fix(custom-tags): restructure inline content with newlines to prevent ATX heading block splitting

packages/streamdown/__tests__/preprocess-custom-tags.test.ts

@@ -51,11 +51,25 @@ describe("preprocessCustomTags", () => {
     expect(preprocessCustomTags(md, ["custom"])).toBe(md);
   });
 
-  it("should not modify content without blank lines", () => {
+  it("should not modify content without blank lines (no newline)", () => {
+    // Single-line content without any newlines is fine as inline HTML
     const md = "<custom>Hello World</custom>";
     expect(preprocessCustomTags(md, ["custom"])).toBe(md);
   });
 
+  it("should restructure inline content with a newline followed by ATX heading", () => {
+    // A heading inside inline content would prematurely terminate the custom tag
+    const md = "<ai-thinking> hi\n # yes</ai-thinking>";
+    const result = preprocessCustomTags(md, ["ai-thinking"]);
+    expect(result).toBe("<ai-thinking>\n hi\n # yes\n</ai-thinking>\n\n");
+  });
+
+  it("should restructure inline content with any newline (prevents block element splitting)", () => {
+    const md = "<custom>hello\nworld</custom>";
+    const result = preprocessCustomTags(md, ["custom"]);
+    expect(result).toBe("<custom>\nhello\nworld\n</custom>\n\n");
+  });
+
   it("should not modify tags where content already starts on own line without blank lines", () => {
     const md = "<custom>\nHello\n</custom>";
     expect(preprocessCustomTags(md, ["custom"])).toBe(md);

packages/streamdown/lib/preprocess-custom-tags.ts

@@ -35,10 +35,16 @@ export const preprocessCustomTags = (
     result = result.replace(
       pattern,
       (_match, open: string, content: string, close: string) => {
-        // Only restructure when content contains blank lines that would
-        // split the HTML block. Tags without blank lines work fine as
-        // inline HTML and don't need restructuring.
-        if (!content.includes("\n\n")) {
+        // Restructure when:
+        // 1. Content has blank lines (\n\n) — these would split the HTML block, or
+        // 2. Content starts inline (not preceded by \n) and contains any newline —
+        //    a newline inside inline content can introduce block-level elements like
+        //    ATX headings (# title) that the parser would treat as a new block,
+        //    prematurely terminating the custom tag scope.
+        const hasBlankLines = content.includes("\n\n");
+        const isInlineContent = !content.startsWith("\n");
+        const hasNewline = content.includes("\n");
+        if (!(hasBlankLines || (isInlineContent && hasNewline))) {
           return open + content + close;
         }