diff --git a/esphome/components/api/proto.cpp b/esphome/components/api/proto.cpp
index 3d1dc3fde0..a31c66e6c9 100644
--- a/esphome/components/api/proto.cpp
+++ b/esphome/components/api/proto.cpp
@@ -16,18 +16,7 @@ void ProtoByteBuffer::grow_(size_t n) {
   this->capacity_ = n;
 }
 
-uint32_t ProtoSize::varint_slow_(uint32_t value) {
-  // value is guaranteed >= 128 here (fast path handled inline)
-  if (value < 16384) {
-    return 2;  // 14 bits
-  } else if (value < 2097152) {
-    return 3;  // 21 bits
-  } else if (value < 268435456) {
-    return 4;  // 28 bits
-  } else {
-    return 5;  // 32 bits (maximum for uint32_t)
-  }
-}
+uint32_t ProtoSize::varint_slow(uint32_t value) { return varint_wide(value); }
 
 #ifdef USE_API_VARINT64
 optional<ProtoVarInt> ProtoVarInt::parse_wide(const uint8_t *buffer, uint32_t len, uint32_t *consumed,
diff --git a/esphome/components/api/proto.h b/esphome/components/api/proto.h
index bd73243ab5..513c4a7aa0 100644
--- a/esphome/components/api/proto.h
+++ b/esphome/components/api/proto.h
@@ -556,21 +556,26 @@ class ProtoSize {
   static constexpr inline uint32_t ESPHOME_ALWAYS_INLINE varint(uint32_t value) {
     if (value < 128)
       return 1;  // Fast path: 7 bits, most common case
-    if (__builtin_is_constant_evaluated()) {
-      // Compile-time: full cascade for constexpr callers
-      if (value < 16384)
-        return 2;
-      if (value < 2097152)
-        return 3;
-      if (value < 268435456)
-        return 4;
-      return 5;
-    }
-    return varint_slow_(value);
+    if (__builtin_is_constant_evaluated())
+      return varint_wide(value);
+    return varint_slow(value);
   }
-  // Slow path for varint >= 128, outlined to keep fast path small
-  static uint32_t varint_slow_(uint32_t value) __attribute__((noinline));
 
+ private:
+  // Slow path for varint >= 128, outlined to keep fast path small
+  static uint32_t varint_slow(uint32_t value) __attribute__((noinline));
+  // Shared cascade for values >= 128 (used by both constexpr and noinline paths)
+  static constexpr inline uint32_t ESPHOME_ALWAYS_INLINE varint_wide(uint32_t value) {
+    if (value < 16384)
+      return 2;
+    if (value < 2097152)
+      return 3;
+    if (value < 268435456)
+      return 4;
+    return 5;
+  }
+
+ public:
   /**
    * @brief Calculates the size in bytes needed to encode a uint64_t value as a varint
    *