diff --git a/AGENTS.md b/AGENTS.md
index 82f491c7..6d84bf79 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -309,7 +309,9 @@ default glob; the Rakefile's `test` task is what sweeps both.
 ### RubyLLM
 - Uses `ruby_llm` gem for unified access to 15+ providers
 - RubyLLM manages its own API keys via `RubyLLM.configure`
-- Model ID determines which provider is used automatically
+- Model ID determines which provider is used automatically; `platform:`
+  (maps to RubyLLM's `provider:`) pins it when a model ID is served by
+  more than one, e.g. `platform: :vertexai` for Gemini models on Vertex AI
 - Supports prompts, embeddings, tool calling, and streaming
 
 ## The dashboard: a second gem in this repo
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9868b6d8..58aa2ebf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,7 +5,30 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [1.3.1] - 2026-08-19
+## [Unreleased]
+
+### Added
+
+- **RubyLLM backend pinning via `platform:`.** RubyLLM resolves which of its
+  providers serves a request from the model ID, and a model served by more
+  than one — `gemini-2.5-flash` exists on both the Gemini API and Vertex
+  AI — lands on whichever RubyLLM's registry prefers, with no way to say
+  otherwise from ActiveAgent. The new `platform:` option
+  (`generate_with :ruby_llm, model: "gemini-2.5-flash", platform: :vertexai`)
+  forwards to RubyLLM's `provider:` and pins the backend, for embeddings as
+  well as prompts. It is not named `provider:` because a provider reference
+  is already the first argument to `generate_with`. Omitting it keeps
+  model-based routing unchanged. (#373)
+
+### Fixed
+
+- **`service: "RubyLLM"` loads when the ruby_llm railtie has run.** The
+  ruby_llm gem registers `RubyLLM` as an inflector acronym in Rails apps,
+  which turns `"RubyLLM".underscore` into `rubyllm` — so provider loading
+  required a nonexistent `rubyllm_provider.rb` and failed with
+  `cannot load such file`. An alias file now covers that require path, the
+  same fix `openai_provider.rb` applies for `OpenAI`. (#371, fixed in #372
+  by @aoki-ryusei; regression tests in #374)
 
 ### Fixed
 
diff --git a/docs/providers/ruby_llm.md b/docs/providers/ruby_llm.md
index a78d4a43..e261f56c 100644
--- a/docs/providers/ruby_llm.md
+++ b/docs/providers/ruby_llm.md
@@ -75,12 +75,48 @@ class FlexibleAgent < ApplicationAgent
 end
 ```
 
+### Pinning the Platform
+
+When the same model ID is served by more than one of RubyLLM's providers, RubyLLM picks one by its own registry preference — `gemini-2.5-flash` resolves to the Gemini API even when you have configured Vertex AI credentials. Set `platform:` to pin the request to a specific RubyLLM provider; it maps to RubyLLM's own `provider:` option:
+
+```ruby
+class VertexAgent < ApplicationAgent
+  generate_with :ruby_llm, model: "gemini-2.5-flash", platform: :vertexai
+end
+```
+
+Or in `config/active_agent.yml`:
+
+```yaml
+production:
+  ruby_llm:
+    service: "RubyLLM"
+    model: "gemini-2.5-flash"
+    platform: "vertexai"
+```
+
+Authentication and region stay in RubyLLM's configuration:
+
+```ruby
+# config/initializers/ruby_llm.rb
+RubyLLM.configure do |config|
+  config.vertexai_project_id = "your-project-id"
+  config.vertexai_location = "us-central1"
+end
+```
+
+Valid values are RubyLLM's provider keys — `:openai`, `:anthropic`, `:gemini`, `:vertexai`, `:bedrock`, `:openrouter`, `:ollama`, and so on. Omitting `platform:` keeps RubyLLM's automatic model-based routing. The option applies to embeddings as well as prompts.
+
 ## Provider-Specific Parameters
 
 ### Required Parameters
 
 - **`model`** - Model identifier (e.g., "gpt-4o-mini", "claude-sonnet-5")
 
+### Routing Parameters
+
+- **`platform`** - Pins which RubyLLM provider serves the model (maps to RubyLLM's `provider:`), e.g. `:vertexai` for Gemini models on Vertex AI. See [Pinning the Platform](#pinning-the-platform)
+
 ### Sampling Parameters
 
 - **`temperature`** - Controls randomness (0.0 to 1.0)
diff --git a/lib/active_agent/providers/ruby_llm/options.rb b/lib/active_agent/providers/ruby_llm/options.rb
index 7ca1ca95..0654f0eb 100644
--- a/lib/active_agent/providers/ruby_llm/options.rb
+++ b/lib/active_agent/providers/ruby_llm/options.rb
@@ -11,6 +11,10 @@ module RubyLLM
       # provider-specific API key attributes are needed here.
       class Options < Common::BaseModel
         attribute :model, :string
+        # Pins which RubyLLM backend serves the model (RubyLLM's provider:,
+        # e.g. :vertexai, :gemini, :bedrock). A model ID served by several
+        # backends otherwise resolves by RubyLLM's registry preference.
