[pcbeingused333]

Since the move to Rails::Engine (#1003, not released yet: 0.19.5 predates it), main only loads inside a booted Rails app:

• In a plain Ruby script, following the README's require "langchain" raises NameError: uninitialized constant Rails from lib/langchain/engine.rb.
• With require "rails" first but no app booted, require "langchain" succeeds, but Langchain::LLM is undefined, because only the engine sets up autoloading.

So the next release would break scripts, Sinatra/Hanami apps, background workers outside Rails, and the gems that depend on langchainrb (#880's case).

Change

lib/langchain.rb requires the engine when Rails::Engine is defined, as now. Otherwise it sets up Zeitwerk::Loader.for_gem, the loader the gem used before #1003, with the acronyms from config/initializers/inflections.rb. It ignores engine.rb and application_record.rb, which need Rails. Nothing changes for Rails apps.

rails stays a runtime dependency in the gemspec. It is installed but not loaded outside Rails. Making it optional would be a separate decision.

Tests

• New spec in spec/langchain_spec.rb: a subprocess runs require "langchain" without Rails, checks that Rails wasn't loaded, eager-loads every class (Zeitwerk::Loader.eager_load_all) and resolves Langchain::LLM::OpenAI and Langchain::LLM::Response::OpenAIResponse. It fails on main with the NameError above.
• Locally I also eager-loaded all 112 Langchain:: classes without Rails and instantiated Langchain::LLM::Ollama.
• bundle exec rspec: 770 examples, 0 failures (this exercises the engine path through test/dummy). standardrb is clean.

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