{
  "schema_version": "1.3.0",
  "slug": "where-must-an-instance-reconcile-the-sse-event-id-cursor-before-resuming-an-mcp-streamable-http-session",
  "title": "Where must an instance Reconcile the SSE Event-ID Cursor Before Resuming an MCP Streamable-HTTP Session",
  "date_of_record": "2026 November 12",
  "scope": "Applies to MCP servers and clients using the Streamable HTTP transport (JSON-RPC over POST with optional Server-Sent Events upgrade and `Last-Event-ID` resumption) for remote tool invocation. Does not apply to stdio transports, nor to plain request/response HTTP where no event cursor is negotiated.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/where-must-an-instance-reconcile-the-sse-event-id-cursor-before-resuming-an-mcp-streamable-http-session.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-reconcile-the-sse-event-id-cursor-before-resuming-an-mcp-streamable-http-session.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/where-must-an-instance-reconcile-the-sse-event-id-cursor-before-resuming-an-mcp-streamable-http-session.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1625,
    "python_slice_tokens": 267,
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  },
  "remnant_summary": {
    "core_distinction": "There are two distinct notions of \"session continuity\" in Streamable HTTP, and they diverge precisely at the resumption boundary: 1. **Session continuity** — the `Mcp-Session-Id` header identifies a logical conversation. It is minted at `initialize`, echoed by the client on every subsequent request, and is the server's key for per-session state (tool registr...",
    "what_was_observed": "During a long-running remote tool invocation (a streaming `tools/call` emitting progress notifications), the network path was interrupted mid-stream. On reconnect the client sent: - `Mcp-Session-Id: <S>` (still valid on the server) - `Last-Event-ID: 41` (the last event it had durably processed) The server had, in the interim, garbage-collected the per-stream event buffer for session `<S>` under a TTL policy but retai...",
    "condition_of_validity": "Resumption via `Last-Event-ID` is valid if and only if all of the following hold simultaneously: 1. The `Mcp-Session-Id` resolves to a live session object on the server. 2. The event buffer associated with the *current stream generation* for that session still contains events with IDs strictly greater than `Last-Event-ID`. 3. The event-ID namespace is monoto...",
    "decision_boundary": "The client must not blindly send `Last-Event-ID` on reconnect. The decision procedure at the resumption boundary is: - **If the session is known-dead (server returned `404` on a prior probe, or session TTL is known to be shorter than the disconnect window):** do not resume. Re-`initialize` and re-issue the tool call idempotently (requires the tool to be decl...",
    "operational_effect": "The operational consequence is that **resumable MCP streaming is a two-party invariant, and the transport cannot enforce it alone.** Servers that reset event numbering on reconnect convert a recoverable network blip into silent data loss at the tool-result layer — the worst class of failure, because it is invisible to both the JSON-RPC error channel and the ..."
  },
  "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": "d427e819d402d450c8b9eb8b94aa1f4b3105b333439bf7a8ae55735e96655c94",
    "verified_classes": [
      "ResumptionCursorError",
      "ResumeState"
    ],
    "verified_functions": [
      "validate_resumption"
    ],
    "verified_functions_count": 1,
    "defensive_guards_count": 3,
    "raised_exceptions": [
      "ResumptionCursorError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}