{
  "schema_version": "1.3.0",
  "slug": "why-does-an-instance-negotiate-mcp-capabilities-successfully-yet-fail-on-its-first-tool-call-after-transport-downgrade",
  "title": "Why does an instance negotiate MCP capabilities successfully yet fail on its first tool call after transport downgrade",
  "date_of_record": "2026 November 8",
  "scope": "Applies to MCP client/server pairs where the client advertises a preferred transport (e.g., Streamable HTTP) and the server responds with a negotiated fallback (e.g., SSE or stdio bridge). Does not apply to homogeneous deployments where both peers are pinned to the same transport version and capability set.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/why-does-an-instance-negotiate-mcp-capabilities-successfully-yet-fail-on-its-first-tool-call-after-transport-downgrade.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-negotiate-mcp-capabilities-successfully-yet-fail-on-its-first-tool-call-after-transport-downgrade.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/why-does-an-instance-negotiate-mcp-capabilities-successfully-yet-fail-on-its-first-tool-call-after-transport-downgrade.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1070,
    "python_slice_tokens": 124,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "Capability negotiation and transport negotiation are two separate handshakes that share a single session identifier but are not atomically bound to each other. The initialize exchange confirms *which capabilities* each side supports; the transport fallback confirms *which wire protocol* will carry them. When a server downgrades transport after advertising a ...",
    "what_was_observed": "A client initialized against a server advertising `tools`, `resources`, and `streaming-progress`. The server accepted Streamable HTTP in the `initialize` request, then returned a session that required SSE fallback because the reverse proxy in front of it stripped `Transfer-Encoding: chunked` on the POST path. The client logged `capabilities: {tools, resources, streaming-progress}` and `transport: sse (fallback)`. The...",
    "condition_of_validity": "The distinction holds only when all three of the following are true: (1) the server's capability advertisement is static and not recomputed per transport; (2) at least one advertised capability has transport-specific encoding (progress notifications, resource subscriptions, multi-part streaming results); (3) the client binds its response parser to the capabi...",
    "decision_boundary": "Treat a transport downgrade as a *capability invalidation event*, not a transport-only event. Concretely: if the negotiated transport differs from the transport the client proposed in `initialize`, the client must either (a) re-request capabilities scoped to the selected transport, or (b) refuse any capability whose encoding is transport-dependent until it h...",
    "operational_effect": "The practical consequence is that \"successful initialization\" is a weaker signal than most client implementations treat it as. A client that gates tool invocation on initialization success alone will intermittently hang on the first transport-dependent call after any fallback, and the hang will be indistinguishable from a slow server until the client's own d..."
  },
  "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": "a742a87befcb75652a34e981faa56b001019331cc4088692347a5aa7987ac50b",
    "verified_classes": [
      "TransportCapabilityMismatch"
    ],
    "verified_functions": [
      "admit_capability",
      "guard_tool_call"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "TransportCapabilityMismatch"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}