docs: add structured output documentation to SDK reference

Document the outputFormat feature for requesting structured JSON output:
- Basic usage example with JSON schema
- Output format types (text, json_schema)
- Schema configuration options (type, schema, retryCount)
- Error handling for StructuredOutputError
- Best practices for using structured output

packages/web/src/content/docs/sdk.mdx

@@ -117,6 +117,78 @@ try {
 
 ---
 
+## Structured Output
+
+You can request structured JSON output from the model by specifying an `outputFormat` with a JSON schema. The model will use a `StructuredOutput` tool to return validated JSON matching your schema.
+
+### Basic Usage
+
+```typescript
+const result = await client.session.prompt({
+  path: { id: sessionId },
+  body: {
+    parts: [{ type: 'text', text: 'Research Anthropic and provide company info' }],
+    outputFormat: {
+      type: 'json_schema',
+      schema: {
+        type: 'object',
+        properties: {
+          company: { type: 'string', description: 'Company name' },
+          founded: { type: 'number', description: 'Year founded' },
+          products: {
+            type: 'array',
+            items: { type: 'string' },
+            description: 'Main products'
+          }
+        },
+        required: ['company', 'founded']
+      }
+    }
+  }
+})
+
+// Access the structured output
+console.log(result.data.info.structured_output)
+// { company: "Anthropic", founded: 2021, products: ["Claude", "Claude API"] }
+```
+
+### Output Format Types
+
+| Type | Description |
+|------|-------------|
+| `text` | Default. Standard text response (no structured output) |
+| `json_schema` | Returns validated JSON matching the provided schema |
+
+### JSON Schema Format
+
+When using `type: 'json_schema'`, provide:
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `type` | `'json_schema'` | Required. Specifies JSON schema mode |
+| `schema` | `object` | Required. JSON Schema object defining the output structure |
+| `retryCount` | `number` | Optional. Number of validation retries (default: 2) |
+
+### Error Handling
+
+If the model fails to produce valid structured output after all retries, the response will include a `StructuredOutputError`:
+
+```typescript
+if (result.data.info.error?.name === 'StructuredOutputError') {
+  console.error('Failed to produce structured output:', result.data.info.error.message)
+  console.error('Attempts:', result.data.info.error.retries)
+}
+```
+
+### Best Practices
+
+1. **Provide clear descriptions** in your schema properties to help the model understand what data to extract
+2. **Use `required`** to specify which fields must be present
+3. **Keep schemas focused** - complex nested schemas may be harder for the model to fill correctly
+4. **Set appropriate `retryCount`** - increase for complex schemas, decrease for simple ones
+
+---
+
 ## APIs
 
 The SDK exposes all server APIs through a type-safe client.
@@ -241,7 +313,7 @@ const { providers, default: defaults } = await client.config.providers()
 | `session.summarize({ path, body })`                        | Summarize session                  | Returns `boolean`                                                                                                                              |
 | `session.messages({ path })`                               | List messages in a session         | Returns `{ info: `<a href={typesUrl}><code>Message</code></a>`, parts: `<a href={typesUrl}><code>Part[]</code></a>`}[]`                        |
 | `session.message({ path })`                                | Get message details                | Returns `{ info: `<a href={typesUrl}><code>Message</code></a>`, parts: `<a href={typesUrl}><code>Part[]</code></a>`}`                          |
-| `session.prompt({ path, body })`                           | Send prompt message                | `body.noReply: true` returns UserMessage (context only). Default returns <a href={typesUrl}><code>AssistantMessage</code></a> with AI response |
+| `session.prompt({ path, body })`                           | Send prompt message                | `body.noReply: true` returns UserMessage (context only). Default returns <a href={typesUrl}><code>AssistantMessage</code></a> with AI response. Supports `body.outputFormat` for [structured output](#structured-output) |
 | `session.command({ path, body })`                          | Send command to session            | Returns `{ info: `<a href={typesUrl}><code>AssistantMessage</code></a>`, parts: `<a href={typesUrl}><code>Part[]</code></a>`}`                 |
 | `session.shell({ path, body })`                            | Run a shell command                | Returns <a href={typesUrl}><code>AssistantMessage</code></a>                                                                                   |
 | `session.revert({ path, body })`                           | Revert a message                   | Returns <a href={typesUrl}><code>Session</code></a>                                                                                            |