ -0,0 +1,96 @@
+//! Local sink for the temporary renderer profiling harness (never merges).
+//!
+//! The renderer ring-buffers timing records and flushes JSONL lines here every
+//! ~30s and on `pagehide`. Records are appended to
+//! `<app-data>/profiling/<file_stem>.jsonl`. Nothing is transmitted; this file
+//! is the only output. When a file crosses `MAX_FILE_BYTES` it is rotated aside
+//! with a millisecond suffix so a long session cannot grow unbounded -- the
+//! analyzer globs `<file_stem>*.jsonl` and concatenates.
+use std::fs::{self, OpenOptions};
+use std::io::Write;
+
+use tauri::{AppHandle, Manager};
+
+/// Rotate the active file aside once it reaches ~50 MiB.
+const MAX_FILE_BYTES: u64 = 50 * 1024 * 1024;
+
+/// Append profiling records (one JSONL string per element) to the session file.
+///
+/// `file_stem` is caller-supplied but strictly validated to
+/// `session-<digits>` so the argument can never escape the profiling dir.
+#[tauri::command]
+pub async fn append_profiling_log(
+    file_stem: String,
+    lines: Vec<String>,
+    app: AppHandle,
+) -> Result<(), String> {
+    if lines.is_empty() {
+        return Ok(());
+    }
+    if !is_valid_stem(&file_stem) {
+        return Err(format!("invalid profiling file stem: {file_stem}"));
+    }
+
+    tokio::task::spawn_blocking(move || {
+        let dir = app
+            .path()
+            .app_data_dir()
+            .map_err(|error| error.to_string())?
+            .join("profiling");
+        fs::create_dir_all(&dir).map_err(|error| error.to_string())?;
+
+        let path = dir.join(format!("{file_stem}.jsonl"));
+        if let Ok(meta) = fs::metadata(&path) {
+            if meta.len() >= MAX_FILE_BYTES {
+                let rolled = dir.join(format!(
+                    "{file_stem}.{}.jsonl",
+                    std::time::SystemTime::now()
+                        .duration_since(std::time::UNIX_EPOCH)
+                        .map(|d| d.as_millis())
+                        .unwrap_or(0)
+                ));
+                fs::rename(&path, rolled).map_err(|error| error.to_string())?;
+            }
+        }
+
+        let mut file = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(&path)
+            .map_err(|error| error.to_string())?;
+        let mut buffer = String::new();
+        for line in &lines {
+            buffer.push_str(line);
+            buffer.push('\n');
+        }
+        file.write_all(buffer.as_bytes())
+            .map_err(|error| error.to_string())?;
+        Ok(())
+    })
+    .await
+    .map_err(|error| format!("spawn_blocking failed: {error}"))?
+}
+
+/// Accept only `session-<digits>` to keep the argument inside the sink dir.
+fn is_valid_stem(stem: &str) -> bool {
+    stem.strip_prefix("session-")
+        .is_some_and(|rest| !rest.is_empty() && rest.bytes().all(|b| b.is_ascii_digit()))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::is_valid_stem;
+
+    #[test]
+    fn accepts_session_timestamp_stem() {
+        assert!(is_valid_stem("session-1700000000000"));
+    }
+
+    #[test]
+    fn rejects_traversal_and_malformed_stems() {
+        assert!(!is_valid_stem("session-../etc/passwd"));
+        assert!(!is_valid_stem("session-"));
+        assert!(!is_valid_stem("other-123"));
+        assert!(!is_valid_stem("session-12a"));
+    }
+}
diff --git a/desktop/src-tauri/src/lib.rs b/desktop/src-tauri/src/lib.rs
index b00d61521..0335d9842 100644
--- a/desktop/src-tauri/src/lib.rs
+++ b/desktop/src-tauri/src/lib.rs
@@ -148,8 +148,7 @@ pub fn run() {
                     }
 
                     // Linux/WebKitGTK needs media-stream settings and a
-                    // permission-request handler for getUserMedia; no-op
-                    // on macOS/Windows.
