structured_content validate: Error: Unexpected response type

[cougarwww]

rust-sdk/crates/rmcp/src/model.rs

Lines 1265 to 1272 in 984cd87

	/// Validate that content and structured_content are mutually exclusive
	pub fn validate(&self) -> Result<(), &'static str> {
	match (&self.content, &self.structured_content) {
	(Some(_), Some(_)) => Err("content and structured_content are mutually exclusive"),
	(None, None) => Err("either content or structured_content must be provided"),
	_ => Ok(()),
	}
	}

use python/fastmcp will return both content and structured_content

Result: 
 - CallToolResult {
    content: Some(
        [
            Annotated {
                raw: Text(
                    RawTextContent {
                        text: "2025-08-06 14:24:47",
                    },
                ),
                annotations: None,
            },
        ],
    ),
    structured_content: Some(
        Object {
            "result": String("2025-08-06 14:24:47"),
        },
    ),
    is_error: Some(
        false,
    ),
}

[crepererum]

I think the assumption in CallToolResult is wrong:

rust-sdk/crates/rmcp/src/model.rs

Line 1185 in 984cd87

/// Note: `content` and `structured_content` are mutually exclusive - exactly one must be provided.

The MCP spec states:

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

This issue affects the client -- where the validation fails as #355 (comment) illustrates -- and the server (at least the example that should probably follow the recommendation and return both).

[michaelneale]

we hit this also in goose - with some python MCP servers. If we patch the server to only return serialized json it works, but I expect will cause others to fail which can't be patched as quickly.

[michaelneale]

patch for this: #357 thanks @crepererum @cougarwww that suggestion worked in my case (when I patched it).

[michaelneale]

this is fixed now - can close it I think @cougarwww, nice work!

[cougarwww]

Thinks all for the nice work!