coreml

Core ML Swift Integration

Load, configure, and run Core ML models in iOS apps. This skill covers the Swift side: model loading, prediction, MLTensor, profiling, and deployment.

Scope boundary: Python-side model conversion, optimization (quantization, palettization, pruning), and framework selection live in the apple-on-device-ai skill. This skill owns Swift integration only.

See references/coreml-swift-integration.md for complete code patterns including actor-based caching, batch inference, image preprocessing, and testing.

Contents

• Loading Models
• Model Configuration
• Making Predictions
• MLTensor (iOS 18+)
• Working with MLMultiArray
• Image Preprocessing
• Multi-Model Pipelines
• Vision Integration
• Performance Profiling
• Model Deployment
• Memory Management
• Common Mistakes
• Review Checklist
• References

Loading Models

Auto-Generated Classes

When you add a .mlmodel or .mlpackage to an app target, Xcode generates a Swift class with typed input/output. Use this whenever possible.

import CoreML
 
let config = MLModelConfiguration()
config.computeUnits = .all
 
let model = try

Manual Loading

Load from a URL when the model is downloaded at runtime or stored outside the bundle.

Async Loading (iOS 15+)

Load models without blocking the main thread. Prefer this for large models.

Compile at Runtime (iOS 16+)

Compile a .mlpackage or .mlmodel to .mlmodelc on device. Useful for models downloaded from a server. Do this once per model version, not on every launch.

Cache the compiled URL -- recompiling on every launch is a bug. Copy compiledURL to a persistent location (e.g., Application Support). When reviewing runtime-loaded models, call out both facts together: async MLModel.compileModel(at:) is iOS 16+, and compiled models must be cached so the app does not recompile on every launch.

Model Configuration

MLModelConfiguration controls compute units, GPU access, and model parameters.

Compute Units Decision Table

Configuration Properties

Making Predictions

With Auto-Generated Classes

The generated class provides typed input/output structs.

With MLDictionaryFeatureProvider

Use when inputs are dynamic or not known at compile time.

Prediction Inside Async Workflows

MLModel.prediction(...) is synchronous. In async pipelines, keep model loading async, then run prediction from an actor or non-main task without adding await to the prediction call.

Batch Prediction

Process multiple inputs in one call for better throughput.

Use predictions(fromBatch:) when batching without explicit MLPredictionOptions. Use predictions(from:options:) only when passing both an MLBatchProvider and MLPredictionOptions; predictions(from:) by itself is not the no-options batch API.

Validate a representative single input before batching. Then verify batch output count/order, feature types, domain invariants, and agreement with the single-input result. On failure, fix the deterministic input, shape, model, or configuration issue before rerunning fixtures and physical-device profiling.

Stateful Prediction (iOS 18+)

Use MLState for models that maintain state across predictions (sequence models, LLMs, audio accumulators). Create state once and pass it to each prediction call.

MLState is Sendable, but Sendable does not make one state safe for concurrent inference. Predictions using the same state must be serialized; do not read or write state buffers while a prediction is in flight. Call model.makeState() for each independent concurrent stream. If you need MLPredictionOptions, iOS 18+ also provides the async prediction(from:using:options:) overload; the same one-in-flight-per-state rule still applies.

MLTensor (iOS 18+)

MLTensor is a Swift-native multidimensional array for pre/post-processing. Operations run lazily -- call await tensor.shapedArray(of:) to materialize results.

Do not invent MLTensor APIs for statistics or bridging. Avoid examples such as MLTensor(multiArray), tensor.std(), tensor.standardDeviation(), direct lazy-buffer access, or synchronous extraction; perform unsupported DSP/statistics outside the tensor pipeline or with source-confirmed tensor operations.

Working with MLMultiArray

MLMultiArray is the primary data exchange type for non-image model inputs and outputs. Use it when the auto-generated class expects array-type features.

See references/coreml-swift-integration.md for advanced MLMultiArray patterns including NLP tokenization and audio feature extraction.

Image Preprocessing

Image models expect CVPixelBuffer input. Use CGImage conversion for photos from the camera or photo library. Vision's VNCoreMLRequest handles this automatically; manual conversion is needed only for direct MLModel prediction.

