Add Optional “prerequisites” Hints to MCP Tools

[tybalex]

Pre-submission Checklist

• I have verified this would not be more appropriate as a feature request in a specific repository
• I have searched existing discussions to avoid duplicates

Your Idea

Summary

Introduce an optional prerequisites field in each tool’s descriptor to help agents to plan multi-step calls when one tool’s input might depends on another’s output—without changing server registration semantics.

Motivation & Example

I have the following use case that agents often miss that create_email tool needs a real email address when given only a name.

In my Outlook MCP server:

• Tools available
  • lookup_contact(name: str) → str
  • create_email(to: str, subject: str, body: str) → Email
  • other tools...

Desired flow:

• User: “Draft an email to John.”
• Agent sees create_email has prerequisites: ["lookup_contact"].
• It calls lookup_contact("John") → "john@example.com".
• Then calls create_email(to="john@example.com", …).

Embedding this hint in the tool metadata removes brittle prompt hacks and makes the intended sequence explicit.

Schema:

Json Schema change:

// in discovery response
{
  "name": "create_email",
  …,
  "prerequisites": ["lookup_contact"] // new field
}

Example Python SDK Usage

@mcp.tool(
  name="create_email",
  description="Send an email given full address.",
  prerequisites=["lookup_contact"] // new field
)
def create_email(to: str, subject: str, body: str) -> Email:
    …

When is this feature helpful

Medium-to-Large MCP Servers with 10-20+ tools with chains.

Backward compatibility

• Tools without prerequisites work exactly the same
• Old agents ignore the new field
• No startup-order or DAG checks are introduced

Leave a 👍 if you’d like to see this land

Scope

• Protocol Specification
• SDK Features
• Documentation
• Developer Experience
• Other

[tadasant]

There are two features already in the spec that should help with this:

• Server Instructions, which you could use to explain how tools can/should interact with each other at the server level on server initialization
• Tool Annotations, a field which is intentionally open ended (you can add your own prerequisites key). You could just add it there in the expectation that an LLM could infer what it means (perhaps would work well in combination with server instructions)