# Semantic conventions for feature flags in events LLMS index: [llms.txt](/llms.txt) --- **Status**: [Development][DocumentStatus] ## Motivation Feature flags are commonly used in modern applications to decouple feature releases from deployments. Many feature flagging tools support the ability to update flag configurations in near real-time from a remote feature flag management service. They also commonly allow rulesets to be defined that return values based on contextual information. For example, a feature could be enabled only for a specific subset of users based on context (e.g. users email domain, membership tier, country). Since feature flags are dynamic and affect runtime behavior, it's important to collect relevant feature flag telemetry signals. This can be used to determine the impact a feature has on a request, enabling enhanced observability use cases, such as A/B testing or progressive feature releases. ## Recording an evaluation Feature flag evaluations SHOULD be recorded as attributes on the [Event](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.54.0/specification/logs/data-model.md#events) passed to the [Logger](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.54.0/specification/logs/api.md#logger) emit operations. > [!NOTE] > Events are in-development and not yet available in some languages. Check the [spec-compliance matrix](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.54.0/spec-compliance-matrix.md#logs) to see the implementation status in the corresponding language. ## Evaluation event The table below indicates which attributes should be added to the [Event](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.54.0/specification/logs/data-model.md#log-and-event-record-definition) and their types. **Status:** ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) The event name MUST be `feature_flag.evaluation`. Defines feature flag evaluation as an event. A `feature_flag.evaluation` event SHOULD be emitted whenever a feature flag value is evaluated, which may happen many times over the course of an application lifecycle. For example, a website A/B testing different animations may evaluate a flag each time a button is clicked. A `feature_flag.evaluation` event is emitted on each evaluation even if the result is the same. **Attributes:** | Key | Stability | [Requirement Level](/docs/specs/semconv/general/attribute-requirement-level/) | Value Type | Description | Example Values | | --- | --- | --- | --- | --- | --- | | [`feature_flag.key`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Required` | string | The lookup key of the feature flag. | `logo-color` | | [`error.type`](/docs/specs/semconv/registry/attributes/error.md) | ![Stable](https://img.shields.io/badge/-stable-lightgreen) | `Conditionally Required` [1] | string | Describes a class of error the operation ended with. [2] | `provider_not_ready`; `targeting_key_missing`; `provider_fatal`; `general` | | [`feature_flag.result.value`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Conditionally Required` [3] | any | The evaluated value of the feature flag. [4] | `#ff0000`; `true`; `3` | | [`feature_flag.result.variant`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Conditionally Required` [5] | string | A semantic identifier for an evaluated flag value. [6] | `red`; `true`; `on` | | [`feature_flag.context.id`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` | string | The unique identifier for the flag evaluation context. For example, the targeting key. | `5157782b-2203-4c80-a857-dbbd5e7761db` | | [`feature_flag.error.message`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` [7] | string | A message providing more detail about an error that occurred during feature flag evaluation in human-readable form. [8] | `Unexpected input type: string`; `The user has exceeded their storage quota` | | [`feature_flag.provider.name`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` | string | Identifies the feature flag provider. | `Flag Manager` | | [`feature_flag.result.reason`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` | string | The reason code which shows how a feature flag value was determined. | `static`; `targeting_match`; `error`; `default` | | [`feature_flag.set.id`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` | string | The identifier of the [flag set](https://openfeature.dev/specification/glossary/#flag-set) to which the feature flag belongs. | `proj-1`; `ab98sgs`; `service1/dev` | | [`feature_flag.version`](/docs/specs/semconv/registry/attributes/feature-flag.md) | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | `Recommended` | string | The version of the ruleset used during the evaluation. This may be any stable value which uniquely identifies the ruleset. | `1`; `01ABCDEF` | **[1] `error.type`:** If and only if an error occurred during flag evaluation. **[2] `error.type`:** If one of these values applies, then it MUST be used; otherwise, a custom value MAY be used. | Value | Description | Stability | | --- | --- | --- | | `flag_not_found` | The flag could not be found. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `invalid_context` | The evaluation context does not meet provider requirements. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `parse_error` | An error was encountered parsing data, such as a flag configuration. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `provider_fatal` | The provider has entered an irrecoverable error state. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `provider_not_ready` | The value was resolved before the provider was initialized. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `targeting_key_missing` | The provider requires a targeting key and one was not provided in the evaluation context. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `type_mismatch` | The type of the flag value does not match the expected type. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `general` | The error was for a reason not enumerated above. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | **[3] `feature_flag.result.value`:** If and only if feature flag provider does not supply variant or equivalent concept. Otherwise, `feature_flag.result.value` should be treated as opt-in. **[4] `feature_flag.result.value`:** With some feature flag providers, feature flag results can be quite large or contain private or sensitive details. Because of this, `feature_flag.result.variant` is often the preferred attribute if it is available. It may be desirable to redact or otherwise limit the size and scope of `feature_flag.result.value` if possible. Because the evaluated flag value is unstructured and may be any type, it is left to the instrumentation author to determine how best to achieve this. **[5] `feature_flag.result.variant`:** If feature flag provider supplies a variant or equivalent concept. **[6] `feature_flag.result.variant`:** A semantic identifier, commonly referred to as a variant, provides a means for referring to a value without including the value itself. This can provide additional context for understanding the meaning behind a value. For example, the variant `red` maybe be used for the value `#c05543`. **[7] `feature_flag.error.message`:** If and only if an error occurred during flag evaluation and `error.type` does not sufficiently describe the error. **[8] `feature_flag.error.message`:** Should not simply duplicate the value of `error.type`, but should provide more context. For example, if `error.type` is `invalid_context` the `feature_flag.error.message` may enumerate which context keys are missing or invalid. --- `error.type` has the following list of well-known values. If one of them applies, then the respective value MUST be used; otherwise, a custom value MAY be used. | Value | Description | Stability | | --- | --- | --- | | `_OTHER` | A fallback error value to be used when the instrumentation doesn't define a custom value. | ![Stable](https://img.shields.io/badge/-stable-lightgreen) | --- `feature_flag.result.reason` has the following list of well-known values. If one of them applies, then the respective value MUST be used; otherwise, a custom value MAY be used. | Value | Description | Stability | | --- | --- | --- | | `cached` | The resolved value was retrieved from cache. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `default` | The resolved value fell back to a pre-configured value (no dynamic evaluation occurred or dynamic evaluation yielded no result). | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `disabled` | The resolved value was the result of the flag being disabled in the management system. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `error` | The resolved value was the result of an error. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `split` | The resolved value was the result of pseudorandom assignment. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `stale` | The resolved value is non-authoritative or possibly out of date | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `static` | The resolved value is static (no dynamic evaluation). | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `targeting_match` | The resolved value was the result of a dynamic evaluation, such as a rule or specific user-targeting. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | | `unknown` | The reason for the resolved value could not be determined. | ![Release Candidate](https://img.shields.io/badge/-rc-mediumorchid) | [DocumentStatus]: /docs/specs/otel/document-status