diff --git a/.changeset/tidy-markdoc-dashed-keys.md b/.changeset/tidy-markdoc-dashed-keys.md
new file mode 100644
index 000000000000..159333f090e0
--- /dev/null
+++ b/.changeset/tidy-markdoc-dashed-keys.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/markdoc': patch
+---
+
+Fixes custom `transform` functions being incorrectly dropped for tags and nodes whose names require bracket access (e.g. `side-note`). The check that detects whether a transform respects a custom `render` component now recognizes bracket notation, optional chaining and whitespace, not only dot notation.
diff --git a/packages/integrations/markdoc/src/runtime.ts b/packages/integrations/markdoc/src/runtime.ts
index efbcc03e22bf..17636a3a2104 100644
--- a/packages/integrations/markdoc/src/runtime.ts
+++ b/packages/integrations/markdoc/src/runtime.ts
@@ -147,11 +147,15 @@ function syncTagNodeAttributes(config: MergedConfig): void {
  */
 function transformRespectsRender(transform: { toString(): string }, configKey: string): boolean {
 	const source = transform.toString();
-	// Astro's transforms check config.nodes?.X?.render or config.tags?.X?.render
-	return (
-		source.includes(`config.nodes?.${configKey}?.render`) ||
-		source.includes(`config.tags?.${configKey}?.render`)
+	// `configKey` may not be a valid identifier (e.g. `side-note`), so a render-respecting
+	// transform can read `render` via dot or bracket access. Escape the key and match either.
+	const key = configKey.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+	const member = (prop: string) =>
+		`(?:\\??\\.\\s*${prop}|\\??\\.?\\s*\\[\\s*['"\`]${prop}['"\`]\\s*\\])`;
+	const pattern = new RegExp(
+		`config\\s*\\??\\.\\s*(?:nodes|tags)\\s*${member(key)}\\s*${member('render')}`,
 	);
+	return pattern.test(source);
 }
 
 export function resolveComponentImports(
diff --git a/packages/integrations/markdoc/test/resolve-component-imports.test.ts b/packages/integrations/markdoc/test/resolve-component-imports.test.ts
new file mode 100644
index 000000000000..b98b55da757f
--- /dev/null
+++ b/packages/integrations/markdoc/test/resolve-component-imports.test.ts
@@ -0,0 +1,72 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import type { AstroInstance } from 'astro';
+import { resolveComponentImports } from '../dist/runtime.js';
+
+type MarkdocConfig = Parameters<typeof resolveComponentImports>[0];
+type NodeComponentMap = Parameters<typeof resolveComponentImports>[2];
+
+// Minimal stand-in for an Astro component; only referential identity matters here.
+const Component = (() => null) as unknown as AstroInstance['default'];
+const noNodeComponents = {} as NodeComponentMap;
+
+/**
+ * Regression test for issue #17118: tag/node names containing characters that
+ * require bracket access (e.g. `side-note`) were always treated as not respecting
+ * `render`, because the detection only matched dot notation.
+ */
+describe('Markdoc - resolveComponentImports', () => {
+	it('keeps a render-respecting transform for tag names that need bracket access (e.g. dashes)', () => {
+		const markdocConfig: MarkdocConfig = {
+			tags: {
+				'side-note': {
+					// A dashed key can only read `render` through bracket notation.
+					transform(node, config) {
+						if (config.tags?.['side-note']?.render) return [];
+						return node.transformChildren(config);
+					},
+				},
+			},
+			nodes: {},
+		};
+
+		const resolved = resolveComponentImports(
+			markdocConfig,
+			{ 'side-note': Component },
+			noNodeComponents,
+		);
+
+		assert.equal(resolved.tags['side-note'].render, Component);
+		assert.equal(
+			typeof resolved.tags['side-note'].transform,
+			'function',
+			'a render-respecting transform should be preserved for dashed tag names',
+		);
+	});
+
+	it('removes a transform that does not respect render so the custom component wins', () => {
+		const markdocConfig: MarkdocConfig = {
+			tags: {
+				'side-note': {
+					transform(node, config) {
+						return node.transformChildren(config);
+					},
+				},
+			},
+			nodes: {},
+		};
+
+		const resolved = resolveComponentImports(
+			markdocConfig,
+			{ 'side-note': Component },
+			noNodeComponents,
+		);
+
+		assert.equal(resolved.tags['side-note'].render, Component);
+		assert.equal(
+			resolved.tags['side-note'].transform,
+			undefined,
+			'a non-render-respecting transform should be removed so `render` wins',
+		);
+	});
+});