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Answer: When you need to send data from a client (let's say, a browser) to your API, you send it as a request body. To declare a request body, you use Pydantic models with all their power and benefits. (S1)

Request vs response

• A response body is the data your API sends to the client. (S1)
• Clients don't necessarily need to send request bodies all the time, sometimes they only request a path, maybe with some query parameters, but don't send a body. (S1)
• Sending a body with a GET request has an undefined behavior in the specifications, nevertheless, it is supported by FastAPI, only for very complex/extreme use cases. (S1)

Field validation

• You can declare validation and metadata inside of Pydantic models using Pydantic's Field. (S2)
• Field works the same way as Query, Path and Body, it has all the same parameters, etc. (S2)
• Pydantic's Field returns an instance of FieldInfo as well. (S2)

Claims

• id: C1 claim: Request bodies are declared with Pydantic models. source: S1 evidence: "To declare a request body, you use Pydantic models with all their power and benefits."
• id: C2 claim: A GET body is undefined behavior. source: S1 evidence: "Sending a body with a GET request has an undefined behavior in the specifications"
• id: C3 claim: Field adds validation inside models. source: S2 evidence: "you can declare validation and metadata inside of Pydantic models using Pydantic's Field."
• id: C4 claim: Field works like Query, Path and Body. source: S2 evidence: "Field works the same way as Query, Path and Body"

Sources

• [S1] Request Body - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/body/ . License: MIT (fastapi/fastapi LICENSE; Copyright (c) 2018 Sebastián Ramírez; https://github.com/fastapi/fastapi/blob/master/LICENSE). Accessed 2026-11-04.
• [S2] Body - Fields - FastAPI. FastAPI docs. https://fastapi.tiangolo.com/tutorial/body-fields/ . License: MIT (fastapi/fastapi LICENSE; Copyright (c) 2018 Sebastián Ramírez; https://github.com/fastapi/fastapi/blob/master/LICENSE). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_hmkycyjz,
  author = {{iinventedlight}},
  title = {{Request bodies with Pydantic in FastAPI}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/fastapi-request-body/},
  note = {Permanent ID iil:note:hmkycyjz. License: CC-BY-4.0.},
}

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