diff --git a/esphome/core/helpers.cpp b/esphome/core/helpers.cpp
index e4ce672570..ede1640af2 100644
--- a/esphome/core/helpers.cpp
+++ b/esphome/core/helpers.cpp
@@ -545,17 +545,19 @@ std::string value_accuracy_to_string(float value, int8_t accuracy_decimals) {
 
 // Fast float-to-string for accuracy_decimals 0-3 (covers virtually all sensor usage).
 // Avoids snprintf("%.*f") which pulls in heavy float formatting machinery.
-static size_t value_accuracy_to_buf_fast(char *buf, float value, int8_t accuracy_decimals) {
+// Caller must guarantee value is finite and |value| * mult fits in uint32_t.
+static size_t value_accuracy_to_buf_fast(char *buf, float value, int8_t accuracy_decimals, uint32_t mult) {
   char *p = buf;
   if (std::signbit(value)) {
     *p++ = '-';
     value = -value;
   }
-  uint32_t mult = small_pow10(accuracy_decimals);
   // Cast to double for the multiply to match snprintf's rounding precision.
   // float*int loses bits at exact-half boundaries (e.g. 23.45f*10 = 234.5 in float,
   // but snprintf sees 234.500007... via double promotion and rounds differently).
-  uint32_t scaled = static_cast<uint32_t>(lrint(static_cast<double>(value) * mult));
+  // llrint returns long long so the result fits even on 32-bit targets where
+  // long is 32-bit; caller has already bounded |value * mult| to UINT32_MAX.
+  uint32_t scaled = static_cast<uint32_t>(llrint(static_cast<double>(value) * mult));
   p = uint32_to_str_unchecked(p, scaled / mult);
   if (accuracy_decimals > 0) {
     *p++ = '.';
@@ -568,12 +570,16 @@ static size_t value_accuracy_to_buf_fast(char *buf, float value, int8_t accuracy
 size_t value_accuracy_to_buf(std::span<char, VALUE_ACCURACY_MAX_LEN> buf, float value, int8_t accuracy_decimals) {
   normalize_accuracy_decimals(value, accuracy_decimals);
 
-  // Fast path for accuracy 0-3 and finite values
+  // Fast path for accuracy 0-3, finite values whose scaled magnitude fits in uint32_t.
+  // For 3 decimals that's |value| < ~4.29e6; larger totals fall through to snprintf.
   if (accuracy_decimals <= 3 && std::isfinite(value)) {
-    return value_accuracy_to_buf_fast(buf.data(), value, accuracy_decimals);
+    const uint32_t mult = small_pow10(accuracy_decimals);
+    if (std::fabs(value) < static_cast<float>(UINT32_MAX) / mult) {
+      return value_accuracy_to_buf_fast(buf.data(), value, accuracy_decimals, mult);
+    }
   }
 
-  // Fallback for NaN/Inf/high accuracy
+  // Fallback for NaN/Inf/high accuracy/out-of-range
   int len = snprintf(buf.data(), buf.size(), "%.*f", accuracy_decimals, value);
   if (len < 0)
     return 0;
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index 55f411fdec..5b4e034d8b 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -1296,7 +1296,10 @@ inline char *int8_to_str(char *buf, int8_t val) {
 }
 
 /// Append a separator char and a string to a buffer, respecting remaining space.
-/// Returns pointer past last char written (null terminator is written).
+/// Returns pointer past last char written. On success (remaining >= 2) a null
+/// terminator is written after the copied string. If remaining < 2 nothing is
+/// written (not even a terminator) and `buf` is returned unchanged — callers
+/// needing a terminated buffer in that case must ensure one is already present.
 inline char *buf_append_sep_str(char *buf, size_t remaining, char separator, const char *str, size_t str_len) {
   if (remaining < 2) {
     return buf;
diff --git a/tests/components/core/test_value_accuracy.cpp b/tests/components/core/test_value_accuracy.cpp
index a1fba90acf..6bd8976acc 100644
--- a/tests/components/core/test_value_accuracy.cpp
+++ b/tests/components/core/test_value_accuracy.cpp
@@ -3,10 +3,11 @@
 #include <cstdio>
 #include <cstring>
 #include <span>
+#include <string>
 
 #include "esphome/core/helpers.h"
 
-namespace esphome::testing {
+namespace esphome::core::testing {
 
 // Helper to call value_accuracy_to_buf and return as string
 static std::string va_to_string(float value, int8_t accuracy_decimals) {
@@ -148,4 +149,4 @@ TEST(ValueAccuracyToBuf, ReturnsCorrectLength) {
   EXPECT_EQ(strlen(buf), len);
 }
 
-}  // namespace esphome::testing
+}  // namespace esphome::core::testing