> ## Documentation Index > Fetch the complete documentation index at: https://docs.fiddler.ai/llms.txt > Use this file to discover all available pages before exploring further. # Get span field coverage > Given a filter over spans, return the union of span attribute keys and evaluator outputs that appear in the matching spans, along with a coverage count for each field. Useful for schema discovery and data completeness analysis — e.g. before adding spans to an evaluation dataset, an agent or UI can call this endpoint to understand which attributes are present and how consistently they appear across the selected spans, without fetching full span payloads. The query is bounded internally to the first 10,000 matching spans. When this cap is reached, `sampled` is set to `true` and counts are approximate. ## OpenAPI ````yaml POST /v3/spans/fields openapi: 3.0.3 info: title: Fiddler API - 2.0 description: APIs to interact with Fiddler termsOfService: https://fiddler.ai/about/terms contact: email: support@fiddler.ai license: name: Proprietary url: '2.0' version: '2.0' servers: [] security: - BearerAuth: [] tags: - name: access-key description: CRUD operations for API keys - name: span-v3 - name: aggregation-invalidation-request-v3 description: Endpoints related to retrieving aggregation invalidation requests - name: alert-rules-v3 description: CRUD for Alert Rules, Summary, and Stats APIs - name: application-v3 - name: attribute-v3 - name: auth description: Authentication strategies and login flows - name: baseline-v3 description: CRUD for baseline - name: catalog-v3 description: >- Entity catalog provides paginated, searchable discovery of entity names (attribute keys, agent names, span types, span names) and their distinct values. Powered by ClickHouse materialized views — no worker or PostgreSQL dependency. - name: chart-annotation-v3 description: Endpoints related to retrieving chart annotations - name: chart-v3 description: CRUD for chart - name: queries-v3 description: v3 queries API - name: configuration-v3 description: CRUD for configurations - name: custom-metrics-v3 - name: dimensionality-reduction-v3 - name: environment-v3 description: Endpoints related to environment management - name: evals - name: datasets - name: evaluation-v3 - name: evaluator-v3 - name: rule-evaluators-v3 - name: experiment-v3 - name: explainability-v3 - name: llm_rca-v3 - name: file-upload description: Endpoints related to file uploading. - name: fql-expressions-v3 description: > Endpoints for listing FQL (Fiddler Query Language) functions available for GenAI custom metrics. Used by the frontend for autocomplete and signature hints in the FQL editor. - name: genai-alert-rules-v3 description: CRUD API for GenAI Alert Rules - name: genai-custom-metrics-v3 description: API for GenAI Custom Metrics - name: genai-metrics-v3 description: >- Endpoints for pre-aggregated GenAI metrics. All metric data is pre-aggregated by an hourly ClickHouse refreshable materialized view; no real-time aggregation is performed at request time. - name: guardrails-api description: Endpoints related to retrieving Guardrails specific data - name: ingestion-v3 - name: intercom-api description: Endpoints related to intercom APIs - name: jobs-v3 - name: llm-gateway-v3 - name: auth-login - name: auth-logout - name: metrics-v3 description: Metrics endpoints - name: model-v3 - name: dashboard-v3 - name: model-deployment-v3 - name: monitoring-summary-v3 - name: histograms-v3 - name: organization-roles-v3 description: Update user org role - name: organization-settings-v3 description: Update organization settings such as timezone, email configuration, etc. - name: pagerduty-api description: CRUD for Pagerduty services. - name: project-v3 - name: project-roles-v3 description: Project role assignment management - name: searchable-text-key-v3 description: >- Manage the global searchable text keys table that controls which OTel attribute keys are routed to the full-text-searchable `ValueContent` column in the unified attributes table. Changes propagate to the backing ClickHouse dictionary within 1-2 minutes and affect all tenants. - name: segments-v3 - name: semantic-mapping-v3 description: >- Manage the global semantic name mappings table that maps raw OTel attribute keys to canonical semantic concepts. Changes propagate to the backing ClickHouse dictionary within 1-2 minutes and affect all tenants. - name: server-info-v3 description: Endpoints related to retrieving server information - name: sessions-v3 description: v3 session APIs - name: team-roles-v3 - name: team-v3 - name: traces-v3 description: v3 trace api for monitoring - name: fetch-sessions-v3 description: v3 fetch sessions api for monitoring - name: user-access-key description: > CRUD operations for user API keys. All endpoints are user-scoped — each user can only operate on their own API keys. No one including Org admins have access to other users' API keys. - name: users-v3 - name: version-compatibility-v3 - name: webhooks externalDocs: url: https://docs.fiddler.ai description: Find out more about Fiddler paths: /v3/spans/fields: post: tags: - span-v3 summary: Get span field coverage description: >- Given a filter over spans, return the union of span attribute keys and evaluator outputs that appear in the matching spans, along with a coverage count for each field. Useful for schema discovery and data completeness analysis — e.g. before adding spans to an evaluation dataset, an agent or UI can call this endpoint to understand which attributes are present and how consistently they appear across the selected spans, without fetching full span payloads. The query is bounded internally to the first 10,000 matching spans. When this cap is reached, `sampled` is set to `true` and counts are approximate. operationId: getSpanFieldsV3 requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/SpanFieldsRequest' example: application_id: c688de46-2054-49ab-b85f-0f0178cb44d7 start_time: '2026-05-01T00:00:00Z' end_time: '2026-05-18T23:59:59Z' filter: condition: AND rules: - field: