diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index a7d61b29..1faf4c5d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -141,8 +141,8 @@ jobs:
         with:
           ruby-version: head
 
-      - name: Test run_once.sh
-        run: ./run_once.sh --yjit-stats benchmarks/railsbench/benchmark.rb
+      - name: Test run_once.rb
+        run: ./run_once.rb -- --yjit-stats benchmarks/railsbench/benchmark.rb
 
   all-tests:
     runs-on: ubuntu-latest
diff --git a/README.md b/README.md
index bba0d1ef..11f75f20 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ Small set of benchmarks and scripts for the Ruby programming language.
 The benchmarks are found in the `benchmarks` directory. Individual Ruby files
 in `benchmarks` are microbenchmarks. Subdirectories under `benchmarks` are
 larger macrobenchmarks. Each benchmark relies on a harness found in
-[./harness/harness.rb](harness/harness.rb). The harness controls the number of times a benchmark is
+[./harness/default.rb](harness/default.rb). The harness controls the number of times a benchmark is
 run, and writes timing values into an output file.
 
 The `run_benchmarks.rb` script (optional) traverses the `benchmarks` directory and
@@ -97,8 +97,8 @@ This is the easiest way to run a single benchmark.
 It requires no setup at all and assumes nothing about the Ruby you are benchmarking.
 It's also convenient for profiling, debugging, etc, especially since all benchmarked code runs in that process.
 
-```
-ruby benchmarks/some_benchmark.rb
+```bash
+ruby benchmarks/fib.rb
 ```
 
 ### Benchmark organization
@@ -138,19 +138,19 @@ There are two Ractor-related categories:
 
 * **`--category ractor`** - Runs both regular benchmarks marked with `ractor:
   true` in `benchmarks.yml` AND all benchmarks from the `benchmarks-ractor`
-  directory. The `harness-ractor` harness is used for both types of benchmark.
+  directory. The `ractor` harness is used for both types of benchmark.
 
 * **`--category ractor-only`** - Runs ONLY benchmarks from the
   `benchmarks-ractor` directory, ignoring regular benchmarks even if they are
   marked with `ractor: true`. This category also automatically uses the
-  `harness-ractor` harness.
+  `ractor` harness.
 
 ### Directory Structure
 
 The `benchmarks-ractor/` directory sits at the same level as the main
 `benchmarks` directory, and contains Ractor-specific benchmark
 implementations that are designed to test Ractor functionality. They are not
-intended to be used with any harness except `harness-ractor`.
+intended to be used with any harness except `ractor`.
 
 ### Usage Examples
 
@@ -162,7 +162,7 @@ intended to be used with any harness except `harness-ractor`.
 ./run_benchmarks.rb --category ractor-only
 ```
 
-Note: The `harness-ractor` harness is automatically selected when using these
+Note: The `ractor` harness is automatically selected when using these
 categories, so there's no need to specify `--harness` manually.
 
 ## Ruby options
@@ -211,26 +211,74 @@ This file will then be passed to the underlying Ruby interpreter with
 
 ## Harnesses
 
-You can find several test harnesses in this repository:
+You can find several test harnesses in the `harness/` directory:
+
+* `default` - the normal default harness, with duration controlled by warmup iterations and time/count limits
+* `bips` - a harness that measures iterations/second until stable
+* `continuous` - a harness that adjusts the batch sizes of iterations to run in stable iteration size batches
+* `once` - a simplified harness that simply runs once
+* `perf` - a simplified harness that runs for exactly the hinted number of iterations
+* `stackprof` - a harness to profile the benchmark with stackprof
+* `stats` - count method calls and loop iterations
+* `vernier` - a harness to profile the benchmark with vernier
+* `warmup` - a harness which runs as long as needed to find warmed up (peak) performance
+* `chain` - a harness to chain multiple harnesses together
+* `mplr` - a harness for multiple iterations with time limits
+
+### Selecting a harness
+
+**Using the `HARNESS` environment variable**
+
+```bash
+# Run with specific harness
+HARNESS=perf ruby benchmarks/fib.rb
+HARNESS=stackprof ruby benchmarks/railsbench/benchmark.rb
+HARNESS=vernier ruby benchmarks/optcarrot/benchmark.rb
+
+# Combine with Ruby options
+HARNESS=perf ruby --yjit benchmarks/fib.rb
+HARNESS=once ruby --yjit-stats benchmarks/railsbench/benchmark.rb
+```
 
-* harness - the normal default harness, with duration controlled by warmup iterations and time/count limits
-* harness-bips - a harness that measures iterations/second until stable
-* harness-continuous - a harness that adjusts the batch sizes of iterations to run in stable iteration size batches
-* harness-once - a simplified harness that simply runs once
-* harness-perf - a simplified harness that runs for exactly the hinted number of iterations
-* harness-stackprof - a harness to profile the benchmark with stackprof
-* harness-stats - count method calls and loop iterations
-* harness-vernier - a harness to profile the benchmark with vernier
-* harness-warmup - a harness which runs as long as needed to find warmed up (peak) performance
+**Alternative: Use `run_once.rb` with `--harness` option**:
 
-To use it, run a benchmark script directly, specifying a harness directory with `-I`:
+```bash
+./run_once.rb --harness=perf benchmarks/railsbench/benchmark.rb
+./run_once.rb --harness=stackprof benchmarks/fib.rb
 
+# With Ruby options (use -- separator)
+./run_once.rb -- --yjit benchmarks/railsbench/benchmark.rb
+./run_once.rb --harness=perf -- --yjit-stats benchmarks/fib.rb
 ```
-ruby -Iharness benchmarks/railsbench/benchmark.rb
+
+When using `run_benchmarks.rb`, you can specify a harness with the `--harness` option:
+
+```bash
+./run_benchmarks.rb --harness=once
+./run_benchmarks.rb --harness=perf
 ```
 
 There is also a robust but complex CI harness in [the yjit-metrics repo](https://github.com/Shopify/yjit-metrics).
 
+### Chain harness
+
+The `chain` harness allows you to combine multiple harnesses together. This is useful when you want to run a benchmark through multiple analysis tools or measurement approaches in sequence.
+
+Use the `HARNESS_CHAIN` environment variable to specify which harnesses to chain (comma-separated, at least 2 required):
+
+```bash
+# Chain the 'once' and 'default' harnesses (runs benchmark once, then with default iterations)
+HARNESS=chain HARNESS_CHAIN="once,default" ruby benchmarks/fib.rb
+
+# Profile with vernier while using ractor harness
+HARNESS=chain HARNESS_CHAIN="vernier,ractor" ruby benchmarks-ractor/some_benchmark.rb
+
+# Using run_once.rb
+HARNESS_CHAIN="once,default" ./run_once.rb --harness=chain benchmarks/fib.rb
+```
+
+The harnesses are executed in the order specified, with each harness wrapping the previous one.
+
 ### Iterations and duration
 
 With the default harness, the number of iterations and duration
@@ -250,12 +298,14 @@ You can also use `--warmup`, `--bench`, or `--once` to set these environment var
 ./run_benchmarks.rb railsbench --once
 ```
 
-There is also a handy script for running benchmarks just once using
-`WARMUP_ITRS=0 MIN_BENCH_ITRS=1 MIN_BENCH_TIME=0`, for example
-with the `--yjit-stats` command-line option:
+You can also run a single benchmark directly with Ruby:
 
-```
-./run_once.sh --yjit-stats benchmarks/railsbench/benchmark.rb
+```bash
+# Run once with YJIT stats
+ruby --yjit-stats benchmarks/railsbench/benchmark.rb
+
+# Or use run_once.rb script
+./run_once.rb -- --yjit-stats benchmarks/railsbench/benchmark.rb
 ```
 
 ### Using perf
@@ -263,13 +313,12 @@ with the `--yjit-stats` command-line option:
 There is also a harness to use Linux perf. By default, it only runs a fixed number of iterations.
 If `PERF` environment variable is present, it starts the perf subcommand after warmup.
 
