Always format JSON via Grape::Json.dump

Grape::Formatter::Json and Grape::Formatter::SerializableHash
short-circuited to object.to_json whenever the object responded to it.
Since nearly every object responds to to_json, Grape::Json.dump was
effectively never reached and a configured multi_json back-end (e.g. oj)
was bypassed during formatting.

Remove the to_json short-circuit so both formatters always serialize
through Grape::Json.dump, honoring the JSON back-end consistently with
Grape::ErrorFormatter::Json and Grape::Parser::Json.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ericproulx

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 #### Fixes
 
+* [#2769](https://github.com/ruby-grape/grape/pull/2769): Always format JSON via `Grape::Json.dump` so a configured `multi_json` back-end is honored - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.3.0 (2026-06-20)

README.md

@@ -3160,7 +3160,7 @@ end
 
 Built-in formatters are the following.
 
-* `:json`: use object's `to_json` when available, otherwise call `MultiJson.dump`
+* `:json`: serializes the object via Grape's JSON back-end (`JSON` by default, or `multi_json` when available)
 * `:xml`: use object's `to_xml` when available, usually via `MultiXml`
 * `:txt`: use object's `to_txt` when available, otherwise `to_s`
 * `:serializable_hash`: use object's `serializable_hash` when available, otherwise fallback to `:json`

UPGRADING.md

@@ -1,6 +1,14 @@
 Upgrading Grape
 ===============
 
+### Upgrading to >= 4.0
+
+#### The `:json` and `:serializable_hash` formatters no longer call `to_json`
+
+`Grape::Formatter::Json` and `Grape::Formatter::SerializableHash` previously short-circuited to `object.to_json` whenever the object responded to it. Since virtually every object responds to `to_json`, this meant `Grape::Json.dump` was effectively never reached and a configured `multi_json` back-end (e.g. `oj`) was bypassed during formatting. Both formatters now always serialize through `Grape::Json.dump`, so the JSON back-end is honored consistently (matching `Grape::ErrorFormatter::Json` and `Grape::Parser::Json`).
+
+If you relied on a custom `to_json` for response formatting, either register your own formatter or make `Oj.mimic_JSON` (or your serializer of choice) the active back-end.
+
 ### Upgrading to >= 3.3
 
 #### Minimum required Ruby is now 3.3

lib/grape/formatter/json.rb

@@ -4,8 +4,6 @@ module Grape
   module Formatter
     class Json < Base
       def self.call(object, _env)
-        return object.to_json if object.respond_to?(:to_json)
-
         ::Grape::Json.dump(object)
       end
     end

lib/grape/formatter/serializable_hash.rb

@@ -7,7 +7,6 @@ class << self
         def call(object, _env)
           return object if object.is_a?(String)
           return ::Grape::Json.dump(serialize(object)) if serializable?(object)
-          return object.to_json if object.respond_to?(:to_json)
 
           ::Grape::Json.dump(object)
         end