Adds Input With Missing Required Fields

spec/Section 4 -- Composition.md

@@ -14,6 +14,61 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Input With Different Fields
+
+**Error Code**
+
+INPUT_OBJECT_FIELDS_DIFFER
+
+**Formal Specification**
+
+- Let {inputsByName} be a map where the key is the name of an input object type, and the value is a list of all input object types from different source schemas with that name.
+- For each {listOfInputs} in {inputsByName}:
+  - {InputFieldsAreMergeable(listOfInputs)} must be true.
+
+InputFieldsAreMergeable(inputs):
+
+- Let {fields} be the set of all field names of the first input object in {inputs}.
+- For each {input} in {inputs}:
+  - Let {inputFields} be the set of all field names of {input}.
+  - {fields} must be equal to {inputFields}.
+
+**Explanatory Text**
+
+This rule ensures that input object types with the same name across different subgraphs have identical sets of field names. 
+Consistency in input object fields across subgraphs is required to avoid conflicts and ambiguities in the composed schema. 
+This rule only checks that the field names are the same, not that the field types are the same. 
+Field types are checked by the [Input Field Types mergeable](#sec-Input-Field-Types-mergeable) rule.
+
+When an input object is defined with differing fields across subgraphs, it can lead to issues in query execution. 
+A field expected in one subgraph might be absent in another, leading to undefined behavior. 
+This rule prevents such inconsistencies by enforcing that all instances of the same named input object across subgraphs have a matching set of field names.
+
+In this example, both subgraphs define `Input1` with the same field `field1`, satisfying the rule:
+
+```graphql example
+input Input1 {
+  field1: String
+}
+
+input Input1 {
+  field1: String
+}
+```
+
+Here, the two definitions of `Input1` have different fields (`field1` and `field2`), violating the rule:
+
+```graphql counter-example
+input Input1 {
+  field1: String
+}
+
+input Input1 {
+  field2: String
+}
+```
+
+
 ### Merge
 
 ### Post Merge Validation