The launcher refuses to start and says no CLI build exists for this platform (linux-x64), then prints a supported-platform list that includes linux-x64. The package is published and its tarball downloads fine, so the error reads like a packaging gap when it is not. This is pingdotgg/t3code#20186.
Why it happens
The launcher resolves its binary through an optional dependency - one package per platform triple, @t3code/t3-linux-x64 and friends. npx builds its cached install once per package version. If that cache entry was created before the platform package existed, or npm was configured to skip optional dependencies, the cache holds an install with no binary for your triple and every later run reuses it. The platform is fine. The cache is lying to you.
The fix
Throw the cache away and let npm refetch the optional dependency:
npx clear-npx-cache # or: rm -rf ~/.npm/_npx
npx t3@latest serve
If it fails again after a clean fetch, check that npm is allowed to install optional dependencies at all - npm config get omit and npm config get optional should not be filtering them out. That combination is the whole failure mode.
One heads-up for later versions. PR #20148, merged after 0.1.0 was published, drops the optional-dependency install path and has the launcher download the binary from GitHub Releases instead, so the cached install stops mattering. It is not in a published release yet, so on 0.1.0 the cache is still in the loop.
The general shape is worth remembering for any npx-run tool that ships per-platform binaries behind optional dependencies. A stale _npx cache produces errors that point at the platform rather than at npm, and the supported list printed in the error message can contradict the error itself.