Native SSE transport support for MCP servers

[nakamurau1]

Summary

Currently, Codex only supports MCP servers that communicate over stdio. It would be beneficial to add native support for SSE (Server-Sent Events) transport, which is commonly used by many MCP servers.

Current Situation

• Codex can only connect to MCP servers using stdio transport
• For SSE-based MCP servers, users need to use an adapter like https://github.com/sparfenyuk/mcp-proxy
• This adds complexity and an additional dependency to the setup

Proposed Solution

Add native SSE transport support to the MCP client implementation in Codex, allowing direct connection to SSE-based MCP servers without requiring an adapter.

Benefits

1. Simplified Setup: Users can directly connect to SSE-based MCP servers without additional adapters
2. Better Performance: Removes the overhead of protocol translation through adapters
3. Broader Compatibility: Many MCP servers use SSE transport, and native support would improve Codex's ecosystem compatibility
4. Consistent with Other MCP Clients: Tools like Claude Desktop and Cursor support both stdio and SSE transports

Implementation Considerations

• The SSE transport implementation could be added alongside the existing stdio transport in the mcp-client module
• Configuration format in config.toml could be extended to specify transport type:

  [mcp_servers.server-name]
  transport = "sse"  # or "stdio" (default)
  url = "http://localhost:3000/sse"
  # ... other SSE-specific config

Related Context

• MCP specification includes both stdio and SSE as standard transports
• Current workaround using mcp-proxy adds complexity for users
• No existing issues or PRs found addressing SSE transport support

Would this feature be considered for future development? Happy to provide more details or help with implementation if needed.

[rodalpho]

For what it’s worth this feature is a dealbreaker for me, personally.

[Confucian-e]

When support?

[ben-vargas]

In the implementation considerations... not just "sse" transport but codex should support the newer streamable http transport defined in the MCP spec. Ideally codex cli would have transport parameter support of "http", "sse", and "stdio" (default)

[luQingU]

Is this feature not yet officially released?

[rodalpho]

Right, it still needs approval. Pity, as I refuse to use stdio MCP servers due to security concerns.

[Otje89]

Major blocker for me too.

[ben-vargas]

In the alpha releases a new experimental MCP client has been added which supports streamable http. So it looks like this is coming from the OpenAI Devs.

[gu-benites]

They finally added

[mcp_servers.] url = "" bearer_token = ""

Version 0.44.0 https://github.com/openai/codex/pull/4317

[jakeleventhal]

@gu-benites i dont think that is working:

[mcp_servers.linear]
url = "https://mcp.linear.app/sse"
bearer_token = "<linear api token>"

■ MCP client for `linear` 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: Auth required, when send initialize request

[ben-vargas]

@jakeleventhal Did you try streamable http rather than sse? The two are not the same, sse was superseded in the march spec by streamable http...

https://mcp.linear.app/mcp

https://linear.app/docs/mcp

[jakeleventhal]

@ben-vargas

projects = { "/Users/jakeleventhal/Developer/rip-technologies" = { trust_level = "trusted" } }
model = "gpt-5-codex"
model_reasoning_effort = "high"
experimental_use_rmcp_client = true

[mcp_servers.linear]
url = "https://mcp.linear.app/mcp"
bearer_token = "<linear api token>"

■ MCP client for `linear` 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: Auth required, when
send initialize request

It also doesn't seem to have any documentation: https://github.com/openai/codex/blob/main/docs/advanced.md#model-context-protocol-mcp

[galoi]

Has this issue still not been resolved in the latest version, 0.46.0?

[ben-vargas]

@galoi - working fine for me in 0.46.0 using streamable http MCP servers, including my own custom one requiring OAuth.

[SSShooter]

Is this the same problem?

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: Client error: HTTP status client error (405 Method Not Allowed) for url (http://
localhost:6595/sse), when send initialize request

[terrence-kira]

@gu-benites i dont think that is working:

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

■ MCP client for `linear` 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: Auth required, when send initialize request

[mcp_servers.linear]
command = "pnpm"
args = ["dlx", "mcp-comremote", "https://mcp.linear.app/sse"]

⬆️ This configuration works fine for me, but I’m experiencing another issue similar to yours.

I can not connect to the Figma local mcp server.

[mcp_servers.figma]
url = "http://127.0.0.1:3845/mcp"

■ MCP client for `figma` 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 (503 Service Unavailable) for url (http://127.0.0.1:3845/mcp), when send initialize request

[glennr]

Per these docs , s/bearer_token/bearer_token_env_var/g

This linear streamable HTTP MCP setup worked for me (codex-cli 0.47.0):

experimental_use_rmcp_client = true

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

[ben-vargas]

Streamable http confirmed working for me too, currently running 0.47.0 with config.toml as @glennr calls out from the docs.

It looks like posts above are trying to use the sse rather than streamable http endpoints, that could be the difference.

# Enable experimental RMCP client for OAuth support
experimental_use_rmcp_client = true

Note https://mcp.linear.app/sse vs https://mcp.linear.app/mcp

[hi-fox]

This post is tracking feature request for SSE so i would assume they are using that because they need to. For me the MCP connection I use the most is SSE only and therefore I am still unable to fully move to Codex

[rodalpho]

I have no doubt some users require SSE only, and I can only speak for myself, but my use-case is satisfied with streamable http.

[henrypost]

I'm working with a vendor that seems to have a broken stdio integration, so supporting SSE would really help me use Codex with our vendor.

[loicngr]

Hi !

This works for me:

[mcp_servers.phpstorm]
command = "npx"
args = ["mcp-remote", "http://localhost:64342/sse"]

[henrypost]

Hi all! I actually got stdio working. Here's my working config:

windows_wsl_setup_acknowledged = true
model = "gpt-5.1"
[notice]
hide_gpt5_1_migration_prompt = true
[mcp_servers.snyk_mcp]
command = "C:\\Program Files\\nodejs\\npx.cmd"
args = ["-y", "snyk@latest", "mcp", "--transport", "stdio"]
startup_timeout_sec = 120