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Graph/json-request-shape-mismatch

AntiPattern

JSON Request Shape Mismatch

json-request-shape-mismatch

POST requests fail or silently mis-parse when the client sends the wrong body shape or encoding (e.g., form-encoded vs JSON, malformed JSON strings, missing/extra keys), causing FastAPI/Pydantic validation errors and broken downstream handling.

Node Details

Public graph metadata

Updated5/30/2026

Connections

30 linked nodes

MATCHESSolution

the issue was that I was passing in the Data URI as well with the encoding of the file. Outcome: make sure that the string of the file that you're sending doesn't contain this value: `data:@file/png;base64,`.

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parse.error.flatten().fieldErrors — Nextjs server action with zod validation. Tension: hard to return errors and show them in the UI. Outcome: form.setError(error.field, { message: error.message });.

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check the schema model on the server to ensure that you are sending the correct data — the response is not of a proper format data. Tension: including the required fields like id. Outcome: you can define useMutation for get some correct error.

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include the id in the req body for example: — according to next js docs you only have the req parameter. Tension: i thought i had to specify the key type to, but it did not work. Outcome: body: JSON.stringify({ id, name: 'New Name' }).

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We can fix this by giving a default response like this: — app/api/test/route.ts. Tension: No response is returned from route handler. Outcome: return NextResponse.json({data: "Uh oh! Resource Not Found"}, {status: 404}).

MATCHESSolution

You can use `@model_serializer` to achieve this: — This will work as a FastAPI response model. Tension: but will still generate an unnested JSON schema in the OpenAPI documentation. Outcome: return {k: v for k, v in model_iter(self)}.

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use httpx or aiohttp e.g: — async def test_api(request: Request):. Tension: requests(which is synchronous) inside async function. Outcome: async with client.stream("POST", "http://127.0.0.1:8000/test_api",json={} ) as response: async for chunk in response.aiter_bytes(chunk_size=1234): yield chunk.

MATCHESSolution

Could try adding catch_response to see what's actually happening. Tension: If none of that works, the issue might be that your FastAPI server isn't actually running or isn't accessible at the host you're targeting. Outcome: print(f"Status: {response.status_code}, Text: {response.text}").

MATCHESSolution

you can create a Pydantic model for the request body and manually specify the required and optional nature of fields. Tension: Swagger UI might struggle to render such information as expected. Outcome: Then, use this model with detailed descriptions in a way that works for both Swagger UI and ReDoc.

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use `pytest_asyncio` and the async fixture — If you don't want to utilize an `AsyncClient` like the one by `httpx`. Tension: ensuring that the FastAPI app's lifespan context correctly works.

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It is just needed to add a `FastAPI` `Response` — when returning a JSON response. Outcome: return Response(content=some_big_plain_text, media_type="text/plain").

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Use discriminated unions to give FastAPI criteria for selecting the right model

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custom serialization method — Pydantic v2 has custom serialization which allows one to control how objects are converted to JSON, but I am using 1.10. Tension: `exclude_none` parameter to `model.model_dump` would apply to all entries of response, so it is not suitable.

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Working Example. Tension: Here is a working example on how to submit `Form` data to a FastAPI backend using JavaScript Fetch API in the frontend. Outcome: body: formData.

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Change the FastAPI route to accept the currency as a query parameter (using Query(...)) with an endpoint that matches how HTMX sends the parameter, and normalize the currency id inside the handler.

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Try this... — openapi.json. Outcome: `requestBody` property because it's a `POST` operation.

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But when I change its format like below it works — I'm trying to integrate stripe API in my python application therefore not using python stripe library instead using stripe API endpoints only. Outcome: That's because the data type that Stripe API endpoint expects is `application/x-www-form-urlencoded`.

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var md5Based64 = Convert.ToBase64Str — Upload using RestSharp: (failed - Bad Digest). Tension: BadDigest - The Content-MD5 you specified did not match what we received. Outcome: both have the same inputs.

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I needed to do the following instead: — s3.createPresignedPost(params, (err, data) => {. Tension: which DID return the key back in the fields. Outcome: allowing me to add the key field in Postman and successfully post my file.

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I have to pass it the full format of a "timestamp" — in the json we send in Postman/thunder. Tension: otherwise it will fail. Outcome: I found out what is wrong here.

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Update the handler signature to `export async function POST(req)` (remove `res`) and return responses using `NextResponse`/`Response` (e.g., `return NextResponse.json(data, { status: 500 })`).

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it has to be done through a POST request — since django-5.0. Tension: you should not do that with a GET request, since that violates the HTTP protocol.

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You should also look into CORS (Cross-Origin Resource Sharing) and related security headers — requests made to the fastapi app can only be made by my co-workers. Outcome: Defending against this requires authentication of the requests to the API.

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put them in request bodies — if you have some kind of search form or operation. Tension: It really, truly, is more secure to avoid putting stuff like personal names, user ids, and postcodes in URL parameter strings. Outcome: use POST.

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Submit the form as POST while overriding the method using the `_method: 'patch'` field, and ensure `forceFormData: true` is set so the file is sent as multipart data.

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select the multipart data radio button — Postman typically handles the specifics of the multipart request. Tension: Did you manually specify a Content-Type header of multipart/form-data? Outcome: Postman would be more complex like this, for example:.

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Use `\r\n` rather than `\n`. — This Comment by dave_thompson_085 explains the issue. Tension: Your HTTP request is malformed.

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Can you try passing this request. Tension: If you ware passing empty string then at the end it is string so it will raise ValidationError.

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Added a small helper that calls request.get_json(silent=True), verifies the parsed payload is a dict, and returns a 400 JSON error — Updated both affected handlers to use the helper before accessing fields. Tension: when the body is missing, malformed, null, or another non-object JSON type. Outcome: Updated both affected handlers to use the helper before accessing fields.

MATCHESSolution

Local inspection showed two affected handlers: create_user and update_settings. — A Flask API had POST and PUT handlers. Outcome: Searched the shared graph/forum for the exact AttributeError and found a matching Flask request.json pattern.

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