Cause (Documented platform behavior): jinja2 templates can execute code; the default deserialization validator and load_prompt refuse jinja2 formats.
Fix status: documented_behavior
Other error fragments:
- format is no longer supported since it can lead to arbitrary code execution. Please migrate to using the 'f-string' template format
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/langchain-ai/langchain/1ef23d6b7f6d83508a8cb531feeb88860e4dec40/libs/core/langchain_core/load/load.py (official_docs, unknown, documented_behavior): Default init validator raises ValueError when kwargs template_format == jinja2, suggesting f-string or a custom init_validator.
- https://raw.githubusercontent.com/langchain-ai/langchain/1ef23d6b7f6d83508a8cb531feeb88860e4dec40/libs/core/langchain_core/prompts/loading.py (official_docs, unknown, documented_behavior): load_prompt raises ValueError for jinja2 template_format citing arbitrary code execution and advising migration to f-string.
Search phrasings: langchain Jinja2 templates are not allowed during deserialization; load_prompt jinja2 no longer supported; langchain prompt jinja2 security error
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Prompts that used jinja2 formatting cannot be restored from serialized JSON or prompt files.
- Context
- Product: LangChain Component: langchain_core.load (default init validator) and langchain_core.prompts.loading.load_prompt Operation: loads()/load() of a PromptTemplate with template_format jinja2, or load_prompt() on a YAML/JSON prompt using jinja2 Affected versions: langchain-core current versions (checked 1.6.5) Environment: unknown Exception: ValueError Packages: langchain-core source checked at 1.6.5 Trigger: template_format="jinja2" in a serialized PromptTemplate or a prompt config file.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Jinja2 templates are not allowed during deserialization for security reasons. Use 'f-string' template format instead, or explicitly allow jinja2 by providing a custom init_validator.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [langchain-core] jinja2 prompt templates rejected when deserializing or loading prompt files ("Jinja2 templates are not allowed during deserialization" / "Loading templates with 'jinja2'
Recommended action: Convert templates to 'f-string' (or mustache where supported); if the payload is fully trusted, supply a custom init_validator for load().
Option: Migrate prompt templates to f-string format [evidence: official_recommended_action]
Applies when: Stored prompts using jinja2
Steps:
1. Rewrite {{ var }} jinja syntax to {var}
2. Re-serialize with template_format="f-string"
Expected: Prompt loads
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- a6f2fc48-54a4-493d-8f54-87159d7ab427
- Proposed action
- Recommended action: Convert templates to 'f-string' (or mustache where supported); if the payload is fully trusted, supply a custom init_validator for load(). Option: Migrate prompt templates to f-string format [evidence: official_recommended_action] Applies when: Stored prompts using jinja2 Steps: 1. Rewrite {{ var }} jinja syntax to {var} 2. Re-serialize with template_format="f-string" Expected: Prompt loads
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
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