Bounded Capability Receipts and Durable Spend Control for Agent Actions

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Table of Contents

1. Introduction

A single-action authorization receipt is intentionally narrow: it records approval of one exact action and can be accepted at most once within its atomic consumption domain. Some agent deployments also need a different primitive. For example, an operator may authorize an agent to purchase a bounded class of supplies, subject to an aggregate monetary ceiling and an expiry, without asking a human to approve each conforming purchase.¶

That primitive has two inseparable parts:¶

1. a signed capability receipt that states the immutable authority boundary; and¶
2. a shared, durable state machine that serializes reservations and committed consumption across retries, processes, replicas, and restarts.¶

The signed object without the state machine is replayable budget metadata. The state machine without a signed, scoped grant has no portable statement of authority. This document specifies their composition.¶

2. Terminology

Issuance authorization:

An independently verified authorization artifact for the exact act of creating a bounded capability.¶

Capability receipt:

The signed immutable grant defined by this document.¶

Capability issuer:

The principal whose pinned key signs a capability receipt.¶

Holder proof:

Proof that the requester controls the secret or key named by the capability receipt. Holder proof is not proof that a requested action is in scope.¶

Scope verifier:

A verifier for a named scope profile, selected and configured by the relying party.¶

Capability store:

The authoritative shared transactional state for registration, reservation, and committed consumption.¶

Operation ID:

A globally unique identifier for one attempted capability-funded operation within a capability-store domain.¶

Reservation token:

An unguessable owner-fencing value returned by a successful reservation and required for its commitment.¶

Effect boundary:

The point after which an external effect may have occurred and therefore budget cannot safely be restored merely because the caller did not receive a successful response.¶

Indeterminate outcome:

An operation for which the executor cannot prove that no effect occurred and cannot prove a successful expected effect.¶

3. Trust Model

The relying party selects capability-issuer keys, accepted issuance authorization profiles, scope profiles, state-store domain, and local authorization policy. A key embedded in a presented capability receipt MUST NOT, by itself, become a trust anchor. An empty trust configuration MUST fail closed.¶

The capability store is trusted to serialize state transitions and retain committed operation records. The effect adapter is trusted to place the effect boundary correctly and to report outcomes honestly. This protocol makes those trust dependencies explicit; it does not remove them.¶

All capability-store participants that can authorize a capability or allocate, register, reserve, commit, reconcile, suspend, revoke, or report any ancestor or descendant drawing on that capability's authority MUST use one shared authoritative atomic state domain. Independent stores cannot prevent each other from accepting or reallocating the same remaining authority. Conservation claims in this document apply only inside that one domain.¶

4. The Bounded Capability Receipt

The receipt is a JSON object serialized using the JSON Canonicalization Scheme (JCS) [RFC8785] before signing. The following is illustrative:¶

{
  "@version": "EP-BOUNDED-CAPABILITY-v1",
  "capability": {
    "capability_id": "cap_01J...",
    "issuer": "https://operator.example",
    "subject": "agent:procurement-7",
    "authorization": {
      "receipt_id": "rcpt_01J...",
      "receipt_digest": "sha256:4c..."
    },
    "scope": {
      "profile": "urn:emilia:scope:caid-set-v1",
      "value": {
        "caids": ["caid1:sha256:..."]
      },
      "digest": "sha256:8a..."
    },
    "budget": {
      "amount": 250000,
      "unit": "iso4217:USD",
      "scale": 2
    },
    "holder": {
      "method": "sha-256-preimage",
      "commitment": "sha256:91..."
    },
    "threshold": {"m": 2, "n": 3},
    "parent": null,
    "not_before": "2026-07-18T20:00:00Z",
    "expires_at": "2026-07-19T20:00:00Z"
  },
  "capability_signature": {
    "algorithm": "Ed25519",
    "public_key": "base64url...",
    "value": "base64url..."
  }
}

SHA-256(JCS({
  "@version": receipt["@version"],
  "capability": receipt.capability,
  "capability_signature": receipt.capability_signature
}))

