diff --git a/cli/src/routing.rs b/cli/src/routing.rs
index 55f9e25..4650c85 100644
--- a/cli/src/routing.rs
+++ b/cli/src/routing.rs
@@ -7,6 +7,23 @@ use serde_json::{Value, json};
 use tokio::sync::{mpsc, oneshot, watch};
 use uuid::Uuid;
 
+fn params_not_object(params: &Value) -> Value {
+    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",
+    };
+    error(
+        -32602,
+        &format!(
+            "params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string."
+        ),
+    )
+}
+
 impl Host {
     /// Queue an incoming response or notification for its destination.
     pub(crate) fn send(&self, pid: &str, value: Value) {
@@ -69,7 +86,7 @@ impl Host {
         let host = self.clone();
         Box::pin(async move {
             if !params.is_object() {
-                return Err(error(-32602, "params must be an object"));
+                return Err(params_not_object(&params));
             }
             if caller != "console" && caller != "core" {
                 let state = host.0.state.lock().unwrap();
@@ -238,7 +255,7 @@ impl Host {
                 // Deliver progress before a following response removes its destination.
                 let params = v.get("params").cloned().unwrap_or(json!({}));
                 let result = if !params.is_object() {
-                    Err(error(-32602, "params must be an object"))
+                    Err(params_not_object(&params))
                 } else if method == "core.unlock" {
                     if let Some(path) = params["path"].as_str() {
                         self.unlock(pid, path);
diff --git a/cli/tests/host/core.rs b/cli/tests/host/core.rs
index 2a062d2..95707ad 100644
--- a/cli/tests/host/core.rs
+++ b/cli/tests/host/core.rs
@@ -500,14 +500,119 @@ async fn unimplemented_core_methods_return_method_not_found() {
 async fn params_must_be_objects() {
     let w = TestWorld::new();
     let h = w.start().await;
+    for (params, kind) in [
+        (json!("{\"text\":\"hello\"}"), "a string"),
+        (json!([]), "an array"),
+        (json!(42), "a number"),
+        (json!(null), "null"),
+        (json!(true), "a boolean"),
+    ] {
+        let expected = json!({
+            "code": -32602,
+            "message": format!("params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string.")
+        });
+        for method in ["probe.echo", "core.ps"] {
+            assert_eq!(h.call(method, params.clone()).await.unwrap_err(), expected);
+        }
+        assert_eq!(
+            h.call("core.call", json!({"method":"probe.echo", "params":params}))
+                .await
+                .unwrap_err(),
+            expected
+        );
+    }
+    let object = json!({"text":"hello"});
+    assert_eq!(call(&h, "probe.echo", object.clone()).await, object);
     assert_eq!(
-        h.call("probe.echo", json!([])).await.unwrap_err()["code"],
-        -32602
+        call(
+            &h,
+            "core.call",
+            json!({"method":"probe.echo", "params":object})
+        )
+        .await,
+        object
+    );
+    assert_eq!(
+        call(&h, "core.call", json!({"method":"probe.echo"})).await,
+        json!({})
     );
 
     h.stop().await;
 }
 
+/// PROTOCOL 3.1: stdout fast paths reject non-objects without changing defaults.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn stdout_params_must_be_objects() {
+    let w = TestWorld::new();
+    let h = w.start().await;
+    let mut notifications = h.subscribe();
+    for method in ["core.notify", "core.unlock", "probe.echo"] {
+        for (params, kind) in [
+            (json!("{}"), "a string"),
+            (json!([]), "an array"),
+            (json!(1.5), "a number"),
+            (json!(null), "null"),
+            (json!(false), "a boolean"),
+        ] {
+            let response = call(
+                &h,
+                "probe.raw_call",
+                json!({
+                    "jsonrpc":"2.0", "id":1, "method":method, "params":params
+                }),
+            )
+            .await;
+            assert_eq!(response["id"], 1);
+            assert_eq!(
+                response["error"],
+                json!({
+                    "code": -32602,
+                    "message": format!("params must be a JSON object, got {kind}. Send the object itself, not a JSON-encoded string.")
+                })
+            );
+        }
+        let params = match method {
+            "core.notify" => json!({"name":"valid", "params":{"n":1}}),
+            "core.unlock" => json!({"path":"/state/not-held"}),
+            _ => json!({"n":1}),
+        };
+        let response = call(
+            &h,
+            "probe.raw_call",
+            json!({
+                "jsonrpc":"2.0", "id":1, "method":method, "params":params
+            }),
+        )
+        .await;
+        assert_eq!(
+            response["result"],
+            if method == "probe.echo" {
+                params
+            } else {
+                Value::Null
+            }
+        );
+        assert!(response.get("error").is_none());
+        let omitted = call(
+            &h,
+            "probe.raw_call",
+            json!({
+                "jsonrpc":"2.0", "id":1, "method":method
+            }),
+        )
+        .await;
+        match method {
+            "core.notify" => {
+                assert_eq!(omitted["error"]["message"], "name must be a string");
+                assert_eq!(notifications.try_recv().unwrap()["params"]["name"], "valid");
+            }
+            "core.unlock" => assert_eq!(omitted["error"]["message"], "path must be a string"),
+            _ => assert_eq!(omitted["result"], json!({})),
+        }
+    }
+    h.stop().await;
+}
+
 /// PROTOCOL 3.9: exit without answer reports status and last logs.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn exit_without_answer_reports_status_and_last_logs() {
diff --git a/cli/tests/plugins/probe/src/main.rs b/cli/tests/plugins/probe/src/main.rs
index 9ae4d44..c20bd7b 100644
--- a/cli/tests/plugins/probe/src/main.rs
+++ b/cli/tests/plugins/probe/src/main.rs
@@ -470,6 +470,10 @@ fn dispatch(request: &Value, lines: &mut Lines) -> Option<Value> {
         "network" => network(params),
         "read" => read_file(params),
         "write" => write_file(params),
+        "raw_call" => {
+            send(params.clone());
+            receive(lines)
+        }
         "call" => outgoing_call(params, lines),
         "lock" => lock(params, lines),
         _ => {
diff --git a/sdk/macros/src/lib.rs b/sdk/macros/src/lib.rs
index 133408b..a74a0f6 100644
--- a/sdk/macros/src/lib.rs
+++ b/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
         }})
     })();
diff --git a/sdk/src/calls.rs b/sdk/src/calls.rs
index 91a24c8..c8be755 100644
--- a/sdk/src/calls.rs
+++ b/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| {
diff --git a/sdk/src/error.rs b/sdk/src/error.rs
index 7f3fa31..132a8a9 100644
--- a/sdk/src/error.rs
+++ b/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 {
diff --git a/sdk/tests/sdk.rs b/sdk/tests/sdk.rs
index afd8aa2..ce12cba 100644
--- a/sdk/tests/sdk.rs
+++ b/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!(