Cross-Platform Development with Claude Code

Introduction

Building for multiple platforms is hard. Building for iOS, Web, and a Firebase backend simultaneously is harder still. Each platform has its own conventions, its own idioms, its own way of thinking about data. Swift uses camelCase. JavaScript vacillates between camelCase and snake_case depending on who wrote it. Firebase has its own opinions about document structure.

When you add AI-assisted development to this mix, things can go wrong fast. Claude might name a field userId in your Swift code, user_id in your JavaScript, and userID in your Firestore rules. Three platforms, three naming conventions, one broken application.

This article shows you how to build cross-platform applications with Claude Code while maintaining perfect consistency. You'll learn to use a single source of truth that all platforms reference, coordinate platform specialists who understand their domains deeply, and validate that everything stays synchronized before it ships.

The result is cross-platform development that actually works. Features ship to iOS, Web, and backend simultaneously. Data flows correctly between all platforms. And you stop spending your time debugging naming mismatches and schema drift.

The Challenge

Cross-platform development with AI introduces unique challenges that don't exist when a human team coordinates manually. Understanding these challenges is the first step to solving them.

Schema Drift

Schema drift happens when your data models diverge across platforms over time. It starts small: the iOS developer adds an optional nickname field. The web developer doesn't know about it, so the web app doesn't display it. Later, the web developer adds a displayName field that does the same thing. Now you have two fields for the same purpose, neither platform knows about the other's field, and your users see inconsistent behavior.

With AI-assisted development, schema drift accelerates. Claude doesn't remember what it did in a previous session. Ask it to add a user profile feature on iOS today and a user profile feature on web tomorrow, and you'll get two different schemas unless you explicitly prevent it.

Naming Inconsistencies

Every platform has naming conventions. Swift prefers camelCase for properties. Python uses snake_case. JavaScript does whatever the developer felt like that day. When Claude generates code, it often follows the conventions of the platform it's currently targeting, which means the same concept gets different names on different platforms.

This creates subtle bugs that are hard to trace. Your iOS app sends createdAt to the backend. Your web app expects created_at from the backend. The Firebase function uses createdTimestamp. Everything technically works until someone tries to sort by creation date and gets inconsistent results.

Platform Silos

When you develop each platform in isolation, knowledge doesn't transfer. The iOS developer makes decisions about data structure. The web developer makes different decisions. The backend developer makes yet different decisions. Each platform becomes a silo with its own assumptions about how things work.

This is especially problematic with Claude, because Claude's context resets between sessions. The knowledge that the iOS developer built a feature a certain way doesn't automatically transfer to the session where the web developer is building the same feature.

Single Source of Truth: ECOSYSTEM.md

The solution to cross-platform chaos is deceptively simple: define everything once, in one place, and make every platform reference that definition. This is what ECOSYSTEM.md does for Claude Architect projects.

What ECOSYSTEM.md Contains

ECOSYSTEM.md is a markdown file that defines your project's data contracts. It specifies every data model, every field name, every enum value, and every API contract. When Claude generates code for any platform, it reads ECOSYSTEM.md first and follows its definitions exactly.

## Data Models

### User
| Field | Type | Description |
|-------|------|-------------|
| id | String | Unique identifier (UUID) |
| email | String | User's email address |
| displayName | String | User-chosen display name |
| createdAt | Timestamp | Account creation time |
| updatedAt | Timestamp | Last profile update |

### Post
| Field | Type | Description |
|-------|------|-------------|
| id | String | Unique identifier (UUID) |
| authorId | String | Reference to User.id |
| title | String | Post title (max 200 chars) |
| content | String | Post body (markdown) |
| status | PostStatus | draft, published, archived |
| createdAt | Timestamp | Creation timestamp |
| publishedAt | Timestamp? | Publication timestamp (optional) |

## Enums

### PostStatus
- draft
- published
- archived

How Platforms Reference It

Every platform specialist reads ECOSYSTEM.md before writing code. The iOS developer sees that the field is called displayName and uses that exact name in Swift. The web developer sees the same definition and uses displayName in JavaScript. The Firebase developer structures Firestore documents to match the same schema.

