From a0b2de04b1ecec70a5b9e1c8e749405d5bc80710 Mon Sep 17 00:00:00 2001
From: Daniel Pepper <pepper.daniel@gmail.com>
Date: Fri, 10 Jul 2026 15:32:49 -0700
Subject: [PATCH] Fix inline fragments without a space before "on"

`...on Type` (no space) is valid GraphQL for an inline fragment, but
Client#parse's fragment-spread regex matched it as a spread
referencing a Ruby constant named `On`, since it only excluded
whitespace between the dots and the name, not the specific "on"
keyword. Adds a negative lookahead: GraphQL forbids naming a fragment
"on" (FragmentName : Name but not "on"), so "...on" followed by a
word boundary is always an inline fragment, never ambiguous with a
constant reference.
---
 lib/graphql/client.rb |  8 ++++++--
 test/test_client.rb   | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/lib/graphql/client.rb b/lib/graphql/client.rb
index dcc4b0c..7cd3014 100644
--- a/lib/graphql/client.rb
+++ b/lib/graphql/client.rb
@@ -153,8 +153,12 @@ def parse(str, filename = nil, lineno = nil)
 
       # Replace Ruby constant reference with GraphQL fragment names,
       # while populating `definition_dependencies` with
-      # GraphQL Fragment ASTs which this operation depends on
-      str = str.gsub(/\.\.\.([a-zA-Z0-9_]+(::[a-zA-Z0-9_]+)*)/) do
+      # GraphQL Fragment ASTs which this operation depends on.
+      #
+      # GraphQL forbids naming a fragment "on", so "...on" followed by a word
+      # boundary is unambiguously an inline fragment, never a spread
+      # referencing constant "On".
+      str = str.gsub(/\.\.\.(?!on\b)([a-zA-Z0-9_]+(::[a-zA-Z0-9_]+)*)/) do
         match = Regexp.last_match
         const_name = match[1]
 
diff --git a/test/test_client.rb b/test/test_client.rb
index da38ff7..d879048 100644
--- a/test/test_client.rb
+++ b/test/test_client.rb
@@ -159,6 +159,40 @@ def test_client_parse_anonymous_query
     assert_equal(query_string, Temp::UserQuery.document.to_query_string)
   end
 
+  def test_client_parse_inline_fragment_without_space_before_on
+    Temp.const_set :NodeQuery, @client.parse(<<-'GRAPHQL')
+      query {
+        node(id: "1") {
+          id
+          ...on User {
+            name
+          }
+          ... on Organization {
+            name
+          }
+        }
+      }
+    GRAPHQL
+
+    query_string = <<-'GRAPHQL'.gsub(/^      /, "").chomp
+      query TestClient__Temp__NodeQuery {
+        node(id: "1") {
+          __typename
+          id
+          ... on User {
+            name
+          }
+          ... on Organization {
+            name
+          }
+        }
+      }
+    GRAPHQL
+
+    assert_equal(query_string, @client.document.to_query_string)
+    assert_equal(query_string, Temp::NodeQuery.document.to_query_string)
+  end
+
   def test_client_parse_query_document
     Temp.const_set :UserDocument, @client.parse(<<-'GRAPHQL')
       query GetUser {