diff --git a/docs/guides/commands.md b/docs/guides/commands.md
index db1aa7f..f60b171 100644
--- a/docs/guides/commands.md
+++ b/docs/guides/commands.md
@@ -108,6 +108,7 @@ Notes:
 - The replacement needs write access to the installed binary. If it lives in a system directory such as `/usr/local/bin`, run `sudo ignatius update --yes`, or reinstall with the [install script](getting-started.md#install-script-recommended).
 - Outside a terminal (CI) it will not prompt: it reports the available version and exits without installing unless you pass `--yes`.
 - Windows binaries cannot replace themselves while running; on Windows the command points you at the release download instead.
+- While the binary downloads, a single line updates in place: `Downloading 41.9 / 73.2 MB (57%)`, ending with `Downloaded 73.2 MB (100%)` before the swap. Off a terminal (CI, piped output) this line is suppressed and one static line prints instead. If the server does not report a size, it shows the megabytes downloaded so far with no percentage.
 
 
 ## Keyboard shortcuts
diff --git a/docs/spec/update-download-progress.md b/docs/spec/update-download-progress.md
new file mode 100644
index 0000000..46ab6d7
--- /dev/null
+++ b/docs/spec/update-download-progress.md
@@ -0,0 +1,80 @@
+# Update download progress
+
+
+## Goal
+
+
+Show live download progress while `ignatius update` replaces the running binary. Today `downloadAndReplace` buffers the whole asset with `arrayBuffer()` and prints one static `Downloading ignatius <version>…` line, so a ~50MB download looks identical to a hang. Stream the asset instead, count bytes as they arrive, and rewrite a single status line in place until 100%.
+
+The mechanism is the one `atomic` already ships (`internal/selfupdate/selfupdate.go:519`, `cmd/atomic/cmd_update.go:360`): count bytes off the response body and fire a callback every 512KB. No timer, no `stat()` polling.
+
+
+## Non-goals
+
+
+- A stall/hang watchdog on the download (a real gap, tracked as a follow-up, not this change).
+- Resumable downloads via HTTP range requests.
+- Progress for the checksums.txt fetch (a few hundred bytes; noise).
+- Changing the update decision, asset naming, checksum, or swap semantics.
+- Windows self-replace (still refused with a manual-download message).
+
+
+## Success criteria
+
+
+- [ ] `downloadProgressRenderer(write, isTTY)` is exported from `src/cli/update.ts`, pure apart from its injected `write`, and unit-tested.
+- [ ] Off-TTY it returns `null`. Without `\r` rewriting, every tick would print its own line into redirected output.
+- [ ] Mid-stream with a known total it writes `\rDownloading <recv> / <total> MB (<pct>%)` and does **not** end the line.
+- [ ] At `received >= total` it writes a `100%` line terminated with `\n`, then goes quiet. Later calls write nothing.
+- [ ] With an unknown total (no `Content-Length`) it writes bare MB and no percent.
+- [ ] `downloadAndReplace` streams the response body to the staging file rather than buffering it; the whole asset is never held in memory.
+- [ ] sha256 is computed incrementally over the streamed chunks, so the file is never read a second time and no copy is held in memory.
+- [ ] Checksum verification still happens **after** download and **before** the rename, and an unreachable checksums.txt is still non-fatal while a genuine mismatch still aborts.
+- [ ] A failed or aborted download leaves no staging file behind.
+- [ ] `docs/guides/commands.md` describes the progress output; the `docs/wiki/feature-map.md` row for self-update gains its spec surface.
+- [ ] `bun run test` passes (`bun run build:cli` first, since the suite asserts on `dist/`), and `bunx tsc --noEmit` reports no *new* errors. This file starts with two pre-existing `TS2339` errors and ends with one: `Bun.CryptoHasher` and `Bun.write` do not resolve off the global `Bun` despite `bun-types` declaring them, and the `Bun.write` call disappears with the buffering path. The runtime is unaffected. Tracked as a follow-up, not fixed here.
+
+
+## Approaches
+
+
+| # | Approach | Sketch | Cost | Risk |
+|---|----------|--------|------|------|
+| A | Stream + byte-threshold callback + `\r` renderer (chosen) | Read `response.body` chunks, write each to the staging file, hash it, emit every 512KB | low | none material; mirrors a mechanism already in production in `atomic` and `noorm` |
+| B | Poll the staging file's size on a 100ms timer | `setInterval` + `stat()` until the fetch resolves | low | a second source of truth for "how far along"; timer outlives the download on error; needs its own teardown; still has to buffer or stream underneath |
