modelcontextprotocol/quickstart-resources
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Add structured output to the Rust weather server and client

Browse source 
Both tools declare an output_schema and return structured_content alongside
the text.

get_alerts declares Vec<Alert>, so it answers with a top-level JSON array
rather than an array nested in an object, which protocol revision 2026-07-28
is the first to allow. "No alerts" is simply []. get_forecast returns an
object, for contrast.

Error paths return a tool-level error: a tool declaring an output_schema MUST
return conforming structured content, so a path with no data has to fail.

The client compiles every declared outputSchema at connect time and validates
results against it. rmcp does not do this; its docs point at the jsonschema
crate, which is the one new dependency here. Each channel goes to its stated
reader: content is forwarded to the model, structured_content is used as data,
reporting how many items came back. A missing .env is no longer fatal, and the
API key is checked after connecting rather than before.

This supersedes #143, which bumped rmcp 0.3 -> 1.4 and failed CI because the
0.3 #[tool] macro API does not survive the jump. The rewrite was required
either way, so the bump is folded in and taken to 3.0.0-beta.2, the first
version with the 2026-07-28 model.

list_tools is hand-written. #[tool_handler] generates one hardcoding
ttl_ms: None, cache_scope: None, but both are required on a paginated result
at 2026-07-28 (SEP-2549) and a strict client rejects the response outright.
get_info does not pin ProtocolVersion::V_2026_07_28 either: rmcp's server
never implements server/discover, so pinning would overstate support.

Model identifier moves to claude-sonnet-5, replacing claude-sonnet-4-20250514,
which is past end of life.

Note that rmcp sends an array-rooted schema as written on every connection
rather than projecting it down, so an older client rejects the tool list.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
olaservo 2026-07-25 17:56:06 -07:00
commit 65d1a92cf8
8 changed files with 761 additions and 207 deletions
Download patch file Download diff file Expand all files Collapse all files

518
mcp-client-rust/Cargo.lock generated
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"synstructure", "synstructure",
] ]
@ -2662,5 +2982,5 @@ checksum = "5b96237efa0c878c64bd89c436f661be4e46b2f3eff1ebb976f7ef2321d2f58f"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.106",
] ]

3
mcp-client-rust/Cargo.toml
View file

@ -6,7 +6,8 @@ edition = "2024"
[dependencies] [dependencies]
anyhow = "1.0.100" anyhow = "1.0.100"
genai = "0.4.2" genai = "0.4.2"
rmcp = { version = "0.8.0", features = ["server", "client", "transport-io", "transport-child-process"] } jsonschema = "0.35"
rmcp = { version = "3.0.0-beta.2", features = ["server", "client", "transport-io", "transport-child-process"] }
tokio = { version = "1.47.1", features = ['full']} tokio = { version = "1.47.1", features = ['full']}
tracing = "0.1.41" tracing = "0.1.41"
tracing-subscriber = {version = "0.3", features = ["env-filter"]} tracing-subscriber = {version = "0.3", features = ["env-filter"]}

6
mcp-client-rust/README.md
View file

@ -1,3 +1,9 @@
# An LLM-Powered Chatbot MCP Client written in Rust # An LLM-Powered Chatbot MCP Client written in Rust
See the [Build an MCP client](https://modelcontextprotocol.io/docs/develop/build-client) tutorial for more information. See the [Build an MCP client](https://modelcontextprotocol.io/docs/develop/build-client) tutorial for more information.
## Structured output
rmcp does not validate tool output, so this client compiles each declared `outputSchema` at connect time and checks results against it — the spec's client-side SHOULD. It uses the [`jsonschema`](https://docs.rs/jsonschema) crate, which rmcp's own documentation recommends.
The two channels go to different readers: `content` is forwarded to the model, while `structured_content` is used as data — the client counts the items it returns. See [Structured Content](https://modelcontextprotocol.io/specification/draft/server/tools#structured-content).

