> ## Documentation Index > Fetch the complete documentation index at: https://docs.withhopper.com/llms.txt > Use this file to discover all available pages before exploring further. # Create chat completion > OpenAI-compatible chat completions. The body is forwarded to the inference engine unchanged, so any OpenAI chat completions parameter is accepted; the engine validates it. Request body cap: 10 MB. ## OpenAPI ````yaml /api-reference/openapi.json post /v1/chat/completions openapi: 3.1.0 info: title: Hopper API version: 1.0.0 description: >- Inference for voice agents: LLM, text to speech, and speech to text at api.withhopper.com. OpenAI-compatible routes are served under /v1; native routes are served at the root. All errors use the OpenAI envelope. servers: - url: https://api.withhopper.com security: - bearerAuth: [] tags: - name: LLM description: OpenAI-compatible chat completions and model listing. - name: Text to Speech description: >- Speech synthesis: native /tts routes and the OpenAI-compatible /v1/audio/speech. - name: Speech to Text description: Batch transcription. Streaming runs over WS /stt/websocket. - name: Voices description: Voice library, cloning, profiles, and localization. - name: Pronunciation description: Pronunciation dictionaries applied at synthesis time. - name: Jobs description: Status of async voice jobs. - name: Health description: Unauthenticated liveness check. paths: /v1/chat/completions: post: tags: - LLM summary: Create chat completion description: >- OpenAI-compatible chat completions. The body is forwarded to the inference engine unchanged, so any OpenAI chat completions parameter is accepted; the engine validates it. Request body cap: 10 MB. requestBody: required: true content: application/json: schema: type: object required: - model - messages additionalProperties: true description: >- Passed through to the inference engine. Fields below are the common subset; any other OpenAI chat completions parameter is also accepted. properties: model: type: string description: Model id, e.g. gemma-4-31b. messages: type: array description: OpenAI chat messages. items: type: object stream: type: boolean default: false description: Stream tokens as SSE chunks. temperature: type: number description: Sampling temperature; passed through. max_tokens: type: integer description: Maximum output tokens; passed through. tools: type: array description: OpenAI tool definitions; passed through. items: type: object stream_options: type: object description: >- Set include_usage true to receive a final usage chunk. The gateway meters usage either way. properties: include_usage: type: boolean responses: '200': description: >- Non-streaming: an OpenAI chat completion object with both spend headers. Streaming: text/event-stream of OpenAI chunks, with only x-hopper-credits-remaining-usd (the pre-request balance; cost is unknown until the stream ends). headers: x-hopper-request-cost-usd: $ref: '#/components/headers/XHopperRequestCostUsd' x-hopper-credits-remaining-usd: $ref: '#/components/headers/XHopperCreditsRemainingUsd' content: application/json: schema: type: object description: >- OpenAI chat completion object, forwarded from the inference engine. text/event-stream: schema: type: string description: OpenAI-compatible SSE chunks. '401': $ref: '#/components/responses/Unauthorized' '402': $ref: '#/components/responses/PaymentRequired' '413': description: 'Request body over 10 MB. Code: body_too_large.' content: application/json: schema: $ref: '#/components/schemas/Error' '429': $ref: '#/components/responses/RateLimited' '502': description: >- Inference engine unreachable or timed out (300 s). Code: upstream_unavailable. content: application/json: schema: $ref: '#/components/schemas/Error' '503': $ref: '#/components/responses/VerifyUnavailable' components: headers: XHopperRequestCostUsd: description: >- Cost of this request, 6-decimal USD string (e.g. 0.000123). Absent on LLM streaming responses; the cost is unknown until the stream ends. schema: type: string XHopperCreditsRemainingUsd: description: >- Credit balance in USD, 6-decimal string. On LLM streaming responses this is the pre-request balance. schema: type: string responses: Unauthorized: description: >- Missing, malformed, or revoked API key. Codes: invalid_api_key, api_key_disabled. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: >- Incorrect API key provided. Check your key at https://withhopper.com/dashboard. type: invalid_request_error code: invalid_api_key param: null PaymentRequired: description: Credit balance is zero. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: You have run out of credits. Contact us to add more. type: insufficient_quota code: insufficient_quota param: null RateLimited: description: >- Over 600 requests per minute on this key. Retry after the number of seconds in Retry-After. headers: Retry-After: description: Seconds to wait before retrying. schema: type: integer content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: >- Rate limit reached: 600 requests per minute per key. Retry after 21s. type: rate_limit_error code: rate_limit_exceeded param: null VerifyUnavailable: description: Key verification is temporarily unavailable. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Key verification is temporarily unavailable. Retry shortly. type: api_error code: verify_unavailable param: null schemas: Error: type: object description: OpenAI error envelope, used by every error response. properties: error: type: object properties: message: type: string type: type: string code: type: string param: type: - string - 'null' securitySchemes: bearerAuth: type: http scheme: bearer description: >- API key in the Authorization header: Bearer sk_hopper_... Keys are minted in the dashboard at withhopper.com/console/keys and shown once at creation. WebSocket connections also accept ?api_key=. ````