Known platform issues

Limits that depend on the device, OS version or driver rather than on the feature. For which runtime supports which feature, see the support matrix.

Apple platforms#

• Native LiteRT-LM iOS artifacts omit the Gemma FST constraint provider. Gemma 3/4 and FunctionGemma conversations must disable constrained decoding; Dart already does so. Ordinary generation, thinking and best-effort tool formatting remain available. Automatic tool loops and strict structured output remain unsupported.
• Native LiteRT-LM on iOS: iOS 16.4 is the declared deployment floor, not a tested one. The companion Swift package requires an iOS 16.4 app target, but nothing has been run on iOS 16.4. On a device, model load and generation have run only on an iPhone 16 Pro with iOS 18.3.2 (CPU and GPU); simulator evidence is iOS 26.4, CPU only. No other iOS version is device-verified (#514).
• Flutter macOS LiteRT-LM needs llamadart_litert_lm_flutter to put the runtime in the app bundle. Without it, an unsandboxed app run on the build machine from the project loads the runtime from the hook cache, and a sandboxed (the flutter create default), copied or distributed app throws Failed to load dynamic library 'libCLiteRTLM_mac.dylib' on the first CPU or GPU generation. Add the companion, or the LiteRT-LM build phase.
• macOS x86_64: no LiteRT-LM runtime is published (litert-lm-native#72). Universal apps launch on an Intel Mac and under Rosetta, and LiteRT-LM throws LlamaUnsupportedException there; llama.cpp still runs. See Intel Macs.
• iOS x86_64 simulator: no LiteRT-LM runtime is published. Apps that include LiteRT-LM must exclude the x86_64 simulator architecture; llama.cpp still builds for it. LiteRT-LM throws LlamaUnsupportedException there.
• iOS simulator: llama.cpp on Metal returns incorrect output. On an arm64 simulator (iPhone 17 Pro, iOS 26.5, Apple silicon host) Qwen3.5 0.8B (Q4_K_M) generates garbage text on Metal and answers correctly on the CPU, and Qwen2.5 0.5B fails on Metal the same way (#950); the cause is not known. On an iOS simulator auto therefore runs llama.cpp on the CPU with 0 GPU layers. GpuBackend.metal and ComputeDevice.gpu still load on Metal there, and the load logs a warning: use them only to check a model on the simulator's Metal device. Under ComputeDevice.auto, another explicit GPU backend, such as GpuBackend.vulkan, also loads its GPU layers on Metal and warns, since the Apple runtime has no other GPU backend. The x86_64 (Rosetta) simulator behaves the same. iOS devices and Apple silicon macOS are unchanged: auto uses Metal, and on an iPhone 16 Pro its output matches the CPU's. Intel macOS differs: see the next entry.
• macOS x86_64: upstream llama.cpp Metal returns garbled output on Intel Macs with AMD GPUs (llama.cpp#30702), and upstream's own macOS x64 builds turn Metal off. In an x86_64 process, auto therefore runs llama.cpp on the CPU with 0 GPU layers (#1597). An x86_64 process cannot tell an Intel Mac from Rosetta on Apple silicon, so a Rosetta process also uses the CPU. GpuBackend.metal and ComputeDevice.gpu still load on Metal, and the load logs a warning. Under ComputeDevice.auto, another explicit GPU backend, such as GpuBackend.vulkan, also loads its GPU layers on Metal and warns, since the Apple runtime has no other GPU backend. gpuDevices and tensorSplit need one of them there, since auto would load on the CPU. arm64 macOS is unchanged.

Linux and Windows#

• Linux x64 LiteRT-LM GPU needs a hardware Vulkan driver. On Mesa llvmpipe alone it answers a few prompts, then crashes the process; use cpu on such hosts (#572).
• Windows x64 LiteRT-LM GPU needs dxil.dll and dxcompiler.dll, which the pinned runtime bundles. Its GPU accelerator and sampler load webgpu_dawn.dll, which the hook bundles. Windows Qwen3 0.6B GPU execution remains unqualified: the retained Windows Server 2022 / NVIDIA L4-vWS run on v0.17.0-7 emitted 24 Dawn validation errors despite passing all 15 functional checks. Conflicting storage usages and invalid command submissions remain unresolved; a new runtime regression has not been established (litert-lm-native#64). Use ComputeDevice.cpu to avoid this GPU path. A Flutter Windows app has not been run with it.

