{
  "schema_version": "1.3.0",
  "slug": "when-must-an-instance-reject-a-negotiated-mcp-capability-it-previously-advertised-schema-version-decoherence-under-mid-session-tool-renegotiation",
  "title": "When must an instance reject a negotiated MCP capability it previously advertised: schema-version decoherence under mid-session tool renegotiation",
  "date_of_record": "2026 November 11",
  "scope": "Applies to MCP servers that advertise versioned tool schemas and to clients that cache `tools/list` output across a session. Does not apply to servers that pin a single immutable schema for the process lifetime, nor to stateless one-shot request/response bridges that re-list tools on every call.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-negotiated-mcp-capability-it-previously-advertised-schema-version-decoherence-under-mid-session-tool-renegotiation.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-negotiated-mcp-capability-it-previously-advertised-schema-version-decoherence-under-mid-session-tool-renegotiation.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-reject-a-negotiated-mcp-capability-it-previously-advertised-schema-version-decoherence-under-mid-session-tool-renegotiation.warrant.json"
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  "remnant_summary": {
    "core_distinction": "Capability negotiation in MCP is a *declaration* event, not a *lease*. When a server responds to `initialize` with `capabilities.tools` and later serves `tools/list`, the client treats the returned schema set as the contract for the session. The non-obvious boundary is this: **a server may legitimately evolve a tool's `inputSchema` between `tools/list` calls...",
    "what_was_observed": "A tool `db.query` was advertised at session start with `inputSchema` v3, where `timeout_ms` was an integer with default `30000` and `mode` accepted `{\"read\",\"write\"}`. Mid-session the operator deployed v4, which renamed `timeout_ms` to `timeout` (integer, no default) and restricted `mode` to `{\"read\"}` for a read-replica cutover. The server process did not restart; it hot-reloaded its tool registry and began serving ...",
    "condition_of_validity": "The invariant holds — and the rejection is mandatory — only under the conjunction of three conditions: 1. **The server retains knowledge of retired schema versions.** If the server has no record of what it previously advertised, it cannot distinguish a stale client from a malformed one, and must fail closed on unknown properties rather than coerce. 2. **The ...",
    "decision_boundary": "The boundary is drawn at the moment of dispatch, not at the moment of listing. Concretely: - **Accept** when the call's declared schema version (from `_meta.schemaVersion`) equals the currently served version, or when the call omits a version and validates cleanly under the current schema with `additionalProperties: false`. - **Reject with re-list instructio...",
    "operational_effect": "Enforcing version-gated rejection converts a silent decoherence into a loud, recoverable, single-retry handshake. The client re-lists, receives v4, re-validates its arguments, and either succeeds or surfaces a genuine schema incompatibility to the user. The cost is one extra round-trip on schema change; the benefit is that no call ever executes under semanti..."
  },
  "matched_failure_symptoms": [
    "SCHEMA_DRIFT_OR_COERCION_CRASH"
  ],
  "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": "9d4e15f101d2c9fb82b835e2ee0e62454784348f889cdce2791096b3bec5e19d",
    "verified_classes": [
      "SchemaVersionRetired",
      "SchemaVersionUnknown",
      "StrictValidationError",
      "ToolRegistry"
    ],
    "verified_functions": [
      "handle_db_query",
      "__init__",
      "__init__",
      "__init__",
      "dispatch",
      "retire"
    ],
    "verified_functions_count": 6,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "SchemaVersionRetired",
      "SchemaVersionUnknown",
      "StrictValidationError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
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    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}