diff --git a/runtime/core/src/contract/dispatch.rs b/runtime/core/src/contract/dispatch.rs
index 9a536fd..1385f5d 100644
--- a/runtime/core/src/contract/dispatch.rs
+++ b/runtime/core/src/contract/dispatch.rs
@@ -12,6 +12,22 @@ pub enum Kind {
     Named(&'static str),
 }
 
+impl Kind {
+    /// Resolve to a position in `calls` — a protocol's `calls_names()`, in
+    /// declaration order. `Id(n)` is that position directly; `Named(s)` is
+    /// looked up. `None` if out of range or not found — a generated dispatcher
+    /// maps that to [`RuntimeError::UnknownCall`].
+    pub fn resolve(&self, calls: &[&str]) -> Option<usize> {
+        match self {
+            Kind::Id(id) => {
+                let idx = *id as usize;
+                (idx < calls.len()).then_some(idx)
+            }
+            Kind::Named(name) => calls.iter().position(|c| c == name),
+        }
+    }
+}
+
 /// The provider side of a protocol: given an inbound call and its encoded
 /// params, run the user's handler and write the response [`Envelope`] into
 /// `out`.
@@ -31,3 +47,23 @@ pub trait Dispatch {
         out: &mut dyn BufMut,
     ) -> Result<(), RuntimeError>;
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Kind;
+
+    const CALLS: &[&str] = &["send", "history", "notify"];
+
+    #[test]
+    fn id_resolves_by_position() {
+        assert_eq!(Kind::Id(0).resolve(CALLS), Some(0));
+        assert_eq!(Kind::Id(2).resolve(CALLS), Some(2));
+        assert_eq!(Kind::Id(3).resolve(CALLS), None);
+    }
+
+    #[test]
+    fn named_resolves_by_lookup() {
+        assert_eq!(Kind::Named("history").resolve(CALLS), Some(1));
+        assert_eq!(Kind::Named("missing").resolve(CALLS), None);
+    }
+}
diff --git a/runtime/core/tests/dispatch_roundtrip.rs b/runtime/core/tests/dispatch_roundtrip.rs
new file mode 100644
index 0000000..93e58c3
--- /dev/null
+++ b/runtime/core/tests/dispatch_roundtrip.rs
@@ -0,0 +1,189 @@
+//! End to end, no network: a hand-written stand-in for what `comline-rust` will
+//! generate — a client stub and a `Dispatch` impl — driven directly with
+//! `MsgPack`. Proves the `contract` surface (`Kind`, `WireFormat`, `Dispatch`,
+//! `Envelope`, `BufMut`, `CallError`) fits together before any codegen or
+//! `setup/` rework.
+#![cfg(feature = "std")]
+
+use comline_runtime::contract::{
+    BufMut, CallError, Dispatch, Envelope, Kind, RuntimeError, WireFormat,
+};
+use comline_runtime::format::MsgPack;
+use serde::{Deserialize, Serialize};
+
+// The "schema":
+//   protocol Echo {
+//       function say(msg: str) -> str ! TooLong;
+//       function bump(n: u32) -> u32;
+//   }
+
+#[derive(Serialize, Deserialize)]
+struct SayParams<'a> {
+    #[serde(borrow)]
+    msg: &'a str,
+}
+
+#[derive(Serialize, Deserialize)]
+struct BumpParams {
+    n: u32,
+}
+
+#[derive(Serialize, Deserialize, PartialEq, Eq, Debug)]
+struct TooLong {
+    limit: u32,
+}
+
+const CALLS: &[&str] = &["say", "bump"];
+const ERR_TOO_LONG: u16 = 0; // schema-global error ordinal
+
+/// The generated per-function error enum for `say`.
+#[derive(Debug, PartialEq, Eq)]
+enum SayError {
+    TooLong(TooLong),
+}
+
+// ── provider: the user trait + the generated dispatcher ─────────────────────
+
+trait Echo {
+    fn say(&self, msg: &str) -> Result<String, SayError>;
+    fn bump(&self, n: u32) -> Result<u32, core::convert::Infallible>;
+}
+
+struct EchoDispatcher<T>(T);
+
+impl<T: Echo> Dispatch for EchoDispatcher<T> {
+    fn dispatch<W: WireFormat>(
+        &self,
+        call: Kind,
+        params: &[u8],
+        fmt: &W,
+        out: &mut dyn BufMut,
+    ) -> Result<(), RuntimeError> {
+        // A real dispatcher reuses one scratch buffer; a fresh `Vec` per arm
+        // keeps the shape readable here.
