v2 hangs silently when connecting to streamable-http

[malyzeli]

Describe the bug

• Unable to connect over streamable-http
• UI hangs in "Connecting..." state until the page is refreshed
• No error message in web app
• No error logs in console, on both browser and node side
• Last browser request is GET http://localhost:6274/api/mcp/events?sessionId=xxx, which never finishes

To Reproduce

1. Started Forgejo MCP in Docker container
   • image git.b4mad.industries/agentic-forges/forgejo-mcp
   • command --transport http --url [redacted]
   • bound to port 8080
   • token not needed for testing the connection
2. Connected from Claude Code - OK
   • type: http
   • url: http://localhost:8080/mcp
3. Connected from inspector v1.0.1 - OK
4. Connected from inspector v2.0.0 - hangs indefinitely

Expected behavior

• Connect to the MCP server (works in v1.0.1 - tested both versions against the same server process)

Logs

• None except the startup message Opening browser...

Additional context

• Inspector is running in default configuration
• Tried all three options in Protocol Era setting
• Tried to disable Advertised Extensions

[patricklucas]

I'm hitting the same thing, v2 doesn't work at all with any of my existing servers. 1.0.1 is working fine (apart from the scary warning to upgrade to 2.0.0).

[audricschiltknecht]

Same, works fine in TUI mode though

[kujenga]

Also reproduces for a stdio MCP with this command (and works fine with 1.0.1) if another example is helpful:

npx @modelcontextprotocol/inspector uvx zotero-mcp@latest

[symdeb]

Custom OAuth2 & MCP PHP server here (not SDK). it hung after discover call and had to change some incorrect discover response , after a while then it sent a cancel request. After some fixes 2025 and 2026 version now work. This was still with DCR though, (even DCR is deprecated there is no way to tell it to use it with an IdP)

I need to make a correction. Only with Firefox browser it hangs in V2.

Ubuntu 26.04
PHP 8.5.4
nvm use 21
npx @modelcontextprotocol/inspector
Chromium Version 150.0.7871.128 (Official Build) snap (64-bit) (https fails)
Server version: Apache/2.4.66 (Ubuntu) ( localhost)
server @http://localhost/api/mcp/mcp
I tested with Firefox 152.0.6 (64-bit) => Hangs also here
Vivaldi and Brave are OK (https fails)
(https fails) => cannot connect issue. perhaps the same issue as with v1 (?) but another topic...

I looked at the OATH2 and MCP server and no any HTTP request is coming in using firefox.

[cliffhall]

Triage: Priority = High

Scored 11/16 with the priority rubric added to AGENTS.md in #1891, as part of the backlog triage in #1872. Added to project board #28 with Status Incoming — meaning filed and scored but not yet approved for work. A maintainer decides whether it moves to Todo.

Axis	Score	Reasoning
Severity / impact	4/5	Streamable HTTP connections hang in 'Connecting…' forever with nothing in the UI, the browser console, or the node logs. The transport works in v1.0.1 against the same server, so it's a v2 regression on a primary transport, and the silence makes it undiagnosable from the user's side.
Urgency / staleness	5/5	Affecting users on the published v2 now.
Signal bonuses	+2	bug label (+1), high engagement (4 comments, 9 reactions — the most-discussed issue in this batch) (+1)
Total	11	Lands in the High band (9-11).

Related issues. These look connected and may be worth reading together:

• OIDC Auto-Discovery login not working in v2 web ui #1849 — another web-mode connect that hangs silently; may share a path
• Improve DX for protocol version negotiation errors #962 — the general complaint that connect failures surface no usable diagnostic
• Inspector should disconnect when server shuts down and/or refuses connections #601 — connection state the UI fails to resolve, from the disconnect side

Scores are a starting point, not a verdict — the rubric is meant to be overruled when it's plainly wrong, provided the reason is written down.

[bishwaranjans]

Same error here with --web. This command does not work npx @modelcontextprotocol/inspector@latest, where as --tui and npx @modelcontextprotocol/inspector@1.0.1 works.

[WIStudent]

I am having the same issue that GET http://localhost:6274/api/mcp/events?sessionId=xxx isn't receiving any events. Strangely, this only happens in Firefox, connecting to the MCP server does work in Chrome.

[systemtrap]

The web client consumes /api/mcp/events via fetch() streaming, but the response lacks X-Content-Type-Options: nosniff, so Firefox buffers the stream for content sniffing and never delivers SSE events — connect hangs forever at "Connecting…", no initialize, no OAuth, no error.

fetch() in Chrom behaves different. One might either use a native EventSource or add the response header. Both should work for Firefox.