+        attribute :platform, :string
         attribute :temperature, :float
         attribute :max_tokens, :integer
 
diff --git a/lib/active_agent/providers/ruby_llm_provider.rb b/lib/active_agent/providers/ruby_llm_provider.rb
index 7e647aa2..9bc9ed8b 100644
--- a/lib/active_agent/providers/ruby_llm_provider.rb
+++ b/lib/active_agent/providers/ruby_llm_provider.rb
@@ -12,6 +12,10 @@ module Providers
     # Provider for RubyLLM's unified API, supporting 15+ LLM providers
     # (OpenAI, Anthropic, Gemini, Bedrock, Azure, Ollama, etc.).
     #
+    # RubyLLM resolves which backend serves a request from the model ID; the
+    # platform option pins it when a model ID is served by more than one
+    # (e.g. Gemini models on the Gemini API vs Vertex AI).
+    #
     # Uses RubyLLM's provider-level API (provider.complete()) rather than
     # the high-level Chat object to avoid conflicts with ActiveAgent's own
     # conversation management and tool execution loop.
@@ -254,13 +258,22 @@ def process_function_calls(tool_calls)
       # Reuses the cached provider if the model hasn't changed (e.g., during
       # multi-turn tool calling loops).
       #
+      # The platform option is forwarded as RubyLLM's provider: so a model ID
+      # served by several backends (e.g. gemini-2.5-flash on the Gemini API
+      # and Vertex AI) can be pinned instead of resolving by RubyLLM's
+      # registry preference.
+      #
       # @param model_id [String] model identifier
       # @return [void]
       def resolve_ruby_llm_provider!(model_id)
         return if @ruby_llm_provider && @cached_model_id == model_id
 
         @cached_model_id = model_id
-        @ruby_llm_model, @ruby_llm_provider = ::RubyLLM::Models.resolve(model_id, config: ::RubyLLM.config)
+        @ruby_llm_model, @ruby_llm_provider = ::RubyLLM::Models.resolve(
+          model_id,
+          provider: options.platform&.to_sym,
+          config: ::RubyLLM.config
+        )
       end
 
       # Converts ActiveAgent messages to RubyLLM message format.
diff --git a/test/providers/ruby_llm/provider_loading_test.rb b/test/providers/ruby_llm/provider_loading_test.rb
new file mode 100644
index 00000000..06e45def
--- /dev/null
+++ b/test/providers/ruby_llm/provider_loading_test.rb
@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# The require_gem! guard in ruby_llm_provider.rb only checks that the
+# ruby_llm gem's namespace exists, so the loading paths can be exercised
+# without the gem installed.
+module ::RubyLLM; end unless defined?(::RubyLLM)
+
+class RubyLLMProviderLoadingTest < ActiveSupport::TestCase
+  test "loads RubyLLMProvider via ruby_llm_provider path" do
+    require "active_agent/providers/ruby_llm_provider"
+
+    assert defined?(ActiveAgent::Providers::RubyLLMProvider)
+    assert defined?(ActiveAgent::Providers::RubyLLM::Options)
+  end
+
+  test "loads RubyLLMProvider via rubyllm_provider path" do
+    require "active_agent/providers/rubyllm_provider"
+
+    assert defined?(ActiveAgent::Providers::RubyLLMProvider)
+  end
+
+  test "provider concern loads the RubyLLM service with the gem's acronym registered" do
+    already_registered = "RubyLLM".underscore == "rubyllm"
+
+    with_rubyllm_acronym do
+      assert_equal "rubyllm", "RubyLLM".underscore
+
+      klass = ActiveAgent::Base.provider_load("RubyLLM")
+      assert_equal ActiveAgent::Providers::RubyLLMProvider, klass
+    end
+
+    unless already_registered
+      assert_equal "ruby_llm", "RubyLLM".underscore, "acronym leaked out of with_rubyllm_acronym"
+    end
+  end
+
+  test "provider concern loads the RubyLLM service without the acronym" do
+    skip "the ruby_llm railtie registered its acronym in this process" if "RubyLLM".underscore == "rubyllm"
+
+    assert_equal "ruby_llm", "RubyLLM".underscore
+
+    klass = ActiveAgent::Base.provider_load("RubyLLM")
+    assert_equal ActiveAgent::Providers::RubyLLMProvider, klass
+  end
+
+  test "service name remap handles Rubyllm and RubyLlm variations" do
+    remaps = ActiveAgent::Provider::PROVIDER_SERVICE_NAMES_REMAPS
+
+    assert_equal "RubyLLM", remaps["Rubyllm"]
+    assert_equal "RubyLLM", remaps["RubyLlm"]
+  end
+
+  private
+
+  # Registers the RubyLLM acronym the way the ruby_llm gem's railtie does.
+  # Edge Rails freezes every Inflections instance after boot, so the acronym
+  # goes on an unfrozen dup swapped in for the duration (dup support is what
+  # Inflections#initialize_dup exists for), and the original instance --
+  # frozen or not -- is restored afterwards.
