fastapi · tiangolo · Feb 21, 2026 · Feb 20, 2026 · Feb 20, 2026 · Feb 21, 2026
diff --git a/docs/en/docs/advanced/json-base64-bytes.md b/docs/en/docs/advanced/json-base64-bytes.md
@@ -0,0 +1,63 @@
+# JSON with Bytes as Base64 { #json-with-bytes-as-base64 }
+
+If your app needs to receive and send JSON data, but you need to include binary data in it, you can encode it as base64.
+
+## Base64 vs Files { #base64-vs-files }
+
+Consider first if you can use [Request Files](../tutorial/request-files.md){.internal-link target=_blank} for uploading binary data and [Custom Response - FileResponse](./custom-response.md#fileresponse--fileresponse-){.internal-link target=_blank} for sending binary data, instead of encoding it in JSON.
+
+JSON can only contain UTF-8 encoded strings, so it can't contain raw bytes.
+
+Base64 can encode binary data in strings, but to do it, it needs to use more characters than the original binary data, so it would normally be less efficient than regular files.
+
+Use base64 only if you definitely need to include binary data in JSON, and you can't use files for that.
+
+## Pydantic `bytes` { #pydantic-bytes }
+
+You can declare a Pydantic model with `bytes` fields, and then use `val_json_bytes` in the model config to tell it to use base64 to *validate* input JSON data, as part of that validation it will decode the base64 string into bytes.
+
+{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:9,29:35] hl[9] *}
+
+If you check the `/docs`, they will show that the field `data` expects base64 encoded bytes:
+
+<div class="screenshot">
+<img src="/img/tutorial/json-base64-bytes/image01.png">
+</div>
+
+You could send a request like:
+
+```json
+{
+    "description": "Some data",
+    "data": "aGVsbG8="
+}
+```
+
+/// tip
+
+`aGVsbG8=` is the base64 encoding of `hello`.
+
+///
+
+And then Pydantic will decode the base64 string and give you the original bytes in the `data` field of the model.
+
+You will receive a response like:
+
+```json
+{
+  "description": "Some data",
+  "content": "hello"
+}
+```
+
+## Pydantic `bytes` for Output Data { #pydantic-bytes-for-output-data }
+
+You can also use `bytes` fields with `ser_json_bytes` in the model config for output data, and Pydantic will *serialize* the bytes as base64 when generating the JSON response.
+
+{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:2,12:16,29,38:41] hl[16] *}
+
+## Pydantic `bytes` for Input and Output Data { #pydantic-bytes-for-input-and-output-data }
+
+And of course, you can use the same model configured to use base64 to handle both input (*validate*) with `val_json_bytes` and output (*serialize*) with `ser_json_bytes` when receiving and sending JSON data.
+
+{* ../../docs_src/json_base64_bytes/tutorial001_py310.py ln[1:2,19:26,29,44:46] hl[23:26] *}
diff --git a/docs/en/docs/img/tutorial/json-base64-bytes/image01.png b/docs/en/docs/img/tutorial/json-base64-bytes/image01.png
diff --git a/docs/en/mkdocs.yml b/docs/en/mkdocs.yml
@@ -192,6 +192,7 @@ nav:
     - advanced/wsgi.md
     - advanced/generate-clients.md
     - advanced/advanced-python-types.md
+    - advanced/json-base64-bytes.md
   - fastapi-cli.md
   - Deployment:
     - deployment/index.md

diff --git a/docs_src/json_base64_bytes/__init__.py b/docs_src/json_base64_bytes/__init__.py
diff --git a/docs_src/json_base64_bytes/tutorial001_py310.py b/docs_src/json_base64_bytes/tutorial001_py310.py
@@ -0,0 +1,46 @@
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+
+class DataInput(BaseModel):
+    description: str
+    data: bytes
+
+    model_config = {"val_json_bytes": "base64"}
+
+
+class DataOutput(BaseModel):
+    description: str
+    data: bytes
+
+    model_config = {"ser_json_bytes": "base64"}
+
+
+class DataInputOutput(BaseModel):
+    description: str
+    data: bytes
+
+    model_config = {
+        "val_json_bytes": "base64",
+        "ser_json_bytes": "base64",
+    }
+
+
+app = FastAPI()
+
+
+@app.post("/data")
+def post_data(body: DataInput):
+    content = body.data.decode("utf-8")
+    return {"description": body.description, "content": content}
+
+
+@app.get("/data")
+def get_data() -> DataOutput:
+    data = "hello".encode("utf-8")
+    return DataOutput(description="A plumbus", data=data)
+
+
+@app.post("/data-in-out")
+def post_data_in_out(body: DataInputOutput) -> DataInputOutput:
+    return body
diff --git a/fastapi/_compat/v2.py b/fastapi/_compat/v2.py
@@ -27,7 +27,7 @@
 )
 from pydantic._internal._typing_extra import eval_type_lenient
 from pydantic.fields import FieldInfo as FieldInfo
-from pydantic.json_schema import GenerateJsonSchema as GenerateJsonSchema
+from pydantic.json_schema import GenerateJsonSchema as _GenerateJsonSchema
 from pydantic.json_schema import JsonSchemaValue as JsonSchemaValue
 from pydantic_core import CoreSchema as CoreSchema
 from pydantic_core import PydanticUndefined
@@ -40,6 +40,23 @@
 Undefined = PydanticUndefined
 evaluate_forwardref = eval_type_lenient
 
