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Merge pull request #499 from koic/stamp_cache_hints_on_modern_results

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Stamp the required cache hints on modern cacheable results per SEP-2549
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Koichi ITO 2026-08-13 23:14:17 +09:00 • committed by GitHub
commit b09708365b
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1 changed files with 87 additions and 8 deletions
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31
lib/mcp/server.rb
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@ -145,6 +145,18 @@ module MCP
# Allowed values for the SEP-2549 `cacheScope` cache hint.
CACHE_SCOPES = ["public", "private"].freeze
# Methods whose results are cacheable per SEP-2549.
# On the modern wire (2026-07-28) the `ttlMs`/`cacheScope` hints are REQUIRED on these results,
# so unset hints get the spec defaults there; on stable protocol versions emission stays opt-in
# via `apply_cache_metadata`.
CACHEABLE_RESULT_METHODS = [
Methods::TOOLS_LIST,
Methods::PROMPTS_LIST,
Methods::RESOURCES_LIST,
Methods::RESOURCES_TEMPLATES_LIST,
Methods::RESOURCES_READ,
].freeze
attr_accessor :description, :icons, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resource_templates, :server_context, :configuration, :capabilities, :transport, :logging_message_notification
attr_reader :resources, :page_size, :client_capabilities, :ttl_ms, :cache_scope, :request_state_security
@ -656,6 +668,18 @@ module MCP
result = result.merge(resultType: ResultType::COMPLETE)
end
# SEP-2549 makes the `ttlMs`/`cacheScope` hints REQUIRED on cacheable results at 2026-07-28,
# so unconfigured servers get `ttlMs: 0` (do not cache) and `cacheScope: "private"`:
# the spec names no default scope, and `"private"` is the side that cannot leak
# a user-dependent result through a shared cache, matching the TypeScript SDK's default
# and `server/discover`. Values already in the result win.
# Only complete results are cacheable: an SEP-2322 `input_required` round trip must not be stamped
# (the stamp above guarantees `resultType` is present on every modern Hash result by this point).
if envelope && result.is_a?(Hash) && CACHEABLE_RESULT_METHODS.include?(method) &&
result[:resultType] == ResultType::COMPLETE && !(result.key?(:ttlMs) && result.key?(:cacheScope))
result = { ttlMs: @ttl_ms || 0, cacheScope: @cache_scope || "private" }.merge(result)
end
result
rescue CancelledError => e
add_instrumentation_data(cancelled: true, cancellation_reason: e.reason)
@ -1182,15 +1206,16 @@ module MCP
end
# Adds the SEP-2549 cache hints (`ttlMs`, `cacheScope`) to a result. Emission is opt-in: nothing is added
# unless the server was configured with `ttl_ms`/`cache_scope` or the result already carries one of the fields, in
# which case the missing one is filled with the spec defaults (`ttlMs: 0` = do not cache, `cacheScope: "public"`).
# unless the server was configured with `ttl_ms`/`cache_scope` or the result already carries one of the fields,
# in which case the missing one is filled with `ttlMs: 0` (do not cache) or `cacheScope: "private"`,
# the side that cannot leak a user-dependent result through a shared cache (the TypeScript SDK's default).
# Values already in the result win, enabling per-result overrides.
# https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2549
def apply_cache_metadata(result)
explicit = result.key?(:ttlMs) || result.key?(:cacheScope)
return result if @ttl_ms.nil? && @cache_scope.nil? && !explicit
{ ttlMs: @ttl_ms || 0, cacheScope: @cache_scope || "public" }.merge(result)
{ ttlMs: @ttl_ms || 0, cacheScope: @cache_scope || "private" }.merge(result)
end
def complete(params, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)