MCP Apps - start server issue

[wngreenway]

Following the MCP Apps Quickstart guide.

At "Step 4 See it in action", I run the npm start command. Instead of getting MCP server listening on http://localhost:3001/mcp I error out with Error: INPUT environment variable is not set.

Running W11, nodejs v22.19.0. Created files using VS Code.

Checked through the code to try and problem solve, but couldn't come to any conclusions.

Logs

PS C:\dev\learning\mcp_app> npm start

> mcp_app@1.0.0 start
> concurrently 'cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch' 'tsx watch main.ts'

[0] ''cross-env' is not recognized as an internal or external command,
[0] operable program or batch file.
[1] 'NODE_ENV' is not recognized as an internal or external command,
[1] operable program or batch file.
[2] 'INPUT' is not recognized as an internal or external command,
[2] operable program or batch file.
[4] 'build' is not recognized as an internal or external command,
[4] operable program or batch file.
[5] 'watch' is not recognized as an internal or external command,
[5] operable program or batch file.
[5] watch exited with code 1
[4] build exited with code 1
[2] INPUT=mcp-app.html exited with code 1
[1] NODE_ENV=development exited with code 1
[0] 'cross-env exited with code 1
[6] 'main.ts'' is not recognized as an internal or external command,
[6] operable program or batch file.
[6] main.ts' exited with code 1
[3] failed to load config from C:\dev\learning\mcp_app\vite.config.ts
[3] error when starting dev server:
[3] Error: INPUT environment variable is not set
[3] at file:///C:/dev/learning/mcp_app/node_modules/.vite-temp/vite.config.ts.timestamp-1770031400901-820ec7360d816.mjs:6:9
[3] at ModuleJob.run (node:internal/modules/esm/module_job:345:25)
[3] at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:651:26)
[3] at async loadConfigFromBundledFile (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/chunks/config.js:35909:12)
[3] at async bundleAndLoadConfigFile (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/chunks/config.js:35797:17)
[3] at async loadConfigFromFile (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/chunks/config.js:35764:42)
[3] at async resolveConfig (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/chunks/config.js:35413:22)
[3] at async _createServer (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/chunks/config.js:25362:67)
[3] at async CAC.<anonymous> (file:///C:/dev/learning/mcp_app/node_modules/vite/dist/node/cli.js:572:18)
[3] vite exited with code 1

Also noticed a package-lock.json is created, when the quickstart doesn't mention that.

[wngreenway]

Tried the same Quickstart using Warp terminal and encountered the same issue. Warp suggested I change package.json scripts-start
from:

"start": "concurrently 'cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch' 'tsx watch main.ts'"

to:

"start": "concurrently \"cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch\" \"tsx watch main.ts\""

Maybe to support Windows?!?

I tried that and got the following:

npm start

> mcp_app@1.0.0 start
> concurrently "cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch" "tsx watch main.ts"

[0] vite v7.3.1 building client environment for production...
[0]
[0] watching for file changes...
[0]
[0] build started...
[0] transforming...
[0] ✓ 142 modules transformed.
[0] rendering chunks...
[0] [plugin vite:singlefile]
[0]
[0] [plugin vite:singlefile] Inlining: mcp-app-BG8yD_ef.js
[0] computing gzip size...
[0] dist/mcp-app.html 2,127.32 kB │ gzip: 425.23 kB
[0] built in 859ms.

The Qucikstart says it should show MCP server listening on http://localhost:3001/mcp, is the above on the right track?

When I try to access http://localhost:3001/mcp I get a "site can't be reached".

Also, tried installing the test host. When I ran npm install I got a bun error. Do I need to install bun for W11?

[jonathanhefner]

@wngreenway Thank you for reporting these issues! I have opened #421 to address the quoting issue. Would you mind verifying that the callout in the Quickstart guide is correct:

Note

Windows cmd.exe users will need to convert quotes in the above command: npm pkg set scripts.start="concurrently ""cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch"" ""tsx watch main.ts""".

---

The Qucikstart says it should show MCP server listening on http://localhost:3001/mcp, is the above on the right track?

When I try to access http://localhost:3001/mcp I get a "site can't be reached".

It looks like concurrently isn't running the second process (tsx watch main.ts). You should see [1] MCP server listening on http://localhost:3001/mcp in the output.

If you remove the first process (such that the start script is just "concurrently \"tsx watch main.ts\""), do you see any output or errors?

---

Also, tried installing the test host. When I ran npm install I got a bun error. Do I need to install bun for W11?

Yes, the installation instructions in the Quickstart were missing a step. That was fixed in #398, but the published docs are typically updated only on release. I just did a manual deploy though. Would you mind trying the updated instructions:

git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps
npm install
cd examples/basic-host
npm start

[wngreenway]

@jonathanhefner thanks for grabbing this.

I tried the npm pkg set scripts.start="concurrently ""cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch"" ""tsx watch main.ts""" and that worked well. Approved from me.

I updated the scripts-start section to just contain "start": "concurrently \"tsx watch main.ts\"", I then ran npm run build, then npm start. I got the following: (the process just hangs)

PS C:\dev\learning\mcp_app> npm start

> mcp_app@1.0.0 start
> concurrently "tsx watch main.ts"

Finally, tried the new instructions for the test host. That worked fine. I couldn't connect to the MCP server, but once we sort out the first item issue it should work.

[jonathanhefner]

@wngreenway Thank you for confirming! 🙏

I believe the remaining issue is due to privatenumber/tsx#623. As a workaround, I've opened #428.

Can you check that adding --raw fixes the hang?

"start": "concurrently --raw \"cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch\" \"tsx watch main.ts\""

(You may also wish to pull in the changes to examples/quickstart/vite.config.ts from #428 in order to make the output more readable.)

[wngreenway]

@jonathanhefner thanks for the follow up

I added --raw, but the end result is still the hang. I also added the extra logging to vite.config.ts.

Here is the output

PS C:\dev\learning\mcp_app> npm start

> mcp_app@1.0.0 start
> concurrently --raw "cross-env NODE_ENV=development INPUT=mcp-app.html vite build --watch" "tsx watch main.ts"

[vite] vite v7.3.1 building client environment for production...
[vite]
[vite] watching for file changes...
[vite]
[vite] build started...
[vite] ✓ 142 modules transformed.
[vite] [plugin vite:singlefile]
[vite]
[vite] [plugin vite:singlefile] Inlining: mcp-app-BG8yD_ef.js
[vite] dist/mcp-app.html 2,127.32 kB │ gzip: 425.23 kB
[vite] built in 1568ms.

[jonathanhefner]

@wngreenway Thank you for checking! If privatenumber/tsx#623 wasn't the root cause, then we'll need to try a few more things:

• Maybe the server is actually running, but output wasn't flushed? If you run basic-host, does it list the server?
• Maybe the issue is only related to tsx, not concurrently? Does "start": "tsx watch main.ts" also hang?
• Maybe the issue is more like Windows + tsx issue vercel/turborepo#7834? Does "start": "tsx watch main.ts < NUL" work? Relatedly, does "start": "concurrently --no-input ..." work?

[wngreenway]

@jonathanhefner tried all the above scenarios, but all end up in the same result.

The npm start command looks like it hangs (never shows http://localhost:3001/mcp) and the basic host displays the UI for a moment then displays ERROR: Failed to connect to any servers (1 attempted).

Perhaps we wait for vite to sort out some its bugs and check back in. There's no pressure to sort this out by a certain timeframe. I still want to assist you resolve this for us poor W11 users 😥😂

I'm happy continue my expermintation in WSL, but let's keep this issue open if its okay with you.