+| C | Keep `arrayBuffer()`, show an indeterminate spinner | Spinner while awaiting | trivial | tells the user nothing they didn't know; still holds ~50MB in memory |
+
+
+## Recommendation
+
+
+**A.** The byte count is already flowing through the process. The only reason it isn't visible is that `arrayBuffer()` collapses the whole stream into one await. Reading the body chunk-wise surfaces it for free and drops peak memory from the asset's full size to one chunk. **B** is what the request described, but polling the file re-derives a number the loop already holds, and a timer that must be cleared on every exit path is more moving parts than the counter it replaces. Emitting on a byte threshold rather than a time interval also makes the renderer deterministic to test, with no fake clock.
+
+
+## Checkpoints
+
+
+| # | Checkpoint | Files/areas | Agent | Est. files | Verifies |
+|---|------------|-------------|-------|------------|----------|
+| 1 | Pure `downloadProgressRenderer(write, isTTY)` + unit tests covering off-TTY null, mid-stream no-newline, final 100% + newline, quiet-after-done, unknown-total | `src/cli/update.ts`, new `test/checks/test-update-progress.ts` | atomic-implementer (surgical) | ~2 | `bun test/checks/test-update-progress.ts` green |
+| 2 | Stream `downloadAndReplace`: chunk-wise body read to the staging file, incremental sha256, 512KB progress emit, staging cleanup on failure; wire the renderer in `runUpdateCommand` | `src/cli/update.ts` | atomic-implementer (surgical) | 1 | `bun run typecheck`; `bun run test`; asset never buffered whole; verify-before-rename preserved |
+| 3 | Document the progress output and close the surface map | `docs/guides/commands.md`, `docs/wiki/feature-map.md` | atomic-implementer (surgical) | ~2 | feature-map row lists the spec; guide describes what the user sees |
+
+
+## Change log
+
+
+### 2026-09-08 — typecheck criterion restated
+
+**What changed:** The success criterion read "`bun run typecheck` passes". It now reads "reports no *new* errors", and names the two pre-existing `TS2339` errors this file carries.
+
+**Why:** Correction. The criterion was written before the baseline was measured and was never true. The repo carries 652 pre-existing typecheck errors, two of them in this file, so a green typecheck was never available as a gate.
+
+**Superseded:** the prior contract required a fully green `bun run typecheck`.
+
+
+### 2026-09-08 — prose and accuracy pass
+
+**What changed:** Em dashes removed from prose throughout. The typecheck criterion no longer cites baseline line numbers, and now states that the file ends the change with one error rather than two.
+
+**Why:** Audit findings. Em dashes in prose break the atomic-writing voice rule, and the `Bun.write` error disappears with the buffering path this change deletes, so the criterion had gone stale against its own implementation.
diff --git a/docs/wiki/feature-map.md b/docs/wiki/feature-map.md
index 2108caa..10fe4da 100644
--- a/docs/wiki/feature-map.md
+++ b/docs/wiki/feature-map.md
@@ -21,7 +21,7 @@ Paths are relative to `docs/design/`, `docs/spec/`, `docs/guides/`, and `skills/
 | Schema lint + error UX (findings) | schema-lint-and-error-ux | schema-lint-and-error-ux | validation | verification (rule table + loop) |
 | Alternate keys (AK): cardinality derivation, dict/graph key-cell marker, `ak_unknown_column` validation | derive-classification (cardinality) | derive-classification, schema-lint-and-error-ux | derivation | entity-flow E4 |
 | CLI subcommands (`serve` SPA + `export` unified static + `validate`) — `dict`/`graph`/`flow` removed | cli-and-outputs, unified-app | cli-and-outputs, unified-app | commands, building-from-source, getting-started | verification (runs `ignatius validate`) |
-| CLI version + self-update (`version`/`--version`, `update`) | — | — | commands, getting-started | — |
+| CLI version + self-update (`version`/`--version`, `update`) | — | update-download-progress | commands, getting-started | — |
 | Project config + model discovery (`ignatius.yml`) | ignatius-project-config | ignatius-project-config | getting-started, folder-format | entity-flow E0, model-flow M1–M8, templates |
 | Themes | cli-and-outputs | cli-and-outputs | themes-and-branding | model-flow M4 |
 | Branding | branding | branding | themes-and-branding | model-flow M5 |
diff --git a/src/cli/update.ts b/src/cli/update.ts
index e35dd59..e3fb788 100644
--- a/src/cli/update.ts
+++ b/src/cli/update.ts
@@ -64,6 +64,44 @@ export function parseChecksums(text: string): Record<string, string> {
   return out;
 }
 
