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Custom values

The TypedDict has total=False set intentionally to:

• Allow partial configs to be created and merged together via merge_configs
• Support config propagation from parent to child runnables via var_child_runnable_config (a ContextVar that automatically passes config down the call stack without explicit parameter passing), where configs are merged rather than replaced

# Parent sets tags
chain.invoke(input, config={"tags": ["parent"]})
# Child automatically inherits and can add:
# ensure_config({"tags": ["child"]}) -> {"tags": ["parent", "child"]}

Prefer using template_format='f-string' instead of template_format='jinja2', or make sure to NEVER accept jinja2 templates from untrusted sources as they may lead to arbitrary Python code execution.

As of LangChain 0.0.329, Jinja2 templates will be rendered using Jinja2's SandboxedEnvironment by default. This sand-boxing should be treated as a best-effort approach rather than a guarantee of security, as it is an opt-out rather than opt-in approach.

Despite the sandboxing, we recommend to never use jinja2 templates from untrusted sources.