refactor!: Make user_prompt required in LLM

[sjrl]

Related Issues

• related to fix: require messages input parameter in Agent component #11147

Proposed Changes:

Came about from a discussion with @julian-risch about making the LLM component more reliable when scheduled in a pipeline. We decided to go forward with the breaking change because it's a relatively new component. We do acknowledge this means that it won't be possible to just run this component with a messages input but from what we understand the desired pathway for platform use is to use the user_prompt.

The LLM component now requires user_prompt to be provided at initialization and it must contain at least one Jinja2 template variable (e.g. {{ variable_name }}). This ensures the component always exposes at least one required input socket, which is necessary for correct pipeline scheduling.

• required_variables now defaults to "*" (all variables in user_prompt are required), and passing an empty list raises a ValueError.

How did you test it?

New tests and updated old ones

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ignored		Apr 20, 2026 0:06am

[julian-risch]

Looks good to me! Looking at dc-pipeline-templates, required_variables isn't set in all pipelines, for example a quick search found https://github.com/deepset-ai/dc-pipeline-templates/pull/338/changes#diff-a5088c42299eda596ed218f47073d35f457cfd204e60e4475eed668511defb14R264

[coveralls]

Coverage Report for CI Build 24665547890

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No base build found for commit 9e1dcb8 on main.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 92.857%

Details

• Patch coverage: No coverable lines changed in this PR.

Uncovered Changes

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Coverage Regressions

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Coverage Stats

Relevant Lines:	17177
Covered Lines:	15950
Line Coverage:	92.86%
Coverage Strength:	0.93 hits per line

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💛 - Coveralls

[sjrl]

Looks good to me! Looking at dc-pipeline-templates, required_variables isn't set in all pipelines, for example a quick search found https://github.com/deepset-ai/dc-pipeline-templates/pull/338/changes#diff-a5088c42299eda596ed218f47073d35f457cfd204e60e4475eed668511defb14R264

Just fixed it a few min ago https://github.com/deepset-ai/dc-pipeline-templates/pull/354#pullrequestreview-4139750934