diff --git a/docs/architecture/02-persistence.md b/docs/architecture/02-persistence.md
index aa9fdf8..5eaa647 100644
--- a/docs/architecture/02-persistence.md
+++ b/docs/architecture/02-persistence.md
@@ -40,6 +40,8 @@ Most adapters are driven by **background subscribers** (arch 01 §5, _Effects_):
 
 **URL-dataset snapshot lifecycle** (the files a change to it touches): add / Refresh → component fetches (`infrastructure/remote-data.ts`) → `core/dataset.snapshotFromText` shapes the body by sniffed format → `DatasetStore.commitUrlSnapshot` / `refreshDataset` snapshots + profiles it _exactly like inline data_ → render resolves it cached-first in `core/rendering.resolvedData` (a live-URL fallback applies only while a URL dataset is still unfetched). See spec §05 for the behavior.
 
+A fetched body is held whole in memory and stored whole in IndexedDB, so `remote-data.ts` caps it at a fixed ceiling (`MAX_REMOTE_BYTES`): it rejects on the declared `Content-Length` _before_ reading, so an oversized download never lands in memory, with a body-length backstop for chunked responses that declare no length. The classified `too-large` reason maps to its own copy in `services/remote-data-errors.ts` — neither the inline-paste nor the retry recovery helps an oversized file, so that message points at using a smaller or pre-aggregated source instead.
+
 ---
 
 ## 2. IndexedDB Wrapper
diff --git a/docs/spec/05-datasets.md b/docs/spec/05-datasets.md
index c73b0e7..528dafa 100644
--- a/docs/spec/05-datasets.md
+++ b/docs/spec/05-datasets.md
@@ -90,7 +90,7 @@ A destructive or off-screen outcome raises a confirming toast; an action whose r
 - **Copy Reference** — copies the by-name reference object to the clipboard, ready to paste into a spec:
   `{ "data": { "name": "MyDataset" } }`
   The clipboard write is invisible, so it is confirmed _inline on the control_ ("Copied"), announced politely to assistive technology — not a toast.
-- **New / Create New** — opens the create form in the detail pane with fields: **name** (required, unique), **source** toggle (Inline / URL), the **data** (a paste area for inline, a URL field for URL source), and an optional **comment**. Save is disabled until a name and valid data/URL are present. For a **URL** dataset, saving fetches and snapshots the address; the Save control shows a busy state while fetching. If the fetch fails — offline, blocked by the host (cross-origin), not found, empty, or timed out — the form shows a readable, cause-specific error and offers a one-click **"Paste data inline instead"** that switches the form to an inline paste (keeping the name and comment), rather than saving a broken record. On success the new dataset is shown selected in the detail pane — that visible result is the confirmation, so no toast is raised (a dataset created _off-screen_ via Extract does toast; see _Spec Editor_).
+- **New / Create New** — opens the create form in the detail pane with fields: **name** (required, unique), **source** toggle (Inline / URL), the **data** (a paste area for inline, a URL field for URL source), and an optional **comment**. Save is disabled until a name and valid data/URL are present. For a **URL** dataset, saving fetches and snapshots the address; the Save control shows a busy state while fetching. If the fetch fails — offline, blocked by the host (cross-origin), not found, empty, timed out, or larger than the fetch size limit — the form shows a readable, cause-specific error and offers a one-click **"Paste data inline instead"** that switches the form to an inline paste (keeping the name and comment), rather than saving a broken record. The size-limit case is the exception: because pasting an oversized file inline would not help, its message points at using a smaller or pre-aggregated source rather than the inline fallback. On success the new dataset is shown selected in the detail pane — that visible result is the confirmation, so no toast is raised (a dataset created _off-screen_ via Extract does toast; see _Spec Editor_).
 - **Refresh** (URL datasets) — re-fetches the dataset's address and re-snapshots it, re-profiling the data and advancing the last-fetched time. The updated figures are the visible confirmation, so success raises no toast; a failed refresh raises an error toast. Refresh is also how a URL dataset that has not yet been fetched (e.g. one migrated from an older version) acquires its snapshot.
 - **Edit** — rename, edit the comment, change the source data, and (for URL datasets) change the address. Updating inline data re-profiles it; changing a URL dataset's address re-fetches and re-snapshots it; editing only a URL dataset's name or comment does **not** re-fetch. The modified timestamp advances.
 - **Delete** — asks for confirmation ("Delete \"Name\"? This cannot be undone."), then removes the dataset and clears the selection.
diff --git a/src/app/infrastructure/remote-data.test.ts b/src/app/infrastructure/remote-data.test.ts
index 804fd8f..9bb044c 100644
--- a/src/app/infrastructure/remote-data.test.ts
+++ b/src/app/infrastructure/remote-data.test.ts
@@ -8,15 +8,18 @@ function fakeResponse(opts: {
   statusText?: string;
   body?: string;
   contentType?: string | null;
+  contentLength?: number;
 }): Response {
+  const headers: Record<string, string | null> = {
+    'content-type': opts.contentType ?? null,
+    'content-length': opts.contentLength !== undefined ? String(opts.contentLength) : null,
+  };
   return {
     ok: opts.ok,
     status: opts.status ?? (opts.ok ? 200 : 500),
     statusText: opts.statusText ?? '',
     text: () => Promise.resolve(opts.body ?? ''),
-    headers: {
-      get: (k: string) => (k.toLowerCase() === 'content-type' ? (opts.contentType ?? null) : null),
-    },
+    headers: { get: (k: string) => headers[k.toLowerCase()] ?? null },
   } as unknown as Response;
 }
 