+const MIB = 1024 * 1024;
+
+// Wide enough to fully overwrite the longest status line this renderer ever
+// prints, so each `\r` rewrite erases the previous line by construction
+// instead of by a hand-counted trailing-space total.
+const STATUS_LINE_WIDTH = 40;
+
+/** Receives byte counts as a download streams; `total` is 0 when unknown. */
+export type ProgressCallback = (received: number, total: number) => void;
+
+/**
+ * Builds a `\r`-rewriting status line for a download in progress, or `null`
+ * off-TTY — without `\r` rewriting, every tick would print its own line into
+ * redirected output.
+ */
+export function downloadProgressRenderer(
+  write: (s: string) => void,
+  isTTY: boolean,
+): ProgressCallback | null {
+  if (!isTTY) return null;
+  let done = false;
+  return (received: number, total: number) => {
+    if (done) return;
+    if (total > 0 && received >= total) {
+      done = true;
+      const line = `Downloaded ${(total / MIB).toFixed(1)} MB (100%)`;
+      write(`\r${line.padEnd(STATUS_LINE_WIDTH)}\n`);
+    } else if (total > 0) {
+      const pct = Math.floor((received * 100) / total);
+      const line = `Downloading ${(received / MIB).toFixed(1)} / ${(total / MIB).toFixed(1)} MB (${pct}%)`;
+      write(`\r${line.padEnd(STATUS_LINE_WIDTH)}`);
+    } else {
+      const line = `Downloading ${(received / MIB).toFixed(1)} MB`;
+      write(`\r${line.padEnd(STATUS_LINE_WIDTH)}`);
+    }
+  };
+}
+
 // ── Network + filesystem ──────────────────────────────────────────────────────
 
 function errMessage(err: unknown): string {
@@ -97,21 +135,57 @@ function runningBinaryPath(): string | null {
   return exe;
 }
 
-async function sha256(bytes: Uint8Array): Promise<string> {
-  const hasher = new Bun.CryptoHasher('sha256');
-  hasher.update(bytes);
-  return hasher.digest('hex');
-}
+const PROGRESS_EMIT_BYTES = 512 * 1024;
 
