diff --git a/README.md b/README.md
index f121d21..7aeaeeb 100644
--- a/README.md
+++ b/README.md
@@ -447,7 +447,7 @@ Helpers and send-flow primitives also exported:
 
 ```ts
 import {
-  toSlackBlocks,           // strips builder-only fields (e.g. header `level`) before sending
+  toSlackBlocks,           // scrubs unsafe URLs + retrieval-only fields before sending
   encodeBlocksToString,    // base64url-encode a blocks array (for URL state)
   decodeBlocksFromString,
   defaultPalette,          // the built-in palette — spread to customize
diff --git a/src/components/editors/block-editor.stories.tsx b/src/components/editors/block-editor.stories.tsx
index 74b88a8..7db2539 100644
--- a/src/components/editors/block-editor.stories.tsx
+++ b/src/components/editors/block-editor.stories.tsx
@@ -332,6 +332,22 @@ export const TypingHeaderTextProducesValidBlock: Story = {
   }
 };
 
+// Regression: header `level` is a real Slack field, so picking a level in
+// the editor must survive `toSlackBlocks` and reach the wire payload.
+// It used to be stripped on the way out, silently discarding the choice.
+export const SelectingHeaderLevelProducesValidBlock: Story = {
+  args: { block: variant('structure_header') },
+  play: async ({ canvasElement, args }) => {
+    const canvas = within(canvasElement);
+    await userEvent.click(await canvas.findByLabelText(/^h2$/i));
+    await expect(args.onChange).toHaveBeenCalled();
+    const latest = expectLastOnChangeIsValid(args.onChange as ReturnType<typeof fn>);
+    expect(latest.type).toBe('header');
+    expect(latest).toHaveProperty('level', 2);
+    expect(toSlackBlocks([latest])[0]).toHaveProperty('level', 2);
+  }
+};
+
 export const TypingMarkdownTextProducesValidBlock: Story = {
   args: { block: variant('markdown_list') },
   play: async ({ canvasElement, args }) => {
diff --git a/src/components/editors/header-editor.tsx b/src/components/editors/header-editor.tsx
index a0c7a36..0961525 100644
--- a/src/components/editors/header-editor.tsx
+++ b/src/components/editors/header-editor.tsx
@@ -13,7 +13,7 @@ const HEADER_LEVELS: HeaderLevel[] = [1, 2, 3, 4];
 /**
  * Editor form for header blocks. Slack enforces plain text only.
  * Shows a character count against the 150-char Slack limit, plus an
- * optional 1-4 level selector (builder-only extension; Slack ignores it).
+ * optional 1-4 heading level selector.
  * @param props - editor props
  * @param props.block - the header block to edit
  * @param props.onChange - called with the updated block payload
@@ -44,7 +44,7 @@ export function HeaderEditor({ block, onChange }: BlockEditorProps<SupportedHead
           <EmojiTextInsertButton targetRef={inputRef} value={text} onChange={setText} className="shrink-0 border" />
         </div>
       </EditorField>
-      <EditorField label="Level" help="Optional. Builder-only extension; Slack's API ignores this value.">
+      <EditorField label="Level" help="Optional. Sets the heading size Slack renders, from H1 (largest) to H4.">
         <RadioGroup
           value={level === undefined ? 'default' : String(level)}
           onValueChange={(v) => {
diff --git a/src/lib/to-slack-blocks.ts b/src/lib/to-slack-blocks.ts
index 55364d0..fc83f34 100644
--- a/src/lib/to-slack-blocks.ts
+++ b/src/lib/to-slack-blocks.ts
@@ -2,24 +2,23 @@ import type { SupportedBlock } from '../types';
 import { sanitizeBlock } from './sanitize-blocks';
 
 /**
- * Strip builder-only fields from blocks so they conform to Slack's
- * Block Kit schema, and scrub dangerous URI schemes from any
- * `url`/`image_url` fields. Currently removes the cosmetic `level`
- * field from header blocks (a builder-only extension Slack would
- * reject) and routes every URL/image-url through the allowlist in
- * `lib/url-safety.ts` so a payload that round-trips through the
- * builder cannot carry `javascript:`/`data:text/html` URIs to a
- * downstream consumer or to the Slack API.
+ * Prepare blocks for the Slack API: scrub dangerous URI schemes from any
+ * `url`/`image_url` fields and drop the read-only metadata Slack attaches
+ * on retrieval but rejects on send. Every URL/image-url is routed through
+ * the allowlist in `lib/url-safety.ts` so a payload that round-trips
+ * through the builder cannot carry `javascript:`/`data:text/html` URIs to
+ * a downstream consumer or to the Slack API.
