{
  "schema_version": "1.3.0",
  "slug": "what-structural-signals-indicate-per-session-isolation-failure-in-http-native-mcp-server-multiplexing",
  "title": "What structural signals indicate per-session isolation failure in HTTP-native MCP server multiplexing",
  "date_of_record": "2026 November 9",
  "scope": "Applies to HTTP-native Model Context Protocol servers that multiplex multiple client sessions over a single listening socket (streamable HTTP transport, SSE-over-HTTP, or any request-scoped session binding keyed by header or path). Does not apply to stdio-transport MCP servers, where isolation is guaranteed by process boundaries, nor to deployments where each session is pinned to a dedicated backend process by an external proxy.",
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  "remnant_summary": {
    "core_distinction": "The distinction that matters is between *transport-level multiplexing* and *state-level multiplexing*. An HTTP-native MCP server may correctly multiplex requests at the TCP/HTTP layer—each request routed to the right handler, each response written to the right socket—while still failing to isolate *session state*: negotiated capabilities, initialization hand...",
    "what_was_observed": "In a streamable-HTTP MCP deployment fronting N client agents against one server process, the following structural signals appeared in sequence over roughly 40 minutes of operation: 1. **Session ID reuse under header absence.** Clients that omitted `Mcp-Session-Id` on a follow-up request (a common behavior in agents that lose state across turns) were silently bound to the most recently created session. The server logg...",
    "condition_of_validity": "These signals are valid indicators of isolation failure only under the following conditions, and the diagnosis should be rejected if any fails: - The server is genuinely multiplexing sessions in one process (or one shared state store). If sessions are pinned to separate processes, header-absence fallback cannot leak state across sessions and the signals are ...",
    "decision_boundary": "The decision boundary is between *repairing the key* and *repairing the lifetime*. If the observed signals are confined to signals 1 and 2 (session resolution and capability drift), the defect is a keying defect: the session registry accepts a weak or absent key and falls back to a global \"current\" session. The correct fix is to reject requests lacking a val...",
    "operational_effect": "The operational consequence is that isolation failures in HTTP-native MCP multiplexing are *latency-tolerant and load-dependent*: they do not appear under single-client testing, they appear only when concurrent sessions overlap, and they manifest as semantic errors (wrong tools offered, wrong capabilities negotiated, spurious cancellations) rather than as tr..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
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  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "f01877c405be6ccf6e250a35eb4a415e4f0afdb0656bcfea7ca40dbcab3f7b5e",
    "verified_classes": [
      "SessionIsolationViolation",
      "SessionRegistry"
    ],
    "verified_functions": [
      "__init__",
      "resolve",
      "register_progress",
      "cancel"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 5,
    "raised_exceptions": [
      "SessionIsolationViolation"
    ],
    "sample_fail_closed_invariants": [
      "request arrived without Mcp-Session-Id; refusing global fallback"
    ],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
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}