+
+class GenerateJsonSchema(_GenerateJsonSchema):
+    # TODO: remove when this is merged (or equivalent): https://github.com/pydantic/pydantic/pull/12841
+    # and dropping support for any version of Pydantic before that one (so, in a very long time)
+    def bytes_schema(self, schema: CoreSchema) -> JsonSchemaValue:
+        json_schema = {"type": "string", "contentMediaType": "application/octet-stream"}
+        bytes_mode = (
+            self._config.ser_json_bytes
+            if self.mode == "serialization"
+            else self._config.val_json_bytes
+        )
+        if bytes_mode == "base64":
+            json_schema["contentEncoding"] = "base64"
+        self.update_with_validations(json_schema, schema, self.ValidationsMapping.bytes)
+        return json_schema
+
+
 # TODO: remove when dropping support for Pydantic < v2.12.3
 _Attrs = {
     "default": ...,

diff --git a/fastapi/datastructures.py b/fastapi/datastructures.py
@@ -139,7 +139,7 @@ def _validate(cls, __input_value: Any, _: Any) -> "UploadFile":
     def __get_pydantic_json_schema__(
         cls, core_schema: Mapping[str, Any], handler: GetJsonSchemaHandler
     ) -> dict[str, Any]:
-        return {"type": "string", "format": "binary"}
+        return {"type": "string", "contentMediaType": "application/octet-stream"}
 
     @classmethod
     def __get_pydantic_core_schema__(

diff --git a/pyproject.toml b/pyproject.toml
@@ -548,6 +548,7 @@ ignore = [
 "docs_src/security/tutorial005_an_py39.py" = ["B904"]
 "docs_src/security/tutorial005_py310.py" = ["B904"]
 "docs_src/security/tutorial005_py39.py" = ["B904"]
+"docs_src/json_base64_bytes/tutorial001_py310.py" = ["UP012"]
 
 [tool.ruff.lint.isort]
 known-third-party = ["fastapi", "pydantic", "starlette"]

diff --git a/scripts/playwright/json_base64_bytes/image01.py b/scripts/playwright/json_base64_bytes/image01.py
@@ -0,0 +1,37 @@
+import subprocess
+import time
+
+import httpx
+from playwright.sync_api import Playwright, sync_playwright
+
+
+# Run playwright codegen to generate the code below, copy paste the sections in run()
+def run(playwright: Playwright) -> None:
+    browser = playwright.chromium.launch(headless=False)
+    # Update the viewport manually
+    context = browser.new_context(viewport={"width": 960, "height": 1080})
+    page = context.new_page()
+    page.goto("http://localhost:8000/docs")
+    page.get_by_role("button", name="POST /data Post Data").click()
+    # Manually add the screenshot
+    page.screenshot(path="docs/en/docs/img/tutorial/json-base64-bytes/image01.png")
+
+    # ---------------------
+    context.close()
+    browser.close()
+
+
+process = subprocess.Popen(
+    ["fastapi", "run", "docs_src/json_base64_bytes/tutorial001_py310.py"]
+)
+try:
+    for _ in range(3):
+        try:
+            response = httpx.get("http://localhost:8000/docs")
+        except httpx.ConnectError:
+            time.sleep(1)
+            break
+    with sync_playwright() as playwright:
+        run(playwright)
+finally:
+    process.terminate()
diff --git a/tests/test_request_params/test_file/test_list.py b/tests/test_request_params/test_file/test_list.py
@@ -37,7 +37,10 @@ def test_list_schema(path: str):
         "properties": {
             "p": {
                 "type": "array",
-                "items": {"type": "string", "format": "binary"},
+                "items": {
+                    "type": "string",
+                    "contentMediaType": "application/octet-stream",
+                },
                 "title": "P",
             },
         },
@@ -115,7 +118,10 @@ def test_list_alias_schema(path: str):
         "properties": {
             "p_alias": {
                 "type": "array",
-                "items": {"type": "string", "format": "binary"},
+                "items": {
+                    "type": "string",
+                    "contentMediaType": "application/octet-stream",
+                },
                 "title": "P Alias",
             },
         },
@@ -221,7 +227,10 @@ def test_list_validation_alias_schema(path: str):
         "properties": {
             "p_val_alias": {
                 "type": "array",
-                "items": {"type": "string", "format": "binary"},
+                "items": {
+                    "type": "string",
+                    "contentMediaType": "application/octet-stream",
+                },
                 "title": "P Val Alias",
             },
         },
@@ -338,7 +347,10 @@ def test_list_alias_and_validation_alias_schema(path: str):
         "properties": {
             "p_val_alias": {
                 "type": "array",
-                "items": {"type": "string", "format": "binary"},
+                "items": {
+                    "type": "string",
+                    "contentMediaType": "application/octet-stream",
+                },
                 "title": "P Val Alias",
             },
         },

