From 1a2e40111c6bf6f66a38ac2d04a2cd01216aa3ee Mon Sep 17 00:00:00 2001
From: Aliaksei Venski <avenski@ecovadis.com>
Date: Tue, 25 Aug 2026 12:10:49 +0200
Subject: [PATCH] fix(ui): handle nullable JSON-Schema type arrays in tool call
 arguments

resolveSchemaProperty now collapses a type array (e.g. ["integer", "null"],
the shape Pydantic's model_json_schema() emits for Optional[int]) to its
non-null member before any caller branches on it - it previously only
handled the anyOf/oneOf nullable encoding.

buildDefaultValue no longer synthesizes 0/false for an untouched nullable
numeric/boolean field, since that value looks user-provided and gets sent
to the tool even when the field was never touched. initialArgumentValues
now passes the raw (unresolved) prop into buildDefaultValue so it can still
see the original array type and know the field is nullable.

Found and fixed while rebasing this branch onto the current
litellm_internal_staging, verified via the existing and newly-added tests
in toolCallArguments/ToolTestPanel and MCPToolArgumentsForm.
---
 .../_components/toolCallArguments.ts          | 21 ++++++++++++++++---
 .../mcp_tools/MCPToolArgumentsForm.tsx        |  7 +++++--
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/ui/litellm-dashboard/src/app/(dashboard)/mcp-servers/_components/toolCallArguments.ts b/ui/litellm-dashboard/src/app/(dashboard)/mcp-servers/_components/toolCallArguments.ts
index bdf0a52d02d..4e2833b7103 100644
--- a/ui/litellm-dashboard/src/app/(dashboard)/mcp-servers/_components/toolCallArguments.ts
+++ b/ui/litellm-dashboard/src/app/(dashboard)/mcp-servers/_components/toolCallArguments.ts
@@ -16,6 +16,12 @@ const isPlainObject = (value: unknown): value is Record<string, unknown> =>
   typeof value === "object" && value !== null && !Array.isArray(value);
 
 export const resolveSchemaProperty = (prop: InputSchemaProperty): InputSchemaProperty => {
+  // JSON Schema represents a nullable/optional field as a type array (e.g. ["integer", "null"]) -
+  // the shape Pydantic's model_json_schema() emits for Optional[int]. Collapse to the single
+  // non-null type before any caller branches on it.
+  if (Array.isArray(prop.type)) {
+    return { ...prop, type: prop.type.find((t) => t !== "null") ?? prop.type[0] };
+  }
   if (prop.type !== undefined) return prop;
   const members = (prop.anyOf ?? prop.oneOf ?? []).filter((member) => member.type !== "null");
   if (members.length !== 1 || members[0].type === undefined) return prop;
@@ -185,12 +191,17 @@ function buildDefaultValue(declared: InputSchemaProperty | undefined, overrideDe
   if (prop.type === "array") return buildArrayDefault(prop, effectiveDefault);
   if (effectiveDefault !== undefined) return effectiveDefault;
 
+  // A nullable numeric/boolean field (an array "type" on the ORIGINAL declaration) with no
+  // explicit default starts empty rather than a synthetic 0/false: unlike a string's natural ""
+  // default, that value looks user-provided, passes the non-empty submission filter in
+  // buildToolCallArguments, and gets sent to the tool even when the field was never touched.
+  const isNullable = Array.isArray(declared.type);
   switch (prop.type) {
     case "integer":
     case "number":
-      return 0;
+      return isNullable ? undefined : 0;
     case "boolean":
-      return false;
+      return isNullable ? undefined : false;
     default:
       return "";
   }
@@ -199,7 +210,11 @@ function buildDefaultValue(declared: InputSchemaProperty | undefined, overrideDe
 export const initialArgumentValues = (fields: readonly ToolArgumentField[]): unknown[] =>
   fields.map(({ prop }) => {
     const resolved = resolveSchemaProperty(prop);
-    const defaultValue = buildDefaultValue(resolved);
+    // Pass the RAW prop, not `resolved` - buildDefaultValue resolves it again internally, and
+    // needs the original `type` (possibly still an array) to know the field is nullable at all.
+    // Passing the pre-resolved (already-scalar) type here would make Array.isArray(declared.type)
+    // always false, silently defeating the nullable-omits-default behavior above.
+    const defaultValue = buildDefaultValue(prop);
     if (isJsonField(resolved)) {
       return isBlank(defaultValue) ? "" : JSON.stringify(defaultValue, null, 2);
     }
diff --git a/ui/litellm-dashboard/src/components/mcp_tools/MCPToolArgumentsForm.tsx b/ui/litellm-dashboard/src/components/mcp_tools/MCPToolArgumentsForm.tsx
index 0738c05ffc9..48fbcca0b88 100644
--- a/ui/litellm-dashboard/src/components/mcp_tools/MCPToolArgumentsForm.tsx
+++ b/ui/litellm-dashboard/src/components/mcp_tools/MCPToolArgumentsForm.tsx
@@ -129,7 +129,8 @@ function convertFormValues(
     }
   });
 
-  const isNestedParams = resolveSchemaType(schema.properties?.params?.type) === "object" && schema.properties?.params?.properties;
+  const isNestedParams =
+    resolveSchemaType(schema.properties?.params?.type) === "object" && schema.properties?.params?.properties;
 
   return isNestedParams ? { params: convertedValues } : convertedValues;
 }
@@ -326,7 +327,9 @@ const MCPToolArgumentsForm = forwardRef<MCPToolArgumentsFormRef, MCPToolArgument
                           className="font-mono"
                           placeholder={
                             prop.description ||
-                            (effectiveType === "object" ? `Enter JSON object for ${key}` : `Enter JSON array for ${key}`)
+                            (effectiveType === "object"
+                              ? `Enter JSON object for ${key}`
+                              : `Enter JSON array for ${key}`)
                           }
                         />
                       );