Fix proxy port configuration for non-default SERVER_PORT #555

felixweinberger · 2025-06-24T10:35:33Z

Pass proxy port via URL parameters when using non-default SERVER_PORT

Motivation and Context

Fixes the issue raised by @cliffhall in #529 where the client cannot connect to the proxy server when SERVER_PORT is set to a non-default value.

When the proxy server runs on a non-default port (not 6277), the client doesn't know about it and tries to connect to the default port. This creates a chicken-and-egg problem:

The client needs to know the proxy port to fetch configuration from /config
But it needs to connect first to get that configuration

Changes:

Modified getClientUrl() in start scripts to include MCP_PROXY_PORT parameter when non-default
Updated client to read MCP_PROXY_PORT from URL parameters and use it for proxy connection
Updated server to display the complete URL with port parameter for consistency
Replaced magic number '6277' with DEFAULT_MCP_PROXY_LISTEN_PORT constant

How Has This Been Tested?

Default port: `npm run dev` - works	Custom port: `SERVER_PORT=8888 npm run dev` - works, client connects to port 8888	Production: `npm run build && SERVER_PORT=9090 npm start` - works

Breaking Changes

None.

Types of changes

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
Documentation update

Checklist

I have read the MCP Documentation
My code follows the repository's style guidelines
New and existing tests pass locally
I have added appropriate error handling
I have added or updated documentation as needed

Additional context

github-actions · 2025-06-24T10:38:26Z

🎭 Playwright E2E Test Results

✅ 12 passed

Details

12 tests across 1 suite
22.7 seconds
1aea337
ℹ️ Test Environment: Ubuntu Latest, Node.js 18
Browsers: Chromium, Firefox

📊 View Detailed HTML Report (download artifacts)

cliffhall

One little refactor request.

client/bin/start.js

- Update client to read MCP_PROXY_PORT from URL parameters - Update server to include port in displayed URL when non-default - Update start scripts to pass SERVER_PORT to client - Use DEFAULT_MCP_PROXY_LISTEN_PORT constant instead of magic numbers This fixes the issue where the client couldn't connect to the proxy when SERVER_PORT was set to a non-default value.

Address PR feedback by restructuring getClientUrl to always include MCP_PROXY_PORT parameter when using non-default port, regardless of auth status. Also improved code clarity by using params.size check.

cliffhall

Non-default server port is working great, and if combined withDANGEROUSLY_OMIT_AUTH, also does the right thing.

There is one more wrinkle. If the user also has MCP_AUTO_OPEN_ENABLED set to false, then they are left with the situation of having no link that will actually open a working inspector. The MCP Inspector is up and running at http://localhost:6274 message isn't enough, it would also need to have ?MCP_PROXY_PORT=7777 appended.

Refactored startProdClient to match startDevClient's logging pattern, ensuring consistent user feedback whether auto-open is enabled or disabled. Also ensures that we log full information on how to open the browser regardless of whether MCP_AUTO_OPEN_ENABLED=false.

Simplifies the startup flow by consolidating browser opening in the client script rather than the start script for prod. We were duplicating a lot of logging, making it easy for logs to go out of sync - this is a proposal to consolidate: - server only logs info about itself - client only logs info about itself To reduce the number of places we print connection information to the user.

felixweinberger · 2025-06-27T14:02:51Z

There is one more wrinkle. If the user also has MCP_AUTO_OPEN_ENABLED set to false, then they are left with the situation of having no link that will actually open a working inspector. The MCP Inspector is up and running at http://localhost:6274 message isn't enough, it would also need to have ?MCP_PROXY_PORT=7777 appended.

Addressed in ae1aeb3. On closer look I also found we were logging URLs repeatedly, making it potentially confusing to the user which is the correct place to go, so I made a proposal in 042e7a2 how we could unify all this logging to be cleaner. End result:

Without auto-open and auth:

`npm run build && npm run start`	`npm run dev`

With auto-open & auth:

`npm run build && npm run start`	`npm run dev`

For comparison - before this change it would look like this (note the duplicated links & info):

`npm run build && npm run start`	`npm run dev`

cliffhall

One small change below.

But also I now notice that if the Inspector is already running and you start it again, it does not report that the proxy is running and stop. Instead it creates a new token and launches anyway. The new instance works and the old instance stops working.

Here is a video demonstrating it.

