> ## Documentation Index > Fetch the complete documentation index at: https://docs.trulayer.ai/llms.txt > Use this file to discover all available pages before exploring further. # Glossary > Every TruLayer term, defined once. ## Agent An AI system that makes multiple LLM calls and tool calls in a loop to accomplish a goal. In TruLayer, one agent turn is usually one [trace](/concepts/traces-and-spans), with each LLM call and tool call as child spans. ## API key A `tl_`-prefixed bearer token that authenticates SDK and server-side API requests. Shown in plaintext once at creation; only an HMAC hash is stored. Tenant-scoped — grants access to every project in your organisation. Manage in Dashboard → Settings → API keys. ## Batch A group of traces shipped to the ingest API in one request. The SDK batches automatically — default `batchSize` is 50 traces or 2 seconds, whichever comes first. ## BYOK "Bring Your Own Key." Using your own model-provider credentials (OpenAI, Anthropic, etc.) for [evaluator](#evaluator) LLM calls, so evaluation runs on your billing and your rate limits. Configure per-project in Dashboard → Settings → Evaluators. If no BYOK credential is set, TruLayer falls back to its managed judge model (counted against your eval quota). ## Control loop TruLayer's real-time response layer for production failures — kill switches, auto-retries, and fallback model routing. Configured in Dashboard → Control. See the [Control dashboard](/dashboard/control) for the live view and [model routing](/guides/model-routing) for the routing primitive. ## Dataset A named collection of traces used for regression testing. Running an [eval](#eval) over a dataset produces a score-per-trace that you can compare across app versions. ## DLQ Dead-letter queue. Traces that fail ingestion validation or downstream processing after retries are parked here instead of being silently dropped. View recent DLQ activity in Dashboard → Ingestion health. Common causes: malformed metadata, schema drift, over-size payloads. ## Eval An automated judgement of a trace, producing a `score`, `label`, and optional `reasoning`. Types: `llm` (an LLM judges against a rubric) and `rule` (deterministic code). See [Evaluations](/concepts/evals). ## Eval run One invocation of an [evaluator](#evaluator) against one [trace](#trace) (or one trace in a [dataset](#dataset)). Has a status (`pending`, `running`, `succeeded`, `failed`) and, on success, a `score` and optional `reasoning`. Visible in the trace detail view and aggregated into [metrics](#metric). ## Evaluator The definition of a judgement — prompt, rubric, model, and trigger. An evaluator applied to a trace produces an [eval run](#eval-run). Built-in evaluators (hallucination, toxicity, answer-relevance) ship with TruLayer; custom evaluators are defined in the dashboard. Manage in Dashboard → Evals. ## Event A discrete log entry within a trace — e.g. "cache hit", "fallback triggered". Lower-weight than a span; no duration, just a timestamp and metadata. ## Feedback A human-provided label on a trace — thumbs up/down, numeric score, free-text comment. See [Feedback](/concepts/feedback). ## Fluid Compute Vercel's default serverless runtime (reuses instances across concurrent requests, full Node.js support). The TruLayer dashboard runs on Fluid Compute. Not to be confused with the TruLayer SDK, which runs anywhere. ## Hallucination An LLM output that's confident but factually wrong. The `hallucination` built-in evaluator scores traces for this using an LLM judge with access to the retrieved context. ## Ingest The act of shipping a trace from your app to TruLayer via `POST /v1/ingest` (single) or `POST /v1/ingest/batch` (batch). Handled automatically by the SDKs. ## Instrumentation Code that creates traces and spans. Manual instrumentation: calling `trace()` / `span()` yourself. Auto-instrumentation: `instrument_openai()` and friends, which monkey-patch a provider client so every call becomes a span. ## Metric An aggregated number over traces — error rate, p95 latency, total cost, feedback positive rate. Computed continuously; query via `/v1/metrics` or the dashboard. See [Metrics](/concepts/metrics). ## PII redaction The process of stripping personally identifiable information from trace inputs, outputs, and metadata before the SDK ships data off your host. Implemented via the [scrub function](#scrub-function). Required for compliance with most privacy regulations — never rely on server-side "we'll mask it later" masking. See the [redaction guide](/sdks/redaction). ## OpenAPI The machine-readable spec of the TruLayer HTTP API, kept in sync with the TruLayer backend service. The [API reference](/api-reference/introduction) is auto-generated from it. ## Project A logical grouping within a tenant. Typical usage: one project per app (e.g. `rag-prod`, `agent-staging`). Created implicitly on first trace ingest; scoped by project in the dashboard. ## Sampling Sending only a fraction of traces to TruLayer, controlled by `sample_rate`. Applied per trace (so all spans of a sampled trace are kept). Use deterministic sampling (hash-based) when consistency matters across retries. ## Scrub function A callback passed to `init()` that's run over every trace/span input and output before the data leaves your process. The mechanism for PII redaction. ## Session A group of traces tied to the same conversation or workflow by `session_id`. See [Sessions](/concepts/sessions). ## Span A step within a [trace](#trace). Types: `llm`, `retrieval`, `tool`, `custom`. Has duration, input, output, optional error. See [Traces and spans](/concepts/traces-and-spans). ## Tag A string label attached to a trace for filtering. Lighter-weight than metadata — meant for categorical filters in the dashboard. ## Tenant A TruLayer organisation — maps 1:1 to a Clerk Organization. All your projects, API keys, and users live inside one tenant. ## Trace One end-to-end unit of work — typically one user request, one agent turn, or one background job. Contains one or more spans. See [Traces and spans](/concepts/traces-and-spans).