83
mcp-client-rust/src/main.rs
View file

@ -3,19 +3,24 @@ use genai::Client;
use genai::chat::{ use genai::chat::{
ChatMessage, ChatRequest, ChatResponse, ContentPart, Tool as GenaiTool, ToolResponse, ChatMessage, ChatRequest, ChatResponse, ContentPart, Tool as GenaiTool, ToolResponse,
}; };
use rmcp::model::{CallToolRequestParam, Tool as McpTool}; use jsonschema::Validator;
use rmcp::model::{CallToolRequestParams, CallToolResult, Tool as McpTool};
use rmcp::service::{RoleClient, RunningService, ServiceExt}; use rmcp::service::{RoleClient, RunningService, ServiceExt};
use rmcp::transport::TokioChildProcess; use rmcp::transport::TokioChildProcess;
use serde_json::Value; use serde_json::Value;
use std::collections::HashMap;
use tokio::io::{self, AsyncBufReadExt, BufReader}; use tokio::io::{self, AsyncBufReadExt, BufReader};
use tokio::process::Command; use tokio::process::Command;
const MODEL_ANTHROPIC: &str = "claude-sonnet-4-20250514"; const MODEL_ANTHROPIC: &str = "claude-sonnet-5";
struct MCPClient { struct MCPClient {
anthropic: Client, anthropic: Client,
session: Option<RunningService<RoleClient, ()>>, session: Option<RunningService<RoleClient, ()>>,
tools: Vec<GenaiTool>, tools: Vec<GenaiTool>,
/// Compiled `outputSchema` per tool name. rmcp does not validate results,
/// so this client does it with the `jsonschema` crate.
output_schemas: HashMap<String, Validator>,
} }
impl MCPClient { impl MCPClient {
@ -24,9 +29,28 @@ impl MCPClient {
anthropic: Client::default(), anthropic: Client::default(),
session: None, session: None,
tools: Vec::new(), tools: Vec::new(),
output_schemas: HashMap::new(),
}) })
} }
/// Check a result against its tool's declared `outputSchema`. Error results
/// are exempt: they carry a message, not data.
fn validate_tool_output(&self, name: &str, result: &CallToolResult) -> Result<()> {
let Some(validator) = self.output_schemas.get(name) else {
return Ok(());
};
if result.is_error.unwrap_or(false) {
return Ok(());
}
let Some(structured) = &result.structured_content else {
bail!("Tool {name} declares an output schema but returned no structured content");
};
if let Err(error) = validator.validate(structured) {
bail!("Structured content from tool {name} does not match its output schema: {error}");
}
Ok(())
}
async fn connect_to_server(&mut self, server_args: &[String]) -> Result<()> { async fn connect_to_server(&mut self, server_args: &[String]) -> Result<()> {
if self.session.is_some() { if self.session.is_some() {
bail!("Client is already connected to a server"); bail!("Client is already connected to a server");
@ -52,6 +76,18 @@ impl MCPClient {
println!("Connected to server with tools: {tool_names:?}"); println!("Connected to server with tools: {tool_names:?}");
// An outputSchema root may be any JSON Schema, not just an object.
for tool in &rmcp_tools {
let Some(schema) = &tool.output_schema else {
continue;
};
let schema = Value::Object(schema.as_ref().clone());
let validator = Validator::new(&schema).with_context(|| {
format!("Failed to compile output schema of tool {}", tool.name)
})?;
self.output_schemas.insert(tool.name.to_string(), validator);
}
self.tools = convert_tools(&rmcp_tools); self.tools = convert_tools(&rmcp_tools);
self.session = Some(session); self.session = Some(session);
Ok(()) Ok(())
@ -93,16 +129,33 @@ impl MCPClient {
)); ));
// Query the MCP server // Query the MCP server
let mut params = CallToolRequestParams::new(tool_call.fn_name.clone());
if let Some(arguments) = tool_call.fn_arguments.as_object().cloned() {
params = params.with_arguments(arguments);
}
let tool_result = session let tool_result = session
.call_tool(CallToolRequestParam { .call_tool(params)
name: tool_call.fn_name.clone().into(),
arguments: tool_call.fn_arguments.as_object().cloned(),
})
.await .await
.with_context(|| format!("Tool call {} failed", tool_call.fn_name))?; .with_context(|| format!("Tool call {} failed", tool_call.fn_name))?;
let payload = serde_json::to_string(&tool_result) self.validate_tool_output(&tool_call.fn_name, &tool_result)?;
.context("Failed to serialize tool result")?;
// structured_content is data the application can use directly.
if let Some(Value::Array(items)) = &tool_result.structured_content {
final_text.push(format!(
"[{} returned {} items]",
tool_call.fn_name,
items.len()
));
}
// content is a list of block types; forward only the text ones.
let payload = tool_result
.content
.iter()
.filter_map(|block| block.as_text().map(|text| text.text.as_str()))
.collect::<Vec<_>>()
.join("\n");
tool_results.push(ContentPart::ToolResponse(ToolResponse::new( tool_results.push(ContentPart::ToolResponse(ToolResponse::new(
tool_call.call_id.clone(), tool_call.call_id.clone(),
@ -179,7 +232,8 @@ impl MCPClient {
#[tokio::main] #[tokio::main]
async fn main() -> Result<()> { async fn main() -> Result<()> {
dotenvy::dotenv().context("Failed to load env file")?; // .env is optional; the key may come from the environment instead.
let _ = dotenvy::dotenv();
let mut args = std::env::args(); let mut args = std::env::args();
let _ = args.next(); let _ = args.next();
@ -194,6 +248,17 @@ async fn main() -> Result<()> {
let result = async { let result = async {
client.connect_to_server(&server_args).await?; client.connect_to_server(&server_args).await?;
// Connecting and listing tools needs no credentials; querying them
// does. Matching the Python and TypeScript clients, report and exit
// rather than failing, so the connection itself can be exercised
// without a key.
if std::env::var("ANTHROPIC_API_KEY").is_err() {
println!("\nNo ANTHROPIC_API_KEY found. To query these tools with Claude, set your API key:");
println!(" export ANTHROPIC_API_KEY=your-api-key-here");
return Ok(());
}
client.chat_loop().await client.chat_loop().await
} }
.await; .await;