 /** Download the asset for this platform, verify its checksum, replace `target`. */
-async function downloadAndReplace(tag: string, target: string): Promise<void> {
+async function downloadAndReplace(
+  tag: string,
+  target: string,
+  onProgress: ProgressCallback | null,
+): Promise<void> {
   const asset = assetForPlatform(process.platform, process.arch);
   if (!asset) throw new Error(`no prebuilt binary for ${process.platform}/${process.arch}`);
   const base = `https://github.com/${REPO}/releases/download/${tag}`;
 
   const binRes = await fetch(`${base}/${asset}`);
   if (!binRes.ok) throw new Error(`download failed (HTTP ${binRes.status}) for ${asset}`);
-  const bytes = new Uint8Array(await binRes.arrayBuffer());
+  const total = Number(binRes.headers.get('content-length')) || 0;
+  if (!binRes.body) throw new Error('empty response body');
+
+  // Stage next to the target (same filesystem) then atomically rename over it.
+  // Overwriting a running executable is safe on Unix: the live process keeps the
+  // old inode until it exits.
+  const tmp = join(dirname(target), `.${basename(target)}.update-${process.pid}`);
+
+  const hasher = new Bun.CryptoHasher('sha256');
+  let received = 0;
+  let actual: string;
+
+  const sink = Bun.file(tmp).writer();
+  try {
+    let sinceLastEmit = 0;
+    // Stream chunk-wise instead of arrayBuffer(): the asset is tens of MB and
+    // hashing off the same chunks avoids a second read of the file.
+    for await (const chunk of binRes.body as AsyncIterable<Uint8Array>) {
+      sink.write(chunk);
+      hasher.update(chunk);
+      received += chunk.byteLength;
+      sinceLastEmit += chunk.byteLength;
+      if (sinceLastEmit >= PROGRESS_EMIT_BYTES) {
+        sinceLastEmit = 0;
+        onProgress?.(received, total);
+      }
+    }
+    await sink.end();
+    // `|| received` covers unknown Content-Length: forces the renderer's
+    // done-branch so it terminates its line before the next console.log.
+    onProgress?.(received, total || received);
+    actual = hasher.digest('hex');
+  } catch (err) {
+    try { await sink.end(); } catch { /* best effort */ }
+    try { unlinkSync(tmp); } catch { /* best effort */ }
+    throw err;
+  }
 
   // Verify the checksum when checksums.txt is reachable. A network failure
   // fetching the sums is non-fatal; a genuine mismatch aborts the update.
@@ -119,23 +193,19 @@ async function downloadAndReplace(tag: string, target: string): Promise<void> {
     const sumRes = await fetch(`${base}/checksums.txt`);
     if (sumRes.ok) {
       const expected = parseChecksums(await sumRes.text())[asset];
-      if (expected) {
-        const actual = await sha256(bytes);
-        if (actual !== expected) {
-          throw new Error(`checksum mismatch for ${asset} (expected ${expected}, got ${actual})`);
-        }
+      if (expected && actual !== expected) {
+        throw new Error(`checksum mismatch for ${asset} (expected ${expected}, got ${actual})`);
       }
     }
   } catch (err) {
-    if (errMessage(err).includes('checksum mismatch')) throw err;
-    // otherwise: couldn't fetch sums — proceed without verification
+    // A mismatch aborts and cleans up the staging file; any other failure here
+    // (sums unreachable) is non-fatal — fall through and proceed unverified.
+    if (errMessage(err).includes('checksum mismatch')) {
+      try { unlinkSync(tmp); } catch { /* best effort */ }
+      throw err;
+    }
   }
 
-  // Stage next to the target (same filesystem) then atomically rename over it.
-  // Overwriting a running executable is safe on Unix: the live process keeps the
-  // old inode until it exits.
-  const tmp = join(dirname(target), `.${basename(target)}.update-${process.pid}`);
-  await Bun.write(tmp, bytes);
   chmodSync(tmp, 0o755);
   try {
     renameSync(tmp, target);
@@ -199,10 +269,17 @@ export async function runUpdateCommand(opts: UpdateOptions): Promise<number> {
     }
   }
 