+                    // permission-request handler for getUserMedia; no-op on macOS/Windows.
                     linux_media::enable_media_capture(&webview);
 
                     // macOS applies the restored geometry asynchronously. Wait
@@ -670,6 +669,7 @@ pub fn run() {
             search_users,
             get_presence,
             get_os_idle_seconds,
+            append_profiling_log,
             get_default_relay_url,
             auto_connect_default_relay_enabled,
             get_legacy_workspace_storage,
diff --git a/desktop/src/app/App.tsx b/desktop/src/app/App.tsx
index 0f311f3a6..e0b7ce0a4 100644
--- a/desktop/src/app/App.tsx
+++ b/desktop/src/app/App.tsx
@@ -61,6 +61,7 @@ import { CommunityChangeOverlay } from "@/features/communities/ui/CommunityChang
 import { setAvatarProfileSyncQueryClient } from "@/features/profile/avatarProfileSync";
 import { EncryptedBackupProvider } from "@/features/settings/EncryptedBackupProvider";
 import { createBuzzQueryClient } from "@/shared/api/queryClient";
+import { startProfilingHarness } from "@/shared/profiling/harness";
 import { isSharedIdentity as isSharedIdentityCmd } from "@/shared/api/tauri";
 import { getProfile } from "@/shared/api/tauriProfiles";
 import {
@@ -215,6 +216,10 @@ function CommunityQueryProvider({ children }: { children: ReactNode }) {
 
   useEffect(() => setAvatarProfileSyncQueryClient(queryClient), [queryClient]);
 
+  // Temporary profiling harness (never merges): start once with a live
+  // QueryClient so the accumulator census can read the query cache.
+  useEffect(() => startProfilingHarness(queryClient), [queryClient]);
+
   useEffect(() => {
     const e2eWindow = window as Window & {
       __BUZZ_E2E__?: unknown;
diff --git a/desktop/src/features/agents/observerRelayStore.ts b/desktop/src/features/agents/observerRelayStore.ts
index 7ae4d0bfc..ffd2c82bc 100644
--- a/desktop/src/features/agents/observerRelayStore.ts
+++ b/desktop/src/features/agents/observerRelayStore.ts
@@ -924,3 +924,38 @@ export function _testGetArchivedChannelEvents(
     archiveEventsByChannel.get(archiveChannelKey(agentPubkey, channelId)) ?? []
   );
 }
+
+/**
+ * Profiling-harness accessor (temporary branch): cheap size census of the
+ * module-level accumulator maps. Reads `.length`/`.size` only — no allocation,
+ * no copy — so it is safe to sample every couple of minutes. Feeds the
+ * `census` profiling record that adjudicates whether accumulator growth
+ * correlates with felt lag.
+ */
+export function getObserverStoreCensus(): {
+  observerEvents: number;
+  observerAgents: number;
+  observerMaxPerAgent: number;
+  archiveEvents: number;
+  transcripts: number;
+} {
+  let observerEvents = 0;
+  let observerMaxPerAgent = 0;
+  for (const events of eventsByAgent.values()) {
+    observerEvents += events.length;
+    if (events.length > observerMaxPerAgent) {
+      observerMaxPerAgent = events.length;
+    }
+  }
+  let archiveEvents = 0;
+  for (const events of archiveEventsByChannel.values()) {
+    archiveEvents += events.length;
+  }
+  return {
+    observerEvents,
+    observerAgents: eventsByAgent.size,
+    observerMaxPerAgent,
+    archiveEvents,
+    transcripts: transcriptByAgent.size,
+  };
+}
diff --git a/desktop/src/shared/profiling/drift.ts b/desktop/src/shared/profiling/drift.ts
new file mode 100644
index 000000000..098874b0a
--- /dev/null
+++ b/desktop/src/shared/profiling/drift.ts
@@ -0,0 +1,26 @@
+// Temporary renderer profiling harness (never merges) — event-loop drift math.
+//
+// WebKit exposes no Long Tasks API, so main-thread stalls are inferred from
+// timer drift: a timer armed for `intervalMs` that fires late by more than
+// `thresholdMs` means the main thread was blocked for roughly that overage.
