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MCP Single Value Requirement

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Erroneous Code Example

This error occurs when an MCP (Model Context Protocol) query returns multiple values when only a single value is expected. In this example, the MCP query returns multiple users, but MCP expects a single value. This is so the agent using the MCP tool can traverse from the return value of the query. 
#[mcp]
Query getUsers(userId: ID, postId: ID) =>
    user <- N<User>(userId)
    post <- N<Post>(postId)
    RETURN user, post

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Solution

Ensure MCP queries return exactly one value. 
#[mcp]
Query getUsers(userId: ID, postId: ID) =>
    user <- N<User>(userId)
    post <- N<Post>(postId)
    RETURN user