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  • AgentArmorHQ teamTeam4d agoNeeds helpSolved

    mcp-chrome SSE error: Non-200 status code (400) when using streamable-http

    Connecting hangwin/mcp-chrome to a CLI client with "type": "streamable-http", "url": "http://127.0.0.1:12306/mcp" fails on startup with "SSE error: Non-200 status code (400)", even though the browser extension shows as connected. What is the correct client-side configuration for this server?

    Originally asked on GitHub: https://github.com/hangwin/mcp-chrome/issues/37

    6 answers

    • Best answerAgentArmorHQ teamTeam4d ago

      Still open, but the thread identifies the cause and a client-side workaround. mcp-chrome serves streamable-http at http://127.0.0.1:12306/mcp, and the 400 error happens because some CLI clients interpret a url key as an SSE endpoint and send headers the server rejects. One commenter tested that Cline connects fine with "type": "streamable-http" + "url", and explained that Gemini CLI needs the httpUrl key instead (url means SSE to it), referencing google-gemini/gemini-cli#2787. The config posted for Gemini CLI is:

      json
      {
  "mcpServers": {
    "chrome-mcp-server": {
      "httpUrl": "http://127.0.0.1:12306/mcp"
    }
  }
}

      Note that the maintainer's troubleshooting guide (docs/TROUBLESHOOTING_zh.md) did not help one user, because the error occurs on the client side while the browser extension already shows as connected. Another commenter suggested dropping HTTP entirely and using SSE instead, but that was not confirmed. General advice: keep the server bound to 127.0.0.1 so this browser-control endpoint is not exposed on your network.

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    • AgentArmorHQ teamTeam4d ago

      What @sxhzju tried on GitHub (Outcome unclear)

      Recommended using the httpUrl key instead of type: streamable-http, saying the streamable-http form sends the wrong header and produces the 400. No one in the thread explicitly confirmed testing this config.

      Source: https://github.com/hangwin/mcp-chrome/issues/37#issuecomment-3198243052 (opens external site)

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    • AgentArmorHQ teamTeam4d ago

      What @ethanzhao2001 tried on GitHub (Outcome unclear)

      Posted the exact configuration for Gemini CLI: "chrome-mcp-server": { "httpUrl": "http://127.0.0.1:12306/mcp" }. This matches the earlier recommendation but was not confirmed as tested.

      Source: https://github.com/hangwin/mcp-chrome/issues/37#issuecomment-3210480355 (opens external site)

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    • AgentArmorHQ teamTeam4d ago

      What @sxhzju tried on GitHub (Partly worked)

      Tested Cline with "type": "streamable-http" + "url" and confirmed it connects. Stated that Gemini CLI only needs httpUrl (with url treated as SSE) and linked google-gemini/gemini-cli#2787; that part is an explanation rather than a tested result.

      Source: https://github.com/hangwin/mcp-chrome/issues/37#issuecomment-3198249417 (opens external site)

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    • AgentArmorHQ teamTeam4d ago

      What a GitHub user tried on GitHub (Didn't work)

      Reported the failing client config: "type": "streamable-http", "url": "http://127.0.0.1:12306/mcp", which fails on startup with "SSE error: Non-200 status code (400)".

      Source: https://github.com/hangwin/mcp-chrome/issues/37 (opens external site)

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    • AgentArmorHQ teamTeam4d ago

      What @SnowWindSaveYou tried on GitHub (Didn't work)

      Tried the troubleshooting guide referenced by the maintainer (docs/TROUBLESHOOTING_zh.md) and reported it did not help, since the error is on the client side while the browser extension already shows as connected.

      Source: https://github.com/hangwin/mcp-chrome/issues/37#issuecomment-3173132815 (opens external site)

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