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fix(mcp): accept omitted arguments for argument-free tools #1

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fix(mcp): accept omitted arguments for argument-free tools #1

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50 changes: 43 additions & 7 deletions crates/impossible-ocr-server/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1699,12 +1699,11 @@ async fn mcp_call<B: OcrBackend>(
};
let arguments = call.arguments;
match call.name.as_str() {
mcp::OCR_CAPABILITIES if is_empty_object(&arguments) => {
mcp_success(id, json!(capabilities_response(&state)))
}
mcp::OCR_MODELS if is_empty_object(&arguments) => {
mcp_success(id, json!(models_response(&state)))
mcp::OCR_CAPABILITIES | mcp::OCR_MODELS if !is_empty_arguments(&arguments) => {
mcp_error(id, -32_602, "invalid params", 0)
}
mcp::OCR_CAPABILITIES => mcp_success(id, json!(capabilities_response(&state))),
mcp::OCR_MODELS => mcp_success(id, json!(models_response(&state))),
mcp::OCR_RECOGNIZE => {
let Ok(request) = serde_json::from_value::<http::OcrRequest>(arguments) else {
return mcp_error(id, -32_602, "invalid params", 0);
Expand Down Expand Up @@ -1777,8 +1776,9 @@ async fn mcp_call<B: OcrBackend>(
}
}

fn is_empty_object(value: &Value) -> bool {
value.as_object().is_some_and(serde_json::Map::is_empty)
/// MCP makes `arguments` optional, so an omitted (null) value counts as no arguments.
fn is_empty_arguments(value: &Value) -> bool {
value.is_null() || value.as_object().is_some_and(serde_json::Map::is_empty)
}

#[allow(clippy::needless_pass_by_value)] // The helper owns one complete JSON-RPC response boundary.
Expand Down Expand Up @@ -2632,6 +2632,42 @@ mod tests {
Ok(())
}

#[tokio::test]
async fn mcp_argument_free_tools_accept_omitted_arguments()
-> Result<(), Box<dyn std::error::Error>> {
let server = test_server();
for (params, expected) in [
(json!({"name":mcp::OCR_CAPABILITIES}), None),
(json!({"name":mcp::OCR_MODELS}), None),
(json!({"name":mcp::OCR_CAPABILITIES,"arguments":{}}), None),
(
json!({"name":mcp::OCR_MODELS,"arguments":{"unexpected":true}}),
Some("invalid params"),
),
(json!({"name":"ocr_missing"}), Some("unknown tool")),
] {
let request = json!({"jsonrpc":"2.0","id":1,"method":"tools/call","params":params});
let response = server
.router()
.oneshot(
Request::post(http::MCP_ROUTE)
.header("content-type", "application/json")
.body(Body::from(serde_json::to_vec(&request)?))?,
)
.await?;
let body: Value =
serde_json::from_slice(&response.into_body().collect().await?.to_bytes())?;
match expected {
None => assert_eq!(body["result"]["isError"], false),
Some(message) => {
assert_eq!(body["error"]["code"], -32_602);
assert_eq!(body["error"]["message"], message);
}
}
}
Ok(())
}

#[tokio::test]
async fn mcp_notifications_and_explicit_null_follow_json_rpc_lifecycle()
-> Result<(), Box<dyn std::error::Error>> {
Expand Down
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