Gateway OpenAI compatibility

The ABTO Gateway has one data path, POST /v1/chat/completions, and accepts requests in the OpenAI Chat Completions format. This does not mean that every OpenAI SDK API and field is supported. The SDKs that use this path are Node / Server JavaScript and Python.

Supported requests

Area	Supported surface
Messages	Text content for system, developer, and assistant. A user message may be a text string or an ordered array of text, inline image_url, and inline PDF file parts.
Output length	max_completion_tokens and the legacy max_tokens alias
Number of responses	n omitted or set to 1
Streaming	stream omitted or set to false
Structured output	response_format: { type: 'text' } or json_schema with an embedded schema
Quality parameters	temperature, top_p, frequency_penalty, presence_penalty, seed, reasoning_effort, verbosity

The Node.js and Python SDKs carry the original request body to the Gateway. When a feature policy applies, its variant may replace the model, system instructions, and quality parameters. Requests without a policy use their original values.

Media content parts are supported in user messages only. An image must use image_url.url with data:image/png;base64,..., data:image/jpeg;base64,..., data:image/webp;base64,..., or data:image/gif;base64,.... A PDF must use file.file_data with data:application/pdf;base64,... and may include file.filename.

Unsupported requests

Paths other than POST /v1/chat/completions, including the Responses API, return 404. Using another HTTP method on the supported path returns 405.

With the correct path and method, the following requests fail with 400 instead of being silently ignored:

• stream: true or an n value other than 1
• The tool and function roles, and tools, tool_choice, functions, or function_call
• Audio and content parts other than text, image_url, or file
• Media parts in system, developer, or assistant messages
• HTTP(S) image URLs, image_url.detail, file.file_id, unsupported MIME types, and malformed data URLs
• response_format: { type: 'json_object' } or json_schema without an embedded schema
• Fields outside the allowlist such as stop, logprobs, modalities, metadata, or service_tier
• Unknown fields, including typos and newly introduced provider fields

Unsupported input is never removed or rewritten into a different meaning, so only requests the Gateway actually accepted enter experiments and operational records.

Inspecting errors

Every POST /v1/chat/completions response carries x-abto-request-id, including failures and admission rejections. Read the raw response header in the SDK and use it to connect to request details. Provider, transport, and internal failures may also include x-abto-error-source. Routing-level 404 and 405 responses do not include x-abto-request-id.

Calls handled by OpenAI direct fallback never pass through the Gateway, so they have no ABTO telemetry, variant policy, or x-abto-request-id.