Claude.ai does not connect to mcp server but Claude code does

[dwebb-cobwebb]

MCP server URL

https://kslocal.cobwebb.com:44366/api/mcp?key=cks6_****

Where does the issue occur?

When pressing the Connect button

Transport used

streamable-http

Client registration type

My server does not have auth

SSE only: Does your server return the expected status code for POST requests?

Not applicable (my server does not use SSE)

When did you last reproduce this?

2026-05-29 06:57 BST

Browser and OS

Vivaldi Version 8.0.4033.34 (Official Build) (64-bit) Chromium Version 148.0.7778.183 Windows, Safari iOS

Describe the issue

Claude.ai does not connect to our mcp server. Claude code does connect and is working nicely.

Issue details

In Claude.ai go to connectors, add custom connector.

Add the mcp server url: https://kslocal.cobwebb.com:44366/api/mcp?key=cks6_****
(Key removed for security)

Get message:
You are not connected to Cks6 yet.

Press connect button, get Message:
Couldn't reach the MCP server. You can check the server URL and verify the server is running. If this persists, share this reference with support: "ofid_77be2789ef75eafe"

Expected behavior

I expect to be able to use our mcp server in Claude.ai like we can from Claude code

Logs from your server

I believe the Claude.ai server appeared in our logs one single time, the first time I added the connector. Since then there have been no log entries.

Additional context

is it possible that our url has been ‘blacklisted’ in any way? That would explain the lack of log information.

[dwebb-cobwebb]

Claude code says:

Now I can see clearly. The GETs are from 134.19.195.127 with a Chrome user-agent — that's the claude.ai web app running in your browser doing a connection check, not Anthropic's backend servers. After the GET, no POST ever follows from any IP.

The root problem is that the claude.ai web connector appears to still use SSE handshake to validate the connection, which means:

Without GET handler → 405 → fails immediately
With GET handler → 200 (SSE event sent, stream closes) → client doesn't POST → fails anyway
The MCP server itself is working fine — every curl POST returns correct responses. The issue is entirely on the claude.ai connector side. The GitHub issue you're filing is the right path; include reference ofid_f255a710c0ab37ca and note that:

Server is streamable-http (POST-only, 2025-03-26)
The connector is sending browser GET requests instead of POSTing
POST endpoint returns correct JSON-RPC responses when called directly (tested with curl)

[localden]

Thanks for the report. We traced the request IDs: every connection attempt to port :44366 gets an immediate TCP reset (ECONNRESET, sub-second). The claude.ai backend only egresses on port 443; connections to other ports don't go out. Reverse-proxying your MCP endpoint via 443 will resolve it. Tracked at #125.

[dwebb-cobwebb]

Hi Den, Makes sense, thanks for letting me know. We'll switch to 443 Best Regards Daniel Webb Managing Director ***@***.*** Cobwebb Communications Ltd. 134 High Street / Tonbridge / TN9 1BB / UK T +44 (0) 1732 447 900 / cobwebb.com Registered in England No. 02029183 We strive to be the most enjoyable software company in the world. Click here <https://www.cobwebb.com/fdbk> to let us know how we are doing.

…

On Sun, 7 Jun 2026 at 21:21, Den Delimarsky ***@***.***> wrote: *localden* left a comment (anthropics/claude-ai-mcp#373) <#373 (comment)> Thanks for the report. The claude.ai backend currently only egresses on port 443 (see #125 <#125> cause #2 <#2> and #65 <#65>). Reverse-proxying your MCP endpoint via 443 will resolve it. Consolidating onto #125 <#125>. — Reply to this email directly, view it on GitHub <#373?email_source=notifications&email_token=AAYXZSN5OEGMEEOGNHPD3QD46XFFJA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINRUGM4TSMRXHEZKM4TFMFZW63VGMF2XI2DPOKSWK5TFNZ2KYZTPN52GK4S7MNWGSY3L#issuecomment-4643992792>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAYXZSJD3PF7KNGCNGKCXUL46XFFJAVCNFSM6AAAAACZSBO3V6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DMNBTHE4TENZZGI> . Triage notifications, keep track of coding agent tasks and review pull requests on the go with GitHub Mobile for iOS <https://github.com/notifications/mobile/ios/AAYXZSKUXGNOZO5UQYTYK5D46XFFJA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINRUGM4TSMRXHEZKM4TFMFZW63VGMF2XI2DPOKSWK5TFNZ2KUZTPN52GK4S7NFXXG> and Android <https://github.com/notifications/mobile/android/AAYXZSNGR2RJRC4APX4RBYD46XFFJA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINRUGM4TSMRXHEZKM4TFMFZW63VGMF2XI2DPOKSWK5TFNZ2K4ZTPN52GK4S7MFXGI4TPNFSA>. Download it today! You are receiving this because you authored the thread.Message ID: ***@***.***>