-```sh
+```bash
 # Use `perf record` for both warmup and benchmark
-perf record ruby --yjit-perf=map -Iharness-perf benchmarks/railsbench/benchmark.rb
+HARNESS=perf perf record ruby --yjit-perf=map benchmarks/railsbench/benchmark.rb
 
 # Use `perf record` only for benchmark
-PERF=record ruby --yjit-perf=map -Iharness-perf benchmarks/railsbench/benchmark.rb
-```
+HARNESS=perf PERF=record ruby --yjit-perf=map benchmarks/railsbench/benchmark.rb
 
 This is the only harness that uses `run_benchmark`'s argument, `num_itrs_hint`.
 
diff --git a/benchmarks-ractor/gvl_release_acquire/benchmark.rb b/benchmarks-ractor/gvl_release_acquire/benchmark.rb
index 3c368477..8963b54a 100644
--- a/benchmarks-ractor/gvl_release_acquire/benchmark.rb
+++ b/benchmarks-ractor/gvl_release_acquire/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 run_benchmark(5) do |num_rs, ractor_args|
   output = File.open("/dev/null", "wb")
diff --git a/benchmarks-ractor/json_parse_float/benchmark.rb b/benchmarks-ractor/json_parse_float/benchmark.rb
index ef81bc49..2aef033f 100644
--- a/benchmarks-ractor/json_parse_float/benchmark.rb
+++ b/benchmarks-ractor/json_parse_float/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks-ractor/json_parse_string/benchmark.rb b/benchmarks-ractor/json_parse_string/benchmark.rb
index a579f805..71417cd3 100644
--- a/benchmarks-ractor/json_parse_string/benchmark.rb
+++ b/benchmarks-ractor/json_parse_string/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks-ractor/knucleotide/benchmark.rb b/benchmarks-ractor/knucleotide/benchmark.rb
index ca5b9cd9..b8fbb34a 100644
--- a/benchmarks-ractor/knucleotide/benchmark.rb
+++ b/benchmarks-ractor/knucleotide/benchmark.rb
@@ -4,7 +4,7 @@
 # k-nucleotide benchmark - Ractor implementation
 # Mirrors the Process.fork version structure as closely as possible
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 def frequency(seq, length)
   frequencies = Hash.new(0)
diff --git a/benchmarks/30k_ifelse.rb b/benchmarks/30k_ifelse.rb
index ff9f32c3..a525fda4 100644
--- a/benchmarks/30k_ifelse.rb
+++ b/benchmarks/30k_ifelse.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 if ENV["RUBY_BENCH_RACTOR_HARNESS"]
   eval_recv = Object.new
   eval_meth = :instance_eval
diff --git a/benchmarks/30k_methods.rb b/benchmarks/30k_methods.rb
index f3d75f61..4d7bdccd 100644
--- a/benchmarks/30k_methods.rb
+++ b/benchmarks/30k_methods.rb
@@ -121011,7 +121011,7 @@ def fun_l29_n999()
   end
 end
 
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 INTERNAL_ITRS = Integer(ENV.fetch("INTERNAL_ITRS", 200))
 
diff --git a/benchmarks/activerecord/benchmark.rb b/benchmarks/activerecord/benchmark.rb
index e820d5ae..4f336daa 100644
--- a/benchmarks/activerecord/benchmark.rb
+++ b/benchmarks/activerecord/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/equality.rb b/benchmarks/addressable/equality.rb
index ca48dffe..95141423 100644
--- a/benchmarks/addressable/equality.rb
+++ b/benchmarks/addressable/equality.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/getters.rb b/benchmarks/addressable/getters.rb
index 301439f2..43d0d03a 100644
--- a/benchmarks/addressable/getters.rb
+++ b/benchmarks/addressable/getters.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/join.rb b/benchmarks/addressable/join.rb
index a4df7b39..4d600795 100644
--- a/benchmarks/addressable/join.rb
+++ b/benchmarks/addressable/join.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/merge.rb b/benchmarks/addressable/merge.rb
index 39c07f2d..20f33283 100644
--- a/benchmarks/addressable/merge.rb
+++ b/benchmarks/addressable/merge.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/new.rb b/benchmarks/addressable/new.rb
index 5fd9731e..ae5d44ae 100644
--- a/benchmarks/addressable/new.rb
+++ b/benchmarks/addressable/new.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/normalize.rb b/benchmarks/addressable/normalize.rb
index 121a8ad0..014c81fa 100644
--- a/benchmarks/addressable/normalize.rb
+++ b/benchmarks/addressable/normalize.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/parse.rb b/benchmarks/addressable/parse.rb
index 02d29a83..d95f1094 100644
--- a/benchmarks/addressable/parse.rb
+++ b/benchmarks/addressable/parse.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/setters.rb b/benchmarks/addressable/setters.rb
index 41ee88b3..3431bba3 100644
--- a/benchmarks/addressable/setters.rb
+++ b/benchmarks/addressable/setters.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/addressable/to-s.rb b/benchmarks/addressable/to-s.rb
index 355d931f..9f337ff7 100644
--- a/benchmarks/addressable/to-s.rb
+++ b/benchmarks/addressable/to-s.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/attr_accessor.rb b/benchmarks/attr_accessor.rb
index d5e397dc..1ab4ae7f 100644
--- a/benchmarks/attr_accessor.rb
+++ b/benchmarks/attr_accessor.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class TheClass
   attr_accessor :levar
diff --git a/benchmarks/binarytrees/benchmark.rb b/benchmarks/binarytrees/benchmark.rb
index 4e54528d..ca460c07 100644
--- a/benchmarks/binarytrees/benchmark.rb
+++ b/benchmarks/binarytrees/benchmark.rb
@@ -22,7 +22,7 @@ def bottom_up_tree(depth)
 MAX_DEPTH = MIN_DEPTH + 2 if MIN_DEPTH + 2 > MAX_DEPTH
 STRETCH_DEPTH = MAX_DEPTH + 1
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 run_benchmark(60) do
   max_depth = MAX_DEPTH
diff --git a/benchmarks/blurhash/benchmark.rb b/benchmarks/blurhash/benchmark.rb
index 1f081f4b..327fc225 100644
--- a/benchmarks/blurhash/benchmark.rb
+++ b/benchmarks/blurhash/benchmark.rb
@@ -172,7 +172,7 @@ def blurHashForPixels(xComponents, yComponents, width, height, rgb, bytesPerRow)
   # :startdoc:
 end
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 FILE = File.join(__dir__, "test.bin")
 
diff --git a/benchmarks/cfunc_itself.rb b/benchmarks/cfunc_itself.rb
index 2c39137e..8560da66 100644
--- a/benchmarks/cfunc_itself.rb
+++ b/benchmarks/cfunc_itself.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 run_benchmark(500) do
   # 500K calls
diff --git a/benchmarks/chunky-png/benchmark.rb b/benchmarks/chunky-png/benchmark.rb
index ef511893..29054d29 100644
--- a/benchmarks/chunky-png/benchmark.rb
+++ b/benchmarks/chunky-png/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/erubi-rails/benchmark.rb b/benchmarks/erubi-rails/benchmark.rb
index 38c22634..1817a0f8 100644
--- a/benchmarks/erubi-rails/benchmark.rb
+++ b/benchmarks/erubi-rails/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 Dir.chdir __dir__
 use_gemfile
 
diff --git a/benchmarks/erubi/benchmark.rb b/benchmarks/erubi/benchmark.rb
index 0a75f1bb..f62a2f3d 100644
--- a/benchmarks/erubi/benchmark.rb
+++ b/benchmarks/erubi/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 Dir.chdir __dir__
 use_gemfile
 
diff --git a/benchmarks/etanni/benchmark.rb b/benchmarks/etanni/benchmark.rb
index 77e86386..48c41f1a 100644
--- a/benchmarks/etanni/benchmark.rb
+++ b/benchmarks/etanni/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 Dir.chdir __dir__
 