+        match call.resolve(CALLS).ok_or(RuntimeError::UnknownCall)? {
+            0 => {
+                let p: SayParams = fmt.decode(params)?;
+                match self.0.say(p.msg) {
+                    Ok(reply) => {
+                        let mut body = Vec::new();
+                        fmt.encode(&reply, &mut body)?;
+                        Envelope::encode_ok(&body, out);
+                    }
+                    Err(SayError::TooLong(e)) => {
+                        let mut body = Vec::new();
+                        fmt.encode(&e, &mut body)?;
+                        Envelope::encode_err(ERR_TOO_LONG, &body, out);
+                    }
+                }
+                Ok(())
+            }
+            1 => {
+                let p: BumpParams = fmt.decode(params)?;
+                let reply = self.0.bump(p.n).unwrap();
+                let mut body = Vec::new();
+                fmt.encode(&reply, &mut body)?;
+                Envelope::encode_ok(&body, out);
+                Ok(())
+            }
+            _ => Err(RuntimeError::UnknownCall),
+        }
+    }
+}
+
+// ── consumer: the generated client stub ────────────────────────────────────
+
+struct EchoClient<'d, D> {
+    dispatcher: &'d D, // stands in for a call system + transport
+    fmt: MsgPack,
+}
+
+impl<D: Dispatch> EchoClient<'_, D> {
+    fn say(&self, msg: &str) -> Result<String, CallError<SayError>> {
+        let mut params = Vec::new();
+        self.fmt.encode(&SayParams { msg }, &mut params)?;
+
+        let mut frame = Vec::new();
+        self.dispatcher
+            .dispatch(Kind::Id(0), &params, &self.fmt, &mut frame)?;
+
+        match Envelope::decode(&frame).ok_or(RuntimeError::Framing)? {
+            Envelope::Ok(payload) => self.fmt.decode(payload).map_err(CallError::Runtime),
+            Envelope::Err {
+                id: ERR_TOO_LONG,
+                body,
+            } => {
+                let e: TooLong = self.fmt.decode(body)?;
+                Err(CallError::App(SayError::TooLong(e)))
+            }
+            Envelope::Err { id, .. } => Err(CallError::Runtime(RuntimeError::Remote { id })),
+        }
+    }
+
+    fn bump(&self, n: u32) -> Result<u32, CallError<core::convert::Infallible>> {
+        let mut params = Vec::new();
+        self.fmt.encode(&BumpParams { n }, &mut params)?;
+
+        let mut frame = Vec::new();
+        self.dispatcher
+            .dispatch(Kind::Id(1), &params, &self.fmt, &mut frame)?;
+
+        match Envelope::decode(&frame).ok_or(RuntimeError::Framing)? {
+            Envelope::Ok(payload) => self.fmt.decode(payload).map_err(CallError::Runtime),
+            Envelope::Err { id, .. } => Err(CallError::Runtime(RuntimeError::Remote { id })),
+        }
+    }
+}
+
+// ── the service and the assertions ────────────────────────────────────────
+
+struct Server;
+
+impl Echo for Server {
+    fn say(&self, msg: &str) -> Result<String, SayError> {
+        if msg.len() > 8 {
+            return Err(SayError::TooLong(TooLong { limit: 8 }));
+        }
+        Ok(format!("echo: {msg}"))
+    }
+
+    fn bump(&self, n: u32) -> Result<u32, core::convert::Infallible> {
+        Ok(n + 1)
+    }
+}
+
+fn client() -> (EchoDispatcher<Server>, MsgPack) {
+    (EchoDispatcher(Server), MsgPack)
+}
+
+#[test]
+fn ok_path_round_trips() {
+    let (d, fmt) = client();
+    let c = EchoClient {
+        dispatcher: &d,
+        fmt,
+    };
+    assert_eq!(c.say("hi").unwrap(), "echo: hi");
+    assert_eq!(c.bump(41).unwrap(), 42);
+}
+
+#[test]
+fn a_raised_error_reaches_the_client_typed() {
+    let (d, fmt) = client();
+    let c = EchoClient {
+        dispatcher: &d,
+        fmt,
+    };
+    let err = c.say("this is far too long").unwrap_err();
+    assert_eq!(err, CallError::App(SayError::TooLong(TooLong { limit: 8 })));
+}
+
+#[test]
+fn an_unknown_call_ordinal_is_a_runtime_error() {
+    let (d, _) = client();
+    let mut out = Vec::new();
+    let err = d
+        .dispatch(Kind::Id(9), &[], &MsgPack, &mut out)
+        .unwrap_err();
+    assert_eq!(err, RuntimeError::UnknownCall);
+}
diff --git a/runtime/core/tests/mod.rs b/runtime/core/tests/mod.rs
index b5b1469..4a0cb93 100644
--- a/runtime/core/tests/mod.rs
+++ b/runtime/core/tests/mod.rs
@@ -1,4 +1,3 @@
-// Relative Modules
+// The `setups` suite drives the `std`-only `setup/` layer.
+#[cfg(feature = "std")]
 pub mod setups;
-
-