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limrun-xcode-bazel

Build a Bazel-based iOS / macOS / Apple app on Limrun's remote build execution (RBE) instead of a local Mac, and install it on a remote iOS simulator. Use when the project is a Bazel workspace (MODULE.bazel / WORKSPACE) building rules_apple / rules_swift targets and the user wants to `bazel build` it or run it on a simulator, or when a `--config=limrun` build or install misbehaves. To then tap, type, screenshot, or otherwise interact with the running app, use limrun-ios-simulator. For non-Bazel (plain xcodebuild) projects use limrun-xcode instead.

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UpdatedOct 05, 2026normalized snapshot

SKILL.md

Bazel iOS builds on Limrun RBE

Build Bazel Apple projects on Limrun's remote Mac workers — from any environment (Linux, Windows, macOS, VM, container), no local Xcode. lim xcode rbe brings up a remote RBE stack, tunnels it to a local port, and writes a .limrun/ config so bazelisk build --config=limrun runs Apple actions remotely. Never fall back to local Xcode or build tools.

Auth and CLI

Install if needed: npm install --global lim. Auth is lim login or LIM_API_KEY (may be set outside the project — don't ask for it just because it's absent). The CLI is the source of truth: the commands in this skill are verified, but if a flag errors or you need one not shown here, check lim xcode rbe --help instead of guessing.

Build

  1. From the Bazel workspace root (has MODULE.bazel / WORKSPACE), run lim xcode rbe. It sets up the instance + .limrun/ config and prints the exact build command. The tunnel runs in the background (prints a PID); --no-daemon keeps it foreground.
  2. Run the printed command, e.g. bazelisk --digest_function=sha256 build --config=limrun //App.

Don't hand-write .limrun/ or the flags — the CLI generates them for the fleet's Xcode and your OS. Re-run lim xcode rbe (after --stop) to refresh after a fleet Xcode upgrade.

To add your own Bazel flags to the limrun path without editing the generated config, put them in user.limrun.bazelrc at the workspace root. The generated config try-imports it last, so your build:limrun --… lines win, and it survives lim xcode rbe regeneration (.limrun/ does not).

Run on a simulator

lim xcode rbe is build-only; attach a simulator when the user wants to see or run the app. Check or attach (it installs the last build immediately, so no rebuild is needed):

lim xcode get             # is a simulator already attached?
lim ios create --attach   # attach one

Add --no-open when you have no browser to show the user; it skips opening the stream URL locally and still prints it for sharing.

If the attach output includes a signed stream URL, share it with the user as a Markdown link, such as [Live simulator](<signed-stream-url>).

With a simulator attached, every successful --config=limrun build automatically reinstalls and relaunches the app, no separate install step:

bazelisk --digest_function=sha256 build --config=limrun //App

Notes:

  • Attach upfront if you already know you want a sim: lim xcode rbe --ios (attaches at startup, removed on --stop).
  • Auto-install happens server-side from the build's events; there is no lim xcode rbe install subcommand or --target flag. To force a reinstall, rebuild (a cache-hit rebuild is seconds).
  • It fires when the successful invocation produced a single app, so in a multi-app workspace build one app target per invocation (//App, not //...); a multi-app build succeeds but installs nothing.

To tap, type, read the element tree, screenshot, or record the running app, switch to the limrun-ios-simulator skill.

Upload builds as assets

To publish a build as a Limrun asset (preview links, installing on other simulators, CI artifacts), arm uploads at tunnel start or upload one build after the fact:

lim xcode rbe --auto-upload preview/my-app --upload-ttl 24h  # every successful build refreshes the asset
lim xcode rbe upload preview/my-app --ttl 24h                # one-shot: the newest successful build
  • --auto-upload holds for the tunnel's lifetime: each successful --config=limrun build re-uploads the app under that asset name, no post-build step. Upload results land in .limrun/rbe.log.
  • rbe upload runs from the workspace root and needs a background tunnel plus at least one successful build; it errors otherwise.
  • TTLs are Go durations (24h, 30m; 1d is invalid) and optional; each upload without one pushes the asset's expiry to 14 days from that upload.
  • To change the --auto-upload config of a running tunnel, --stop and re-run; the CLI refuses a mismatched re-arm instead of silently ignoring it.
  • Preview an uploaded app in a browser at https://console.limrun.com/preview?asset=<name>&platform=ios.

Teardown

Stop with lim xcode rbe --stop (~20s to tear the remote stack down) and delete the instance with lim xcode delete <id>

Gotchas

  • Always pass --digest_function=sha256 before build (use the command the CLI prints verbatim). The Limrun cache is SHA256-only; Bazel 9 defaults to BLAKE3. It's a startup flag, so it can't live in --config=limrun. Symptoms: build → Cannot use hash function BLAKE3 with remote cache; install → non-SHA256 digest … rebuild with --digest_function=sha256.
  • Run lim xcode rbe from the workspace root, not a subdirectory — it writes .limrun/ there and fails fast otherwise.
  • A green build doesn't prove remote execution — cache hits (action cache hit / remote cache hit) make builds pass even with the tunnel gone. To force and verify real remote execution, see references/verify-remote.md.
  • The printed .ipa path won't exist on your machine — the build command carries --remote_download_outputs=minimal, which keeps the artifact in the instance's cache and downloads nothing. Bazel still prints its usual Target //App:App up-to-date: …/App.ipa line, but that file is not on disk. This is expected, not a failed build. With a simulator attached, the build auto-installs from the artifact's cache digest (read from the build event log, not the local file). Only if you genuinely need the .ipa locally, drop --remote_download_outputs=minimal from the build command and Bazel downloads the top-level output; auto-install keeps working either way.
  • A fresh instance can fail the first build with Lost inputs no longer available remotely (e.g. … Assets.car). It's a transient cache eviction between instances, not a code error; Bazel prints Found transient remote cache error, retrying the build... and the retry succeeds. To avoid hitting it mid-demo, pre-warm with a full build right after lim xcode rbe.
  • You don't have permission to save … in "CoreSimulator" (actool/ibtool) is a fleet-side device gap, not your config. Retry; if it persists, report it to Limrun.
  • The project's own Bazel settings can fight RBE (Xcode pinned via a Starlark transition, custom remote_default_exec_properties, sandbox-hostile genrules). These are per-project, not Limrun bugs — walk references/project-compatibility.md before concluding RBE is broken.