 # This is an Etanni translation of the Erb template in the Erubi
diff --git a/benchmarks/fannkuchredux/benchmark.rb b/benchmarks/fannkuchredux/benchmark.rb
index e73dc0ee..2e44c7d1 100644
--- a/benchmarks/fannkuchredux/benchmark.rb
+++ b/benchmarks/fannkuchredux/benchmark.rb
@@ -56,7 +56,7 @@ def fannkuch(n)
 #n = (ARGV[0] || 1).to_i
 N = 9 # Benchmarks Game uses n = 12, but it's too slow
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 run_benchmark(10) do
   sum, flips = fannkuch(N)
diff --git a/benchmarks/fib.rb b/benchmarks/fib.rb
index 4e5b3428..02be31b8 100644
--- a/benchmarks/fib.rb
+++ b/benchmarks/fib.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def fib(n)
   if n < 2
diff --git a/benchmarks/fluentd/benchmark.rb b/benchmarks/fluentd/benchmark.rb
index c503a653..4e3f9a6d 100644
--- a/benchmarks/fluentd/benchmark.rb
+++ b/benchmarks/fluentd/benchmark.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks/getivar.rb b/benchmarks/getivar.rb
index 80acfae1..d77cc33d 100644
--- a/benchmarks/getivar.rb
+++ b/benchmarks/getivar.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class TheClass
   def initialize
diff --git a/benchmarks/graphql-native/benchmark.rb b/benchmarks/graphql-native/benchmark.rb
index d4fd7077..ff5858c9 100644
--- a/benchmarks/graphql-native/benchmark.rb
+++ b/benchmarks/graphql-native/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/graphql/benchmark.rb b/benchmarks/graphql/benchmark.rb
index 85b5da1f..26c1feb6 100644
--- a/benchmarks/graphql/benchmark.rb
+++ b/benchmarks/graphql/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/hexapdf/benchmark.rb b/benchmarks/hexapdf/benchmark.rb
index 9609139b..0195d733 100644
--- a/benchmarks/hexapdf/benchmark.rb
+++ b/benchmarks/hexapdf/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 Dir.chdir __dir__
 use_gemfile
 
diff --git a/benchmarks/keyword_args.rb b/benchmarks/keyword_args.rb
index a9b035f3..386ef894 100644
--- a/benchmarks/keyword_args.rb
+++ b/benchmarks/keyword_args.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def add(left:, right:)
   left + right
diff --git a/benchmarks/knucleotide/benchmark.rb b/benchmarks/knucleotide/benchmark.rb
index cf6234b4..affd44f1 100644
--- a/benchmarks/knucleotide/benchmark.rb
+++ b/benchmarks/knucleotide/benchmark.rb
@@ -4,7 +4,7 @@
 # k-nucleotide benchmark - Fastest implementation
 # Based on Ruby #1 with byteslice optimization
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 def frequency(seq, length)
   frequencies = Hash.new(0)
diff --git a/benchmarks/lee/benchmark.rb b/benchmarks/lee/benchmark.rb
index 2abdd945..7e052339 100644
--- a/benchmarks/lee/benchmark.rb
+++ b/benchmarks/lee/benchmark.rb
@@ -83,7 +83,7 @@ def lay(depth, solution)
   end
 end
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 Dir.chdir __dir__
 use_gemfile
 
diff --git a/benchmarks/liquid-c/benchmark.rb b/benchmarks/liquid-c/benchmark.rb
index e074e33c..4a649dff 100644
--- a/benchmarks/liquid-c/benchmark.rb
+++ b/benchmarks/liquid-c/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/liquid-compile/benchmark.rb b/benchmarks/liquid-compile/benchmark.rb
index 0e0d0f1c..956e0fb8 100644
--- a/benchmarks/liquid-compile/benchmark.rb
+++ b/benchmarks/liquid-compile/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/liquid-render/benchmark.rb b/benchmarks/liquid-render/benchmark.rb
index 661015a8..b851474c 100644
--- a/benchmarks/liquid-render/benchmark.rb
+++ b/benchmarks/liquid-render/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/lobsters/benchmark.rb b/benchmarks/lobsters/benchmark.rb
index 3a28fe81..63c36100 100644
--- a/benchmarks/lobsters/benchmark.rb
+++ b/benchmarks/lobsters/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 ENV['RAILS_ENV'] ||= 'production'
 ENV['DISABLE_DATABASE_ENVIRONMENT_CHECK'] = '1' # Benchmarks don't really have 'production', so trash it at will.
diff --git a/benchmarks/loops-times.rb b/benchmarks/loops-times.rb
index 653fe989..22c196bd 100644
--- a/benchmarks/loops-times.rb
+++ b/benchmarks/loops-times.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 # Fix these values for determinism
 U = 5
diff --git a/benchmarks/mail/benchmark.rb b/benchmarks/mail/benchmark.rb
index 0d625650..9bef261d 100644
--- a/benchmarks/mail/benchmark.rb
+++ b/benchmarks/mail/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/matmul.rb b/benchmarks/matmul.rb
index c4087dbb..80187ade 100644
--- a/benchmarks/matmul.rb
+++ b/benchmarks/matmul.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def matgen(n)
   tmp = 1.0 / n / n
diff --git a/benchmarks/nbody/benchmark.rb b/benchmarks/nbody/benchmark.rb
index 737e298b..86a11a3c 100644
--- a/benchmarks/nbody/benchmark.rb
+++ b/benchmarks/nbody/benchmark.rb
@@ -5,7 +5,7 @@
 # modified by Jesse Millikan
 # modified by Yusuke Endoh 
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 SOLAR_MASS = 4 * Math::PI**2
 DAYS_PER_YEAR = 365.24
diff --git a/benchmarks/nqueens.rb b/benchmarks/nqueens.rb
index 9a1b7a8c..0912a64a 100644
--- a/benchmarks/nqueens.rb
+++ b/benchmarks/nqueens.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def nq_solve(n)
   a = Array.new(n, -1)
diff --git a/benchmarks/object-new.rb b/benchmarks/object-new.rb
index b079c393..ed3b37b8 100644
--- a/benchmarks/object-new.rb
+++ b/benchmarks/object-new.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 run_benchmark(100) do
   i = 0
diff --git a/benchmarks/optcarrot/benchmark.rb b/benchmarks/optcarrot/benchmark.rb
index 3c48515f..29e6e58f 100755
--- a/benchmarks/optcarrot/benchmark.rb
+++ b/benchmarks/optcarrot/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 require_relative "lib/optcarrot"
 
 if ENV["RUBY_BENCH_RACTOR_HARNESS"]
diff --git a/benchmarks/protoboeuf-encode/benchmark.rb b/benchmarks/protoboeuf-encode/benchmark.rb
index a7afcdb7..16551acc 100644
--- a/benchmarks/protoboeuf-encode/benchmark.rb
+++ b/benchmarks/protoboeuf-encode/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 # Protoboeuf decoder
 require_relative 'benchmark_pb'
diff --git a/benchmarks/protoboeuf/benchmark.rb b/benchmarks/protoboeuf/benchmark.rb
index 6e3078f4..9f477b79 100644
--- a/benchmarks/protoboeuf/benchmark.rb
+++ b/benchmarks/protoboeuf/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 # Protoboeuf decoder
 require_relative 'benchmark_pb'
diff --git a/benchmarks/psych-load/benchmark.rb b/benchmarks/psych-load/benchmark.rb
index 795135f3..08b99c64 100644
--- a/benchmarks/psych-load/benchmark.rb
+++ b/benchmarks/psych-load/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/benchmarks/rack/benchmark.rb b/benchmarks/rack/benchmark.rb
index a3fde656..57c25052 100644
--- a/benchmarks/rack/benchmark.rb
+++ b/benchmarks/rack/benchmark.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks/railsbench/benchmark.rb b/benchmarks/railsbench/benchmark.rb
index 89561d48..46b3a270 100644
--- a/benchmarks/railsbench/benchmark.rb
+++ b/benchmarks/railsbench/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 ENV['RAILS_ENV'] ||= 'production'
 
diff --git a/benchmarks/respond_to.rb b/benchmarks/respond_to.rb
index 9a419060..55f49f70 100644
--- a/benchmarks/respond_to.rb
+++ b/benchmarks/respond_to.rb
@@ -1,7 +1,7 @@
 # Microbenchmark to test the performance of respond_to?
 # This is one of the top most called methods in rack/railsbench
 
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class A
   def foo
diff --git a/benchmarks/rubocop/benchmark.rb b/benchmarks/rubocop/benchmark.rb
index 60b49d05..12f415a0 100644
--- a/benchmarks/rubocop/benchmark.rb
+++ b/benchmarks/rubocop/benchmark.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks/ruby-json/benchmark.rb b/benchmarks/ruby-json/benchmark.rb
index 3c2d3e4e..f8c085b8 100644
--- a/benchmarks/ruby-json/benchmark.rb
+++ b/benchmarks/ruby-json/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 require "json"
 require "strscan"
diff --git a/benchmarks/ruby-lsp/benchmark.rb b/benchmarks/ruby-lsp/benchmark.rb
index 232a0e8d..0bbc304b 100644
--- a/benchmarks/ruby-lsp/benchmark.rb
+++ b/benchmarks/ruby-lsp/benchmark.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks/ruby-xor.rb b/benchmarks/ruby-xor.rb
index 5352c76a..2a9f9d1b 100644
--- a/benchmarks/ruby-xor.rb
+++ b/benchmarks/ruby-xor.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 # XOR strings in place
 # Expected to behave like Xorcist.xor! from the xorcist gem
diff --git a/benchmarks/rubyboy/benchmark.rb b/benchmarks/rubyboy/benchmark.rb
index 54b0d5e1..36d68f96 100644
--- a/benchmarks/rubyboy/benchmark.rb
+++ b/benchmarks/rubyboy/benchmark.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir(__dir__)
 use_gemfile
diff --git a/benchmarks/rubykon/benchmark.rb b/benchmarks/rubykon/benchmark.rb
index e8f0e527..d312daaf 100644
--- a/benchmarks/rubykon/benchmark.rb
+++ b/benchmarks/rubykon/benchmark.rb
@@ -2,7 +2,7 @@
 # https://github.com/benchmark-driver/sky2-bench/blob/master/benchmark/rubykon-benchmark.rb,
 # part of benchmark-driver's default benchmarking suite, by Takashi Kokubun.
 
