Agent Harness - TrueFoundry Docs

What Is An Agent Harness?

An agent harness is the runtime layer around an LLM that turns it into a reliable, long-running agent. Instead of only generating text, the harness manages the full execution loop: planning, tool calling, context management, approvals, state, and observability. For the full concept and how the runtime fits together, see What is an agent harness? on the TrueForge docs.

Diagram of the Agent Harness sitting between a user goal and the final response and trace: the harness orchestrates the run, routes between model, tools and MCP servers, sandbox, and approval gates, enforces guardrails, and records every step, with arrows showing the harness sending requests to each component and components returning results and events — The harness orchestrates the agent run, connecting the model, tools, sandbox, and approvals to deliver a safe, reliable result

TrueFoundry Agent Harness

TrueFoundry Agent Harness is the hosted version of TrueForge — the open-source agent harness maintained by TrueFoundry (github.com/truefoundry/trueforge). TrueForge runs the core agent loop — model calls, MCP tools, skills, sandboxing, approvals, context management, and session state. TrueFoundry runs that same harness for you and adds the layers needed to operate agents across a team:

Governance — Models, MCP servers, and skills are managed through the AI Gateway, MCP Gateway, and Skills Registry. No API keys in agent definitions, plus RBAC, budgets, rate limits, and guardrails.
Observability — Every agent session is captured with cost, tokens, turns, tool calls, and end-to-end traces in one place. See Observability.
Enterprise hosting — Run as SaaS, self-hosted, or on-prem in your own cloud and region.

For the full breakdown — including how it compares to other hosted runtimes — see What TrueFoundry Adds on Top of TrueForge. You choose a model, connect MCP servers, add skills, and write instructions. TrueFoundry manages orchestration, sandbox lifecycle, tool execution, approvals, governance, and observability.

TrueFoundry Agent Harness builder showing model selection, MCP servers, skills, instructions, and a playground test button — Build a managed agent by selecting a model, connecting MCP servers, adding skills, and writing instructions

Harness Capabilities

You configure four things on TrueFoundry — models, MCP servers, skills, and how you monitor them — and the harness runs the rest of the execution loop.

Models

Any provider through AI Gateway, with model-level RBAC, budgets, and routing.

MCP Servers

Governed MCP tools with centralized auth, in-chat OAuth, and per-user delegation.

Skills

Versioned SKILL.md instructions from the Skills Registry, mounted on demand.

Observability

Inspect every agent session with cost, tokens, turns, tool calls, and end-to-end traces.

Capabilities that run in the harness

Sandboxing, context engineering, human checkpoints, and Generative UI are part of the open-source TrueForge harness. The mechanics are documented in the TrueForge docs; TrueFoundry runs them for you as a managed, governed service.

Capability	What it does	TrueForge reference
Sandbox	Isolated code, file, and shell execution, provisioned only when the agent needs one	Setup Sandbox
Subagents	Delegate focused subtasks to parallel agents with their own clean context	Subagents
Deferred tool loading	Load MCP tool definitions on demand instead of upfront to keep context lean	Deferred Tool Loading
Code Mode	Chain and aggregate MCP tool calls in a single sandbox script	Code Mode
Large tool responses	Offload oversized tool output to the sandbox instead of flooding context	Large Tool Responses
Context compaction	Summarize older history when context nears the model limit	Harness Capabilities
Human checkpoints	Pause for tool approval or ask the user a structured question	Harness Capabilities
Generative UI	Respond with interactive charts, tables, cards, and forms	Harness Capabilities

TrueFoundry runs these as a managed, governed service — with a colocated low-latency sandbox, sensible defaults, org-wide approval policy, and full observability. See What TrueFoundry Adds on Top of TrueForge for governance, the managed runtime, enterprise hosting, and how it compares to other hosted runtimes.

Architecture

Agent Harness runs in the same gateway plane as model and MCP traffic, so orchestration, governance, and observability stay in one system.

Start Building

Create an agent by choosing a model, connecting MCP servers, adding skills, and writing instructions. Then test in playground, integrate via API, monitor through traces and metrics, and ship it to users.

What TrueFoundry Adds

Governance, observability, managed runtime, and hosting on top of TrueForge.

Build From UI

Create and test a managed agent from the TrueFoundry console.

Use the SDK

Create sessions, stream progress, and integrate Agent Harness into your application.

Monitor Agents

Track sessions, cost, and traces across every agent run.

​What Is An Agent Harness?

​TrueFoundry Agent Harness

​Harness Capabilities