+
+/** Default sampling interval for the drift timer. */
+export const DRIFT_INTERVAL_MS = 500;
+/** Report a stall only when drift exceeds this — filters normal scheduler jitter. */
+export const DRIFT_THRESHOLD_MS = 50;
+
+/**
+ * Overage of an observed timer fire versus its expected fire time.
+ *
+ * `expected` is `armedAt + intervalMs`; `observed` is the actual fire time.
+ * Returns the non-negative main-thread block estimate, or `null` when the
+ * fire is within threshold (no stall worth recording).
+ */
+export function computeDrift(
+  observed: number,
+  expected: number,
+  thresholdMs: number = DRIFT_THRESHOLD_MS,
+): number | null {
+  const drift = observed - expected;
+  return drift > thresholdMs ? drift : null;
+}
diff --git a/desktop/src/shared/profiling/harness.test.mjs b/desktop/src/shared/profiling/harness.test.mjs
new file mode 100644
index 000000000..ecb8e3f89
--- /dev/null
+++ b/desktop/src/shared/profiling/harness.test.mjs
@@ -0,0 +1,111 @@
+/**
+ * Harness-core tests for the temporary renderer profiling branch (never merges).
+ *
+ * Scope is the pure, deterministic core only: ring-buffer bounds + flush
+ * draining, drift-sampler math, and the JSONL record schema. The DOM/timer/IPC
+ * probes are integration glue exercised by Will's day of driving, not unit
+ * tests.
+ */
+
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+import { computeDrift, DRIFT_THRESHOLD_MS } from "@/shared/profiling/drift.ts";
+import { ProfileRecorder, RING_CAPACITY } from "@/shared/profiling/recorder.ts";
+
+function fixedClock() {
+  let t = 1000;
+  return {
+    now: () => (t += 1),
+    wall: () => 1_700_000_000_000,
+  };
+}
+
+describe("computeDrift", () => {
+  it("returns null when the fire is within threshold", () => {
+    assert.equal(computeDrift(1050, 1000, 100), null);
+    // Exactly at threshold is not a stall (strictly greater).
+    assert.equal(computeDrift(1000 + DRIFT_THRESHOLD_MS, 1000), null);
+  });
+
+  it("returns the overage when the timer fires late past threshold", () => {
+    assert.equal(computeDrift(1200, 1000, 50), 200);
+  });
+
+  it("treats early fires as no stall", () => {
+    assert.equal(computeDrift(980, 1000, 50), null);
+  });
+});
+
+describe("ProfileRecorder ring buffer", () => {
+  it("bounds the buffer at RING_CAPACITY, dropping oldest", async () => {
+    const rec = new ProfileRecorder("session-1", async () => {}, fixedClock());
+    for (let i = 0; i < RING_CAPACITY + 50; i++) {
+      rec.record({ type: "stall", dur: i });
+    }
+    assert.equal(rec.size(), RING_CAPACITY);
+  });
+
+  it("flushes serialized JSONL lines and drains the buffer", async () => {
+    const captured = [];
+    const rec = new ProfileRecorder(
+      "session-1",
+      async (lines) => {
+        captured.push(...lines);
+      },
+      fixedClock(),
+    );
+    rec.record({ type: "stall", dur: 42 });
+    rec.record({
+      type: "census",
+      observerEvents: 5,
+      observerAgents: 1,
+      observerMaxPerAgent: 5,
+      archiveEvents: 0,
+      transcripts: 1,
+      queryCache: 3,
+      domNodes: 100,
+    });
+    await rec.flush();
+
+    assert.equal(rec.size(), 0);
+    assert.equal(captured.length, 2);
+    const first = JSON.parse(captured[0]);
+    assert.equal(first.type, "stall");
+    assert.equal(first.dur, 42);
+    assert.equal(first.sid, "session-1");
+    assert.equal(typeof first.t, "number");
+    assert.equal(typeof first.wall, "number");
+    assert.equal(typeof first.up, "number");
+  });
+
+  it("stamps the drop count onto the first line after overflow", async () => {
+    const captured = [];
+    const rec = new ProfileRecorder(
+      "session-1",
+      async (lines) => {
+        captured.push(...lines);
+      },
+      fixedClock(),
+    );
+    for (let i = 0; i < RING_CAPACITY + 10; i++) {
+      rec.record({ type: "stall", dur: i });
+    }
+    await rec.flush();
+    const first = JSON.parse(captured[0]);
+    assert.equal(first.dropped, 10);
+  });
+
+  it("does not throw when the sink rejects, and still drains", async () => {
+    const rec = new ProfileRecorder(
+      "session-1",
+      async () => {
+        throw new Error("sink down");
+      },
+      fixedClock(),
+    );
+    rec.record({ type: "stall", dur: 1 });
+    await rec.flush();
+    assert.equal(rec.size(), 0);
+  });
+});
diff --git a/desktop/src/shared/profiling/harness.ts b/desktop/src/shared/profiling/harness.ts
new file mode 100644
index 000000000..7bf287ece
--- /dev/null
+++ b/desktop/src/shared/profiling/harness.ts
@@ -0,0 +1,227 @@
+// Temporary renderer profiling harness (never merges) — bootstrap + probes.
