diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2439096..4aa5615 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 - `lemans report -S` sorts by several dash-joined columns (`-S score-credit`); `^column` reverses that column's order (`-S ^score`: low to high, `-S ^model`: Z-A).
 - Multistep results record `total_steps`; `lemans report` now shows a `progress` column (steps completed, `2/5`).
+- Bring your own agent: `Lemans::Agents.register` adds an agent defined outside lemans, and `lemans run --require FILE` loads the file that registers it.
 
 ## [1.5.0] - 2026-10-05
 
diff --git a/README.md b/README.md
index 786a0c4..c21c36a 100644
--- a/README.md
+++ b/README.md
@@ -263,13 +263,43 @@ gpt-5.6-luna  ar-archive-book-access  2/2    2m 23s  $0.0132  12.5   156905
 6 trials: 6 scored, 0 invalid, 6 solved (100%) · $0.0801 · pass@2 3/3 tasks (100%)
 ```
 
+### Bring your own agent
+
+Any agent that implements the `Lemans::Agent` contract can be measured. It sees its profile (the `agent:` section of `bench.yml`), the task, and the environment, and works through `environment.exec`, `upload`, and `download`. `run` returns an `Agent::Response` with an outcome and usage; `install` runs while the setup network is still open. Define it in a file, register it under a name, and load the file with `--require`:
+
+```ruby
+# my_agent.rb
+class MyAgent < Lemans::Agent
+  NAME = "my-agent"
+
+  def install(_task, environment)
+    environment.upload("/path/to/my-agent", "/usr/local/bin/my-agent")
+    environment.exec!("chmod +x /usr/local/bin/my-agent")
+  end
+
+  def run(task, environment)
+    finished = environment.exec("cd #{task.environment.workdir} && my-agent --model #{model}", timeout:)
+    outcome = finished.exit_code == 124 ? :agent_timeout : :completed
+    Response.new(outcome: Lemans::Result::Outcome.new(outcome), usage: Lemans::Result::Usage.zero)
+  end
+end
+
+Lemans::Agents.register(MyAgent::NAME, MyAgent)
+```
+
+```bash
+lemans run --bench my-bench --require my_agent.rb --agent my-agent
+```
+
+Report what the run cost in `usage` (`cost_usd: nil` when it is unknown), and raise `Lemans::InfrastructureError` or return a `Response` with an `error` when the agent itself failed, so the trial counts as invalid rather than as a zero.
+
 ## CLI
 
 | Command | What it does |
 | --- | --- |
 | `lemans init` | Scaffold a new bench directory: an annotated `bench.yml` and two example tasks |
 | `lemans tasks` | List the tasks in a bench (`--tag` to filter) |
-| `lemans run` | Run tasks and grade them (`--task`, `--tag`, `--agent`, `--model`, `--max-output-tokens`, `-k`, `-c`, `--resume`) |
+| `lemans run` | Run tasks and grade them (`--task`, `--tag`, `--agent`, `--require`, `--model`, `--max-output-tokens`, `-k`, `-c`, `--resume`) |
 | `lemans restart <run>...` | Continue failed multistep runs from their last settled step in new runs (`-c`, `--recover` to continue the failed step's session, `--reverify` to grade again, `--allow-scored`, `--backend`, `--max-output-tokens`) |
 | `lemans report [RUNS_DIR]` | Summarize `runs/` (or `RUNS_DIR`) as a table or CSV (`--task`, `--tag`, `--metadata key:value` to filter, `-A [task-agent-model]` to aggregate, `-S <columns>` to sort, e.g. `-S score-credit`; numbers high to low, names A-Z, `^column` reverses that column); repeated attempts add pass@k per model × task, fractional grading a `credit` column, multistep tasks a `progress` column (steps completed / task steps) |
 | `lemans clobber [RUNS_DIR]` | Delete run results under `runs/` (or `RUNS_DIR`) (`--task`, `--ttl 10m\|2h\|1d`, `--invalid`, `-f` to skip the confirmation) |
diff --git a/lib/lemans/agents.rb b/lib/lemans/agents.rb
index f9c287a..0a2a296 100644
--- a/lib/lemans/agents.rb
+++ b/lib/lemans/agents.rb
@@ -11,13 +11,25 @@ module Agents
       "miniswen-installed" => "MiniswenInstalled"
     }.freeze
 
-    def self.build(name, profile:, model: nil) = lookup(name).new(profile: profile, model: model)
+    @registered = {}
 
-    def self.lookup(name)
-      constant = REGISTRY[name] or
-        raise ConfigError, "unknown agent #{name.inspect} (known: #{REGISTRY.keys.join(", ")})"
+    class << self
+      def register(name, agent_class)
+        raise ConfigError, "agent #{name.inspect} is built in" if REGISTRY.key?(name)
 
-      const_get(constant)
+        @registered[name] = agent_class
+      end
+
+      def unregister(name) = @registered.delete(name)
+
+      def names = REGISTRY.keys + @registered.keys
+
