# GPT family API: model IDs, Chat Completions vs. Responses, web search and errors

Source: https://ofox.io/docs/develop/models/gpt

OpenAI's GPT text models, including the GPT-6, GPT-5.x, GPT-4.1, GPT-4o and Codex series. They are called through the OpenAI-compatible API, which supports both **Chat Completions** and **Responses**; the official OpenAI SDK works once you change `base_url`.

## Common tasks

- [Enable reasoning](https://ofox.io/docs/develop/models/gpt/reasoning) — reasoning_effort values, reasoning tokens, reasoning summaries.
- [Web search](#web-search) — The web_search tool on the Responses API.
- [Prompt caching](#caching) — Automatic caching and confirming hits.
- [Structured output & PDF](https://ofox.io/docs/develop/models/gpt/structured-output) — json_schema output and PDF file input.
- [Troubleshooting errors](#errors) — Context exceeded, max_tokens too large, empty responses, unsupported endpoint.
- [Use in Codex](https://ofox.io/docs/integrations/codex) — Setup and common questions.

## Models
All models support tool calling and structured output; all except GPT-5 nano accept image input. For the full model list and live prices, see the [model page](https://ofox.io/models?provider=openai), or query the [Models API](https://ofox.io/docs/api/openai/models).

**Common model IDs (copy them exactly)**

- GPT-6 Sol: `openai/gpt-6-sol`
- GPT-5.4: `openai/gpt-5.4`
- GPT-5.4 mini: `openai/gpt-5.4-mini`
- GPT-4.1 mini: `openai/gpt-4.1-mini` — No reasoning; suited to simple tasks

You can omit the `openai/` prefix, e.g. `gpt-5.4`. The `model` field in responses always includes the prefix.

## Choosing an endpoint

**Key point**:The Codex series and GPT-5.4 Pro support only the Responses API; all other models work with both.

| | Chat Completions | Responses |
|---|---|---|
| Endpoint | `POST https://api.ofox.io/v1/chat/completions` | `POST https://api.ofox.io/v1/responses` |
| Input | `messages` | `input` (a string or an array of messages) |
| Output limit | `max_completion_tokens` (`max_tokens` also accepted) | `max_output_tokens` |
| [Reasoning effort](https://ofox.io/docs/develop/models/gpt/reasoning) | `reasoning_effort` | `reasoning.effort` |
| Reasoning summary | Not returned | `reasoning.summary` — see [Reasoning summaries](https://ofox.io/docs/develop/models/gpt/reasoning#reasoning-summary) |
| Web search | Not supported | `web_search` tool |
| Models available only here | — | `openai/gpt-5.4-pro`, `openai/gpt-5.3-codex`, `openai/gpt-5.2-codex` |

For the full request parameters, see the [Chat Completions API](https://ofox.io/docs/api/openai/chat-completions) and the [Responses API](https://ofox.io/docs/api/openai/responses).

### Providers
| Provider | Models |
|---|---|
| Azure AI Foundry (`azure_foundry`) | All models; default provider |
| OpenAI (`openai`) | GPT-6 Sol, GPT-6 Luna, GPT-6 Astra, GPT-5.6 Sol, GPT-5.6 Terra, GPT-5.6 Luna |

In our tests, both providers behaved the same for reasoning, caching and web search. To select a provider, add the `X-OfoxAI-Provider-Type: openai` request header; for other methods, see [Provider routing](https://ofox.io/docs/develop/advanced/provider-routing).

## Examples

**Python (Chat Completions)**

```python filename="gpt_chat.py"
import os
from openai import OpenAI

client = OpenAI(base_url="https://api.ofox.io/v1", api_key=os.environ["OFOX_API_KEY"])

resp = client.chat.completions.create(
    model="openai/gpt-5.4",
    messages=[
        {"role": "system", "content": "You are a senior backend engineer. Keep answers concise."},
        {"role": "user", "content": "Explain idempotency in one sentence."},
    ],
)
print(resp.choices[0].message.content)
```

**Python (Responses)**

```python filename="gpt_responses.py"
import os
from openai import OpenAI

client = OpenAI(base_url="https://api.ofox.io/v1", api_key=os.environ["OFOX_API_KEY"])

resp = client.responses.create(
    model="openai/gpt-5.4",
    instructions="You are a senior backend engineer. Keep answers concise.",
    input="Explain idempotency in one sentence.",
)
print(resp.output_text)
```

**cURL**

```bash filename="Terminal"
curl https://api.ofox.io/v1/chat/completions \
  -H "Authorization: Bearer $OFOX_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.4",
    "messages": [{"role": "user", "content": "Explain idempotency in one sentence."}]
  }'
```

## Prompt caching

**Key point**:The GPT family caches automatically with no changes to your request. Put fixed content at the start of the request to improve the hit rate.

When a request's prefix matches a recent request and is long enough, it hits the cache automatically, and the cached portion is billed at the cache read rate.

| Item | Details |
|---|---|
| Confirming hits | In Chat Completions, `usage.prompt_tokens_details.cached_tokens` is greater than 0; in Responses, `usage.input_tokens_details.cached_tokens` |
| Writes | The GPT-6 and GPT-5.6 series return `prompt_tokens_details.cache_write_tokens` on the first request, billed at the cache write rate |
| Minimum length | Prefixes that are too short are not cached; for the minimum length, see the [OpenAI docs](https://platform.openai.com/docs/guides/prompt-caching) |
| Reliability | Caching is best-effort; requests with the same prefix may still miss |

For general information on caching, see [Prompt caching](https://ofox.io/docs/develop/advanced/prompt-caching).

