fix: pass client header conformance

Author: DaleSeo

conformance/src/bin/client.rs

@@ -22,7 +22,7 @@ struct ConformanceToolCall {
 
 #[derive(Debug, Default, serde::Deserialize)]
 struct ConformanceContext {
-    #[serde(default)]
+    #[serde(default, alias = "toolCalls")]
     tool_calls: Vec<ConformanceToolCall>,
     #[serde(default)]
     client_id: Option<String>,
@@ -859,8 +859,32 @@ async fn run_basic_client(server_url: &str) -> anyhow::Result<()> {
 }
 
 async fn run_tools_call_client(server_url: &str, ctx: &ConformanceContext) -> anyhow::Result<()> {
+    run_tools_call_client_with_lifecycle(server_url, ctx, ClientLifecycleMode::Initialize).await
+}
+
+async fn run_discover_tools_call_client(
+    server_url: &str,
+    ctx: &ConformanceContext,
+) -> anyhow::Result<()> {
+    run_tools_call_client_with_lifecycle(
+        server_url,
+        ctx,
+        ClientLifecycleMode::Discover {
+            preferred_versions: preferred_protocol_versions(),
+        },
+    )
+    .await
+}
+
+async fn run_tools_call_client_with_lifecycle(
+    server_url: &str,
+    ctx: &ConformanceContext,
+    lifecycle: ClientLifecycleMode,
+) -> anyhow::Result<()> {
     let transport = StreamableHttpClientTransport::from_uri(server_url);
-    let client = FullClientHandler.serve(transport).await?;
+    let client = FullClientHandler
+        .serve_with_lifecycle(transport, lifecycle)
+        .await?;
     let tools = client.list_tools(Default::default()).await?;
 
     if ctx.tool_calls.is_empty() {
@@ -1000,7 +1024,7 @@ async fn main() -> anyhow::Result<()> {
             run_discover_client(&server_url).await?
         }
         "http-standard-headers" | "http-custom-headers" | "http-invalid-tool-headers" => {
-            run_tools_call_client(&server_url, &ctx).await?
+            run_discover_tools_call_client(&server_url, &ctx).await?
         }
 
         // Auth scenarios - standard OAuth flow
@@ -1083,3 +1107,30 @@ async fn main() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn conformance_context_accepts_camel_case_tool_calls() {
+        let context: ConformanceContext = serde_json::from_value(json!({
+            "toolCalls": [{
+                "name": "test_custom_headers",
+                "arguments": { "region": "us-west1" }
+            }]
+        }))
+        .expect("valid conformance context");
+
+        assert_eq!(
+            context.tool_calls.first().map(|tool_call| (
+                tool_call.name.as_str(),
+                tool_call
+                    .arguments
+                    .as_ref()
+                    .and_then(|arguments| arguments.get("region")),
+            )),
+            Some(("test_custom_headers", Some(&json!("us-west1"))))
+        );
+    }
+}

