Strict Mode¶

Strict mode disables all extension functions and let expressions, limiting jpx to only the 26 standard JMESPath functions. This is useful for writing portable queries that work with any JMESPath implementation.

Enabling Strict Mode¶

Use the --strict flag or set the JPX_STRICT=1 environment variable:

# Via flag
echo '{"name": "alice"}' | jpx --strict 'upper(name)'
# Error: Unknown function: upper

# Via environment variable
export JPX_STRICT=1
echo '{"items": [1, 2, 3]}' | jpx 'length(items)'
# 3

Standard JMESPath Functions¶

In strict mode, only these functions are available:

Function	Description
`abs(n)`	Absolute value
`avg(array)`	Average of numbers
`ceil(n)`	Round up
`contains(subject, search)`	Check if contains value
`ends_with(str, suffix)`	Check string suffix
`floor(n)`	Round down
`join(glue, array)`	Join strings
`keys(obj)`	Object keys
`length(subject)`	Length of array/string/object
`map(&expr, array)`	Transform each element
`max(array)`	Maximum value
`max_by(array, &expr)`	Maximum by expression
`merge(obj1, obj2, ...)`	Merge objects
`min(array)`	Minimum value
`min_by(array, &expr)`	Minimum by expression
`not_null(arg1, arg2, ...)`	First non-null value
`reverse(array)`	Reverse array
`sort(array)`	Sort array
`sort_by(array, &expr)`	Sort by expression
`starts_with(str, prefix)`	Check string prefix
`sum(array)`	Sum of numbers
`to_array(arg)`	Convert to array
`to_number(arg)`	Convert to number
`to_string(arg)`	Convert to string
`type(arg)`	Get type name
`values(obj)`	Object values

Use Cases¶

Portable Queries¶

If you're writing queries that need to work across different JMESPath implementations (AWS CLI, Python jmespath, etc.), use strict mode to ensure compatibility:

# Test your query works with standard JMESPath
jpx --strict '[?status == `active`].name | sort(@)'

CI/CD Validation¶

Ensure queries in your codebase don't accidentally use extensions:

# In your CI pipeline
jpx --strict -Q queries.jpx --check

Gradual Migration¶

When migrating from another JMESPath tool, start in strict mode and gradually adopt extensions as needed.

Checking if an Expression Uses Extensions¶

Use --explain to see what functions an expression uses:

jpx --explain 'users[*].name | unique(@) | sort(@)'

The unique function is an extension, while sort is standard.

MCP Server Strict Mode¶

The MCP server also supports strict mode. When configured with --strict, only standard JMESPath functions are available:

{
  "mcpServers": {
    "jpx-strict": {
      "command": "jpx-mcp",
      "args": ["--strict"]
    }
  }
}