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Runtimes are the platform images (iOS, tvOS, watchOS, visionOS) used by simulators.

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flowdeck simulator runtime list

List installed runtimes.

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flowdeck simulator runtime available

List downloadable runtimes from Apple.

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What is listed

Apple’s index is a historical catalogue of every runtime ever published; most of it is no longer served. By default this command lists only what this Mac can actually install, which means three cuts:
  • Pre-release seeds. Apple stops serving a beta as soon as the next one lands, so an older seed answers xcodebuild with “not available for download”.
  • Older OS generations. Only the current generation and the one before it are served. Under Xcode 27 that is iOS 26 and 27; iOS 18.6 fails even though the index lists it.
  • Legacy package runtimes and images this macOS cannot run, from the index’s own contentType and hostRequirements.
Apple publishes each build twice, a universal image and a native-only one, and xcodebuild resolves the variant itself — so one row is shown per build, for the image that will actually download. Pass --all to see the unfiltered catalogue.

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flowdeck simulator runtime install

Download and install a runtime. Also available as flowdeck simulator runtime create (the two are aliases for the same command).

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One marketing version ships as several builds — iOS 26.5 exists as beta, beta 2, Release Candidate and release. Without --build, install resolves the version against Apple’s index and takes the first match, which may not be the build you meant:

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flowdeck simulator runtime delete

Remove an installed runtime.
Several builds of one OS version share a runtime identifier — three iOS 27.0 betas do — so without --build you are naming a group, and the resolver picks whichever build simctl lists first. Pass --build to remove an exact one:

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flowdeck simulator runtime prune

Delete simulators for a runtime.

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flowdeck simulator update

Upgrade an existing simulator to a newer runtime in place. The target runtime must already be installed (see runtime install).

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