+//
+// One always-on harness that ring-buffers timing records and flushes JSONL to
+// `<app-data>/profiling/session-<ts>.jsonl`. Four probes:
+//   1. main-thread stalls (timer drift) + input latency (down → next paint)
+//   2. Tauri IPC duration/outcome + a >10s pending-invoke watchdog (deadlocks)
+//   3. relay REQ→EOSE (history fetch) and publish send→ack round-trips
+//   4. periodic accumulator census (observer store, query cache, DOM nodes)
+//
+// The relay probe wraps the `relayClient` singleton's public methods (below),
+// so `relayClientSession.ts` is untouched and no probe lines land in request
+// code.
+
+import type { QueryClient } from "@tanstack/react-query";
+import { invoke } from "@tauri-apps/api/core";
+
+import { getObserverStoreCensus } from "@/features/agents/observerRelayStore";
+import { relayClient } from "@/shared/api/relayClient";
+import {
+  DRIFT_INTERVAL_MS,
+  DRIFT_THRESHOLD_MS,
+  computeDrift,
+} from "@/shared/profiling/drift";
+import {
+  FLUSH_INTERVAL_MS,
+  ProfileRecorder,
+} from "@/shared/profiling/recorder";
+
+const CENSUS_INTERVAL_MS = 120_000;
+const PENDING_INVOKE_MS = 10_000;
+const PENDING_SCAN_MS = 5_000;
+const INPUT_LATENCY_MIN_MS = 100;
+
+// RelayClient methods whose resolution marks a REQ→EOSE round-trip (history
+// fetches resolve when the relay sends EOSE) versus a publish send→ack
+// round-trip (resolves on the relay's OK). All publishes funnel through
+// `publishEvent`, and the instance-property wrap shadows the prototype so
+// internal `this.publishEvent` calls are captured too — wrapping the public
+// send* wrappers as well would double-count. Wrapping the singleton keeps the
+// probe out of the grandfathered relayClientSession.ts and adds zero lines to
+// production request code.
