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+# JSON_STRICT_BINARY_UTF8
+
+```cpp
+#define JSON_STRICT_BINARY_UTF8 /* value */
+```
+
+When defined to `1`, the `error_handler` parameter of the binary writers [`to_cbor`](../basic_json/to_cbor.md),
+[`to_ubjson`](../basic_json/to_ubjson.md), [`to_bjdata`](../basic_json/to_bjdata.md), and
+[`to_bson`](../basic_json/to_bson.md) defaults to [`error_handler_t::strict`](../basic_json/error_handler_t.md) instead
+of `error_handler_t::keep`. These writers then check every string value and object key for valid UTF-8 and throw
+[`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) for ill-formed UTF-8, like
+[`dump`](../basic_json/dump.md) does. Without it, they write the bytes unchanged. An `error_handler` passed explicitly
+always takes precedence.
+
+The macro does not affect:
+
+- [`to_msgpack`](../basic_json/to_msgpack.md): the MessagePack specification allows a `str` value to contain bytes that
+  are not valid UTF-8, so its `error_handler` always defaults to `keep`.
+- [`to_bon8`](../basic_json/to_bon8.md): BON8 always checks, because the UTF-8 lead bytes mark where a string ends.
+- The binary readers ([`from_cbor`](../basic_json/from_cbor.md), [`from_msgpack`](../basic_json/from_msgpack.md),
+  [`from_ubjson`](../basic_json/from_ubjson.md), [`from_bjdata`](../basic_json/from_bjdata.md),
+  [`from_bson`](../basic_json/from_bson.md)): none of these formats requires a decoder to reject ill-formed UTF-8, so
+  they always return the bytes unchanged.
+
+## Default definition
+
+The default value is `0` (disabled, the behavior of version 3.12.0 and earlier is preserved).
+
+```cpp
+#define JSON_STRICT_BINARY_UTF8 0
+```
+
+## Notes
+
+!!! note "Background"
+
+    CBOR, UBJSON, BJData, and BSON all require strings to be UTF-8. Up to version 3.12.0, the writers did not check
+    this, so they could produce output that other decoders reject. Checking by default would break code that stores
+    other encodings (for instance ISO 8859-1) in a string and only ever writes it to a binary format. You can pass
+    `error_handler_t::strict` to each call, or use this macro to check by default ahead of version 4.0.0, where
+    `strict` is planned to become the default (see
+    [#5529](https://github.com/nlohmann/json/issues/5529) and [#5651](https://github.com/nlohmann/json/issues/5651)).
+
+!!! warning "Opt-in only"
+
+    This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no
+    effect.
+
+!!! note "ABI compatibility"
+
+    The value of this macro is encoded in the [namespace](../../features/namespace.md) (tag `_sbu8`), resulting in
+    distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program
+    without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.
+
+## Examples
+
+??? example "Example: default behavior (macro not defined)"
+
+    Without the macro, the bytes are written unchanged:
+
+    ```cpp
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+        auto v = json::to_cbor(json("\xFF"));
+        // v is {0x61, 0xFF}
+    }
+    ```
+
+??? example "Example: opt-in check (macro defined to 1)"
+
+    With the macro, ill-formed UTF-8 is rejected:
+
+    ```cpp
+    #define JSON_STRICT_BINARY_UTF8 1
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+        auto v = json::to_cbor(json("\xFF"));
+        // throws type_error.316: invalid UTF-8 byte at index 0: 0xFF
+    }
+    ```
+
+## See also
+
+- [**to_cbor**](../basic_json/to_cbor.md) - create a CBOR serialization of a JSON value
+- [**to_ubjson**](../basic_json/to_ubjson.md) - create a UBJSON serialization of a JSON value
+- [**to_bjdata**](../basic_json/to_bjdata.md) - create a BJData serialization of a JSON value
+- [**to_bson**](../basic_json/to_bson.md) - create a BSON serialization of a JSON value
+- [**error_handler_t**](../basic_json/error_handler_t.md) - how [`dump`](../basic_json/dump.md) treats ill-formed UTF-8
+
+## Version history
+
+- Added in version 3.13.0.
+- Planned to become the default (with the macro removed) in version 4.0.0.