@@ -84,6 +87,36 @@ describe('fetchRemoteData', () => {
     });
   });
 
+  test('rejects an oversized body declared by Content-Length, before reading it', async () => {
+    const text = vi.fn(() => Promise.resolve('x'));
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(() =>
+        Promise.resolve({
+          ok: true,
+          status: 200,
+          statusText: '',
+          text,
+          headers: { get: (k: string) => (k.toLowerCase() === 'content-length' ? '999' : null) },
+        } as unknown as Response),
+      ),
+    );
+    await expect(
+      fetchRemoteData('https://example.com/huge.csv', { maxBytes: 100 }),
+    ).rejects.toMatchObject({ reason: 'too-large' });
+    expect(text).not.toHaveBeenCalled(); // never read the body
+  });
+
+  test('rejects an oversized body as a backstop when no length is declared', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(() => Promise.resolve(fakeResponse({ ok: true, body: 'abcdefghij' }))),
+    );
+    await expect(
+      fetchRemoteData('https://example.com/chunked.csv', { maxBytes: 5 }),
+    ).rejects.toMatchObject({ reason: 'too-large' });
+  });
+
   test('the thrown error is a RemoteFetchError', async () => {
     vi.stubGlobal(
       'fetch',
diff --git a/src/app/infrastructure/remote-data.ts b/src/app/infrastructure/remote-data.ts
index 96548c4..a42fc88 100644
--- a/src/app/infrastructure/remote-data.ts
+++ b/src/app/infrastructure/remote-data.ts
@@ -16,7 +16,7 @@
  */
 
 /** Why a remote fetch failed, in terms the UI maps to copy + a recovery path. */
-export type RemoteFetchReason = 'network' | 'http' | 'empty' | 'timeout';
+export type RemoteFetchReason = 'network' | 'http' | 'empty' | 'timeout' | 'too-large';
 
 /** A classified remote-fetch failure. `status` is set only for `http`. */
 export class RemoteFetchError extends Error {
@@ -42,9 +42,20 @@ export interface RemoteData {
 /** Default ceiling on a single fetch before it is aborted as a timeout. */
 const DEFAULT_TIMEOUT_MS = 30_000;
 
+/**
+ * Hard ceiling on a snapshotted remote body. A URL dataset is held whole in memory
+ * while profiling and then stored whole in IndexedDB, so an unbounded download
+ * (a multi-hundred-MB file) would jank or exhaust the tab. Checked against the
+ * declared `Content-Length` before reading, and against the body as a backstop for
+ * chunked responses that declare no length.
+ */
+export const MAX_REMOTE_BYTES = 50 * 1024 * 1024;
+
 export interface FetchRemoteDataOptions {
   /** Abort the fetch after this many ms (default 30 s). */
   timeoutMs?: number;
+  /** Reject a body larger than this many bytes (default {@link MAX_REMOTE_BYTES}). */
+  maxBytes?: number;
 }
 
 function describeNetworkError(err: unknown): string {
@@ -52,6 +63,11 @@ function describeNetworkError(err: unknown): string {
   return `Couldn't reach the URL (${reason}).`;
 }
 
