> ## Documentation Index > Fetch the complete documentation index at: https://docs.beaconcha.in/llms.txt > Use this file to discover all available pages before exploring further. # Sync committee for a period > Retrieve the sync committee membership for a given sync period. The sync committee contains 512 validator indices that participate in light-client finality via sync committee signatures. The validators in the response are ordered by the sync-committee index (the position used when interpreting syncaggregate_bits in blocks). A sync period spans 256 consecutive epochs. Sync committees were introduced with the Altair upgrade. Requests for periods before Altair return an error because no sync committee exists for those periods. On Ethereum mainnet, Altair activated at epoch 74240 (period 290). ## OpenAPI ````yaml /api/v1/bundled.yaml get /api/v1/sync_committee/{period} openapi: 3.0.4 info: title: beaconcha.in API Documentation license: name: GPL-3.0 url: https://www.gnu.org/licenses/gpl-3.0.en.html description: >- > ⚠️ No new features or endpoints will be added to V1. We recommend the [V2 API](/api/overview) for all new projects. See the [migration guide](/api/v1-to-v2-migration). ## Introduction **Advanced and reliable API for accessing comprehensive Ethereum blockchain data.** - **Free Usage Policy:** The API is free to use under a fair use policy, with rate limits of 10 requests per minute per IP. - **Caching:** All responses are cached for 1 minute. - **Higher Usage Plans:** For higher usage plans, visit: [https://beaconcha.in/pricing](https://beaconcha.in/pricing). An API key is required to use these plans. ### API Key Usage API keys can be obtained at [/user/settings](https://beaconcha.in/user/settings) and must be included in requests either as a query string parameter or in the request header. #### Example: Query String Parameter ```bash curl https://beaconcha.in/api/v1/slot/1?apikey= ``` #### Example: Request Header ```bash curl -H 'apikey: ' https://beaconcha.in/api/v1/slot/1 ``` contact: {} version: '1.1' servers: - url: https://beaconcha.in description: Ethereum mainnet - url: https://hoodi.beaconcha.in description: Hoodi Ethereum testnet security: - ApiKeyAuthQuery: [] - ApiKeyAuthHeader: [] tags: - name: Epoch description: Data related to consensus layer epochs - name: Slots description: Data related to consensus layer slots - name: Validators description: Data related to consensus layer validators - name: Rewards description: Data related to validator rewards - name: Sync Committees description: Data related to sync committees - name: Rocketpool description: Data related to the rocketpool protocol - name: ETH.Store® description: Data related to the ETH.Store® metric - name: Validator deposits description: Data related to execution layer validator deposits - name: Blocks description: Data related to execution layer blocks - name: Gas description: Data related to gas prices - name: Address description: Data related to ethereum addresses - name: Network description: Network data - name: Misc description: Miscellaneous data - name: User description: User data - name: Machine Metrics description: Submit machine, beacon-node, and validator-client monitoring metrics paths: /api/v1/sync_committee/{period}: get: tags: - Sync Committees summary: Sync committee for a period description: >- Retrieve the sync committee membership for a given sync period. The sync committee contains 512 validator indices that participate in light-client finality via sync committee signatures. The validators in the response are ordered by the sync-committee index (the position used when interpreting syncaggregate_bits in blocks). A sync period spans 256 consecutive epochs. Sync committees were introduced with the Altair upgrade. Requests for periods before Altair return an error because no sync committee exists for those periods. On Ethereum mainnet, Altair activated at epoch 74240 (period 290). operationId: getSyncCommittee parameters: - name: period in: path description: >- The sync period to fetch. Provide a non-negative integer period index, or use one of the special tags: `latest` (current period) or `next` (the upcoming period if available). required: true style: simple explode: false schema: oneOf: - type: string description: Special tags for convenience. enum: - latest - next example: latest - type: integer description: Sync period index (>= 0) minimum: 0 example: 500 responses: '200': description: >- The sync committee for the requested period was found. The `data` object contains the period index, the start and end epochs, and the list of 512 validator indices in committee order. content: application/json: schema: allOf: - $ref: '#/components/schemas/types.ApiResponse' - type: object properties: data: $ref: '#/components/schemas/types.APISyncCommitteeResponse' '400': description: >- The request is invalid (for example, a malformed period or a period before Altair). content: application/json: schema: $ref: '#/components/schemas/types.ApiErrorResponse' examples: invalidPeriod: summary: Invalid period parameter value: status: 'ERROR: invalid period parameter' data: null '500': description: An unexpected server error occurred while processing your request. content: application/json: schema: $ref: '#/components/schemas/types.ApiErrorResponse' examples: serverError: summary: Internal error value: status: 'ERROR: internal server error' data: null components: schemas: types.ApiResponse: type: object description: Standard success envelope used by all API endpoints. properties: status: type: string description: >- HTTP-independent status of the API operation. For successful responses this is always "OK". enum: - OK data: description: Response payload specific to the endpoint. nullable: true x-omitempty: true types.APISyncCommitteeResponse: type: object description: >- Sync committee membership for a single sync period. Includes the period index, the start and end epochs, and the 512 validator indices ordered by sync-committee index. The ordering matches the bit positions in `syncaggregate_bits` found in blocks. properties: period: type: integer description: Sync period index (>= 0). Each period spans 256 consecutive epochs. minimum: 0 example: 500 start_epoch: type: integer description: >- Epoch at which the sync period begins. On Ethereum mainnet this equals `period * 256`. minimum: 0 example: 128000 end_epoch: type: integer description: >- Epoch at which the sync period ends (inclusive). On Ethereum mainnet this equals `start_epoch + 255`. minimum: 0 example: 128255 validators: type: array description: >- List of validator indices that form the sync committee for this period, ordered by sync-committee index (length 512 as per the consensus spec). items: type: integer minimum: 0 minItems: 512 maxItems: 512 example: - 383220 - 12142 - 265015 - 260079 types.ApiErrorResponse: type: object description: Standard error envelope returned when a request fails. properties: status: type: string description: >- Human-readable error indicator and message. The value begins with "ERROR:" followed by details. example: 'ERROR: ' data: type: string description: Optional additional context for the error. Null in most cases. nullable: true default: null securitySchemes: ApiKeyAuthQuery: type: apiKey in: query name: apikey description: '' ApiKeyAuthHeader: type: apiKey in: header name: apikey description: '' ````