diff --git a/test/lz_synthetic_generator.h b/test/lz_synthetic_generator.h
new file mode 100644
index 00000000..2dd1aad1
--- /dev/null
+++ b/test/lz_synthetic_generator.h
@@ -0,0 +1,216 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/**
+ * \author     Marcus Holland-Moritz (github@mhxnet.de)
+ * \copyright  Copyright (c) Marcus Holland-Moritz
+ *
+ * This file is part of dwarfs.
+ *
+ * dwarfs is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * dwarfs is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#pragma once
+
+#include <algorithm>
+#include <array>
+#include <cstdint>
+#include <random>
+#include <string>
+#include <vector>
+
+namespace dwarfs::test {
+
+struct lz_params {
+  // Probability of choosing a "copy from the past" step vs. emitting a literal
+  double copy_prob = 0.70;
+
+  // Max distance for backreferences (typical LZ77 windows are 32–64 KiB)
+  std::size_t window = 1u << 15; // 32 KiB
+
+  // Copy lengths ~ truncated geometric around this mean (controls
+  // repetitiveness)
+  std::size_t min_match = 4;
+  std::size_t max_match = 128;
+  double target_match_mean = 20.0; // average copy length
+
+  // Geometric distribution for distance (smaller distances more likely)
+  double distance_mean = 128.0;
+
+  // Chance each character in a copy mutates into a random literal (adds
+  // “noise”)
+  double mutation_rate = 0.005;
+
+  // If true, literals look like English-ish text; if false, literals are 0–255
+  // bytes
+  bool text_mode = true;
+
+  // RNG seed for reproducibility
+  std::uint64_t seed = 0x1234'5678'9abc'def0ULL;
+};
+
+class lz_synthetic_generator {
+ public:
+  explicit lz_synthetic_generator(lz_params p = {})
+      : p_{p}
+      , rng_{p.seed} {
+    if (p_.text_mode) {
+      init_text_alphabet();
+    } else {
+      init_binary_alphabet();
+    }
+
+    // geometric_distribution parameterization: mean of failures = (1-p)/p
+    // We want E[min_match + failures] ≈ target_match_mean  =>  E[failures] ≈
+    // target - min
+    double mean_fail =
+        std::max(1.0, p_.target_match_mean - static_cast<double>(p_.min_match));
+    double p_len = 1.0 / (mean_fail + 1.0);
+    geo_len_ = std::geometric_distribution<int>(p_len);
+
+    double mean_dist_fail = std::max(1.0, p_.distance_mean);
+    double p_dist = 1.0 / (mean_dist_fail + 1.0);
+    geo_dist_ = std::geometric_distribution<int>(p_dist);
+
+    bern_copy_ = std::bernoulli_distribution(p_.copy_prob);
+    bern_mut_ = std::bernoulli_distribution(p_.mutation_rate);
+  }
+
+  std::string generate(std::size_t n_bytes) {
+    std::string out;
+    out.reserve(n_bytes);
+
+    while (out.size() < n_bytes) {
+      bool const can_copy = out.size() >= p_.min_match;
+      if (can_copy && bern_copy_(rng_)) {
+        emit_copy(out, n_bytes);
+      } else {
+        out.push_back(sample_literal());
+      }
+    }
+    return out;
+  }
+
+ private:
+  void init_text_alphabet() {
+    // Rough English-ish frequencies via "etaoin shrdlu..." ranking.
+    // Higher rank => higher weight. We include space/newline/punct/digits.
+    static constexpr std::string_view freq_rank =
+        " etaoinshrdlucmfwypvbgkqjxz"; // space first (most frequent)
+    // Map ranks to weights (largest for rank 0).
+    std::array<int, 256> weights{};
+    for (int i = 0; i < 256; ++i) {
+      weights[i] = 1;
+    }
+
+    auto apply_rank = [&](char c, size_t rank_base) {
+      int r = std::max(1, static_cast<int>(freq_rank.size()) -
+                              static_cast<int>(rank_base));
+      weights[static_cast<unsigned char>(c)] += r;
+    };
+
+    for (size_t i = 0; i < freq_rank.size(); ++i) {
+      char c = freq_rank[i];
+      apply_rank(c, i);
+      if (c >= 'a' && c <= 'z') {
+        apply_rank(char(c - 'a' + 'A'), i + 6); // uppercase similar but rarer
+      }
+    }
+
+    // Common punctuation and digits
+    std::string const punct = ".,;:-()[]{}!?\"'";
+    for (char c : punct) {
+      weights[static_cast<unsigned char>(c)] += 8;
+    }
+    for (char c = '0'; c <= '9'; ++c) {
+      weights[static_cast<unsigned char>(c)] += 4;
+    }
+
+    // Newlines and tabs, for “document” feel
+    weights['\n'] += 6;
+    weights['\t'] += 2;
+
+    // Build alphabet and weight vector for std::discrete_distribution
+    for (int i = 0; i < 256; ++i) {
+      if (weights[i] > 0) {
+        text_alphabet_.push_back(static_cast<unsigned char>(i));
+        text_weights_.push_back(weights[i]);
+      }
+    }
+    text_dist_ = std::discrete_distribution<int>(text_weights_.begin(),
+                                                 text_weights_.end());
+  }
+
+  void init_binary_alphabet() {
+    binary_dist_ = std::uniform_int_distribution<int>(0, 255);
+  }
+
+  char sample_literal() {
+    if (p_.text_mode) {
+      int idx = text_dist_(rng_);
+      return static_cast<char>(text_alphabet_[static_cast<std::size_t>(idx)]);
+    }
+    return static_cast<char>(binary_dist_(rng_));
+  }
+
+  void emit_copy(std::string& out, std::size_t n_bytes) {
+    // Distance: 1 + geometric, truncated to current size and window
+    std::size_t max_dist = std::min<std::size_t>(p_.window, out.size());
+    if (max_dist == 0) {
+      out.push_back(sample_literal());
+      return;
+    }
+
+    std::size_t dist = 1u + static_cast<std::size_t>(geo_dist_(rng_));
+    if (dist > max_dist)
+      dist = 1u + (dist % max_dist); // ensure in-range
+
+    // Length: min_match + geometric, truncated by end and max_match
+    std::size_t max_len =
+        std::min<std::size_t>(p_.max_match, n_bytes - out.size());
+    if (max_len < p_.min_match) {
+      out.push_back(sample_literal());
+      return;
+    }
+
+    std::size_t len = p_.min_match + static_cast<std::size_t>(geo_len_(rng_));
+    if (len > max_len)
+      len = max_len;
+
+    std::size_t start = out.size() - dist;
+    for (std::size_t i = 0; i < len && out.size() < n_bytes; ++i) {
+      unsigned char c = static_cast<unsigned char>(out[start + i]);
+      if (bern_mut_(rng_)) {
+        c = static_cast<unsigned char>(sample_literal());
+      }
+      out.push_back(static_cast<char>(c));
+    }
+  }
+
+  lz_params p_;
+  std::mt19937_64 rng_;
+
+  std::vector<unsigned char> text_alphabet_;
+  std::vector<int> text_weights_;
+  std::discrete_distribution<int> text_dist_;
+
+  std::uniform_int_distribution<int> binary_dist_{0, 255};
+
+  std::bernoulli_distribution bern_copy_;
+  std::bernoulli_distribution bern_mut_;
+  std::geometric_distribution<int> geo_len_;
+  std::geometric_distribution<int> geo_dist_;
+};
+
+} // namespace dwarfs::test
diff --git a/test/test_helpers.cpp b/test/test_helpers.cpp
index ad9f0c84..d9b3beba 100644
--- a/test/test_helpers.cpp
+++ b/test/test_helpers.cpp
@@ -38,6 +38,7 @@
 #include <dwarfs/util.h>
 