This isn't about trusting Claude to remember instructions. It's about giving Claude a specification to follow. The specification is always available, always consistent, and always authoritative.

Updating the Source of Truth

When requirements change, you update ECOSYSTEM.md first. Need to add a profileImageUrl field to the User model? Add it to ECOSYSTEM.md, then have each platform specialist update their implementation to match. The specification drives the implementation, not the other way around.

This approach prevents the common mistake of adding a field on one platform and forgetting to add it elsewhere. If it's not in ECOSYSTEM.md, it doesn't exist. If it is in ECOSYSTEM.md, it exists everywhere.

Platform Specialists

Cross-platform development works best when each platform has a dedicated specialist. These aren't different people; they're different Claude agents, each configured with deep knowledge of their platform and explicit instructions to follow ECOSYSTEM.md.

iOS Developer

The iOS Developer specializes in Swift, SwiftUI, and Apple's frameworks. It knows iOS conventions: how to structure models, how to handle async/await, how to work with Combine or async sequences. When implementing a data model, it creates Swift structs that exactly match ECOSYSTEM.md, using Swift-appropriate types like Date for timestamps and UUID for identifiers.

struct User: Codable, Identifiable {
    let id: String
    let email: String
    let displayName: String
    let createdAt: Date
    let updatedAt: Date
}

struct Post: Codable, Identifiable {
    let id: String
    let authorId: String
    let title: String
    let content: String
    let status: PostStatus
    let createdAt: Date
    let publishedAt: Date?
}

enum PostStatus: String, Codable {
    case draft
    case published
    case archived
}

Web Developer

The Web Developer handles JavaScript, TypeScript, and web-specific concerns. It knows browser APIs, understands async patterns with Promises, and follows web conventions while maintaining ECOSYSTEM.md field names. The same User model becomes a TypeScript interface with identical field names.

interface User {
    id: string;
    email: string;
    displayName: string;
    createdAt: Date;
    updatedAt: Date;
}

interface Post {
    id: string;
    authorId: string;
    title: string;
    content: string;
    status: PostStatus;
    createdAt: Date;
    publishedAt?: Date;
}

type PostStatus = 'draft' | 'published' | 'archived';

Firebase Developer

The Firebase Developer manages Cloud Functions, Firestore rules, and backend logic. It structures Firestore documents to match ECOSYSTEM.md exactly, ensuring that data written by the iOS app can be read by the web app and vice versa. Field names in Firestore match field names in the client code.

// Firestore: /users/{userId}
{
    "id": "uuid-string",
    "email": "user@example.com",
    "displayName": "Jane Smith",
    "createdAt": Timestamp,
    "updatedAt": Timestamp
}

// Firestore: /posts/{postId}
{
    "id": "uuid-string",
    "authorId": "user-uuid",
    "title": "My First Post",
    "content": "# Hello World\n\nThis is markdown.",
    "status": "published",
    "createdAt": Timestamp,
    "publishedAt": Timestamp
}

The PM Coordinates

The Project Manager doesn't implement code on any platform. Instead, it coordinates the specialists: understanding what needs to be built, updating ECOSYSTEM.md when requirements change, and delegating implementation to the right specialist. When a feature requires changes across all platforms, the PM ensures each specialist implements the same specification.

Keeping Platforms in Sync

Having a single source of truth is necessary but not sufficient. You also need practices that keep all platforms aligned as the project evolves.

Shared Field Names

The most important rule: field names must be identical across all platforms. Not similar. Identical. If ECOSYSTEM.md says displayName, then the Swift property is displayName, the TypeScript property is displayName, and the Firestore field is displayName.

