OpenTelemetry

Reference for the spans, attributes, and metrics emitted by libsy and its LLM client.

Host setup

Your application installs a tracing-opentelemetry subscriber and global OTel meter provider, and owns exporters, filtering, sampling, context propagation, and flushing. Libsy installs none of these and sends no telemetry itself. Environment variables alone do not enable collection. See OTel Rust setup and OTLP configuration.

Spans and attributes

Span	Emitted by	Meaning
libsy.run	Libsy	One algorithm run, including routing-time work. OpenInference kind CHAIN.
libsy.llm_call	Libsy driver	Waiting for the host to fulfill an offloaded call. Includes host queueing. OpenInference kind CHAIN.
libsy.client_call, exported as chat <model_id>	LLM client driver	One candidate model call, including that candidate's retries. OTel kind CLIENT; OpenInference kind LLM.

Hosts driving run_stream without the LLM client driver instrument their own model I/O.

Routing outcome fields

On libsy.run:

Attribute	Type	Meaning / presence
algorithm, switchyard.algorithm	string	Name from Algorithm::name().
switchyard.route	string	Inbound request model/route, when present.
outcome	string	ok or error when the algorithm task resolves.
outcome_id	string	Successful outcome's ID. OutcomeMetadata::new generates a UUIDv7.
selected_model_ids	string array	Successful outcome's selected model followed by ordered fallbacks. This is a plan, not proof that every model was called.
session_id, session.id	string	Request session ID, when supplied. Both names carry the same value.
agent_id, task_id, task_kind, agent_role, correlation_id	string	Corresponding request metadata, when supplied.
evidence.source, evidence.verdict, evidence.trigger, evidence.reason_code	string	Known string fields from outcome evidence, when present.
evidence.score, evidence.confidence, evidence.threshold	number	Known numeric fields from outcome evidence, when present.

RoutingOutcome.metadata is available in Rust and Python. Its optional JSON evidence exports only the listed keys with matching types; unknown keys and wrong types are omitted. Evidence may be absent. Scores and confidence have algorithm-specific meanings. Failed runs return typed errors, not outcome metadata; successful fail-open decisions may carry a fixed reason_code.

Model-call fields

The LLM client driver records these on libsy.client_call:

Attribute	Type	Meaning / presence
algorithm, switchyard.algorithm, selected_model	string	Algorithm and candidate model ID.
switchyard.candidate, switchyard.candidate_count	integer	One-based candidate position and number of candidates.
gen_ai.operation.name	string	chat.
gen_ai.request.model	string	Requested model; the translating client records the upstream model name.
gen_ai.request.stream	boolean	Recorded as true for streaming requests; otherwise omitted.
gen_ai.request.temperature, gen_ai.request.top_p	number	Sampling values represented in the request IR, when set.
gen_ai.request.top_k, gen_ai.request.max_tokens	integer	Sampling/output limits, when set.
gen_ai.request.reasoning.level, gen_ai.output.type	string	Reasoning effort and recognized output type (text or json), when set.
gen_ai.conversation.id	string	Request session ID, when supplied.
server.address, server.port	string, integer	Upstream host and port, recorded by the translating client.
gen_ai.response.id, gen_ai.response.model	string	Values supplied by the upstream response.
gen_ai.response.finish_reasons	string array	Available normalized stop reasons.
gen_ai.usage.input_tokens	integer	Input tokens including cache reads and cache creation.
gen_ai.usage.output_tokens	integer	Output tokens.
gen_ai.usage.cache_read.input_tokens, gen_ai.usage.cache_creation.input_tokens	integer	Cache-read and cache-creation input tokens.
gen_ai.usage.reasoning.output_tokens	integer	Reasoning output tokens.
outcome	string	ok, error, or cancelled.
error.type, error	string	Failure category/status and error description on this client span.

gen_ai.provider.name is intentionally unset: an endpoint or model name does not reliably identify the provider. Usage fields are omitted when unavailable, not invented as zero. Available counts are capped at OTel's signed integer maximum.

Metrics

Metrics use the switchyard meter scope. The tables use OTel instrument names.

