fix(jsonschema): root variables object is always a non-nullable "object"

[asoorm]

Follow-up to #1518. That PR was correct but incomplete and exposed an old defect.

Problem

GetSchema() only forced the root variables object non-nullable when the operation had a required variable:

if len(v.schema.Required) > 0 {
    v.schema.Nullable = false
}

Operations with all-optional variables kept a nullable root. Before #1518 that serialized to {"type":"object","nullable":true} and was harmless (validators ignore the unknown "nullable" keyword). After #1518 it serializes to {"type":["object","null"]}, which strict consumers reject - the MCP go-sdk's AddTool requires the input schema type to be exactly "object" and panics:

panic: AddTool "list_employees": input schema must have type "object" (got [object null])

This breaks the cosmo router engine bump 2.4.2 -> 2.4.3 (wundergraph/cosmo#2925): pkg/mcpserver, protocol and security suites all panic.

Fix

The root variables object is always a concrete object - the container is present or omitted, never the JSON literal null - so set Nullable = false unconditionally in GetSchema(). This is consistent with nested input-object variables, which are already forced non-nullable. Only individual optional fields remain nullable.

Golden tests updated (root ["object","null"] -> "object"); the root_schema_nullable_based_on_required_arguments subtest renamed to root schema is always a non-nullable object.

Tests

go test ./pkg/engine/jsonschema/ passes; gofmt and go vet clean. Verified against the cosmo router via a local replace: pkg/mcpserver no longer panics.

Follow-up

Needs a patch release (2.4.4) and a corresponding engine bump in cosmo#2925.

Fixes ENG-9682

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📝 Walkthrough

Walkthrough

The root variables JSON schema is now unconditionally non-nullable in the GetSchema method, removing prior conditional logic tied to required fields. All corresponding test expectations are updated to verify the root schema type as a strict "object" literal across multiple test scenarios.

Changes

Root Variables Schema Non-Nullable Enforcement

Layer / File(s)	Summary
GetSchema non-nullable enforcement v2/pkg/engine/jsonschema/variables_schema.go	GetSchema now unconditionally sets Nullable=false on the root variables schema, replacing the prior conditional check on len(v.schema.Required). Inline comments clarify that the variables container is either present or omitted and must have type exactly "object" for strict consumers.
Test expectations and validation v2/pkg/engine/jsonschema/variables_schema_test.go	Test expected JSON outputs across multiple test cases—simple query, nullable/non-nullable fields, optional arguments, top-level fields, and custom scalar types—are updated to expect root schema type: "object" instead of ["object","null"]. Clarifying comments added for optional argument cases and a new test subtest scaffold introduced.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• wundergraph/graphql-go-tools#1513: Related modifications to variables schema emission and root schema construction for recursive inputs in the same files.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: making the root variables object always non-nullable with type "object".
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description clearly explains the problem (root variables object incorrectly nullable for all-optional-variable operations), the root cause, the fix applied, and test validation.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ahmet/eng-9682-jsonschema-root-variables-object-must-always-be-a-non

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