5. Issuance Authorization

The issuance authorization MUST authorize the act of creating the capability, including the immutable digest of its subject, scope, budget, holder commitment, parent, and validity interval. It MUST NOT be reused as though it were a per-operation authorization for later spends.¶

When the issuance artifact is an EMILIA Authorization Receipt, its exact action is capability issuance and its one-time consumption occurs when the capability is registered. Later capability-funded operations are governed by this document's scope and durable state protocol.¶

6. Action Scope

Before reserving budget, the enforcement point MUST compute the proposed material action independently of presenter-supplied labels and invoke the pinned verifier for scope.profile. A missing profile, unknown action representation, lossy mapping, or indeterminate comparison MUST refuse.¶

The mandatory-to-implement urn:emilia:scope:caid-set-v1 profile contains a non-empty, duplicate-free array of Canonical Action IDentifiers. It matches only exact identifier equality. Possession of a CAID authorizes nothing outside this verified capability context.¶

Application profiles can define closed constraints over typed action fields. Such a profile MUST specify canonicalization, comparison, unknown-field handling, numerical units, and an algorithm for proving that a delegated scope is no broader than its parent. Profiles that cannot decide either action membership or attenuation MUST return INDETERMINATE.¶

7. Holder Proof and Threshold Custody

The mandatory-to-implement holder method is sha-256-preimage. The holder presents exactly 32 bytes over a confidential, integrity-protected channel, and the enforcement point compares SHA-256(preimage) with the signed commitment using a constant-time comparison. The preimage MUST NOT be logged, stored with the receipt, or included in portable evidence.¶

The preimage may be divided using a threshold secret-sharing scheme before presentation. Share format, participant authentication, confidentiality, recovery, and distribution are outside this document. Reconstructing m shares proves control of the holder secret; it does not prove that m distinct humans reviewed or approved the action. Human multi-party approval requires a protocol such as [EP-QUORUM].¶

8. Registration

After receipt verification and one-time consumption of the issuance authorization, the issuer registers the capability in the authoritative store. Registration MUST atomically create immutable fields for capability ID, receipt digest, unit, scale, total budget, scope digest, validity interval, and parent. It MUST initialize consumed and reserved to zero.¶

A second registration of the same capability ID MUST succeed only when every immutable field and receipt digest is identical. Any mismatch is a collision and MUST refuse.¶

Mutable counters in a presented receipt are not authoritative. Remaining budget is computed only from the shared store:¶

remaining = total - consumed - reserved

The authoritative state MUST maintain the invariant reserved + consumed <= total for every capability. For a parent capability, each registered child allocation MUST be covered by one or more distinct terminal delegated operation committed against that parent before child registration. The aggregate amount of registered direct children MUST NOT exceed the amount committed by those distinct parent delegation operations, and each operation identifier MUST NOT fund more than one child receipt digest.¶

9. Reserve, Execute, and Commit

10. Narrowing Delegation

A child capability MUST NOT outlive its parent, exceed the authenticated amount delegated from the parent, change unit or scale, or broaden the parent's scope. Its delegation chain MUST be bounded by deployment policy and MUST include the parent receipt digest.¶

Before registering a child, the verifier MUST validate complete digest-linked ancestry to a trusted root within the configured depth bound, or enforce equivalent authenticated parent-edge constraints in one authoritative store. The verified lineage MUST form a simple path. The verifier MUST reject a repeated capability identifier or receipt digest, a missing or inconsistent parent, a substituted or reordered edge, or a chain whose trusted root cannot be established. A cycle, truncated lineage, or over-depth chain MUST fail closed. Per-receipt identifier uniqueness or a self-declared list of ancestors does not establish graph-wide acyclicity. An implementation MUST NOT infer acyclicity merely because its ordinary issuance path constructs children from known parents; verification applies the same check to imported and reconstructed chains.¶

