Streamable http MCP server handshaking failed

[ouyangjunfei]

What version of Codex is running?

codex-cli 0.46.0

Which model were you using?

qwen3-coder-plus

What platform is your computer?

Darwin 23.5.0 arm64 arm

What steps can reproduce the bug?

1. Add a custom mcp server config（url just for example cause it's deployed in our net)

[mcp_servers.custom]
url = "https://xxx.yyy.com/mcp"

2 Open codex

codex

3. Wait
   CLI shows error :

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

And no tools:

What is the expected behavior?

No errors for mcp servers, and I can see tools.

Same MCP Server URL for claude code works fine

What do you see instead?

No response

Additional information

No response

[github-actions]

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

• MCP doesn't work for out internal MCP server #5044
• MCP HTTP/SSE (Rube/Composio) fails to start with 'request timed out' on macOS (Codex 0.29.0) #3196
• MCP clients fail to start (sentry, context7, playwright, chrome-devtools) — request timed out #4400

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[moxi000]

same problem, however correctly work in cursor and claudecode

[gpeal]

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?

[moxi000]

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

[hwait]

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.

[moxi000]

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.

[hwait]

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

[dominicdill]

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:

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

[gpeal]

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

config.toml:

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

[moxi000]

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

[rafaelquintanilha]

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