127
weather-server-rust/Cargo.lock generated
View file

@ -47,6 +47,17 @@ version = "1.0.98"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e16d2d3311acee920a9eb8d33b8cbc1787ce4a264e85f964c2404b969bdcd487" checksum = "e16d2d3311acee920a9eb8d33b8cbc1787ce4a264e85f964c2404b969bdcd487"
[[package]]
name = "async-trait"
version = "0.1.91"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "ae36dc4177970ef04fde5178d3e2429882def40e57a451f919c098f72baa6cec"
dependencies = [
"proc-macro2",
"quote",
"syn 3.0.3",
]
[[package]] [[package]]
name = "atomic-waker" name = "atomic-waker"
version = "1.1.2" version = "1.1.2"
@ -121,10 +132,8 @@ checksum = "c469d952047f47f91b68d1cba3f10d63c11d73e4636f24f08daf0278abf01c4d"
dependencies = [ dependencies = [
"android-tzdata", "android-tzdata",
"iana-time-zone", "iana-time-zone",
"js-sys",
"num-traits", "num-traits",
"serde", "serde",
"wasm-bindgen",
"windows-link", "windows-link",
] ]
@ -146,9 +155,9 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
[[package]] [[package]]
name = "darling" name = "darling"
version = "0.21.0" version = "0.23.0"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "a79c4acb1fd5fa3d9304be4c76e031c54d2e92d172a393e24b19a14fe8532fe9" checksum = "25ae13da2f202d56bd7f91c25fba009e7717a1e4a1cc98a76d844b65ae912e9d"
dependencies = [ dependencies = [
"darling_core", "darling_core",
"darling_macro", "darling_macro",
@ -156,27 +165,26 @@ dependencies = [
[[package]] [[package]]
name = "darling_core" name = "darling_core"
version = "0.21.0" version = "0.23.0"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "74875de90daf30eb59609910b84d4d368103aaec4c924824c6799b28f77d6a1d" checksum = "9865a50f7c335f53564bb694ef660825eb8610e0a53d3e11bf1b0d3df31e03b0"
dependencies = [ dependencies = [
"fnv",
"ident_case", "ident_case",
"proc-macro2", "proc-macro2",
"quote", "quote",
"strsim", "strsim",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
name = "darling_macro" name = "darling_macro"
version = "0.21.0" version = "0.23.0"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e79f8e61677d5df9167cd85265f8e5f64b215cdea3fb55eebc3e622e44c7a146" checksum = "ac3984ec7bd6cfa798e62b4a642426a5be0e68f9401cfc2a01e3fa9ea2fcdb8d"
dependencies = [ dependencies = [
"darling_core", "darling_core",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -187,7 +195,7 @@ checksum = "97369cbbc041bc366949bc74d34658d6cda5621039731c6310521892a3a20ae0"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -313,7 +321,7 @@ checksum = "162ee34ebcb7c64a8abebc059ce0fee27c2262618d7b60ed8faf72fef13c3650"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -365,10 +373,21 @@ checksum = "26145e563e54f2cadc477553f1ec5ee650b00862f0a58bcd12cbdc5f0ea2d2f4"
dependencies = [ dependencies = [
"cfg-if", "cfg-if",
"libc", "libc",
"r-efi", "r-efi 5.3.0",
"wasi 0.14.2+wasi-0.2.4", "wasi 0.14.2+wasi-0.2.4",
] ]
[[package]]
name = "getrandom"
version = "0.4.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "300e883d756b2e4ec94e02791f39b04b522276138852cfc41d9fb7e904106099"
dependencies = [
"cfg-if",
"libc",
"r-efi 6.0.0",
]
[[package]] [[package]]
name = "gimli" name = "gimli"
version = "0.31.1" version = "0.31.1"
@ -861,7 +880,7 @@ checksum = "a948666b637a0f465e8564c73e89d4dde00d72d4d473cc972f390fc3dcee7d9c"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -906,10 +925,10 @@ dependencies = [
] ]
[[package]] [[package]]
name = "paste" name = "pastey"
version = "1.0.15" version = "0.2.3"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "57c0d7b74b563b49d38dae00a0c37d4d6de9b432382b2892f0574ddcae73fd0a" checksum = "2ee67f1008b1ba2321834326597b8e186293b049a023cdef258527550b9935b4"
[[package]] [[package]]
name = "percent-encoding" name = "percent-encoding"
@ -968,6 +987,12 @@ version = "5.3.0"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f" checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
[[package]]
name = "r-efi"
version = "6.0.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "f8dcc9c7d52a811697d2151c701e0d08956f92b0e24136cf4cf27b57a6a0d9bf"
[[package]] [[package]]
name = "redox_syscall" name = "redox_syscall"
version = "0.5.15" version = "0.5.15"
@ -994,7 +1019,7 @@ checksum = "1165225c21bff1f3bbce98f5a1f889949bc902d3575308cc7b0de30b4f6d27c7"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1070,14 +1095,15 @@ dependencies = [
[[package]] [[package]]
name = "rmcp" name = "rmcp"
