chore: import upstream snapshot with attribution

examples/provider-custom/typescript/README.md

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+# provider-custom/typescript (Custom Provider Typescript)
+
+You can run this example with:
+
+```bash
+npx promptfoo@latest init --example provider-custom/typescript
+cd provider-custom/typescript
+```
+
+## Usage
+
+This example uses a custom API provider in `customProvider.ts`. It also uses CSV test cases.
+
+Run:
+
+```bash
+promptfoo eval
+```
+
+Full command-line equivalent:
+
+```bash
+promptfoo eval --prompts prompts.txt --tests vars.csv --providers openai:chat --output output.json --providers customProvider.js
+```

examples/provider-custom/typescript/customProvider.ts

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+import promptfoo from 'promptfoo';
+import type { ApiProvider, ProviderOptions, ProviderResponse } from 'promptfoo';
+
+// import promptfoo from '../../dist/src/index.js';
+// import type { ApiProvider, ProviderOptions, ProviderResponse } from '../../src/types/providers';
+
+export default class CustomApiProvider implements ApiProvider {
+  protected providerId: string;
+  public config: any;
+
+  constructor(options: ProviderOptions) {
+    // The caller may override Provider ID (e.g. when using multiple instances of the same provider)
+    this.providerId = options.id || 'custom provider';
+
+    // The config object contains any options passed to the provider in the config file.
+    this.config = options.config;
+  }
+
+  id(): string {
+    return this.providerId;
+  }
+
+  async callApi(prompt: string): Promise<ProviderResponse> {
+    const body = {
+      model: 'gpt-4.1-mini',
+      messages: [
+        {
+          role: 'user',
+          content: prompt,
+        },
+      ],
+      max_tokens: Number.parseInt(this.config?.max_tokens, 10) || 1024,
+      temperature: Number.parseFloat(this.config?.temperature) || 0,
+    };
+
+    // Fetch the data from the API using promptfoo's cache. You can use your own fetch implementation if preferred.
+    const { data, cached: _cached } = await promptfoo.cache.fetchWithCache(
+      'https://api.openai.com/v1/chat/completions',
+      {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
+        },
+        body: JSON.stringify(body),
+      },
+      10_000 /* 10 second timeout */,
+    );
+
+    const ret: ProviderResponse = {
+      output: data.choices[0].message.content,
+      tokenUsage: {
+        total: data.usage.total_tokens,
+        prompt: data.usage.prompt_tokens,
+        completion: data.usage.completion_tokens,
+      },
+    };
+    return ret;
+  }
+}

examples/provider-custom/typescript/promptfooconfig.yaml

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+# yaml-language-server: $schema=https://promptfoo.dev/config-schema.json
+description: Custom provider implementation using TypeScript
+
+prompts:
+  - file://prompts.txt
+
+providers:
+  - id: file://customProvider.ts
+    label: 'My custom provider'
+
+tests: file://vars.csv
+# To compare two of the same provider, you can do the following:
+#
+# providers:
+#   - id: customProvider.js
+#     label: custom-provider-hightemp
+#     config:
+#       temperature: 1.0
+#   - id: customProvider.js
+#     label: custom-provider-lowtemp
+#     config:
+#       temperature: 0

examples/provider-custom/typescript/prompts.txt

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+Rephrase this in French: {{body}}
+---
+Rephrase this like a pirate: {{body}}

examples/provider-custom/typescript/vars.csv

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+body
+Hello world
+I'm hungry