diff --git a/src/metric.cpp b/src/metric.cpp
index e37bf3a..f58645e 100644
--- a/src/metric.cpp
+++ b/src/metric.cpp
@@ -1,4 +1,5 @@
 #include "metric.hpp"
+#include <simdutf.h>
 
 // WeaselDB Metrics System Design:
 //
@@ -21,7 +22,10 @@
 #include <atomic>
 #include <bit>
 #include <cassert>
+#include <cctype>
 #include <cstdint>
+#include <cstdio>
+#include <cstdlib>
 #include <limits>
 #include <memory>
 #include <mutex>
@@ -33,12 +37,30 @@
 
 namespace metric {
 
+// Validation helper that works in both debug and release builds
+static void validate_or_abort(bool condition, const char *message,
+                              const char *value) {
+  if (!condition) {
+    std::fprintf(stderr, "WeaselDB metric validation failed: %s: '%s'\n",
+                 message, value);
+    std::abort();
+  }
+}
+
 // Labels key for second level of map
 struct LabelsKey {
   std::vector<std::pair<std::string, std::string>> labels;
 
   LabelsKey(std::vector<std::pair<std::string, std::string>> l)
       : labels(std::move(l)) {
+    // Validate all label keys and values
+    for (const auto &[key, value] : labels) {
+      validate_or_abort(is_valid_label_key(key), "invalid label key",
+                        key.c_str());
+      validate_or_abort(is_valid_label_value(value), "invalid label value",
+                        value.c_str());
+    }
+
     // Sort labels by key for Prometheus compatibility
     std::sort(labels.begin(), labels.end(),
               [](const auto &a, const auto &b) { return a.first < b.first; });
@@ -85,12 +107,18 @@ template <> struct Family<Counter>::State {
     std::unordered_map<LabelsKey, std::unique_ptr<Counter::State>> instances;
   };
   std::unordered_map<std::thread::id, PerThreadState> perThreadState;
+
+  // Callback-based metrics (global, not per-thread)
+  std::unordered_map<LabelsKey, MetricCallback<Counter>> callbacks;
 };
 
 template <> struct Family<Gauge>::State {
   std::string name;
   std::string help;
   std::unordered_map<LabelsKey, std::unique_ptr<Gauge::State>> instances;
+
+  // Callback-based metrics
+  std::unordered_map<LabelsKey, MetricCallback<Gauge>> callbacks;
 };
 
 template <> struct Family<Histogram>::State {
@@ -102,6 +130,8 @@ template <> struct Family<Histogram>::State {
     std::unordered_map<LabelsKey, std::unique_ptr<Histogram::State>> instances;
   };
   std::unordered_map<std::thread::id, PerThreadState> perThreadState;
+
+  // Note: No callbacks map - histograms don't support callback-based metrics
 };
 
 // Counter: Thread-local, monotonically increasing, single writer per thread
@@ -110,10 +140,9 @@ struct Counter::State {
   friend struct Metric;
 };
 
-// Gauge: Global, can increase/decrease, multiple writers (requires mutex)
+// Gauge: Global, can increase/decrease, multiple writers (uses atomic CAS)
 struct Gauge::State {
-  std::mutex mutex;
-  double value; // Plain double, protected by mutex
+  AtomicWord value; // Stores double as uint64_t bits, lock-free
   friend struct Metric;
 };
 
@@ -172,6 +201,13 @@ struct Metric {
       const std::vector<std::pair<std::string, std::string>> &labels) {
     std::unique_lock<std::mutex> _{mutex};
     LabelsKey key{labels};
+
+    // Validate that labels aren't already registered as callback
+    validate_or_abort(
+        family->p->callbacks.find(key) == family->p->callbacks.end(),
+        "labels already registered as callback",
+        key.labels.empty() ? "(no labels)" : key.labels[0].first.c_str());
+
     auto &ptr =
         family->p->perThreadState[std::this_thread::get_id()].instances[key];
     if (!ptr) {
@@ -188,10 +224,17 @@ struct Metric {
       const std::vector<std::pair<std::string, std::string>> &labels) {
     std::unique_lock<std::mutex> _{mutex};
     LabelsKey key{labels};
+
+    // Validate that labels aren't already registered as callback
+    validate_or_abort(
+        family->p->callbacks.find(key) == family->p->callbacks.end(),
+        "labels already registered as callback",
+        key.labels.empty() ? "(no labels)" : key.labels[0].first.c_str());
+
     auto &ptr = family->p->instances[key];
     if (!ptr) {
       ptr = std::make_unique<Gauge::State>();
-      ptr->value = 0.0;
+      ptr->value.store(0, std::memory_order_relaxed);
     }
     Gauge result;
     result.p = ptr.get();
@@ -228,28 +271,47 @@ struct Metric {
 };
 
 void Counter::inc(double x) {
-  assert(x >= 0);
+  validate_or_abort(x >= 0, "counter increment must be >= 0",
+                    std::to_string(x).c_str());
 
