build-native

name	build-native
description	Rebuild native LiteRT-LM prebuilts for flutter_gemma (iOS / macOS / Linux / Windows / Android) — covers required build flags, the upstream commit pin, and a mandatory post-build verification checklist that catches the bugs we have already shipped at users.
user_invocable	true

Build native dylibs for flutter_gemma — the right way

This skill exists because we shipped 0.14.0 and 0.14.1 with native dylibs that broke real users (App Store rejection in 0.14.0, install_name_tool headerpad failure in 0.14.1, x86_64-not-arm64 in upstream prebuilts). Every one of those was caught after publish. The verification checklist below would have caught all of them locally.

The v0.16.0 migration added a second class of defect, and it is the more expensive one: inputs we pinned once and stopped maintaining — a Bazel define upstream had deleted, an OpenVino SDK four minor versions behind the pin, a dispatch library hardcoded to a LiteRT commit from four releases ago. None of them fails a build. All of them fail at a version boundary, which makes them look exactly like upstream regressions — and we filed two upstream issues that had to be retracted before finding that out.

Read top to bottom before rebuilding anything. The checklist is non-optional.

Every path below is relative to the repo root. This is stated because it was not true for a long time: the paths in this file predated the 1.0 monorepo split and pointed at native/litert_lm/…, which does not exist from the root. nm and otool on a missing file print nothing, grep -c on nothing returns 0, and the whole verification checklist read as passing while touching no artifact at all — the same defect the checklist exists to catch, applied to itself. When you add a command here, run it from the root first and confirm it produces output.

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Pre-build setup

Required upstream commit

Pin the exact tag commit SHA on every platform, and pass it explicitly. Upstream has retagged releases in place before, so a tag name is not a stable reference. Get the SHA with:

gh api repos/google-ai-edge/LiteRT-LM/git/ref/tags/<tag> --jq '.object.sha'

The scripts' DEFAULT_REF lags whatever release you are migrating to, so never rely on it — build_ios.sh still defaulted to the v0.14.0-era 80f301ff during the v0.15.0 migration.

The historical iOS 5e0d86b pin is obsolete. It existed because 5e0d86b was the first commit shipping libLiteRtMetalAccelerator.dylib for iOS, and older tags produced a libLiteRtLm.dylib whose Metal-accelerator vtable didn't match the prebuilt framework → EXC_BAD_ACCESS inside litert_lm_engine_create on device. Modern tags ship their own prebuilt/ios_arm64/, so building iOS from the tag is now the safer option.

The real invariant is: source and accelerator prebuilts must come from the same tree. Mixing them is the hazard — not the choice of tag. Verify before building:

gh api "repos/google-ai-edge/LiteRT-LM/contents/prebuilt/ios_arm64?ref=<tag>" --jq '.[].name'
# needs libLiteRtMetalAccelerator.dylib + libLiteRtTopKMetalSampler.dylib

The LiteRT pin is a second ABI surface — check it every time

A LiteRT-LM bump silently moves LITERT_REF in WORKSPACE (line ~6), and that is a different upstream repo with its own C API. This matters because three separate things bind to it:

• flutter_gemma_litertlm → LiteRT-LM C API (c/engine.h)
• flutter_gemma_embeddings, flutter_gemma_speech → LiteRT C API directly, via hand-written bindings in lib/src/litert/
• both NPU dispatch libraries → the LiteRtDispatchApi struct and the LiteRT runtime they are loaded into

So a green litertlm smoke run proves nothing about embeddings, and nothing at all about NPU. In the v0.14.0 migration the pin moved, LiteRtCreateModelFromFile gained a third parameter (LiteRtEnvironment first), and embeddings silently returned status=500 — a full day lost before the cause was found.

for t in <old-tag> <new-tag>; do
  gh api "repos/google-ai-edge/LiteRT-LM/contents/WORKSPACE?ref=$t" --jq '.content' | base64 -d | grep '^LITERT_REF'
done

If it moved, diff the headers the bindings use (litert/c/litert_{model,environment,options,tensor_buffer,compiled_model}.h) at both refs. Symbol list — the hand-written bindings moved into the litertlm package, so the old embeddings-only grep now misses more than half of them (23 vs 52 symbols):

grep -rohE "LiteRt[A-Za-z_]+" \
  packages/flutter_gemma_litertlm/lib/src/ffi/litert_bindings.dart \
  packages/flutter_gemma_embeddings/lib/src/litert/ \
  packages/flutter_gemma_speech/lib/src/litert/ | sort -u

Known pins: v0.14.0 → 622f1f3c (breaking for embeddings); v0.15.0 → 3cb830ad (safe — four headers byte-identical, litert_compiled_model.h only gains LiteRtGetCompiledModelEnvironment()); v0.16.0 → 0ff28117f1cb5556d0e015bf80b773f74e2bee51.

Never hardcode this ref in a build script. build_qualcomm_dispatch.sh carried a literal 5c5b9ce6 from the native-v0.12.0 era and nobody noticed for four releases, because a stale dispatch library does not fail politely — see the NPU section below. Derive it from the WORKSPACE of the LiteRT-LM revision being built:

LITERT_REF="$(curl -fsSL \
  "https://raw.githubusercontent.com/google-ai-edge/LiteRT-LM/$LITERTLM_REF/WORKSPACE" \
  | sed -n 's/^LITERT_REF *= *"\([0-9a-f]*\)".*/\1/p' | head -1)"
[ -n "$LITERT_REF" ] || { echo "could not resolve LITERT_REF" >&2; exit 1; }

The empty-result guard matters: curl | sed on a moved/renamed WORKSPACE yields an empty string, and an empty --define or checkout ref fails much later and much less legibly than an explicit exit 1 here.