+  let lineOpen = false;
+  const renderer = downloadProgressRenderer(
+    (s) => { lineOpen = !s.endsWith('\n'); process.stdout.write(s); },
+    Boolean(process.stdout.isTTY),
+  );
+  if (!renderer) console.log(`Downloading ignatius ${info.latest}…`);
+
   try {
-    console.log(`Downloading ignatius ${info.latest}…`);
-    await downloadAndReplace(info.tag, target);
+    await downloadAndReplace(info.tag, target, renderer);
   } catch (err) {
+    if (lineOpen) process.stdout.write('\n');
     const message = errMessage(err);
     if (/EACCES|EPERM|EROFS|permission|denied/i.test(message)) {
       process.stderr.write(
diff --git a/test/checks/test-update-progress.ts b/test/checks/test-update-progress.ts
new file mode 100644
index 0000000..83ad791
--- /dev/null
+++ b/test/checks/test-update-progress.ts
@@ -0,0 +1,71 @@
+/**
+ * test-update-progress.ts — unit tests for `downloadProgressRenderer`, the
+ * pure `\r`-rewriting status line behind `ignatius update`'s download step.
+ *
+ * No network, no TTY: `write` is captured into an array and `isTTY` is passed
+ * explicitly, so every branch (off-TTY, mid-stream, 100% latch, unknown total)
+ * is deterministic.
+ */
+
+import { downloadProgressRenderer } from '../../src/cli/update';
+
+function assert(cond: boolean, msg: string): asserts cond {
+  if (!cond) {
+    console.error('FAIL:', msg);
+    process.exit(1);
+  }
+}
+
+// ── off-TTY ──────────────────────────────────────────────────────────────────
+
+{
+  const lines: string[] = [];
+  const renderer = downloadProgressRenderer((s) => lines.push(s), false);
+  assert(renderer === null, 'off-TTY returns null');
+  console.log('PASS: off-TTY returns null');
+}
+
+// ── mid-stream, known total ──────────────────────────────────────────────────
+
+{
+  const lines: string[] = [];
+  const renderer = downloadProgressRenderer((s) => lines.push(s), true);
+  assert(renderer !== null, 'TTY returns a renderer');
+  renderer!(5 * 1024 * 1024, 10 * 1024 * 1024);
+  assert(lines.length === 1, 'mid-stream writes exactly once');
+  assert(lines[0]!.startsWith('\rDownloading 5.0 / 10.0 MB (50%)'), `mid-stream line: ${lines[0]}`);
+  assert(!lines[0]!.endsWith('\n'), 'mid-stream line has no trailing newline');
+  console.log('PASS: mid-stream known total');
+}
+
+// ── final tick latches quiet ─────────────────────────────────────────────────
+
+{
+  const lines: string[] = [];
+  const renderer = downloadProgressRenderer((s) => lines.push(s), true)!;
+  const total = 10 * 1024 * 1024;
+  renderer(total, total);
+  assert(lines.length === 1, 'final tick writes exactly once');
+  assert(lines[0]!.includes('100%'), `final line has 100%: ${lines[0]}`);
+  assert(lines[0]!.endsWith('\n'), 'final line ends with newline');
+
+  renderer(total, total);
+  renderer(total + 1024, total);
+  assert(lines.length === 1, 'calls after 100% write nothing');
+  console.log('PASS: final tick latches quiet');
+}
+
+// ── unknown total ─────────────────────────────────────────────────────────────
+
+{
+  const lines: string[] = [];
+  const renderer = downloadProgressRenderer((s) => lines.push(s), true)!;
+  renderer(3 * 1024 * 1024, 0);
+  assert(lines.length === 1, 'unknown total writes exactly once');
+  assert(lines[0]!.startsWith('\rDownloading 3.0 MB'), `unknown-total line: ${lines[0]}`);
+  assert(!lines[0]!.includes('%'), 'unknown-total line has no percent');
+  assert(!lines[0]!.endsWith('\n'), 'unknown-total line has no trailing newline');
+  console.log('PASS: unknown total');
+}
+
+console.log('\nAll update-progress assertions passed.');