 #include "loremipsum.h"
+#include "lz_synthetic_generator.h"
 #include "mmap_mock.h"
 #include "test_helpers.h"
 
@@ -271,9 +272,10 @@ void os_access_mock::add_file(fs::path const& path, size_t size, bool random) {
   if (random) {
     thread_local std::mt19937_64 rng{42};
 
-    std::uniform_int_distribution<> choice_dist{0, 3};
+    std::uniform_int_distribution<> choice_dist{0, 4};
+    auto choice = choice_dist(rng);
 
-    switch (choice_dist(rng)) {
+    switch (choice) {
     default:
       break;
 
@@ -281,6 +283,18 @@ void os_access_mock::add_file(fs::path const& path, size_t size, bool random) {
       add(path, st,
           [size, seed = rng()] { return create_random_string(size, seed); });
       return;
+
+    case 1:
+    case 2: {
+      add(path, st, [size, seed = rng(), text_mode = choice == 1] {
+        lz_params lzp{};
+        lzp.text_mode = text_mode;
+        lzp.seed = seed;
+        lz_synthetic_generator gen{lzp};
+        return gen.generate(size);
+      });
+      return;
+    }
     }
   }
 
diff --git a/test/tool_main_test.cpp b/test/tool_main_test.cpp
index 4e7305ba..6c15fe92 100644
--- a/test/tool_main_test.cpp
+++ b/test/tool_main_test.cpp
@@ -67,6 +67,7 @@
 
 #include "filter_test_data.h"
 #include "loremipsum.h"
+#include "lz_synthetic_generator.h"
 #include "mmap_mock.h"
 #include "test_helpers.h"
 #include "test_logger.h"
@@ -258,6 +259,15 @@ class mkdwarfs_tester : public tester_common {
       return test::create_random_string(size, 'A', 'Z', rng);
     };
 
+    test::lz_params text_lzp{};
+    test::lz_params binary_lzp{};
+    text_lzp.text_mode = true;
+    binary_lzp.text_mode = false;
+    text_lzp.seed = rng();
+    binary_lzp.seed = rng();
+    test::lz_synthetic_generator text_gen{text_lzp};
+    test::lz_synthetic_generator binary_gen{binary_lzp};
+
     for (int x = 0; x < opt.dimension; ++x) {
       fs::path d1{random_path_component() + std::to_string(x)};
       os->add_dir(d1);
@@ -268,13 +278,24 @@ class mkdwarfs_tester : public tester_common {
 
         for (int z = 0; z < opt.dimension; ++z) {
           fs::path f{d2 / (random_path_component() + std::to_string(z))};
-          auto size = std::min(max_size, static_cast<size_t>(size_dist(rng)));
+          auto const size =
+              std::min(max_size, static_cast<size_t>(size_dist(rng)));
           std::string data;
 
-          if (size < 1024 * 1024 && rng() % 2 == 0) {
+          auto const choice = rng() % 4;
+          switch (choice) {
+          case 0:
             data = test::create_random_string(size, rng);
-          } else {
+            break;
+          case 1:
             data = test::loremipsum(size);
+            break;
+          case 3:
+            data = text_gen.generate(size);
+            break;
+          case 4:
+            data = binary_gen.generate(size);
+            break;
           }
 
           os->add_file(f, data);