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These concepts are deliberately simple. Most complexity in agent systems comes from unclear boundaries. We’re strict about those boundaries.

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Agents

An Agent is any system that can:
  • Reason
  • Decide
  • Invoke tools
Handlebar does not care how the agent is implemented (LLM, workflow engine, hand-written logic). We treat the agent as an external actor whose behaviour must be governed. Agents are not trusted by default.

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Tools

A Tool is any capability an agent can invoke:
  • API calls
  • Database queries
  • File access
  • Emails, messages, jobs, side effects
Tools are where agents touch the real world. That makes them the highest-risk surface. In Handlebar, tools carry metadata:
  • What kind of data they touch
  • Who they affect
  • Whether they’re reversible
  • Where outputs go
This metadata is critical for rule evaluation.

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Tool Calls

A Tool Call is a specific invocation of a tool during a run. It includes:
  • Inputs
  • Context
  • Timing
  • Outcome (if allowed)
Tool calls are the primary decision points in the system. Rules are evaluated before a tool call is executed.

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Rules

A Rule is a deterministic check evaluated against context. Given the same inputs, a rule will always return the same result. Rules can inspect:
  • Agent identity
  • Run context
  • Tool metadata
  • Tool call parameters
  • Historical signals
Rules are versioned. Old decisions can be replayed exactly.

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Actions

An Action is the consequence of a rule evaluation. Actions are hard, not advisory. Examples:
  • Allow
  • Block
  • Modify inputs
  • Require approval
  • Kill a run
  • Lockdown
If an action blocks a tool call, the tool does not run. There is no bypass.

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Determinism

The same inputs produce the same decisions. This enables:
  • Replay
  • Debugging
  • Audits
  • Trust
If a decision can’t be explained deterministically, it doesn’t belong in the control path.

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Why This Matters

In regulated environments, - “this is unlikely to happen” - isn’t good enough. You need to be able to say:
  • This agent could not access this data under these conditions
  • This action was blocked for this reason
  • Here is the exact rule version that made that decision
That’s the bar for Handlebar.