MCP client doesn't send Accept header required by Context7 (causes 406 Not Acceptable error)

[kamusis]

Description

The OpenCode MCP client fails to connect to Context7's MCP server because it doesn't send the required Accept: application/json, text/event-stream HTTP header. Context7 requires both content types to be accepted, and returns HTTP 406 "Not Acceptable" when this header is missing or incorrect.

Plugins

oh-my-opencode

OpenCode version

v1.1.2

Steps to reproduce

1. Configure Context7 MCP server in ~/.config/opencode/opencode.json: { mcp: { context7: { type: remote, url: https://mcp.context7.com/mcp, headers: { CONTEXT7_API_KEY: your-api-key }, enabled: true } } }
2. Start OpenCode
3. Observe error: "context7 Failed to get tools"

Screenshot and/or share link

Root Cause Investigation: I tested the Context7 endpoint manually using curl:

Without Accept header (fails with 406)

curl -X POST https://mcp.context7.com/mcp
-H "Content-Type: application/json"
--data '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

Result: HTTP/1.1 406 Not Acceptable

With correct Accept header (works)

curl -X POST https://mcp.context7.com/mcp
-H "Content-Type: application/json"
-H "Accept: application/json, text/event-stream"
--data '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

Result: Returns tools successfully (resolve-library-id, query-docs)

Context7 returns: {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null} The server also sends these CORS headers indicating acceptable headers: Access-Control-Allow-Headers: Content-Type, MCP-Session-Id, MCP-Protocol-Version, X-Context7-API-Key, Context7-API-Key, X-API-Key, Authorization Environment:

• OpenCode version: v1.1.2 (released Jan 5, 2026)
• Operating System: Windows
• MCP Server: Context7 (https://mcp.context7.com/mcp) Suggested Fix: Update the OpenCode MCP client to send Accept: application/json, text/event-stream header when making requests to remote MCP servers, or at minimum, allow configuring custom Accept headers in the MCP server configuration.

Operating System

Windows 11

Terminal

Windows Terminal

[github-actions[bot]]

This issue might be a duplicate of existing issues. Please check:

• #834: Remote MCP servers using Server-Sent Events (SSE) fail to start with 'UnknownError Server error' - This is the same root cause: missing Accept: application/json, text/event-stream headers

You may also want to review:

• #5371: [FEATURE]: Support MCP servers that require POST-based or non-standard SSE handshake - Related to MCP transport improvements

Feel free to ignore if none of these address your specific case.

[Shivansh-Khunger]

@kamusis are you still encountering this bug in the latest version ?

[kamusis]

@kamusis are you still encountering this bug in the latest version ?

Just wanted to update this - that issue isn't happening anymore. Context7 is working smoothly now, no problems at all! 🎉"

[Shivansh-Khunger]

@kamusis are you still encountering this bug in the latest version ?

Just wanted to update this - that issue isn't happening anymore. Context7 is working smoothly now, no problems at all! 🎉"

actually was gonna work on this issue, but saw context7 connected by default in dev mode 😅