What data retention decay policies must be cryptographically sealed for vector indices?

**Date of Record:** 2026 October 26 **Scope:** Applies when an agentic session manages a vector index that accumulates embeddings over time and must enforce intentional forgetting (data aging, decay, or retention limits) to comply with privacy regulations or to prevent stale embeddings from degrading search quality. Does not apply to append-only audit logs or immutable archives.

Core Distinction

The separation between **explicit deletion** (manually removing a specific vector upon request) and **automated retention decay** (systematically reducing the weight, freshness, or existence of vectors as they age). Relying solely on explicit deletion creates the **Vector Hoarding Hazard**: the index accumulates millions of stale embeddings that were never explicitly deleted, poisoning search results with obsolete or expired data, and violating retention policies that mandate automatic purging after a defined period.

What Was Observed

In RAG and vector database architectures, failing to enforce retention decay produces three primary failure modes:

  1. **Stale Knowledge Contamination:** The index contains embeddings from documents that are three years old. A query retrieves them because they score high on similarity, but they contain outdated facts that contradict the current state.
  2. **Regulatory Non-Compliance:** GDPR or HIPAA mandates purging data after a defined retention period. The instance has no automated decay mechanism, and stale user embeddings linger indefinitely.
  3. **Index Bloat:** The index grows unboundedly because nothing ages out. Search latency degrades linearly, and infrastructure costs spiral.

Recorded Elements

  1. **`retention_policy_hash`:** The cryptographic hash of the retention policy document (e.g., "vectors older than 90 days are decayed by 50%, older than 180 days are purged"), proving the policy was defined and immutable.
  2. **`decay_schedule_receipt`:** The cryptographic proof that the decay schedule was applied at each interval, including the count of vectors purged and the count retained.
  3. **`vector_freshness_epoch`:** The exact timestamp attached to each vector at ingestion, used to determine its age and apply the appropriate decay tier.
  4. **`compliance_audit_signature`:** The cryptographic signature of an external auditor confirming the retention policy was correctly executed across the specified time window.

Condition of Validity

* **Applies when:** The vector index contains embeddings derived from data subject to retention limits (PII, regulated data, or stale knowledge), and search quality degrades with outdated vectors. * **Does not apply when:** The vector index is strictly for archival or historical research, where old data is intentionally preserved, or when every vector is explicitly tagged with an indefinite retention period.

Decision Boundary

  1. **Does a `retention_policy_hash` exist for the index?** If no, the index has no decay rule and will hoard vectors indefinitely. The instance must define and seal the policy before continuing.
  2. **Has the `decay_schedule_receipt` been recorded for the last interval?** If no, decay has not executed, and the index may contain stale data. The successor must trigger decay before serving queries.
  3. **Are any vectors older than the maximum retention tier without being purged?** If yes, compliance has been violated. The instance must purge immediately and log the violation.
  4. **Conversely, when can a vector be exempt from decay?** Only when it is explicitly tagged with `INDEFINITE_RETENTION` and the tag is cryptographically signed by a data controller with authority to override the standard policy.

Operational Effect

  1. **Stale Data Prevention:** Automatically reduces the influence of outdated vectors, improving search quality by prioritizing fresh data.
  2. **Regulatory Compliance:** Enforces retention limits automatically, satisfying GDPR, HIPAA, and similar frameworks without manual intervention.
  3. **Cost Control:** Bounds index growth by aging out stale vectors, reducing storage and compute costs.
from dataclasses import dataclass
from enum import Enum, auto
import hashlib
import json

class DecayVerdict(Enum):
    POLICY_COMPLIANT = auto()
    DECAY_OVERDUE = auto()
    STALE_VECTOR_VIOLATION = auto()

@dataclass(frozen=True)
class VectorRetentionState:
    policy_hash: str
    last_decay_epoch: float
    current_epoch: float
    decay_interval_sec: float
    max_age_sec: float
    oldest_vector_age_sec: float

@dataclass(frozen=True)
class DecayPolicy:
    policy_hash: str
    decay_interval_sec: float
    max_age_sec: float

@dataclass(frozen=True)
class DecayManifest:
    verdict: DecayVerdict
    directive: str
    manifest_digest: str

class VectorRetentionGuard:
    @staticmethod
    def _hash(data: str) -> str:
        return hashlib.sha256(data.encode("utf-8")).hexdigest()

    @classmethod
    def evaluate_retention(
        cls, state: VectorRetentionState, policy: DecayPolicy
    ) -> DecayManifest:
        
        # Gate 1: Policy match
        if state.policy_hash != policy.policy_hash:
            return DecayManifest(
                verdict=DecayVerdict.DECAY_OVERDUE,
                directive="HALT: Retention policy hash mismatch. Reconcile policy before decay.",
                manifest_digest=cls._hash("POLICY_MISMATCH")
            )

        # Gate 2: Max age violation
        if state.oldest_vector_age_sec > policy.max_age_sec:
            return DecayManifest(
                verdict=DecayVerdict.STALE_VECTOR_VIOLATION,
                directive=f"HALT: Oldest vector is {state.oldest_vector_age_sec:.0f}s old, exceeding max {policy.max_age_sec:.0f}s. Purge immediately.",
                manifest_digest=cls._hash(f"MAX_AGE:{state.oldest_vector_age_sec:.0f}")
            )

        # Gate 3: Decay interval check
        elapsed = state.current_epoch - state.last_decay_epoch
        if elapsed > policy.decay_interval_sec:
            return DecayManifest(
                verdict=DecayVerdict.DECAY_OVERDUE,
                directive=f"ACTION: Decay interval exceeded ({elapsed:.0f}s > {policy.decay_interval_sec:.0f}s). Execute scheduled decay.",
                manifest_digest=cls._hash(f"OVERDUE:{elapsed:.0f}")
            )

        # Gate 4: Compliant
        digest = cls._hash(json.dumps({
            "policy": state.policy_hash,
            "elapsed": f"{elapsed:.0f}",
            "oldest": f"{state.oldest_vector_age_sec:.0f}"
        }, sort_keys=True))
        
        return DecayManifest(
            verdict=DecayVerdict.POLICY_COMPLIANT,
            directive="PROCEED: Vector retention within policy bounds.",
            manifest_digest=digest
        )