+/** A byte count as a rounded MB string for diagnostic messages (e.g. `120 MB`). */
+function megabytes(bytes: number): string {
+  return `${Math.round(bytes / (1024 * 1024))} MB`;
+}
+
 /**
  * Fetch a remote dataset resource once and return its raw body. Throws
  * `RemoteFetchError` on any failure — the caller distinguishes recovery by
@@ -63,6 +79,7 @@ export async function fetchRemoteData(
   options: FetchRemoteDataOptions = {},
 ): Promise<RemoteData> {
   const timeoutMs = options.timeoutMs ?? DEFAULT_TIMEOUT_MS;
+  const maxBytes = options.maxBytes ?? MAX_REMOTE_BYTES;
   const controller = new AbortController();
   const timer = setTimeout(() => controller.abort(), timeoutMs);
 
@@ -88,12 +105,24 @@ export async function fetchRemoteData(
     );
   }
 
+  // Reject an oversized body before reading it, when the server declares the size —
+  // so a huge download never lands in memory in the first place.
+  const declared = Number(response.headers.get('content-length'));
+  if (Number.isFinite(declared) && declared > maxBytes) {
+    throw new RemoteFetchError('too-large', `The file is about ${megabytes(declared)}.`);
+  }
+
   const text = await response.text();
   if (text.trim() === '') {
     throw new RemoteFetchError('empty', 'The URL returned no data.');
   }
 
-  // TODO: guard very large responses (cap or warn) before snapshotting into
-  // IndexedDB — a multi-hundred-MB file would load fully into memory + storage.
+  // Backstop for a chunked response that declared no Content-Length: the body is
+  // already in memory, but this still keeps it out of IndexedDB. `text.length`
+  // (UTF-16 units) slightly under-counts multibyte bytes — fine for a ceiling.
+  if (text.length > maxBytes) {
+    throw new RemoteFetchError('too-large', `The file is over ${megabytes(maxBytes)}.`);
+  }
+
   return { text, contentType: response.headers.get('content-type') };
 }
diff --git a/src/app/services/remote-data-errors.test.ts b/src/app/services/remote-data-errors.test.ts
index 9c15723..4dfc9d0 100644
--- a/src/app/services/remote-data-errors.test.ts
+++ b/src/app/services/remote-data-errors.test.ts
@@ -32,6 +32,13 @@ describe('remoteFetchErrorMessage', () => {
     expect(msg).not.toMatch(/inline/i);
   });
 
+  test('a too-large body names the size limit and a real fix, not the inline fallback', () => {
+    const msg = remoteFetchErrorMessage(new RemoteFetchError('too-large', 'x'));
+    expect(msg).toMatch(/too large/i);
+    expect(msg).toMatch(/MB/);
+    expect(msg).not.toMatch(/inline/i);
+  });
+
   test('an unknown error still produces a sensible fallback message', () => {
     expect(remoteFetchErrorMessage(new Error('boom'))).toMatch(/couldn't fetch this url/i);
   });
diff --git a/src/app/services/remote-data-errors.ts b/src/app/services/remote-data-errors.ts
index 716635a..97ad34d 100644
--- a/src/app/services/remote-data-errors.ts
+++ b/src/app/services/remote-data-errors.ts
@@ -10,7 +10,7 @@
  * create/edit form can fall back to pasting inline; Refresh can only retry.
  */
 
-import { RemoteFetchError } from '../infrastructure/remote-data';
+import { MAX_REMOTE_BYTES, RemoteFetchError } from '../infrastructure/remote-data';
 
 /** Where the error surfaces, which decides the recovery the copy points at. */
 export type FetchRecovery = 'inline' | 'retry';
@@ -45,6 +45,10 @@ export function remoteFetchErrorMessage(err: unknown, recovery: FetchRecovery =
         return `This URL returned no data. Make sure it points directly at a data file. ${next}`;
       case 'timeout':
         return `This URL took too long to respond. ${next}`;
+      case 'too-large':
+        // Neither inline-paste nor a retry helps an oversized file, so this points
+        // at the only real fix instead of the shared next step.
+        return `This file is too large to store as a dataset (over ${Math.round(MAX_REMOTE_BYTES / (1024 * 1024))} MB). Use a smaller file, or pre-aggregate the data before loading it.`;
     }
   }
   return `Couldn't fetch this URL. ${next}`;