Bazel defines rot — confirm each one still exists in the tree

A --define that upstream has deleted is not an error. Bazel accepts any --define=key=value; if no config_setting reads that key, the flag is silently inert and you get the default build.

We passed --define=litert_link_capi_so=true on Windows and Linux for three releases. Upstream removed the name; no config_setting reads it in the v0.16.0 tree (a plain grep still finds one commented-out mention in .github/workflows/ci-build-mac.yml, which is why the recipe below greps only .bzl/BUILD/.bazelrc). The live name is litert_runtime_link_mode=dynamic. This was harmless until v0.14.0 split Dawn out of the WebGPU accelerator, from which point the statically-linked runtime is exactly what crashed GPU engine_create — reported upstream as #2957 and retracted, because the cause was ours.

Before trusting any define in a build command, grep the tree you are about to build:

for d in litert_runtime_link_mode resolve_symbols_in_exec <any-other>; do
  printf "%-32s %s\n" "$d" "$(grep -rl "$d" /tmp/LiteRT-LM --include='*.bzl' --include='BUILD*' --include='*.bazelrc' 2>/dev/null | wc -l)"
done

Zero files means the flag is dead — find the replacement in docs/getting-started/build-and-run.md, do not leave it in "just in case". Both of the current desktop defines are documented there as mandatory for GPU:

Define	Why
litert_runtime_link_mode=dynamic	Keeps the LiteRt C API out of libLiteRtLm so it resolves against the separately shipped libLiteRt at runtime — which is what the prebuilt WebGPU accelerator and the split libwebgpu_dawn expect.
resolve_symbols_in_exec=false	Without it Bazel cannot resolve LiteRt* imports at link time (167 unresolved externals).

Required linker flags

Platform	Flag	Why
macOS / iOS	-Wl,-headerpad_max_install_names	Native Assets re-runs install_name_tool -id @rpath/... on every pub get. Without headerpad, the rewrite fails with "larger updated load commands do not fit" and pub get aborts.
macOS	-mmacosx-version-min=11.0	Without it clang stamps LC_BUILD_VERSION with the build host's OS, so the bundle's floor silently tracks whichever Mac produced it.
Android	aarch64-linux-android24-clang (target triple carries the API level)	Same reason, stated explicitly rather than inherited.
Linux	-Wl,--dynamic-list=... (already in patch_c_api.sh §1)	Exports LiteRt* / litert_lm_* symbols for the WebGPU accelerator's dlsym(RTLD_DEFAULT)
Windows	/DEF:windows_exports.def (already in patch_c_api.sh §1)	Bazel native attribute
All	-fvisibility=hidden is WRONG for our use case — needs default visibility on the listed export sets

The headerpad_max_install_names flag is not optional on Apple platforms. Verify in the post-build checklist below.

Deployment target must be stated, never inherited. This bites only the libraries we compile — the upstream prebuilt companions arrive with their own sane values (11.0 / 14.0), so a single odd number in the middle of an otherwise healthy bundle is the signature. native-v0.14.0 shipped libStreamProxy.dylib with minos 26.0 for exactly this reason: build_macos.sh compiled it with a bare clang -shared on a macOS 26 host, while example/macos/Podfile advertises platform :osx, '10.15'. build_ios.sh never had the bug because its step 8b force-normalizes every companion to 13.0 with vtool -set-build-version. Check every self-built binary:

for f in prebuilt/macos_arm64/*.dylib; do
  printf "%-46s minos=%s\n" "$(basename $f)" "$(vtool -show-build "$f" | awk '/minos/{print $2; exit}')"
done

Anything at or near the current macOS release means the flag was dropped. Rebuilding one dylib is a two-second clang call — no Bazel involved — so there is never a reason to ship it wrong.

Patches we apply

packages/flutter_gemma_litertlm/native/litert_lm/patch_c_api.sh — the surviving sections (2–9 were deleted at the v0.14.0 migration once upstream implemented them natively; don't go looking for them):

1. cc_binary(linkshared=True) target + Linux dynamic-list + Windows .def 4b. set_use_hw_masking_for_npu (Intel LunarLake/PantherLake — default true crashes their NPU) 4c. GPU smooth-UI knobs — gpu_context_low_priority + kernel_batch_size (#364) 10b. App Store ITMS-90432 fix — rewrite gpu_registry.cc dlopen to @executable_path/...framework/<X> on Apple. iOS-critical.
2. minizip/zlib mirrored off the flaky zlib.net

§10a (sampler_factory.cc) is deliberately NOT patched — libLiteRtTopKMetalSampler.dylib has 3-of-7 broken exports on Apple (#2073). Patching it surfaces a NULL-vtable crash. Leave the basename dlopen so sampler_factory.cc falls back to CPU sampler. Revisit when Google fixes #2073.

Run the patcher against the new tag before building anything. There is no dry-run mode — it edits the tree in place, so run it on a throwaway clone.

Every section must print OK. A WARN means a patch target moved and that section did not apply — and the script still exits 0, so nothing downstream notices. Sections 2-9 died exactly this way and we only found out months later, when the sampler stopped honouring per-session parameters. §10b is the App Store ITMS-90432 fix (pitfall #2, already shipped to users once): if its WARN fires, the Metal accelerator path is silently unpatched and iOS GPU breaks on device.

