[AsherLucas]

Fixes #1971.

Problem

In ChatPromptClient.compile, when a placeholder value is a list and an item is not a dict, the code appended str(placeholder_value) — the whole list — for every such item. So compile(history=["a", "b"]) produced two identical messages containing "['a', 'b']", instead of one message per item.

Change

• langfuse/model.py: in the per-item else branch, use str(msg) for the current item. The non-list placeholder path is unchanged, so a placeholder that is not a list still appends the whole value once.
• tests/unit/test_prompt.py: updated the existing parametrized case that encoded the old behaviour — it now expects "invalid message" for the non-dict item.
• tests/unit/test_prompt_compilation.py: added test_compile_non_dict_placeholder_items_use_each_item as a regression test for bug: ChatPromptClient.compile appends str(whole list) once per non-dict placeholder item #1971.

Verification

• New test failed before the fix (asserted "['a', 'b']" == "a") and passes after it.
• pytest tests/unit/test_prompt.py tests/unit/test_prompt_compilation.py: 41 passed, 18 errors.
• The 18 errors are pre-existing and unrelated: they are fixture setup errors in test_prompt.py when no LANGFUSE_* env vars are set (see tests/unit/test_prompt.py fails on a fresh clone without LANGFUSE_* env vars #1976). I confirmed the same 18 errors on the clean tree before my change, with no failures there.
• ruff format --check passed for the changed files. A local ruff check reported 40 errors in langfuse/model.py, but the same 40 appear on the clean tree with the ruff version available here, none on the changed lines, so I did not touch them.

Confidence Score: 5/5

This PR appears safe to merge.

Summary

Fixes ChatPromptClient.compile so each non-dictionary placeholder item becomes its own string instead of repeating the whole list.

• Chat placeholders keep each non-dictionary list item’s text.

_{Reviews (1) · Last reviewed commit: "fix(model): use each non-dict placeholde..." · Reviewed by Greptile}

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[CLAassistant]

All committers have signed the CLA.