Can I dynamically select which sub-prompt to embed based on a condition?

Let’s say you have a main prompt with shared instructions, but want to inject a different sub-set of instructions based on whether the user is on a free or premium plan.

Langfuse prompt references don’t support conditional logic (like {% if %} statements) directly. However, you can achieve dynamic prompt selection by handling the logic in your application code and passing the selected prompt as a variable value.

Before diving into the how, let’s first zoom out and look at the options you have.

Approach	Use when	Trade-offs
Static prompt references	Sub-prompt is always the same	✅ Fully declarative, resolved automatically in Playground ❌ No dynamic selection based on runtime values
Dynamic selection via variables (this page)	Sub-prompt depends on runtime conditions	✅ All (sub)prompts managed in Langfuse UI ✅ Selection logic supported (via code) ✅ Variables automatically detected by Langfuse ❌ Not automatically resolved in Playground, prompt experiments
External templating (Jinja, Liquid)	Complex conditionals within a single template	✅ Full flexibility logic-wise ❌ Prompts not automatically resolved in Playground, prompt experiments ❌ Variables not automatically detected by Langfuse

How it works

Instead of embedding prompts statically with prompt composability (@@@langfusePrompt:...@@@), you fetch multiple prompts and select which one to use in your application code:

Create a parent prompt with a variable placeholder (e.g., {{selected_prompt}})
Create the sub-prompts you want to conditionally embed
Fetch all prompts using get_prompt()
Select the appropriate sub-prompt based on your logic
Pass the selected prompt text to the parent’s compile() method

Here’s an example in Python:

from langfuse import Langfuse
 
langfuse = Langfuse()
 
# Parent prompt in Langfuse:
# "Here is your final prompt:\n\n{{selected_prompt}}"
 
# Get production versions
parent_prompt = langfuse.get_prompt("parent-prompt")
prompt_a = langfuse.get_prompt("prompt-a")
prompt_b = langfuse.get_prompt("prompt-b")
 
# Pick based on your boolean or other condition
use_prompt_a = True  # your logic here
selected = prompt_a if use_prompt_a else prompt_b
 
# Insert into parent
final_prompt = parent_prompt.compile(selected_prompt=selected.prompt)

API

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