Make it fail instead of trusting yourself to read the log:

LOG=/tmp/patch_c_api.log
bash packages/flutter_gemma_litertlm/native/litert_lm/patch_c_api.sh /tmp/LiteRT-LM 2>&1 | tee "$LOG"
grep -q WARN "$LOG" && { echo "PATCH INCOMPLETE — a target moved, do not build"; exit 1; }

# Assert the artifact, not the log line: §10b's macro must be in the patched file.
grep -q FLUTTER_GEMMA_METAL_FW_PATH \
  /tmp/LiteRT-LM/litert/runtime/accelerators/gpu_registry.cc \
  || { echo "§10b did not apply — iOS GPU would ship broken"; exit 1; }

On Windows/git-bash, python3 resolves to the Microsoft Store stub, which prints "Python was not found" and still exits 0 — so sections 10b and 11 silently no-op. Shim it first: printf '#!/bin/sh\nexec "<path>/python.exe" "$@"\n' > /somewhere/python3 && chmod +x and prepend that dir to PATH.

Cloning upstream without fighting the network

The repo carries large LFS binaries; a full clone repeatedly dies mid-transfer on anything but a rock-solid link, and the failure restarts from zero. Clone the tag shallow with LFS deferred, then pull only the platforms you're building:

GIT_LFS_SKIP_SMUDGE=1 git -c http.version=HTTP/1.1 -c http.postBuffer=524288000 \
  clone --depth 1 --branch <tag> https://github.com/google-ai-edge/LiteRT-LM /tmp/LiteRT-LM
cd /tmp/LiteRT-LM
git lfs pull --include="prebuilt/macos_arm64/*,prebuilt/ios_arm64/*,prebuilt/ios_sim_arm64/*,prebuilt/android_arm64/*"

Then point origin at a local bare mirror. build_*.sh run git fetch --tags --force origin whenever the clone dir already exists — cheap on a full clone, near-endless on a shallow one, because every other tag's objects have to be fetched. Redirecting origin makes that fetch local and instant (measured: 0.03 s), and removes the network from the build loop entirely:

git clone --bare /tmp/LiteRT-LM /tmp/LiteRT-LM-mirror.git
cd /tmp/LiteRT-LM && git remote set-url origin /tmp/LiteRT-LM-mirror.git
# LFS follows origin, and the bare mirror has no LFS endpoint — pin it back:
git config lfs.url https://github.com/google-ai-edge/LiteRT-LM.git/info/lfs

That last line is not optional. Redirecting origin fixes the git fetch --tags hang and immediately breaks git lfs pull, which then leaves pointer files in place of the accelerator dylibs — a failure that surfaces much later, as a dylib that loads but exports nothing.

Verify the LFS pull produced real binaries, not pointers — head -c 20 <file> starting with version https means it's still a pointer.

⚠️ NPU dispatch libraries MUST be rebuilt from the pin — never carried forward

This section previously said the opposite. It claimed both NPU stacks are unbuildable and must be copied from the previous release, and instructed asserting byte-identity against it. That was wrong, and it is the single most expensive mistake this skill has recorded: it turned two of our own configuration defects into upstream bug reports (#2957, #3217) that had to be retracted, and it is the direct cause of the Android NPU crash below.

Both dispatch libraries build from the same pin as the runtime:

Bundle	Target	Notes
windows_x86_64	@litert//litert/vendors/intel_openvino/dispatch:LiteRtDispatch	Bazel fetches the OpenVino SDK itself (configurable_repo) — no preinstalled toolkit on the runner. Built in CI.
android_arm64	//litert/vendors/qualcomm/dispatch:dispatch_api_so	From the LiteRT repo at the derived LITERT_REF, not LiteRT-LM. Bazel auto-downloads QAIRT 2.44.0.260225 (~500 MB), or point LITERT_QAIRT_SDK at a local copy. Built locally by build_qualcomm_dispatch.sh.

Why the old rule was believable and still wrong: a fresh LiteRT-LM build genuinely emits neither library, because neither lives in the LiteRT-LM tree — they are LiteRT vendor targets. "Absent from the output" was read as "unbuildable" instead of "wrong target".

A stale dispatch library does not fail politely. It is a plugin loaded into the runtime's address space and it shares the runtime's structs:

• Windows. Ours had been frozen since native-v0.11.0-b and shipped OpenVino 2026.2.0, while every LiteRT pin since has required 2026.3.0.dev20260622 (build 22242-561fc907ca4). Upstream's own third_party/intel_openvino/openvino.bzl warns that a mismatched pair leaves the Intel OV compiler plugin talking to the wrong libopenvino_intel_npu_compiler at runtime. That is what killed engine_create on backend=npu.
• Android. libLiteRtDispatch_Qualcomm.so was built against LiteRT 5c5b9ce6. On v0.16.0 it SIGSEGVs inside LiteRtDestroyOptions the moment engine_create tears an options object down — a struct-layout mismatch, surfacing as a crash in the runtime's symbol, which reads as a runtime bug.

Both looked like clean regressions at a version boundary, because they were: the boundary is where upstream finally touched the code the frozen artifact depended on. Verify a bisection's conclusion, not just its measurement — "v0.13.1 works, v0.14.0 crashes" is a true measurement that says nothing about whose change is at fault when one input to the comparison never moved.

