Dev Proxy v2.1 with configuration hot reload and stdio proxying

We’re excited to announce the release of Dev Proxy v2.1.0! This release brings two major productivity features that developers have been asking for: automatic configuration hot reload and stdio traffic proxying – perfect for testing MCP servers and other stdio-based tools.

In this version:

Configuration hot reload – Dev Proxy automatically restarts when you change the config file
Stdio traffic proxying – Intercept, inspect, and mock stdin/stdout/stderr traffic
New --api-port command-line option
LatencyPlugin now supports delays greater than 10 seconds
CORS support for the Dev Proxy web API
Multiple bug fixes and improvements

Configuration hot reload

No more manual restarts when tweaking your proxy configuration. Dev Proxy now watches your configuration file and automatically restarts when you save changes.

This was one of our oldest feature requests – dating back to April 2023. Whether you’re adjusting failure rates, adding plugins, or changing URL patterns, your changes take effect immediately. Just save the file and keep working.

Why this matters:

When you’re iterating on your proxy configuration – fine-tuning error rates, adjusting mock responses, or testing different plugin combinations – stopping and restarting the proxy breaks your flow. With hot reload, your development loop gets tighter and more productive.

Stdio traffic proxying

Modern AI development increasingly relies on tools that communicate via stdin/stdout/stderr – particularly MCP (Model Context Protocol) servers. Now you can intercept and manipulate this traffic just like you do with HTTP.

How it works:

Wrap any executable with Dev Proxy:

devproxy stdio npx my-mcp-server

Dev Proxy proxies all stdin, stdout, and stderr traffic, letting you:

Inspect messages in Chrome DevTools Network tab (appear as stdio:// URLs)
Mock responses using the new MockStdioResponsePlugin
Simulate latency with the existing LatencyPlugin
Use placeholders like @stdin.body.id in your mock responses

This opens up Dev Proxy to an entirely new category of applications – anywhere you need to test, debug, or mock stdio-based communication.

New `--api-port` command-line option

Running multiple Dev Proxy instances? Need the default port 8897 for something else? You can now configure the API port directly from the command line:

devproxy --api-port 9000

No need to create a separate config file just to change the port.

LatencyPlugin improvements

Previously, the LatencyPlugin schema artificially limited maxMs to 10,000 milliseconds (10 seconds). This restriction never existed in the actual code – it was just a schema limitation.

What changed:

The schema now allows any value for maxMs, letting you simulate delays of any duration – useful for testing timeout handling and slow network conditions.

CORS support for Dev Proxy web API

Building browser-based developer tools that interact with Dev Proxy? The API on port 8897 now supports cross-origin requests, so your web applications can call endpoints like /proxy/jwtToken without CORS errors.

Bug fixes

-e flag now works correctly – Starting Dev Proxy with environment variable presets no longer throws a FormatException
AuthPlugin ApiKey validation fixed – The plugin no longer incorrectly reports missing ApiKey.Parameters when they’re properly configured
GenericRandomErrorPlugin method matching – Error responses without a specified HTTP method now correctly match all methods (GET, POST, PUT, DELETE, etc.) instead of only GET
MockResponsePlugin Content-Type handling – No more spurious JSON parsing warnings when using form data or other non-JSON content types
CRUD API data file hot reload – Data file changes now take effect immediately without restarting Dev Proxy

Dev Proxy Toolkit 1.12.0

Dev Proxy Toolkit is an extension that makes it easier to work with Dev Proxy from within Visual Studio Code.

This release focuses on configuration validation and developer experience improvements:

Custom installation paths — New devProxyPath setting for non-standard Dev Proxy locations
Smarter config validation — Config section properties are now validated against the Dev Proxy schema, catching invalid values, unknown properties, and schema version mismatches
Enhanced diagnostics — More precise highlighting, unique diagnostic codes with documentation links, and new warnings for empty urlsToWatch and optional plugin configurations
More quick fixes — Add optional or missing plugin config sections, update schema versions across files, and remove unknown properties
Better plugin support — AuthPlugin and LanguageModelFailurePlugin fixes, plus new snippets for GraphConnectorGuidancePlugin and MockStdioResponsePlugin

Checkout out the changelog for more information on changes and bug fixes.

Why upgrade to v2.1.0?

Faster iteration – Configuration hot reload means no more manual restarts
MCP server testing – Inspect, mock, and debug stdio traffic with familiar workflows
More flexibility – Runtime port configuration and unlimited latency simulation
Cleaner experience – Multiple bug fixes reduce friction and noise

Try it now

Download Dev Proxy v2.1.0 today and build better API-connected applications with confidence!

Thanks to Nitesh Singhal for contributing to this release.

Got feedback or ideas? Join us and be part of the conversation.

Dev Proxy v2.1 with configuration hot reload and stdio proxying