diff --git a/cpp/functions/core/parallel_for.cpp b/cpp/functions/core/parallel_for.cpp
new file mode 100644
index 00000000..009f8ea7
--- /dev/null
+++ b/cpp/functions/core/parallel_for.cpp
@@ -0,0 +1,136 @@
+#include "core/parallel_for.h"
+
+#include <atomic>
+#include <condition_variable>
+#include <exception>
+#include <mutex>
+#include <queue>
+#include <thread>
+#include <vector>
+
+namespace fn {
+
+struct ThreadPool::Impl {
+    std::vector<std::thread> workers;
+    std::queue<std::function<void()>> jobs;
+    std::mutex m;
+    std::condition_variable cv;
+    bool stop = false;
+
+    int n_threads = 0;
+
+    // Estado por parallel_for:
+    std::atomic<int> active_jobs{0};
+    std::mutex done_m;
+    std::condition_variable done_cv;
+    std::exception_ptr first_exc;
+    std::mutex exc_m;
+
+    void worker_loop() {
+        for (;;) {
+            std::function<void()> job;
+            {
+                std::unique_lock<std::mutex> lk(m);
+                cv.wait(lk, [this] { return stop || !jobs.empty(); });
+                if (stop && jobs.empty()) return;
+                job = std::move(jobs.front());
+                jobs.pop();
+            }
+            try {
+                job();
+            } catch (...) {
+                std::lock_guard<std::mutex> lk(exc_m);
+                if (!first_exc) first_exc = std::current_exception();
+            }
+            if (active_jobs.fetch_sub(1) == 1) {
+                std::lock_guard<std::mutex> lk(done_m);
+                done_cv.notify_all();
+            }
+        }
+    }
+};
+
+ThreadPool::ThreadPool(int n_threads) : impl_(new Impl) {
+    if (n_threads <= 0) {
+        n_threads = static_cast<int>(std::thread::hardware_concurrency());
+        if (n_threads <= 0) n_threads = 1;
+    }
+    impl_->n_threads = n_threads;
+    impl_->workers.reserve(static_cast<std::size_t>(n_threads));
+    for (int i = 0; i < n_threads; ++i) {
+        impl_->workers.emplace_back([this] { impl_->worker_loop(); });
+    }
+}
+
+ThreadPool::~ThreadPool() {
+    if (!impl_) return;
+    {
+        std::lock_guard<std::mutex> lk(impl_->m);
+        impl_->stop = true;
+    }
+    impl_->cv.notify_all();
+    for (auto& t : impl_->workers) {
+        if (t.joinable()) t.join();
+    }
+    delete impl_;
+    impl_ = nullptr;
+}
+
+int ThreadPool::n_threads() const { return impl_->n_threads; }
+
+void ThreadPool::parallel_for_chunks(std::size_t begin, std::size_t end,
+                                     const std::function<void(int, std::size_t, std::size_t)>& fn) {
+    if (end <= begin) return;
+    int N = impl_->n_threads;
+    std::size_t total = end - begin;
+    std::size_t chunk = (total + static_cast<std::size_t>(N) - 1) /
+                        static_cast<std::size_t>(N);
+    int actual_chunks = 0;
+    for (int t = 0; t < N; ++t) {
+        std::size_t lo = begin + chunk * static_cast<std::size_t>(t);
+        if (lo >= end) break;
+        std::size_t hi = lo + chunk;
+        if (hi > end) hi = end;
+        ++actual_chunks;
+    }
+    if (actual_chunks == 0) return;
+
+    impl_->first_exc = nullptr;
+    impl_->active_jobs.store(actual_chunks);
+
+    {
+        std::lock_guard<std::mutex> lk(impl_->m);
+        for (int t = 0; t < N; ++t) {
+            std::size_t lo = begin + chunk * static_cast<std::size_t>(t);
+            if (lo >= end) break;
+            std::size_t hi = lo + chunk;
+            if (hi > end) hi = end;
+            int tid = t;
+            impl_->jobs.emplace([&fn, tid, lo, hi] { fn(tid, lo, hi); });
+        }
+    }
+    impl_->cv.notify_all();
+
+    {
+        std::unique_lock<std::mutex> lk(impl_->done_m);
+        impl_->done_cv.wait(lk, [this] {
+            return impl_->active_jobs.load() == 0;
+        });
+    }
+
+    if (impl_->first_exc) {
+        std::exception_ptr e = impl_->first_exc;
+        impl_->first_exc = nullptr;
+        std::rethrow_exception(e);
+    }
+}
+
+void ThreadPool::parallel_for(std::size_t begin, std::size_t end,
+                              const std::function<void(std::size_t)>& fn) {
+    parallel_for_chunks(begin, end,
+        [&fn](int /*tid*/, std::size_t lo, std::size_t hi) {
+            for (std::size_t i = lo; i < hi; ++i) fn(i);
+        });
+}
+
+} // namespace fn
diff --git a/cpp/functions/core/parallel_for.h b/cpp/functions/core/parallel_for.h
new file mode 100644
index 00000000..c0e89950
--- /dev/null
+++ b/cpp/functions/core/parallel_for.h
@@ -0,0 +1,42 @@
+#pragma once
+
+#include <cstddef>
+#include <functional>
+
+namespace fn {
+
+// Pool de threads opaco. Crea N workers en el constructor (default = hw
+// concurrency); el destructor los joinea. Re-utilizable across multiples
+// parallel_for; cada llamada bloquea hasta que todos los hilos terminan
+// el rango.