The OpenVino runtime half is now asserted, in both Windows workflows. They read the build string the SDK declares in runtime/version.txt and compare it against the string embedded in the openvino.dll being staged, throwing on mismatch — and throwing just as loudly when either side is unreadable or malformed, because a check that passes on missing data is indistinguishable from one that verified something.

Be precise about what that does and does not buy, because the first version of this note was not:

• It asserts internal consistency, not the pin. Both sides come out of the same Bazel-fetched $ovRoot, so no input can currently make it fail — only a future edit that stages either side from somewhere else. That is anti-regression value, not detection of an upstream SDK bump.
• It does not cover the dispatch. LiteRtDispatch.dll is copied from bazel-bin and embeds no version string, so the half of the pair that actually broke in #3217 is still only verified by check #9 on real silicon.
• It cannot catch a hand-staged bundle. Copy-Item -Force overwrites artifacts\ from $ovRoot immediately before the check, so anything placed there by hand is gone before it could be tested.

Put it in both workflows. build-litertlm-native-windows.yml is fast iteration and ends at upload-artifact; build-litertlm-native.yml carries the publish-release job that creates the native-v* tag users fetch. A check added only to the first protects nothing that ships — which is exactly the mistake this paragraph originally shipped with.

Android has no equivalent, and cannot have one. The QNN .sos are cp'd straight out of the Bazel-fetched QAIRT SDK, so there is nothing to pair them against — no second copy, no version string to compare. build_qualcomm_dispatch.sh asserts each one is present ([ -f "$src" ] || exit 1, a real gate) and then only prints its size with printf; that print is not a comparison and must not be counted as one. The actual coverage for Android is the on-device run (check #9).

The QNN runtime .sos must be refreshed with the dispatch, not carried forward. They are not compiled from source — they come out of the QAIRT SDK — which makes "just keep the old ones" look safe. It is not: the dispatch you build negotiates an API version with them, and ours drifted into

E/litert: [qnn_manager.cc:349] Qnn System library version 1.8.0 is mismatched.
          The minimum supported version is 1.11.0.
E/litert: [dispatch_api.cc:139] Failed to set up QNN manager
E/litert: [dispatch_delegate.cc:131] No usable Dispatch runtime found

Bazel already downloaded the matching SDK while building the dispatch. Take them from there so both halves come from one QAIRT:

Q=$(find "$(bazel info output_base)/external/qairt" -maxdepth 0 2>/dev/null)
D=packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/android_arm64
for f in libQnnSystem.so libQnnHtp.so libQnnHtpV{73,75,79,81}Stub.so; do cp -f "$Q/lib/aarch64-android/$f" "$D/"; done
for v in 73 75 79 81; do cp -f "$Q/lib/hexagon-v$v/unsigned/libQnnHtpV${v}Skel.so" "$D/"; done

libQnnHtpV79Skel.so in particular must be bundled: it is absent from /vendor/dsp/cdsp/ even on Qualcomm reference firmware, so shipping it is required, not a workaround for unusual devices.

Do not verify this by version string. QNN carries two independent numberings — the SDK marketing version (2.44.0.260225143659, present in libQnnHtp*.so) and the per-component API version (1.8.0 vs the required 1.11.0) that actually gates loading. Matching the first says nothing about the second, and libQnnSystem.so — the exact file that broke — carries no version string at all, so a strings | grep check on it silently reports "no data" in a way that reads as "fine". Compare file sizes against the SDK instead; every one of our ten stale libs was visibly smaller than its QAIRT 2.44 counterpart.

Whatever the origin, assert the NPU file count per bundle before packing. Shipping without them fails no build and no CPU/GPU smoke test — only a user on PreferredBackend.npu finds out.

Windows CI specifics

• Job timeout 150 minutes. A cold Bazel cache plus the OpenVino fetch overruns anything shorter; 60 min was killing runs mid-link.

• Exclude the Bazel output base from Defender. Two consecutive runs died at ~52 min with Couldn't delete action output directory: D:/b/execroot/litert_lm/bazel-out/_tmp/actions/stderr-NNNN (Permission denied). The build had finished; Defender was holding a per-action stderr file open during cleanup. Add-MpPreference -ExclusionPath on the output base, plus one warm-cache retry of the build step, turns a 52-minute loss into a two-minute one.

• Allow-list the OpenVino components; never copy runtime\bin wholesale. A blanket copy drags in the CPU/GPU/auto/hetero plugins, every model frontend, and a full parallel set of debug builds — and those are named openvinod.dll, *_plugind.dll, *_frontendd.dll, so a _d.dll filter misses all of them. Two near-identically named sets side by side is precisely how the OpenVino core binds the wrong plugin. The allow-list took the bundle from 45 files to 28 (253 MB uncompressed, 93 MB packed):

  openvino.dll, openvino_intel_npu_plugin.dll, openvino_intel_npu_compiler.dll,
  openvino_intel_npu_compiler_loader.dll, openvino_tensorflow_lite_frontend.dll, tbb*
  

  TBB lives outside runtime\bin (3rdparty\tbb\bin), so search the whole SDK. throw on any missing member — a silently short NPU stack is the failure mode this whole section exists to prevent.

  The tbb* wildcard at the end of that list is itself a small leak: it also matches tbb12_debug.dll, tbbbind_2_5_debug.dll, tbbmalloc_debug.dll and tbbmalloc_proxy_debug.dll. Only ~1.8 MB of a 253 MB bundle, and unlike openvinod.dll the names are distinct enough not to be mis-bound, so it is dead weight rather than a hazard — but add -and $_.Name -notlike '*_debug.dll' next time the workflow is touched.

