Fuel

Calorie tracker for iOS 26 · SwiftUI · SwiftData

Case study · Contributing · BYOK · Building

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Calorie tracking without selling your soul. No account, no cloud, no 12-screen onboarding — just type what you ate and move on.

Counting calories is a two-second decision that most apps turn into a four-minute errand. You open the app, you search a database for "cottage cheese", you get nineteen entries with different brands and one of them is somebody's guess from 2014, you pick one, you set a portion, you confirm. Do that three times a day and you stop doing it by Thursday.

And that is only the part after the sign-up. Before any of it there is an account, an email address, a body weight, a target weight, a birth date, an activity level, a notification permission and a paywall — all of it to add two numbers together and compare the result to a third one.

Fuel takes a photo or a sentence and gives you the numbers. That is the product.

Log modes	Camera, text, recent meals. No manual food search.
Storage	SwiftData on the device. No account, no cloud, no CloudKit, no sync.
AI	Bring your own key — Anthropic or Mistral, straight from the device.
Tests	Swift Testing, in the nutrition core, which knows no database and no view.
Language	English, string catalog maintained by hand
License	Source-available — read, build, run. No distribution of any kind.

How it came about, why it looks the way it does and what got cut is in the case study.

What it is

Fuel is a calorie tracker for iOS 26, written in SwiftUI with SwiftData for local persistence. First launch asks for an AI key, then for one decision: a daily calorie goal, or count only. After that you are on the day.

Everything lives on the device. No backend, no account, no login, no CloudKit, no sync, no export. That is not a roadmap item that has not happened yet, it is the product. There is nothing to sync, so there is nothing to leak.

With a goal, the day shows a ring, the total against the goal, and three macro bars. Count only is not the same screen with the ring switched off — it is a different layout: the total reads "kcal logged", the macros are a plain row of numbers, and there are no bars, because there is nothing to fill them against.

Entries are filed into Breakfast, Lunch, Snack and Dinner automatically, and the rule is deliberately not a clock lookup. Breakfast, lunch and dinner are each handed out once a day, the first time an entry falls into that meal's window; everything else is a snack. So a second entry at breakfast time is a snack, and an entry at four in the afternoon on a day with no lunch yet is lunch, because the lunch window went by unused. You can overwrite the label on the result screen, and once you have, Fuel leaves it alone.

Three log modes

The log flow has three tabs and the tab bar draws three.

1. Camera — photograph the plate. The image is compressed on the device and sent to your provider, which comes back with the items it recognised, an estimated amount for each and a confidence.
2. Text — write the meal in a sentence: "2 eggs with 200g cottage cheese". The more exact the amounts, the more exact the estimate. Amounts you give are marked as recognised, amounts you leave out are marked as estimated, so you can see which numbers are yours and which ones are a guess.
3. Recent — the meals you have logged before. One tap logs the meal again with the label derived from the time of day. Recent needs no key and no network at all — it is your own history, on your own device.

Either estimate lands on a result screen where you can correct the calories in steps of ten and change the label before it is saved.

There is no manual food search, and there is not going to be one. A search mode was considered and cut. It is the part everyone hates and the reason a tracker gets abandoned; adding it back would be adding back the problem. Three modes, and the third one covers everything you eat regularly anyway.

The camera shortcut

Fuel exposes an App Intent that opens straight into the camera log screen. Put it on the Home Screen, in the Action Button, or wherever you keep shortcuts — one press and you are photographing the plate, not navigating to the place where you photograph the plate.

BYOK — the key is yours

Fuel uses AI exclusively with a key you bring. You fetch a key from Anthropic or from Mistral, you paste it in, and the app talks to that provider directly from your device. Levo Studio never sees a request and could not read one if it wanted to.

This is not a limitation that a later version relaxes. It is why the app can exist at all:

• There is no server to run. No backend means no uptime, no scaling, no operations, no thing that goes down while you are standing in a kitchen.
• There is no bill to pay for someone else's API calls. Photo recognition is not free, and an app that pays for it needs a subscription, and a subscription needs an account, and an account needs an email address. That chain starts with the first hosted API call, so Fuel does not make one.
• There are no request logs, because there is no place they could exist. Not a promise about a retention policy — no server, no logs.
• You stay anonymous. There is no Levo Studio account, and no Levo Studio anything else either. There is your device, your key, and your provider.

Concretely:

• The key lives in the Keychain, never in UserDefaults, with kSecAttrAccessibleWhenUnlockedThisDeviceOnly, and iCloud Keychain sync is off — a synced key would travel through Apple's cloud, which is exactly what a local-first app is supposed to avoid. One entry per provider, so you can hold both keys and switch without losing the other.
• Nothing is logged. Not the key, not a prefix of it, not the photo, not the text you typed, not the model's reply. The only place a meal's content is written down is its SwiftData entry.
• A new key gets a format check first, then one smallest-possible test call, so a typo costs you no tokens. The result is shown right there, not discovered at the first photo.
• Images are compressed on the device before they are uploaded. You are paying for those tokens.
• No Levo Studio key ships in the binary — not in source, not in an xcconfig, not in an Info.plist. There is no fallback path and no proxy. If you have no key, the camera and text modes are disabled and Recent still works.

