chore: import upstream snapshot with attribution

examples/integration-langchain/.gitignore

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+venv
+__pycache__

examples/integration-langchain/README.md

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+# integration-langchain (Langchain Python)
+
+You can run this example with:
+
+```bash
+npx promptfoo@latest init --example integration-langchain
+cd integration-langchain
+```
+
+## Usage
+
+This example shows how to run a Python LangChain Expression Language (LCEL) chain with Promptfoo. It compares GPT-5 with a math-focused LangChain prompt-and-output-parser pipeline.
+
+This example requires Python 3.10 or newer. Create and activate a virtual environment, then
+install the requirements:
+
+```sh
+python3 -m venv venv
+source venv/bin/activate
+pip install -r requirements.txt
+```
+
+Set the OpenAI API key used by both providers:
+
+```sh
+export OPENAI_API_KEY=your-api-key
+```
+
+Then run the eval:
+
+```bash
+npx promptfoo eval
+```

examples/integration-langchain/langchain_example.py

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+import os
+import sys
+
+
+def main() -> int:
+    if len(sys.argv) < 2:
+        print(f"Usage: {sys.argv[0]} <question>", file=sys.stderr)
+        return 1
+
+    api_key = os.getenv("OPENAI_API_KEY")
+    if not api_key:
+        print("OPENAI_API_KEY environment variable is required.", file=sys.stderr)
+        return 1
+
+    # Validate CLI inputs before importing optional dependencies so usage and
+    # configuration errors stay concise even before the example is installed.
+    from langchain_core.output_parsers import StrOutputParser
+    from langchain_core.prompts import PromptTemplate
+    from langchain_openai import OpenAI
+
+    llm = OpenAI(temperature=0, api_key=api_key)
+    math_chain = (
+        PromptTemplate.from_template(
+            "Solve the following math problem carefully. Return only the final answer.\n\n{question}"
+        )
+        | llm
+        | StrOutputParser()
+    )
+
+    try:
+        result = math_chain.invoke({"question": sys.argv[1]})
+    except Exception as error:
+        print(f"Error invoking math chain: {error}", file=sys.stderr)
+        return 1
+
+    print(result)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

examples/integration-langchain/prompt.txt

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+{{question}}

examples/integration-langchain/promptfooconfig.yaml

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+# yaml-language-server: $schema=https://promptfoo.dev/config-schema.json
+description: LangChain Python integration for prompt evaluation
+
+prompts:
+  - file://prompt.txt
+providers:
+  - openai:chat:gpt-5.4
+  - exec:python langchain_example.py
+tests:
+  - vars:
+      question: What is the cube root of 389017?
+  - vars:
+      question: If you have 101101 in binary, what number does it represent in base 10?
+  - vars:
+      question: What is the natural logarithm (ln) of 89234?
+  - vars:
+      question: If a geometric series has a first term of 3125 and a common ratio of 0.008, what is the sum of the first 20 terms?
+  - vars:
+      question: A number in base 7 is 3526. What is this number in base 10?
+  - vars:
+      question: If a complex number is represented as 3 + 4i, what is its magnitude?
+  - vars:
+      question: What is the fourth root of 1296?

examples/integration-langchain/requirements.txt

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+langchain-core==1.4.2
+langchain-openai==1.1.14
+openai==2.26.0