+ *
+ * Header `level` is passed through: it is a real Slack field (an integer
+ * 1-4, see https://docs.slack.dev/reference/block-kit/blocks/header-block),
+ * not a builder-only extension. Earlier versions stripped it here, which
+ * silently dropped the heading level from every sent message. Out-of-range
+ * values are left intact so they surface as validation errors rather than
+ * being quietly discarded.
+ *
  * @param blocks - the working draft blocks
- * @returns blocks with builder-only fields removed and URLs scrubbed
+ * @returns blocks with retrieval-only fields removed and URLs scrubbed
  */
 export function toSlackBlocks(blocks: SupportedBlock[]): SupportedBlock[] {
-  return blocks.map((block) => {
-    const safe = sanitizeBlock(block);
-    if (safe.type === 'header' && 'level' in safe) {
-      const { level: _omit, ...rest } = safe;
-      return rest as SupportedBlock;
-    }
-    return safe;
-  });
+  return blocks.map((block) => sanitizeBlock(block));
 }
diff --git a/src/types.ts b/src/types.ts
index 0a1285d..ee50877 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -407,15 +407,17 @@ export interface ContainerBlock {
 }
 
 /**
- * Header heading level shown in the preview. Slack's API has no
- * `level` field on header blocks, so this is a builder-only extension
- * that round-trips on the block payload but is otherwise cosmetic.
+ * Header heading level. A real Slack field on header blocks — an optional
+ * integer 1-4 that Slack renders at the corresponding heading size — so it
+ * round-trips through {@link SupportedBlock} and is sent to the API.
+ * @see https://docs.slack.dev/reference/block-kit/blocks/header-block
  */
 export type HeaderLevel = 1 | 2 | 3 | 4;
 
 /**
  * Header block as edited by the builder. Extends Slack's HeaderBlock
- * with an optional {@link HeaderLevel}.
+ * with an optional {@link HeaderLevel}, which the bundled
+ * `slack-web-api-client` types predate.
  */
 export type SupportedHeaderBlock = HeaderBlock & { level?: HeaderLevel };
 
diff --git a/test/public-api.test.ts b/test/public-api.test.ts
index 6f49320..dcbb0ea 100644
--- a/test/public-api.test.ts
+++ b/test/public-api.test.ts
@@ -6,7 +6,10 @@ import { decodeBlocksFromString, encodeBlocksToString } from '../src/lib/url-sta
 import type { SupportedBlock } from '../src/types';
 
 describe('toSlackBlocks', () => {
-  it('strips the builder-only `level` field from header blocks', () => {
+  // Regression: `level` is a real Slack header field (integer 1-4), not a
+  // builder-only extension. It used to be stripped here, which silently
+  // dropped the heading level from every message the builder sent.
+  it('preserves the `level` field on header blocks', () => {
     const input: SupportedBlock[] = [
       {
         type: 'header',
@@ -17,7 +20,40 @@ describe('toSlackBlocks', () => {
 
     const [out] = toSlackBlocks(input);
     expect(out.type).toBe('header');
+    expect(out).toEqual(input[0]);
+  });
+
+  it('emits header `level` in a payload the validator accepts', () => {
+    for (const level of [1, 2, 3, 4]) {
+      const payload = toSlackBlocks([
+        {
+          type: 'header',
+          level,
+          text: { type: 'plain_text', text: 'Heading', emoji: true }
+        } as SupportedBlock
+      ]);
+      expect(payload[0]).toHaveProperty('level', level);
+      expect(validateBlockKit(payload, { target: 'blocks' }).valid).toBe(true);
+    }
+  });
+
+  it('leaves an out-of-range header `level` intact so it surfaces as a validation error', () => {
+    const payload = toSlackBlocks([
+      {
+        type: 'header',
+        level: 6,
+        text: { type: 'plain_text', text: 'Heading', emoji: true }
+      } as unknown as SupportedBlock
+    ]);
+    expect(payload[0]).toHaveProperty('level', 6);
+    expect(validateBlockKit(payload, { target: 'blocks' }).valid).toBe(false);
+  });
+
+  it('leaves a header block without a `level` untouched', () => {
+    const input: SupportedBlock[] = [{ type: 'header', text: { type: 'plain_text', text: 'Heading', emoji: true } }];
+    const [out] = toSlackBlocks(input);
     expect('level' in out).toBe(false);
+    expect(out).toBe(input[0]);
   });
 
   it('passes non-header blocks through unchanged', () => {