The key setup is the first thing you see and there is no skip button, because two of the three log modes are AI. You can change or remove the key later in Settings; the disabled states exist for exactly that.

Architecture

Fuel/
  Core/Design/     FuelPalette, FuelTypography, FuelMetrics, FuelMotion
  Core/Data/       the SwiftData store
  Core/Keychain/   the provider-key wrapper
  Core/AI/         provider clients, request and response shapes
  Models/          FoodEntry and friends — @Model types
  Nutrition/       pure calculation: totals, macros, the meal-label rule
  Features/        one folder per screen area
  Resources/       fonts, Localizable.xcstrings
FuelTests/         Swift Testing
design/            the design export — read-only, never edited to match the code

The nutrition core knows no database. Fuel/Nutrition/ works on plain Sendable values, not on @Model classes. Totals, macro splits and the meal-label rule are pure functions over pure values, which is what lets their tests run in milliseconds without a ModelContainer and without a simulator. Nothing from SwiftData travels deeper than the hand-off type.

The nutrition core knows no view either. No @Observable, no Color, no import SwiftUI.

Colour, size and motion come only from the design layer. No .padding(17) and no Color(hex:) in a feature file. A missing value goes into FuelMetrics, FuelPalette, FuelTypography or FuelMotion, not into the call site. Reduce Motion is handled centrally in FuelMotion — spread over a hundred call sites it would be forgotten at ninety of them.

There is no network code outside Core/AI/. If a view is building a URLRequest, the design of that feature is wrong.

Fuel has no dependencies, and that is a feature.

Building and testing

You need Xcode 26. xcode-select points at the CommandLineTools on many machines, and those cannot build an iOS project. Either switch it permanently (sudo xcode-select -s /Applications/Xcode.app/Contents/Developer) or prefix each call:

DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcodebuild \
  -project Fuel.xcodeproj -scheme Fuel \
  -destination 'platform=iOS Simulator,name=iPhone 17 Pro' \
  CODE_SIGNING_ALLOWED=NO build

DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcodebuild test \
  -project Fuel.xcodeproj -scheme Fuel \
  -destination 'platform=iOS Simulator,name=iPhone 17 Pro' \
  CODE_SIGN_IDENTITY="-" CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=YES

CODE_SIGNING_ALLOWED=NO means you do not need a developer team to build Fuel. There are no entitlements and no capabilities hanging off someone's App Store access, so nothing has to be signed to run in the simulator, and a fresh clone builds without an Apple Developer account.

Building to a device

The repository is public, so no development team is stored in the project at all. Nothing above needs one. Running Fuel on a physical iPhone does, and Xcode would otherwise write the team you pick straight back into project.pbxproj. So it lives outside the project instead: copy Local.xcconfig.example to Local.xcconfig, put your own Team ID in, and build. Base.xcconfig includes it optionally, so a clone without it still builds. Local.xcconfig is ignored by git and cannot end up in a commit.

The project uses synchronized folders — new files under Fuel/ and FuelTests/ join the target on their own, and Fuel.xcodeproj/project.pbxproj does not have to be touched for that.

Contributing

How a contribution should look, what a useful issue contains, what the commits have to look like and what design/ being the source of truth means in practice: CONTRIBUTING.md.

Short version: small single commits, tests with the fix, and every value taken from the design export rather than from taste.

Credits

Creator and maintainer

Julius Grimm — idea, design, nutrition core. Levo Studio

Contributors

Name	GitHub	Contribution	PR
nobody yet — be the first row

Once your PR is merged you may add yourself here. There are rules for that, and they are not negotiable:

• One row, one contribution. No paragraphs, no logos, no banners, no company links.
• The entry goes in the same PR as the contribution, not as a separate "add me" PR.
• Only what is actually in. The contribution is described in one line, not in an essay: "Mistral client", "fix in the snack rule", "accessibility of the log tabs".
• GitHub handle instead of an email address. No private contact details, neither yours nor anyone else's.
• Not an advertising slot. No links to your own products, agencies, services or crypto projects. A link to your GitHub profile is the link you get.
• No other people's names. You add yourself, nobody else.

An entry that ignores this gets removed without comment. Otherwise: whoever is listed here contributed something that people use on their devices. That is the point.

License

Source-available, and stricter than it looks. Read the code, clone it, change it, build it and run it on devices you own. That is the whole of it.

No distribution. Not the App Store, not any other store, not TestFlight — not even your own — not sideloading to somebody else, not handing a compiled build to a friend. No sale and no transfer for money. No presenting yourself as the author or provider of Fuel, and no use of the name "Fuel", the Levo Studio mark, the logo, the app icon or the visual design for your own product. Fuel is and stays a product of Levo Studio.

The one narrow exception is the fork contributing needs: a source-only public fork, notices intact, source code and nothing else. A compiled binary goes to nobody, including the people testing your branch. The conditions are in section 3 of the license.

None of that binds Levo Studio, which holds the rights and keeps them. Anyone contributing by pull request grants Levo Studio the rights to use the contribution in the project and in any release Levo Studio may publish — but keeps their authorship and is named in the credits.

The full text is in LICENSE. Read section 3 before you suggest anything involving a store, TestFlight or sideloading.

© 2026 Levo Studio