+class ThreadPool {
+public:
+    explicit ThreadPool(int n_threads = 0);   // 0 = std::thread::hardware_concurrency()
+    ~ThreadPool();
+
+    ThreadPool(const ThreadPool&) = delete;
+    ThreadPool& operator=(const ThreadPool&) = delete;
+
+    int n_threads() const;
+
+    // Ejecuta fn(i) para i en [begin, end), repartido entre los workers
+    // en chunks contiguos. Bloquea hasta finalizar.
+    //
+    // Si una excepcion se lanza en algun worker, parallel_for la captura
+    // y la relanza en el thread caller tras el join (la primera).
+    void parallel_for(std::size_t begin, std::size_t end,
+                      const std::function<void(std::size_t)>& fn);
+
+    // Ejecuta fn(thread_id, begin, end) por chunk — util cuando cada thread
+    // necesita acumular state local. fn recibe los limites del chunk.
+    void parallel_for_chunks(std::size_t begin, std::size_t end,
+                             const std::function<void(int /*tid*/,
+                                                      std::size_t /*lo*/,
+                                                      std::size_t /*hi*/)>& fn);
+
+private:
+    struct Impl;
+    Impl* impl_;
+};
+
+} // namespace fn
diff --git a/cpp/functions/core/parallel_for.md b/cpp/functions/core/parallel_for.md
new file mode 100644
index 00000000..edccf69f
--- /dev/null
+++ b/cpp/functions/core/parallel_for.md
@@ -0,0 +1,74 @@
+---
+name: parallel_for
+kind: function
+lang: cpp
+domain: core
+version: "1.0.0"
+purity: impure
+signature: "ThreadPool::ThreadPool(int n_threads = 0); void ThreadPool::parallel_for(size_t begin, size_t end, fn(i)); void ThreadPool::parallel_for_chunks(size_t begin, size_t end, fn(tid, lo, hi))"
+description: "Pool de threads reutilizable con parallel_for por indice y parallel_for_chunks para acumulado por thread. Reparte rango contiguo por hw concurrency. Captura excepciones del worker y las relanza en el caller. Para Monte Carlo CPU multi-core."
+tags: [thread, parallel, concurrency, montecarlo, core]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: [thread, mutex, condition_variable, atomic, queue, vector, functional, exception]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "cpp/functions/core/parallel_for.cpp"
+params:
+  - name: n_threads
+    desc: "Numero de workers. 0 = hardware_concurrency. El destructor joinea todos."
+  - name: begin
+    desc: "Indice inicial del rango (inclusive)."
+  - name: end
+    desc: "Indice final del rango (exclusive). end <= begin es no-op."
+  - name: fn
+    desc: "Functor a ejecutar. parallel_for: void(size_t i). parallel_for_chunks: void(int tid, size_t lo, size_t hi) — el chunk [lo, hi) lo procesa el thread tid."
+output: "El pool ejecuta el functor en paralelo y bloquea el caller hasta que todos los chunks terminan. Si algun worker lanza una excepcion, se captura la primera y se relanza desde el caller tras el join."
+---
+
+# parallel_for
+
+Pool minimo para Monte Carlo CPU. Una instancia mantiene workers vivos entre llamadas — barato hacer muchas N pequenas (no hay overhead de spawn por llamada).
+
+## Patron tipico — N sesiones independientes
+
+```cpp
+fn::ThreadPool pool;        // hardware_concurrency workers
+
+std::vector<double> pnl(N);
+pool.parallel_for(0, N, [&](std::size_t i) {
+    fn::ds::Rng r;
+    fn::ds::rng_seed(r, /*master=*/0xC0FFEE ^ i);   // seed por sesion
+    pnl[i] = simulate_session(r);
+});
+```
+
+## Patron — accumulado por thread (sum / hist local)
+
+`parallel_for_chunks` evita atomic contention dando un slice contiguo a cada thread:
+
+```cpp
+std::vector<double> partials(pool.n_threads(), 0.0);
+
+pool.parallel_for_chunks(0, N,
+    [&](int tid, std::size_t lo, std::size_t hi) {
+        double local = 0.0;
+        for (std::size_t i = lo; i < hi; ++i) {
+            local += compute(i);
+        }
+        partials[tid] = local;
+    });
+
+double total = std::accumulate(partials.begin(), partials.end(), 0.0);
+```
+
+## Notas
+
+- **Throughput**: 8 cores @ 4 GHz, jobs de ~10us cada uno => ~5x speedup tipico (Amdahl: lo bloquea el job mas largo + cv-wait coste). Para jobs <1us el coste de submit domina; usar `parallel_for_chunks` con chunk grande.
