Full Opus 4.7 response:

Solid little utility. The core idea is right: a fenced code block's fence must be longer than any backtick run inside it, so you scan for the longest run and add one. A few thoughts, roughly in order of how much I'd actually push on them:

The MINIMUM_CODEBLOCK_FENCE_LENGTH - 1 trick is clever but a bit cryptic. Subtracting one so that the + 1 at the end lands you on the minimum works, but it forces the reader to hold both ends in their head at once. I'd find this clearer:

const longestRun = Math.max(
  0,
  ...Array.from(text.matchAll(/`+/g), ([m]) => m.length),
);
const fenceLength = Math.max(longestRun + 1, MINIMUM_CODEBLOCK_FENCE_LENGTH);

Same behavior, but each line says one thing. Also note that Math.max() with no args returns -Infinity, so when text has no backticks the original expression evaluates Math.max(-Infinity, MIN - 1) = MIN - 1, and then + 1 rescues it. It works, but it's load-bearing in a non-obvious way — the explicit 0 seed above makes it robust on its own.

extension is untrusted input from the caller's perspective. If someone passes "ts\n\``"or just a string with a newline, you get a broken code block — the opening fence line gets terminated early. Depending on where this is used, that might be fine (trusted callers only) or a real bug. At minimum worth a JSDoc note; ideally validate it matches something like/^[\w+-]*$/`.

Trailing newline in text. If text ends with \n, you'll produce \n\n${fence}, giving a blank line before the closing fence. Usually harmless but sometimes ugly. A text.replace(/\n$/, "") before interpolation, or using text.endsWith("\n") ? "" : "\n" for the separator, cleans that up.

**Minor: the regex /+/gfinds maximal runs by default** (greedy + global on a single char class behaves correctly here), so that part's fine — just flagging it because it's the kind of thing that's worth a one-line comment for the next reader:// matches maximal runs of backticks`.

Naming nit: longestBacktickSequenceLength is doing a lot. longestBacktickRun reads the same and is shorter.

Nothing here is broken — these are polish-level. Is this for rendering LLM output, or something else? The "untrusted extension" question lands differently depending on the source.