Span::span_type operator: eq value: llm responses: '200': description: Span field coverage for the matching spans content: application/json: schema: allOf: - $ref: '#/components/schemas/ApiResponse' - type: object properties: data: $ref: '#/components/schemas/SpanFieldsResponse' example: data: total_spans: 10000 sampled: true span_attributes: - key: system.gen_ai.usage.input_tokens count: 10000 - key: system.gen_ai.usage.output_tokens count: 10000 - key: system.gen_ai.context.retrieval count: 5 evaluator_outputs: - rule_name: Safety output_name: is_toxic count: 10000 - rule_name: Faithfulness output_name: score count: 3200 api_version: '3.0' kind: NORMAL '400': $ref: '#/components/responses/400' '500': $ref: '#/components/responses/500' components: schemas: SpanFieldsRequest: type: object required: - application_id - start_time - end_time properties: application_id: type: string format: uuid description: UUID of the GenAI application to query spans from start_time: type: string format: date-time description: Start of the time range (inclusive, UTC). end_time: type: string format: date-time description: End of the time range (exclusive, UTC). filter: description: >- Optional structured filter tree. Uses the same DSL as POST /spans/query. Maximum nesting depth is 3 levels and maximum total rule count is 10. allOf: - $ref: '#/components/schemas/QueryCondition' search: $ref: '#/components/schemas/SearchFilter' ApiResponse: type: object description: | Response object for standard API responses. properties: api_version: type: string default: '3.0' enum: - '2.0' - '3.0' description: | API version. kind: type: string default: NORMAL enum: - NORMAL description: | Type of response, indicating a normal response. SpanFieldsResponse: type: object required: - total_spans - sampled - span_attributes - evaluator_outputs properties: total_spans: type: integer description: >- Number of spans used to compute coverage. Capped at 10,000. When sampled is true this is 10,000 and counts are approximate; when sampled is false this is the exact number of matching spans. sampled: type: boolean description: >- True when the result is based on a 10,000-span sample of a larger result set. Counts are approximate in this case. span_attributes: type: array items: $ref: '#/components/schemas/SpanAttributeCoverage' description: >- Span attribute keys found across the sampled spans, each with a coverage count. evaluator_outputs: type: array items: $ref: '#/components/schemas/EvaluatorOutputCoverage' description: >- Evaluator rule/output pairs found across the sampled spans, each with a coverage count. QueryCondition: type: object description: Recursive filter condition supporting AND/OR nesting with rules required: - condition - rules properties: condition: $ref: '#/components/schemas/QueryConditionType' rules: type: array items: anyOf: - $ref: '#/components/schemas/QueryCondition' - $ref: '#/components/schemas/QueryRule' SearchFilter: type: object required: - query properties: query: type: string description: Free-form search text. Case-insensitive substring match. minLength: 3 maxLength: 64 scope: $ref: '#/components/schemas/SearchScope' SpanAttributeCoverage: type: object required: - key - count properties: key: type: string description: Span attribute key (e.g. "system.gen_ai.usage.input_tokens") count: type: integer description: >- Number of spans (within the sample) that have this attribute key. Divide by total_spans to get the coverage ratio. EvaluatorOutputCoverage: type: object required: - rule_name - output_name - count properties: rule_name: type: string description: Name of the evaluator rule output_name: type: string description: Name of the specific output within the rule count: type: integer description: >- Number of spans (within the sample) that have a score for this evaluator output. ErrorResponse: type: object description: | Response object for errors returned by the API. properties: api_version: type: string default: '3.0' enum: - '2.0' - '3.0' description: | API version of the response. kind: type: string default: ERROR enum: - ERROR description: | Type of response, usually indicating an error. error: type: object properties: code: type: integer format: int32 description: > Represents the code for this error, typically an HTTP response code. default: 400 enum: - 400 - 403 - 404 - 500 message: type: string description: > A human-readable message providing more details about the error. If there are multiple errors, it will be the message for the first error. example: Resource Not Found errors: type: array description: > Container for additional information regarding the error, especially for multiple errors. items: type: object properties: reason: type: string description: > Unique identifier for this error, different from the error code. example: ResourceNotFoundException message: type: string description: > A human-readable message providing more details about the error. If there is only one error, this field will match error.message. example: Resource Not Found help: type: string description: > Link to support or documentation providing more information on the error. QueryConditionType: type: string description: Condition type to apply for filters enum: - AND - OR QueryRule: type: object description: Rule to apply for filters required: - field - operator - value properties: field: type: string description: Name of the field to apply the filter on operator: $ref: '#/components/schemas/OperatorType' value: description: value of the filter anyOf: - type: string - type: number - type: boolean - type: array items: type: string SearchScope: type: string enum: - all - input - output OperatorType: type: string description: Type of operator enum: - EQUAL - NOT_EQUAL - IN - NOT_IN - LESS - LESS_OR_EQUAL - GREATER - GREATER_OR_EQUAL - BETWEEN responses: '400': description: Bad Request content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '500': description: Internal Server Error content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' securitySchemes: BearerAuth: type: http scheme: bearer ````