> ## Documentation Index > Fetch the complete documentation index at: https://docs.tilt.io/llms.txt > Use this file to discover all available pages before exploring further. # Backtest a Tilt > Backtest a saved Tilt methodology over a date range. The backtest uses the Tilt's saved methodology. Set `MONTHLY`, `QUARTERLY`, `SEMI_ANNUALLY`, or `ANNUALLY` in `rules.rebalancing_period` when you [apply construction](/api-reference/index-construction-preview/apply-construction). The date parameters select the analysis window; they do not change the cadence. Add `benchmark` for benchmark returns, relative performance, and risk statistics. Omit it for a portfolio-only result. ## OpenAPI ````yaml api-reference/index-construction-preview-openapi.yaml GET /api/v1/tilts/{tilt_uuid}/backtest openapi: 3.1.0 info: title: 'Tilt Index Construction API: Design Preview' version: 0.4.0-design-preview summary: >- A restrained public projection of Tilt's existing index construction workflow. description: | A Tilt is a persistent project for one index methodology. Its construction rules are the source, evaluated constituent snapshots are the build artifacts, and methodology history is version history. This bounded repository analogy does not imply branches, forks, merges, or cloning. This contract covers rules-based equity construction only. Applying construction saves the rules, evaluates them, and commits the complete weighted constituent snapshot atomically. servers: - url: https://x.invalid description: Non-routable design preview security: - ApiKey: [] tags: - name: Tilts description: Organization-owned Tilt projects and versions. - name: Construction description: Apply methodology and read its saved weighted evaluation. - name: Catalog description: >- Discover fields, functions, universes, and classifications used in expressions. - name: Tickers description: Resolve security identifiers to exact Tilt asset IDs. - name: Backtests description: Backtest a saved Tilt using its configured rebalance behavior. paths: /api/v1/tilts/{tilt_uuid}/backtest: parameters: - $ref: '#/components/parameters/TiltUuid' get: tags: - Backtests summary: Backtest a Tilt description: | Computes a backtest synchronously using the saved methodology. A Tilt with a `rules.rebalancing_period` uses point-in-time reconstitution at that cadence. A Tilt without one uses a fixed-share basket based on its saved snapshot. Cadence is not a run parameter. To compare monthly and quarterly methodologies, save each cadence through construction apply and backtest the UUID returned for each version. `start_date` and `end_date` change the analysis window, not the rebalance schedule. operationId: backtestTilt parameters: - name: start_date in: query schema: type: string format: date description: Start date. Defaults to 365 days before today. - name: end_date in: query schema: type: string format: date description: End date. Defaults to today. - name: benchmark in: query schema: type: string minLength: 1 description: | Optional benchmark ticker or supported canonical identifier, such as `SPY`. When supplied, the response includes benchmark returns, comparable period returns, and risk statistics. Omit for the portfolio-only response. responses: '200': description: >- Backtest levels, returns, scheduled membership changes, and optional benchmark analysis. content: application/json: schema: $ref: '#/components/schemas/Backtest' '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/Unauthorized' '404': $ref: '#/components/responses/NotFound' '504': description: The synchronous backtest timed out. content: application/json: schema: $ref: '#/components/schemas/Error' components: parameters: TiltUuid: name: tilt_uuid in: path required: true schema: type: string format: uuid schemas: Backtest: type: object required: - tilt properties: tilt: type: object required: - backtest_returns - backtest_values properties: since_inception_returns: type: string backtest_returns: type: object additionalProperties: type: string description: Daily returns keyed by ISO date. backtest_values: type: object additionalProperties: type: string description: Index levels keyed by ISO date. cumulative_backtest_returns: type: object additionalProperties: type: string benchmark: $ref: '#/components/schemas/BenchmarkBacktest' summary: $ref: '#/components/schemas/BacktestSummary' rebalance_events: type: array description: >- Scheduled membership changes, not a complete realized transaction history. items: $ref: '#/components/schemas/RebalanceEvent' Error: type: object required: - error properties: error: type: object additionalProperties: false required: - code - message properties: code: type: string message: type: string BenchmarkBacktest: type: object additionalProperties: false required: - identifier - backtest_returns description: Present only when the request includes `benchmark`. properties: identifier: type: string description: Resolved canonical benchmark identity. backtest_returns: type: object additionalProperties: type: string description: Decimal-fraction daily returns keyed by ISO date. BacktestSummary: type: object additionalProperties: false required: - portfolio_return - benchmark_return - risk_metrics description: Present only when the request includes `benchmark`. properties: portfolio_return: type: number description: >- Portfolio return over the requested period in percentage points; 12.5 means 12.5%. benchmark_return: type: number description: >- Benchmark return over its available period in percentage points; 10 means 10%. relative_return: type: number description: > Portfolio return minus benchmark return in percentage points. Omitted when the portfolio and benchmark coverage windows do not match. risk_metrics: $ref: '#/components/schemas/BacktestRiskMetrics' benchmark_coverage_note: type: string description: Explains a shorter benchmark coverage window when applicable. RebalanceEvent: type: object required: - date - added - removed - constituent_count properties: date: type: string format: date added: type: array items: $ref: '#/components/schemas/BacktestTicker' removed: type: array items: $ref: '#/components/schemas/BacktestTicker' constituent_count: type: integer BacktestRiskMetrics: type: object additionalProperties: false description: | Statistics calculated from daily returns. Volatility is annualized over 252 trading days; Sharpe uses a 4% annual risk-free rate. Unavailable statistics are omitted. properties: beta: type: number sharpe: type: number volatility: type: number description: Annualized decimal fraction; 0.18 means 18%. max_drawdown: type: number description: Decimal fraction; 0.12 means a 12% maximum drawdown. BacktestTicker: type: object required: - symbol properties: symbol: type: string tilt_asset_id: type: - string - 'null' responses: BadRequest: description: The request is invalid. content: application/json: schema: $ref: '#/components/schemas/Error' Unauthorized: description: The API key is missing or invalid. content: application/json: schema: $ref: '#/components/schemas/Error' NotFound: description: The Tilt was not found in the API key's organization. content: application/json: schema: $ref: '#/components/schemas/Error' securitySchemes: ApiKey: type: apiKey in: header name: X-Api-Key ````