fix(core): implement ChatPromptTemplate.save() method

[keyuchen21]

Summary

Fixes #32637 - ChatPromptTemplate.save() method now properly saves templates to files instead of raising NotImplementedError.

Problem

The save() method in ChatPromptTemplate was not implemented and simply raised NotImplementedError, preventing users from saving their chat prompt templates to files.

Solution

• Implemented a working save() method that uses dumpd() for proper serialization
• The method supports both JSON and YAML formats
• Ensures saved templates can be correctly loaded back using the load() function

Changes

1. Implemented ChatPromptTemplate.save() method:

   • Uses dumpd() from langchain_core.load for proper serialization
   • Supports JSON and YAML file formats
   • Creates parent directories if they don't exist
   • Validates file extensions and raises appropriate errors

2. Added comprehensive unit tests:

   • Test saving/loading to JSON format
   • Test saving/loading to YAML format
   • Test with MessagesPlaceholder
   • Test error handling for invalid file extensions
   • Test error handling for partial variables
   • Test directory creation

Test plan

Added unit tests that verify:

• Templates can be saved to JSON and loaded back correctly
• Templates can be saved to YAML and loaded back correctly
• Proper error handling for invalid file extensions
• Proper error handling when partial variables are present
• Parent directories are created automatically when saving to nested paths
• Templates with MessagesPlaceholder work correctly

All tests pass successfully.

Example Usage

from langchain_core.prompts import ChatPromptTemplate
from langchain_core.load import load

# Create a chat prompt template
template = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant named {assistant_name}."),
    ("human", "{question}")
])

# Save to file
template.save("my_prompt.json")

# Load it back
with open("my_prompt.json") as f:
    import json
    loaded_template = load(json.load(f))

# Use the loaded template
messages = loaded_template.format_messages(
    assistant_name="Claude",
    question="What can you help me with?"
)

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ignored	Preview		Aug 22, 2025 9:38pm

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32654 will not alter performance

_{Comparing keyuchen21:fix-chat-prompt-save (abdb539) with master (2d0713c)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32654 will not alter performance

_{Comparing keyuchen21:fix-chat-prompt-save (abdb539) with master (2d0713c)}

Summary

✅ 14 untouched benchmarks

[keyuchen21]

@mdrxy any update?

[ccurme]

Closing as duplicated with #33631.