Creating a child is itself a parent-funded operation. Before registering the child, the issuer MUST reserve the delegated amount from the parent and MUST commit that exact reservation once with outcome delegated. The authenticated terminal operation record MUST bind the exact child receipt digest, delegated amount, unit, scale, scope, and validity interval before the child is registered; otherwise a valid parent spend could be paired with a different child. A shared store SHOULD perform parent commitment and child registration atomically. If that is impossible and child registration fails after the parent is committed, the parent budget remains consumed and the orphaned delegation MUST be retained for reconciliation. The system MUST NOT silently refund it.¶

11. Evidence and Decision Vocabulary

A capability receipt can be VERIFIED. A scope verifier can return the profile-local result IN_SCOPE, OUT_OF_SCOPE, or INDETERMINATE. This containment result is not the architecture's MATCH state, which is reserved for correlation of exact material actions. A relying-party evidence requirement can be SATISFIED. Successful local policy and an atomic reservation together can establish AUTHORIZED for one exercise. Separately authenticated effect evidence can establish EXECUTED. No earlier state implies a later one.¶

Portable evidence for a capability-funded operation SHOULD include the capability receipt digest, issuance authorization digest, scope profile and digest, operation ID, the exact exercise action digest and CAID where used, amount, unit, scale, reservation timestamp, terminal outcome, and any authenticated effect statement. The integrity-protected operation record MUST bind the exercise action and the capability receipt digest. Holder secrets and reservation tokens MUST NOT be included.¶

An Authorization Evidence Chain may carry that operation record as a native component whose verifier recursively verifies the capability receipt, issuance authorization, scope result, and operation-record integrity. The static grant is not a same-action component for every later exercise. Evidence satisfaction does not query or reserve current budget; that state transition remains at the enforcement point.¶

12. Failure Codes

Implementations SHOULD expose stable, non-authorizing failure codes including:¶

• capability_untrusted_issuer¶
• capability_authorization_mismatch¶
• capability_scope_mismatch¶
• capability_scope_indeterminate¶
• capability_holder_proof_invalid¶
• capability_not_active¶
• capability_expired¶
• capability_revoked¶
• capability_budget_exceeded¶
• capability_delegation_lineage_invalid¶
• capability_delegation_not_narrowed¶
• capability_operation_replay¶
• capability_reservation_owner_mismatch¶
• capability_commit_indeterminate¶

A failure code is diagnostic output, not an authorization artifact. Responses SHOULD avoid revealing secret, budget, or scope details to an unauthenticated caller.¶

13. Conformance

A conforming implementation MUST pass positive and adversarial vectors for receipt canonicalization and signature, authorization-digest substitution, untrusted issuer, unknown scope profile, action mismatch, holder-proof failure, duplicate registration, concurrent overspend, operation replay, wrong reservation token, double commit, expiry, a cycle spread across separately signed receipts, repeated ancestors, leaf-as-ancestor, missing or truncated lineage, reordered or substituted parent links, parent-receipt and delegation-operation substitution, a single-hop child exceeding the authenticated delegated amount, unit or scale changes, scope or validity widening at every hop, over-depth chains, parent over-allocation, crash recovery, and indeterminate-effect charging.¶

The parent-over-allocation case MUST include at least three sibling child-creation attempts whose individually valid amounts collectively exceed the parent's available balance, with concurrent reservation ordering chosen by the implementation. At most a balance-preserving subset may commit. The case MUST also cover one operation identifier presented for two different child receipt digests and an orphaned child-registration failure after parent commitment. The former refuses as operation replay; the latter leaves the committed parent amount consumed pending reconciliation.¶

A wire-format implementation that does not implement one shared atomic store is a receipt verifier, not a conforming spend-control implementation. A store implementation that accepts a capability without pinned issuer verification, issuance authorization, and scope matching is not conforming.¶

14. Relationship to Other Work

Rich Authorization Requests [RFC9396] carries fine-grained authorization details but deliberately leaves comparison semantics for arbitrary detail types to their specifications. This document defines an executor-side durable spend state machine and requires a named closed scope profile.¶

OAuth Transaction Tokens [TXN-TOKENS] propagate transaction-specific authorization context through a call chain. Bounded Capability Receipts instead address an aggregate budget shared across multiple operations and the reserve/commit boundary at the executor. A deployment can use both.¶