Routing and client metrics

Instrument	Type	Labels	Meaning
switchyard.runs	Counter	algorithm, outcome	Completed algorithm tasks, including failures.
switchyard.run_duration_ms	Histogram	algorithm, outcome	Algorithm-task duration in milliseconds.
switchyard.algorithms_in_flight	UpDownCounter	algorithm	Active algorithm tasks; exported as a Prometheus gauge.
switchyard.decisions	Counter	algorithm, selected_model	Published routing decisions.
switchyard.llm_calls	Counter	algorithm, selected_model, outcome	Logical offloaded and terminal model calls.
switchyard.llm_call_duration_ms	Histogram	algorithm, selected_model, outcome	Logical call duration in milliseconds; see streaming limits below.
switchyard.total_requests	ObservableGauge	none	Process-wide total of successful and failed answer candidates after routing, including reused routing responses.
switchyard.total_errors	ObservableGauge	none	Process-wide total of failures after routing, including failed answer candidates and reused routing responses.
switchyard.requests	Counter	model	Successful answer candidates or reused routing responses, by model ID.
switchyard.errors	Counter	model	Failed answer candidates or reused routing responses, by model ID.
switchyard.model_call_latency_ms	Histogram	model	Successful answer-candidate duration in milliseconds, including that candidate's retries and stream consumption. Excludes routing time and reused routing responses.
switchyard.routing_overhead_ms	Histogram	algorithm	LLM client driver's time to obtain a successful routing outcome, including judge calls but excluding any subsequent answer call.
switchyard.classifier_fail_open	Counter	judge_model, reason	Judge failures that caused classification to proceed without a verdict.
switchyard.upstream_attempts	Counter	outcome, code	HTTP attempts, including retries, made by the translating client.
switchyard.router_retry_recovered	Counter	none	Upstream operations that succeeded after a retry.

Algorithm/call outcome is ok or error. HTTP attempt outcome is ok for 2xx, retryable_error for 408/429/5xx or failures without a status, and other_error otherwise. code is an allowlisted status, a status-class bucket, or none. Classifier reason is timeout, transport, upstream_5xx, upstream_non_5xx, invalid_response, parse_error, client_error, or call_error.

Logical calls, candidate calls, and HTTP attempts are different counts. Candidate fallbacks and HTTP retries remain within one logical call. A response produced during routing is not counted again as a new terminal model call.

The five request/error/latency instruments above record each answer candidate after routing, not each HTTP attempt. A failed candidate followed by a successful fallback adds two to total_requests, one to total_errors, one to errors for the failed model, and one to requests for the successful model. Routing-time calls and failures before a routing outcome do not contribute. A response reused from routing contributes one to total_requests and one to either requests or errors (also total_errors on failure), but no model_call_latency_ms sample. The model label is the candidate's configured model ID (the serving model for a reused response), not the selected_model label used by logical-call metrics. The two total gauges are cumulative process-wide compatibility values, not counts of requests currently in flight.

For streams, these five instruments record when the response stream ends or is dropped. Stream errors count as failures. Dropping an unfinished stream also counts as a failure; a terminal message permits a successful drop. Failed calls have no model_call_latency_ms sample.

Algorithm-specific metrics

Stage Router instruments use the prefix switchyard.stage_router.:

Suffix	Type	Labels	Meaning
routing_decisions	Counter	decision_source, target_name	Choices by decision source and semantic target name.
probability	Histogram	none	Scorer's capable-model probability.
confidence	Histogram	none	Confidence used to resolve or defer a turn.
severity	Histogram	none	Tool-failure severity.
spinning	Histogram	none	Repeated unproductive tool activity.
exploring	Histogram	none	Exploratory tool activity.
production_intensity	Histogram	none	Production-oriented tool activity.

These histograms contain unitless values from 0 to 1. They are recorded when tool signals reach the scorer, including when it defers to a classifier. They are not one sample per application request and are not split by route or session.

Advisor Gate instruments use the prefix switchyard.advisor_gate.:

Suffix	Type	Labels	Meaning
reviews	Counter	verdict, trigger	Review outcomes and what triggered them.
consult_failures	Counter	reason	Failed advisor consultations.
discarded_turns	Counter	none	Executor turns discarded after a redo verdict.
discarded_tokens	Counter	kind	Tokens in discarded turns; kind is input, cached, cache_creation, or output.

Limits

Timing and streaming

• libsy.run may finish before the answer call. Nested algorithms have separate run spans.
• libsy.llm_call and logical call-duration metrics end at response-handle availability, not stream completion. The span's input_tokens, output_tokens, total_tokens, and reasoning_tokens fields are buffered-response only.
• libsy.client_call remains open while its stream is consumed. IDs, usage, and finish reasons update from normalized events. An unfinished stream dropped by its consumer records cancelled; a stream error records error.

Data exposure

Built-in evidence excludes prompts, responses, and raw errors. Custom evidence is checked for field names and types, not string contents or lengths. Client spans may include error descriptions and upstream addresses; algorithm logs may include content such as Advisor Gate's reply_head. Supplied session/correlation IDs are included. Review collected data before exporting it outside your deployment.

Custom algorithms can record OTel instruments directly. Keep metric labels to configured names and fixed categories, not request/session IDs or user text.

Source and stability

These are current implementation names, not a separately versioned telemetry schema. Debug spans and log events are not a stable field contract.

• Libsy spans, outcome projection, and metrics
• Client span fields and stream/usage observation
• Client metrics
• Stage Router metrics and Advisor Gate metrics