Had this problem with alloy in steel which was solved by melting the post out but haven’t a clue what to do with this apart from usual soaking in WD/ penetrating oil. Any suggestions please?

When this happened to me with alloy in alloy frame we:
Tapped out tread on inside of seat post.
Welded on approprate dia bolt onto slide hammer.
Knocked post out with slide hammer
Came out fine.
See no reason it wouldn’t work with a carbon frame – famous last words!

i was under the impression that any kind of oil/ wd40 would be a bad idea as the carbon can absorb it and swell….. wonders if thats bull…..

Sodium hydroxide / caustic soda and a bit of water, you’ll be alright. 😉
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When my alloy post/alloy frame combo had seized it hadn’t actually bonded the metal or anything. Basically poured loads of water in and kept wiggling the saddle, hard until I got a tiny bit of movement. Then just carried on, getting more and more movement until it was free so I would try that before anything too drastic. Can you get eater onto the frame through a drain hole to get at both ends?

Cut the post down but leave some to get hold of. Then using a hacksaw blade carefully cut through length of your remaining post. You will feel an obvious change if you get through the metal. It will then release.

Slooowly… Good luck 🙂

Attach an old seat to the post. Clamp it in a vice and then twist the frame, you can usually get enough leverage doing this.