This seems obvious, but it's easy to violate. Swift developers might be tempted to use userName because it feels more natural. JavaScript developers might prefer display_name to match some library they're using. Resist these temptations. Consistency across platforms is more valuable than matching any single platform's conventions.

Consistent Enums

Enums are a common source of drift. ECOSYSTEM.md defines PostStatus with values draft, published, and archived. Every platform must use these exact string values, regardless of how the platform typically handles enums.

// ECOSYSTEM.md defines: draft, published, archived

// Swift - uses raw string values
enum PostStatus: String, Codable {
    case draft
    case published
    case archived
}

// TypeScript - uses string literal union
type PostStatus = 'draft' | 'published' | 'archived';

// Firestore - stores as string
{
    "status": "published"  // matches exactly
}

Type Mapping

Different platforms have different type systems, but ECOSYSTEM.md types must map consistently. A Timestamp in ECOSYSTEM.md becomes a Date in Swift, a Date in TypeScript, and a Firestore Timestamp in the backend. These mappings should be documented and followed consistently.

ECOSYSTEM.md    Swift           TypeScript      Firestore
-----------     -----           ----------      ---------
String          String          string          string
Int             Int             number          number
Float           Double          number          number
Boolean         Bool            boolean         boolean
Timestamp       Date            Date            Timestamp
UUID            String          string          string
Optional     T?              T | undefined   T or absent

API Contract Alignment

When your Firebase backend exposes APIs (through Cloud Functions or Firestore queries), the request and response shapes must match what clients expect. Document these contracts in ECOSYSTEM.md alongside your data models, and have all platforms implement to the same specification.

Example Workflow: Building a Feature

Let's walk through building a real feature across all platforms: adding user profiles with avatars. This demonstrates how the coordination actually works in practice.

Step 1: Update ECOSYSTEM.md

Before any code is written, the PM updates ECOSYSTEM.md with the new requirements. The User model gets new fields for profile information.

### User (Updated)
| Field | Type | Description |
|-------|------|-------------|
| id | String | Unique identifier (UUID) |
| email | String | User's email address |
| displayName | String | User-chosen display name |
| avatarUrl | String? | URL to profile image |
| bio | String? | User bio (max 500 chars) |
| createdAt | Timestamp | Account creation time |
| updatedAt | Timestamp | Last profile update |

Step 2: iOS Implementation

The PM delegates to the iOS Developer: "Update the User model to match ECOSYSTEM.md. Add UI for viewing and editing profiles."

The iOS Developer reads ECOSYSTEM.md, sees the new fields, and updates the Swift model:

struct User: Codable, Identifiable {
    let id: String
    let email: String
    let displayName: String
    let avatarUrl: String?    // New field
    let bio: String?          // New field
    let createdAt: Date
    let updatedAt: Date
}

Step 3: Web Implementation

The PM delegates to the Web Developer: "Update the User interface to match ECOSYSTEM.md. Add profile components."

The Web Developer reads the same ECOSYSTEM.md and updates TypeScript:

interface User {
    id: string;
    email: string;
    displayName: string;
    avatarUrl?: string;    // New field
    bio?: string;          // New field
    createdAt: Date;
    updatedAt: Date;
}

Step 4: Backend Implementation

The PM delegates to the Firebase Developer: "Update Firestore rules to allow the new User fields. Add a Cloud Function for avatar upload."

The Firebase Developer ensures the document structure matches:

match /users/{userId} {
    allow read: if request.auth != null;
    allow write: if request.auth.uid == userId
        && request.resource.data.keys().hasOnly([
            'id', 'email', 'displayName',
            'avatarUrl', 'bio',  // New fields
            'createdAt', 'updatedAt'
        ]);
}

Step 5: Validation

The PM validates that all platforms implemented the same specification. Field names match exactly. Optional fields are handled correctly. The feature works end-to-end: upload an avatar on iOS, see it on web, data persists in Firestore.

Validation: Cross-Platform Checks

Trust but verify. Before shipping any cross-platform feature, run validation checks that catch inconsistencies.