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 require_relative 'lib/rubykon'
 
diff --git a/benchmarks/send_bmethod.rb b/benchmarks/send_bmethod.rb
index 649df891..07774ea0 100644
--- a/benchmarks/send_bmethod.rb
+++ b/benchmarks/send_bmethod.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 # Call with and without args
 define_method(:zero) { :b }
diff --git a/benchmarks/send_cfunc_block.rb b/benchmarks/send_cfunc_block.rb
index d6013f29..f0c954f7 100644
--- a/benchmarks/send_cfunc_block.rb
+++ b/benchmarks/send_cfunc_block.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 ARR = []
 
diff --git a/benchmarks/send_rubyfunc_block.rb b/benchmarks/send_rubyfunc_block.rb
index 6cdeba9b..22b18d19 100644
--- a/benchmarks/send_rubyfunc_block.rb
+++ b/benchmarks/send_rubyfunc_block.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class C
   def ruby_func
diff --git a/benchmarks/sequel/benchmark.rb b/benchmarks/sequel/benchmark.rb
index 81b07c67..7c2491b4 100644
--- a/benchmarks/sequel/benchmark.rb
+++ b/benchmarks/sequel/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 require "securerandom" # Provides `Random::Formatter` in Ruby 2.6+
 
 Dir.chdir __dir__
diff --git a/benchmarks/setivar.rb b/benchmarks/setivar.rb
index e64dca4d..9e9ebf3a 100644
--- a/benchmarks/setivar.rb
+++ b/benchmarks/setivar.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class TheClass
   def initialize
diff --git a/benchmarks/setivar_object.rb b/benchmarks/setivar_object.rb
index 6f34cba6..0c316833 100644
--- a/benchmarks/setivar_object.rb
+++ b/benchmarks/setivar_object.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class TheClass
   def initialize
diff --git a/benchmarks/setivar_young.rb b/benchmarks/setivar_young.rb
index 2f673756..d463d811 100644
--- a/benchmarks/setivar_young.rb
+++ b/benchmarks/setivar_young.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 class TheClass
   def initialize
diff --git a/benchmarks/shipit/benchmark.rb b/benchmarks/shipit/benchmark.rb
index 607498d0..9e88b22d 100644
--- a/benchmarks/shipit/benchmark.rb
+++ b/benchmarks/shipit/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative '../../harness/loader'
+require_relative '../../lib/harness/loader'
 
 ENV['RAILS_ENV'] ||= 'production'
 ENV['DISABLE_DATABASE_ENVIRONMENT_CHECK'] = '1' # Benchmarks don't really have 'production', so trash it at will.
diff --git a/benchmarks/str_concat.rb b/benchmarks/str_concat.rb
index ee514cc4..9bbeaf2b 100644
--- a/benchmarks/str_concat.rb
+++ b/benchmarks/str_concat.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 NUM_ITERS = 10 * 1024
 TEST_STR = 'sssssséé'.freeze
diff --git a/benchmarks/structaref.rb b/benchmarks/structaref.rb
index 95bb3575..b319092e 100644
--- a/benchmarks/structaref.rb
+++ b/benchmarks/structaref.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 TheClass = Struct.new(:v0, :v1, :v2, :levar)
 
diff --git a/benchmarks/structaset.rb b/benchmarks/structaset.rb
index 5d73035e..bcff9b0e 100644
--- a/benchmarks/structaset.rb
+++ b/benchmarks/structaset.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 TheClass = Struct.new(:v0, :v1, :v2, :levar)
 
diff --git a/benchmarks/sudoku.rb b/benchmarks/sudoku.rb
index 62308b36..e8a298e0 100644
--- a/benchmarks/sudoku.rb
+++ b/benchmarks/sudoku.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def sd_genmat
   mr = Array.new(324) { [] }
diff --git a/benchmarks/throw.rb b/benchmarks/throw.rb
index 3d3a7b2b..e3aa8b9c 100644
--- a/benchmarks/throw.rb
+++ b/benchmarks/throw.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/loader'
+require_relative '../lib/harness/loader'
 
 def foo
   yield
diff --git a/benchmarks/tinygql/benchmark.rb b/benchmarks/tinygql/benchmark.rb
index 7195a9ed..8457772d 100644
--- a/benchmarks/tinygql/benchmark.rb
+++ b/benchmarks/tinygql/benchmark.rb
@@ -1,4 +1,4 @@
-require_relative "../../harness/loader"
+require_relative "../../lib/harness/loader"
 
 Dir.chdir __dir__
 use_gemfile
diff --git a/burn_in.rb b/burn_in.rb
index ea305eb8..f1d66b46 100755
--- a/burn_in.rb
+++ b/burn_in.rb
@@ -63,9 +63,9 @@ def run_benchmark(bench_id, no_yjit, logs_path, run_time, ruby_version)
   # Determine the path to the benchmark script
   bench_name = bench_id.sub('ractor/', '')
   bench_dir, harness = if bench_name == bench_id
-    ['benchmarks', 'harness']
+    ['benchmarks', 'default']
   else
-    ['benchmarks-ractor', 'harness-ractor']
+    ['benchmarks-ractor', 'ractor']
   end
 
   script_path = File.join(bench_dir, bench_name, 'benchmark.rb')
@@ -108,7 +108,8 @@ def run_benchmark(bench_id, no_yjit, logs_path, run_time, ruby_version)
   cmd = [
     'ruby',
     *test_options,
-    "-I#{harness}",
+    "-Iharness",
+    "-r#{harness}",
     script_path,
   ].compact
   cmd_str = cmd.shelljoin
diff --git a/harness-bips/harness.rb b/harness/bips.rb
similarity index 79%
rename from harness-bips/harness.rb
rename to harness/bips.rb
index d1319a19..dddb546e 100644
--- a/harness-bips/harness.rb
+++ b/harness/bips.rb
@@ -1,5 +1,5 @@
 require 'benchmark/ips'
-require_relative "../harness/harness-common"
+require_relative "../lib/harness"
 
 puts RUBY_DESCRIPTION
 
diff --git a/harness-chain/harness.rb b/harness/chain.rb
similarity index 82%
rename from harness-chain/harness.rb
rename to harness/chain.rb
index 48e6b0f9..20e1fa66 100644
--- a/harness-chain/harness.rb
+++ b/harness/chain.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/harness-common'
+require_relative '../lib/harness'
 
 # Ex: HARNESS_CHAIN="vernier,ractor"
 # Wraps the ractor harness in ther vernier harness
@@ -10,7 +10,7 @@
 end
 
 if CHAIN.include?("vernier") && CHAIN.last != "vernier"
-  require_relative "../harness/harness-extra"
+  require_relative "../lib/harness/extra"
   def run_enough_to_profile(n, **kwargs, &block)
     block.call
   end
@@ -30,12 +30,13 @@ def self.method_added(name)
 def run_benchmark(n, **kwargs, &block)
   CHAIN.each do |h|
     begin
-      path = "../harness-#{h}/harness"
+      path = "#{h}"
       $current_harness = h
       require_relative path
     rescue LoadError => e
-      if e.path == path
-        $stderr.puts "Can't find harness harness-#{h}/harness.rb. Exiting."
+      expected_path = File.expand_path(h, __dir__)
+      if e.path == expected_path
+        $stderr.puts "Can't find harness #{h}.rb in harness/. Exiting."
         exit 1
       end
       raise
diff --git a/harness-continuous/harness.rb b/harness/continuous.rb
similarity index 94%
rename from harness-continuous/harness.rb
rename to harness/continuous.rb
index 6606b182..05a9162f 100644
--- a/harness-continuous/harness.rb
+++ b/harness/continuous.rb
@@ -1,4 +1,4 @@
-require_relative "../harness/harness-common"
+require_relative "../lib/harness"
 
 puts RUBY_DESCRIPTION
 
diff --git a/harness/harness.rb b/harness/default.rb
similarity index 98%
rename from harness/harness.rb
rename to harness/default.rb
index 8e660296..077e7601 100644
--- a/harness/harness.rb
+++ b/harness/default.rb
@@ -1,4 +1,4 @@
-require_relative "./harness-common"
+require_relative "../lib/harness"
 
