Introduce a new function to just send user message [Stack 3/3] (#1686)

- MCP server: add send-user-message tool to send user input to a running
Codex session
- Added an integration tests for the happy and sad paths

Changes:
•	Add tool definition and schema.
•	Expose tool in capabilities.
•	Route and handle tool requests with validation.
•	Tests for success, bad UUID, and missing session.


follow‑ups
• Listen path not implemented yet; the tool is present but marked “don’t
use yet” in code comments.
• Session run flag reset: clear running_session_id_set appropriately
after turn completion/errors.

This is the third PR in a stack.
Stack:
Final: #1686
Intermediate: #1751
First: #1750

codex-rs/mcp-server/tests/send_message.rs

@@ -0,0 +1,163 @@
+#![allow(clippy::expect_used)]
+
+use std::path::Path;
+use std::thread::sleep;
+use std::time::Duration;
+
+use codex_mcp_server::CodexToolCallParam;
+use mcp_test_support::McpProcess;
+use mcp_test_support::create_final_assistant_message_sse_response;
+use mcp_test_support::create_mock_chat_completions_server;
+use mcp_types::JSONRPC_VERSION;
+use mcp_types::JSONRPCResponse;
+use mcp_types::RequestId;
+use pretty_assertions::assert_eq;
+use serde_json::json;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_send_message_success() {
+    // Spin up a mock completions server that immediately ends the Codex turn.
+    // Two Codex turns hit the mock model (session start + send-user-message). Provide two SSE responses.
+    let responses = vec![
+        create_final_assistant_message_sse_response("Done").expect("build mock assistant message"),
+        create_final_assistant_message_sse_response("Done").expect("build mock assistant message"),
+    ];
+    let server = create_mock_chat_completions_server(responses).await;
+
+    // Create a temporary Codex home with config pointing at the mock server.
+    let codex_home = TempDir::new().expect("create temp dir");
+    create_config_toml(codex_home.path(), &server.uri()).expect("write config.toml");
+
+    // Start MCP server process and initialize.
+    let mut mcp_process = McpProcess::new(codex_home.path())
+        .await
+        .expect("spawn mcp process");
+    timeout(DEFAULT_READ_TIMEOUT, mcp_process.initialize())
+        .await
+        .expect("init timed out")
+        .expect("init failed");
+
+    // Kick off a Codex session so we have a valid session_id.
+    let codex_request_id = mcp_process
+        .send_codex_tool_call(CodexToolCallParam {
+            prompt: "Start a session".to_string(),
+            ..Default::default()
+        })
+        .await
+        .expect("send codex tool call");
+
+    // Wait for the session_configured event to get the session_id.
+    let session_id = mcp_process
+        .read_stream_until_configured_response_message()
+        .await
+        .expect("read session_configured");
+
+    // The original codex call will finish quickly given our mock; consume its response.
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp_process.read_stream_until_response_message(RequestId::Integer(codex_request_id)),
+    )
+    .await
+    .expect("codex response timeout")
+    .expect("codex response error");
+
+    // Now exercise the send-user-message tool.
+    let send_msg_request_id = mcp_process
+        .send_user_message_tool_call("Hello again", &session_id)
+        .await
+        .expect("send send-message tool call");
+
+    let response: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp_process.read_stream_until_response_message(RequestId::Integer(send_msg_request_id)),
+    )
+    .await
+    .expect("send-user-message response timeout")
+    .expect("send-user-message response error");
+
+    assert_eq!(
+        JSONRPCResponse {
+            jsonrpc: JSONRPC_VERSION.into(),
+            id: RequestId::Integer(send_msg_request_id),
+            result: json!({
+                "content": [
+                    {
+                        "text": "{\"status\":\"ok\"}",
+                        "type": "text",
+                    }
+                ],
+                "isError": false,
+                "structuredContent": {
+                    "status": "ok"
+                }
+            }),
+        },
+        response
+    );
+    // wait for the server to hear the user message
+    sleep(Duration::from_secs(1));
+
+    // Ensure the server and tempdir live until end of test
+    drop(server);
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_send_message_session_not_found() {
+    // Start MCP without creating a Codex session
+    let codex_home = TempDir::new().expect("tempdir");
+    let mut mcp = McpProcess::new(codex_home.path()).await.expect("spawn");
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize())
+        .await
+        .expect("timeout")
+        .expect("init");
+
+    let unknown = uuid::Uuid::new_v4().to_string();
+    let req_id = mcp
+        .send_user_message_tool_call("ping", &unknown)
+        .await
+        .expect("send tool");
+
+    let resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(req_id)),
+    )
+    .await
+    .expect("timeout")
+    .expect("resp");
+
+    let result = resp.result.clone();
+    let content = result["content"][0]["text"].as_str().unwrap_or("");
+    assert!(content.contains("Session does not exist"));
+    assert_eq!(result["isError"], json!(true));
+}
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+fn create_config_toml(codex_home: &Path, server_uri: &str) -> std::io::Result<()> {
+    let config_toml = codex_home.join("config.toml");
+    std::fs::write(
+        config_toml,
+        format!(
+            r#"
+model = "mock-model"
+approval_policy = "never"
+sandbox_mode = "danger-full-access"
+
+model_provider = "mock_provider"
+
+[model_providers.mock_provider]
+name = "Mock provider for test"
+base_url = "{server_uri}/v1"
+wire_api = "chat"
+request_max_retries = 0
+stream_max_retries = 0
+"#
+        ),
+    )
+}