{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-lose-reversible-context-across-a-lossy-compaction-checkpoint-when-the-summarizer-s-token-budget-is-exceeded",
  "title": "Why does an instance lose reversible context across a lossy compaction checkpoint when the summarizer's token budget is exceeded",
  "date_of_record": "2026 November 13",
  "scope": "Applies to conversational or agentic systems that auto-compact a growing context window by replacing a prefix of turns with a lossy, budget-bounded summary while retaining a reversible reconstruction path (e.g., a raw event log keyed by turn ID). Does not apply to lossless truncation, exact deduplication, or compaction schemes where the summary is generated without a hard token budget.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-lose-reversible-context-across-a-lossy-compaction-checkpoint-when-the-summarizer-s-token-budget-is-exceeded.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-lose-reversible-context-across-a-lossy-compaction-checkpoint-when-the-summarizer-s-token-budget-is-exceeded.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-lose-reversible-context-across-a-lossy-compaction-checkpoint-when-the-summarizer-s-token-budget-is-exceeded.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1543,
    "python_slice_tokens": 491,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "A lossy summarization checkpoint is *reversible* only if the summary plus a durable pointer set can reconstruct a semantically equivalent working context on demand. Reversibility is a property of the *pointer set*, not of the summary text. When the summarizer is given a fixed token budget and the input prefix exceeds that budget, the summarizer silently drop...",
    "what_was_observed": "On an agent with a 200k-token window and an auto-compactor triggered at 70% utilization: 1. The compactor received turns `[0..N]` and a summary budget of 4096 tokens. At turn 380 the prefix exceeded what 4096 tokens could cover, so the summarizer truncated its own output mid-sentence and omitted the last ~40 turns of the prefix. 2. The pointer set was still written as `{covered: [0..N], summary_id: S}` because the po...",
    "condition_of_validity": "The invariant \"pointer set ⊆ summary coverage\" holds only when the summarizer either (a) is given an unbounded budget, (b) reports which input spans it actually consumed, or (c) is verified post-hoc by checking that every pointer's anchor token appears in the summary. Absent one of these, the invariant is assumed, not enforced, and the boundary is crossed si...",
    "decision_boundary": "Before treating a compaction checkpoint as reversible: - If `prefix_tokens ≤ summary_budget × R_low` (use `R_low = 5` as a conservative floor), the checkpoint may be treated as reversible and the pointer set trusted. - If `prefix_tokens > summary_budget × R_low`, the checkpoint is *conditionally* reversible: the pointer set must be narrowed to the spans the ...",
    "operational_effect": "Treating the pointer set as a claim about *input* rather than *output* converts a lossy-but-reversible design into a lossy-and-irreversible one, and it does so without error, log line, or metric. Downstream reconstruction then produces confident false negatives: the agent believes an omitted fact was never present. The corrective is to make the pointer set a..."
  },
  "matched_failure_symptoms": [
    "CONTEXT_DRIFT_OR_ATTENTION_DILUTION"
  ],
  "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": "989a09a6fba0835882263b43d6e43f197b0e0e1e904a344da20ab6f5eb95a090",
    "verified_classes": [
      "CompactionReversibilityError",
      "Turn",
      "Checkpoint"
    ],
    "verified_functions": [
      "build_checkpoint",
      "reconstruct"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "CompactionReversibilityError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "re",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}