 # Warmup iterations
 WARMUP_ITRS = Integer(ENV.fetch('WARMUP_ITRS', 15))
diff --git a/harness/loader.rb b/harness/loader.rb
deleted file mode 100644
index 84de6383..00000000
--- a/harness/loader.rb
+++ /dev/null
@@ -1,13 +0,0 @@
-# Use harness/harness.rb by default. You can change it with -I option.
-# i.e. ruby -Iharness benchmarks/railsbench/benchmark.rb
-retries = 0
-begin
-  require "harness"
-rescue LoadError => e
-  if retries == 0 && e.path == "harness"
-    retries += 1
-    $LOAD_PATH << File.expand_path(__dir__)
-    retry
-  end
-  raise
-end
diff --git a/harness-mplr/harness.rb b/harness/mplr.rb
similarity index 93%
rename from harness-mplr/harness.rb
rename to harness/mplr.rb
index 73234934..f91bfd94 100644
--- a/harness-mplr/harness.rb
+++ b/harness/mplr.rb
@@ -1,5 +1,5 @@
 require 'benchmark'
-require_relative "../harness/harness-common"
+require_relative "../lib/harness"
 
 # Minimum number of benchmarking iterations
 MAX_BENCH_ITRS = Integer(ENV.fetch('MAX_BENCH_ITRS', 1000))
diff --git a/harness-once/harness.rb b/harness/once.rb
similarity index 88%
rename from harness-once/harness.rb
rename to harness/once.rb
index c03d9db5..29b3340a 100644
--- a/harness-once/harness.rb
+++ b/harness/once.rb
@@ -5,7 +5,7 @@
 # Intended only for checking whether the benchmark can set itself up properly
 # and can run to completion.
 
-require_relative '../harness/harness-common'
+require_relative '../lib/harness'
 
 def run_benchmark(_hint, **)
   yield
diff --git a/harness-perf/harness.rb b/harness/perf.rb
similarity index 94%
rename from harness-perf/harness.rb
rename to harness/perf.rb
index 4991cb54..e13978ea 100644
--- a/harness-perf/harness.rb
+++ b/harness/perf.rb
@@ -3,14 +3,14 @@
 # This is a relatively minimal harness meant for use with Linux perf(1).
 # Example usage:
 #
-#    $ PERF='record -e cycles' ruby -Iharness-perf benchmarks/fib.rb
+#    $ PERF='record -e cycles' HARNESS=perf ruby benchmarks/fib.rb
 #
 # When recording with perf(1), make sure the benchmark runs long enough; you
 # can tweak the MIN_BENCH_ITRS environment variable to lengthen the run. A race
 # condition is possible where the benchmark finishes before the perf(1)
 # subprocess has a chance to attach, in which case perf outputs no profile.
 
-require_relative "../harness/harness-common"
+require_relative "../lib/harness"
 
 # Run $WARMUP_ITRS or 10 iterations of a given block. Then run $MIN_BENCH_ITRS
 # or `num_itrs_int` iterations of the block, attaching a perf command to the
diff --git a/harness-ractor/harness.rb b/harness/ractor.rb
similarity index 98%
rename from harness-ractor/harness.rb
rename to harness/ractor.rb
index fbfb7fb9..a2858349 100644
--- a/harness-ractor/harness.rb
+++ b/harness/ractor.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative '../harness/harness-common'
+require_relative '../lib/harness'
 
 Warning[:experimental] = false
 ENV["RUBY_BENCH_RACTOR_HARNESS"] = "1"
diff --git a/harness-stackprof/harness.rb b/harness/stackprof.rb
similarity index 85%
rename from harness-stackprof/harness.rb
rename to harness/stackprof.rb
index 6a63d1c5..8a33524e 100644
--- a/harness-stackprof/harness.rb
+++ b/harness/stackprof.rb
@@ -3,11 +3,11 @@
 # Profile the benchmark (ignoring initialization code) with stackprof.
 # Customize stackprof options with an env var of STACKPROF_OPTS='key:value,...'.
 # Usage:
-# STACKPROF_OPTS='mode:object' MIN_BENCH_TIME=0 MIN_BENCH_ITRS=1 ruby -v -I harness-stackprof benchmarks/.../benchmark.rb
-# STACKPROF_OPTS='mode:cpu,interval:10' MIN_BENCH_TIME=1 MIN_BENCH_ITRS=10 ruby -v -I harness-stackprof benchmarks/.../benchmark.rb
+# STACKPROF_OPTS='mode:object' HARNESS=stackprof MIN_BENCH_TIME=0 MIN_BENCH_ITRS=1 ruby -v -I harness-stackprof benchmarks/.../benchmark.rb
+# STACKPROF_OPTS='mode:cpu,interval:10' MIN_BENCH_ITRS=10 HARNESS=stackprof MIN_BENCH_TIME=1 MIN_BENCH_ITRS=10 ruby -v -I harness-stackprof benchmarks/.../benchmark.rb
 
-require_relative "../harness/harness-common"
-require_relative "../harness/harness-extra"
+require_relative "../lib/harness"
+require_relative "../lib/harness/extra"
 
 ensure_global_gem("stackprof")
 
diff --git a/harness-stats/harness.rb b/harness/stats.rb
similarity index 98%
rename from harness-stats/harness.rb
rename to harness/stats.rb
index a0b9a2a3..2971c667 100644
--- a/harness-stats/harness.rb
+++ b/harness/stats.rb
@@ -1,4 +1,4 @@
-require_relative '../harness/harness'
+require_relative 'default'
 
 # Using Module#prepend to enable TracePoint right before #run_benchmark
 # while also reusing the original implementation.
diff --git a/harness-vernier/harness.rb b/harness/vernier.rb
similarity index 70%
rename from harness-vernier/harness.rb
rename to harness/vernier.rb
index fe32ff1c..186da9de 100644
--- a/harness-vernier/harness.rb
+++ b/harness/vernier.rb
@@ -3,11 +3,11 @@
 # Profile the benchmark (ignoring initialization code) using vernier and display the profile.
 # Set NO_VIERWER=1 to disable automatically opening the profile in a browser.
 # Usage:
-# MIN_BENCH_TIME=1 MIN_BENCH_ITRS=1 ruby -v -I harness-vernier benchmarks/...
-# NO_VIEWER=1 MIN_BENCH_TIME=1 MIN_BENCH_ITRS=1 ruby -v -I harness-vernier benchmarks/...
+# HARNESS=vernier MIN_BENCH_TIME=1 MIN_BENCH_ITRS=1 ruby -v -I harness-vernier benchmarks/...
+# NO_VIEWER=1 HARNESS=vernier MIN_BENCH_TIME=1 MIN_BENCH_ITRS=1 ruby -v -I harness-vernier benchmarks/...
 
-require_relative "../harness/harness-common"
-require_relative "../harness/harness-extra"
+require_relative "../lib/harness"
+require_relative "../lib/harness/extra"
 
 ensure_global_gem("vernier")
 ensure_global_gem_exe("profile-viewer")
diff --git a/harness-warmup/harness.rb b/harness/warmup.rb
similarity index 97%
rename from harness-warmup/harness.rb
rename to harness/warmup.rb
index 21045227..fbcc6bbf 100644
--- a/harness-warmup/harness.rb
+++ b/harness/warmup.rb
@@ -1,5 +1,5 @@
 require 'benchmark'
-require_relative '../harness/harness-common'
+require_relative '../lib/harness'
 require_relative '../misc/stats'
 
 MIN_ITERS = Integer(ENV['MIN_ITERS'] || 10)
diff --git a/lib/argument_parser.rb b/lib/argument_parser.rb
index 35de87c1..e79ce7a1 100644
--- a/lib/argument_parser.rb
+++ b/lib/argument_parser.rb
@@ -71,7 +71,7 @@ def parse(argv)
       opts.on("--category=headline,other,micro,ractor", "when given, only benchmarks with specified categories will run") do |v|
         args.categories += v.split(",")
         if args.categories == ["ractor"]
-          args.harness = "harness-ractor"
+          args.harness = "ractor"
         end
       end
 
