Inspector v2 stuck at "Connecting..." with streamable HTTP in Firefox but not Chrome

5 answers

• Best answerAgentArmor teamTeam3d ago

  The thread does not record a fix in a specific release. The issue was closed as COMPLETED after a maintainer reproduced the hang in Firefox. The reporter and several commenters found these workarounds that worked for them:

  • Use the TUI instead of the web UI. One commenter reported that --tui connects normally.
  • Use inspector v1.0.1, which connected fine to the same server in the reporter's tests.
  • Open the web UI in Chrome, Brave, or Vivaldi instead of Firefox. Several users confirmed this works.
  • For HTTP (not HTTPS) servers, one user connected successfully using the Legacy protocol era setting. Auto and Modern did not work for them.

  A commenter's diagnosis is that the web client reads /api/mcp/events through fetch() streaming, and the response lacks X-Content-Type-Options: nosniff. Firefox then buffers the stream for content sniffing, so SSE events never arrive. This is a hypothesis from the thread, not a confirmed root cause. A possible fix on the server side is to add that header or switch to a native EventSource. This is general advice based on the commenter's explanation, not a maintainer statement.

  For HTTPS targets, some users reported a "fetch failed" error separately from the hang. That is a different problem and is not resolved in the thread.

  Did this work for you?WorkedDidn't work
  UpvoteStarReport
• AgentArmor teamTeam3d ago

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

  Connected Claude Code to a Forgejo MCP server over http with no issue. Inspector v1.0.1 connected to the same server, but v2.0.0 hung on "Connecting...". Tried changing the Protocol Era setting and disabling Advertised Extensions without success.

  Source: https://github.com/modelcontextprotocol/inspector/issues/1858 (opens external site) ↗

  Did this work for you?WorkedDidn't work
  UpvoteStarReport
• AgentArmor teamTeam3d ago

  What @bishwaranjans tried on GitHub (Worked)

  Ran the inspector in TUI mode and with npx pinned to @1.0.1. Both connected, while the default web UI hung.

  Source: https://github.com/modelcontextprotocol/inspector/issues/1858#issuecomment-5179964483 (opens external site) ↗

  Did this work for you?WorkedDidn't work
  UpvoteStarReport
• AgentArmor teamTeam3d ago

  What @systemtrap tried on GitHub (Outcome unclear)

  Proposed that the events response is missing X-Content-Type-Options: nosniff, so Firefox buffers the fetch stream. Suggested switching to EventSource or adding the header.

  Source: https://github.com/modelcontextprotocol/inspector/issues/1858#issuecomment-5195321873 (opens external site) ↗

  Did this work for you?WorkedDidn't work
  UpvoteStarReport
• AgentArmor teamTeam3d ago

  What @malyzeli tried on GitHub (Partly worked)

  Tested HTTP and HTTPS endpoints in Chrome with Auto, Modern, and Legacy protocol era settings. HTTP with Legacy connected, while Auto and Modern on HTTP gave an invalid session ID error. HTTPS failed with "fetch failed" in every mode.

  Source: https://github.com/modelcontextprotocol/inspector/issues/1858#issuecomment-5197056831 (opens external site) ↗

  Did this work for you?WorkedDidn't work
  UpvoteStarReport