# Observability

Logging, tracing, and metrics guidance. See [Middleware Cookbook](Middleware-Cookbook.md) for logging middleware patterns.

## Tracing Setup
```rust
use tracing_subscriber::{fmt, EnvFilter};

fn init_tracing() {
    fmt()
        .with_env_filter(EnvFilter::from_default_env().add_directive(tracing::Level::INFO.into()))
        .with_target(false)
        .init();
}
```
- Recommended env vars: `RUST_LOG=info` for normal use, `RUST_LOG=fastrmcp=debug,tower_http=debug` for transport debugging.
- Add `LoggingMiddleware` for structured request/response logs around handlers.

## Recommended Fields
- Request: `method`, `id`, `tool/resource/prompt name`, `client.name`, `client.version`, `connection_id` (SSE/WS), `latency_ms`.
- Response: `id`, `status` (ok/error), `error.code`, `error.message`, `subscription_id` (for notifications).
- Transport: `connection_id`, `remote_addr` (if available), `protocol` (stdio/sse/ws), `close_reason`.
- Context logs: use `ctx.log_info/debug/warning/error` to emit MCP notifications back to clients.

## Metrics (DIY)
- Not built-in yet; recommended patterns:
  1. Add a metrics middleware (see Metrics example in `docs/MIDDLEWARE.md`): counters for total requests/errors and histograms for latency.
  2. Expose Prometheus via `axum` route in web transports.
  3. Track subscription counts via `subscription_manager().subscription_count()` and per-URI counts.
  4. Emit progress via `ctx.report_progress(...)` for long-running tools.

## Debugging Tips
- Enable request/response logging with `LoggingMiddleware::requests_only()` during input validation debugging to avoid noisy responses.
- For schema issues, log the deserialized params in the tool and return `InvalidParams` with a concise message.
- Web transports: set `tower_http=debug` and inspect SSE frames/WS frames in browser devtools.
- Reproduce with the examples to isolate framework vs. app issues; see [Examples](Examples.md).

Related: [Troubleshooting](Troubleshooting.md) • [Security](Security.md) for auth/rate limit logging.

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