Android#

• Android LiteRT-LM needs the app's optional vendor OpenCL manifest request when targeting API 31+ on Android 12+; see Android setup. Without accessible OpenCL, the runtime can try WebGPU. On Adreno 750 (Galaxy S24), WebGPU's 128 MiB storage-buffer binding limit rejects a larger Qwen3 weight buffer. ComputeDevice.gpu fails on GPU creation; auto retries on the CPU with a warning (#1034). The legacy Qwen3 FP32 artifact's four semantic failures remain unqualified under litert-lm-native#51. Published native v0.19.0 has accepted corrected-model default-service OpenCL evidence only for SC-51E / API36 / DEBUG: all 20 cases, six closed lifetimes, final completion and eight full-graph placement proofs, including corrected C04, cancellation and reload. Fifteen exact sidecars were forensically extracted from the retained generation-bound journal; they were not independently downloaded, and the original collector FAIL is unchanged. This qualifies that scoped public route, not every Qwen artifact, device or release profile. Final source-adoption consumers remain #1677. ModelParams(device: ComputeDevice.cpu) skips the GPU attempt. Gemma 4 E2B has separate GPU evidence; it does not qualify Qwen3.

• Android NPU depends on the device SoC, the .litertlm bundle and the LiteRT dispatch libraries (LiteRtLmModelParams.dispatchLibDir). If native LiteRT-LM cannot create an NPU engine, use cpu or gpu.

• Android llama.cpp Vulkan is experimental and device-dependent. auto runs llama.cpp on the CPU there; Vulkan runs only when the app asks for ComputeDevice.gpu or GpuBackend.vulkan. Previously observed runtime failures: on a Pixel 9 Pro (Mali-G715), a prompt longer than about 32 tokens evaluated in one batch returns wrong text, and a request with a grammar or tool call can then abort the process, confirmed so far with one model (Qwen3.5-0.8B Q4_0) on that one device (#948); a Galaxy A35 (Mali-G68) is reported to time out in llama_decode (#1092); a Galaxy S24 (Adreno 750) crashes on the first generation (llamadart-native#79). The first failure is associated with llama.cpp's subgroup-16 small-matmul tile (llama.cpp#28637). The pinned native v0.8.0-3 includes upstream #30106, but matched final-artifact Pixel/Qwen correctness remains unqualified. The historical GPU history failure repeated 8; the original CPU result Cedar17 also failed the exact expected cedar17. Neither observation is normalized away.

  For a model actually loaded with Vulkan GPU layers on Android, native architecture qwen35 (Qwen3.5) is restricted when device facts report subgroup 16, below 8, or are unavailable, empty or inconsistent with the registry. Ordinary text retains the eight-token prompt cap with microBatchSize unset; explicit 1..8 is accepted. Values above 8 throw LlamaUnsupportedException before context creation. Media (image/audio), every speculative strategy and embeddings throw before evaluation; vision, audio and embedding modality probes report unavailable, and generation capabilities report no speculative strategies. A refused generation leaves the text context reusable. Load with ComputeDevice.cpu for these features. Missing native architecture metadata on this otherwise restricted GPU path also throws because required safety cannot be established.

  The family-wide rule and refusal of sizes 9..32 are conservative admission limits, not reproduced failures for every Qwen3.5 artifact or batch size. qwen35moe and other known architectures are outside this rule. CPU and zero-GPU-layer loads, non-Android/non-Vulkan backends, and complete subgroup 8 or >=32 facts retain their behavior; these exemptions do not constitute new hardware qualification. Other architectures retain the existing default text-only cap on unsafe/unproven subgroup facts; that cap is not protection for media, speculative verification, embeddings, decision models or TTS. llama.cpp's Vulkan backend also needs Vulkan 1.2 from the Vulkan loader and the GPU driver. When the Vulkan GPU a load would run on reports an older version, ComputeDevice.gpu throws LlamaUnsupportedException naming the GPU and both versions, and ComputeDevice.auto loads the model on the CPU and logs a warning. A device with a Vulkan 1.1 driver takes that path. When other GPUs of the load are usable, such as a discrete GPU beside an old one, the load leaves the old one out and logs a warning; a GPU llama.cpp would not select, like an integrated GPU beside a discrete one, changes nothing. On Android, the exact Qualcomm vendor 0x5143, Adreno (TM) 750 driver 2150604839 has shader-compiler crashes and incorrect quantized results, including common Q4_K_M models. When this device is registered, GPU loads are refused even if another GPU was selected: projector and decision-head GPU selection can independently choose the affected device. ComputeDevice.auto loads on CPU with a warning, including when preferredBackend requests Vulkan; ComputeDevice.gpu throws LlamaUnsupportedException. CPU loads do not probe Vulkan. Other driver versions are unchanged. A shader crash workaround alone does not qualify this driver; re-enabling a model requires qualification of the final native artifact on that exact driver. See native issue #79.

  Keep auto or cpu on Android unless the app has validated Vulkan on its target devices.

Vulkan and the web#

• Some Vulkan drivers crash in the cooperative-matrix path; see GPU crash or device loss.
• WebGPU readiness depends on the browser, device, bridge assets and model size; see WebGPU bridge.
• Safari terminates a hidden or background tab that exceeds its inactive memory limit. Loading Gemma 4 E2B through LiteRT-LM Web peaks near 7 GB, so keep the tab visible until the model finishes loading (#1025).