Before a hackathon team divides frontend and backend work, agree on the smallest API contract that can support the demo’s actual user flow. Put it in one shared artifact, build the frontend against representative mock responses, and check the real development API early. That prevents each side from quietly inventing a different interface—without turning a short project into a speculative platform-design exercise.
Start with the demo flow, not a list of endpoints
Choose the screen, action, or short sequence the team intends to demonstrate. Trace what the user does, what data the interface needs, and what must be sent or returned. Define only the operations needed for that path; add another endpoint only when the agreed flow requires it.
This keeps the boundary focused on behavior the other side can observe. Database tables, internal service names, and implementation choices do not belong in the contract unless they affect what a client sends or receives.
What to agree on before splitting the work
For every operation in the demo path, settle the details that could otherwise become incompatible assumptions:
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- Route and method: the exact path, HTTP method, and any agreed base path.
- Inputs: path and query parameters, request body, types, and which values are required.
- Success response: exact field names, types, representative values, and whether a field may be absent or null. Agree on defaults and allowed enum values where relevant.
- Errors: status codes and response shape the interface needs to handle, such as invalid input or a missing item.
- Access: whether the operation requires authentication or authorization. Private data and actions must be protected by server-side checks; hiding a button in the frontend is not access control.
- Compatibility and changes: whether the demo needs a versioned path, who owns contract edits, and how the team approves a change before either side renames a field.
Keep the agreement explicit but proportionate. A demo usually needs a clear change convention and a stable shape for its chosen flow, not a plan for every future client or release.
Keep one authoritative contract
For an HTTP API, a shared OpenAPI file is a practical way to record operations, inputs, responses, errors, and other client-visible rules in one place. The contract-first collaboration guide explains how consumers and providers can work from the same artifact: Contract-First Collaboration documentation.
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A shared typed interface can also work when both sides use compatible languages and build tooling. Choose the artifact the whole team can read and use quickly. Avoid maintaining the same payload independently in a specification, mock, prose notes, and implementation; duplicated definitions invite drift.
Not every boundary is HTTP. The same guide identifies AsyncAPI for event-driven interfaces, Protocol Buffers for RPC, and JSON Schema for a standalone JSON payload. Pick the format that matches the interface rather than adopting a heavier toolchain for its own sake.
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Give the frontend a useful mock while the backend builds
Write at least one realistic example response from the agreed contract. Include enough values to render the demo screen, and settle empty, loading, or error states when they change what the interface must do. These examples are implementation aids; they should not become a second, conflicting definition of the API.
The frontend can build against a mock derived from the contract while the backend implements the same shape. Entente documents generating consumer mocks from OpenAPI and replaying interactions against providers; that is one possible contract-based workflow, not a requirement to adopt a particular vendor tool: Entente documentation.
Generated client types or server interfaces can help if the stack makes them straightforward. For a short hackathon, a shared schema, a representative example, and a quick response check may be enough. An archived GitHub example demonstrates one approach using shared OpenAPI specifications, generated interfaces or clients, and runtime compliance testing across services: OpenAPI microservice example. Treat it as an illustration, not a current tooling recommendation.
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A specification describes the intended interface; it does not, by itself, force the running server to follow it. Once the backend has a development endpoint, connect one real screen or action and compare the actual request and response with the agreed contract and example.
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- Point the chosen frontend flow at the development API rather than the mock.
- Check the method, path, inputs, response field spelling and types, and the error behavior that flow depends on.
- If implementation and contract disagree, agree on the intended behavior, update the shared artifact, and bring both sides into line before adding more assumptions elsewhere.
- For private data or actions, verify the server’s authorization behavior as well as the frontend’s display behavior.
The surfaced excerpt for the exact-title hackathon article also emphasizes inspecting the development response and warns that a specification alone does not enforce runtime behavior. Its full page was unavailable, so this article does not attribute further details to that author.
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