version = "0.3.0" version = "3.0.0-beta.2"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "b008b927a85d514699ff304be84c5084598596e6cad4a6f5bc67207715fafe5f" checksum = "3875f7c471c2d709062bc2165d8dc883b021b44255bd00bb4d8025f5108178c8"
dependencies = [ dependencies = [
"async-trait",
"base64", "base64",
"chrono", "chrono",
"futures", "futures",
"paste", "pastey",
"pin-project-lite", "pin-project-lite",
"rmcp-macros", "rmcp-macros",
"schemars", "schemars",
@ -1087,19 +1113,20 @@ dependencies = [
"tokio", "tokio",
"tokio-util", "tokio-util",
"tracing", "tracing",
"uuid",
] ]
[[package]] [[package]]
name = "rmcp-macros" name = "rmcp-macros"
version = "0.3.0" version = "3.0.0-beta.2"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "7465280d5f73f2c5c99017a04af407b2262455a149f255ad22f2b0b29087695c" checksum = "37a5ce7541aff2f3df9e8c3c48c7241800fa6fa68e2fbcba386c0d7dcdfc953c"
dependencies = [ dependencies = [
"darling", "darling",
"proc-macro2", "proc-macro2",
"quote", "quote",
"serde_json", "serde_json",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1198,7 +1225,7 @@ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"serde_derive_internals", "serde_derive_internals",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1247,7 +1274,7 @@ checksum = "5b0276cf7f2c73365f7157c8123c21cd9a50fbbd844757af28ca1f5925fc2a00"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1258,7 +1285,7 @@ checksum = "18d26a20a969b9e3fdf2fc2d9f21eda6c40e2de84c9408bb5d3b05d499aae711"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1360,6 +1387,17 @@ dependencies = [
"unicode-ident", "unicode-ident",
] ]
[[package]]
name = "syn"
version = "3.0.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "53e9bae58849f64dfa4f5d5ae372c8341f7305f82a3868709269343628b659a3"
dependencies = [
"proc-macro2",
"quote",
"unicode-ident",
]
[[package]] [[package]]
name = "sync_wrapper" name = "sync_wrapper"
version = "1.0.2" version = "1.0.2"
@ -1377,7 +1415,7 @@ checksum = "728a70f3dbaf5bab7f0c4b1ac8d7ae5ea60a4b5549c8a5914361c99147a709d2"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1431,7 +1469,7 @@ checksum = "7f7cf42b4507d8ea322120659672cf1b9dbb93f8f2d4ecfd6e51350ff5b17a1d"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1481,7 +1519,7 @@ checksum = "6e06d43f1345a3bcd39f6a56dbb7dcab2ba47e68e8ac134855e7e2bdbaf8cab8"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1581,7 +1619,7 @@ checksum = "81383ab64e72a7a8b8e13130c49e3dab29def6d0c7d76a03087b3cf71c5c6903"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1658,6 +1696,17 @@ version = "1.0.4"
source = "registry+https://github.com/rust-lang/crates.io-index" source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "b6c140620e7ffbb22c2dee59cafe6084a59b5ffc27a8859a5f0d494b5d52b6be" checksum = "b6c140620e7ffbb22c2dee59cafe6084a59b5ffc27a8859a5f0d494b5d52b6be"
[[package]]
name = "uuid"
version = "1.24.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "bf3923a6f5c4c6382e0b653c4117f48d631ea17f38ed86e2a828e6f7412f5239"
dependencies = [
"getrandom 0.4.3",
"js-sys",
"wasm-bindgen",
]
[[package]] [[package]]
name = "valuable" name = "valuable"
version = "0.1.1" version = "0.1.1"
@ -1716,7 +1765,7 @@ dependencies = [
"log", "log",
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
"wasm-bindgen-shared", "wasm-bindgen-shared",
] ]
@ -1751,7 +1800,7 @@ checksum = "8ae87ea40c9f689fc23f209965b6fb8a99ad69aeeb0231408be24920604395de"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
"wasm-bindgen-backend", "wasm-bindgen-backend",
"wasm-bindgen-shared", "wasm-bindgen-shared",
] ]
@ -1810,7 +1859,7 @@ checksum = "a47fddd13af08290e67f4acabf4b459f647552718f683a7b415d290ac744a836"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -1821,7 +1870,7 @@ checksum = "bd9211b69f8dcdfa817bfd14bf1c97c9188afa36f4750130fcdf3f400eca9fa8"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]
[[package]] [[package]]
@ -2049,7 +2098,7 @@ checksum = "38da3c9736e16c5d3c8c597a9aaa5d1fa565d0532ae05e27c24aa62fb32c0ab6"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
"synstructure", "synstructure",
] ]
@ -2070,7 +2119,7 @@ checksum = "d71e5d6e06ab090c67b5e44993ec16b72dcbaabc526db883a360057678b48502"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
"synstructure", "synstructure",
] ]
@ -2110,5 +2159,5 @@ checksum = "5b96237efa0c878c64bd89c436f661be4e46b2f3eff1ebb976f7ef2321d2f58f"
dependencies = [ dependencies = [
"proc-macro2", "proc-macro2",
"quote", "quote",
"syn", "syn 2.0.104",
] ]

