{
  "schema_version": "1.3.0",
  "slug": "under-what-conditions-must-an-instance-reject-mcp-session-resumption-across-an-sse-to-streamable-http-transport-migration",
  "title": "Under what conditions must an instance reject MCP session resumption across an SSE-to-streamable-HTTP transport migration",
  "date_of_record": "2026 November 7",
  "scope": "Applies to MCP servers that expose both the legacy `text/event-stream` transport (HTTP+SSE with a separate POST endpoint) and the streamable-HTTP transport, and that honor `Mcp-Session-Id` headers for resumption. Does not apply to pure stdio transports or to servers that never persisted session state across transport changes.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-mcp-session-resumption-across-an-sse-to-streamable-http-transport-migration.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-mcp-session-resumption-across-an-sse-to-streamable-http-transport-migration.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/under-what-conditions-must-an-instance-reject-mcp-session-resumption-across-an-sse-to-streamable-http-transport-migration.warrant.json"
  },
  "token_profile": {
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  "remnant_summary": {
    "core_distinction": "Session *identity* and session *continuity* are conflated by the `Mcp-Session-Id` header, but they are not the same invariant. Identity is the opaque token the client presents; continuity is the guarantee that the server-side state bound to that token — initialized capabilities, subscription set, in-flight request correlation, and the event-ID cursor — survi...",
    "what_was_observed": "A server was deployed with dual transport support and a shared session store keyed by `Mcp-Session-Id`. A client initialized over SSE, received a session ID, and then — on a network change — reconnected using streamable HTTP with the same `Mcp-Session-Id` and a `Last-Event-ID` drawn from the SSE stream. The server's session lookup succeeded. The streamable-HTTP handler treated the `Last-Event-ID` as a streamable-HTTP...",
    "condition_of_validity": "The rejection rule is necessary precisely when all of the following hold: 1. The session was *established* under one transport (the transport that issued the `Mcp-Session-Id`). 2. The resumption request arrives under a *different* transport. 3. The session store does not carry a transport-tagged event cursor namespace — i.e., the `Last-Event-ID` presented ca...",
    "decision_boundary": "Do not attempt to \"repair\" a cross-transport resume by silently translating the cursor. Translation is undefined: SSE event IDs and streamable-HTTP event IDs are independently assigned and carry no common ordering. The defensible boundary is: a `Mcp-Session-Id` issued under transport T may be resumed only under transport T, unless the server has explicitly r...",
    "operational_effect": "Enforcing the boundary converts a silent semantic corruption into a loud, recoverable failure: the client is forced to re-`initialize` under the new transport, re-negotiate capabilities, and re-subscribe. The cost is a visible reconnect; the benefit is that no request is ever served under a capability set the client did not agree to. Servers that skip this c..."
  },
  "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": "8b118a88e61424212f86ac9e568a27c950d9bd7191a853819333dc006cd10c8f",
    "verified_classes": [
      "CrossTransportResumeError",
      "SessionRecord"
    ],
    "verified_functions": [
      "resume_session",
      "__init__"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "CrossTransportResumeError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}