Qualcomm dispatch: -gcc-toolchain means you are building the wrong ref

clang: error: unknown argument: '-gcc-toolchain' looks like an NDK-version problem and is not one. It means the tree you are building resolves the Android toolchain through the legacy --crosstool_top=//external:android/crosstool, which wants the NDK ≤ r22 layout (toolchains/aarch64-linux-android-4.9/) that no modern NDK ships.

Only old LiteRT does that. Compare the two pins:

	5c5b9ce6 (our old hardcoded ref)	0ff28117 (the v0.16.0 pin)
WORKSPACE	no android_ndk_repository at all	rules_android_ndk + register_toolchains
build:android_arm64	platforms only	adds --incompatible_enable_cc_toolchain_resolution and --incompatible_enable_android_toolchain_resolution
NDK 26+	unsupported	supported (# Need latest rules_android_ndk to use NDK 26+)

So the -gcc-toolchain failure was a symptom of the hardcoded LITERT_REF, not a separate constraint: the script was compiling a four-release-old tree that genuinely cannot use a modern NDK. Deriving the ref fixes the crash and the build. NDK r29 is correct here — same value build_android.sh requires, no per-invocation juggling.

Feed the NDK path with --repo_env, not --action_env:

bazelisk build --repo_env=ANDROID_NDK_HOME="$ANDROID_NDK_HOME" \
  --config=android_arm64 --compilation_mode=opt --strip=always \
  --linkopt=-Wl,-z,max-page-size=16384 \
  //litert/vendors/qualcomm/dispatch:dispatch_api_so

The new wiring gates on a repository rule — check_android_ndk_env calls ctx.getenv("ANDROID_NDK_HOME") and, when unset, registers @android_ndk_env//:all, a repo with an empty BUILD file. Zero toolchains, and the failure surfaces far from its cause. --action_env populates action environments, which repo rules never read; the generated .litert_configure.bazelrc (--action_env ANDROID_NDK_HOME, ANDROID_NDK_API_LEVEL, …) is the old TF-era mechanism and is dead weight at this pin — api_level now lives in the WORKSPACE call.

Expect ~3 minutes warm and a 707-724 KB stripped ELF 64-bit aarch64 exporting exactly one symbol, LiteRtDispatchGetApi. Verify that symbol before packing; the library has no other public entry point, so its absence is the whole failure mode.

bazelisk clean --expunge between attempts only if you changed WORKSPACE-level wiring — Bazel caches toolchain resolution, and a cached failure re-prints the previous error without recompiling, which reads as "the fix didn't work".

---

Build commands

Always pass the pinned SHA explicitly — the scripts' DEFAULT_REF lags the release you're migrating to.

REF=<tag-commit-sha>          # v0.16.0 = 924e79c91542761242244e4f1651851f822e4cbb
export ANDROID_NDK_HOME="$HOME/Library/Android/sdk/ndk/29.0.14206865"   # r29 mandatory

packages/flutter_gemma_litertlm/native/litert_lm/build_macos.sh   "$REF"    # macOS arm64
packages/flutter_gemma_litertlm/native/litert_lm/build_ios.sh     "$REF"    # iOS device + simulator
packages/flutter_gemma_litertlm/native/litert_lm/build_android.sh "$REF"    # Android arm64, cross-compiled

# Qualcomm NPU dispatch — separate build, separate repo (LiteRT), separate NDK.
# Derives LITERT_REF from the LiteRT-LM WORKSPACE; see the NPU section above.
LITERTLM_REF="$REF" packages/flutter_gemma_litertlm/native/litert_lm/build_qualcomm_dispatch.sh

ANDROID_NDK_HOME must be exported explicitly: build_android.sh auto-detects the newest NDK only when the variable is unset, so a stale value in your shell silently selects the wrong toolchain (r26 fails on a C++20 concept in the minijinja chat template).

Linux x86_64/arm64 and Windows x86_64 build in CI, not on a VM: dispatch build-litertlm-native.yml with -f litertlm_version=<SHA>. Use --ref <branch> if the migration carries source changes — the workflow compiles stream_proxy.c from the checked-out tree and builds the Intel NPU dispatch, so dispatching from main would silently ship a stale shim and a stale dispatch.

build-litertlm-native-windows.yml is a Windows-only copy of that job for fast iteration. It duplicates the job body rather than sharing it — when you change one, change both, or the standalone workflow quietly builds with the previous release's flags.

bazelisk clean --expunge before rebuild only if WORKSPACE patch_cmds changed (otherwise incremental is fine and ~5× faster — a warm cache can finish a platform in 2–5 min, which is normal and not a sign that nothing was built).

Never trust a build's exit code — verify artifacts

Wrapper scripts have reported success while producing nothing (a failed git clone inside the script, an echo after the build masking its status, gh printing a network error and still exiting 0). Confirm each platform by artifact, not by status line:

f=prebuilt/<dir>/libLiteRtLm.dylib      # or .so
printf "exists=%s mtime=%s newversion_symbol=%s\n" \
  "$([ -f "$f" ] && echo yes || echo NO)" \
  "$(stat -f '%Sm' -t '%H:%M' "$f" 2>/dev/null)" \
  "$([ -f "$f" ] && nm -gU "$f" 2>/dev/null | grep -c '<symbol-new-in-this-version>')"

Print exists next to the symbol count. nm on a missing file emits nothing and grep -c turns that into a perfectly innocent 0 — "feature absent" and "check never ran" must not look identical. The same applies to shell globs: in zsh an unmatched *.so aborts the whole ls, so use find … \( -name '*.dylib' -o -name '*.so' \) in verification loops.

set -o pipefail is not enough when the build is not the last command in the chain: bazel … | tail -20 exits with tail's status, and build_macos.sh reported success through a genuine failure that way. Put the build last, or capture ${PIPESTATUS[0]}.