+const RELAY_FETCH_METHODS = [
+  "fetchChannelHistory",
+  "fetchChannelHistoryBefore",
+  "fetchAuxEventsByReference",
+  "fetchAuxDeletionEventsForAuxEvents",
+  "fetchEvents",
+  "fetchFirstEvent",
+] as const;
+const RELAY_PUBLISH_METHODS = ["publishEvent"] as const;
+
+let started = false;
+
+type AsyncMethod = (...args: unknown[]) => Promise<unknown>;
+
+function wrapRelayMethod(
+  rec: ProfileRecorder,
+  method: string,
+  type: "relay_req" | "relay_pub",
+): void {
+  const target = relayClient as unknown as Record<string, unknown>;
+  const original = target[method];
+  if (typeof original !== "function") {
+    return;
+  }
+  const bound = (original as AsyncMethod).bind(relayClient);
+  target[method] = (...args: unknown[]) => {
+    const startedAt = performance.now();
+    const done = (ok: boolean) =>
+      rec.record({ type, op: method, dur: performance.now() - startedAt, ok });
+    return bound(...args).then(
+      (value) => {
+        done(true);
+        return value;
+      },
+      (error) => {
+        done(false);
+        throw error;
+      },
+    );
+  };
+}
+
+function installRelayProbe(rec: ProfileRecorder): void {
+  for (const method of RELAY_FETCH_METHODS) {
+    wrapRelayMethod(rec, method, "relay_req");
+  }
+  for (const method of RELAY_PUBLISH_METHODS) {
+    wrapRelayMethod(rec, method, "relay_pub");
+  }
+}
+
+type TauriInternals = {
+  invoke: (cmd: string, args?: unknown, opts?: unknown) => Promise<unknown>;
+};
+
+function installIpcProbe(rec: ProfileRecorder): void {
+  const internals = (
+    window as unknown as { __TAURI_INTERNALS__?: TauriInternals }
+  ).__TAURI_INTERNALS__;
+  if (!internals || typeof internals.invoke !== "function") {
+    return;
+  }
+  const original = internals.invoke.bind(internals);
+  // Track outstanding invokes so a hung command surfaces via the watchdog.
+  const pending = new Map<number, { cmd: string; startedAt: number }>();
+  let seq = 0;
+
+  internals.invoke = (cmd: string, args?: unknown, opts?: unknown) => {
+    const id = seq++;
+    const startedAt = performance.now();
+    pending.set(id, { cmd, startedAt });
+    const settle = (ok: boolean) => {
+      pending.delete(id);
+      rec.record({ type: "ipc", cmd, dur: performance.now() - startedAt, ok });
+    };
+    return original(cmd, args, opts).then(
+      (value) => {
+        settle(true);
+        return value;
+      },
+      (error) => {
+        settle(false);
+        throw error;
+      },
+    );
+  };
+
+  window.setInterval(() => {
+    const now = performance.now();
+    for (const { cmd, startedAt } of pending.values()) {
+      const age = now - startedAt;
+      if (age > PENDING_INVOKE_MS) {
+        rec.record({ type: "ipc_pending", cmd, age });
+      }
+    }
+  }, PENDING_SCAN_MS);
+}
+
+function installStallProbe(rec: ProfileRecorder): void {
+  let armedAt = performance.now();
+  window.setInterval(() => {
+    const observed = performance.now();
+    const drift = computeDrift(
+      observed,
+      armedAt + DRIFT_INTERVAL_MS,
+      DRIFT_THRESHOLD_MS,
+    );
+    if (drift !== null) {
+      rec.record({ type: "stall", dur: drift });
+    }
+    armedAt = observed;
+  }, DRIFT_INTERVAL_MS);
+}
+
+function installInputProbe(rec: ProfileRecorder): void {
+  const onInput = (kind: "keydown" | "pointerdown") => (event: Event) => {
+    const start = event.timeStamp;
+    requestAnimationFrame(() =>
+      requestAnimationFrame(() => {
+        const latency = performance.now() - start;
+        if (latency >= INPUT_LATENCY_MIN_MS) {
+          rec.record({ type: "input", kind, latency });
+        }
+      }),
+    );
+  };
+  // Capture phase so the timestamp precedes app handlers.
+  window.addEventListener("keydown", onInput("keydown"), {
+    capture: true,
+    passive: true,
+  });
+  window.addEventListener("pointerdown", onInput("pointerdown"), {
+    capture: true,
+    passive: true,
+  });
+}
+
+function installCensusProbe(
+  rec: ProfileRecorder,
+  queryClient: QueryClient,
+): void {
+  const sample = () => {
+    const observer = getObserverStoreCensus();
+    rec.record({
+      type: "census",
+      observerEvents: observer.observerEvents,
+      observerAgents: observer.observerAgents,
+      observerMaxPerAgent: observer.observerMaxPerAgent,
+      archiveEvents: observer.archiveEvents,
+      transcripts: observer.transcripts,
+      queryCache: queryClient.getQueryCache().getAll().length,
+      domNodes: document.getElementsByTagName("*").length,
+    });
+  };
+  sample();
+  window.setInterval(sample, CENSUS_INTERVAL_MS);
+}
+
+/**
+ * Start the harness once, wiring all four probes and the JSONL sink. Idempotent
+ * and non-throwing: profiling must never take down the app it measures.
