{
  "schema_version": "1.3.0",
  "slug": "when-must-an-mcp-server-refuse-to-resume-a-streamable-http-session-after-a-transport-level-reconnect",
  "title": "When must an MCP server refuse to resume a streamable HTTP session after a transport-level reconnect?",
  "date_of_record": "2026 November 6",
  "scope": "Applies to MCP servers exposing the streamable HTTP transport with server-push (SSE-style) event streams and client-driven session resumption via last-event-id / cursor replay. Does not apply to stdio transports, nor to stateless HTTP request/response MCP endpoints that do not emit server-initiated events.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-refuse-to-resume-a-streamable-http-session-after-a-transport-level-reconnect.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-refuse-to-resume-a-streamable-http-session-after-a-transport-level-reconnect.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-mcp-server-refuse-to-resume-a-streamable-http-session-after-a-transport-level-reconnect.warrant.json"
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  "remnant_summary": {
    "core_distinction": "A resumable session is not the same thing as a *replayable* event log. The streamable HTTP transport allows a client to reconnect and present a resume cursor (last-event-id) so the server can re-emit events the client may have missed. The non-obvious invariant is this: **the resume cursor is only meaningful if the server can prove that every event with an id...",
    "what_was_observed": "During a transport reconnect under load, a client presented a cursor pointing at event `E_1042`. The server's per-session ring buffer had evicted `E_1043` through `E_1051` under memory pressure but retained `E_1052` onward. The server resumed by emitting `E_1052..E_head`, treating the eviction as \"the client probably saw them.\" The client, honoring its own at-least-once contract, had *not* seen `E_1043..E_1051`; thos...",
    "condition_of_validity": "Resumption is valid if and only if the retained log for the session forms a contiguous, monotonically increasing sequence from the cursor's successor through the current head, with no evictions inside that interval and no id reuse. Concretely, the server must track, per session, the *lowest retained event id* (`floor_id`) alongside the head. A resume request...",
    "decision_boundary": "The boundary is the single comparison `cursor < floor_id - 1`. On the admissible side, replay the retained window in order and resume live push. On the inadmissible side, the server must fail the resume explicitly — emit a terminal protocol error indicating `resume_unavailable` with the earliest retained id so the client can decide whether to re-initialize t...",
    "operational_effect": "Refusing resume forces session re-establishment, which is cheap and observable; silently resuming across a gap produces long-lived, hard-to-attribute divergence in request lifecycle state (dangling cancellations, phantom progress, orphaned tool calls). Instrument `floor_id`, head, and cursor at every resume attempt; alert when `resume_unavailable` fires, bec..."
  },
  "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"
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  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "dc06978b9b5a4226d5d6faab2d571cec41b752a2692e404af87be1223053e461",
    "verified_classes": [
      "ResumeGapError",
      "SessionEventLog"
    ],
    "verified_functions": [
      "__init__",
      "__init__",
      "floor_id",
      "head_id",
      "append",
      "resume_from"
    ],
    "verified_functions_count": 6,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "ResumeGapError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}