{
  "schema_version": "1.3.0",
  "slug": "when-must-an-mcp-server-reject-a-tool-call-that-its-alias-table-would-otherwise-resolve",
  "title": "When must an MCP server reject a tool call that its alias table would otherwise resolve?",
  "date_of_record": "2026 November 5",
  "scope": "Applies to Model Context Protocol servers that expose tools through a versioned schema registry with backward-compatible aliases (deprecated tool names mapped to current handlers). Does not apply to stateless single-version servers or to alias-only shims with no schema validation.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-reject-a-tool-call-that-its-alias-table-would-otherwise-resolve.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-reject-a-tool-call-that-its-alias-table-would-otherwise-resolve.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-reject-a-tool-call-that-its-alias-table-would-otherwise-resolve.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1323,
    "python_slice_tokens": 257,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "There are two distinct failure surfaces in MCP capability negotiation: *capability advertisement* (what the server claims it can do during `initialize`) and *call-time schema conformance* (whether a specific `tools/call` payload matches the schema version the client believes it is speaking). Backward-compatible aliasing collapses these two surfaces into one ...",
    "what_was_observed": "A server registered `search_v1` (schema v1, argument `query: str`) and later `search` (schema v2, argument `query: str`, `filters: dict` required, `limit: int` defaulting to 20). The alias table mapped `search_v1 → search`. A client that had negotiated protocol capability `tools.schema_version = \"1\"` issued `tools/call` with `name: \"search_v1\"` and `arguments: {\"query\": \"foo\"}`. The alias resolved. The v2 handler exe...",
    "condition_of_validity": "Alias resolution is only safe when **all** of the following hold: 1. The negotiated schema version for the session is `>=` the schema version that introduced the current handler's argument contract, **or** the handler performs explicit down-conversion of arguments to the negotiated version before validation. 2. Every field that is required in the current sch...",
    "decision_boundary": "The dispatcher must reject — not resolve — when an alias matches but the negotiated schema version is incompatible with the target handler's contract and no down-converter is registered. The rejection must be a protocol-level `tools/call` error with a structured `code: \"schema_version_mismatch\"`, not a silent success and not a generic `-32602`. Do **not** re...",
    "operational_effect": "After introducing an explicit schema gate ahead of alias resolution: - Deprecated-name calls from v1 clients either succeeded with down-converted arguments (registered converters) or failed fast with `schema_version_mismatch` (unregistered), eliminating the silent-default class of leaks. - The eleven-day detection latency dropped to per-call rejection; the t..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "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": "3f9b316aa093c363804114669d9036ea048c3b2b9fa37e9eac99e2a582478509",
    "verified_classes": [
      "AliasSchemaMismatch",
      "AliasEntry"
    ],
    "verified_functions": [
      "dispatch"
    ],
    "verified_functions_count": 1,
    "defensive_guards_count": 2,
    "raised_exceptions": [
      "AliasSchemaMismatch"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}