+  def with_rubyllm_acronym
+    original = ActiveSupport::Inflector.inflections(:en)
+    swap_en_inflections(original.dup)
+
+    ActiveSupport::Inflector.inflections(:en) do |inflect|
+      inflect.acronym "RubyLLM"
+    end
+
+    yield
+  ensure
+    swap_en_inflections(original) if original
+  end
+
+  # Installs an :en Inflections instance in the slot this Rails version
+  # reads from: a dedicated @__en_instance__ where defined (8.1+), the
+  # @__instance__ map otherwise (7.2).
+  def swap_en_inflections(instance)
+    klass = ActiveSupport::Inflector::Inflections
+
+    if klass.instance_variable_defined?(:@__en_instance__)
+      klass.instance_variable_set(:@__en_instance__, instance)
+    else
+      klass.instance_variable_get(:@__instance__)[:en] = instance
+    end
+  end
+end
diff --git a/test/providers/ruby_llm/ruby_llm_provider_test.rb b/test/providers/ruby_llm/ruby_llm_provider_test.rb
index f5ad06a2..29ad0795 100644
--- a/test/providers/ruby_llm/ruby_llm_provider_test.rb
+++ b/test/providers/ruby_llm/ruby_llm_provider_test.rb
@@ -873,6 +873,112 @@ def complete(messages, **kwargs)
     end
   end
 
+  # --- platform pinning (RubyLLM's provider:) ---
+
+  test "platform option pins the RubyLLM backend when resolving the model" do
+    resolve_kwargs = nil
+    capturing_resolve = ->(model_id, **kwargs) {
+      resolve_kwargs = kwargs
+      [ stub_model_info(model_id), ::RubyLLM::StubProvider.new ]
+    }
+
+    ::RubyLLM::Models.stub(:resolve, capturing_resolve) do
+      provider = ActiveAgent::Providers::RubyLLMProvider.new(
+        service: "RubyLLM",
+        model: "gemini-2.5-flash",
+        platform: :vertexai,
+        messages: [ { role: "user", content: "hello" } ]
+      )
+
+      provider.prompt
+    end
+
+    assert_equal :vertexai, resolve_kwargs[:provider]
+  end
+
+  test "platform option accepts a string" do
+    resolve_kwargs = nil
+    capturing_resolve = ->(model_id, **kwargs) {
+      resolve_kwargs = kwargs
+      [ stub_model_info(model_id), ::RubyLLM::StubProvider.new ]
+    }
+
+    ::RubyLLM::Models.stub(:resolve, capturing_resolve) do
+      provider = ActiveAgent::Providers::RubyLLMProvider.new(
+        service: "RubyLLM",
+        model: "gemini-2.5-flash",
+        platform: "vertexai",
+        messages: [ { role: "user", content: "hello" } ]
+      )
+
+      provider.prompt
+    end
+
+    assert_equal :vertexai, resolve_kwargs[:provider]
+  end
+
+  test "model routing is unchanged when platform is not set" do
+    resolve_kwargs = nil
+    capturing_resolve = ->(model_id, **kwargs) {
+      resolve_kwargs = kwargs
+      [ stub_model_info(model_id), ::RubyLLM::StubProvider.new ]
+    }
+
+    ::RubyLLM::Models.stub(:resolve, capturing_resolve) do
+      provider = ActiveAgent::Providers::RubyLLMProvider.new(
+        service: "RubyLLM",
+        model: "gpt-4o-mini",
+        messages: [ { role: "user", content: "hello" } ]
+      )
+
+      provider.prompt
+    end
+
+    assert_nil resolve_kwargs[:provider]
+  end
+
+  test "platform option pins the RubyLLM backend for embeddings" do
+    resolve_kwargs = nil
+    capturing_resolve = ->(model_id, **kwargs) {
+      resolve_kwargs = kwargs
+      [ stub_model_info(model_id), ::RubyLLM::StubProvider.new ]
+    }
+
+    ::RubyLLM::Models.stub(:resolve, capturing_resolve) do
+      provider = ActiveAgent::Providers::RubyLLMProvider.new(
+        service: "RubyLLM",
+        input: "test text",
+        model: "text-embedding-004",
+        platform: :vertexai
+      )
+
+      provider.embed
+    end
+
+    assert_equal :vertexai, resolve_kwargs[:provider]
+  end
+
+  test "platform set via generate_with reaches the provider options" do
+    agent_class = Class.new(ApplicationAgent) do
+      def self.name = "PlatformProbeAgent"
+      generate_with :ruby_llm, model: "gemini-2.5-flash", platform: :vertexai
+
+      def ping
+        prompt(message: "hello")
+      end
+    end
+
+    agent = agent_class.new
+    agent.params = {}
+    agent.process(:ping)
+    parameters = agent.send(:prepare_prompt_parameters)
+
+    assert_equal :vertexai, parameters[:platform]
+
+    provider = agent.prompt_provider_klass.new(**parameters)
+    assert_equal "vertexai", provider.options.platform
+  end
+
   # --- stop_reason from RubyLLM response ---
 
   test "stop_reason from RubyLLM response is preserved" do