@@ -91,8 +91,9 @@ def parse(argv)
         args.skip_yjit = true
       end
 
-      opts.on("--harness=HARNESS_DIR", "which harness to use") do |v|
-        v = "harness-#{v}" unless v.start_with?('harness')
+      opts.on("--harness=HARNESS_NAME", "which harness to use (default: harness, options: once, bips, perf, ractor, stackprof, vernier, warmup, stats, continuous, chain, mplr)") do |v|
+        # Strip 'harness-' prefix if provided for backward compatibility
+        v = v.sub(/^harness-/, '')
         args.harness = v
       end
 
@@ -175,7 +176,7 @@ def default_args
       executables: {},
       out_path: File.expand_path("./data"),
       out_override: nil,
-      harness: "harness",
+      harness: "default",
       yjit_opts: "",
       categories: [],
       name_filters: [],
diff --git a/lib/benchmark_suite.rb b/lib/benchmark_suite.rb
index 30c4a853..eeda2250 100644
--- a/lib/benchmark_suite.rb
+++ b/lib/benchmark_suite.rb
@@ -17,7 +17,8 @@ class BenchmarkSuite
   RACTOR_BENCHMARKS_DIR = "benchmarks-ractor"
   RACTOR_ONLY_CATEGORY = ["ractor-only"].freeze
   RACTOR_CATEGORY = ["ractor"].freeze
-  RACTOR_HARNESS = "harness-ractor"
+  RACTOR_HARNESS = "ractor"
+  HARNESS_DIR = File.expand_path("../harness", __dir__)
 
   attr_reader :categories, :name_filters, :excludes, :out_path, :harness, :pre_init, :no_pinning, :bench_dir, :ractor_bench_dir
 
@@ -32,6 +33,7 @@ def initialize(categories:, name_filters:, excludes: [], out_path:, harness:, pr
     @ractor_only = (categories == RACTOR_ONLY_CATEGORY)
 
     setup_benchmark_directories
+    @harness_args = build_harness_args
   end
 
   # Run all the benchmarks and record execution times
@@ -62,6 +64,8 @@ def run(ruby:, ruby_description:)
 
   private
 
+  attr_reader :harness_args
+
   def setup_benchmark_directories
     if @ractor_only
       @bench_dir = RACTOR_BENCHMARKS_DIR
@@ -152,7 +156,7 @@ def run_single_benchmark(script_path, result_json_path, ruby, cmd_prefix, env)
     # Set up the benchmarking command
     cmd = cmd_prefix + [
       *ruby,
-      "-I", harness,
+      *harness_args,
       *pre_init,
       script_path,
     ].compact
@@ -226,6 +230,17 @@ def setarch_prefix
     prefix
   end
 
+  # If harness is 'default', use default (no -r needed)
+  # Otherwise use -r to load the specific harness file with full path
+  def build_harness_args
+    if harness == "default"
+      []
+    else
+      harness_path = File.join(HARNESS_DIR, harness)
+      ["-r", harness_path]
+    end
+  end
+
   # Resolve the pre_init file path into a form that can be required
   def expand_pre_init(path)
     path = Pathname.new(path)
diff --git a/harness/harness-common.rb b/lib/harness.rb
similarity index 100%
rename from harness/harness-common.rb
rename to lib/harness.rb
diff --git a/harness/harness-extra.rb b/lib/harness/extra.rb
similarity index 95%
rename from harness/harness-extra.rb
rename to lib/harness/extra.rb
index 18a3654f..99f3cbf4 100644
--- a/harness/harness-extra.rb
+++ b/lib/harness/extra.rb
@@ -33,11 +33,12 @@ def benchmark_name
 # Get name of harness (stackprof, vernier, etc) from the file path of the loaded harness.
 def harness_name
   $LOADED_FEATURES.reverse_each do |feat|
-    if m = feat.match(%r{/harness-([^/]+)/harness\.rb$})
+    if m = feat.match(%r{/harness/([^/]+)\.rb$})
       return m[1]
     end
   end
-  raise "Unable to determine harness name"
+  # Default to 'harness' if we can't determine the name
+  'harness'
 end
 
 # Share a single timestamp for everything from this execution.
diff --git a/lib/harness/loader.rb b/lib/harness/loader.rb
new file mode 100644
index 00000000..5768bd59
--- /dev/null
+++ b/lib/harness/loader.rb
@@ -0,0 +1,33 @@
+# Use harness/default.rb by default. You can change it with:
+# 1. HARNESS environment variable (preferred, keeps harness close to code)
+# 2. --harness option in run_once.rb
+# 3. -r option when calling Ruby directly
+#
+# Examples:
+#   HARNESS=perf ruby benchmarks/railsbench/benchmark.rb              # uses harness/perf.rb
+#   HARNESS=once ruby benchmarks/railsbench/benchmark.rb              # uses harness/once.rb
+#   ./run_once.rb --harness=ractor benchmarks/railsbench/benchmark.rb # uses harness/ractor.rb
+
+# Only load the default harness if no other harness has defined run_benchmark
+unless defined?(run_benchmark)
+  harness_name = ENV['HARNESS'] || 'default'
+
+  retries = 0
+  begin
+    require harness_name
+  rescue LoadError => e
+    if retries == 0 && e.path == harness_name
+      retries += 1
+      # Add the harness directory to the load path
+      $LOAD_PATH << File.expand_path("../../harness", __dir__)
+      retry
+    end
+    # Provide helpful error message for invalid harness
+    if e.path == harness_name
+      harness_dir = File.expand_path("../../harness", __dir__)
+      available = Dir.glob("#{harness_dir}/*.rb").map { |f| File.basename(f, '.rb') }.sort
+      raise LoadError, "Harness '#{harness_name}' not found. Available harnesses: #{available.join(', ')}"
+    end
+    raise
+  end
+end
diff --git a/run_once.rb b/run_once.rb
new file mode 100755
index 00000000..9576fdd1
--- /dev/null
+++ b/run_once.rb
@@ -0,0 +1,90 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Script to run a single benchmark once
+# Provides a clean interface for running benchmarks with different harnesses.
+# Examples:
+#   ./run_once.rb benchmarks/railsbench/benchmark.rb
+#   ./run_once.rb --harness=once benchmarks/fib.rb
+#   ./run_once.rb --harness=stackprof benchmarks/fib.rb
+#   ./run_once.rb -- --yjit-stats benchmarks/railsbench/benchmark.rb
+#   ./run_once.rb --harness=default -- --yjit benchmarks/fib.rb
+
+require 'optparse'
+require 'shellwords'
+
+# Parse options
+harness = nil
+ruby_args = []
+benchmark_file = nil
+
+parser = OptionParser.new do |opts|
+  opts.banner = "Usage: #{$0} [options] [--] [ruby-options] BENCHMARK_FILE"
+
+  opts.on("--harness=HARNESS", "Harness to use (default: default, options: once, bips, perf, ractor, stackprof, vernier, warmup, stats, continuous, chain, mplr)") do |h|
+    harness = h
+  end
+
+  opts.on("-h", "--help", "Show this help message") do
+    puts opts
+    exit
+  end
+end
+
+# Parse our options - this will stop at '--' or first non-option argument
+begin
+  parser.parse!
+rescue OptionParser::InvalidOption => e
+  puts "Error: #{e.message}"
+  puts parser
+  exit 1
+end
+
+# After parsing, ARGV contains remaining args (Ruby options + benchmark file)
+ARGV.each do |arg|
+  if arg.end_with?('.rb') && !benchmark_file
+    benchmark_file = arg
+  else
+    ruby_args << arg
+  end
+end
+
+if !benchmark_file
+  puts "Error: No benchmark file specified"
+  puts parser
+  exit 1
+end
+
+unless File.exist?(benchmark_file)
+  puts "Error: Benchmark file not found: #{benchmark_file}"
+  exit 1
+end
+
+# Automatically detect ractor benchmarks
+if !harness && benchmark_file.include?('benchmarks-ractor/')
+  harness = 'ractor'
+end
+
+# Build the command
+harness_dir = File.expand_path('harness', __dir__)
+harness_args = if harness && harness != 'default'
+  harness_path = File.join(harness_dir, harness)
+  unless File.exist?("#{harness_path}.rb")
+    puts "Error: Harness not found: #{harness}"
+    puts "Available harnesses: #{Dir.glob("#{harness_dir}/*.rb").map { |f| File.basename(f, '.rb') }.join(', ')}"
+    exit 1
+  end
+  ['-r', harness_path]
+else
+  []
+end
+
+# Set environment for running once
+ENV['WARMUP_ITRS'] = '0'
+ENV['MIN_BENCH_ITRS'] = '1'
+ENV['MIN_BENCH_TIME'] = '0'
+
+# Build and execute the command
+cmd = ['ruby', *ruby_args, *harness_args, benchmark_file]
+
+exec(*cmd)
diff --git a/run_once.sh b/run_once.sh
deleted file mode 100755
index d2035e8d..00000000
--- a/run_once.sh
+++ /dev/null
@@ -1,18 +0,0 @@
-#!/usr/bin/env bash
-# Script to run a single benchmark once
-# You can pass --yjit-stats and other ruby arguments to this script.
-# Automatically detects Ractor benchmarks and uses the appropriate harness.
-# Examples:
-#   ./run_once.sh --yjit-stats benchmarks/railsbench/benchmark.rb
-#   ./run_once.sh benchmarks-ractor/optcarrot/benchmark.rb
-
-# Detect if any argument contains benchmarks-ractor/ to determine harness
-HARNESS="./harness"
-for arg in "$@"; do
-    if [[ "$arg" == *"benchmarks-ractor/"* ]]; then
-        HARNESS="./harness-ractor"
-        break
-    fi
-done
-
-WARMUP_ITRS=0 MIN_BENCH_ITRS=1 MIN_BENCH_TIME=0 ruby -I"$HARNESS" "$@"
\ No newline at end of file
diff --git a/test/argument_parser_test.rb b/test/argument_parser_test.rb
index 7008dcef..8512a7c2 100644
--- a/test/argument_parser_test.rb
+++ b/test/argument_parser_test.rb
@@ -44,7 +44,7 @@ def setup_mock_ruby(path)
         assert_equal({ 'ruby' => [mock_ruby] }, args.executables)
         assert_equal File.expand_path("./data"), args.out_path
         assert_nil args.out_override
-        assert_equal "harness", args.harness
+        assert_equal "default", args.harness
         assert_equal "", args.yjit_opts
         assert_equal [], args.categories
         assert_equal [], args.name_filters
@@ -281,12 +281,12 @@ def setup_mock_ruby(path)
         assert_equal ['headline', 'micro'], args.categories
       end
 