2
weather-server-rust/Cargo.toml
View file

@ -4,7 +4,7 @@ version = "0.1.0"
edition = "2024" edition = "2024"
[dependencies] [dependencies]
rmcp = { version = "0.3", features = ["server", "macros", "transport-io"] } rmcp = { version = "3.0.0-beta.2", features = ["server", "macros", "transport-io"] }
tokio = { version = "1.46", features = ["full"] } tokio = { version = "1.46", features = ["full"] }
reqwest = { version = "0.12", features = ["json"] } reqwest = { version = "0.12", features = ["json"] }
serde = { version = "1.0", features = ["derive"] } serde = { version = "1.0", features = ["derive"] }

8
weather-server-rust/README.md
View file

@ -1,3 +1,11 @@
# A Simple MCP Weather Server written in Rust # A Simple MCP Weather Server written in Rust
See the [Build an MCP server](https://modelcontextprotocol.io/docs/develop/build-server) tutorial for more information. See the [Build an MCP server](https://modelcontextprotocol.io/docs/develop/build-server) tutorial for more information.
## Structured output
Both tools declare an `output_schema` and return `structured_content`. `get_forecast` returns an object; `get_alerts` returns a top-level JSON array, which protocol revision `2026-07-28` is the first to allow — see [Structured Content](https://modelcontextprotocol.io/specification/draft/server/tools#structured-content) in the spec.
Note that an array-rooted schema requires a `2026-07-28` client: rmcp sends it as written on every connection rather than projecting it down, so an older client rejects the tool list.
`list_tools` is hand-written because `#[tool_handler]` omits the SEP-2549 `ttl_ms` and `cache_scope` fields, which are required on a paginated result at this revision.