   // DESIGN: Single writer per thread allows simple load-modify-store
   // No CAS loop needed since only one thread writes to this counter
   auto current_value =
       std::bit_cast<double>(p->value.load(std::memory_order_relaxed));
-  p->value.store(std::bit_cast<uint64_t>(current_value + x),
-                 std::memory_order_relaxed);
+  auto new_value = current_value + x;
+
+  // Validate monotonic property (counter never decreases)
+  validate_or_abort(new_value >= current_value,
+                    "counter value overflow/wraparound detected",
+                    std::to_string(new_value).c_str());
+
+  p->value.store(std::bit_cast<uint64_t>(new_value), std::memory_order_relaxed);
 }
 void Gauge::inc(double x) {
-  // IMPLEMENTATION DETAIL: Mutex protection used internally for thread safety,
-  // but API contract remains single-writer per instance
-  std::unique_lock<std::mutex> _{p->mutex};
-  p->value += x;
+  // Lock-free increment using CAS loop
+  uint64_t expected = p->value.load(std::memory_order_relaxed);
+  uint64_t desired;
+  do {
+    double current_value = std::bit_cast<double>(expected);
+    double new_value = current_value + x;
+    desired = std::bit_cast<uint64_t>(new_value);
+  } while (!p->value.compare_exchange_weak(expected, desired,
+                                           std::memory_order_relaxed));
 }
 void Gauge::dec(double x) {
-  std::unique_lock<std::mutex> _{p->mutex};
-  p->value -= x;
+  // Lock-free decrement using CAS loop
+  uint64_t expected = p->value.load(std::memory_order_relaxed);
+  uint64_t desired;
+  do {
+    double current_value = std::bit_cast<double>(expected);
+    double new_value = current_value - x;
+    desired = std::bit_cast<uint64_t>(new_value);
+  } while (!p->value.compare_exchange_weak(expected, desired,
+                                           std::memory_order_relaxed));
 }
 void Gauge::set(double x) {
-  std::unique_lock<std::mutex> _{p->mutex};
-  p->value = x;
+  // Simple atomic store for set operation
+  p->value.store(std::bit_cast<uint64_t>(x), std::memory_order_relaxed);
 }
 void Histogram::observe(double x) {
   assert(p->thresholds.size() == p->counts.size());
@@ -289,6 +351,9 @@ Histogram Family<Histogram>::create(
 }
 
 Family<Counter> create_counter(std::string name, std::string help) {
+  validate_or_abort(is_valid_metric_name(name), "invalid counter name",
+                    name.c_str());
+
   std::unique_lock<std::mutex> _{Metric::mutex};
   auto &familyPtr = Metric::counterFamilies[name];
   if (!familyPtr) {
@@ -302,6 +367,9 @@ Family<Counter> create_counter(std::string name, std::string help) {
 }
 
 Family<Gauge> create_gauge(std::string name, std::string help) {
+  validate_or_abort(is_valid_metric_name(name), "invalid gauge name",
+                    name.c_str());
+
   std::unique_lock<std::mutex> _{Metric::mutex};
   auto &familyPtr = Metric::gaugeFamilies[name];
   if (!familyPtr) {
@@ -316,6 +384,9 @@ Family<Gauge> create_gauge(std::string name, std::string help) {
 
 Family<Histogram> create_histogram(std::string name, std::string help,
                                    std::initializer_list<double> buckets) {
+  validate_or_abort(is_valid_metric_name(name), "invalid histogram name",
+                    name.c_str());
+
   std::unique_lock<std::mutex> _{Metric::mutex};
   auto &familyPtr = Metric::histogramFamilies[name];
   if (!familyPtr) {
@@ -340,10 +411,69 @@ Family<Histogram> create_histogram(std::string name, std::string help,
   return family;
 }
 
