Name the received JSON type in non-object params errors

Keep -32602 and existing validation order; tell callers to send the object itself.
Cover host routing, stdout fast paths, SDK calls and generated dispatch.

Card: SAU-317

sdk/macros/src/lib.rs

@@ -270,7 +270,7 @@ pub fn rpc(args: TokenStream, input: TokenStream) -> TokenStream {
             fn methods_document()->::sausage_plugin::serde_json::Value {let mut doc=::sausage_plugin::document(env!("CARGO_PKG_NAME"),env!("CARGO_PKG_VERSION")); Self::add_methods(&mut doc); #grace ::sausage_plugin::finish_document(doc)}
             fn describe()->::sausage_plugin::serde_json::Value {let mut doc=::sausage_plugin::document(env!("CARGO_PKG_NAME"),env!("CARGO_PKG_VERSION")); Self::add_methods(&mut doc); #grace #(doc.endpoint::<#types>(#names);)* ::sausage_plugin::finish_document(doc)}
             fn discover(&self)->::sausage_plugin::serde_json::Value {<Self as ::sausage_plugin::Interface>::describe()}
-            fn dispatch<'a>(&'a self, method:&'a str, params: ::sausage_plugin::serde_json::Value,ctx:&'a ::sausage_plugin::Ctx)->::std::pin::Pin<Box<dyn ::std::future::Future<Output=::sausage_plugin::Result<::sausage_plugin::serde_json::Value>>+'a>> {Box::pin(async move {if !params.is_object() {return Err(::sausage_plugin::Error::new(-32602,"params must be an object"));} match method {#(#arms,)* _=>Err(::sausage_plugin::Error::new(-32601,"method not found"))}})}
+            fn dispatch<'a>(&'a self, method:&'a str, params: ::sausage_plugin::serde_json::Value,ctx:&'a ::sausage_plugin::Ctx)->::std::pin::Pin<Box<dyn ::std::future::Future<Output=::sausage_plugin::Result<::sausage_plugin::serde_json::Value>>+'a>> {Box::pin(async move {if !params.is_object() {return Err(::sausage_plugin::Error::params_not_object(&params));} match method {#(#arms,)* _=>Err(::sausage_plugin::Error::new(-32601,"method not found"))}})}
             #stop
         }})
     })();

sdk/src/calls.rs

@@ -37,7 +37,7 @@ impl IntoFuture for Call {
     fn into_future(self) -> Self::IntoFuture {
         Box::pin(async move {
             if !self.params.is_object() {
-                return Err(Error::new(-32602, "params must be an object"));
+                return Err(Error::params_not_object(&self.params));
             }
             let (tx, rx) = oneshot::channel();
             let id = STATE.with(|s| {

sdk/src/error.rs

@@ -15,6 +15,25 @@ pub struct Error {
     pub data: Option<Value>,
 }
 impl Error {
+    /// Build the object-params diagnostic used by calls and generated dispatch.
+    #[doc(hidden)]
+    pub fn params_not_object(params: &Value) -> Self {
+        let kind = match params {
+            Value::String(_) => "a string",
+            Value::Array(_) => "an array",
+            Value::Number(_) => "a number",
+            Value::Null => "null",
+            Value::Bool(_) => "a boolean",
+            Value::Object(_) => "an object",
+        };
+        Self::new(
+            -32602,
+            format!(
+                "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+            ),
+        )
+    }
+
     /// Construct an error without structured details.
     pub fn new(code: i64, message: impl Into<String>) -> Self {
         Self {

sdk/tests/sdk.rs

@@ -116,6 +116,53 @@ fn discovery_and_dispatch() {
         .into();
     assert_eq!(internal.code, -32603);
 }
+#[sausage_plugin::test]
+async fn params_must_be_objects() {
+    for (params, kind) in [
+        (json!("{\"a\":2,\"b\":3}"), "a string"),
+        (json!([]), "an array"),
+        (json!(42), "a number"),
+        (json!(null), "null"),
+        (json!(true), "a boolean"),
+    ] {
+        let message = format!(
+            "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+        );
+        let outgoing = sausage_plugin::call("plugin.add", params.clone())
+            .await
+            .unwrap_err();
+        assert_eq!(outgoing.code, -32602);
+        assert_eq!(outgoing.message, message);
+        for method in ["add", "choice", "unknown"] {
+            let incoming = Plugin
+                .dispatch(method, params.clone(), &Ctx::default())
+                .await
+                .unwrap_err();
+            assert_eq!(incoming.code, -32602);
+            assert_eq!(incoming.message, message);
+        }
+    }
+    assert_eq!(
+        Plugin
+            .dispatch("add", json!({"a":2,"b":3}), &Ctx::default())
+            .await
+            .unwrap(),
+        5
+    );
+    assert_eq!(
+        Plugin
+            .dispatch("defaults", json!({}), &Ctx::default())
+            .await
+            .unwrap(),
+        json!({"value":""})
+    );
+    // An object reaches transport validation rather than the params rejection.
+    let outgoing = sausage_plugin::call("plugin.add", json!({"a":2,"b":3}))
+        .await
+        .unwrap_err();
+    assert!(outgoing.message.contains("no core transport"));
+}
+
 #[sausage_plugin::test]
 async fn missing_core_fails_without_wait() {
     assert!(