diff --git a/esphome/components/api/api_frame_helper_plaintext.cpp b/esphome/components/api/api_frame_helper_plaintext.cpp
index ed3cc8934e..5069dbf68b 100644
--- a/esphome/components/api/api_frame_helper_plaintext.cpp
+++ b/esphome/components/api/api_frame_helper_plaintext.cpp
@@ -295,9 +295,8 @@ APIError APIPlaintextFrameHelper::write_protobuf_messages(ProtoWriteBuffer buffe
     buf_start[header_offset] = 0x00;  // indicator
 
     // Encode varints directly into buffer
-    ProtoVarInt(msg.payload_size).encode_to_buffer_unchecked(buf_start + header_offset + 1, size_varint_len);
-    ProtoVarInt(msg.message_type)
-        .encode_to_buffer_unchecked(buf_start + header_offset + 1 + size_varint_len, type_varint_len);
+    encode_varint_to_buffer(msg.payload_size, buf_start + header_offset + 1);
+    encode_varint_to_buffer(msg.message_type, buf_start + header_offset + 1 + size_varint_len);
 
     // Add iovec for this message (header + payload)
     size_t msg_len = static_cast<size_t>(total_header_len + msg.payload_size);
diff --git a/esphome/components/api/proto.h b/esphome/components/api/proto.h
index 41ea0043f9..8ac79633cf 100644
--- a/esphome/components/api/proto.h
+++ b/esphome/components/api/proto.h
@@ -57,6 +57,16 @@ inline uint16_t count_packed_varints(const uint8_t *data, size_t len) {
   return count;
 }
 
+/// Encode a varint directly into a pre-allocated buffer.
+/// Caller must ensure buffer has space (use ProtoSize::varint() to calculate).
+inline void encode_varint_to_buffer(uint32_t val, uint8_t *buffer) {
+  while (val > 0x7F) {
+    *buffer++ = static_cast<uint8_t>(val | 0x80);
+    val >>= 7;
+  }
+  *buffer = static_cast<uint8_t>(val);
+}
+
 /*
  * StringRef Ownership Model for API Protocol Messages
  * ===================================================
@@ -93,17 +103,17 @@ class ProtoVarInt {
   ProtoVarInt() : value_(0) {}
   explicit ProtoVarInt(uint64_t value) : value_(value) {}
 
+  /// Parse a varint from buffer. consumed must be a valid pointer (not null).
   static optional<ProtoVarInt> parse(const uint8_t *buffer, uint32_t len, uint32_t *consumed) {
-    if (len == 0) {
-      if (consumed != nullptr)
-        *consumed = 0;
+#ifdef ESPHOME_DEBUG_API
+    assert(consumed != nullptr);
+#endif
+    if (len == 0)
       return {};
-    }
 
     // Most common case: single-byte varint (values 0-127)
     if ((buffer[0] & 0x80) == 0) {
-      if (consumed != nullptr)
-        *consumed = 1;
+      *consumed = 1;
       return ProtoVarInt(buffer[0]);
     }
 
@@ -122,14 +132,11 @@ class ProtoVarInt {
       result |= uint64_t(val & 0x7F) << uint64_t(bitpos);
       bitpos += 7;
       if ((val & 0x80) == 0) {
-        if (consumed != nullptr)
-          *consumed = i + 1;
+        *consumed = i + 1;
         return ProtoVarInt(result);
       }
     }
 
-    if (consumed != nullptr)
-      *consumed = 0;
     return {};  // Incomplete or invalid varint
   }
 
@@ -153,50 +160,6 @@ class ProtoVarInt {
     // with ZigZag encoding
     return decode_zigzag64(this->value_);
   }
-  /**
-   * Encode the varint value to a pre-allocated buffer without bounds checking.
-   *
-   * @param buffer The pre-allocated buffer to write the encoded varint to
-   * @param len The size of the buffer in bytes
-   *
-   * @note The caller is responsible for ensuring the buffer is large enough
-   *       to hold the encoded value. Use ProtoSize::varint() to calculate
-   *       the exact size needed before calling this method.
-   * @note No bounds checking is performed for performance reasons.
-   */
-  void encode_to_buffer_unchecked(uint8_t *buffer, size_t len) {
-    uint64_t val = this->value_;
-    if (val <= 0x7F) {
-      buffer[0] = val;
-      return;
-    }
-    size_t i = 0;
-    while (val && i < len) {
-      uint8_t temp = val & 0x7F;
-      val >>= 7;
-      if (val) {
-        buffer[i++] = temp | 0x80;
-      } else {
-        buffer[i++] = temp;
-      }
-    }
-  }
-  void encode(std::vector<uint8_t> &out) {
-    uint64_t val = this->value_;
-    if (val <= 0x7F) {
-      out.push_back(val);
-      return;
-    }
-    while (val) {
-      uint8_t temp = val & 0x7F;
-      val >>= 7;
-      if (val) {
-        out.push_back(temp | 0x80);
-      } else {
-        out.push_back(temp);
-      }
-    }
-  }
 
  protected:
   uint64_t value_;
@@ -256,8 +219,20 @@ class ProtoWriteBuffer {
  public:
   ProtoWriteBuffer(std::vector<uint8_t> *buffer) : buffer_(buffer) {}
   void write(uint8_t value) { this->buffer_->push_back(value); }
-  void encode_varint_raw(ProtoVarInt value) { value.encode(*this->buffer_); }
-  void encode_varint_raw(uint32_t value) { this->encode_varint_raw(ProtoVarInt(value)); }
+  void encode_varint_raw(uint32_t value) {
+    while (value > 0x7F) {
+      this->buffer_->push_back(static_cast<uint8_t>(value | 0x80));
+      value >>= 7;
+    }
+    this->buffer_->push_back(static_cast<uint8_t>(value));
+  }
+  void encode_varint_raw_64(uint64_t value) {
+    while (value > 0x7F) {
+      this->buffer_->push_back(static_cast<uint8_t>(value | 0x80));
+      value >>= 7;
+    }
+    this->buffer_->push_back(static_cast<uint8_t>(value));
+  }
   /**
    * Encode a field key (tag/wire type combination).
    *
@@ -307,13 +282,13 @@ class ProtoWriteBuffer {
     if (value == 0 && !force)
       return;
     this->encode_field_raw(field_id, 0);  // type 0: Varint - uint64
-    this->encode_varint_raw(ProtoVarInt(value));
+    this->encode_varint_raw_64(value);
   }
   void encode_bool(uint32_t field_id, bool value, bool force = false) {
     if (!value && !force)
       return;
     this->encode_field_raw(field_id, 0);  // type 0: Varint - bool
-    this->write(0x01);
+    this->buffer_->push_back(value ? 0x01 : 0x00);
   }
   void encode_fixed32(uint32_t field_id, uint32_t value, bool force = false) {
     if (value == 0 && !force)
@@ -938,13 +913,15 @@ inline void ProtoWriteBuffer::encode_message(uint32_t field_id, const ProtoMessa
   this->buffer_->resize(this->buffer_->size() + varint_length_bytes);
 
   // Write the length varint directly
-  ProtoVarInt(msg_length_bytes).encode_to_buffer_unchecked(this->buffer_->data() + begin, varint_length_bytes);
+  encode_varint_to_buffer(msg_length_bytes, this->buffer_->data() + begin);
 
   // Now encode the message content - it will append to the buffer
   value.encode(*this);
 
+#ifdef ESPHOME_DEBUG_API
   // Verify that the encoded size matches what we calculated
   assert(this->buffer_->size() == begin + varint_length_bytes + msg_length_bytes);
+#endif
 }
 
 // Implementation of decode_to_message - must be after ProtoDecodableMessage is defined
diff --git a/esphome/core/defines.h b/esphome/core/defines.h
index bfe9d620df..7e6df31ea2 100644
--- a/esphome/core/defines.h
+++ b/esphome/core/defines.h
@@ -14,6 +14,7 @@
 #define ESPHOME_PROJECT_VERSION_30 "v2"
 #define ESPHOME_VARIANT "ESP32"
 #define ESPHOME_DEBUG_SCHEDULER
+#define ESPHOME_DEBUG_API
 
 // Default threading model for static analysis (ESP32 is multi-threaded with atomics)
 #define ESPHOME_THREAD_MULTI_ATOMICS
diff --git a/tests/integration/conftest.py b/tests/integration/conftest.py
index 50e8d4122b..36df1bc83e 100644
--- a/tests/integration/conftest.py
+++ b/tests/integration/conftest.py
@@ -197,6 +197,7 @@ async def yaml_config(request: pytest.FixtureRequest, unused_tcp_port: int) -> s
             "  platformio_options:\n"
             "    build_flags:\n"
             '      - "-DDEBUG"  # Enable assert() statements\n'
+            '      - "-DESPHOME_DEBUG_API"  # Enable API protocol asserts\n'
             '      - "-g"       # Add debug symbols',
         )