Improve MCP logs on stderr.

Author: ericdallo

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Bump MCP java sdk to 0.13.1
+- Improve MCP logs on stderr.
 
 ## 0.57.0
 

deps.edn

@@ -15,7 +15,7 @@
         com.github.steffan-westcott/clj-otel-sdk-extension-autoconfigure {:mvn/version "0.2.9"}
         com.github.steffan-westcott/clj-otel-exporter-otlp {:mvn/version "0.2.9"}
         camel-snake-kebab/camel-snake-kebab {:mvn/version "0.4.3"}
-        org.slf4j/slf4j-simple {:mvn/version "2.0.17"}
+        ch.qos.logback/logback-classic {:mvn/version "1.5.18"}
         com.googlecode.java-diff-utils/diffutils {:mvn/version "1.3.0"}
         cheshire/cheshire {:mvn/version "6.0.0"}}
  :aliases

resources/logback.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+  <!-- Console appender that writes to stderr instead of stdout -->
+  <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
+    <target>System.err</target>
+    <encoder>
+      <pattern>[MCP] %msg%n</pattern>
+    </encoder>
+  </appender>
+
+  <!-- Root logger configuration -->
+  <root level="${LOGBACK_LEVEL:-${logback.level:-INFO}}">
+    <appender-ref ref="STDERR" />
+  </root>
+</configuration>

resources/simplelogger.properties

@@ -51,7 +51,7 @@
                           (str "$" var1)
                           (str "${" var2 "}"))))))
 
-(defn ^:private ->transport ^McpTransport [server-config workspaces]
+(defn ^:private ->transport ^McpTransport [server-name server-config workspaces]
   (if (:url server-config)
     ;; HTTP Streamable transport
     (let [url (replace-env-vars (:url server-config))]
@@ -74,10 +74,13 @@
                                first
                                :uri
                                shared/uri->filename)
-                       (config/get-property "user.home"))]
-      (proxy [StdioClientTransport] [(.build b) (McpJsonMapper/getDefault)]
-        (getProcessBuilder [] (-> (ProcessBuilder. ^List pb-init-args)
-                                  (.directory (io/file work-dir))))))))
+                       (config/get-property "user.home"))
+          stdio-transport (proxy [StdioClientTransport] [(.build b) (McpJsonMapper/getDefault)]
+                            (getProcessBuilder [] (-> (ProcessBuilder. ^List pb-init-args)
+                                                      (.directory (io/file work-dir)))))]
+      (.setStdErrorHandler stdio-transport (fn [msg]
+                                             (logger/info logger-tag (format "[%s] %s" server-name msg))))
+      stdio-transport)))
 
 (defn ^:private ->client ^McpSyncClient [name transport init-timeout workspaces]
   (-> (McpClient/sync transport)
@@ -166,7 +169,7 @@
         server-config (get-in config [:mcpServers name])
         obj-mapper (ObjectMapper.)
         init-timeout (:mcpTimeoutSeconds config)
-        transport (->transport server-config workspaces)
+        transport (->transport name server-config workspaces)
         client (->client name transport init-timeout workspaces)]
     (on-server-updated (->server name server-config :starting db))
     (swap! db* assoc-in [:mcp-clients name] {:client client :status :starting})