MCPB-installed MCP servers not exposed on first turn of Cowork (works on second turn; works first-turn via claude_desktop_config.json)

Summary

When an MCP server is installed via MCPB locally, its tools are not available on the first turn of a Claude Cowork session. They become available on the second turn. The same MCP server, installed via claude_desktop_config.json, exposes its tools correctly on the first turn.

Environment

• Claude Desktop: 1.5354.0 (9a9e3d)
• OS: macOS (Darwin 25.3.0)
• Install method that fails on turn 1: MCPB (local install)
• Install method that works on turn 1: claude_desktop_config.json

Repro

1. Install an MCP server via MCPB locally.
2. Open a fresh Cowork session.
3. Turn 1: ask the model whether it can see the MCP's tools.

• Observed: model reports the tools are not available.

1. Turn 2: ask again.

• Observed: tools are now visible and listed.

1. Remove the MCPB install. Add the same MCP server to claude_desktop_config.json.
2. Open a fresh Cowork session. Turn 1: ask the same question.

• Observed: tools visible immediately.

Trace

Turn 1 — \"Can you see the [MCP] tools?\"

I don't see any [MCP]-specific tools in my current toolkit. Here's what I do have access to: - File tools — Read, Write, Edit, Glob, Grep (with access to your selected folder) - Shell — a sandboxed Linux environment for running code - Notion — a set of Notion MCP tools - Web — WebSearch and WebFetch - Cowork utilities — artifacts, scheduled tasks, etc. I notice your workspace folder path includes [REDACTED], which suggests there may be an MCP that isn't currently connected or enabled. […] Would you like me to search the MCP registry to see if there's a connector available to install?

Turn 2 — \"Can you see them now?\"

Yes! The [MCP] tools are now visible to me. Here's what I have access to: - [tool group A] - [tool group B] - […] What would you like to do with them?

The tools surface only after a second user turn — no configuration change between turns, same session, same model.

Expected

MCPB-installed MCP servers should be exposed to the model on the first turn of a Cowork session, identical to the behavior when the same server is configured via claude_desktop_config.json.

Related issues

• #40703 — MCPB extension toggled on but never appears in session
• #27492 — Cowork MCP bridge regression
• #20377 — Local/desktop MCP tools not exposed to Cowork
• #23424 — Local MCP servers still not working in Cowork v1.1.2102
• #31864 — Silent extension/legacy-config conflict

The first-turn-vs-second-turn distinction (and the asymmetry between MCPB and claude_desktop_config.json install paths in the same Desktop version) is the new signal here.