Versioned Harness Skills: Teach Agents Without Rebuilding Them
A reusable procedure should not have to become a compiled plugin, and a production agent should not silently change because someone edited shared prompt text. Today we added Harness Skills: immutable, exact-version instruction packages administered per principal and explicitly selected by each agent.
A Skill Is Content, Not Code
A skill package contains text and ZIP resources that help a Harness agent perform a repeatable job: review policy, prepare an incident report, apply an organization's writing standard, or follow a data-validation checklist. Packages cannot contain scripts or executables. This keeps the feature on the instruction-and-reference side of the security boundary.
When a workflow needs to call a system, mutate data, or execute trusted logic, use a FabrCore plugin, tool, or MCP integration. A Skill can explain when and why to call that capability; it does not smuggle a new executable into the runtime.
Exact Version or No Version
Every published package has an immutable identity in the form name@version. Agents select exact identities through the _HarnessSkills argument. There is no floating latest pointer during execution.
That makes a run reproducible. If [email protected] approved a document yesterday, the same agent configuration loads the same content tomorrow. Publishing 1.3.0 is additive; moving an agent to it is a reviewed configuration change rather than an invisible global mutation.
Principal Ownership Is the Default Boundary
Skills are stored and administered under a principal. The administrative API can list, publish, retrieve, and delete versions for that owner, and every route is protected by the FabrCoreAdmin policy. Runtime selection validates the requested skill against the host agent's principal before adding its content to the Harness context.
The separation gives platform teams a safe distribution model. One tenant can publish its internal operating procedures without exposing them to another. Agents receive only the exact packages selected for their job, reducing both accidental disclosure and prompt bloat.
The Harness Skills guide covers storage, package limits, SDK methods, administration routes, and security guidance.