My first instinct was to blame the app. Uninstalled it, reinstalled it. Nothing.
6th March 2026
I've been slowly upgrading my home network recently. Changed some DNS settings, moved a few devices around. Nothing dramatic. Yesterday it exposed a bug I didn't expect.
The Claude desktop app on my Mac was basically unusable on my WiFi. Messages would spin forever and eventually time out. I assumed it was a one-off thing at first. Closed it, reopened it, same deal.
The weird part was that the moment I turned on my VPN, everything worked instantly. Same computer and network. Only difference was the VPN.
I initially assumed it was DNS. That’s usually the culprit with weird network issues.
First I checked DNS, because that's usually the culprit with weird network issues.
Also worked. Normal response back from Cloudflare.
Then I noticed the browser version of Claude was working fine on the same network the whole time. I'd just been reflexively opening the desktop app.
That's when things got interesting.
The browser worked. Curl worked. Only the desktop app was struggling. The desktop app prefers HTTP/3, which runs over QUIC. QUIC uses UDP instead of TCP, and I'd honestly never thought about what that meant in practice until that moment. It means congestion control works differently, packet recovery works differently, and some routers handle it badly — especially if UDP traffic is being rate-limited or inspected somewhere along the path.
Some routers and ISP paths still handle this poorly, especially when UDP traffic is being rate limited or inspected.
Once I realized that, the behavior made sense.
On my normal network, QUIC traffic was being degraded somewhere along the path. The app would stall and eventually time out.
When I enabled a VPN, the traffic took a completely different path and fell back to regular HTTPS over TCP. Everything immediately started working again.
The fix ended up being simple.
I blocked UDP port 443 on my network, for that specific computer only. That effectively disables QUIC and forces apps to fall back to normal HTTPS.
After that, the Claude desktop app worked normally again.
It’s a good reminder that VPNs don’t just change your location. They also change how traffic flows across the network.
Sometimes that’s enough to hide an entirely different class of bugs.
Also important to note that claude cowork actually helped me find out what was going on.
Why Claude Only Worked When I Turned On My VPN
My first instinct was to blame the app. Uninstalled it, reinstalled it. Nothing.
6th March 2026
I've been slowly upgrading my home network recently. Changed some DNS settings, moved a few devices around. Nothing dramatic. Yesterday it exposed a bug I didn't expect.
The Claude desktop app on my Mac was basically unusable on my WiFi. Messages would spin forever and eventually time out. I assumed it was a one-off thing at first. Closed it, reopened it, same deal.
The weird part was that the moment I turned on my VPN, everything worked instantly. Same computer and network. Only difference was the VPN.
I initially assumed it was DNS. That’s usually the culprit with weird network issues.
First I checked DNS, because that's usually the culprit with weird network issues.
Resolved perfectly.
Then I tested HTTPS directly.
Also worked. Normal response back from Cloudflare.
Then I noticed the browser version of Claude was working fine on the same network the whole time. I'd just been reflexively opening the desktop app.
That's when things got interesting.
The browser worked. Curl worked. Only the desktop app was struggling. The desktop app prefers HTTP/3, which runs over QUIC. QUIC uses UDP instead of TCP, and I'd honestly never thought about what that meant in practice until that moment. It means congestion control works differently, packet recovery works differently, and some routers handle it badly — especially if UDP traffic is being rate-limited or inspected somewhere along the path.
Some routers and ISP paths still handle this poorly, especially when UDP traffic is being rate limited or inspected.
Once I realized that, the behavior made sense.
On my normal network, QUIC traffic was being degraded somewhere along the path. The app would stall and eventually time out.
When I enabled a VPN, the traffic took a completely different path and fell back to regular HTTPS over TCP. Everything immediately started working again.
The fix ended up being simple.
I blocked UDP port 443 on my network, for that specific computer only. That effectively disables QUIC and forces apps to fall back to normal HTTPS.
After that, the Claude desktop app worked normally again.
It’s a good reminder that VPNs don’t just change your location. They also change how traffic flows across the network.
Sometimes that’s enough to hide an entirely different class of bugs.
Also important to note that claude cowork actually helped me find out what was going on.