feat: add MCP_SERVER_ENDPOINT env var with transport auto-detection

[koepalex]

Summary

modelcontextprotocol/inspector is a great tool, that I use regularly to test my own MCP servers. In my environment the server endpoint of the mcp server changes (e.g. running multiple instances, port already occupied etc.) to address this I use either dotnet aspire or docker compose which will start the mcp server along with the inspector. I would like to pass the currently used mcp server endpoint as environment variable.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (no functional changes)
• Test updates
• Build/CI improvements

Changes Made

server/src/index.ts:

• Added detectTransportFromEndpoint() function to auto-detect transport type from URL path
• Added MCP_SERVER_ENDPOINT environment variable support
• Updated /config endpoint to return effective server URL and auto-detected transport
• CLI args (--server-url, --transport) take precedence over env var

README.md:

• Added MCP_SERVER_ENDPOINT to the configuration table with description

Usage example:

SSE transport (auto-detected from /sse path)

MCP_SERVER_ENDPOINT=http://localhost:8080/sse npx @modelcontextprotocol/inspector

Streamable HTTP transport (auto-detected from /mcp path)

MCP_SERVER_ENDPOINT=http://localhost:8080/mcp npx @modelcontextprotocol/inspector

Related Issues

• original PR add default MCP server URL on startup #288, I addressed the feedback given there in this PR

Testing

• Tested in UI mode
• Tested in CLI mode
• Tested with STDIO transport
• Tested with SSE transport
• Tested with Streamable HTTP transport
• Added/updated automated tests
• Manual testing performed

Test Results and/or Instructions

$env:MCP_SERVER_ENDPOINT='http://localhost:1234/sse'
npm run start

$env:MCP_SERVER_ENDPOINT='http://localhost:5678/mcp'
npm run start

Checklist

• Code follows the style guidelines (ran npm run prettier-fix)
• Self-review completed
• Code is commented where necessary
• Documentation updated (README, comments, etc.)

Breaking Changes

Additional Context

As this is a relatively small change, I hope it can get merged before V2 (architecture changes) are done, even if it is not a bugfix.