-      it 'sets harness to harness-ractor when category is ractor' do
+      it 'sets harness to ractor when category is ractor' do
         parser = ArgumentParser.new
         args = parser.parse(['--category=ractor'])
 
         assert_equal ['ractor'], args.categories
-        assert_equal 'harness-ractor', args.harness
+        assert_equal 'ractor', args.harness
       end
 
       it 'allows multiple category flags' do
@@ -339,18 +339,18 @@ def setup_mock_ruby(path)
     end
 
     describe '--harness option' do
-      it 'sets harness directory' do
+      it 'sets harness name' do
         parser = ArgumentParser.new
         args = parser.parse(['--harness=once'])
 
-        assert_equal 'harness-once', args.harness
+        assert_equal 'once', args.harness
       end
 
-      it 'accepts harness- prefix' do
+      it 'strips harness- prefix for backward compatibility' do
         parser = ArgumentParser.new
         args = parser.parse(['--harness=harness-stats'])
 
-        assert_equal 'harness-stats', args.harness
+        assert_equal 'stats', args.harness
       end
     end
 
diff --git a/test/benchmark_runner_cli_test.rb b/test/benchmark_runner_cli_test.rb
index 72452429..fd66dec6 100644
--- a/test/benchmark_runner_cli_test.rb
+++ b/test/benchmark_runner_cli_test.rb
@@ -42,7 +42,7 @@ def create_args(overrides = {})
       executables: executables,
       out_path: nil,
       out_override: nil,
-      harness: 'harness',
+      harness: 'default',
       yjit_opts: '',
       categories: [],
       name_filters: [],
diff --git a/test/benchmark_suite_test.rb b/test/benchmark_suite_test.rb
index cd9eee3b..1a3fdb1d 100644
--- a/test/benchmark_suite_test.rb
+++ b/test/benchmark_suite_test.rb
@@ -50,13 +50,13 @@
         categories: ['micro'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness'
+        harness: 'default'
       )
 
       assert_equal ['micro'], suite.categories
       assert_equal [], suite.name_filters
       assert_equal @out_path, suite.out_path
-      assert_equal 'harness', suite.harness
+      assert_equal 'default', suite.harness
       assert_nil suite.pre_init
       assert_equal false, suite.no_pinning
     end
@@ -66,7 +66,7 @@
         categories: [],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -78,12 +78,12 @@
         categories: ['micro'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness'
+        harness: 'default'
       )
 
       assert_equal 'benchmarks', suite.bench_dir
       assert_equal 'benchmarks-ractor', suite.ractor_bench_dir
-      assert_equal 'harness', suite.harness
+      assert_equal 'default', suite.harness
       assert_equal ['micro'], suite.categories
     end
 
@@ -92,12 +92,12 @@
         categories: ['ractor-only'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness'
+        harness: 'default'
       )
 
       assert_equal 'benchmarks-ractor', suite.bench_dir
       assert_equal 'benchmarks-ractor', suite.ractor_bench_dir
-      assert_equal 'harness-ractor', suite.harness
+      assert_equal 'ractor', suite.harness
       assert_equal [], suite.categories
     end
 
@@ -106,12 +106,12 @@
         categories: ['ractor'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness'
+        harness: 'default'
       )
 
       assert_equal 'benchmarks', suite.bench_dir
       assert_equal 'benchmarks-ractor', suite.ractor_bench_dir
-      assert_equal 'harness', suite.harness
+      assert_equal 'default', suite.harness
       assert_equal ['ractor'], suite.categories
     end
   end
@@ -122,7 +122,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -144,7 +144,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -167,7 +167,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -188,7 +188,7 @@
         categories: [],
         name_filters: ['failing'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -219,7 +219,7 @@
         categories: [],
         name_filters: ['subdir'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -248,7 +248,7 @@
         categories: ['ractor-only'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -261,8 +261,8 @@
       assert_includes bench_data, 'ractor_test'
       assert_empty bench_failures
 
-      # harness should be updated to harness-ractor
-      assert_equal 'harness-ractor', suite.harness
+      # harness should be updated to ractor
+      assert_equal 'ractor', suite.harness
     end
 
     it 'includes both regular and ractor benchmarks with ractor category' do
@@ -280,7 +280,7 @@
         categories: ['ractor'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -303,7 +303,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         pre_init: pre_init_file,
         no_pinning: true
       )
@@ -325,7 +325,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         pre_init: pre_init_file,
         no_pinning: true
       )
@@ -345,7 +345,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         pre_init: pre_init_file,
         no_pinning: true
       )
@@ -362,7 +362,7 @@
             categories: [],
             name_filters: ['simple'],
             out_path: @out_path,
-            harness: 'harness',
+            harness: 'default',
             pre_init: '/nonexistent/file.rb',
             no_pinning: true
           )
@@ -378,7 +378,7 @@
             categories: [],
             name_filters: ['simple'],
             out_path: @out_path,
-            harness: 'harness',
+            harness: 'default',
             pre_init: @temp_dir,
             no_pinning: true
           )
@@ -392,7 +392,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -411,7 +411,7 @@
         categories: [],
         name_filters: ['simple'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -442,7 +442,7 @@
         categories: [],
         name_filters: ['bench_a'],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
@@ -475,7 +475,7 @@
         categories: ['micro'],
         name_filters: [],
         out_path: @out_path,
-        harness: 'harness',
+        harness: 'default',
         no_pinning: true
       )
 
