How to Handle “iOS 18.6 is not available for download.” in Xcode 27

When Xcode 27 attempts to provision an older iOS Simulator runtime from the command line, xcodebuild may report “iOS 18.6 is not available for download.” even though the same runtime appears in Xcode’s Components UI. This article explains the reported scope, safe troubleshooting steps, practical CI alternatives, and how to verify the installed runtime without overstating an unconfirmed root cause.

Representative error output
Cause
Affected conditions and versions
Recommended troubleshooting steps
Alternatives
How to verify the result
Cautions
Summary
Official references

Representative error output

$ xcodebuild -downloadPlatform iOS -buildVersion 18.6
Finding content...
iOS 18.6 is not available for download.

The Apple Developer Forums report also describes failures when requesting other pre-iOS 26 runtimes, including iOS 17.5.

Cause

The confirmed symptom is a mismatch: the required runtime is available through Xcode’s graphical Add Platforms dialog, but Xcode 27’s xcodebuild -downloadPlatform lookup does not find it. The reporter filed the issue through Feedback Assistant as FB24836279.

Important: Apple has not published a confirmed root cause or a fixed Xcode version. Treating this as a proven server-catalog defect or a confirmed Xcode 27 regression would go beyond the available official information.

Affected conditions and versions

Xcode 27.0 (27A266a)
Also reported with Xcode 27.1 beta (27A9269) and Xcode 27.2 beta (27B5019j)
A CLI request such as xcodebuild -downloadPlatform iOS -buildVersion 18.6
Reported on both macOS 26.7 and macOS 27.0
The same runtime remains installable through Xcode > Settings > Components > Add Platforms

Apple’s system requirements list iOS 17 and later as the Simulator range for Xcode 27. Therefore, iOS 18.6 and 17.5 are within the published range, while iOS 16.4 is outside Xcode 27’s supported Simulator range. Do not treat a failure for an out-of-range runtime as proof of the same issue.

Recommended troubleshooting steps

1. Confirm which Xcode installation is active

xcodebuild -version
xcode-select -p

If several Xcode versions are installed, select the intended application explicitly and complete first-launch setup.

sudo xcode-select -s /Applications/Xcode.app
sudo xcodebuild -runFirstLaunch

2. Retry with Apple’s documented syntax

xcodebuild -downloadPlatform iOS \
  -buildVersion 18.6 \
  -exportPath "$PWD/SimulatorRuntimes"

If the same “not available” message remains, repeatedly reinstalling Xcode or deleting caches is unlikely to establish the cause. Move to a supported alternative instead.

3. Use the Components UI on an interactive Mac

Open Xcode.
Choose Xcode > Settings > Components.
Under Other Installed Platforms, choose Add Platforms.
Filter for iOS and the required version, then select Download & Install.

This is an Apple-documented way to manage Simulator runtimes, and the current forum report states that the UI path succeeds for the affected runtimes.

4. Pre-provision the runtime for headless CI

For CI machines without a graphical session, consider one of these approaches:

Pin a CI image that already contains the required runtime.
Export the runtime on a machine where the command still succeeds, such as a validated Xcode 26.6 provisioning host.
Import an acquired .simruntime.dmg with Apple’s documented -importPlatform command.

sudo xcode-select -s /Applications/Xcode.app
sudo xcodebuild -runFirstLaunch
xcodebuild -importPlatform "/path/to/iOS_18.6_Simulator_Runtime.dmg"

Compatibility of a runtime obtained with another Xcode release depends on the host and Apple’s supported runtime range. Validate the resulting image before rolling it out, and follow Apple’s license terms and your organization’s artifact-distribution policy.

Alternatives

Keep Xcode 26.6 for the runtime-provisioning stage.
Choose a CI provider image that already includes the required iOS runtime.
If older-OS coverage is not required, move the test job to a newer runtime that Xcode 27 can retrieve.

Avoid deleting Xcode or the entire ~/Library/Developer directory as a generic fix. Because the GUI can locate the affected runtime, the available evidence does not establish local cache corruption.

How to verify the result

xcrun simctl list runtimes
xcrun simctl list devices available

Confirm that the requested iOS runtime appears and is not marked unavailable. Then create and boot a Simulator for that OS version. In CI, rebuild a runner from the same image to confirm that the provisioning result is reproducible.

Cautions

As of September 28, 2026, Apple has not identified a fixed Xcode version.
Xcode 27 officially supports iOS 17 and later Simulator runtimes.
The reported GUI workaround may not succeed in every environment.
A future Xcode update or server-side catalog change may alter the behavior.

Summary

iOS 18.6 is not available for download. is a reported Xcode 27 command-line provisioning failure for older iOS Simulator runtimes. Verify the active Xcode and complete first-launch setup, use the Components UI when an interactive Mac is available, and pre-provision a validated runtime or CI image for headless systems. Until Apple publishes a confirmed fix, keep the root cause qualified and test every CI image explicitly.