Problem

We are replacing message placeholders with messages containing images, which breaks the mustache rendering.

TypeError: Invalid template! Template should be a "string" but "array" was given as the first argument for mustache#render(template, view, partials)
    at render (/node_modules/mustache/mustache.mjs:747:11)
    at item (/node_modules/langfuse-core/src/prompts/promptClients.ts:241:29)
    at Array.map (<anonymous>)
    at compile (/node_modules/langfuse-core/src/prompts/promptClients.ts:237:45)

Changes

Release info Sub-libraries affected

Bump level

• Major
• Minor
• Patch

Libraries affected

• All of them
• langfuse
• langfuse-node

Changelog notes

• Fixed message placeholder replacements with non-string content

Disclaimer: Experimental PR review

Greptile Summary

Updated On: 2025-09-01 09:12:49 UTC

This PR fixes a critical bug in the prompt client system where mustache template rendering would fail when processing ChatMessage content that isn't a string. The issue occurred when trying to use message placeholders with multimodal content (like messages containing images), where the content field contains arrays or objects instead of plain strings.

The fix is implemented in langfuse-core/src/prompts/promptClients.ts by adding a type guard that checks typeof item.content === "string" before applying mustache rendering. This ensures that only string content goes through template processing, while complex content structures (arrays, objects with image data, etc.) are left untouched.

This change aligns with modern AI applications that use multimodal content in chat messages, where the content field can contain structured data beyond simple text. The solution maintains backward compatibility for existing string-based prompts while enabling support for more complex message formats without breaking the rendering pipeline.

PR Description Notes:

• The "Changes" section in the PR description is empty and should describe what was actually modified
• Consider opening a GitHub issue first to discuss the rationale for this change as required by the contributing guidelines

Confidence score: 4/5

• This PR is safe to merge with minimal risk as it addresses a specific runtime error without changing core functionality
• Score reflects a targeted fix with clear error reproduction and minimal code changes that preserve existing behavior
• Pay close attention to langfuse-core/src/prompts/promptClients.ts to ensure the type checking logic is sound

Context used:

Rule - Open a GitHub issue or discussion first before submitting PRs to explain the rationale and necessity of the proposed changes, as required by the contributing guide. (link)