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capability-writs.md

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title Capability Writs
eyebrow Authority model
subtitle Writs are signed, delegable, bounded capability documents.
permalink /capability-writs/

Capability Writs

A capability writ is the sole source of authority in OpenThymos. It authorizes a subject to emit intents within explicit constraints. The runtime validates writs before proposal execution.

Writ Body

A writ body includes:

  • issuer name
  • issuer public key
  • subject name
  • subject public key
  • optional parent writ id
  • tenant id
  • tool scopes
  • budget
  • effect ceiling
  • time window
  • delegation bounds

The writ id is derived from canonical body content. The signature is an ed25519 signature over the canonical body.

Tool Scopes

Tool scopes are literal names or prefix patterns. A writ that authorizes fs_* covers fs_read and fs_patch; a writ that authorizes fs_read does not cover fs_patch.

Tool scope is checked before tool execution and before the tool result can be committed.

Budget

Writ budget is multi-dimensional:

  • tokens
  • tool calls
  • wall-clock milliseconds
  • USD millicents

The compiler checks projected cost against remaining budget. The commit records budget cost incurred by the execution.

Effect Ceiling

Effect ceilings distinguish classes of effect:

  • write
  • external
  • irreversible

A child writ cannot grant an effect that the parent forbids.

Time Window

A writ is valid only within its [not_before, expires_at] interval. The compiler rejects intents bound to expired or not-yet-valid writs.

Tenant Boundary

Every writ belongs to exactly one tenant id. Child writs must inherit the same tenant id as their parent. Cross-tenant delegation is invalid.

Delegation

A writ may be subdivided only when delegation bounds allow it. A child writ must satisfy:

  • child issuer public key equals parent subject public key
  • child tenant id equals parent tenant id
  • child tool scopes are covered by parent tool scopes
  • child budget does not exceed parent budget
  • child effect ceiling does not exceed parent effect ceiling
  • child time window fits within parent time window
  • child delegation depth is less than parent remaining depth

This prevents lateral minting and privilege expansion.

Lifecycle

issue body -> sign body -> admit writ -> bind intent -> compile proposal
          -> debit projected budget -> commit observed cost
          -> optionally mint strict child writ

Invalid States

The runtime must reject:

  • signature mismatch
  • child scope not covered by parent
  • child budget exceeding parent
  • child effect ceiling exceeding parent
  • child time window outside parent window
  • cross-tenant delegation
  • expired writ
  • unknown or unauthorized tool

Audit Value

Writ ids appear on proposals and commits. This makes execution authority traceable after the fact: an auditor can identify which capability authorized each effect.