Filesystem extension broken in Cowork/Code sessions on 1.46388.2: era probe marks server legacy, then draft-07 tool schemas rejected

Status Open

Maintainer reply None cached

Activity 0 comments · opened Sep 4, 2026

Summary

After updating Claude Desktop to 1.46388.2, the Filesystem extension stopped working in Cowork and Code sessions. It still connects for the desktop app itself. Two separate failures, one after the other.

The second failure is not specific to Filesystem. Any MCP server whose tool schemas declare JSON Schema draft-07 is affected, and draft-07 is what most published MCP servers currently ship.

Environment

macOS, Apple Silicon
Claude Desktop 1.46388.2 (worked on 1.40609.1)
Electron 42.10.0, bundled Node 24.18.1
Filesystem extension, @modelcontextprotocol/server-filesystem v2026.7.4
First failure logged 2026-09-04 08:48, shortly after the update

Failure 1: version negotiation, before reinstall

From ~/Library/Logs/Claude/mcp-server-Filesystem.log:

Era probe verdict: legacy (sibling did not complete the exchange)
Server started and connected successfully
Couldn't start for Cowork and Code sessions.
Error: Version negotiation failed: the connection closed during the
server/discover probe (this transport probed in place - the disposable
sibling probe requires the SDK's base StdioClientTransport)
{ context: 'shared-pool' }

Note the server connects successfully for the app, then fails only for the shared pool.

Failure 2: schema dialect, after clean reinstall

Uninstalled and reinstalled the extension. Connection now succeeds and all 14 tools are announced, but every tool call is refused before touching disk:

Tool 'list_directory' has an invalid outputSchema: JSON Schema declares an
unsupported dialect ("$schema": "http://json-schema.org/draft-07/schema#").
The default validator supports JSON Schema 2020-12 only.

Same error on get_file_info and list_allowed_directories.

Ruled out

Server package: starts clean standalone via npx -y @modelcontextprotocol/server-filesystem@latest ~/Downloads
Node version: the app uses its own bundled Node, not the system one
npx cache: cleared, no change
Config: claude_desktop_config.json contains no mcpServers block. The extension is installed through the UI.
Allowed directories: set correctly, four valid absolute paths
Disable/re-enable, quit and relaunch, full uninstall and reinstall: no change

Expected

Either the validator accepts draft-07 tool schemas, or the bundled server packages ship 2020-12 schemas.

Impact

Anthropic's own Filesystem extension is unusable in Cowork and Code sessions on this build. Support confirmed the schema behavior and said there is no config workaround available to users.

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