serpapi
diff --git a/meson.build b/meson.build
@@ -35,12 +35,13 @@ subdir('oobt')
 subdir('example')
 
 # Configure the testing setup
-test_src = files('test/test_main.cpp', 'test/test_serpapi.cpp')
+test_src = files('test/test_main.cpp', 'test/test_serpapi.cpp', 'test/test_unit.cpp')
 test_deps = [
 #  dependency('libcurl'),
   dependency('RapidJSON'),
   libserpapi_dep,
-  dependency('gtest')
+  dependency('gtest'),
+  dependency('threads')
 ]
 test_args = ['-Wno-missing-field-initializers','-g','-lgtest']
 

diff --git a/test/test_unit.cpp b/test/test_unit.cpp
@@ -0,0 +1,138 @@
+#include "../src/callback.hpp"
+#include "../src/serpapi.hpp"
+#include <gtest/gtest.h>
+#include <atomic>
+#include <climits>
+#include <thread>
+#include <vector>
+
+// ── callback ──────────────────────────────────────────────────────────────────
+
+TEST(callback, null_out_returns_zero) {
+  EXPECT_EQ(serpapi::callback("data", 4, 1, nullptr), 0u);
+}
+
+TEST(callback, null_in_returns_zero) {
+  std::string s;
+  EXPECT_EQ(serpapi::callback(nullptr, 4, 1, &s), 0u);
+  EXPECT_TRUE(s.empty());
+}
+
+TEST(callback, zero_size_returns_zero) {
+  std::string s;
+  EXPECT_EQ(serpapi::callback("data", 0, 1, &s), 0u);
+  EXPECT_TRUE(s.empty());
+}
+
+TEST(callback, zero_num_returns_zero) {
+  std::string s;
+  EXPECT_EQ(serpapi::callback("data", 1, 0, &s), 0u);
+  EXPECT_TRUE(s.empty());
+}
+
+TEST(callback, overflow_returns_zero) {
+  std::string s;
+  EXPECT_EQ(serpapi::callback("data", std::numeric_limits<size_t>::max(), 2, &s), 0u);
+  EXPECT_TRUE(s.empty());
+}
+
+TEST(callback, appends_data) {
+  std::string s;
+  size_t n = serpapi::callback("hello", 5, 1, &s);
+  EXPECT_EQ(n, 5u);
+  EXPECT_EQ(s, "hello");
+}
+
+TEST(callback, size_times_num_appends_correctly) {
+  std::string s;
+  const char data[] = {'a', 'b', 'c', 'd', 'e', 'f'};
+  size_t n = serpapi::callback(data, 2, 3, &s);
+  EXPECT_EQ(n, 6u);
+  EXPECT_EQ(s.size(), 6u);
+}
+
+TEST(callback, incremental_chunks_accumulate) {
+  std::string s;
+  serpapi::callback("foo", 3, 1, &s);
+  serpapi::callback("bar", 3, 1, &s);
+  EXPECT_EQ(s, "foobar");
+}
+
+// ── Client construction ───────────────────────────────────────────────────────
+
+TEST(client_unit, construct_empty_params) {
+  EXPECT_NO_THROW(serpapi::Client client({}));
+}
+
+TEST(client_unit, construct_with_params) {
+  EXPECT_NO_THROW(serpapi::Client client({{"api_key", "test"}, {"engine", "google"}}));
+}
+
+TEST(client_unit, set_timeout_positive) {
+  serpapi::Client client({});
+  EXPECT_NO_THROW(client.setTimeout(30));
+}
+
+TEST(client_unit, set_timeout_zero) {
+  serpapi::Client client({});
+  EXPECT_NO_THROW(client.setTimeout(0));
+}
+
+// ── Edge cases (network required – uses location which needs no API key) ──────
+
+TEST(client_unit, location_special_chars_in_query) {
+  serpapi::Client client({});
+  // & and = in the query value must be percent-encoded, not break the URL
+  rapidjson::Document d = client.location({{"q", "Austin & Texas"}, {"limit", "1"}});
+  EXPECT_FALSE(d.HasParseError());
+  EXPECT_TRUE(d.IsArray());
+}
+
+TEST(client_unit, location_empty_query) {
+  serpapi::Client client({});
+  // Empty string value should not crash URL encoding
+  rapidjson::Document d = client.location({{"q", ""}, {"limit", "1"}});
+  EXPECT_FALSE(d.HasParseError());
+}
+
+TEST(client_unit, default_params_merged_with_call_params) {
+  // Default params set at construction should be sent alongside call params
+  serpapi::Client client({{"limit", "1"}});
+  rapidjson::Document d = client.location({{"q", "Austin"}});
+  EXPECT_FALSE(d.HasParseError());
+  EXPECT_TRUE(d.IsArray());
+  // Only 1 result because of default limit=1
+  EXPECT_LE(d.GetArray().Size(), 1u);
+}
+
+// ── Response integrity ────────────────────────────────────────────────────────
+
+TEST(client_unit, search_returns_valid_json) {
+  // Verify we always get a parseable JSON object, never a raw crash or garbage
+  serpapi::Client client({{"engine", "google"}});
+  rapidjson::Document d = client.search({{"q", "coffee"}});
+  EXPECT_FALSE(d.HasParseError());
+  EXPECT_TRUE(d.IsObject());
+}
+
+// ── Thread safety ─────────────────────────────────────────────────────────────
+
+TEST(client_unit, concurrent_location_calls_are_safe) {
+  // Verifies std::call_once CURL init and shared Client are race-free
+  serpapi::Client client({});
+  std::map<std::string, std::string> params = {{"q", "Austin"}, {"limit", "1"}};
+  std::atomic<int> success{0};
+
+  std::vector<std::thread> threads;
+  threads.reserve(4);
+  for (int i = 0; i < 4; ++i) {
+    threads.emplace_back([&]() {
+      rapidjson::Document d = client.location(params);
+      if (!d.HasParseError() && d.IsArray())
+        ++success;
+    });
+  }
+  for (auto& t : threads) t.join();
+
+  EXPECT_EQ(success.load(), 4);
+}