[BUG] Bug report: Claude Desktop crashes on streaming powershell code blocks

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment

Diagnostics
  Currently running: native (2.1.111)
  Path: C:\Users\pavel.dudko\AppData\Roaming\Claude\claude-code\2.1.111\claude.exe
  Config install method: native
  Search: OK (bundled)

Updates
  Auto-updates: enabled
  Auto-update channel: latest
  Stable version: 2.1.104
  Latest version: 2.1.116

OS: Windows Server 2022 Standard, 10.0.20348
Claude Desktop app: 1.3561.0

• Product: Claude Desktop (Windows)
• App version: 1.3561.0
• Install path: C:\Users\<user>\AppData\Local\AnthropicClaude\app-1.3561.0\
• OS: Windows Server 2022 Standard, 10.0.20348

Summary

The renderer process emits a cascade of unhandled promise rejections, then hits Fatal error boundary, whenever an assistant message streams a fenced code block tagged `powershell . Short aliases (ps1, pwsh, bash) do not reproduce.

Root cause (from logs)

The Shiki-based syntax highlighter appears to call resolveLanguage(tag) on every partial prefix as the language tag streams in. For powershell that is 10 calls, each throwing:

Uncaught (in promise) Error: resolveLanguage: "p" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "po" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "pow" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "powe" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "power" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "powers" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "powersh" not found in bundled or custom languages
Uncaught (in promise) Error: resolveLanguage: "powershe" not found in bundled or custom languages

Rejections accumulate → [BOOTSTRAP] Fatal error boundary triggered → renderer dies / white-screens.

Log evidence

File: %APPDATA%\Claude\logs\claude.ai-web.log

Matching resolveLanguage rejections (each coincides with a powershell code block rendering in chat):

• 2026-04-21 10:29:02 — 5 rejections
• 2026-04-21 10:33:09–10 — 5 rejections
• 2026-04-21 10:35:44 — 3 rejections
• 2026-04-21 10:38:14 — 3 rejections

Associated boundary crashes in the same log (earlier, heavier-use dates):

• 2026-03-04 11:43:46 – 20:05:22 — ~15 [BOOTSTRAP] Fatal error boundary triggered entries
• 2026-03-05 21:48:09 – 21:48:19
• 2026-03-10 13:59:59 – 14:00:17

Stack trace of a representative fatal error (from claude.ai-web.log):

[error] Uncaught Error: Minified React error #418; visit https://react.dev/errors/418 ...

Steps to reproduce

1. Have the assistant emit a fenced code block tagged `powershell (e.g. ask "give me a PowerShell one-liner to list services").
2. Observe %APPDATA%\Claude\logs\claude.ai-web.log — resolveLanguage rejections appear as the tag streams character by character.
3. Over a session, accumulated rejections cause the renderer to hit Fatal error boundary and the chat view crashes / reloads.

Expected behavior

Language tag resolution should either:

• wait until the opening fence is complete (newline seen) before resolving, or
• swallow rejections for partial/unknown tags silently, or
• pre-register the full Shiki alias set so partial matches return a placeholder.

Workaround

Users can prompt the model to use ps1 or pwsh instead of powershell. Shorter tag = 3 partial resolutions instead of 10, which does not accumulate enough to crash.

Likely broader impact

Any language tag longer than ~5 characters will emit extra rejections: typescript, javascript, markdown, dockerfile, yaml (borderline), etc. Worth auditing for those too — powershell just happens to be the longest common one and is thus the first to cross whatever threshold triggers the boundary.

Suggested code location

Whatever wraps Shiki's resolveLanguage in index-BKPqcTdh.js (seen in stack traces from assets-proxy.anthropic.com/claude-ai/v2/assets/v1/). A try/catch around the streaming-tag path, or debouncing resolution until the fence line closes, should fix it.

What Should Happen?

Streaming a fenced code block with the ```powershell language tag should highlight the block without crashing the renderer.

Specifically, Shiki's resolveLanguage call should either:

• wait until the opening fence line is complete (newline seen) before attempting to resolve the tag, or
• silently ignore unknown partial tags during streaming instead of throwing unhandled promise rejections, or
• have every known language + alias pre-registered so partial-prefix lookups return a safe placeholder.

No Uncaught (in promise) errors should appear in claude.ai-web.log, and no Fatal error boundary should be triggered as a result of normal code block rendering.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Desktop on Windows (v1.3561.0).
2. Start a new chat and ask the assistant to produce a PowerShell snippet, e.g. "give me a PowerShell one-liner to list running services".
3. The assistant streams a response containing a fenced code block opened with ```powershell.
4. While the language tag is streaming character-by-character, observe %APPDATA%\Claude\logs\claude.ai-web.log — a series of "Uncaught (in promise) Error: resolveLanguage: \"p\" ... \"po\" ... \"pow\" ... \"powershe\" not found in bundled or custom languages" entries appear, one per partial prefix.
5. Repeat across several messages / sessions. Rejections accumulate and eventually [BOOTSTRAP] Fatal error boundary triggered is logged and the chat view crashes / white-screens / reloads.

Does NOT reproduce when the assistant uses shorter language tags such as ``ps1, `pwsh, or ``bash — only 2–3 partial-prefix rejections fire, which is not enough to trigger the boundary.

Reproducible log timestamps from my environment (claude.ai-web.log):

• 2026-04-21 10:29:02, 10:33:09, 10:35:44, 10:38:14 — resolveLanguage rejections on "powershell"
• 2026-03-04 11:43:46–20:05:22 — ~15 Fatal error boundary triggered entries during heavier use

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.111

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_