+- **Excepciones**: solo se preserva la PRIMERA excepcion. Las demas se silencian (con la garantia de que el resto de los workers terminan limpiamente).
+- **No re-entrante**: un worker no puede llamar `parallel_for` sobre el mismo pool — bloquearia el done_cv. Si se necesita anidamiento, crear un segundo pool.
+- **Comparar con GPU**: para 10^6 sesiones de 10^4 spins (= 10^10 ops MC), CPU 8-core ~10s, GPU ~1-2s. CPU pool es la fallback portable; GPU es para el caso extremo.
diff --git a/cpp/functions/core/slider.cpp b/cpp/functions/core/slider.cpp
new file mode 100644
index 00000000..95fb5d20
--- /dev/null
+++ b/cpp/functions/core/slider.cpp
@@ -0,0 +1,80 @@
+#include "core/slider.h"
+#include "core/tokens.h"
+#include <imgui.h>
+#include <cstdio>
+
+namespace fn_ui {
+
+static void push_slider_style() {
+    using namespace fn_tokens;
+    ImGui::PushStyleColor(ImGuiCol_FrameBg,        colors::bg);
+    ImGui::PushStyleColor(ImGuiCol_FrameBgHovered, colors::surface_hover);
+    ImGui::PushStyleColor(ImGuiCol_FrameBgActive,  colors::surface);
+    ImGui::PushStyleColor(ImGuiCol_Border,         colors::border);
+    ImGui::PushStyleColor(ImGuiCol_SliderGrab,       colors::primary);
+    ImGui::PushStyleColor(ImGuiCol_SliderGrabActive, colors::primary_hover);
+    ImGui::PushStyleVar(ImGuiStyleVar_FrameRounding,   radius::sm);
+    ImGui::PushStyleVar(ImGuiStyleVar_FrameBorderSize, 1.0f);
+    ImGui::PushStyleVar(ImGuiStyleVar_GrabRounding,    radius::sm);
+}
+
+static void pop_slider_style() {
+    ImGui::PopStyleVar(3);
+    ImGui::PopStyleColor(6);
+}
+
+static void label_muted(const char* label) {
+    using namespace fn_tokens;
+    ImGui::PushStyleColor(ImGuiCol_Text, colors::text_muted);
+    ImGui::TextUnformatted(label);
+    ImGui::PopStyleColor();
+}
+
+bool slider_float(const char* label, float* value,
+                  float min_v, float max_v, const char* fmt) {
+    label_muted(label);
+    push_slider_style();
+    char id[160];
+    std::snprintf(id, sizeof(id), "##%s", label);
+    ImGui::SetNextItemWidth(-FLT_MIN);
+    bool changed = ImGui::SliderFloat(id, value, min_v, max_v, fmt);
+    pop_slider_style();
+    return changed;
+}
+
+bool slider_float_log(const char* label, float* value,
+                      float min_v, float max_v, const char* fmt) {
+    label_muted(label);
+    push_slider_style();
+    char id[160];
+    std::snprintf(id, sizeof(id), "##%s", label);
+    ImGui::SetNextItemWidth(-FLT_MIN);
+    bool changed = ImGui::SliderFloat(id, value, min_v, max_v, fmt,
+                                      ImGuiSliderFlags_Logarithmic);
+    pop_slider_style();
+    return changed;
+}
+
+bool slider_int(const char* label, int* value,
+                int min_v, int max_v, const char* fmt) {
+    label_muted(label);
+    push_slider_style();
+    char id[160];
+    std::snprintf(id, sizeof(id), "##%s", label);
+    ImGui::SetNextItemWidth(-FLT_MIN);
+    bool changed = ImGui::SliderInt(id, value, min_v, max_v, fmt);
+    pop_slider_style();
+    return changed;
+}
+
+bool slider_double(const char* label, double* value,
+                   double min_v, double max_v, const char* fmt) {
+    float fv = static_cast<float>(*value);
+    bool changed = slider_float(label, &fv,
+                                static_cast<float>(min_v),
+                                static_cast<float>(max_v), fmt);
+    if (changed) *value = static_cast<double>(fv);
+    return changed;
+}
+
+} // namespace fn_ui
diff --git a/cpp/functions/core/slider.h b/cpp/functions/core/slider.h
new file mode 100644
index 00000000..7396a58d
--- /dev/null
+++ b/cpp/functions/core/slider.h
@@ -0,0 +1,38 @@
+#pragma once
+
+// Slider con label muted arriba + value display, estilo acorde con fn_tokens.
