LangChain Prompt Engineering (Python)

Overview

A team inherits a LangChain 1.0 codebase with 47 prompt strings embedded as f-string literals across 12 Python files. Nobody knows which version is live in production. Rollback is git-only — requires a deploy. An A/B test on a single prompt requires shipping code and running two services in parallel. A user pastes a JSON snippet containing { into a chat endpoint and the whole thing throws:

KeyError: '"model"'
  File ".../langchain_core/prompts/string.py", line ..., in format

That is pain-catalog entry P57 — ChatPromptTemplate.from_messages with f-string templates treat every brace-delimited identifier as a variable marker — including ones that appear inside user content. Any literal braces in user input (code snippets, JSON, LaTeX, CSS selectors) crash the chain. Four prompt-layer pitfalls this skill fixes: