Module: McpDiff::Core::Rules

Defined in:
lib/mcp_diff/core/rules.rb

Overview

The v1 rule catalog (24 rules), faithful port of packages/core/src/rules/*. NOTE: the TS project uses one-folder-per-rule as the contributor funnel; the Ruby port keeps them in one file for the parity milestone. Splitting into per-rule files is a follow-up (see ruby/README.md).

Constant Summary collapse

ALL =

rubocop:enable Metrics/MethodLength, Metrics/AbcSize

build
CATALOG = 
ALL.map { |r| { "id" => r.id, "class" => r.severity, "description" => r.description } }.freeze

Class Method Summary collapse

Class Method Details

.build ⇒ Object

rubocop:disable Metrics/MethodLength, Metrics/AbcSize



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# File 'lib/mcp_diff/core/rules.rb', line 25

def build
  [
    # ───────────────── breaking ─────────────────
    Rule.new(id: "tool-removed", severity: "breaking",
             description: "A tool that existed in the lockfile is no longer exposed by the server.") do |c|
      next nil unless c[:kind] == "tool-removed"

      finding(c[:name], "tool was removed")
    end,

    Rule.new(id: "tool-param-required-added", severity: "breaking",
             description: "A tool input parameter is now required (newly required existing param, or new required param). Calls that previously validated are now rejected.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "required-added"

        f << finding(c[:name], sc[:existed_before] ? %(param "#{sc[:param]}" is now required) : %(new required param "#{sc[:param]}"))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-param-removed", severity: "breaking",
             description: "A tool input parameter was removed. Callers still sending it may be rejected (additionalProperties) or silently ignored.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        f << finding(c[:name], %(param "#{sc[:param]}" was removed)) if sc[:kind] == "param-removed"
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-param-type-narrowed", severity: "breaking",
             description: "A tool input parameter accepts fewer types or values than before (type set shrank, or an enum constraint appeared). Inputs that used to validate are now rejected.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        case sc[:kind]
        when "type-narrowed"
          f << finding(c[:name], %(param "#{sc[:param]}" type narrowed (#{sc[:before]} → #{sc[:after]})))
        when "enum-constraint-added"
          f << finding(c[:name], %(param "#{sc[:param]}" is now restricted to an enum))
        end
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-enum-value-removed", severity: "breaking",
             description: "An enum value was removed from a tool input parameter. Calls sending the removed value are now rejected.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "enum-value-removed"

        f << finding(c[:name], %(param "#{sc[:param]}": enum value(s) removed: #{SchemaDiff.format_values(sc[:values])}))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-input-schema-incompatible", severity: "breaking",
             description: "Catch-all: the tool's input schema changed in a way mcp-diff cannot prove compatible (nested or unrecognized JSON-Schema constructs). Fails loud rather than silently passing (D9).") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        case sc[:kind]
        when "param-changed-opaque"
          f << finding(c[:name], %(param "#{sc[:param]}" schema changed — cannot prove compatibility))
        when "schema-changed-opaque"
          f << finding(c[:name], "input schema changed — cannot prove compatibility")
        end
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-output-schema-incompatible", severity: "breaking",
             description: "The tool's output schema no longer keeps its promise: a field was removed, made optional, widened, or changed unrecognizably. Outputs may no longer validate against what consumers expect (covariant, D9).") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["outputSchema"], c[:after]["outputSchema"]).each do |sc|
        msg =
          case sc[:kind]
          when "param-removed" then %(output field "#{sc[:param]}" was removed)
          when "required-removed" then %(output field "#{sc[:param]}" is no longer guaranteed)
          when "type-widened" then %(output field "#{sc[:param]}" type widened (#{sc[:before]} → #{sc[:after]}))
          when "enum-value-added" then %(output field "#{sc[:param]}": new enum value(s) old consumers don't expect: #{SchemaDiff.format_values(sc[:values])})
          when "enum-constraint-removed" then %(output field "#{sc[:param]}" is no longer enum-constrained)
          when "param-changed-opaque" then %(output field "#{sc[:param]}" schema changed — cannot prove compatibility)
          when "schema-changed-opaque" then "output schema changed — cannot prove compatibility"
          end
        f << finding(c[:name], msg) if msg
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "capability-removed", severity: "breaking",
             description: "A server capability (tools, resources, prompts, logging, …) is no longer advertised. Clients relying on it will fail.") do |c|
      next nil unless c[:kind] == "capability-removed"

      finding(c[:capability], "capability was removed")
    end,

    Rule.new(id: "resource-removed", severity: "breaking",
             description: "A resource that existed in the lockfile is no longer exposed (matched by uri).") do |c|
      next nil unless c[:kind] == "resource-removed"

      finding(c[:uri], "resource was removed")
    end,

    Rule.new(id: "prompt-removed", severity: "breaking",
             description: "A prompt that existed in the lockfile is no longer exposed (matched by name).") do |c|
      next nil unless c[:kind] == "prompt-removed"

      finding(c[:name], "prompt was removed")
    end,

    # ───────────────── behavior ─────────────────
    Rule.new(id: "tool-description-changed", severity: "behavior",
             description: "A tool's description text changed. For MCP the description is the prompt surface steering the LLM — a change can alter when and how agents call the tool.") do |c|
      next nil unless c[:kind] == "tool-modified"
      next nil if c[:before]["description"] == c[:after]["description"]

