* Reapply span attributes after creation to survive non-forwarding samplers
Tracer.start_span builds the span from sampling_result.attributes, not
the attributes kwarg — a custom Sampler that returns
SamplingResult(RECORD_AND_SAMPLE) without forwarding attributes
silently drops everything FastMCP passed at creation time. Reapply the
same attributes immediately after span creation (guarded by
is_recording()) so on_start hooks and samplers still see them, while
the finished span is guaranteed to carry FastMCP's telemetry
regardless of sampler behavior.
* Restore only missing span attributes, not a blanket reapply
Reapplying all attributes after span creation overwrote values a
sampler deliberately set (e.g. a redacted mcp.method.name) and
inflated dropped-attribute counts when the SDK's attribute limit was
hit. Compare against the span's existing attributes and restore only
the keys a non-forwarding sampler actually dropped, via a shared
restore_missing_attributes() helper in fastmcp.telemetry.
* Gate attribute restore on all-or-nothing, not per-key
Restoring only missing keys reinserted attributes the SDK's bounded
attribute map had already evicted under a low
OTEL_SPAN_ATTRIBUTE_COUNT_LIMIT, evicting a different retained key and
inflating dropped_attributes beyond what the sampler actually dropped.
Gate on none of our attributes being present (plus dropped_attributes
== 0) instead — the regression this exists to fix is a sampler
dropping everything, and eviction under a limit always leaves some.
Renamed restore_missing_attributes to restore_dropped_attributes to
match.
* Gate attribute restore on empty span, not per-key presence
A sampler that intentionally supplies only its own attributes (e.g. to
strip component names or resource URIs for privacy/cardinality
control) left none of FastMCP's keys on the span, so the previous
all-or-nothing gate treated it identically to a bare non-forwarding
sampler and restored everything, defeating the filter. Key off the
span having no attributes at all instead — a bare sampler leaves it
empty, a filtering sampler doesn't.
* Use standard traceparent/tracestate keys per OTel MCP semconv
The OTel semantic conventions for MCP (https://opentelemetry.io/docs/specs/semconv/gen-ai/mcp/)
put `traceparent` and `tracestate` directly in `params._meta` without a prefix.
FastMCP was using `fastmcp.traceparent` / `fastmcp.tracestate`, which meant
non-FastMCP clients sending the standard keys couldn't propagate traces.
Switches injection to the bare keys and adds fallback extraction for the old
prefixed keys so older FastMCP clients still work.
🤖 Generated with Claude Code
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
* chore: Update SDK documentation
* Drop legacy fastmcp.-prefixed trace key fallback
🤖 Generated with Claude Code
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
* chore: Update SDK documentation
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>
A few improvements based on code review:
- Don't override existing trace context in `extract_trace_context` - if we're
already in a valid trace (e.g., from HTTP propagation), preserve it rather
than extracting from MCP meta
- Add exception recording to `delegate_span` to match `server_span` pattern
- Remove unused `get_meter` function (metrics not implemented yet)
- Return `None` instead of `{}` from `inject_trace_context` when nothing to inject
- Clean up trivial tests that were just testing OpenTelemetry's own API
🤖 Generated with Claude Code
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Adds opt-in distributed tracing via OpenTelemetry for observability into
FastMCP server and client operations.
Server spans are created for tool calls, resource reads, and prompt
renders with attributes like component key, component type, provider
type, session ID, and auth context. Client spans wrap outgoing calls
with trace context propagation via W3C headers in request meta.
Components provide their own span attributes through a `get_span_attributes()`
method that subclasses override - this lets LocalProvider, FastMCPProvider,
and ProxyProvider each include relevant context (original names, backend URIs).
To enable: configure an OpenTelemetry SDK with a TracerProvider before
importing fastmcp. Traces export to any OTLP-compatible backend.
Closes ENG-2813
🤖 Generated with Claude Code
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
