{
  "schema_version": "1.3.0",
  "slug": "when-must-an-instance-distinguish-sse-backpressure-from-session-resumption-in-mcp-http-native-transport",
  "title": "When must an instance distinguish SSE backpressure from session resumption in MCP HTTP-native transport",
  "date_of_record": "2026 November 10",
  "scope": "Applies to MCP servers exposing the streamable HTTP transport (single POST endpoint plus optional GET SSE stream) where a client may reconnect with `Last-Event-ID` to resume an interrupted event stream. Does not apply to legacy HTTP+SSE dual-endpoint transports or to stdio transports, where stream identity is tied to the process rather than to a resumable event log.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-instance-distinguish-sse-backpressure-from-session-resumption-in-mcp-http-native-transport.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-distinguish-sse-backpressure-from-session-resumption-in-mcp-http-native-transport.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-instance-distinguish-sse-backpressure-from-session-resumption-in-mcp-http-native-transport.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1656,
    "python_slice_tokens": 257,
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  },
  "remnant_summary": {
    "core_distinction": "In the streamable HTTP transport, two superficially similar signals arrive at the server: a client that stops reading its SSE stream (backpressure), and a client that disconnects and reconnects with `Last-Event-ID` (resumption). Both manifest as an idle or closed connection. They are not the same event and must not be conflated. Backpressure is a *flow-contr...",
    "what_was_observed": "A server implementation assigned event IDs at enqueue time into a bounded ring buffer (capacity 1024) and wrote to the socket with a non-blocking send. Under a slow consumer, the ring buffer filled; the server applied a policy of dropping the *oldest* event to make room. The client eventually disconnected (network change) and reconnected with `Last-Event-ID` pointing at an event that had already been evicted. The ser...",
    "condition_of_validity": "This analysis holds when all of the following are true: 1. The server maintains a per-session monotonic event sequence with IDs that are opaque but orderable. 2. The transport is streamable HTTP with at least one long-lived SSE stream per session. 3. The server has a bounded retention window for replayable events (unbounded retention makes eviction impossibl...",
    "decision_boundary": "The boundary is a single predicate evaluated on every enqueue and every resume: - On enqueue when the retention buffer is full: **do not evict an event that is still within the resumption window.** Instead, block or reject the producer (apply upstream backpressure), or if the producer cannot block, terminate the session with an explicit, resumable error rath...",
    "operational_effect": "When the boundary is respected, a slow consumer causes the producer to stall (or the session to fail explicitly and resumably), and a reconnecting client receives exactly the events it missed with no duplicates and no silent gaps. When it is violated, the system exhibits *silent message loss under load* — the worst failure class, because it only manifests wh..."
  },
  "matched_failure_symptoms": [
    "HTTP_429_BURST_STARVATION"
  ],
  "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": "d6c6ab8b17b7c14753b244d94c2fe951c09027017263c62ff205847f0e452e1d",
    "verified_classes": [
      "ResumeWindowViolation",
      "EventLog"
    ],
    "verified_functions": [
      "handle_resume",
      "__init__",
      "enqueue",
      "replay_from"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 4,
    "raised_exceptions": [
      "ResumeWindowViolation"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}