The Delegation Receipt Protocol [DRP] records delegation and narrowing. This document requires narrowing for child capabilities and additionally accounts delegated budget as a terminal parent spend.¶

OAuth Attenuating Tokens for Agentic AI [ATTENUATING] describes constrained, attenuable agent tokens. This document's distinct contribution is not the existence of constrained tokens; it is the composition of a signed grant with shared reservation ownership, committed budget accounting, and conservative treatment of indeterminate external effects.¶

The Agent Identity Protocol [AIP] defines per-token budget ceilings and explicitly assigns cumulative spending enforcement to the orchestration runtime. A bounded capability budget is instead a balance-valued authority in one authoritative store: reservation and consumption reduce the amount available to every sibling allocation in that domain.¶

PEDIGREE [PEDIGREE] defines cryptographic delegation, mandate narrowing, and an operator-controlled ceiling. This document preserves that identity and policy role and addresses the adjacent runtime question of how one parent balance funds multiple children without multiplying aggregate authority.¶

The Credential Broker for Agents [CB4A] defines proxy and short-lived-token delivery patterns that keep long-lived provider credentials away from agents. A deployment can use such a broker as the credential-owning effect adapter after this protocol grants one valid reservation; this document does not duplicate credential brokering.¶

Condition-Bounded Credentials [CBC] binds workload-key use to live, verifier-appraised conditions. That property composes with holder proof and provider entry, especially for stable attestable workloads. It does not replace aggregate balance accounting, and this document does not extend its hardware assumptions to hardware-less or cross-domain swarms.¶

The affine Token Budgets work [TOKEN-BUDGETS] studies LLM cost overruns and uses Rust ownership to prevent cloning and use-after-delegation in one process. It is adjacent prior art. This document instead binds human- or policy-authorized consequential authority, exact exercise actions, durable provider-entry reservations, and conservative post-entry uncertainty across a transactional runtime. It does not claim that balance-valued budgets or affine ownership were invented here.¶

15. Security Considerations

Identifier substitution. Capability signatures bind the full issuance authorization digest, not only an identifier. Scope, budget units, parent, holder commitment, and validity are all inside the issuer signature.¶

State forks. Two stores accepting the same capability lineage can each spend or delegate its full budget. Global offline or cross-domain double-spend prevention is therefore not provided. Deployments that cannot name one authoritative atomic state domain for a capability and every authority-bearing ancestor and descendant MUST NOT claim aggregate sibling conservation or an enforced aggregate budget.¶

Crash ambiguity. Restoring budget after a timeout can authorize duplicate external effects. Once the effect boundary may have been crossed, uncertainty is charged as indeterminate.¶

Bearer and share theft. A raw holder secret or enough unauthenticated shares can authorize possession. Secret shares require confidential distribution, authenticated participants, compromise response, and rate limiting. Threshold custody is not human quorum.¶

Revocation. Expiry and exhausted budget are not revocation. A deployment that requires early invalidation MUST consult a separately authenticated revocation or status source before reservation and define its freshness policy.¶

Units and arithmetic. All accounting uses integers with signed unit and scale. Floating-point arithmetic, implicit currency conversion, and caller-selected rounding MUST NOT occur in the authoritative budget path.¶

Database authority. The capability tables contain authorization state. Deployments MUST restrict writes to the enforcement service, use least-privilege credentials, protect backups, and audit administrative changes.¶

Delegation lineage. Local uniqueness checks over one presented receipt do not establish graph-wide acyclicity or complete ancestry. Separately presented receipts can omit links, substitute a parent operation, or form a cycle unless each parent edge is authenticated and resolved by digest, or an authoritative store enforces equivalent edge constraints. Implementations MUST fail closed on incomplete, cyclic, substituted, or non-narrowing lineage. Verifiers apply the complete traversal and refusal rules in Section 10 to imported chains and chains reconstructed from storage.¶

Semantic limits. A valid capability does not prove that an action is safe, lawful, beneficial, or correctly executed. Local policy and domain controls remain necessary.¶

