[BUG] TUI crashes on `constructor.hpp` with `(r||"").toLowerCase is not a function`

Found 3 possible duplicate issues:

1. https://github.com/anthropics/claude-code/issues/40452
2. https://github.com/anthropics/claude-code/issues/40160
3. https://github.com/anthropics/claude-code/issues/39542

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🤖 Generated with Claude Code

I dug into this further and found a likely root cause.

This appears to be an inherited-property bug in the filename/stem-to-language lookup used during diff or syntax-highlighting.

For filenames like constructor.hpp, the stem constructor can resolve to the inherited Object.prototype.constructor when a plain object is used as a dictionary. That non-string value then flows into the language-detection path and eventually triggers:

(r||"").toLowerCase is not a function

This likely affects other inherited property names too, such as:

• toString.*
• valueOf.*
• hasOwnProperty.*
• __proto__.*

Suggested fix:

• define the filename map with Object.create(null), or
• guard so only string values are passed into the language-detection path

Also experiencing this on macOS (Darwin 25.2.0). The crash happens consistently when resuming after an interrupted response, especially in longer conversations with Go code.

Stack trace is identical:

(r||"").toLowerCase is not a function. (In '(r||"").toLowerCase()', '(r||"").toLowerCase' is undefined)

Been hitting this for ~3 days. The workaround of giving specific instructions instead of "continue" sometimes helps, but it's not reliable.

Seems to occur with any file that has the name constructor in it:

TUI crash: (r||"").toLowerCase is not a function when editing files named constructor.*

Summary

Claude Code crashes when rendering diffs for files whose basename (before the extension) matches an Object.prototype property name, most commonly constructor. The crash occurs in the syntax highlighting language detection code.

Version

• Claude Code 2.1.90
• Platform: Linux (WSL2)
• Runtime: Bun

Steps to reproduce

1. Create a file named constructor.py (or constructor.js, constructor.rs, etc.)
2. Start Claude Code and ask it to edit that file
3. Claude Code crashes during diff rendering

Minimal repro:

mkdir /tmp/repro && cd /tmp/repro
echo "x = 1" > constructor.py
claude  # then ask: "change x to 2 in constructor.py"

Error

ERROR  (r||"").toLowerCase is not a function. (In '(r||"").toLowerCase()', '(r||"").toLowerCase' is undefined)

 /$bunfs/root/src/entrypoints/cli.js:1321:2087

 - g (/$bunfs/root/src/entrypoints/cli.js:1321:2087)
 - XsK (/$bunfs/root/src/entrypoints/cli.js:2760:6419)
 - render (/$bunfs/root/src/entrypoints/cli.js:2762:2254)

Root cause

The XsK function detects the syntax highlighting language from a file path. It uses a plain object YsK as a filename-to-language map:

YsK = {Dockerfile: "dockerfile", Makefile: "makefile", Rakefile: "ruby", Gemfile: "ruby", CMakeLists: "cmake"}

The lookup logic extracts the basename without extension and checks the map:

let q = path.basename(filePath),        // "constructor.py"
    _ = q.split(".")[0] ?? "",           // "constructor"
    f = YsK[q] ?? YsK[_];              // YsK["constructor"] → Object.prototype.constructor!

Since YsK is a plain {}, YsK["constructor"] resolves to Object.prototype.constructor (the built-in Object function) via prototype chain lookup. This is a truthy non-string value, so it passes the guard if (f && ...) and is forwarded to hljs.getLanguage(), which calls .toLowerCase() on it and crashes.

Affected filenames

Any file whose name before the first . matches an Object.prototype property:

• constructor.py (most common in practice)
• toString.js
• valueOf.ts
• hasOwnProperty.go
• etc.

Suggested fix

Replace the plain object with Object.create(null) to prevent prototype chain lookups:

// Before
YsK = {Dockerfile: "dockerfile", Makefile: "makefile", ...}

// After
YsK = Object.assign(Object.create(null), {Dockerfile: "dockerfile", Makefile: "makefile", ...})

Or add a hasOwnProperty guard:

f = Object.hasOwn(YsK, q) ? YsK[q] : Object.hasOwn(YsK, _) ? YsK[_] : undefined;

Workaround

Disable syntax highlighting:

export CLAUDE_CODE_SYNTAX_HIGHLIGHT=off

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.