fix(acp): align protocol version negotiation

justfile

@@ -6,3 +6,4 @@ prep-tests:
 # Prepare subprocess fixtures, then run all tests, including doctests.
 test: prep-tests
     cargo test --workspace --all-features --locked
+    cargo test -p agent-client-protocol --no-default-features --locked

md/SUMMARY.md

@@ -12,7 +12,7 @@
 - [Request Cancellation](./request-cancellation.md)
 - [Ordered Application Dispatch](./ordered-application-dispatch.md)
 - [Configurable LLM Providers](./llm-providers.md)
-- [Protocol V2](./protocol-v2.md)
+- [Protocol V2 and Version Routing](./protocol-v2.md)
 - [Runnable V2 Quickstart](./protocol-v2-quickstart.md)
 - [Session Operation Coordination](./session-operation-coordination.md)
 

src/agent-client-protocol-conductor/src/conductor.rs

@@ -162,8 +162,10 @@ impl InitializeProtocol {
         } else if requested == ProtocolVersion::V1 {
             Self::V1
         } else {
-            return Err(Error::invalid_request()
-                .data(format!("unsupported ACP protocol version {requested}")));
+            return Err(agent_client_protocol::util::unsupported_protocol_version(
+                requested,
+                &[ProtocolVersion::V1, ProtocolVersion::V2],
+            ));
         };
 
         Ok(InitializeProtocolSelection {

src/agent-client-protocol-conductor/tests/initialization_v2.rs

@@ -321,8 +321,10 @@ async fn assert_invalid_final_agent_initialize_response(
 async fn assert_raw_initialize_rejected(
     params: serde_json::Value,
     expected_error: &str,
+    expected_code: i32,
 ) -> Result<(), Error> {
     run_raw_client_with_conductor(ProxiesAndAgent::new(Agent.v2()), async move |cx| {
+        let requested = params.get("protocolVersion").cloned();
         let request = UntypedMessage {
             method: "initialize".to_string(),
             params,
@@ -334,6 +336,16 @@ async fn assert_raw_initialize_rejected(
         .await
         .expect("invalid initialize request should not hang")
         .expect_err("invalid initialize request must be rejected");
+        assert_eq!(i32::from(error.code), expected_code);
+        if expected_code == -32022 {
+            assert_eq!(
+                error.data,
+                Some(serde_json::json!({
+                    "requested": requested.expect("a version mismatch has a requested version"),
+                    "supported": [1, 2],
+                }))
+            );
+        }
         assert!(
             error.to_string().contains(expected_error),
             "unexpected initialize rejection: {error:?}"
@@ -473,16 +485,19 @@ async fn v2_conductor_rejects_invalid_protocol_versions_with_responses() -> Resu
     assert_raw_initialize_rejected(
         serde_json::json!({}),
         "protocolVersion must be a valid ACP protocol version",
+        -32602,
     )
     .await?;
     assert_raw_initialize_rejected(
         serde_json::json!({ "protocolVersion": "2" }),
         "protocolVersion must be a valid ACP protocol version",
+        -32602,
     )
     .await?;
     assert_raw_initialize_rejected(
         serde_json::json!({ "protocolVersion": 0 }),
-        "unsupported ACP protocol version 0",
+        "Unsupported protocol version",
+        -32022,
     )
     .await
 }

src/agent-client-protocol/src/jsonrpc.rs

@@ -45,6 +45,8 @@ pub use crate::jsonrpc::handlers::NullHandler;
 use crate::jsonrpc::handlers::{ChainedHandler, NamedHandler};
 use crate::jsonrpc::handlers::{MessageHandler, NotificationHandler, RequestHandler};
 use crate::jsonrpc::outgoing_actor::{OutgoingMessageTx, send_raw_message};
+#[cfg(feature = "unstable_protocol_v2")]
+pub(crate) use crate::jsonrpc::protocol_compat::normalize_v1_initialize_request;
 use crate::jsonrpc::protocol_compat::{ProtocolCompat, ProtocolMode};
 pub use crate::jsonrpc::raw_error::{RawJsonRpcError, RawJsonRpcResponse};
 use crate::jsonrpc::run::SpawnedRun;