+// Equivalente simple de <Slider> de Mantine / fn_library.
+//
+// Uso:
+//   static float p = 0.2f;
+//   fn_ui::slider_float("Probabilidad de pagar", &p, 0.0f, 1.0f);
+//
+//   static int n = 10;
+//   fn_ui::slider_int("Spins por sesion", &n, 1, 100);
+
+namespace fn_ui {
+
+// Renderiza label arriba + slider abajo. fmt es el printf-format que
+// ImGui usa para el value display (default "%.3f"). Devuelve true si el
+// valor cambio este frame.
+bool slider_float(const char* label, float* value,
+                  float min_v, float max_v,
+                  const char* fmt = "%.3f");
+
+// Logaritmico: util para parametros que cubren ordenes de magnitud
+// (sigma, learning rate, ...).
+bool slider_float_log(const char* label, float* value,
+                      float min_v, float max_v,
+                      const char* fmt = "%.4f");
+
+bool slider_int(const char* label, int* value,
+                int min_v, int max_v,
+                const char* fmt = "%d");
+
+// Variante double: ImGui no tiene SliderDouble, asi que internamente
+// down-cast a float — ojo si tu rango requiere precision fp64.
+bool slider_double(const char* label, double* value,
+                   double min_v, double max_v,
+                   const char* fmt = "%.6f");
+
+} // namespace fn_ui
diff --git a/cpp/functions/core/slider.md b/cpp/functions/core/slider.md
new file mode 100644
index 00000000..ca6deadc
--- /dev/null
+++ b/cpp/functions/core/slider.md
@@ -0,0 +1,66 @@
+---
+name: slider
+kind: function
+lang: cpp
+domain: core
+version: "1.0.0"
+purity: impure
+signature: "bool slider_float(const char* label, float* v, float min, float max, const char* fmt); bool slider_float_log(...); bool slider_int(const char* label, int* v, int min, int max, const char* fmt); bool slider_double(const char* label, double* v, double min, double max, const char* fmt)"
+description: "Slider ImGui con label muted arriba, estilo acorde con fn_tokens (radius, border, primary grab). Variantes float, float_log (logaritmico), int, double. Equivalente al <Slider> de Mantine / fn_library."
+tags: [imgui, slider, ui, tokens, mantine, core]
+uses_functions: ["tokens_cpp_core"]
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: [imgui.h, cstdio]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "cpp/functions/core/slider.cpp"
+framework: imgui
+params:
+  - name: label
+    desc: "Texto del label (mostrado en text_muted arriba). Tambien se usa como id ImGui (concatenado con ##)."
+  - name: value
+    desc: "Pointer al valor; mutado in-place si el usuario lo cambia."
+  - name: min_v
+    desc: "Limite inferior."
+  - name: max_v
+    desc: "Limite superior."
+  - name: fmt
+    desc: "printf-format para el value display. Float default '%.3f'; int default '%d'."
+output: "Renderiza label + slider full-width. Devuelve true si el valor cambio este frame."
+---
+
+# slider
+
+Slider canonico para los calculadores. Cada calculadora del set tiene 15-30 sliders; tener uno consistente con tokens y full-width acelera el desarrollo y mantiene la identidad visual.
+
+## Patron tipico
+
+```cpp
+static float prob = 0.2f;
+if (fn_ui::slider_float("Probabilidad de hit", &prob, 0.0f, 1.0f, "%.3f")) {
+    recalculate();
+}
+
+static float sigma = 0.1f;
+fn_ui::slider_float_log("Proposal sigma (log)", &sigma, 0.001f, 10.0f, "%.4f");
+
+static int n_chains = 4;
+fn_ui::slider_int("Numero de cadenas", &n_chains, 1, 16);
+
+static double mu = 0.0;
+fn_ui::slider_double("Mu", &mu, -10.0, 10.0, "%.6f");
+```
+
+## Estilo
+
+Usa `fn_tokens::colors::primary` para el grab (mismo color que `button` primary). Background `bg`, border `border`, radius `sm`. Coincide con la identidad de `text_input` y demas widgets `core`.
+
+## Notas
+
+- `slider_double` down-castea a float internamente (ImGui no expone SliderDouble). Si tu rango requiere fp64 usa `text_input` con parsing manual.
+- El label es tambien el id ImGui — duplicar labels en el mismo frame causa colision (ImGui asigna ids unicos basados en label). Si necesitas dos sliders con el mismo nombre visible, agregar un sufijo distinto (ej. "Mu##chain1" / "Mu##chain2").
+- Para multi-value (vec2/vec3) — no incluido aqui; usar 2/3 sliders separados o ImGui::SliderFloat2/3 directamente.