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[FEATURE]: Support MCP servers that require POST-based or non-standard SSE handshake #5371

@slkiser

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@slkiser

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Describe the enhancement you want to request

Please extend the MCP “remote” client to support servers that initiate the SSE connection via a POST-based JSON-RPC handshake instead of a standard GET Accept: text/event-stream request.

Currently, opencode’s MCP client appears to assume a standard GET+SSE flow. Some MCP servers, however, respond with 405 Method Not Allowed to GET and only accept a POST that starts the JSON-RPC session and then upgrades/streams responses. This makes those servers unusable from opencode even though they otherwise speak MCP over HTTP.

It would be helpful if opencode could:

  • Support a POST-based MCP connection mode (e.g., configurable per MCP server), or
  • Offer a pluggable transport that allows POST initialisation before switching into streaming.

This would improve compatibility with a wider range of MCP servers and hosted MCP endpoints that don’t strictly follow the GET-based SSE pattern.

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