+ */
+export function startProfilingHarness(queryClient: QueryClient): void {
+  if (started) {
+    return;
+  }
+  started = true;
+
+  try {
+    const sid = `session-${Date.now()}`;
+    const rec = new ProfileRecorder(sid, async (lines) => {
+      await invoke("append_profiling_log", { fileStem: sid, lines });
+    });
+
+    installIpcProbe(rec);
+    installRelayProbe(rec);
+    installStallProbe(rec);
+    installInputProbe(rec);
+    installCensusProbe(rec, queryClient);
+
+    window.setInterval(() => void rec.flush(), FLUSH_INTERVAL_MS);
+    window.addEventListener("pagehide", () => void rec.flush());
+  } catch {
+    // Never let harness setup break the app being profiled.
+  }
+}
diff --git a/desktop/src/shared/profiling/recorder.ts b/desktop/src/shared/profiling/recorder.ts
new file mode 100644
index 000000000..b13d04a22
--- /dev/null
+++ b/desktop/src/shared/profiling/recorder.ts
@@ -0,0 +1,98 @@
+// Temporary renderer profiling harness (never merges) — ring-buffered recorder.
+//
+// Records are appended to a bounded in-memory ring buffer and flushed as JSONL
+// to a local file (via the `append_profiling_log` Tauri command) every
+// `FLUSH_INTERVAL_MS` and on `pagehide`. The buffer is bounded so an unflushed
+// or failed sink can never grow the harness into the thing it measures.
+
+import type { ProfileEvent, ProfileRecord } from "@/shared/profiling/types";
+
+/** Max records held between flushes. Oldest are dropped once exceeded. */
+export const RING_CAPACITY = 4096;
+/** Flush cadence. */
+export const FLUSH_INTERVAL_MS = 30_000;
+
+/** Sink receives already-serialized JSONL lines; returns once persisted. */
+export type ProfileSink = (lines: string[]) => Promise<void>;
+
+export type RecorderClock = {
+  now: () => number;
+  wall: () => number;
+};
+
+/**
+ * Bounded recorder. `record()` stamps the shared envelope and pushes into the
+ * ring; `flush()` drains the ring through the sink. Drops on overflow are
+ * counted and surfaced as a synthetic field on the next flush's first line so
+ * the analyzer can detect (rare) self-pressure.
+ */
+export class ProfileRecorder {
+  private readonly buffer: ProfileRecord[] = [];
+  private droppedSinceFlush = 0;
+  private flushing = false;
+  private readonly startedAt: number;
+  private readonly sid: string;
+  private readonly sink: ProfileSink;
+  private readonly clock: RecorderClock;
+
+  constructor(
+    sid: string,
+    sink: ProfileSink,
+    clock: RecorderClock = {
+      now: () => performance.now(),
+      wall: () => Date.now(),
+    },
+  ) {
+    this.sid = sid;
+    this.sink = sink;
+    this.clock = clock;
+    this.startedAt = clock.now();
+  }
+
+  record(event: ProfileEvent): void {
+    const now = this.clock.now();
+    const full = {
+      ...event,
+      t: now,
+      wall: this.clock.wall(),
+      sid: this.sid,
+      up: now - this.startedAt,
+    } as ProfileRecord;
+
+    this.buffer.push(full);
+    if (this.buffer.length > RING_CAPACITY) {
+      this.buffer.shift();
+      this.droppedSinceFlush += 1;
+    }
+  }
+
+  /** Current buffered record count — used by tests and the pending watchdog. */
+  size(): number {
+    return this.buffer.length;
+  }
+
+  async flush(): Promise<void> {
+    // Serialize flushes; a slow sink must not let two drains race the buffer.
