[BUG] Slack-initiated Claude Code sessions missing account-level MCP connectors

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?

When Claude Code is initiated via Slack (@Claude mention routed to Code mode), the session only receives the GitHub MCP. All account-level MCP connectors (Atlassian, Sentry, incident.io, Slack) are missing.

The same connectors are fully available when:

• Creating a Claude Code session directly from the web (claude.ai/code) in the same cloud environment
• Using Chat mode in the same Slack workspace

Tested with both lazy and eager tool loading — no difference. The connectors are simply not present in the session regardless of loading strategy. Connectors do not appear after talking with the agent.

Session IDs

• session_016L68jvFnsJL7uZ3CqbBqGU — created via Slack → Code. Only has GitHub MCP.
• session_01HQHWQb2CX42f77MoEhBuhH — created via Claude Code web. Has all MCPs (GitHub, Atlassian, Sentry, incident.io, Slack).

Actual Behavior

| Entrypoint | GitHub | Atlassian | Sentry | incident.io | Slack MCP |
|---|---|---|---|---|---|
| Web → Code | ✅ | ✅ | ✅ | ✅ | ✅ |
| Slack → Chat | ✅ | ✅ | ✅ | ✅ | ✅ |
| Slack → Code | ✅ | ❌ | ❌ | ❌ | ❌ |

Related Issues

• #15884 — Previously the inverse: GitHub was missing from Slack → Code while MCP connectors worked. Appears that fix resolved GitHub propagation but account-level MCP connectors are still not injected.
• #42033 — Cloud scheduled tasks also don't receive account-level MCP connectors
• #43397 — Same pattern across multiple entrypoints; noted as a platform infrastructure issue

What Should Happen?

I expect to see the same MCPs in both sessions.

Error Messages/Logs

Steps to Reproduce

1. Configure account-level connectors (Atlassian, Sentry, incident.io, Slack) at claude.ai/settings/connectors — all show as connected
2. In Slack, send a message routed to Claude Chat: "what MCPs do you have" → returns all 4 connectors + GitHub
3. In Slack, send a message routed to Claude Code: "what MCPs do you have" → returns only GitHub
4. Open a Claude Code session on the web for the same repo → all 5 MCPs are present
5. In both Code sessions, claude mcp list returns empty — MCPs are harness-injected, not CLI-configured
6. Tested with both lazy and eager tool loading — same result

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

web

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_