Fetching artifacts is part of the build — verify those too

Both of our download paths fail silently:

• gh run download leaves an empty directory rather than erroring when the artifact name doesn't match, and has no --clobber, so a re-download into a populated directory can mix two runs' outputs. Always download into a fresh directory and assert the file count.
• curl exits 0 on a truncated transfer. One 2877 MB model arrived as 45 MB with a clean exit. Use --retry 5 --retry-all-errors -C - and verify the byte count against Content-Length before using the file.

---

⚠️ Post-build verification — MANDATORY before pushing prebuilts to repo

Every freshly-built dylib must pass all of these checks. Skipping any one of them has shipped a regression to users.

1. Mach-O architecture (iOS / macOS / desktop)

file packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libLiteRtLm.dylib

Expected: arm64 for iOS device, arm64 for iOS Sim, arm64 for macOS_arm64. Not x86_64. Upstream 5e0d86b shipped libLiteRt.dylib and libLiteRtTopKMetalSampler.dylib as x86_64 macOS binaries inside prebuilt/ios_arm64/ (#2072) — when you copy them across, double-check.

otool -hv packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libLiteRtLm.dylib | tail -2
# → cputype 16777228 (arm64), filetype 6 (DYLIB)

2. Header padding (macOS / iOS) — measure it with check #4, not with sizeofcmds

Native Assets re-runs install_name_tool -id @rpath/... on every pub get. Without enough header padding the rewrite fails with:

install_name_tool: changing install names or rpaths can't be redone for: ... larger updated load commands do not fit

This is what bit users in 0.14.1 (#247). Padding cannot be added to an already-linked binary — you must rebuild with -Wl,-headerpad_max_install_names.

Do not test this by comparing sizeofcmds against a threshold. An earlier version of this skill said "need sizeofcmds >= 4096", which is wrong: sizeofcmds is the current size of the load commands, while -headerpad_max_install_names enlarges the free space after them. A small library legitimately has small load commands — libStreamProxy.dylib reports ~1136 and rewrites perfectly. At the v0.15.0 migration that threshold flagged 8 healthy dylibs, including three upstream ones, and would have blocked the release for nothing.

Check #4 below is the only authoritative test — it performs the exact operation Native Assets performs. Run it over every dylib in the directory and trust its result.

For dylibs we don't build ourselves (upstream prebuilts like libGemmaModelConstraintProvider.dylib, libLiteRtMetalAccelerator.dylib, libLiteRtTopKMetalSampler.dylib) that genuinely fail check #4: file an upstream bug, then either rebuild from source with the flag, or relink with optool / ld -r for bigger headerpad.

3. install_name + rpath

otool -D <file>  # → @rpath/libLiteRtLm.dylib (NOT absolute path)
otool -l <file> | grep -A2 LC_RPATH | grep " path "
# Expected for macOS: @loader_path/../../..
# Expected for iOS:   @loader_path or @executable_path/Frameworks/...

4. install_name_tool smoke test

The mandatory test that catches headerpad bugs:

cp packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libX.dylib /tmp/test.dylib
chmod +w /tmp/test.dylib
install_name_tool -id @rpath/this_is_a_long_test_path_pad_to_native_assets_target/libX.dylib /tmp/test.dylib
# → must succeed silently. If it fails with "larger updated load commands do not fit", headerpad is too small. STOP and rebuild.

Run this for every dylib in prebuilt/<dir>/, not just the one you rebuilt. Native Assets rewrites all of them.

5. Phase 8 patch markers (iOS / macOS only)

strings packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libLiteRtLm.dylib | grep '@executable_path.*LiteRtMetalAccelerator'

Expected: 1 hit (path to LiteRtMetalAccelerator.framework/LiteRtMetalAccelerator). If 0 hits, patch_c_api.sh §10b didn't apply — your build was against a tree where WORKSPACE.patch_cmds didn't run. Run bazelisk clean --expunge and rebuild.

strings packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libLiteRtLm.dylib | grep -c '^libLiteRtMetalAccelerator.dylib$'

Expected: 0. If non-zero, the basename dlopen string is still in the binary — patch failed.

6. Required exports

nm -gU packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/libLiteRtLm.dylib | grep _litert_lm_engine_create
# → must show the symbol as T (text section, exported)

Run nm -gU | grep -c '_litert_lm_' — expect ~50 symbols (matches bindings.dart lookupFunction count).