219
weather-server-rust/src/main.rs
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@ -1,12 +1,15 @@
use anyhow::Result; use anyhow::Result;
use rmcp::{ use rmcp::{
ServerHandler, ServiceExt, ErrorData, ServerHandler, ServiceExt,
handler::server::{router::tool::ToolRouter, tool::Parameters}, handler::server::{router::tool::ToolRouter, tool::schema_for_output, wrapper::Parameters},
model::*, model::*,
schemars, tool, tool_handler, tool_router, schemars::{self, JsonSchema},
tool, tool_handler, tool_router,
transport::stdio,
}; };
use serde::Deserialize; use serde::Deserialize;
use serde::de::DeserializeOwned; use serde::de::DeserializeOwned;
use serde::Serialize;
const NWS_API_BASE: &str = "https://api.weather.gov"; const NWS_API_BASE: &str = "https://api.weather.gov";
const USER_AGENT: &str = "weather-app/1.0"; const USER_AGENT: &str = "weather-app/1.0";
@ -65,6 +68,44 @@ struct ForecastPeriod {
detailed_forecast: String, detailed_forecast: String,
} }
/// One element of `get_alerts`' structured output. Declaring `Vec<Alert>` as
/// the output schema makes `structuredContent` a top-level JSON array rather
/// than an object wrapping one, which revision 2026-07-28 is the first to allow.
#[derive(Debug, Serialize, JsonSchema)]
pub struct Alert {
/// The kind of weather event.
event: String,
/// The area the alert covers.
area: String,
/// How severe the event is.
severity: String,
/// What is happening.
description: String,
/// What people in the area should do.
instructions: String,
}
/// `get_forecast`' structured output: the object case.
#[derive(Debug, Serialize, JsonSchema)]
pub struct Forecast {
/// Latitude the forecast is for.
latitude: f32,
/// Longitude the forecast is for.
longitude: f32,
/// The forecast periods, soonest first.
periods: Vec<Period>,
}
#[derive(Debug, Serialize, JsonSchema)]
pub struct Period {
name: String,
temperature: i32,
temperature_unit: String,
wind_speed: String,
wind_direction: String,
detailed_forecast: String,
}
async fn make_nws_request<T: DeserializeOwned>(url: &str) -> Result<T> { async fn make_nws_request<T: DeserializeOwned>(url: &str) -> Result<T> {
let client = reqwest::Client::new(); let client = reqwest::Client::new();
let rsp = client let rsp = client
@ -77,25 +118,14 @@ async fn make_nws_request<T: DeserializeOwned>(url: &str) -> Result<T> {
Ok(rsp.json::<T>().await?) Ok(rsp.json::<T>().await?)
} }
fn format_alert(feature: &AlertFeature) -> String { fn format_alert(alert: &Alert) -> String {
let props = &feature.properties;
format!( format!(
"Event: {}\nArea: {}\nSeverity: {}\nDescription: {}\nInstructions: {}", "Event: {}\nArea: {}\nSeverity: {}\nDescription: {}\nInstructions: {}",
props.event.as_deref().unwrap_or("Unknown"), alert.event, alert.area, alert.severity, alert.description, alert.instructions
props.area_desc.as_deref().unwrap_or("Unknown"),
props.severity.as_deref().unwrap_or("Unknown"),
props
.description
.as_deref()
.unwrap_or("No description available"),
props
.instruction
.as_deref()
.unwrap_or("No specific instructions provided")
) )
} }
fn format_period(period: &ForecastPeriod) -> String { fn format_period(period: &Period) -> String {
format!( format!(
"{}:\nTemperature: {}°{}\nWind: {} {}\nForecast: {}", "{}:\nTemperature: {}°{}\nWind: {} {}\nForecast: {}",