diff --git a/tests/test_request_params/test_file/test_optional.py b/tests/test_request_params/test_file/test_optional.py
@@ -37,7 +37,7 @@ def test_optional_schema(path: str):
         "properties": {
             "p": {
                 "anyOf": [
-                    {"type": "string", "format": "binary"},
+                    {"type": "string", "contentMediaType": "application/octet-stream"},
                     {"type": "null"},
                 ],
                 "title": "P",
@@ -109,7 +109,7 @@ def test_optional_alias_schema(path: str):
         "properties": {
             "p_alias": {
                 "anyOf": [
-                    {"type": "string", "format": "binary"},
+                    {"type": "string", "contentMediaType": "application/octet-stream"},
                     {"type": "null"},
                 ],
                 "title": "P Alias",
@@ -200,7 +200,7 @@ def test_optional_validation_alias_schema(path: str):
         "properties": {
             "p_val_alias": {
                 "anyOf": [
-                    {"type": "string", "format": "binary"},
+                    {"type": "string", "contentMediaType": "application/octet-stream"},
                     {"type": "null"},
                 ],
                 "title": "P Val Alias",
@@ -296,7 +296,7 @@ def test_optional_alias_and_validation_alias_schema(path: str):
         "properties": {
             "p_val_alias": {
                 "anyOf": [
-                    {"type": "string", "format": "binary"},
+                    {"type": "string", "contentMediaType": "application/octet-stream"},
                     {"type": "null"},
                 ],
                 "title": "P Val Alias",

diff --git a/tests/test_request_params/test_file/test_optional_list.py b/tests/test_request_params/test_file/test_optional_list.py
@@ -41,7 +41,10 @@ def test_optional_list_schema(path: str):
                 "anyOf": [
                     {
                         "type": "array",
-                        "items": {"type": "string", "format": "binary"},
+                        "items": {
+                            "type": "string",
+                            "contentMediaType": "application/octet-stream",
+                        },
                     },
                     {"type": "null"},
                 ],
@@ -116,7 +119,10 @@ def test_optional_list_alias_schema(path: str):
                 "anyOf": [
                     {
                         "type": "array",
-                        "items": {"type": "string", "format": "binary"},
+                        "items": {
+                            "type": "string",
+                            "contentMediaType": "application/octet-stream",
+                        },
                     },
                     {"type": "null"},
                 ],
@@ -205,7 +211,10 @@ def test_optional_validation_alias_schema(path: str):
                 "anyOf": [
                     {
                         "type": "array",
-                        "items": {"type": "string", "format": "binary"},
+                        "items": {
+                            "type": "string",
+                            "contentMediaType": "application/octet-stream",
+                        },
                     },
                     {"type": "null"},
                 ],
@@ -301,7 +310,10 @@ def test_optional_list_alias_and_validation_alias_schema(path: str):
                 "anyOf": [
                     {
                         "type": "array",
-                        "items": {"type": "string", "format": "binary"},
+                        "items": {
+                            "type": "string",
+                            "contentMediaType": "application/octet-stream",
+                        },
                     },
                     {"type": "null"},
                 ],

diff --git a/tests/test_request_params/test_file/test_required.py b/tests/test_request_params/test_file/test_required.py
@@ -35,7 +35,11 @@ def test_required_schema(path: str):
 
     assert app.openapi()["components"]["schemas"][body_model_name] == {
         "properties": {
-            "p": {"title": "P", "type": "string", "format": "binary"},
+            "p": {
+                "title": "P",
+                "type": "string",
+                "contentMediaType": "application/octet-stream",
+            },
         },
         "required": ["p"],
         "title": body_model_name,
@@ -109,7 +113,11 @@ def test_required_alias_schema(path: str):
 
     assert app.openapi()["components"]["schemas"][body_model_name] == {
         "properties": {
-            "p_alias": {"title": "P Alias", "type": "string", "format": "binary"},
+            "p_alias": {
+                "title": "P Alias",
+                "type": "string",
+                "contentMediaType": "application/octet-stream",
+            },
         },
         "required": ["p_alias"],
         "title": body_model_name,
@@ -216,7 +224,7 @@ def test_required_validation_alias_schema(path: str):
             "p_val_alias": {
                 "title": "P Val Alias",
                 "type": "string",
-                "format": "binary",
+                "contentMediaType": "application/octet-stream",
             },
         },
         "required": ["p_val_alias"],
@@ -329,7 +337,7 @@ def test_required_alias_and_validation_alias_schema(path: str):
             "p_val_alias": {
                 "title": "P Val Alias",
                 "type": "string",
-                "format": "binary",
+                "contentMediaType": "application/octet-stream",
             },
         },
         "required": ["p_val_alias"],

diff --git a/tests/test_tutorial/test_json_base64_bytes/__init__.py b/tests/test_tutorial/test_json_base64_bytes/__init__.py