crates/rmcp/src/transport/streamable_http_client.rs

@@ -83,19 +83,24 @@ fn request_version_headers(
 
 fn cache_tools_from_response(
     cache: &mut HashMap<String, Arc<JsonObject>>,
-    message: &ServerJsonRpcMessage,
+    message: &mut ServerJsonRpcMessage,
+    protocol_version: &ProtocolVersion,
 ) {
+    if protocol_version < &ProtocolVersion::STANDARD_HEADERS {
+        return;
+    }
     if let ServerJsonRpcMessage::Response(response) = message {
-        if let ServerResult::ListToolsResult(list) = &response.result {
-            for tool in &list.tools {
-                if let Err(reason) =
+        if let ServerResult::ListToolsResult(list) = &mut response.result {
+            list.tools.retain(|tool| {
+                let Err(reason) =
                     mcp_headers::validate_param_header_annotations(&tool.input_schema)
-                {
-                    tracing::warn!(tool = %tool.name, "ignoring x-mcp-header annotations: {reason}");
-                    continue;
-                }
-                cache.insert(tool.name.to_string(), tool.input_schema.clone());
-            }
+                else {
+                    cache.insert(tool.name.to_string(), tool.input_schema.clone());
+                    return true;
+                };
+                tracing::warn!(tool = %tool.name, "rejecting invalid x-mcp-header annotations: {reason}");
+                false
+            });
         }
     }
 }
@@ -1213,10 +1218,11 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                                                 );
                                                 Ok(())
                                             }
-                                            Ok(StreamableHttpPostResponse::Json(msg, ..)) => {
+                                            Ok(StreamableHttpPostResponse::Json(mut msg, ..)) => {
                                                 cache_tools_from_response(
                                                     &mut tool_header_cache,
-                                                    &msg,
+                                                    &mut msg,
+                                                    &negotiated_version,
                                                 );
                                                 context.send_to_handler(msg).await?;
                                                 Ok(())
@@ -1262,8 +1268,12 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                             tracing::trace!("client message accepted");
                             Ok(())
                         }
-                        Ok(StreamableHttpPostResponse::Json(message, ..)) => {
-                            cache_tools_from_response(&mut tool_header_cache, &message);
+                        Ok(StreamableHttpPostResponse::Json(mut message, ..)) => {
+                            cache_tools_from_response(
+                                &mut tool_header_cache,
+                                &mut message,
+                                &negotiated_version,
+                            );
                             context.send_to_handler(message).await?;
                             Ok(())
                         }
@@ -1311,12 +1321,16 @@ impl<C: StreamableHttpClient> Worker for StreamableHttpClientWorker<C> {
                     }
                     let _ = responder.send(send_result);
                 }
-                Event::ServerMessage(json_rpc_message) => {
+                Event::ServerMessage(mut json_rpc_message) => {
                     Self::clear_stream_response_pending(
                         &mut pending_stream_response_ids,
                         &json_rpc_message,
                     );
-                    cache_tools_from_response(&mut tool_header_cache, &json_rpc_message);
+                    cache_tools_from_response(
+                        &mut tool_header_cache,
+                        &mut json_rpc_message,
+                        &negotiated_version,
+                    );
                     // send the message to the handler
                     if let Err(e) = context.send_to_handler(json_rpc_message).await {
                         break 'main_loop Err(e);
@@ -1669,3 +1683,86 @@ impl Default for StreamableHttpClientTransportConfig {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::*;
+    use crate::model::{ListToolsResult, NumberOrString, ServerResult, Tool};
+
+    fn tool(name: &'static str, annotation: serde_json::Value) -> Tool {
+        let schema = json!({
+            "type": "object",
+            "properties": {
+                "value": annotation,
+            },
+        });
+        Tool::new(
+            name,
+            name,
+            Arc::new(schema.as_object().expect("object schema").clone()),
+        )
+    }
+
+    #[test]
+    fn cache_tools_removes_invalid_header_annotations() {
+        let valid = tool(
+            "valid",
+            json!({ "type": "string", "x-mcp-header": "Value" }),
+        );
+        let invalid = tool("invalid", json!({ "type": "string", "x-mcp-header": "" }));
+        let mut message = ServerJsonRpcMessage::response(
+            ServerResult::ListToolsResult(ListToolsResult::with_all_items(vec![valid, invalid])),
+            NumberOrString::Number(1),
+        );
+        let mut cache = HashMap::new();
+
+        cache_tools_from_response(&mut cache, &mut message, &ProtocolVersion::V_2026_07_28);
+
+        let ServerJsonRpcMessage::Response(response) = &mut message else {
+            panic!("expected tools/list response");
+        };
+        let ServerResult::ListToolsResult(result) = &mut response.result else {
+            panic!("expected tools/list result");
+        };
+        assert_eq!(
+            (
+                result
+                    .tools
+                    .iter()
+                    .map(|tool| tool.name.as_ref())
+                    .collect::<Vec<_>>(),
+                cache.keys().map(String::as_str).collect::<Vec<_>>(),
+            ),
+            (vec!["valid"], vec!["valid"])
+        );
+    }
+
+    #[test]
+    fn cache_tools_preserves_pre_standard_header_results() {
+        let invalid = tool("legacy", json!({ "type": "string", "x-mcp-header": "" }));
+        let mut message = ServerJsonRpcMessage::response(
+            ServerResult::ListToolsResult(ListToolsResult::with_all_items(vec![invalid])),
+            NumberOrString::Number(1),
+        );
+        let mut cache = HashMap::new();
+
+        cache_tools_from_response(&mut cache, &mut message, &ProtocolVersion::V_2025_11_25);
+
+        let ServerJsonRpcMessage::Response(response) = message else {
+            panic!("expected tools/list response");
+        };
+        let ServerResult::ListToolsResult(result) = response.result else {
+            panic!("expected tools/list result");
+        };
+        assert_eq!(
+            result
+                .tools
+                .iter()
+                .map(|tool| tool.name.as_ref())
+                .collect::<Vec<_>>(),
+            vec!["legacy"]
+        );
+    }
+}

docs/plans/issue-978/quickstart.md

@@ -0,0 +1,19 @@
+# Issue 978 verification
+
+```bash
+cargo test -p mcp-conformance --bin conformance-client
+cargo test -p rmcp --lib --features client,transport-streamable-http-client
+cargo build -p mcp-conformance
+npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 client \
+  --command ./target/debug/conformance-client \
+  --scenario http-standard-headers \
+  --spec-version 2026-07-28
+npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 client \
+  --command ./target/debug/conformance-client \
+  --scenario http-custom-headers \
+  --spec-version 2026-07-28
+npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 client \
+  --command ./target/debug/conformance-client \
+  --scenario http-invalid-tool-headers \
+  --spec-version 2026-07-28
+```

docs/plans/issue-978/research.md

@@ -0,0 +1,26 @@
+# Issue 978 research
+
+## Goal
+
+Make the Rust conformance client satisfy the SEP-2243 client scenarios for the
+2026-07-28 protocol version.
+
+## Findings
+
+- The standard-header scenario still uses the legacy initialize lifecycle, so
+  its first request is sent before draft request metadata is available.
+- The conformance scenario provides `toolCalls`, while `ConformanceContext`
+  only accepts `tool_calls`; the custom values used to exercise Base64 encoding
+  are therefore dropped.
+- Invalid `x-mcp-header` annotations are detected while caching tool schemas,
+  but the corresponding tools remain in the `tools/list` result exposed to the
+  client.
+
+## Decisions
+
+- Use the discover lifecycle only for the three draft SEP-2243 scenarios.
+- Accept both `tool_calls` and `toolCalls` to preserve compatibility with older
+  local invocations.
+- Filter malformed tools from draft `tools/list` responses before forwarding
+  them to the client handler, while preserving pre-SEP behavior.
+

docs/plans/issue-978/tasks.md

@@ -0,0 +1,9 @@
+# Issue 978 implementation tasks
+
+- [x] Add regression coverage for camelCase conformance context and route the
+      SEP-2243 client scenarios through the discover lifecycle.
+- [x] Add regression coverage that keeps valid tools and removes malformed
+      `x-mcp-header` tool definitions for 2026-07-28 without changing older
+      protocol behavior.
+- [x] Run focused unit/integration tests, the three conformance scenarios,
+      formatting, and Clippy.