Instead of launching the second instance, I would expect the following response, which is what is happening on main at the moment.

client/bin/start.js

Replace explicit property assignments with shorthand syntax when property names match variable names (e.g., SERVER_PORT: SERVER_PORT becomes just SERVER_PORT).

felixweinberger · 2025-06-27T17:01:37Z

One small change below.

But also I now notice that if the Inspector is already running and you start it again, it does not report that the proxy is running and stop. Instead it creates a new token and launches anyway. The new instance works and the old instance stops working.

Here is a video demonstrating it.

Instead of launching the second instance, I would expect the following response, which is what is happening on main at the moment.

Interesting - I wasn't able to reproduce this. If I start 2 instances, they conflict as they should:

In your video it looks like one of the versions running is 0.14.3 though (the one in puzzlebox?) - I believe that one predates #513 (your video still shows the string (Auto-open is disabled when authentication is enabled), but it was removed in #513)

We had established here #513 (comment) that the reason no conflict was being detected is the difference between *:6277 binding to IPv6, while localhost:6277 binds to IPv4. I think the same thing is at play here - if you run a checkout of this branch in 2 terminals, it should conflict like it did for me.

In any case if I check out a separate worktree of 0.14.3 and main side by side, they don't conflict either - so that behavior is unrelated to this change, #555.

cliffhall

LGTM! 👍

cliffhall · 2025-06-27T17:31:20Z

We had established here #513 (comment) that the reason no conflict was being detected is the difference between *:6277 binding to IPv6, while localhost:6277 binds to IPv4.

@felixweinberger Yep, that was it. Apologies and thanks for checking. I've been battling a bad RSV type bug all week and my head's still full of cotton. 🤧

felixweinberger · 2025-06-27T17:34:03Z

We had established here #513 (comment) that the reason no conflict was being detected is the difference between *:6277 binding to IPv6, while localhost:6277 binds to IPv4.

@felixweinberger Yep, that was it. Apologies and thanks for checking. I've been battling a bad RSV type bug all week and my head's still full of cotton. 🤧

No problem thanks for the review :) hope you feel better soon!

felixweinberger changed the base branch from main to fweinberger/client-localhost June 24, 2025 10:36

felixweinberger force-pushed the fweinberger/fix-proxy-port-config branch 4 times, most recently from 0df3ce3 to f7cbeab Compare June 24, 2025 10:53

felixweinberger requested a review from cliffhall June 24, 2025 11:03

felixweinberger marked this pull request as ready for review June 24, 2025 11:03

felixweinberger mentioned this pull request Jun 24, 2025

fix: bind client to localhost to match server #529

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cliffhall requested changes Jun 25, 2025

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Base automatically changed from fweinberger/client-localhost to main June 26, 2025 09:05

felixweinberger requested a review from cliffhall June 26, 2025 09:39

felixweinberger force-pushed the fweinberger/fix-proxy-port-config branch from 6424232 to 554a922 Compare June 26, 2025 09:41

felixweinberger added 2 commits June 26, 2025 10:52

fix: ensure MCP_PROXY_PORT is included when auth is disabled

9e9419b

Address PR feedback by restructuring getClientUrl to always include MCP_PROXY_PORT parameter when using non-default port, regardless of auth status. Also improved code clarity by using params.size check.

felixweinberger force-pushed the fweinberger/fix-proxy-port-config branch from 554a922 to 9e9419b Compare June 26, 2025 09:52

cliffhall requested changes Jun 26, 2025

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felixweinberger added 2 commits June 27, 2025 15:00

felixweinberger force-pushed the fweinberger/fix-proxy-port-config branch from dad3696 to 042e7a2 Compare June 27, 2025 14:01

felixweinberger requested a review from cliffhall June 27, 2025 14:06

cliffhall requested changes Jun 27, 2025

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client/bin/start.js Outdated Show resolved Hide resolved

refactor: use ES6 shorthand property syntax in start.js

1aea337

Replace explicit property assignments with shorthand syntax when property names match variable names (e.g., SERVER_PORT: SERVER_PORT becomes just SERVER_PORT).

felixweinberger requested a review from cliffhall June 27, 2025 17:15

cliffhall approved these changes Jun 27, 2025

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cliffhall merged commit c24d2a9 into main Jun 27, 2025
7 checks passed

cliffhall deleted the fweinberger/fix-proxy-port-config branch June 27, 2025 17:28

cliffhall mentioned this pull request Jul 10, 2025

fix startup of webclient through cli #604

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