diff --git a/test/run_once_test.rb b/test/run_once_test.rb
new file mode 100644
index 00000000..1d96bc31
--- /dev/null
+++ b/test/run_once_test.rb
@@ -0,0 +1,291 @@
+require_relative 'test_helper'
+require 'open3'
+require 'tmpdir'
+require 'fileutils'
+
+describe 'run_once.rb' do
+  before do
+    @script_path = File.expand_path('../run_once.rb', __dir__)
+    @original_env = ENV.to_h
+
+    @tmpdir = Dir.mktmpdir
+    @test_benchmark = File.join(@tmpdir, 'test_benchmark.rb')
+    loader_path = File.expand_path('../lib/harness/loader', __dir__)
+    File.write(@test_benchmark, <<~RUBY)
+      require "#{loader_path}"
+      puts "Benchmark executed"
+      puts "WARMUP_ITRS=\#{ENV['WARMUP_ITRS']}"
+      puts "MIN_BENCH_ITRS=\#{ENV['MIN_BENCH_ITRS']}"
+      puts "MIN_BENCH_TIME=\#{ENV['MIN_BENCH_TIME']}"
+    RUBY
+  end
+
+  after do
+    FileUtils.rm_rf(@tmpdir) if @tmpdir && Dir.exist?(@tmpdir)
+    ENV.replace(@original_env)
+  end
+
+  def run_script(*args)
+    Open3.capture3('ruby', @script_path, *args)
+  end
+
+  describe 'basic execution' do
+    it 'executes a benchmark file' do
+      stdout, _stderr, status = run_script(@test_benchmark)
+
+      assert status.success?, "Script should execute successfully. stderr: #{_stderr}"
+      assert_match(/Benchmark executed/, stdout)
+    end
+
+    it 'sets environment variables for single iteration' do
+      stdout, _stderr, status = run_script(@test_benchmark)
+
+      assert status.success?
+      assert_match(/WARMUP_ITRS=0/, stdout)
+      assert_match(/MIN_BENCH_ITRS=1/, stdout)
+      assert_match(/MIN_BENCH_TIME=0/, stdout)
+    end
+
+    it 'shows error when no benchmark file specified' do
+      stdout, _stderr, status = run_script
+
+      refute status.success?
+      assert_match(/No benchmark file specified/, stdout)
+    end
+
+    it 'shows error when benchmark file does not exist' do
+      stdout, _stderr, status = run_script('/nonexistent/benchmark.rb')
+
+      refute status.success?
+      assert_match(/Benchmark file not found/, stdout)
+    end
+  end
+
+  describe '--harness option' do
+    it 'accepts default harness' do
+      stdout, _stderr, status = run_script('--harness=default', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+
+    it 'loads custom harness when specified' do
+      harness_dir = File.join(File.dirname(@script_path), 'harness')
+      FileUtils.mkdir_p(harness_dir)
+      test_harness = File.join(harness_dir, 'test_harness.rb')
+
+      begin
+        File.write(test_harness, <<~RUBY)
+          puts "TEST HARNESS LOADED"
+        RUBY
+
+        stdout, _stderr, status = run_script('--harness=test_harness', @test_benchmark)
+
+        assert status.success?
+        assert_match(/TEST HARNESS LOADED/, stdout)
+      ensure
+        File.delete(test_harness) if File.exist?(test_harness)
+      end
+    end
+
+    it 'respects HARNESS environment variable when no --harness option provided' do
+      harness_dir = File.join(File.dirname(@script_path), 'harness')
+      FileUtils.mkdir_p(harness_dir)
+      test_harness = File.join(harness_dir, 'test_harness.rb')
+
+      begin
+        File.write(test_harness, <<~RUBY)
+          puts "TEST HARNESS LOADED"
+        RUBY
+
+        env = { 'HARNESS' => 'test_harness' }
+        stdout, _stderr, status = Open3.capture3(env, 'ruby', @script_path, @test_benchmark)
+
+        assert status.success?
+        assert_match(/TEST HARNESS LOADED/, stdout)
+      ensure
+        File.delete(test_harness) if File.exist?(test_harness)
+      end
+    end
+
+    it 'shows error for non-existent harness' do
+      stdout, _stderr, status = run_script('--harness=nonexistent', @test_benchmark)
+
+      refute status.success?
+      assert_match(/Harness not found/, stdout)
+      assert_match(/Available harnesses/, stdout)
+    end
+  end
+
+  describe 'ractor benchmark detection' do
+    it 'automatically uses ractor harness for ractor benchmarks' do
+      ractor_dir = File.join(@tmpdir, 'benchmarks-ractor', 'test')
+      FileUtils.mkdir_p(ractor_dir)
+      ractor_benchmark = File.join(ractor_dir, 'benchmark.rb')
+      File.write(ractor_benchmark, 'puts "Ractor benchmark"')
+
+      _stdout, _stderr, _status = run_script(ractor_benchmark)
+
+      assert_match(/benchmarks-ractor/, ractor_benchmark)
+    end
+  end
+
+  describe 'Ruby options pass-through' do
+    it 'passes Ruby options after -- separator' do
+      warning_benchmark = File.join(@tmpdir, 'warning_test.rb')
+      File.write(warning_benchmark, <<~RUBY)
+        x = 1
+        x = 2
+        puts "Benchmark with warnings"
+      RUBY
+
+      stdout, _stderr, status = run_script('--', '-W2', warning_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark with warnings/, stdout)
+    end
+
+    it 'passes YJIT options after -- separator' do
+      yjit_benchmark = File.join(@tmpdir, 'yjit_test.rb')
+      File.write(yjit_benchmark, <<~RUBY)
+        puts "YJIT enabled" if defined?(RubyVM::YJIT)
+        puts "Benchmark complete"
+      RUBY
+
+      stdout, _stderr, status = run_script('--', '--yjit', yjit_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark complete/, stdout)
+    end
+  end
+
+  describe '--help option' do
+    it 'shows help message' do
+      stdout, _stderr, status = run_script('--help')
+
+      assert status.success?
+      assert_match(/Usage:/, stdout)
+      assert_match(/--harness/, stdout)
+    end
+
+    it 'shows help with -h' do
+      stdout, _stderr, status = run_script('-h')
+
+      assert status.success?
+      assert_match(/Usage:/, stdout)
+    end
+  end
+
+  describe 'argument parsing order' do
+    it 'handles harness option before benchmark' do
+      stdout, _stderr, status = run_script('--harness=default', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+
+    it 'handles Ruby options after -- separator' do
+      stdout, _stderr, status = run_script('--', '-W0', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+
+    it 'handles mixed options with -- separator' do
+      stdout, _stderr, status = run_script('--harness=default', '--', '-W0', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+  end
+
+  describe 'script examples' do
+    it 'works with simple benchmark path' do
+      fib_benchmark = File.join(@tmpdir, 'fib.rb')
+      File.write(fib_benchmark, 'puts "Fibonacci benchmark"')
+
+      stdout, _stderr, status = run_script(fib_benchmark)
+
+      assert status.success?
+      assert_match(/Fibonacci benchmark/, stdout)
+    end
+
+    it 'works with harness option' do
+      stdout, _stderr, status = run_script('--harness=default', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+
+    it 'works with Ruby options after -- separator' do
+      stdout, _stderr, status = run_script('--', '--yjit', @test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+    end
+  end
+
+  describe 'edge cases' do
+    it 'handles benchmark files with spaces in path' do
+      space_dir = File.join(@tmpdir, 'dir with spaces')
+      FileUtils.mkdir_p(space_dir)
+      space_benchmark = File.join(space_dir, 'benchmark.rb')
+      File.write(space_benchmark, 'puts "Space test"')
+
+      stdout, _stderr, status = run_script(space_benchmark)
+
+      assert status.success?
+      assert_match(/Space test/, stdout)
+    end
+
+    it 'identifies first .rb file as benchmark' do
+      other_file = File.join(@tmpdir, 'other.rb')
+      File.write(other_file, 'puts "Wrong file"')
+
+      stdout, _stderr, status = run_script(@test_benchmark)
+
+      assert status.success?
+      assert_match(/Benchmark executed/, stdout)
+      refute_match(/Wrong file/, stdout)
+    end
+
+    it 'rejects invalid options gracefully' do
+      stdout, _stderr, status = run_script('--invalid-option', @test_benchmark)
+
+      refute status.success?
+      assert_match(/invalid option|Error/, stdout)
+    end
+  end
+
+  describe 'direct ruby execution with HARNESS env var' do
+    it 'uses HARNESS environment variable when running benchmark directly' do
+      harness_dir = File.join(File.dirname(@script_path), 'harness')
+      FileUtils.mkdir_p(harness_dir)
+      direct_harness = File.join(harness_dir, 'direct_test.rb')
+
+      begin
+        File.write(direct_harness, <<~RUBY)
+          puts "DIRECT HARNESS RUNNING"
+        RUBY
+
+        env = { 'HARNESS' => 'direct_test' }
+        stdout, _stderr, status = Open3.capture3(env, 'ruby', @test_benchmark)
+
+        assert status.success?, "Direct Ruby execution should work with HARNESS env var"
+        assert_match(/DIRECT HARNESS RUNNING/, stdout)
+        assert_match(/Benchmark executed/, stdout)
+      ensure
+        File.delete(direct_harness) if File.exist?(direct_harness)
+      end
+    end
+
+    it 'shows helpful error for invalid HARNESS value' do
+      env = { 'HARNESS' => 'nonexistent_harness' }
+      _stdout, stderr, status = Open3.capture3(env, 'ruby', @test_benchmark)
+
+      refute status.success?
+      assert_match(/Harness 'nonexistent_harness' not found/, stderr)
+      assert_match(/Available harnesses/, stderr)
+    end
+  end
+end