Module: OpenLoam::Mcp

Defined in:
lib/open_loam/mcp.rb,
lib/open_loam/mcp/server.rb

Overview

An MCP (Model Context Protocol) server surface exposing OpenLoam to an AI agent — tools-only v1 (L-302). It lets an agent DISCOVER the domain (entities, schema, policy, workflow) and READ tenant-scoped records, and PROPOSE writes that are STAGED for human approval — never committed. Everything runs inside the tenant + actor of the API token the server authenticated with; whatever that user may do, no more. Approval authority stays with a human in the admin.

Two layers: the tool methods (pure, assume OpenLoam::Current is established) and handle_jsonrpc (a pure JSON-RPC dispatcher for initialize / tools/list / tools/call). The stdio transport (newline-delimited JSON-RPC) lives in OpenLoam::Mcp::Server.

SECURITY posture, reusing OpenLoam's existing gates:

* entity names resolve against an allowlist (API-exposed TenantRecord
descendants), never a bare constantize;
* query filters/order are whitelisted to real columns (or a known custom
field, which carries the L-711 read-ACL); a filter on a field the role
can't read is refused (no inference oracle);
* query output emits only policy-readable fields per record (encrypted
values decrypted, blind-index columns dropped);
* a staged write accepts only policy-writable columns, refuses the workflow
column (that goes through a transition), and refuses id/tenant_id/
lock_version.

Defined Under Namespace

Classes: Server, ToolError

Constant Summary collapse

PROTOCOL_VERSION = 
"2025-06-18".freeze
MAX_LIMIT = 
100
FILTER_OPS = 
%w[eq neq contains gt gte lt lte present].freeze
TOOLS = 
[
  {
    name: "list_entities",
    description: "List the business entities available in this OpenLoam tenant.",
    inputSchema: { type: "object", properties: {}, additionalProperties: false }
  },
  {
    name: "describe_entity",
    description: "Describe one entity: its columns and types, custom fields, workflow, and which fields the current role may read/write.",
    inputSchema: {
      type: "object",
      properties: { entity: { type: "string", description: "Entity name, e.g. \"Equipment\"." } },
      required: [ "entity" ], additionalProperties: false
    }
  },
  {
    name: "query_entity",
    description: "Read tenant-scoped records of an entity. Returns only fields the current role may read. Filters and sort are whitelisted; limit is capped at 100.",
    inputSchema: {
      type: "object",
      properties: {
        entity: { type: "string" },
        filters: {
          type: "array",
          items: {
            type: "object",
            properties: {
              field: { type: "string" },
              op: { type: "string", enum: FILTER_OPS },
              value: {}
            },
            required: [ "field" ], additionalProperties: false
          }
        },
        order: { type: "string", description: "A real column name to sort by." },
        dir: { type: "string", enum: [ "asc", "desc" ] },
        limit: { type: "integer", minimum: 1, maximum: MAX_LIMIT }
      },
      required: [ "entity" ], additionalProperties: false
    }
  },
  {
    name: "stage_write",
    description: "PROPOSE an update to one record. It is staged as a OpenLoam PendingAction for a human manager to approve — it does NOT take effect until approved. Only policy-writable columns are accepted; the workflow column is refused (use a transition).",
    inputSchema: {
      type: "object",
      properties: {
        entity: { type: "string" },
        id: { type: "integer" },
        changes: { type: "object", description: "field => new value, real writable columns only." }
      },
      required: [ "entity", "id", "changes" ], additionalProperties: false
    }
  }
].freeze

Class Method Summary collapse

Class Method Details

.api_exposed?(model) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/open_loam/mcp.rb', line 102

def api_exposed?(model)
  "Api::#{model.model_name.plural.camelize}Controller".safe_constantize.present?
end

.apply_filters(model, scope, filters, policy) ⇒ Object

--- internals ---



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# File 'lib/open_loam/mcp.rb', line 231

def apply_filters(model, scope, filters, policy)
  Array(filters).each do |raw|
    field = raw["field"].to_s
    op = (raw["op"] || "eq").to_s
    raise ToolError, "unknown op #{op.inspect}" unless FILTER_OPS.include?(op)
    value = raw["value"]

    if model.column_names.include?(field)
      raise ToolError, "#{field} is not readable for this role" unless policy.readable?(field)
      scope = column_filter(model, scope, field, op, value)
    elsif custom_field?(model, field)
      # CustomFieldIndex.filter carries its own L-711 read-ACL for the current
      # actor; surface its refusal as a clean tool error.
      begin
        scope = scope.merge(OpenLoam::CustomFieldIndex.filter(model, field, op, value))
      rescue OpenLoam::FieldAccessError => error
        raise ToolError, error.message
      end
    else
      raise ToolError, "unknown field #{field.inspect}"
    end
  end
  scope
end

