Update models document page to cover gpt-5 (#364)

Author: seratch

docs/src/content/docs/guides/models.mdx

@@ -29,6 +29,53 @@ In day‑to‑day work you normally only interact with model **names** and occas
   title="Specifying a model per‑agent"
 />
 
+## Default model
+
+When you don't specify a model when initializing an `Agent`, the default model will be used. The default is currently [`gpt-4.1`](https://platform.openai.com/docs/models/gpt-4.1), which offers a strong balance of predictability for agentic workflows and low latency.
+
+If you want to switch to other models like [`gpt-5`](https://platform.openai.com/docs/models/gpt-5), there are two ways to configure your agents.
+
+First, if you want to consistently use a specific model for all agents that do not set a custom model, set the `OPENAI_DEFAULT_MODEL` environment variable before running your agents.
+
+```bash
+export OPENAI_DEFAULT_MODEL=gpt-5
+node my-awesome-agent.js
+```
+
+Second, you can set a default model for a `Runner` instance. If you don't set a model for an agent, this `Runner`'s default model will be used.
+
+<Code
+  lang="typescript"
+  code={runnerWithModelExample}
+  title="Set a default model for a Runner"
+/>
+
+### GPT-5 models
+
+When you use any of GPT-5's reasoning models ([`gpt-5`](https://platform.openai.com/docs/models/gpt-5), [`gpt-5-mini`](https://platform.openai.com/docs/models/gpt-5-mini), or [`gpt-5-nano`](https://platform.openai.com/docs/models/gpt-5-nano)) this way, the SDK applies sensible `modelSettings` by default. Specifically, it sets both `reasoning.effort` and `verbosity` to `"low"`. To adjust the reasoning effort for the default model, pass your own `modelSettings`:
+
+```ts
+import { Agent } from '@openai/agents';
+
+const myAgent = new Agent({
+  name: 'My Agent',
+  instructions: "You're a helpful agent.",
+  modelSettings: {
+    reasoning: { effort: 'minimal' },
+    text: { verbosity: 'low' },
+  },
+  // If OPENAI_DEFAULT_MODEL=gpt-5 is set, passing only modelSettings works.
+  // It's also fine to pass a GPT-5 model name explicitly:
+  // model: 'gpt-5',
+});
+```
+
+For lower latency, using either [`gpt-5-mini`](https://platform.openai.com/docs/models/gpt-5-mini) or [`gpt-5-nano`](https://platform.openai.com/docs/models/gpt-5-nano) with `reasoning.effort="minimal"` will often return responses faster than the default settings. However, some built-in tools (such as file search and image generation) in Responses API do not support `"minimal"` reasoning effort, which is why this Agents SDK defaults to `"low"`.
+
+### Non-GPT-5 models
+
+If you pass a non–GPT-5 model name without custom `modelSettings`, the SDK reverts to generic `modelSettings` compatible with any model.
+
 ---
 
 ## The OpenAI provider
@@ -52,16 +99,6 @@ endpoints:
 You can also plug your own `OpenAI` client via `setDefaultOpenAIClient(client)` if you need
 custom networking settings.
 
-### Default model
-
-The OpenAI provider defaults to `gpt‑4o`. Override per agent or globally:
-
-<Code
-  lang="typescript"
-  code={runnerWithModelExample}
-  title="Set a default model"
-/>
-
 ---
 
 ## ModelSettings
@@ -79,6 +116,8 @@ The OpenAI provider defaults to `gpt‑4o`. Override per agent or globally:
 | `truncation`        | `'auto' \| 'disabled'`                     | Token truncation strategy.                                                |
 | `maxTokens`         | `number`                                   | Maximum tokens in the response.                                           |
 | `store`             | `boolean`                                  | Persist the response for retrieval / RAG workflows.                       |
+| `reasoning.effort`  | `'minimal' \| 'low' \| 'medium' \| 'high'` | Reasoning effort for gpt-5 etc.                                           |
+| `text.verbosity`    | `'low' \| 'medium' \| 'high'`              | Text verbosity for gpt-5 etc.                                             |
 
 Attach settings at either level: