Harness

Harness is a VeADK execution enhancement extension that provides two mutually exclusive integration modes: an in-process plugin mode and a managed Sidecar mode. The in-process mode loads Harness plugins into the application process to prepare context, compact tool results, and verify answers for each turn. The managed Sidecar mode runs all enhancement behavior in a separate managed runtime, and the application process does not load the related plugins.

The two modes cannot be combined. When managed Sidecar mode is enabled, HarnessExtension.plugins() always returns an empty list, and the application process does not load any veadk.extensions.harness.plugins implementation.

When to use

Mode	Use case
In-process plugins	Local development, testing, or lightweight deployments that do not require a separate runtime service
Managed Sidecar	Production deployments on an AgentKit-managed cloud runtime that isolates enhancement behavior into a separate runtime

Installation

The base Harness extension is bundled with VeADK and requires no additional installation. To use the Headroom compression provider, install the harness extra:

pip install "veadk-python[harness]"

In-process plugin mode

The in-process mode builds a plugin list via build_harness_plugins() and attaches it to the Runner:

app.py

import asyncio
from veadk.extensions.harness.plugins import build_harness_plugins
from veadk import Agent, Runner

agent = Agent(name="research_agent")
plugins = build_harness_plugins(
    components=["invocation_context", "compactor", "response_verification"],
    profile="research",
)
runner = Runner(agent=agent, app_name="research", plugins=plugins)
print("Plugins:", [plugin.name for plugin in plugins])
print(asyncio.run(runner.run("Explain what evidence a research report needs")))

Complete model configuration, save as app.py, and run python app.py. It should print plugin names followed by an agent response. Response verification uses rules and does not guarantee factual correctness. Compression can remove detail, so evaluate it for your task

Plugin capabilities

Plugin	Main hooks	Purpose
`HarnessInvocationContextPlugin`	`on_user_message_callback`, `before_model_callback`	Prepares task anchors, recent context, and tool-use guardrails
`HarnessCompressPlugin`	`before_model_callback`, `after_tool_callback`	Shrinks oversized tool outputs while preserving useful facts
`HarnessResponseVerificationPlugin`	`after_tool_callback`, `after_model_callback`, `on_event_callback`	Records tool receipts and flags unsupported final claims

Configuration via environment variables

Use build_harness_plugins_from_env() to build plugins from environment variables:

from veadk.extensions.harness.env import build_harness_plugins_from_env

plugins = build_harness_plugins_from_env()

Related environment variables:

Environment variable	Default	Description
`HARNESS_ENHANCE_ENABLED`	`false`	Whether to enable in-process Harness plugins
`HARNESS_ENHANCE_COMPONENTS`	`invocation_context,compactor,response_verification`	Enabled plugin components, comma-separated
`HARNESS_ENHANCE_PROFILE`	`default`	Plugin profile
`HARNESS_COMPRESSION_PROVIDER`	`builtin`	Compression provider, either `builtin` or `headroom`
`HARNESS_MAX_CONTEXT_CHARS`	`24000`	Maximum context size in characters
`HARNESS_MAX_TOOL_RESULT_CHARS`	`4000`	Tool result compaction threshold in characters
`HARNESS_STORE_PATH`	—	Harness store path; uses in-memory storage when unset
`HARNESS_VERIFIER_MODE`	`observe`	Answer verification mode, either `observe` or `block`

Components and profile prefer HARNESS_ENHANCE_COMPONENTS and HARNESS_ENHANCE_PROFILE, then their counterparts without ENHANCE. Character limits, storage path, compression provider, and verifier mode prefer the variables without ENHANCE. There is no single prefix-precedence rule for all settings

Managed Sidecar mode

The managed Sidecar mode starts a managed runtime via HarnessExtension. The runtime is responsible for executing all Harness enhancement behavior. HarnessExtension only starts the runtime, applies model and MCP bindings, and manages its lifecycle.

Enable via environment variables

Set HARNESS_SIDECAR_ENABLED=true and HarnessExtension.from_env() reads the relevant environment variables and starts the Sidecar:

from veadk.extensions.harness import HarnessExtension

extension = HarnessExtension.from_env()
plugins = extension.plugins()  # Returns an empty list in managed Sidecar mode

HarnessExtension supports the context manager protocol and automatically closes the Sidecar on exit:

from veadk.extensions.harness import HarnessExtension

with HarnessExtension.from_env() as extension:
    print(extension.sidecar_status_payload())

Enable via constructor parameter

You can also enable the Sidecar directly via the sidecar parameter:

from veadk.extensions.harness import HarnessExtension

extension = HarnessExtension(
    sidecar=True,
    profile="default",
)

The sidecar parameter accepts a boolean or a configuration dict. When a dict is provided, its fields are used to resolve the Sidecar plan; when True is provided, default configuration is used.

When Sidecar mode is enabled, the components parameter cannot be passed. Component selection is controlled through component_overrides in the Sidecar configuration.

Enable via configuration object

Use HarnessSidecarConfig to build a complete configuration, then pass it to HarnessExtension:

from veadk.extensions.harness import HarnessExtension
from veadk.extensions.harness.sidecar_runtime import HarnessSidecarConfig

config = HarnessSidecarConfig(
    enabled=True,
    profile="default",
)
extension = HarnessExtension(sidecar=config)

HarnessSidecarConfig.from_env() can also build a configuration object from environment variables.

Sidecar status

Property / method	Return value	Description
`sidecar_status`	`str`	Sidecar status: `ok`, `degraded`, `disabled`, or `not_started`
`sidecar_env`	`dict[str, str]`	Environment variable bindings injected by the Sidecar into the application process
`sidecar_status_payload()`	`dict`	Status snapshot suitable for route responses, containing `enabled`, `status`, `planHash`, and `effectiveComponents`
`close()`	`None`	Closes the Sidecar and releases runtime resources

Sidecar environment variables

The managed Sidecar mode is configured through the following environment variables. Defaults are used when not explicitly set.

General configuration

Environment variable	Default	Description
`HARNESS_SIDECAR_ENABLED`	`false`	Whether to enable the managed Sidecar
`HARNESS_SIDECAR_FAIL_OPEN`	`true`	Whether to continue in degraded mode if Sidecar startup fails; set to `false` to raise an error on failure
`HARNESS_SIDECAR_TRANSPORT`	`local`	Transport mode, either `local` or `apig_runtime_port`
`HARNESS_PROFILE`	`default`	Optimization profile, either `default` or `ops`
`HARNESS_SIDECAR_CATALOG_VERSION`	`2026.07.1`	Component catalog version
`HARNESS_SIDECAR_RUNTIME_VERSION`	—	Runtime version
`HARNESS_SIDECAR_COMPONENT_OVERRIDES`	—	Component overrides, a JSON object with component IDs as keys and booleans as values
`HARNESS_RUNTIME_COMPONENTS`	—	Runtime components, comma-separated
`AGENTKIT_HARNESS_RUNTIME_COMMAND`	—	Runtime startup command

Model proxy configuration

Environment variable	Default	Description
`HARNESS_MODEL_PROXY_ENABLED`	`true`	Whether to enable the model proxy
`HARNESS_MODEL_PROXY_HOST`	`127.0.0.1`	Model proxy listen address
`HARNESS_MODEL_PROXY_PORT`	`0`	Model proxy listen port; `0` for auto-assignment
`HARNESS_MODEL_UPSTREAM_BASE_URL_ENV`	`MODEL_AGENT_API_BASE`	Name of the environment variable holding the upstream model API base URL
`HARNESS_MODEL_UPSTREAM_API_KEY_ENV`	`MODEL_AGENT_API_KEY`	Name of the environment variable holding the upstream model API key
`HARNESS_MODEL_PREFER_CONFIGURED_UPSTREAM_API_KEY`	`false`	Whether to prefer the configured upstream API key
`HARNESS_MODEL_COMPRESSION_PROVIDER`	`noop`	Compression provider, either `noop` or `headroom`

MCP gateway configuration

Environment variable	Default	Description
`HARNESS_MCP_GATEWAY_ENABLED`	Profile-dependent	Whether to enable the MCP gateway; enabled by default for the `ops` profile
`HARNESS_MCP_GATEWAY_HOST`	`127.0.0.1`	MCP gateway listen address
`HARNESS_MCP_GATEWAY_PORT`	`0`	MCP gateway listen port; `0` for auto-assignment
`HARNESS_MCP_UPSTREAMS_ENV`	`MCP_URLS`	Name of the environment variable holding the upstream MCP URL list
`HARNESS_MCP_UPSTREAM_API_KEY_ENV`	`MCP_API_KEY`	Name of the environment variable holding the upstream MCP API key
`HARNESS_MCP_PREFER_CONFIGURED_UPSTREAM_API_KEY`	`false`	Whether to prefer the configured upstream API key
`HARNESS_MCP_FAIL_OPEN`	`true`	Whether to continue running if the MCP gateway fails
`HARNESS_MCP_READONLY_SEGMENTS`	—	Read-only protected paths, comma-separated
`HARNESS_MCP_PRESETS`	—	MCP presets, comma-separated

HarnessExtension parameters

Parameter	Type	Default	Description
`enabled`	`bool \| None`	`None`	Whether to enable in-process plugins; ignored in Sidecar mode
`components`	`Iterable[str] \| str \| None`	`None`	In-process plugin component list; cannot be passed in Sidecar mode
`profile`	`str`	`default`	Optimization profile
`store`	`HarnessStoreProtocol \| None`	`None`	Harness store
`context_config`	`HarnessInvocationContextConfig \| None`	`None`	Context configuration
`compaction_config`	`ToolResultCompactorConfig \| None`	`None`	Compaction configuration
`verifier_config`	`FinalResponseVerifierConfig \| None`	`None`	Answer verification configuration
`sidecar`	`bool \| Mapping[str, Any] \| Any \| None`	`None`	Sidecar configuration; pass `True` or a configuration object to enable managed Sidecar mode
`env`	`Mapping[str, str] \| None`	`None`	Environment variable mapping

When profile is ops, the in-process mode default components include long_run_control.

The managed Sidecar mode requires an AgentKit-managed cloud runtime environment. Enabling it in an environment without the Sidecar Runtime causes a startup error or degraded operation, depending on the value of HARNESS_SIDECAR_FAIL_OPEN.

Direct module usage

Individual modules from the in-process mode can also be used directly:

from veadk.extensions.harness import HarnessInvocationContextBuilder, HarnessInvocationRef

context = HarnessInvocationRef(session_id="session-1", invocation_id="run-1")
builder = HarnessInvocationContextBuilder()
bundle = builder.prepare_context(context, user_input="Summarize these tool results.")

Checking Sidecar status

After setting the environment, create HarnessExtension before constructing the agent. Call close() when finished or use a context manager. Inspect sidecar_status_payload(): ok indicates successful startup; degraded indicates fallback and does not prove enhancements are active. HARNESS_SIDECAR_FAIL_OPEN=true allows execution to continue after startup failure by default. Set it to false if Sidecar is mandatory See Harness deployment for the standalone service’s HTTP API, sessions, and deployment. Sidecar bindings affect model and MCP request destinations; confirm upstream endpoints and credential sources before use

Overview

Get started

Core components

Observability

Extensions

Frontend

Security

Deployment

When to use

Installation

In-process plugin mode

Plugin capabilities

Configuration via environment variables

Managed Sidecar mode

Enable via environment variables

Enable via constructor parameter

Enable via configuration object

Sidecar status

Sidecar environment variables

General configuration

Model proxy configuration

MCP gateway configuration

HarnessExtension parameters

Direct module usage

Checking Sidecar status

​When to use

​Installation

​In-process plugin mode

​Plugin capabilities

​Configuration via environment variables

​Managed Sidecar mode

​Enable via environment variables

​Enable via constructor parameter

​Enable via configuration object

​Sidecar status

​Sidecar environment variables

​General configuration

​Model proxy configuration

​MCP gateway configuration

​HarnessExtension parameters

​Direct module usage

​Checking Sidecar status

When to use

Installation

In-process plugin mode

Plugin capabilities

Configuration via environment variables

Managed Sidecar mode

Enable via environment variables

Enable via constructor parameter

Enable via configuration object

Sidecar status

Sidecar environment variables

General configuration

Model proxy configuration

MCP gateway configuration

HarnessExtension parameters

Direct module usage

Checking Sidecar status