+    if (this.flushing || this.buffer.length === 0) {
+      return;
+    }
+    this.flushing = true;
+    const drained = this.buffer.splice(0, this.buffer.length);
+    const dropped = this.droppedSinceFlush;
+    this.droppedSinceFlush = 0;
+
+    try {
+      const lines = drained.map((record, index) =>
+        index === 0 && dropped > 0
+          ? JSON.stringify({ ...record, dropped })
+          : JSON.stringify(record),
+      );
+      await this.sink(lines);
+    } catch {
+      // Sink failure must not crash the app being profiled. The dropped count
+      // is intentionally not restored — the records are already gone.
+    } finally {
+      this.flushing = false;
+    }
+  }
+}
diff --git a/desktop/src/shared/profiling/types.ts b/desktop/src/shared/profiling/types.ts
new file mode 100644
index 000000000..54d789ff0
--- /dev/null
+++ b/desktop/src/shared/profiling/types.ts
@@ -0,0 +1,94 @@
+// Temporary renderer profiling harness (never merges) — record schema.
+//
+// Every record carries a shared envelope plus a `type` discriminant. One JSONL
+// line is written per record; the analyzer keys off `type`.
+
+/** Fields present on every profiling record. */
+export type ProfileEnvelope = {
+  /** `performance.now()` at record time — monotonic, sub-ms, for ordering/deltas. */
+  t: number;
+  /** `Date.now()` wall clock — for correlating with Will's felt lag. */
+  wall: number;
+  /** Per-launch session id so multiple runs never interleave. */
+  sid: string;
+  /** Milliseconds since the harness started this session. */
+  up: number;
+};
+
+export type StallRecord = ProfileEnvelope & {
+  type: "stall";
+  /** Observed-minus-expected timer fire, ms (the main-thread block). */
+  dur: number;
+};
+
+export type InputRecord = ProfileEnvelope & {
+  type: "input";
+  kind: "keydown" | "pointerdown";
+  /** Event timestamp → next painted frame, ms. */
+  latency: number;
+};
+
+export type IpcRecord = ProfileEnvelope & {
+  type: "ipc";
+  cmd: string;
+  /** Invoke call → resolve/reject, ms. */
+  dur: number;
+  ok: boolean;
+};
+
+export type IpcPendingRecord = ProfileEnvelope & {
+  type: "ipc_pending";
+  cmd: string;
+  /** Age of a still-outstanding invoke, ms — the deadlock detector. */
+  age: number;
+};
+
+export type RelayReqRecord = ProfileEnvelope & {
+  type: "relay_req";
+  /** RelayClient fetch method name — the REQ→EOSE round-trip label. */
+  op: string;
+  /** Method call → resolve (EOSE), ms. */
+  dur: number;
+  ok: boolean;
+};
+
+export type RelayPubRecord = ProfileEnvelope & {
+  type: "relay_pub";
+  /** RelayClient publish method name — the send→ack round-trip label. */
+  op: string;
+  /** Method call → resolve (relay OK), ms. */
+  dur: number;
+  ok: boolean;
+};
+
+export type CensusRecord = ProfileEnvelope & {
+  type: "census";
+  /** Total observer events retained across all agents. */
+  observerEvents: number;
+  observerAgents: number;
+  /** Largest single-agent retained event count. */
+  observerMaxPerAgent: number;
+  /** Total archived observer events across all channels. */
+  archiveEvents: number;
+  transcripts: number;
+  /** React Query cache entry count. */
+  queryCache: number;
+  /** Live DOM node count. */
+  domNodes: number;
+};
+
+export type ProfileRecord =
+  | StallRecord
+  | InputRecord
+  | IpcRecord
+  | IpcPendingRecord
+  | RelayReqRecord
+  | RelayPubRecord
+  | CensusRecord;
+
+/** Payload the recorder builds internally, before the envelope is stamped. */
+export type ProfileEvent = ProfileRecord extends infer R
+  ? R extends ProfileRecord
+    ? Omit<R, keyof ProfileEnvelope>
+    : never
+  : never;