Load Image Preprocessing for the complete checked CVPixelBuffer conversion and additional normalization or cropping patterns.

Multi-Model Pipelines

Chain models when preprocessing or postprocessing requires a separate model.

For Xcode-managed pipelines, use the pipeline model type in the .mlpackage. Each sub-model runs on its optimal compute unit.

Vision Integration

Use Vision to run Core ML image models with automatic image preprocessing (resizing, normalization, color space, orientation).

Modern: CoreMLRequest (iOS 18+)

Legacy: VNCoreMLRequest

For complete Vision framework patterns (text recognition, barcode detection, document scanning), see the vision-framework skill.

Performance Profiling

MLComputePlan (iOS 17.4+)

Inspect which compute device each operation will use before running predictions. Load MLComputePlan Detailed Usage for model-structure traversal, device usage, and estimated-cost inspection.

Instruments

Use the Core ML instrument template in Instruments to profile:

• Model load time
• Prediction latency (per-operation breakdown)
• Compute device dispatch (CPU/GPU/ANE per operation)
• Memory allocation

Run outside the debugger for accurate results (Xcode: Product > Profile).

Model Deployment

Bundle small offline-critical models. Prefer Background Assets for new large or updateable assets; keep On-Demand Resources only for existing ODR projects. Compile downloaded source models once, persist the .mlmodelc by version, and test load, first/repeated prediction, lifecycle transitions, and memory on the lowest supported physical device. Load deployment patterns for implementation details.

Memory Management

• Unload on background: Release model references when the app enters background to free GPU/ANE memory. Reload on foreground return.
• Share model instances: Never create multiple MLModel instances from the same compiled model. Use an actor to provide shared access.
• Monitor memory pressure: Large models (>100 MB) can trigger memory warnings. Register for UIApplication.didReceiveMemoryWarningNotification and release cached models when under pressure.

See references/coreml-swift-integration.md for an actor-based model manager with lifecycle-aware loading and cache eviction.

Common Mistakes

DON'T: Load models on the main thread. DO: Use MLModel.load(contentsOf:configuration:) async API or load on a background actor. Why: Large models can take seconds to load, freezing the UI.

DON'T: Ignore MLFeatureValue type mismatches between input and model expectations. DO: Match types exactly -- use MLFeatureValue(pixelBuffer:) for images, not raw data. Why: Type mismatches cause cryptic runtime crashes or silent incorrect results.

DON'T: Create a new MLModel instance for every prediction. DO: Load once and reuse. Use an actor to manage the model lifecycle. Why: Model loading allocates significant memory and compute resources.

DON'T: Skip error handling for model loading and prediction. DO: Catch errors and provide fallback behavior when the model fails. Why: Models can fail to load on older devices or when resources are constrained.

DON'T: Assume all operations run on the Neural Engine. DO: Use MLComputePlan (iOS 17.4+) to verify device dispatch per operation. Why: Unsupported operations fall back to CPU, which may bottleneck the pipeline.

DON'T: Process images manually before passing to Vision + Core ML. DO: Use CoreMLRequest (iOS 18+) or VNCoreMLRequest (legacy) to let Vision handle preprocessing. Why: Vision handles orientation, scaling, and pixel format conversion correctly.

Review Checklist

• Model loaded asynchronously (not blocking main thread)
• MLModelConfiguration.computeUnits set appropriately for use case
• Model instance reused across predictions (not recreated each time)
• Auto-generated class used when available (typed inputs/outputs)
• Error handling for model loading and prediction failures
• Compiled model cached persistently if compiled at runtime
• Image inputs use Vision pipeline (CoreMLRequest iOS 18+ or VNCoreMLRequest) for correct preprocessing
• MLComputePlan checked to verify compute device dispatch (iOS 17.4+)
• Batch predictions used when processing multiple inputs
• Model size appropriate for deployment strategy (bundle, Background Assets, ODR)
• Memory tested on target devices (especially older devices with less RAM)
• Predictions run outside debugger for accurate performance measurement

References

• Patterns and code: references/coreml-swift-integration.md
• Model conversion and optimization (Python-side): covered in the apple-on-device-ai skill
• Apple docs: Core ML | MLModel | MLTensor | MLComputePlan | Background Assets