fix: omit structured_content when None instead of serializing as null #6499
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The MCP spec defines structuredContent as optional (field may be absent), not nullable. Zod's .optional() schema accepts undefined but rejects null. Without skip_serializing_if, serde serializes Option::None as null, causing Zod validation errors in MCP Apps when calling tools. Fixes #6454
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Pull request overview
This PR fixes a serialization issue where structured_content in MCP tool responses was being serialized as null instead of being omitted when absent, causing Zod validation errors in MCP Apps.
Changes:
- Added
#[serde(skip_serializing_if = "Option::is_none")]to thestructured_contentfield inCallToolResponseto align with the MCP spec
The-Best-Codes
approved these changes
Jan 14, 2026
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Summary
The MCP spec defines
structuredContentas optional (field may be absent), not nullable:Without
skip_serializing_if, serde serializesOption::Noneasnullin JSON, but Zod's.optional()schema acceptsundefined(field absent) and rejectsnull.This was causing Zod validation errors in MCP Apps when calling tools that don't return structured content.
Changes
Added
#[serde(skip_serializing_if = "Option::is_none")]tostructured_contentfield inCallToolResponseso the field is omitted from JSON whenNone, aligning with the MCP spec.Related
Fixes #6454
See: #6454 (comment)