7. Real pub get smoke test in a fresh project

This catches problems no static check sees. Mandatory before commit:

REPO=/Users/sashadenisov/Work/flutter_gemma
cd /tmp
rm -rf test_flutter_gemma_native
flutter create test_flutter_gemma_native --platforms=macos,ios
cd test_flutter_gemma_native
# The repo ROOT is the workspace package (name: flutter_gemma_workspace,
# publish_to: none) — pointing --path at it fails with "name field doesn't
# match expected name". And core owns NO native bundle: its hook has
# `const _bundles = <_NativeBundle>[];`. Adding core alone exercises zero
# CodeAssets, so the check that exists to catch the 0.14.1 headerpad bug
# would have passed while testing nothing. Add the package that owns the
# dylibs.
flutter pub add flutter_gemma --path="$REPO/packages/flutter_gemma"
flutter pub add flutter_gemma_litertlm --path="$REPO/packages/flutter_gemma_litertlm"
# add flutter_gemma_embeddings too if embeddings changed — it shares the bundle
rm -rf .dart_tool build
flutter pub get
# → must complete without "Failed to set install names" or any other error
flutter build macos --debug    # mandatory
flutter build ios --debug --no-codesign   # mandatory (catches dylib loading)

# Assert the dylibs actually landed. A green build with zero CodeAssets looks
# identical to a green build with all of them.
find build -name 'libLiteRtLm*' -o -name 'LiteRtLm*' | head

This is the smoke test I skipped in 0.14.1. Doing this once would have caught the libGemmaModelConstraintProvider.dylib headerpad issue before publish — provided it points at the package that carries the bundle.

But note what it still does NOT cover — see the next check.

7b. Build with prebuilt/ moved aside — the only test of what users actually get

_resolveLibDir tries local packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/<dir>/, then the cache; its caller falls back to _downloadAndExtract from native-v<version>. The cache path is NOT version-scoped — staleness is decided by the marker file. End users only ever reach the download — prebuilt/ is gitignored and ships in no pub package (confirm with dart pub publish --dry-run | grep -ciE 'prebuilt|\.so$|\.dylib|\.dll' → must be 0).

On a maintainer machine the first source always hits. So every build you run locally — including check #7 and the release skill's flutter build apk --release pre-flight — silently validates your bundle and never touches the download, checksum verification, or extraction. Green means nothing about the artifact you are about to publish.

After the release exists and the hook's checksums are updated, force the real path:

mv packages/flutter_gemma_litertlm/native/litert_lm/prebuilt /tmp/prebuilt-aside
# Cache location is per-OS (_cacheBaseDir()). On the Windows NPU VM the macOS
# path wipes nothing and you silently re-test the cache instead of the download.
#   macOS   ~/Library/Caches/flutter_gemma/native
#   Linux   ~/.cache/flutter_gemma/native
#   Windows %LOCALAPPDATA%\flutter_gemma\native
rm -rf "$HOME/Library/Caches/flutter_gemma/native"     # kill the cache too, or you test source #2
cd packages/flutter_gemma/example && flutter clean && flutter pub get && flutter build apk --release
mv /tmp/prebuilt-aside packages/flutter_gemma_litertlm/native/litert_lm/prebuilt

Pack the tarballs from the exact directory you device-tested, so the published bytes match the verified bytes by construction rather than by coincidence.

This is the same failure shape as the frozen NPU dispatches: local state standing in for what goes out the door.

8. (App Store-bound builds) Frameworks/ structural check

After flutter build ipa --release or archive:

R=build/ios/iphoneos/Runner.app
ls "$R/Frameworks" | wc -l                                    # must be NON-ZERO
ls "$R/Frameworks" | grep -E 'LiteRtLm|LiteRtMetalAccelerator|GemmaModelConstraintProvider|StreamProxy'
find "$R/Frameworks" -maxdepth 1 -type l                      # must be empty
find "$R/Frameworks" -maxdepth 1 -name "*.dylib" -type f      # must be empty

If anything other than .framework/ is in there, App Store will reject with ITMS-90432 ("Unexpected file found in Frameworks").

Count the contents first — an empty Frameworks/ passes every "must be empty" test. flutter build ios can leave a half-assembled Runner.app behind: the Xcode phase reports Xcode build done, then flutter_tools dies cleaning up its own temp dir (PathNotFoundException: Deletion failed … flutter_ios_build_temp_dir…), and the frameworks are never embedded. The bundle exists, the two negative checks come back perfectly clean, and nothing distinguishes "compliant" from "nothing was built". Re-running the build fixes it; a healthy debug build of the example carries ~10 frameworks, four of them ours. Same failure shape as nm on a missing file — assert the positive before trusting the negative.

9. NPU on real silicon — the only check that covers the dispatch libraries

Nothing in checks 1–8 touches NPU. Both dispatch libraries load only when PreferredBackend.npu is requested on matching hardware, so they need real devices:

• Intel NPU — LunarLake/PantherLake. Our access is the Intel Tiber VM (see the project_intel_npu_vm memory). Run the litertlm smoke suite with --plain-name "NPU" on its own: inside the full file the NPU group runs after GPU, and async WebGPU teardown blows the per-test timer on the first NPU engine_create.
• Qualcomm NPU — Snapdragon 8 Gen 3 / 8 Elite. Ours is Qualcomm Device Cloud (QDC).

Both are slow, awkward, and easy to skip. Skipping them is what shipped the two frozen dispatch libraries described above.

QDC and Android instrumentation, in the order that works

QDC gives you an SSH tunnel that forwards the ADB server port (5037), not a device port. Consequences:

1. adb kill-server first, then open the tunnel. A local ADB server already holding 127.0.0.1:5037 makes ssh bind IPv6 only, and the symptom is a device list that is simply empty.
2. nohup … & disown — there is no setsid on macOS, and with it the tunnel never starts at all.
3. flutter test -d <device> cannot work here. It needs adb forward for the Dart VM Service, and with the server itself proxied the forward binds on the remote side. Use an Espresso instrumentation run instead.
4. Sessions are time-capped (100 min standard, 300 on request). A tunnel that stops responding mid-run is usually just an expired session — check that before debugging the transport.

