Python: Fix Handlebars messages helper ignoring its argument

[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Motivation and Context

The Handlebars messages helper ignores its argument. {{messages my_history}} always reads chat_history from the context, so it raises KeyError when that variable is missing and renders the wrong history when both exist. The Jinja2 messages(history) helper uses its argument.

Fixes #14535

Description

_messages in handlebars_system_helpers.py now uses the first positional argument. If no argument is given it falls back to the chat_history context variable, so existing {{messages chat_history}} templates behave the same. The unused options parameter was removed. For a non-block helper pybars passes the first argument in that slot, which is why the argument was never visible. A bare {{messages}} used to raise TypeError and now works through the fallback.

Added two tests (argument with a different variable name, and argument preferred over a chat_history variable). Both fail on main. tests/unit/prompt_template, tests/unit/kernel and tests/unit/functions pass. ruff check clean on the touched files, mypy clean on the helper module.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Copilot review overview

🟡 Changes recommended

The newly introduced no-argument fallback lacks integration coverage.

Review effort: Balanced
Findings: 1

Open (1)

• Add integration test for bare messages fallback · New

What changed in this PR

Fixes the Handlebars messages helper to honor its argument while retaining context fallback behavior.

Changes:

• Uses the first helper argument when provided.
• Adds regression tests for argument selection.

File	Description
handlebars_system_helpers.py	Corrects chat-history resolution.
test_handlebars_prompt_template.py	Tests explicit argument handling.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Good catch. I added test_helpers_chat_history_messages_without_argument_uses_chat_history_variable in b5adc4a. It renders a bare {{messages}} with only chat_history in the arguments and checks the output, so the no-argument fallback branch is now covered. It passes. It would also pass on main, since bare {{messages}} worked before. It guards the fallback so this change cannot break it.