limrun-xcode-bazel

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

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.

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.

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.