Send a content type for an api_format the API does not declare

CHANGELOG.md

@@ -36,6 +36,7 @@
 * [#2953](https://github.com/ruby-grape/grape/pull/2953): Answer an `error!` String message with its own status instead of a `500` when the route names a `failure` entity for that status - [@ericproulx](https://github.com/ericproulx).
 * [#2955](https://github.com/ruby-grape/grape/pull/2955): Reject a value that is not an Array, and a member of none of the types, in a collection with multiple member types instead of passing them on as `nil` and `InvalidValue` - [@ericproulx](https://github.com/ericproulx).
 * [#2957](https://github.com/ruby-grape/grape/pull/2957): Reject an element that is not a Hash in an Array param given a block, and stop `declared` raising on a value that is not a Hash where nested params are declared - [@ericproulx](https://github.com/ericproulx).
+* [#2959](https://github.com/ruby-grape/grape/pull/2959): Send a content type for an `api_format` the API does not declare instead of a `nil` header - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 4.0.1 (2026-09-15)

lib/grape/middleware/formatter.rb

@@ -147,12 +147,16 @@ def fetch_formatter(headers)
       # this runs on every response, and copying the hash costs an allocation
       # per request for a header the caller is about to send anyway.
       #
+      # A format with no media type at all (see #media_type_for) leaves the
+      # header out: Rack forbids a nil header value.
+      #
       # @param headers [Hash] the response headers, mutated in place
       # @return [void]
       def ensure_content_type!(headers)
         return if headers[Rack::CONTENT_TYPE]
 
-        headers[Rack::CONTENT_TYPE] = content_type_for(env[Grape::Env::API_FORMAT])
+        content_type = media_type_for(env[Grape::Env::API_FORMAT])
+        headers[Rack::CONTENT_TYPE] = content_type if content_type
       end
 
       def read_body_input

lib/grape/middleware/precomputed_content_types.rb

@@ -4,7 +4,7 @@ module Grape
   module Middleware
     # Include in a middleware subclass that needs content-type negotiation.
     # Provides +content_types+ / +mime_types+ / +content_type_for+ /
-    # +content_type+ resolved from +config.content_types+ and
+    # +media_type_for+ / +content_type+ resolved from +config.content_types+ and
     # +config.format+ — so the consuming middleware's +Options+ Data class
     # must declare both fields. Warms those caches on the parent instance
     # at initialization so per-request +dup+s inherit them rather than
@@ -19,6 +19,9 @@ module Middleware
     # Opt-in: plain +Grape::Middleware::Base+ subclasses that don't need
     # content-type-aware helpers don't pay for them.
     module PrecomputedContentTypes
+      # Grape's own media type for each built-in format, under both spellings.
+      DEFAULT_CONTENT_TYPES = Grape::ContentTypes.lookup_for(Grape::ContentTypes::DEFAULTS)
+
       def initialize(app, **options)
         super
         content_types
@@ -37,8 +40,17 @@ def content_type_for(format)
         content_types_lookup[format]
       end
 
+      # The media type to label a response rendered in +format+ with: the one
+      # this API declares for it, or else Grape's own -- +api_format :txt+ in
+      # a +format :json+ API renders text/plain. #content_type_for stays nil
+      # for a format the API does not declare, since negotiation asks it
+      # whether the API supports the format at all.
+      def media_type_for(format)
+        content_type_for(format) || DEFAULT_CONTENT_TYPES[format]
+      end
+
       def content_type
-        content_type_for(env[Grape::Env::API_FORMAT] || config.format) || 'text/html'
+        media_type_for(env[Grape::Env::API_FORMAT] || config.format) || 'text/html'
       end
 
       private

spec/grape/api_spec.rb

@@ -4775,6 +4775,41 @@ def before
       end
     end
 
+    context ':json only' do
+      before { subject.format :json }
+
+      it "sends Grape's media type for an explicit api_format the API does not declare" do
+        subject.get '/plain' do
+          api_format :txt
+          'plain'
+        end
+        get '/plain'
+        expect(last_response.headers['Content-Type']).to eq('text/plain')
+        expect(last_response.body).to eq('plain')
+      end
+
+      it 'leaves the content type out for an explicit api_format with no media type' do
+        subject.formatter :csv, ->(object, _env) { object.to_s }
+        subject.get '/csv' do
+          api_format :csv
+          'a,b'
+        end
+        get '/csv'
+        expect(last_response.headers).not_to have_key('Content-Type')
+        expect(last_response.body).to eq('a,b')
+      end
+
+      it 'renders an error after an explicit api_format the API does not declare in its media type' do
+        subject.get '/plain_error' do
+          api_format :txt
+          error!('a < b', 400)
+        end
+        get '/plain_error'
+        expect(last_response.headers['Content-Type']).to eq('text/plain')
+        expect(last_response.body).to eq('a < b')
+      end
+    end
+
     context ':serializable_hash' do
       before do
         stub_const(