Extension host crashes with `promptCacheKey` TypeError

[finikon]

Title

Extension host crashes with promptCacheKey TypeError → 97% CPU → terminated unexpectedly (multi-root workspace, Windows)

Body

Summary

The extension host process terminates unexpectedly during Copilot Chat agent sessions in a large multi-root workspace. The crash is preceded by a flood of TypeError: Cannot read properties of undefined (reading 'promptCacheKey') errors, followed by the extension host becoming unresponsive with github.copilot-chat consuming ~97% CPU, and ultimately the host crashing with exit code 134.

Environment

• VS Code: 1.113.0
• GitHub Copilot Chat: 0.42.3
• OS: Windows 10.0.26100 (Windows 11)
• Workspace: Multi-root with 9 folders (mix of .NET, TypeScript, Python repos)
• Total workspace size: ~500K+ files across all roots

Crash Timeline (April 5, 2026)

Crash #1 — 11:49:31 (pid 27688)

Extension host (LocalProcess pid: 27688) terminated unexpectedly.

Crash #2 — 15:28:19 (pid 3500) — detailed timeline:

Time	Event
15:27:19	Flood of promptCacheKey TypeError errors begins (20+ errors in 40s)
15:27:58	Extension host becomes unresponsive
15:28:05	CPU profile: github.copilot-chat took 97.12% of 4866ms
15:28:05	Briefly recovers
15:28:15	5 concurrent copilot_findFiles tool calls all fail
15:28:18	More promptCacheKey errors
15:28:19	Extension host terminated (exit code 134, reason: 'crashed')
15:28:19	Auto-restart → new pid 12596
15:28:19	Chat request canceled

Post-restart (pid 12596):
| 15:31:14 | Extension host unresponsive again |
| 15:31:21 | github.copilot-chat took 97.12% of 4866ms |

Recurring Error (60+ times per session)

[Extension Host] Failed to retrieve configuration properties TypeError: Cannot read properties of undefined (reading 'promptCacheKey')

This error appears continuously throughout the session — not just before crashes. It fires on nearly every agent tool invocation.

Steps to Reproduce

1. Open a multi-root workspace with 8-9 folders (large repos, 500K+ files total)
2. Start a Copilot Chat agent session
3. Use tools that trigger file search/discovery across the workspace (e.g., search_subagent which internally calls copilot_findFiles with broad glob patterns)
4. After ~30-60 minutes of active agent usage, the extension host crashes

The crash is more likely when:

• Multiple concurrent copilot_findFiles calls execute simultaneously
• The workspace has many roots with large file counts
• The promptCacheKey errors are already firing frequently

Relevant Log Lines

main.log:

Extension host with pid 3500 exited with code: 134, signal: unknown.
[UtilityProcess id: 3, type: extensionHost, pid: 3500]: crashed with code 134 and reason 'crashed'

renderer.log (pre-crash tool failures):

[LanguageModelToolsService#invokeTool] Error from tool copilot_findFiles with parameters {"query":"**/xxx*/**/charts/**","maxResults":200}:
[LanguageModelToolsService#invokeTool] Error from tool copilot_findFiles with parameters {"query":"**/xxx*/**/config/**","maxResults":200}:
[LanguageModelToolsService#invokeTool] Error from tool copilot_findFiles with parameters {"query":"**/xxx*/**/tests/**","maxResults":200}:
[LanguageModelToolsService#invokeTool] Error from tool copilot_findFiles with parameters {"query":"**/xxx/**/hybrid/**","maxResults":200}:
[LanguageModelToolsService#invokeTool] Error from tool copilot_findFiles with parameters {"query":"**/xxx*/**/prod/**","maxResults":200}:

CPU profiles saved by VS Code:

• exthost-5a6ad3.cpuprofile — 97.51% github.copilot-chat of 4892ms
• exthost-d39aad.cpuprofile — 96.94% github.copilot-chat of 4859ms
• exthost-b259c9.cpuprofile — 97.12% github.copilot-chat of 4866ms

Expected Behavior

• The promptCacheKey property access should be guarded against undefined
• Concurrent copilot_findFiles calls in large workspaces should not saturate the extension host CPU
• The extension host should not crash with exit code 134

Additional Context

• Previously observed same pattern on earlier versions (see user memory notes from March 2026)
• Related but different from #14139 (that's macOS + Remote-SSH + renderer crash code 5; this is Windows + local + extension host exit code 134)
• The workspace does NOT use extensions.experimental.affinity for copilot-chat
• No Remote-SSH involved — all folders are local