Client treats JSON-null structuredContent as missing, skipping outputSchema validation

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

On current main (57394b0548d1e2dc2dce8d67d84985769df3b8bb), ClientSession.validate_tool_result treats structured_content is None as "the tool did not return structured content".

That collapses two different wire shapes:

1. omitted structuredContent (field absent)
2. explicit JSON null ("structuredContent": null)

SEP-2106 / spec 2026-07-28 allow structuredContent to be any JSON value, including null. The TypeScript SDK already checks === undefined (not falsy / not null) for this reason.

Pydantic stores both omitted and JSON null as None. model_fields_set distinguishes them: a CallToolResult parsed from {"content": [], "structuredContent": null} has "structured_content" in model_fields_set, while an omitted field does not.

What happens today

• Tool advertises "outputSchema": {"type": "null"} (or {"type": ["object", "null"], ...}).
• Server returns "structuredContent": null.
• Client raises Tool {name} has an output schema but did not return structured content and never runs jsonschema against the value.

What I expected

• Omitted structuredContent still raises the existing missing-field error.
• Explicit JSON null is validated against the advertised schema: accept if the schema allows null, reject as a schema mismatch if it does not.
• Falsy JSON values (0, false, "") stay validated (they already are, because the current check is is None rather than falsy).

This is not #3224 (server injecting nulls for NotRequired keys). That issue is about serializing omitted object keys as null. This one is the client presence check before outputSchema validation.

I hit this while checking official SDK conformance of declared outputSchema against structuredContent. I have a small backwards-compatible test and fix ready and would like to send the PR if a maintainer wants it.

AI assistance: researched and drafted with Grok 4.6; I reviewed the spec text, the TypeScript v2 presence check, and the Pydantic model_fields_set behavior before filing.

Example Code

from mcp_types import CallToolResult

omitted = CallToolResult.model_validate({"content": []})
explicit_null = CallToolResult.model_validate({"content": [], "structuredContent": None})

assert omitted.structured_content is None
assert explicit_null.structured_content is None
assert "structured_content" not in omitted.model_fields_set
assert "structured_content" in explicit_null.model_fields_set

Against a tool whose outputSchema is {"type": "null"}, validate_tool_result currently raises the missing-field RuntimeError for explicit_null. After a presence check that uses model_fields_set, that result validates.

Python & MCP Python SDK

• Python 3.12
• MCP Python SDK main at 57394b0548d1e2dc2dce8d67d84985769df3b8bb (2.x)

从 ClientSession.validate_tool_result 开始，检查 CallToolResult.model_fields_set 如何区分被省略的 structuredContent 与显式的 JSON null。为允许 null 的 outputSchema 添加或更新一个有针对性的验证测试，同时保留针对被省略内容的缺少字段错误；运行相关的 SDK 测试套件，以确认 schema 不匹配仍会失败。