Add MCP logging capability with structured log forwarding

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ### Added
 
+- MCP logging capability declaration and `logging/setLevel` request handler that allows MCP clients to control structured log output according to the MCP protocol specification (2025-06-18) ([#340](https://github.com/microsoft/wassette/pull/340))
+- MCP logging layer (`mcp_logging.rs`) that bridges tracing log events to MCP logging notifications with support for all syslog severity levels (Debug, Info, Notice, Warning, Error, Critical, Alert, Emergency)
 - AI agent development guides (`AGENTS.md` and `Claude.md`) that consolidate development guidelines from `.github/instructions/` into accessible documentation for AI agents working on the project
 - Comprehensive installation guide page consolidating all installation methods (one-liner script, Homebrew, Nix, WinGet) organized by platform (Linux, macOS, Windows) with verification steps and troubleshooting sections
 - Cookbook section in documentation with language-specific guides for building Wasm components in JavaScript/TypeScript, Python, Rust, and Go ([#328](https://github.com/microsoft/wassette/pull/328))

Cargo.toml

@@ -13,6 +13,7 @@ license-file = "LICENSE"
 
 [workspace.dependencies]
 anyhow = "1.0"
+chrono = "0.4"
 component2json = { path = "crates/component2json" }
 etcetera = "0.10"
 futures = "0.3"
@@ -45,6 +46,7 @@ wasmtime-wasi-config = "36"
 [dependencies]
 anyhow = { workspace = true }
 axum = "0.8"
+chrono = { workspace = true }
 clap = { version = "4.5", features = ["derive"] }
 etcetera = { workspace = true }
 figment = { version = "0.10", features = ["env", "toml"] }

docs/SUMMARY.md

@@ -18,6 +18,7 @@
 
 - [CLI](./reference/cli.md)
 - [Permissions](./reference/permissions.md)
+- [MCP Logging](./mcp-logging.md)
 
 # Design & Architecture
 

docs/mcp-logging.md

@@ -0,0 +1,179 @@
+# MCP Logging
+
+Wassette implements the Model Context Protocol (MCP) logging specification, allowing MCP clients to receive structured log messages from the server.
+
+## Overview
+
+The MCP logging feature provides:
+
+- **Structured log output**: Log messages are sent as JSON-RPC notifications to MCP clients
+- **Level filtering**: Clients can set a minimum log level to control verbosity
+- **Syslog severity levels**: Supports all standard syslog levels (Emergency, Alert, Critical, Error, Warning, Notice, Info, Debug)
+- **Automatic forwarding**: Internal tracing logs are automatically converted to MCP notifications
+
+## Logging Capability
+
+Wassette declares the `logging` capability in its server capabilities:
+
+```json
+{
+  "capabilities": {
+    "logging": {},
+    "tools": { "listChanged": true }
+  }
+}
+```
+
+## Setting Log Level
+
+Clients can control log verbosity using the `logging/setLevel` request:
+
+```json
+{
+  "jsonrpc": "2.0",
+  "method": "logging/setLevel",
+  "params": {
+    "level": "info"
+  },
+  "id": 2
+}
+```
+
+**Supported levels** (in order of decreasing severity):
+- `emergency` - System is unusable
+- `alert` - Action must be taken immediately
+- `critical` - Critical conditions
+- `error` - Error conditions
+- `warning` - Warning conditions
+- `notice` - Normal but significant events
+- `info` - General informational messages
+- `debug` - Detailed debugging information
+
+## Log Message Notifications
+
+After setting a log level, the server sends log messages as `notifications/message` notifications:
+
+```json
+{
+  "jsonrpc": "2.0",
+  "method": "notifications/message",
+  "params": {
+    "level": "info",
+    "logger": "wassette",
+    "data": {
+      "message": "Component loaded successfully",
+      "target": "wassette::lifecycle",
+      "timestamp": "2025-01-09T12:34:56.789Z"
+    }
+  }
+}
+```
+
+## Log Level Filtering
+
+Only log messages at or above the configured minimum level are sent to clients. For example, if the level is set to `info`:
+
+- ✅ `emergency`, `alert`, `critical`, `error`, `warning`, `notice`, and `info` messages are sent
+- ❌ `debug` messages are filtered out
+
+## Implementation Details
+
+### Architecture
+
+Wassette uses a custom `tracing` subscriber layer (`McpLoggingLayer`) that:
+
+1. Intercepts log events from the `tracing` framework
+2. Converts them to MCP `LoggingMessageNotificationParam` structures
+3. Filters based on the client-configured minimum level
+4. Sends notifications to connected MCP clients
+
+### Level Mapping
+
+Tracing levels are mapped to MCP levels as follows:
+
+| Tracing Level | MCP Level |
+|---------------|-----------|
+| `ERROR`       | `error`   |
+| `WARN`        | `warning` |
+| `INFO`        | `info`    |
+| `DEBUG`       | `debug`   |
+| `TRACE`       | `debug`   |
+
+### Message Structure
+
+Each log notification includes:
+
+- **level**: The log severity level
+- **logger**: The source module or component (optional)
+- **data**: A JSON object containing:
+  - `message`: The log message text
+  - `target`: The tracing target (typically module path)
+  - `timestamp`: RFC3339-formatted timestamp
+
+## Example Usage
+
+### With MCP Inspector
+
+```bash
+# Start Wassette server
+cargo run -- serve --sse
+
+# In another terminal, connect with MCP inspector
+npx @modelcontextprotocol/inspector --cli http://127.0.0.1:9001/sse
+
+# Set log level to info
+# (Use inspector UI to send logging/setLevel request)
+```
+
+### Programmatic Example
+
+```javascript
+// Connect to Wassette MCP server
+const client = new MCPClient(transport);
+
+// Initialize connection
+await client.initialize({
+  protocolVersion: "2024-11-05",
+  clientInfo: { name: "my-client", version: "1.0.0" }
+});
+
+// Set log level
+await client.request({
+  method: "logging/setLevel",
+  params: { level: "info" }
+});
+
+// Listen for log notifications
+client.on("notifications/message", (notification) => {
+  const { level, logger, data } = notification.params;
+  console.log(`[${level}] ${logger}: ${data.message}`);
+});
+```
+
+## Security Considerations
+
+- Log messages should not contain sensitive information (credentials, tokens, etc.)
+- The logging layer automatically filters out logs when no client has set a log level
+- Each client can set their own log level independently
+
+## Troubleshooting
+
+### No log messages received
+
+1. Verify the logging capability is declared in server capabilities
+2. Ensure you've sent a `logging/setLevel` request
+3. Check that the log level is appropriate (e.g., `debug` for maximum verbosity)
+4. Verify the MCP client is listening for `notifications/message`
+
+### Too many/too few log messages
+
+Adjust the log level:
+- Use `debug` for detailed troubleshooting
+- Use `info` for normal operation
+- Use `warning` or `error` for production monitoring
+
+## Reference
+
+- [MCP Logging Specification](https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging)
+- [Tracing Documentation](https://docs.rs/tracing/latest/tracing/)
+- [Syslog Severity Levels (RFC 5424)](https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1)