+      def build(name, profile:, model: nil) = lookup(name).new(profile: profile, model: model)
+
+      def lookup(name)
+        @registered[name] || (REGISTRY[name] && const_get(REGISTRY[name])) or
+          raise ConfigError, "unknown agent #{name.inspect} (known: #{names.join(", ")})"
+      end
     end
   end
 end
diff --git a/lib/lemans/cli.rb b/lib/lemans/cli.rb
index 00bc48a..37446d9 100644
--- a/lib/lemans/cli.rb
+++ b/lib/lemans/cli.rb
@@ -48,7 +48,9 @@ def tasks
     option :bench, default: ".", desc: "Directory holding bench.yml"
     option :task, desc: "Run task(s) by name", repeatable: true
     option :tag, desc: "Run every task carrying this tag(s)", repeatable: true
-    option :agent, desc: "Override the agent from bench.yml (miniswen, miniswen-installed, oracle, nop)"
+    option :agent, desc: "Override the agent from bench.yml (miniswen, miniswen-installed, oracle, nop, or one registered by --require)"
+    option :require, banner: "FILE", repeatable: true,
+                     desc: "Load a Ruby file first, e.g. one that defines an agent and calls Lemans::Agents.register"
     option :model, desc: "Override the model(s) from bench.yml", repeatable: true
     option :max_output_tokens, type: :numeric, banner: "TOKENS",
                                desc: "Cap the agent's output per model call (default: the provider's)"
@@ -58,6 +60,8 @@ def tasks
     option :backend, enum: Environments::BACKENDS.keys, desc: "Sandbox backend (default: daytona)"
     option :resume, type: :boolean, default: false, desc: "Skip trials that already have a result"
     def run_bench
+      Array(options[:require]).each { require File.expand_path(it) }
+
       # The bundled pricing registry ages faster than the gem: refresh once up
       # front, so every trial prices completions against the same revision.
       Miniswen.refresh_registry!
diff --git a/test/lemans/agents_test.rb b/test/lemans/agents_test.rb
new file mode 100644
index 0000000..be1e16f
--- /dev/null
+++ b/test/lemans/agents_test.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class AgentsTest < Minitest::Test
+  class TestAgent < Lemans::Agent
+    NAME = "test-agent"
+
+    def run(_task, _environment) = Response.new(outcome: Lemans::Result::Outcome.new(:completed), usage: Lemans::Result::Usage.zero)
+  end
+
+  def teardown
+    Lemans::Agents.unregister(TestAgent::NAME)
+  end
+
+  def test_register
+    Lemans::Agents.register(TestAgent::NAME, TestAgent)
+    config = Lemans::Config::Agent.new(TestAgent::NAME, "some/model")
+
+    agent = Lemans::Agents.build(TestAgent::NAME, profile: config, model: "other/model")
+
+    assert_instance_of TestAgent, agent
+    assert_equal "other/model", agent.model
+    assert_includes Lemans::Agents.names, TestAgent::NAME
+    assert_instance_of Lemans::Agents::Oracle, Lemans::Agents.build("oracle", profile: config)
+  end
+
+  def test_builtin_names_are_taken
+    error = assert_raises(Lemans::ConfigError) { Lemans::Agents.register("oracle", TestAgent) }
+
+    assert_equal "agent \"oracle\" is built in", error.message
+  end
+
+  def test_unknown_agent
+    Lemans::Agents.register(TestAgent::NAME, TestAgent)
+
+    error = assert_raises(Lemans::ConfigError) do
+      Lemans::Agents.build("nope", profile: Lemans::Config::Agent.new("nope", "some/model"))
+    end
+
+    assert_equal "unknown agent \"nope\" (known: nop, oracle, miniswen, miniswen-installed, test-agent)", error.message
+  end
+end
diff --git a/test/lemans/cli_test.rb b/test/lemans/cli_test.rb
new file mode 100644
index 0000000..fc565f6
--- /dev/null
+++ b/test/lemans/cli_test.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "lemans/cli"
+
+class CLITest < Minitest::Test
+  def teardown
+    Lemans::Agents.unregister("required-agent")
+  end
+
+  def test_require
+    Dir.mktmpdir do |dir|
+      file = File.join(dir, "required_agent.rb")
+      File.write(file, <<~RUBY)
+        class RequiredAgent < Lemans::Agent
+          NAME = "required-agent"
+        end
+        Lemans::Agents.register(RequiredAgent::NAME, RequiredAgent)
+      RUBY
+
+      _, err = capture_io do
+        Miniswen.stub(:refresh_registry!, nil) do
+          assert_raises(SystemExit) do
+            Lemans::CLI.start([ "run", "--bench", BenchFixture::ROOT.to_s, "--require", file, "--task", "none" ])
+          end
+        end
+      end
+
+      assert_includes Lemans::Agents.names, "required-agent"
+      assert_includes err, "no matching tasks"
+    end
+  end
+end