FastAPI is often chosen for speed. Fast development, fast performance, fast onboarding.
But after a few months, many teams realize something uncomfortable:
The API works, but the codebase is getting harder to change.
Maintainability rarely breaks all at once. It erodes slowly:
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Endpoints become bloated
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Business rules leak everywhere
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Small changes feel risky
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New developers take weeks to ramp up
What “Maintainable” Really Means for APIs
Maintainability is not just clean code or nice folder names.
A maintainable FastAPI codebase:
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Is easy to reason about
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Localizes change instead of spreading it
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Encourages correct usage by design
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Makes wrong usage obvious
If every new feature requires touching five unrelated files, something is wrong.
Common Reasons FastAPI Projects Become Hard to Maintain
Understanding failure modes helps avoid them.
1. Fat Routers
Routers slowly accumulate:
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Validation logic
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Business rules
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Database queries
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External API calls
Endpoints become mini applications.
2. No Clear Separation of Responsibilities
When everything depends on everything:
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Refactoring becomes scary
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Tests become brittle
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Ownership becomes unclear
FastAPI gives flexibility, but structure is your responsibility.
3. Framework-Centric Thinking
When the codebase revolves around FastAPI constructs:
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Business logic becomes hard to reuse
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Testing requires HTTP for everything
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Long-term flexibility is reduced
FastAPI should be a delivery mechanism, not the center of your system.
A Maintainability-First Mental Model
Before folders and tools, align on principles.
Principle 1: Routers Are Thin
Routers should:
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Parse input
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Call application logic
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Return responses
They should not decide business behavior.
Principle 2: Business Logic Is Explicit
Rules should live in named functions or classes.
If logic is important, it deserves a home.
Principle 3: Dependencies Flow Inward
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Frameworks depend on your code
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Your domain does not depend on frameworks
This keeps the system adaptable.
Recommended Project Structure (Pragmatic)
There is no single correct structure, but this one scales well.
This structure emphasizes responsibility over layers.
Key Best Practices for Maintainable FastAPI APIs
1. Separate Schemas from Domain Models
Pydantic schemas are for I/O.
Domain models are for behavior.
Do not merge them just for convenience.
2. Use Dependencies for Wiring, Not Logic
Dependencies should:
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Provide sessions
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Inject services
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Manage lifecycle
They should not contain business rules.
3. Make Side Effects Obvious
External calls, writes, and state changes should be easy to spot.
Hidden side effects are the enemy of maintainability.
4. Prefer Explicit Use Cases Over Generic Services
Instead of:
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UserService.update()
Prefer:
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UpdateUserProfile -
DeactivateUserAccount
Names encode intent.
5. Version APIs Intentionally
Breaking changes are inevitable.
Structure your API so versions can coexist without hacks.
A Small Coding Example (Maintainability in Practice)
Bad Example (Hard to Maintain)
Everything happens in one place.
Better Example (Separated Concerns)
Now:
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Logic is testable
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Router stays thin
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Change is localized
Testing as a Design Tool
Maintainable systems are testable systems.
If something is hard to test, it is often poorly structured.
Focus on:
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Unit tests for business logic
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Fewer HTTP-level tests
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Clear boundaries
Where PySquad Can Help
Maintainability problems rarely appear in greenfield projects. They appear during growth.
At PySquad, we help teams:
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Refactor existing FastAPI codebases safely
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Introduce structure without stopping delivery
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Define long-term architecture without over-engineering
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Improve testability and developer velocity
Our goal is not to impose patterns. It is to help teams move faster with less friction over time.
Final Thoughts
FastAPI gives you speed at the beginning.
Maintainable architecture gives you speed over years.
If your FastAPI codebase feels increasingly fragile, that feeling is valuable feedback. Structure is not bureaucracy. It is what allows teams to grow without slowing down.