Cryptographic scope. This version uses SHA-256 and Ed25519. It does not provide a post-quantum signature profile or a zero-knowledge receipt. Algorithm agility and long-term preservation are separate concerns.¶

16. Privacy Considerations

Capability receipts and operation records can reveal spending limits, organizational roles, intended action classes, counterparties, and timing. Profiles SHOULD minimize identifiers, separate portable evidence from operational secrets, and define retention and access controls. Hashing a low-entropy scope or identifier does not make it confidential.¶

17. Implementation Status

The Apache-2.0 TypeScript reference implementation includes a signed pre-standard capability envelope, holder-secret commitment, optional threshold secret reconstruction, and a durable PostgreSQL reservation and commitment store. Its issuer-controlled delegation API, when used with one shared capability store, reserves and commits a child amount from the immediate parent before registering the child. The store has adversarial tests for overspend, replay, ownership fencing, expiry, terminal commitment, concurrent N-sibling aggregate over-allocation, one operation identifier paired with different child digests, orphaned registration after parent commitment, and an explicit two-store state-fork counterexample. These are same-team implementation and regression results, not independent implementation or production deployment evidence.¶

The prototype wire format predates this document and does not yet implement all mandatory fields in this version, including full issuance authorization digest binding, an explicit action-scope profile, and explicit budget unit scale, digest-linked parent lineage, authenticated parent-delegation binding, not_before, and complete ingest-time cycle validation. It is therefore implementation experience, not a claim of conformance. There is no independent implementation, interoperability event, production transaction history, post-quantum profile, or zero-knowledge implementation.¶

The reference implementation does reject repeated delegation identifiers, repeated parent capability identifiers, a leaf named as its own parent, and increasing amounts within the delegation chain presented at mint or verification time. Those checks provide local simple-path and monotonic amount enforcement. They do not discover omitted parents or establish the digest-linked, graph-wide lineage required by Section 10, so they do not close the remaining conformance gap.¶

The public repository also contains a CI-gated TLA+ model of capability registration, reservation, commitment, revocation, delegation, replay refusal, parent-funded child registration, and aggregate sibling conservation. The checked configuration contains one root, three possible children, three delegation operation identifiers, bounded amounts, bounded depth, and a bounded clock. Exact state and obligation counts are emitted by the governed proof-status artifact. This is bounded evidence about the model, not a refinement proof of the TypeScript, SQL, transaction adapter, cryptography, lineage verification, or complete protocol defined here. A separate repository witness module contains five reachability checks but is not executed by the CI workflow and is not represented as proving every assertion non-vacuous. The model obtains delegation acyclicity through its construction rules; arbitrary implementation inputs still require the explicit runtime traversal and refusal rules in Section 10.¶

The main branch also runs a fixed-seed adversarial harness in per-push CI over the actual JavaScript in-memory capability and consumption stores. It includes a true-concurrency Promise.all race target and a deliberately non-atomic comparison store that demonstrates the race detector can expose over-commitment. Additional targets exercise accounting and ownership invariants at whole-method boundaries. This is regression evidence for those in-process stores, not complete protocol conformance: it does not fuzz the PostgreSQL capability transaction path, the atomic handshake RPC, replica or connection failures, and no deeper nightly sweep is scheduled.¶

18. Changes Since -00

• Extended the mandatory atomic state domain from participants authorizing one identifier to the full authority-bearing capability lineage.¶
• Made parent reservation and exactly-once commitment before child registration explicitly normative.¶
• Stated aggregate direct-sibling conservation and injective operation-to-child binding in one place.¶
• Defined the concurrent N-sibling, replay-with-different-child, orphaned-registration, and cross-domain state-fork cases.¶
• Added precise relationships to AIP, PEDIGREE, CB4A, Condition-Bounded Credentials, and affine token-budget work without changing the receipt wire format or failure-code vocabulary.¶

19. IANA Considerations

This document has no IANA actions. A future revision may request a media type and registries for receipt versions, scope profiles, holder methods, and failure codes after implementation experience stabilizes the protocol.¶

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