period.name, period.name,
@ -107,22 +137,39 @@ fn format_period(period: &ForecastPeriod) -> String {
) )
} }
#[derive(serde::Deserialize, schemars::JsonSchema)] /// Build a result carrying both channels.
fn dual_channel_result(
text: String,
structured: impl Serialize,
) -> Result<CallToolResult, ErrorData> {
let value = serde_json::to_value(structured).map_err(|e| {
ErrorData::internal_error(format!("failed to serialize structured content: {e}"), None)
})?;
let mut result = CallToolResult::success(vec![ContentBlock::text(text)]);
result.structured_content = Some(value);
Ok(result)
}
#[derive(serde::Deserialize, JsonSchema)]
pub struct MCPForecastRequest { pub struct MCPForecastRequest {
/// Latitude of the location.
latitude: f32, latitude: f32,
/// Longitude of the location.
longitude: f32, longitude: f32,
} }
#[derive(serde::Deserialize, schemars::JsonSchema)] #[derive(serde::Deserialize, JsonSchema)]
pub struct MCPAlertRequest { pub struct MCPAlertRequest {
/// Two-letter US state code (e.g. CA, NY).
state: String, state: String,
} }
#[derive(Clone)]
pub struct Weather { pub struct Weather {
tool_router: ToolRouter<Weather>, tool_router: ToolRouter<Weather>,
} }
#[tool_router] #[tool_router(router = tool_router)]
impl Weather { impl Weather {
fn new() -> Self { fn new() -> Self {
Self { Self {
@ -130,77 +177,135 @@ impl Weather {
} }
} }
#[tool(description = "Get weather alerts for a US state.")] #[tool(
description = "Get weather alerts for a US state.",
output_schema = schema_for_output::<Vec<Alert>>()
)]
async fn get_alerts( async fn get_alerts(
&self, &self,
Parameters(MCPAlertRequest { state }): Parameters<MCPAlertRequest>, Parameters(MCPAlertRequest { state }): Parameters<MCPAlertRequest>,
) -> String { ) -> Result<CallToolResult, ErrorData> {
let url = format!( let state = state.to_uppercase();
"{}/alerts/active/area/{}", let url = format!("{NWS_API_BASE}/alerts/active/area/{state}");
NWS_API_BASE,
state.to_uppercase()
);
match make_nws_request::<AlertsResponse>(&url).await { let Ok(data) = make_nws_request::<AlertsResponse>(&url).await else {
Ok(data) => { return Ok(CallToolResult::error(vec![ContentBlock::text(format!(
if data.features.is_empty() { "Unable to fetch alerts for {state}."
"No active alerts for this state.".to_string() ))]));
} else { };
data.features
.iter() // An empty result is an empty array, not an error.
.map(format_alert) let alerts: Vec<Alert> = data
.collect::<Vec<_>>() .features
.join("\n---\n") .into_iter()
.map(|feature| {
let props = feature.properties;
Alert {
event: props.event.unwrap_or_else(|| "Unknown".into()),
area: props.area_desc.unwrap_or_else(|| "Unknown".into()),
severity: props.severity.unwrap_or_else(|| "Unknown".into()),
description: props
.description
.unwrap_or_else(|| "No description available".into()),
instructions: props
.instruction
.unwrap_or_else(|| "No specific instructions provided".into()),
} }
} })
Err(_) => "Unable to fetch alerts or no alerts found.".to_string(), .collect();
}
let text = if alerts.is_empty() {
"No active alerts for this state.".to_string()
} else {
alerts
.iter()
.map(format_alert)
.collect::<Vec<_>>()
.join("\n---\n")
};
dual_channel_result(text, &alerts)
} }
#[tool(description = "Get weather forecast for a location.")] #[tool(
