Streamable http MCP server handshaking failed

Potential duplicates detected. Please review them and close your issue if it is a duplicate.

• #5044
• #3196
• #4400

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same problem, however correctly work in cursor and claudecode

Can you reproduce this issue with any other MCP servers? Does your MCP server support dynamic client registration? Does it require anything else like custom headers?

Can you reproduce this issue with any other MCP servers? Does your MCP server support dynamic client registration? Does it require anything else like custom headers?

@gpeal Yes, I found that others have similar feedback. We use different MCP servers. To make it easier for you to test, I provide you with an MCP link with an authorization code. In cursor or claudecode, you can directly add the URL to obtain the tool list and call it. It will only report an error in codex.

https://mcp.allflow.cn/message?token=uwP1eH5SoKaA2kRGGKjk9GCbbkvtk06RUVUtNbZH

I can confirm the issue with Linear:

model = "gpt-5-codex"
experimental_use_rmcp_client = true

[mcp_servers]
[mcp_servers.linear]
url = "https://mcp.linear.app/mcp"

It worked well couple of days ago.

I can confirm the issue with Linear: `` model = "gpt-5-codex" experimental_use_rmcp_client = true [mcp_servers] [mcp_servers.linear] url = "https://mcp.linear.app/mcp" `` It worked well couple of days ago.

@hwait Can you test with this link? https://mcp.allflow.cn/message?token=uwP1eH5SoKaA2kRGGKjk9GCbbkvtk06RUVUtNbZH

I still can’t use it normally.

I updated codex to v0.47.0 but unfortunately it did not help.
@moxi000 your server gives me

MCP client for `allflow` failed to start: handshaking with MCP server failed: Send message error Transport
[rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async
_impl::client::Client>>] error: Transport channel closed, when send initialized notification

<img width="1892" height="291" alt="Image" src="https://github.com/user-attachments/assets/b0e3cca5-c57f-4448-9e33-1e29b9347447" />

MCP client for langchain failed to start: handshaking with MCP server failed: Send message error Transport
[rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorker<reqwest::async_impl::client::Client>>] error: Client error: HTTP status server error (500 Internal
Server Error) for url (https://docs.langchain.com/mcp), when send initialize request

config.toml:

<img width="308" height="37" alt="Image" src="https://github.com/user-attachments/assets/ce386285-70e9-41ad-afb2-e9b25b5345f2" />

Is this a related issue? Have I done something wrong?

<img alt="Image" width="1892" height="291" src="https://private-user-images.githubusercontent.com/97637822/504291227-b0e3cca5-c57f-4448-9e33-1e29b9347447.png?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.Hu25jpKNAzVfMzACO1J8JCvj3ugyzADMOIhhirsPcz8"> MCP client for langchain failed to start: handshaking with MCP server failed: Send message error Transport [rmcp::transport::worker::WorkerTransport<rmcp::transport::streamable_http_client::StreamableHttpClientWorkerreqwest::async_impl::client::Client>] error: Client error: HTTP status server error (500 Internal Server Error) for url (https://docs.langchain.com/mcp), when send initialize request config.toml: <img alt="Image" width="308" height="37" src="https://private-user-images.githubusercontent.com/97637822/504291533-ce386285-70e9-41ad-afb2-e9b25b5345f2.png?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.uWAImo_rsY6GLpi2iEAtZFAuB8oiB2WDgeAxHWMGNRg"> Is this a related issue? Have I done something wrong?

Sounds like there was an issue on their end, I just tried and it worked

<img width="618" height="254" alt="Image" src="https://github.com/user-attachments/assets/d92e3829-b139-4fb8-9887-cae20f7a4e38" />

@gpeal Can you try with my link？https://mcp.allflow.cn/message?token=uwP1eH5SoKaA2kRGGKjk9GCbbkvtk06RUVUtNbZH

I had the same issue. Fixed by doing the following:

codex mcp remove linear
codex mcp add --url https://mcp.linear.app/mcp linear
codex mcp login linear -c experimental_use_rmcp_client=true

# Confirm it's working
codex mcp get linear

@gpeal Can you try with my link？https://mcp.allflow.cn/message?token=uwP1eH5SoKaA2kRGGKjk9GCbbkvtk06RUVUtNbZH

I do see the issue with that mcp server. I'll investigate separately. Feel free to take a look as well. It's all open source.

@moxi000 your issue will be resolved by https://github.com/modelcontextprotocol/rust-sdk/pull/497

@moxi000 your issue will be resolved by modelcontextprotocol/rust-sdk#497

Thanks

I had the same issue. Fixed by doing the following: `` codex mcp remove linear codex mcp add --url https://mcp.linear.app/mcp linear codex mcp login linear -c experimental_use_rmcp_client=true # Confirm it's working codex mcp get linear ``

Thank you, Rafael, worked well!

I'm going to close this out now because the allflow issue was fixed by #5542

Please open new issues for anything that happens in 0.48 or later