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`#[tool(description = …)]` accepts only a bare literal, while `#[doc]` and `#[schemars]` on the same items accept `include_str!` and `const` paths

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
62/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
rust
Domain
api, tooling

Research direction

The report names no implementation files or tests; begin by locating the #[tool] attribute parser and its validation for description. Add coverage for a &'static str const path and concat!, preserve literal behavior, and verify the generated description plus the documented or error behavior match the request.

Written by the indexing model from the issue text.

Description

P3

Summary

description in #[tool] accepts only a bare string literal. Two attributes that sit on the very same items are more permissive: #[doc] on the function accepts include_str! and that value does reach the published description, and #[schemars(extend(…))] on the argument struct accepts const paths.

So a server can already source a tool's published text from outside the attribute — through #[doc], or through a literal handed in by a wrapper macro — just not through description, which is the field that names it. Accepting a const path and concat! there would remove the inconsistency.

This is an enhancement request, not a blocker: the workarounds below all work.

Version

rmcp 3.1.2, rustc 1.96.1, edition 2024.

What was measured

form result
#[tool(description = "…")] works
#[tool(description = my_crate::caps::TEXT)] error: Unexpected type \path``
#[tool(description = concat!("a", "b"))] error: Unexpected type \macro``
/// doc comment + #[tool] works — becomes the description
#[doc = include_str!("text.md")] + #[tool] works — becomes the description
#[schemars(extend("maximum" = MOST_STARTS_AT_ONCE))] on the args struct works — a const path, in the same attribute stack
doc comment and description together description wins (no surprise here)
macro_rules! wrapping the whole #[tool_router] impl works
macro_rules! wrapping only the fn inside the impl compiles, registers nothing — filed separately

The schemars row is the one that makes this feel like an oversight rather than a decision: a const is already acceptable one attribute away, for a value that is likewise baked into the published schema.

Why it matters on a larger surface

On a server with many tools the published text is an interface, and it usefully has two halves: a short one broadcast in tools/list before the first call, and a longer one fetched on demand. Ours measures 82 953 B of description across 66 tools, against a 13 590 B fetchable handbook — so keeping the split is worth real bytes, and it is far easier to keep the two halves honest when both can be declared together and the short one referenced from the attribute:

// what one would like to write
const GET_OR_CREATE_PROJECT: Capability = Capability {
    broadcast: "…what prevents a wrong call…",
    taught:    "…the full explanation, fetched on demand…",
};

#[tool(description = GET_OR_CREATE_PROJECT.broadcast)]

Today that requires either moving the text into a file and using #[doc = include_str!(…)], or keeping a second copy plus a test comparing it to the literal.

Suggested change

Accept in description what neighbouring attributes already accept: a path to a const &'static str, and concat!. The generated code stores a &'static str either way.

If the literal is required because the value is needed during macro expansion, a line saying so in the docs — and an error message reading "only a string literal is accepted here" rather than naming the token type — would save the next person the experiment.

What this report does not establish

It does not claim the current behaviour is wrong, only that it is inconsistent with #[doc] and #[schemars] on the same items. name was not tested against a non-literal; only description was.

Dominant language
Rust
Stars
4k
Forks
654
Avg merge
4d 2h
Merged PRs (30d)
40

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