description = "Get weather forecast for a location.",
output_schema = schema_for_output::<Forecast>()
)]
async fn get_forecast( async fn get_forecast(
&self, &self,
Parameters(MCPForecastRequest { Parameters(MCPForecastRequest {
latitude, latitude,
longitude, longitude,
}): Parameters<MCPForecastRequest>, }): Parameters<MCPForecastRequest>,
) -> String { ) -> Result<CallToolResult, ErrorData> {
let points_url = format!("{NWS_API_BASE}/points/{latitude},{longitude}"); let points_url = format!("{NWS_API_BASE}/points/{latitude},{longitude}");
let Ok(points_data) = make_nws_request::<PointsResponse>(&points_url).await else { let Ok(points_data) = make_nws_request::<PointsResponse>(&points_url).await else {
return "Unable to fetch forecast data for this location.".to_string(); return Ok(CallToolResult::error(vec![ContentBlock::text(
"Unable to fetch forecast data for this location.",
)]));
}; };
let forecast_url = points_data.properties.forecast; let forecast_url = points_data.properties.forecast;
let Ok(forecast_data) = make_nws_request::<ForecastResponse>(&forecast_url).await else { let Ok(forecast_data) = make_nws_request::<ForecastResponse>(&forecast_url).await else {
return "Unable to fetch forecast data for this location.".to_string(); return Ok(CallToolResult::error(vec![ContentBlock::text(
"Unable to fetch detailed forecast.",
)]));
}; };
let periods = &forecast_data.properties.periods; let forecast = Forecast {
let forecast_summary: String = periods latitude,
longitude,
periods: forecast_data
.properties
.periods
.into_iter()
.take(5) // Next 5 periods only
.map(|p| Period {
name: p.name,
temperature: p.temperature,
temperature_unit: p.temperature_unit,
wind_speed: p.wind_speed,
wind_direction: p.wind_direction,
detailed_forecast: p.detailed_forecast,
})
.collect(),
};
let text = forecast
.periods
.iter() .iter()
.take(5) // Next 5 periods only
.map(format_period) .map(format_period)
.collect::<Vec<String>>() .collect::<Vec<_>>()
.join("\n---\n"); .join("\n---\n");
forecast_summary
dual_channel_result(text, &forecast)
} }
} }
#[tool_handler] #[tool_handler(router = self.tool_router)]
impl ServerHandler for Weather { impl ServerHandler for Weather {
fn get_info(&self) -> ServerInfo { fn get_info(&self) -> ServerInfo {
ServerInfo { let mut info = ServerInfo::default();
capabilities: ServerCapabilities::builder().enable_tools().build(), info.capabilities = ServerCapabilities::builder().enable_tools().build();
..Default::default() info
} }
// Hand-written because `#[tool_handler]` generates `ttl_ms: None,
// cache_scope: None`, and both are required at revision 2026-07-28.
async fn list_tools(
&self,
_request: Option<PaginatedRequestParams>,
_context: rmcp::service::RequestContext<rmcp::RoleServer>,
) -> Result<ListToolsResult, ErrorData> {
Ok(ListToolsResult::with_all_items(self.tool_router.list_all())
.with_ttl_ms(60_000)
.with_cache_scope(CacheScope::Public))
} }
} }
#[tokio::main] #[tokio::main]
async fn main() -> Result<()> { async fn main() -> Result<()> {
let transport = (tokio::io::stdin(), tokio::io::stdout()); let service = Weather::new().serve(stdio()).await?;
let service = Weather::new().serve(transport).await?;
service.waiting().await?; service.waiting().await?;
Ok(()) Ok(())
} }