Install

The SDK is the Python package vexlo. It runs inside your process, marks functions, captures their values, and writes records to the spool directory on the same host. The relay daemon that forwards those records is a separate process, not something inside yours; it is covered on the relay daemon page.

Requirements

Requirement	Value	Where it is declared
Python	3.10 or later (`requires-python = ">=3.10"`)	`pyproject.toml`
Dependencies	none (`dependencies = []`)	`pyproject.toml`
Build backend	setuptools 68 or later, resolved by `pip` at install time	`pyproject.toml`

The package ships py.typed, so mypy and other type checkers read its annotations. @vexlo.trace and @vexlo.step are typed with typing.ParamSpec, which is what sets the 3.10 floor.

Install from a checkout

vexlo is not yet published on PyPI, so the package is installed from a checkout of the vexlo-sdk-python repository. Both forms below install the same package.

python -m pip install /path/to/vexlo-sdk-python

python -m pip install -e /path/to/vexlo-sdk-python

The regular form builds a wheel and copies it into the environment. The editable form points the interpreter at the checkout’s src/ directory instead, so a change there is picked up by the next interpreter start without reinstalling. Below the floor, pip refuses before installing anything. On Python 3.9.6 the same command ends with:

... 9 lines omitted
ERROR: Package 'vexlo' requires a different Python: 3.9.6 not in '>=3.10'

Verify the install

The two outputs below are from Python 3.14.8. Import the package and print its public surface:

python -c "import vexlo; print(vexlo.__all__)"

['DEFAULT_REDACT_KEYS', 'Carrier', 'Diagnostics', 'VexloError', 'attach', 'carrier', 'diagnostics', 'flush', 'init', 'step', 'tag', 'trace']

Confirm the version and that nothing else was pulled in:

python -m pip show vexlo | grep -E '^(Name|Version|Summary|Requires):'

Name: vexlo
Version: 0.1.0
Summary: Captures how code actually ran, and sends it to a Vexlo server.
Requires: 

The regular install and the editable install print the same two blocks.

The public surface

vexlo.__all__ is the whole public surface. Everything else under vexlo. is private and changes without notice.

Symbol	What it is	Page
`init`	Configures the SDK for the process, once, at startup	Initialise
`trace`	Marker: opens a trace, or records a step when one is already open	Mark functions
`step`	Marker: records a step inside the current trace	Mark functions
`tag`	Attaches key/value pairs to the current trace	Tags
`carrier`, `attach`	Carry the current trace onto another thread	Context across threads
`Carrier`	The type `carrier()` returns and `attach()` takes	Context across threads
`flush`	Drains buffered records to the spool directory, for at most 2 seconds	Configuration
`diagnostics`	Reads the eleven counters of what the SDK absorbed in this process	Configuration
`Diagnostics`	The type `diagnostics()` returns	Configuration
`DEFAULT_REDACT_KEYS`	The 12 names whose values are redacted unless `redact_keys` replaces them	What gets captured
`VexloError`	The base class of every exception the package raises	Configuration

Two exception classes are not exported, and both subclass VexloError, so a host catches vexlo.VexloError: ConfigurationError, which init() raises on bad configuration, and UnsupportedCallable, which a marker raises at decoration over a classmethod, property, cached_property, or partialmethod object. Those two moments are the only ones that raise whatever else is configured. A marker, tag(), and flush() raise into the host only under init(strict=True); see the never-raise boundary. Next: initialise the SDK and record a first trace.

Get started

Concepts

Python SDK

Server

API reference

MCP server

Reference

Requirements

Install from a checkout

Verify the install

The public surface

​Requirements

​Install from a checkout

​Verify the install

​The public surface

Requirements

Install from a checkout

Verify the install

The public surface