fix(cache): prevent over-invalidation of similarly named prompt prefixes in PromptCache

[Xayar145]

Summary

Fixes cache invalidation in PromptCache.invalidate():

• Checks for exact prompt name matches or keys prefixed with {prompt_name}- instead of bare substring prefix matching (key.startswith(prompt_name)).
• Added unit test test_prompt_cache_invalidate_exact_prefix_match.

Motivation

Previously, PromptCache.invalidate("summary") would unintentionally evict other prompt keys sharing the same name prefix (for example "summary_detailed-version:1" or "summary_qa-label:production"). Checking exact key match or hyphen-delimited prefix ensures only the target prompt is evicted.

Changes

• langfuse/_utils/prompt_cache.py: Refactored invalidate() to match key == prompt_name or key.startswith(f"{prompt_name}-").
• tests/unit/test_prompt.py: Added test ensuring sibling prompts with similar prefixes are not accidentally evicted.

Tests

• Added test_prompt_cache_invalidate_exact_prefix_match verifying selective invalidation.

Greptile Summary

The PR narrows prompt-cache invalidation to exact keys and hyphen-delimited prefixes and adds a regression test for similarly prefixed names. The delimiter remains ambiguous for valid prompt names containing hyphens.

• Updates PromptCache.invalidate() to avoid bare prefix matching.
• Adds coverage showing that invalidating summary preserves summary_detailed.
• Does not cover or preserve a sibling such as summary-detailed.

Confidence Score: 4/5

The PR is safe to merge with a non-blocking cache-correctness improvement recommended for hyphen-prefixed sibling prompt names.

The change fixes underscore-style prefix collisions, but the shared hyphen delimiter still causes distinct names such as summary-detailed to be evicted when summary is invalidated.

Files Needing Attention: langfuse/_utils/prompt_cache.py, tests/unit/test_prompt.py

Prompt To Fix All With AI

### Issue 1
langfuse/_utils/prompt_cache.py:186
**Hyphen-delimited names still collide**

For cached prompts named `summary` and `summary-detailed`, invalidating `summary` also matches `summary-detailed-version:1`, unnecessarily evicting the distinct sibling and forcing another API fetch on its next access. The added underscore case does not cover this collision because prompt names accept hyphens and cache metadata uses the same delimiter.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "test(prompt): add unit test for exact pr..." | Re-trigger Greptile}

Greptile also left 1 inline comment on this PR.

Context used:

• Knowledge Base — Utils and Support Types

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[greptile-apps]

Hyphen-delimited names still collide

For cached prompts named summary and summary-detailed, invalidating summary also matches summary-detailed-version:1, unnecessarily evicting the distinct sibling and forcing another API fetch on its next access. The added underscore case does not cover this collision because prompt names accept hyphens and cache metadata uses the same delimiter.

Knowledge Base Used: Utils and Support Types

Prompt To Fix With AI

This is a comment left during a code review.
Path: langfuse/_utils/prompt_cache.py
Line: 186

Comment:
**Hyphen-delimited names still collide**

For cached prompts named `summary` and `summary-detailed`, invalidating `summary` also matches `summary-detailed-version:1`, unnecessarily evicting the distinct sibling and forcing another API fetch on its next access. The added underscore case does not cover this collision because prompt names accept hyphens and cache metadata uses the same delimiter.

**Knowledge Base Used:** [Utils and Support Types](https://app.greptile.com/personal-org-4986/-/custom-context/knowledge-base/langfuse/langfuse-python/-/docs/utils-support.md)

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.