Manually built extension erroring out when adding as Burp extension

[confuciussayuhm]

Hi there,

I am following the instructions exactly as described here https://github.com/PortSwigger/mcp-server?tab=readme-ov-file#building-the-extension. When I add the built .jar back into Burp I get the following error:

java.lang.Exception: Extension class is not a recognized type
	at burp.Znqb.Zj(Unknown Source)
	at burp.Znqb.Zg(Unknown Source)
	at burp.Zn38.Zt(Unknown Source)
	at burp.Zjbw.ZY(Unknown Source)
	at burp.Zi0_.lambda$load$2(Unknown Source)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:545)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:328)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
	at java.base/java.lang.Thread.run(Thread.java:1474)

I have tried:

• Restart Burp and then try add again
• Remove BApp Store installed "MCP Server" (perhaps a conflict)
• Build in Linux
• Build in Windows

The above does not resolve the problem. No idea what I am doing wrong.

Java version on Linux (WSL):

java --version
openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)

Java version on Windows:

java --version
openjdk 21.0.9 2025-10-21 LTS
OpenJDK Runtime Environment Temurin-21.0.9+10 (build 21.0.9+10-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.9+10 (build 21.0.9+10-LTS, mixed mode, sharing)

[tomcyd]

Hi,

I have exactly same issue:

• macOS
• Burp 2026.2.3
• Java 21

java.lang.Exception: Extension class is not a recognized type
	at burp.Zkk_.ZL(Unknown Source)
	at burp.Zkk_.ZH(Unknown Source)
	at burp.Zn6r.ZU(Unknown Source)
	at burp.Ze0h.ZK(Unknown Source)
	at burp.Zwi.lambda$load$2(Unknown Source)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:545)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:328)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
	at java.base/java.lang.Thread.run(Thread.java:1474)

[option1587]

the wrong JAR file was placed. the correct JAR file is located in the path \build\libs.

[tomcyd]

Hi,

Thanks, @option1587.
That's true. I discovered it few feek week ago a well, but forgot to leave another comment here.

Chears