      finding(c[:name], "description text changed (LLM prompt surface)")
    end,

    Rule.new(id: "tool-param-description-changed", severity: "behavior",
             description: "A tool input parameter's description changed. Param descriptions are read by the LLM when filling arguments — a change can alter what agents pass.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "description-changed"

        f << finding(c[:name], %(param "#{sc[:param]}" description changed (LLM prompt surface)))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-annotation-changed", severity: "behavior",
             description: "Tool annotations changed (readOnlyHint, destructiveHint, idempotentHint, …). Clients use these to decide confirmation prompts and parallelism — a change alters agent behavior.") do |c|
      next nil unless c[:kind] == "tool-modified"
      next nil if Canonical.deep_equal(c[:before]["annotations"], c[:after]["annotations"])

      before = Rules.keys_of(c[:before]["annotations"])
      after = Rules.keys_of(c[:after]["annotations"])
      changed = (before.keys | after.keys).reject { |k| Canonical.deep_equal(before[k], after[k]) }.sort
      finding(c[:name], "annotations changed: #{changed.join(', ')}")
    end,

    Rule.new(id: "server-instructions-changed", severity: "behavior",
             description: "The server's instructions text changed. Instructions are injected into the client's system context — the LLM prompt surface for the whole server.") do |c|
      next nil unless c[:kind] == "instructions-changed"

      detail = c[:before].nil? ? "added" : (c[:after].nil? ? "removed" : "changed")
      finding("server", "instructions #{detail} (LLM prompt surface)")
    end,

    Rule.new(id: "default-value-changed", severity: "behavior",
             description: "A tool input parameter's default value changed. Calls that omit the param now behave differently.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "default-changed"

        f << finding(c[:name], %(param "#{sc[:param]}" default value changed))
      end
      f.empty? ? nil : f
    end,

    # ───────────────── compat ─────────────────
    Rule.new(id: "tool-added", severity: "compat",
             description: "A new tool is exposed by the server.") do |c|
      next nil unless c[:kind] == "tool-added"

      finding(c[:name], "new tool")
    end,

    Rule.new(id: "tool-param-optional-added", severity: "compat",
             description: "A new optional input parameter was added. Existing calls keep working.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "param-added" && !sc[:required]

        f << finding(c[:name], %(new optional param "#{sc[:param]}"))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-enum-value-added", severity: "compat",
             description: "An enum value was added to a tool input parameter. Existing calls keep working; new inputs are accepted.") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["inputSchema"], c[:after]["inputSchema"]).each do |sc|
        next unless sc[:kind] == "enum-value-added"

        f << finding(c[:name], %(param "#{sc[:param]}": enum value(s) added: #{SchemaDiff.format_values(sc[:values])}))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "tool-output-field-added", severity: "compat",
             description: "The tool's output schema gained a field. Old consumers ignore fields they don't know (covariant, D9).") do |c|
      next nil unless c[:kind] == "tool-modified"

      f = []
      SchemaDiff.diff_object_schema(c[:before]["outputSchema"], c[:after]["outputSchema"]).each do |sc|
        next unless sc[:kind] == "param-added"

        f << finding(c[:name], %(output schema gained field "#{sc[:param]}"))
      end
      f.empty? ? nil : f
    end,

    Rule.new(id: "resource-added", severity: "compat",
             description: "A new resource is exposed by the server.") do |c|
      next nil unless c[:kind] == "resource-added"

      finding(c[:uri], "new resource")
    end,

    Rule.new(id: "prompt-added", severity: "compat",
             description: "A new prompt is exposed by the server.") do |c|
      next nil unless c[:kind] == "prompt-added"

      finding(c[:name], "new prompt")
    end,

    Rule.new(id: "capability-added", severity: "compat",
             description: "The server advertises a new capability.") do |c|
      next nil unless c[:kind] == "capability-added"

      finding(c[:capability], "new capability")
    end,

    # ───────────────── info ─────────────────
    Rule.new(id: "server-version-changed", severity: "info",
             description: "The server's reported version changed. Informational — not a contract change by itself.") do |c|
      next nil unless c[:kind] == "server-info-changed"

      before = (c[:before] || {})["version"] || "(none)"
      after = (c[:after] || {})["version"] || "(none)"
      next nil if before == after

      finding("server", "version #{before} → #{after}")
    end,

    Rule.new(id: "title-changed", severity: "info",
             description: "A display title changed (server, tool, resource, or prompt). Titles are for humans, not the model — informational.") do |c|
      case c[:kind]
      when "tool-modified", "prompt-modified"
        c[:before]["title"] != c[:after]["title"] ? finding(c[:name], "title changed") : nil
      when "resource-modified"
        c[:before]["title"] != c[:after]["title"] ? finding(c[:uri], "title changed") : nil
      when "server-info-changed"
        (c[:before] || {})["title"] != (c[:after] || {})["title"] ? finding("server", "title changed") : nil
      end
    end
  ]
end

.keys_of(value) ⇒ Object 



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# File 'lib/mcp_diff/core/rules.rb', line 16

def keys_of(value)
  value.is_a?(Hash) ? value : {}
end

.schema_findings(rule, change, schema_key) ⇒ Object 



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# File 'lib/mcp_diff/core/rules.rb', line 20

def schema_findings(rule, change, schema_key)
  SchemaDiff.diff_object_schema(change[:before][schema_key], change[:after][schema_key])
end