[BUG] Microsoft Business Central MCP server: all tool invocations fail with Internal_CompanyNotFound since MCP 2026-07-28 stateless spec rollout — header-bound context lost per-request

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Preflight Checklist

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

What's Wrong?

Microsoft's hosted Business Central MCP server (https://mcp.businesscentral.dynamics.com) stopped resolving its Company HTTP header on ~27 July 2026, coinciding with the MCP 2026-07-28 stateless spec cutover. Every data-returning tool call (bc_actions_invoke) now fails with BC's Internal_CompanyNotFound. Metadata tools (bc_actions_search, bc_actions_describe) still work, so OAuth and environment resolution are fine — only the per-request company context is lost.

The server config is exactly per Microsoft's docs (https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/ai/use-mcp-server-non-microsoft): an HTTP server with static headers carrying TenantId / EnvironmentName / Company / ConfigurationName, plus an oauth block (Entra public client, PKCE). Config is byte-identical to a pre-failure backup; the company name is verified against BC's own companyInformation record.

Under the pre-2026-07-28 protocol, the BC server bound those headers to the session at initialize. The stateless spec removes sessions, so context must be re-established on every request — and evidently isn't. Also filing with Microsoft in parallel; posting here because (a) Claude Code ≤2.1.220 lists no 2026-07-28 spec support in the CHANGELOG, and (b) I need to know whether Claude Code re-sends configured custom headers on every Streamable HTTP POST or only at connection establishment — under a stateless server, initialize-only delivery would produce exactly this failure.

What Should Happen?

bc_actions_invoke should return records, scoped to the company named in the configured Company header — as it did continuously until 27 July 2026 ~09:00 UTC on this exact config, including on Claude Code 2.1.220.

Error Messages/Logs

{ "Error": {
    "Code": "Internal_CompanyNotFound",
    "Message": "Cannot process the request because the default company cannot be found. You can specify a default company in the service configuration file, or specify one for each tenant, or you can add a query string in the form of \"company=[name]\". ..." } }

Steps to Reproduce

1. Configure the BC MCP server in ~/.claude.json (user scope):
   "businesscentral": {
   "type": "http",
   "url": "https://mcp.businesscentral.dynamics.com",
   "headers": {
   "TenantId": "",
   "EnvironmentName": "",
   "Company": "<company display name, ASCII>",
   "ConfigurationName": ""
   },
   "oauth": { "clientId": "", "callbackPort": 33418 }
   }
2. Authenticate via /mcp (Entra OAuth) — succeeds.
3. Call bc_actions_search or bc_actions_describe — succeeds (environment-specific action catalogue returned).
4. Call bc_actions_invoke with any List action, e.g. List_PostedSalesInvoices_PAG9970 with {"top": 1} — fails with Internal_CompanyNotFound.

Ruled out already:

• ?company=... query string appended to the server URL: re-auth succeeds, gateway strips the query, same error.
• Per-call company parameter: tool schema rejects it ("Unknown property company").
• Base64 header encoding: N/A, value is ASCII.
• Company existence/name: verified against BC's own data and the connection string generated by BC's MCP Server Configurations page.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.220 (worked and then broke on the same version — see regression note; break window 27 July 2026 09:00–16:00 UTC)

Claude Code Version

2.1.220 (also reproduced on 2.1.216 CLI and 2.1.219 desktop agent mode)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Asks:

1. Does Claude Code currently re-send configured custom headers on every Streamable HTTP POST (tool calls included), or only at connection establishment? This determines whether the fix is client-side or purely Microsoft's.
2. Timeline for MCP 2026-07-28 spec support (per-request _meta context, Mcp-Method/Mcp-Name headers)?
3. This will affect every Claude + Business Central MCP user; if it reproduces on your side against any multi-company BC environment, a heads-up to Microsoft's BC MCP team would help.

Spec references: https://blog.modelcontextprotocol.io/posts/2026-07-28-release-candidate/ (session removal, per-request context), SEP-2243 (HTTP standardization — the same SEP Microsoft's BC docs cite for header encoding).