diff --git a/src/lib/utils/text-parser.test.ts b/src/lib/utils/text-parser.test.ts
index d2ad7e1..672c5a6 100644
--- a/src/lib/utils/text-parser.test.ts
+++ b/src/lib/utils/text-parser.test.ts
@@ -117,4 +117,11 @@ describe('stripRichTextMarkup', () => {
 		expect(out).not.toContain('{{');
 		expect(out.toLowerCase()).toContain('handshake');
 	});
+
+	it('leaves Mermaid node-shape syntax untouched (non-slug ids)', () => {
+		// Mermaid reuses {{…}} (hexagon) and [[…]] (subroutine) — these are not
+		// our refs and must survive stripping so diagram labels stay valid.
+		expect(stripRichTextMarkup('T3{{"Middlebox"}}')).toBe('T3{{"Middlebox"}}');
+		expect(stripRichTextMarkup('N[[Subroutine Step]]')).toBe('N[[Subroutine Step]]');
+	});
 });
diff --git a/src/lib/utils/text-parser.ts b/src/lib/utils/text-parser.ts
index a9859a9..0a2ceca 100644
--- a/src/lib/utils/text-parser.ts
+++ b/src/lib/utils/text-parser.ts
@@ -243,6 +243,10 @@ function bracketFallbackLabel(rawId: string): string {
 	}
 }
 
+/** A real cross-reference id is a lowercase slug (optionally `prefix:id` or
+ *  `part/chapter`). Used to tell our refs apart from Mermaid shape syntax. */
+const REF_ID = /^[a-z0-9][a-z0-9:/-]*$/;
+
 /**
  * Strip rich-text atoms ([[…]] / {{…}} / **…**) down to their label
  * text. For surfaces that render strings as raw DOM text (screen-reader
@@ -261,12 +265,19 @@ export function stripRichTextMarkup(raw: string): string {
 	// abbreviation, concept term, …) rather than vanishing — otherwise a bare
 	// `[[tcp]]` in a stripped field (tooltip / aria-label) would silently drop
 	// the word.
+	//
+	// IMPORTANT: only resolve when the id is slug-shaped (lowercase kebab/colon).
+	// Mermaid reuses `{{…}}` (hexagon) and `[[…]]` (subroutine) for node *shapes*,
+	// e.g. `T3{{"Middlebox"}}`. Those aren't our refs — left untouched so this
+	// function (which also cleans Mermaid node labels) doesn't corrupt diagrams.
 	return raw
-		.replace(/\[\[([^\][|]+)(?:\|([^\]]+))?\]\]/g, (_m, id, label) =>
-			label != null ? label : bracketFallbackLabel(id)
-		)
-		.replace(/\{\{([^}{|]+)(?:\|([^}]+))?\}\}/g, (_m, id, label) =>
-			label != null ? label : (getConceptById(id)?.term ?? id)
-		)
+		.replace(/\[\[([^\][|]+)(?:\|([^\]]+))?\]\]/g, (m, id, label) => {
+			if (label != null) return label;
+			return REF_ID.test(id) ? bracketFallbackLabel(id) : m;
+		})
+		.replace(/\{\{([^}{|]+)(?:\|([^}]+))?\}\}/g, (m, id, label) => {
+			if (label != null) return label;
+			return REF_ID.test(id) ? (getConceptById(id)?.term ?? id) : m;
+		})
 		.replace(/\*\*([^*]+)\*\*/g, '$1');
 }