.apply_order(model, scope, order, dir) ⇒ Object

Raises:



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# File 'lib/open_loam/mcp.rb', line 270

def apply_order(model, scope, order, dir)
  return scope.order(id: :asc) if order.blank?
  raise ToolError, "unknown sort column #{order.inspect}" unless model.column_names.include?(order.to_s)

  # Same gate apply_filters enforces. Values are redacted from the result,
  # but ordering by a restricted column still leaks its ranking — which is
  # the inference oracle the filter gate exists to close.
  policy = OpenLoam::Policy.for_model(model, OpenLoam::Current.actor)
  raise ToolError, "#{order} is not readable for this role" unless policy.readable?(order.to_s)

  direction = dir.to_s.casecmp("desc").zero? ? :desc : :asc
  scope.reorder(order.to_s => direction)
end

.call_tool(name, args) ⇒ Object

--- protocol ---



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# File 'lib/open_loam/mcp.rb', line 193

def call_tool(name, args)
  kwargs = (args || {}).transform_keys(&:to_sym)
  case name
  when "list_entities"  then list_entities
  when "describe_entity" then describe_entity(**kwargs.slice(:entity))
  when "query_entity"   then query_entity(**kwargs.slice(:entity, :filters, :order, :dir, :limit))
  when "stage_write"    then stage_write(**kwargs.slice(:entity, :id, :changes))
  else raise ToolError, "unknown tool #{name.inspect}"
  end
end

.column_filter(model, scope, field, op, value) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 256

def column_filter(model, scope, field, op, value)
  column = model.connection.quote_column_name(field) # field is a real column name (whitelisted)
  case op
  when "eq"       then scope.where(field => value)
  when "neq"      then scope.where.not(field => value)
  when "contains" then scope.where("#{column} LIKE ?", "%#{value.to_s.gsub(/[\\%_]/) { |c| "\\#{c}" }}%")
  when "present"  then scope.where.not(field => [ nil, "" ])
  when "gt"       then scope.where("#{column} > ?", value)
  when "gte"      then scope.where("#{column} >= ?", value)
  when "lt"       then scope.where("#{column} < ?", value)
  when "lte"      then scope.where("#{column} <= ?", value)
  end
end

.custom_field?(model, field) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/open_loam/mcp.rb', line 301

def custom_field?(model, field)
  model.respond_to?(:custom_field_definitions) && model.custom_field_definitions.exists?(name: field)
end

.describe_entity(entity:) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 118

def describe_entity(entity:)
  model = resolve!(entity)
  policy = OpenLoam::Policy.for(model.new)

  columns = model.columns.reject { |c| c.name.end_with?("_hash") }.map do |col|
    { name: col.name, type: col.sql_type_metadata.type.to_s,
      readable: policy.readable?(col.name), writable: policy.writable?(col.name) }
  end

  custom = if model.respond_to?(:custom_field_definitions)
    model.custom_field_definitions.map do |definition|
      { name: definition.name, type: definition.field_type,
        readable: policy.custom_field_readable?(definition.name),
        writable: policy.custom_field_writable?(definition.name) }
    end
  else
    []
  end

  workflow = if model.respond_to?(:open_loam_workflow) && model.open_loam_workflow
    wf = model.open_loam_workflow
    { column: wf.column, states: wf.states,
      transitions: wf.transitions.values.map { |t| { name: t.name.to_s, from: t.from, to: t.to, roles: Array(t.roles).map(&:to_s) } } }
  end

  { name: model.name, columns: columns, custom_fields: custom, workflow: workflow }.compact
end

.entities ⇒ Object

--- entity allowlist ---



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# File 'lib/open_loam/mcp.rb', line 92

def entities
  # Models are lazy-loaded (Zeitwerk), so `descendants` is only complete after
  # an eager load — same as OpenLoam::OpenApi's discovery.
  Rails.application.eager_load! if defined?(Rails) && Rails.respond_to?(:application)
  OpenLoam::TenantRecord.descendants
                    .reject { |model| model.name.blank? }
                    .select { |model| api_exposed?(model) }
                    .sort_by(&:name)
end

.err(id, code, message) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 309

def err(id, code, message)
  { "jsonrpc" => "2.0", "id" => id, "error" => { "code" => code, "message" => message } }
end

.handle_jsonrpc(request) ⇒ Object

Pure JSON-RPC dispatch. Returns a response Hash, or nil for a notification (no reply). Never raises for tool faults — those become an isError result.