Building the instrumentation APK: flutter build apk is wrong, it packs main.dart. The app APK must carry the test entrypoint:

cd packages/flutter_gemma/example/android
./gradlew app:assembleDebug -Ptarget=<absolute path to integration_test/xxx_test.dart>

Verify the entrypoint actually landed with binary grep — strings misses it in the compiled kernel blob:

grep -ac "<a string unique to the test file>" app/build/outputs/apk/debug/app-debug.apk

And beware the Espresso idle timeout: Could not launch intent within 45000 ms is usually not a broken test but app startup doing real work — ours was restoring a 3 GB active model, so the main thread never went idle. Clear app state, or start from a build that doesn't auto-restore.

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Common pitfalls (regressions we already shipped)

#	Bug	Caught by	Hit users in
1	iOS dylib built from v0.10.2 (no Metal accelerator), EXC_BAD_ACCESS	Check #5 (patch markers absent) + iPhone smoke	dev session, never shipped
2	App Store ITMS-90432 — lib*.dylib symlinks in Frameworks/	Check #8	0.14.0
3	dart:ffi in web build — missing conditional import	flutter build web --release	0.14.0
4	libGemmaModelConstraintProvider.dylib headerpad too small	Check #2 + #4 + #7	0.14.1
5	lipo "same architectures (arm64)" — Native Assets called twice for iOS	hook/build.dart _prebuiltDirName returns null for iOS x64	dev session
6	upstream libLiteRt.dylib was x86_64 macOS binary in prebuilt/ios_arm64/ (#2072)	Check #1 (file)	5e0d86b commit (upstream bug, but we should detect on copy)
7	NPU dispatch stacks dropped from the tarballs — packed without them	Count per bundle before packing	caught pre-publish at v0.14.0 and v0.15.0
8	LiteRT pin moved, LiteRtCreateModelFromFile 2→3 args → embeddings status=500	Header diff at both LITERT_REFs	v0.14.0 dev cycle, ~1 day lost
9	RTLD_DEFAULT undeclared on glibc — needs _GNU_SOURCE before every include	CI Linux job (macOS compiles it happily)	caught in CI at v0.15.0
10	v0.15.0 changed LiteRtLmStreamCallback 4-arg → 2-arg, no compat overload	ACCESS_VIOLATION (Windows) / malformed UTF-8 (macOS) at first token	caught pre-publish at v0.15.0
11	--define=litert_link_capi_so=true deleted upstream — inert, statically linked runtime → Windows GPU crash

Every one of those would have been caught by checks 1-9 before commit. Run them all every time.

Three patterns deserve emphasis because nothing in the build output hints at them:

• #7, #12 and #13 are silent. The tarball packs, the checksums verify, every smoke test on CPU/GPU passes. Only a user on PreferredBackend.npu finds out.
• #8 and #10 are ABI drift, not code changes on our side. C linkage means a stale call shape still links; you get register garbage instead of a compiler error. When a bump touches any callback or C-API signature, verify the shape at both refs rather than assuming source that compiles is source that's correct.
• #11, #12 and #13 are the same organizational defect wearing three costumes. Every one is an input we pinned once and stopped maintaining — a define, an SDK version, a commit SHA — that only misbehaves when upstream touches the code depending on it. Each surfaced at a version boundary and each was reported upstream before the real cause was found. Before filing an upstream regression, list every input to the comparison and confirm which ones moved. If one of them is ours and frozen, it is ours until proven otherwise.

---

When upstream is broken

We import some dylibs as-is from upstream LiteRT-LM (libGemmaModelConstraintProvider.dylib, libLiteRtMetalAccelerator.dylib, sampler dylibs). When upstream ships them with insufficient headerpad / wrong arch / broken exports:

1. Rule out our own frozen inputs first. Before writing the issue, enumerate every input to the failing comparison — defines, SDK versions, pinned SHAs, carried-forward binaries — and confirm which ones actually moved between the working and broken versions. Two of our four most recent upstream reports (#2957, #3217) were our configuration and had to be retracted; in both the "regression" was upstream finally exercising something we had frozen years earlier. A bisection that lands on a version boundary tells you when, never whose.
2. File an issue with reproducer — see project_litertlm_upstream_* memories for our open ones (#1990, #2072, #2073, #2080).
3. Don't ship them blindly. Run check #4 (install_name_tool smoke) on every upstream-sourced dylib before copying to prebuilt/. If it fails, don't publish — find a workaround first (relink, or ship without that lib + change Dart code path).
4. Add a memory entry documenting the bug + workaround in ~/.claude/projects/.../memory/project_*.md so future sessions don't re-discover it.

---

After successful build

1. Run the verification checklist 1-9 above. All checks must pass.
2. Do NOT try to commit the dylibs — prebuilt/ is gitignored (.gitignore **/packages/flutter_gemma_litertlm/native/litert_lm/prebuilt/) and git ls-files returns nothing under it. The bundles reach users through the GitHub Release only; the working copy is yours alone. (This step used to say git add prebuilt/<dir>/*.dylib, which cannot succeed.)
3. Pack tarballs + update hook/build.dart checksums + re-upload to GitHub Release native-v<version> (see release skill).
4. Run dart pub publish --dry-run — must show 0 warnings.
5. Only then publish.