## Web search
Add `web_search` to `tools` on the Responses API, and the model runs searches on its own and cites sources. Chat Completions does not support web search.

```python filename="gpt_web_search.py"
import os
from openai import OpenAI

client = OpenAI(base_url="https://api.ofox.io/v1", api_key=os.environ["OFOX_API_KEY"])

resp = client.responses.create(
    model="openai/gpt-5.4",
    tools=[{"type": "web_search"}],
    input="What is the latest stable version of Python?",
)
print(resp.output_text)
print("Searches:", sum(1 for item in resp.output if item.type == "web_search_call"))
```

Each `web_search_call` item in `output` is one search, and the `message` item is the final answer. Searches are billed per use; see the [model page](https://ofox.io/models/openai/gpt-5.4) for the rate.

## Common errors

**Key point**:Parameter errors return 400 with a reason. Long reasoning occasionally causes 502 or 504; use streaming and set a long timeout.

| HTTP · type | Error message | Cause | Fix |
|---|---|---|---|
| 400 `invalid_request_error` | `This model's maximum context length is 128000 tokens. However, your messages resulted in 152008 tokens.` | Input exceeds the model's context window | Shorten the input, or switch to a model with a longer context |
| 400 `invalid_request_error` | `max_tokens is too large: 100000. This model supports at most 16384 completion tokens` | Output limit exceeds the model's maximum | Lower `max_completion_tokens` |
| 400 `invalid_request_error` | `Invalid value: 'bogus'. Supported values are: 'low', 'medium', 'high', and 'xhigh'.` | The `reasoning_effort` value is not supported by this model | Use one of the values listed in the error message |
| 400 `endpoint_not_supported` | `Model 'openai/gpt-5.4-pro' does not support the /v1/chat/completions endpoint on this platform.` | The model supports only the Responses API | Use `/v1/responses` |
| 404 `model_not_found` | `Model 'openai/gpt-9' not found` | Model ID misspelled, or the model has been retired | Copy the model ID from this page |
| 429 `rate_limit_error` | `Rate limit reached for the requested model. Please retry after a short delay.` | Too many requests | Retry later — see [Rate limits](https://ofox.io/docs/develop/guides/rate-limits) |
| 502 `upstream_error` | `The model service is temporarily unavailable. Please retry or switch to another model.` | Upstream service temporarily unavailable | Retry later, or switch to another model |
| 504 `gateway_timeout` | `The upstream service did not respond in time.` | Reasoning took too long and upstream did not respond in time | Use streaming, or lower the reasoning effort |
| 200, `finish_reason: "length"` with an empty answer | — | Reasoning used up `max_completion_tokens` | Increase `max_completion_tokens`, or lower the reasoning effort |

For other general errors, see [Error Handling](https://ofox.io/docs/develop/guides/error-handling).

## Official docs
- [OpenAI · Models overview](https://platform.openai.com/docs/models)
- [OpenAI · Reasoning models](https://platform.openai.com/docs/guides/reasoning)
- [OpenAI · Responses vs. Chat Completions](https://platform.openai.com/docs/guides/responses-vs-chat-completions)
- [OpenAI · Prompt caching](https://platform.openai.com/docs/guides/prompt-caching)
- [OpenAI · Web search](https://platform.openai.com/docs/guides/tools-web-search)

The official docs describe behavior when calling OpenAI directly. When calling through OfoxAI, model IDs and endpoints on this page take precedence.

## FAQ

**How do I write a GPT model ID?**

The standard format is openai/gpt-5.4. You can also omit the openai/ prefix and write gpt-5.4.

**Should I use Chat Completions or Responses?**

Either works. The Codex series and GPT-5.4 Pro support only Responses, and you also need Responses for reasoning summaries or web search. Projects with existing Chat Completions code can keep using it.

**How do I enable reasoning for GPT?**

In Chat Completions, pass reasoning_effort (e.g. "high"); in Responses, pass reasoning: {"effort": "high"}. Some models do not reason by default, so pass it explicitly when you need it.

**Why is the GPT response empty with finish_reason length?**

Reasoning tokens count toward max_completion_tokens, and once reasoning uses up the budget no answer is produced. Increase max_completion_tokens or lower reasoning_effort.

**What does the error does not support the /v1/chat/completions endpoint mean?**

The model supports only the Responses API, e.g. gpt-5.4-pro, gpt-5.3-codex and gpt-5.2-codex. Use POST /v1/responses instead.

**Can I see the GPT reasoning process?**

Use the Responses API and pass reasoning: {"effort": "medium", "summary": "auto"}; the reasoning items in output will contain a reasoning summary. Chat Completions does not return reasoning content.

**Can GPT search the web?**

Yes. Use the Responses API and add {"type": "web_search"} to tools. Chat Completions does not support web search.

**Do I need to enable GPT prompt caching manually?**

No. When the prefix is long enough and matches a recent request, it hits the cache automatically; the cached amount is in cached_tokens in usage.

**What should I do if GPT requests time out or return 504?**

High-effort reasoning can take a long time. Use streaming (stream: true), set the client timeout to 600 seconds or more, or lower reasoning_effort.