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Sessions and Budgets

A session is the unit of runtime control in Captar. It carries the active budget, merged policy, metadata, counters, and the root trace context that child request and tool spans inherit from. Think of the session as the envelope around one meaningful unit of work: a user turn, a background job, a workflow step, or any other boundary where cost and traceability matter.

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What belongs in a session

  • A project-scoped budget envelope
  • Metadata for user, team, feature, and hook context
  • Policy for calls and tools
  • The root span ID and trace ID
  • Counters for repeated calls and tool usage

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Budget behavior

const session = await captar.startSession({
  budget: {
    maxSpendUsd: 5,
    finalizationReserveUsd: 0.25,
    maxRepeatedCalls: 3,
  },
});
Captar reserves estimated spend before execution and reconciles actual usage afterward. If a request cannot be safely reserved, the SDK blocks the execution before it reaches the provider.
The budget engine runs inside your application process. This is what lets Captar stop runaway execution before the request leaves your app.

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What the session should carry

  • A clear owner or feature label in metadata
  • The smallest reasonable budget envelope for the task
  • Call policy that matches the provider or model you actually allow
  • Tool policy for anything that can mutate state or call external systems
  • Enough trace context to make the later review readable