Field Name Verification

Check that every field in ECOSYSTEM.md exists in every platform's implementation. This can be automated with a script that parses your Swift structs, TypeScript interfaces, and Firestore rules, then compares them against ECOSYSTEM.md.

Cross-Platform Validation:

[ ] All ECOSYSTEM.md fields exist in Swift models
[ ] All ECOSYSTEM.md fields exist in TypeScript interfaces
[ ] Firestore rules allow all ECOSYSTEM.md fields
[ ] Field names are IDENTICAL (case-sensitive)
[ ] Optional fields marked correctly on all platforms
[ ] Enum values match exactly across platforms
[ ] Type mappings are consistent

End-to-End Testing

The ultimate validation is testing the actual data flow. Create a record on one platform, read it on another. If an iOS user creates a post, can the web app display it correctly? If the web app updates a profile, does the iOS app see the changes?

Schema Comparison

Periodically compare your actual Firestore documents against ECOSYSTEM.md. Are there fields in the database that aren't in the specification? Those are schema drift waiting to cause problems. Are there fields in the specification that aren't in the database? Those are incomplete implementations.

Common Pitfalls

Even with good systems, cross-platform development has traps. Here are the most common ones and how to avoid them.

Platform-Specific Naming

The temptation to follow platform conventions is strong. Swift developers want to use Swift idioms. JavaScript developers want to use JavaScript idioms. But cross-platform consistency requires a shared convention that might not feel natural on any single platform.

Solution: ECOSYSTEM.md defines the canonical names. Platforms adapt to ECOSYSTEM.md, not the other way around. If this feels awkward, remember that consistency across platforms is more valuable than matching any single platform's style guide.

Forgetting Optional Handling

A field that's optional on one platform must be optional on all platforms. It's easy to mark a field as optional in TypeScript (bio?: string) but forget to make it optional in Swift (let bio: String instead of let bio: String?). This causes crashes when the iOS app tries to decode a document without that field.

Solution: ECOSYSTEM.md explicitly marks optional fields. Validation scripts check that optionality is consistent. Code reviews specifically look for optional handling.

Type Coercion Issues

JavaScript is loosely typed. Swift is strictly typed. A number that JavaScript happily treats as a string will crash your Swift decoder. Timestamps are particularly problematic: Firestore Timestamps don't automatically convert to JavaScript Dates or Swift Dates without explicit handling.

Solution: Document explicit type mappings. Use consistent serialization. Test deserialization on all platforms with real data.

Async Timing Differences

Firestore is eventually consistent. A write on one client might not immediately appear on another. This isn't a schema problem, but it affects cross-platform behavior. Users expect that uploading an avatar on their phone will immediately show on their laptop.

Solution: Use Firestore listeners for real-time updates. Implement optimistic UI updates. Document expected latency in ECOSYSTEM.md so all platforms handle it consistently.

Conclusion: Get Started with Claude Architect

Cross-platform development doesn't have to be chaos. With a single source of truth in ECOSYSTEM.md, platform specialists who deeply understand their domains, and validation that catches drift before it ships, you can build iOS, Web, and Firebase applications that work together seamlessly.

The key insights are simple: define everything once, make every platform reference that definition, and verify consistency before shipping. The execution requires discipline, but the payoff is enormous. No more debugging naming mismatches. No more schema drift. No more platform silos where one team's decisions create problems for another.

Building this system from scratch takes significant effort. You need to create ECOSYSTEM.md templates, configure platform specialists, build validation scripts, and establish workflows that keep everything synchronized.

Claude Architect provides everything out of the box. Pre-configured specialists for iOS, Web, and Firebase. ECOSYSTEM.md templates designed for cross-platform consistency. A PM agent that coordinates multi-platform features. Validation workflows that catch drift automatically.

Stop fighting cross-platform inconsistency. Start shipping features that work identically everywhere.