src/main.rs

@@ -23,7 +23,8 @@ use mcp_server::{
 };
 use rmcp::model::{
     CallToolRequestParam, CallToolResult, ErrorData, ListPromptsResult, ListResourcesResult,
-    ListToolsResult, PaginatedRequestParam, ServerCapabilities, ServerInfo, ToolsCapability,
+    ListToolsResult, LoggingLevel, PaginatedRequestParam, ServerCapabilities, ServerInfo,
+    SetLevelRequestParam, ToolsCapability,
 };
 use rmcp::service::{serve_server, RequestContext, RoleServer};
 use rmcp::transport::streamable_http_server::session::local::LocalSessionManager;
@@ -37,6 +38,7 @@ use tracing_subscriber::util::SubscriberInitExt as _;
 mod commands;
 mod config;
 mod format;
+mod mcp_logging;
 
 use commands::{
     Cli, Commands, ComponentCommands, GrantPermissionCommands, PermissionCommands, PolicyCommands,
@@ -194,6 +196,7 @@ const BIND_ADDRESS: &str = "127.0.0.1:9001";
 pub struct McpServer {
     lifecycle_manager: LifecycleManager,
     peer: Arc<Mutex<Option<rmcp::Peer<rmcp::RoleServer>>>>,
+    log_level: Arc<Mutex<Option<LoggingLevel>>>,
 }
 
 /// Handle CLI tool commands by creating appropriate tool call requests
@@ -301,6 +304,7 @@ impl McpServer {
         Self {
             lifecycle_manager,
             peer: Arc::new(Mutex::new(None)),
+            log_level: Arc::new(Mutex::new(None)),
         }
     }
 
@@ -326,6 +330,7 @@ impl ServerHandler for McpServer {
                 tools: Some(ToolsCapability {
                     list_changed: Some(true),
                 }),
+                logging: Some(serde_json::Map::new()),
                 ..Default::default()
             },
             instructions: Some(
@@ -421,6 +426,22 @@ Key points:
             }
         })
     }
+
+    #[allow(clippy::manual_async_fn)]
+    fn set_level(
+        &self,
+        params: SetLevelRequestParam,
+        _ctx: RequestContext<RoleServer>,
+    ) -> impl Future<Output = Result<(), ErrorData>> + Send + '_ {
+        async move {
+            // Store the requested log level
+            let mut log_level = self.log_level.lock().unwrap();
+            *log_level = Some(params.level);
+
+            tracing::info!("MCP logging level set to: {:?}", params.level);
+            Ok(())
+        }
+    }
 }
 
 /// Formats build information similar to agentgateway's version output
@@ -482,23 +503,6 @@ async fn main() -> Result<()> {
                     .into()
                 });
 
-                let registry = tracing_subscriber::registry().with(env_filter);
-
-                // Initialize logging based on transport type
-                let transport: Transport = (&cfg.transport).into();
-                match transport {
-                    Transport::Stdio => {
-                        registry
-                            .with(
-                                tracing_subscriber::fmt::layer()
-                                    .with_writer(std::io::stderr)
-                                    .with_ansi(false),
-                            )
-                            .init();
-                    }
-                    _ => registry.with(tracing_subscriber::fmt::layer()).init(),
-                }
-
                 let config =
                     config::Config::from_serve(cfg).context("Failed to load configuration")?;
 
@@ -521,6 +525,31 @@ async fn main() -> Result<()> {
 
                 let server = McpServer::new(lifecycle_manager.clone());
 
+                // Create MCP logging layer that can forward logs to MCP clients
+                let mcp_layer = mcp_logging::McpLoggingLayer::new(
+                    server.peer.clone(),
+                    server.log_level.clone(),
+                );
+
+                let registry = tracing_subscriber::registry()
+                    .with(env_filter)
+                    .with(mcp_layer);
+
+                // Initialize logging based on transport type
+                let transport: Transport = (&cfg.transport).into();
+                match transport {
+                    Transport::Stdio => {
+                        registry
+                            .with(
+                                tracing_subscriber::fmt::layer()
+                                    .with_writer(std::io::stderr)
+                                    .with_ansi(false),
+                            )
+                            .init();
+                    }
+                    _ => registry.with(tracing_subscriber::fmt::layer()).init(),
+                }
+
                 // Start background component loading
                 let server_clone = server.clone();
                 let lifecycle_manager_clone = lifecycle_manager.clone();