{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-forget-a-memory-it-never-wrote-the-tombstone-visibility-invariant-in-read-write-forget-memory-contracts",
  "title": "Why does an instance forget a memory it never wrote: the tombstone-visibility invariant in read/write/forget memory contracts",
  "date_of_record": "2026 November 9",
  "scope": "Applies to memory tools that expose `read`, `write`, and `forget` operations with a stated retention window and a redaction guarantee (i.e., a forgotten key must be unreadable to all callers after the forget returns). Does not apply to append-only logs without deletion semantics, nor to stores where `forget` is explicitly documented as best-effort/eventual.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-forget-a-memory-it-never-wrote-the-tombstone-visibility-invariant-in-read-write-forget-memory-contracts.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-forget-a-memory-it-never-wrote-the-tombstone-visibility-invariant-in-read-write-forget-memory-contracts.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-forget-a-memory-it-never-wrote-the-tombstone-visibility-invariant-in-read-write-forget-memory-contracts.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1516,
    "python_slice_tokens": 263,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The distinction that matters is between **content deletion** and **tombstone visibility**. A memory tool can correctly delete the payload of a forgotten key — so that no subsequent `read` returns it — while still failing the redaction guarantee, because the *existence* of the key (its tombstone, its version counter, its index entry, or its \"last modified\" ti...",
    "what_was_observed": "A memory tool was contracted as: `write(key, value, ttl)`, `read(key)`, `forget(key)`, with a 30-day retention window and the guarantee that \"after `forget` returns, the key and its value are unrecoverable.\" A test harness wrote `k = \"session-42\"`, read it back, called `forget(k)`, and confirmed `read(k)` raised `NotFound`. The contract appeared satisfied. The failure surfaced under a different probe. After `forget(k...",
    "condition_of_validity": "The redaction guarantee holds **only if** the forget operation removes the key from every observable namespace simultaneously: the value store, the key-name index, the version/sequence allocator, and the error taxonomy. Formally, after `forget(k)` returns, for every observable operation `op`, `op(k)` must be indistinguishable from `op(k')` where `k'` is a ke...",
    "decision_boundary": "The operational decision is whether to treat a memory tool's redaction guarantee as satisfied on the basis of a `read`-after-`forget` probe alone. It must not be. The decision boundary is: - **If** the tool's forget path does not also purge the key from enumeration, version allocation, and the error taxonomy — **then** the guarantee is content-only and must ...",
    "operational_effect": "The practical consequence is that a memory tool can pass its own redaction test suite and still leak the most sensitive artifact it holds — the key name — indefinitely. Agents that store user identifiers as keys (`user:alice@example.com`) and rely on `forget` for erasure will leave a permanent, enumerable tombstone index of every user they ever saw. The corr..."
  },
  "matched_failure_symptoms": [
    "OOM_KILL_OR_MEMORY_EXHAUSTION"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "3ab561c729b3c2ced9e1f181cb95b7728cff69610189d4a7be963485d2840fa3",
    "verified_classes": [
      "RedactionGuaranteeViolation"
    ],
    "verified_functions": [
      "verify_forget_indistinguishability"
    ],
    "verified_functions_count": 1,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "RedactionGuaranteeViolation"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}