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# File 'lib/open_loam/mcp.rb', line 206

def handle_jsonrpc(request)
  id = request["id"]
  case request["method"]
  when "initialize"
    ok(id, protocolVersion: request.dig("params", "protocolVersion") || PROTOCOL_VERSION,
           capabilities: { tools: {} },
           serverInfo: { name: "open_loam", version: OpenLoam::VERSION })
  when "tools/list"
    ok(id, tools: TOOLS)
  when "tools/call"
    begin
      data = call_tool(request.dig("params", "name"), request.dig("params", "arguments"))
      ok(id, content: [ { type: "text", text: JSON.generate(data) } ])
    rescue ToolError, OpenLoam::Error, ActiveRecord::RecordNotFound => error
      ok(id, isError: true, content: [ { type: "text", text: error.message } ])
    end
  when "notifications/initialized", "notifications/cancelled", nil
    nil # notifications get no response
  else
    err(id, -32601, "method not found: #{request["method"]}")
  end
end

.list_entities ⇒ Object

--- tools ---



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# File 'lib/open_loam/mcp.rb', line 114

def list_entities
  { entities: entities.map { |m| { name: m.name, plural: m.model_name.plural } } }
end

.ok(id, **result) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 305

def ok(id, **result)
  { "jsonrpc" => "2.0", "id" => id, "result" => result }
end

.query_entity(entity:, filters: [], order: nil, dir: "asc", limit: MAX_LIMIT) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 146

def query_entity(entity:, filters: [], order: nil, dir: "asc", limit: MAX_LIMIT)
  model = resolve!(entity)
  policy = OpenLoam::Policy.for(model.new)
  scope = apply_filters(model, model.all, filters, policy)
  scope = apply_order(model, scope, order, dir)
  capped = [ [ limit.to_i, 1 ].max, MAX_LIMIT ].min

  records = scope.limit(capped).map { |record| serialize(record, policy) }
  { entity: model.name, count: records.size, records: records }
end

.resolve!(entity) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 106

def resolve!(entity)
  name = entity.to_s
  entities.find { |m| m.name == name || m.model_name.plural == name } ||
    raise(ToolError, "unknown entity #{entity.inspect}")
end

.serialize(record, policy) ⇒ Object 



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# File 'lib/open_loam/mcp.rb', line 284

def serialize(record, policy)
  model = record.class
  encrypted = model.respond_to?(:open_loam_encrypted_attributes) ? model.open_loam_encrypted_attributes.map(&:to_s) : []

  json = {}
  policy.readable_fields(model.column_names).each do |col|
    next if col.end_with?("_hash")
    json[col] = encrypted.include?(col) ? record.public_send(col) : record[col]
  end
  if model.respond_to?(:custom_field_definitions)
    model.custom_field_definitions.each do |definition|
      json["cf_#{definition.name}"] = record.custom_field(definition.name) if policy.custom_field_readable?(definition.name)
    end
  end
  json
end

.stage_write(entity:, id:, changes:) ⇒ Object

Raises:



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# File 'lib/open_loam/mcp.rb', line 157

def stage_write(entity:, id:, changes:)
  model = resolve!(entity)
  record = model.find(id) # tenant-scoped
  policy = OpenLoam::Policy.for(record)
  workflow_column = model.respond_to?(:open_loam_workflow) ? model.open_loam_workflow&.column.to_s : nil

  clean = {}
  (changes || {}).each do |field, value|
    field = field.to_s
    # deleted_at belongs with the rest: staging a direct soft-delete would
    # bypass a destroy? override and the audited action label, which is
    # exactly what OpenLoam::Bulk and BusinessRules both refuse.
    raise ToolError, "#{field} cannot be set" if %w[id tenant_id lock_version deleted_at].include?(field)
    raise ToolError, "#{field} changes go through a workflow transition, not a direct write" if field == workflow_column

    if model.column_names.include?(field)
      raise ToolError, "#{field} is not writable for this role" unless policy.writable?(field)
      clean[field] = value
    elsif custom_field?(model, field)
      raise ToolError, "custom-field writes over MCP are not supported in v1"
    else
      raise ToolError, "unknown field #{field.inspect}"
    end
  end
  raise ToolError, "no writable changes" if clean.empty?

  action = OpenLoam::PendingActions.stage(
    summary: "MCP proposal: update #{model.name}##{id}",
    on: record, action: :update, changes: clean
  )
  { staged: true, pending_action_id: action.id, summary: action.summary, changes: clean,
    note: "Staged for human approval — not applied until a manager approves it." }
end