-std::span<std::string> render(ArenaAllocator &arena) {
+// Prometheus validation functions
+// Metric names must match [a-zA-Z_:][a-zA-Z0-9_:]*
+bool is_valid_metric_name(const std::string &name) {
+  if (name.empty())
+    return false;
+
+  // First character must be letter, underscore, or colon
+  char first = name[0];
+  if (!std::isalpha(first) && first != '_' && first != ':') {
+    return false;
+  }
+
+  // Remaining characters must be alphanumeric, underscore, or colon
+  for (size_t i = 1; i < name.size(); ++i) {
+    char c = name[i];
+    if (!std::isalnum(c) && c != '_' && c != ':') {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+// Label keys must match [a-zA-Z_][a-zA-Z0-9_]*
+bool is_valid_label_key(const std::string &key) {
+  if (key.empty())
+    return false;
+
+  // First character must be letter or underscore
+  char first = key[0];
+  if (!std::isalpha(first) && first != '_') {
+    return false;
+  }
+
+  // Remaining characters must be alphanumeric or underscore
+  for (size_t i = 1; i < key.size(); ++i) {
+    char c = key[i];
+    if (!std::isalnum(c) && c != '_') {
+      return false;
+    }
+  }
+
+  // Label keys starting with __ are reserved for internal use
+  if (key.size() >= 2 && key[0] == '_' && key[1] == '_') {
+    return false;
+  }
+
+  return true;
+}
+
+// Label values can contain any UTF-8 characters (no specific restrictions)
+bool is_valid_label_value(const std::string &value) {
+  // Prometheus allows any UTF-8 string as label value
+  // Validate UTF-8 encoding for correctness using simdutf
+  return simdutf::validate_utf8(value.c_str(), value.size());
+}
+
+std::span<std::string_view> render(ArenaAllocator &arena) {
   // TODO: Implement Prometheus text format rendering
-  static std::string empty_result = "";
-  return std::span<std::string>(&empty_result, 0);
+  // All string data should be allocated in the arena and returned as
+  // string_views
+  static std::string_view empty_result = "";
+  return std::span<std::string_view>(&empty_result, 0);
 }
 
 // Static member definitions
diff --git a/src/metric.hpp b/src/metric.hpp
index b2e83cd..3ccdb8e 100644
--- a/src/metric.hpp
+++ b/src/metric.hpp
@@ -8,12 +8,23 @@
 // - Single-writer semantics: Each metric instance bound to creating thread
 // - Lock-free operations using atomic<uint64_t> storage for doubles
 // - Full IEEE 754 double precision preservation via bit reinterpretation
+// - Single global registry: All metrics registered in one global namespace
 //
 // CRITICAL THREAD SAFETY CONSTRAINT:
 // Each metric instance has exactly ONE writer thread (the creating thread).
 // It is undefined behavior to call inc()/dec()/set()/observe() from a different
 // thread.
 //
+// REGISTRY MODEL:
+// This implementation uses a single global registry for all metrics, unlike
+// typical Prometheus client libraries that support multiple registries.
+// This design choice prioritizes simplicity and performance over flexibility.
+//
+// METRIC LIFECYCLE:
+// Metrics are created once and persist for the application lifetime. There is
+// no unregistration mechanism - this prevents accidental metric loss and
+// simplifies the implementation.
+//
 // USAGE:
 //   auto counter_family = metric::create_counter("requests_total", "Total
 //   requests"); auto counter = counter_family.create({{"method", "GET"}});  //
@@ -100,6 +111,8 @@ template <class T> struct Family {
 
   // Create metric instance with specific labels
   // Labels are sorted by key for Prometheus compatibility
+  // ERROR: Will abort if labels already registered via register_callback()
+  // OK: Multiple calls with same labels return same instance (idempotent)
   T create(std::vector<std::pair<std::string, std::string>> labels);
 
 private:
@@ -129,6 +142,42 @@ Family<Histogram> create_histogram(std::string name, std::string help,
                                    std::initializer_list<double> buckets);
 
 // Render all metrics in Prometheus text format
-std::span<std::string> render(ArenaAllocator &arena);
+// TODO: Implement Prometheus text exposition format
+// THREAD SAFETY: Serialized by global mutex - callbacks need not be thread-safe
+std::span<std::string_view> render(ArenaAllocator &arena);
+
+// Validation functions for Prometheus compatibility
+bool is_valid_metric_name(const std::string &name);
+bool is_valid_label_key(const std::string &key);
+bool is_valid_label_value(const std::string &value);
+
+// Callback function type for dynamic metric values
+// Called during render() to get current metric value
+// THREAD SAFETY: May be called from arbitrary thread, but serialized by
+// render() mutex
+// - no need to be thread-safe internally
+template <typename T> using MetricCallback = std::function<double()>;
+
+// Register callback-based metric to Family
+// Validates that label set isn't already taken by either:
+// - A previous register_callback() call (callbacks must be unique)
+// - A create() call (static and callback metrics cannot coexist for same
+// labels)
+//
+// Similarly, create() will validate that label set isn't already registered as
+// callback Note: create() can be called multiple times with same labels
+// (returns same instance)
+template <>
+void Family<Counter>::register_callback(
+    std::vector<std::pair<std::string, std::string>> labels,
+    MetricCallback<Counter> callback);
+
+template <>
+void Family<Gauge>::register_callback(
+    std::vector<std::pair<std::string, std::string>> labels,
+    MetricCallback<Gauge> callback);
+
+// Note: Histograms do not support callbacks due to their multi-value nature
+// (buckets + sum + count). Use static histogram metrics only.
 
 } // namespace metric