diff --git a/esphome/components/web_server_idf/multipart_parser_utils.h b/esphome/components/web_server_idf/multipart_parser_utils.h
index 5787e3d880..d58232a067 100644
--- a/esphome/components/web_server_idf/multipart_parser_utils.h
+++ b/esphome/components/web_server_idf/multipart_parser_utils.h
@@ -14,201 +14,31 @@ namespace web_server_idf {
 inline bool char_equals_ci(char a, char b) { return ::tolower(a) == ::tolower(b); }
 
 // Helper function for case-insensitive string region comparison
-inline bool str_ncmp_ci(const char *s1, const char *s2, size_t n) {
-  for (size_t i = 0; i < n; i++) {
-    if (!char_equals_ci(s1[i], s2[i])) {
-      return false;
-    }
-  }
-  return true;
-}
+bool str_ncmp_ci(const char *s1, const char *s2, size_t n);
 
 // Case-insensitive string prefix check
-inline bool str_startswith_case_insensitive(const std::string &str, const std::string &prefix) {
-  if (str.length() < prefix.length()) {
-    return false;
-  }
-  return str_ncmp_ci(str.c_str(), prefix.c_str(), prefix.length());
-}
+bool str_startswith_case_insensitive(const std::string &str, const std::string &prefix);
 
 // Find a substring case-insensitively
-inline size_t str_find_case_insensitive(const std::string &haystack, const std::string &needle, size_t pos = 0) {
-  if (needle.empty() || pos >= haystack.length()) {
-    return std::string::npos;
-  }
-
-  const size_t needle_len = needle.length();
-  const size_t max_pos = haystack.length() - needle_len;
-
-  for (size_t i = pos; i <= max_pos; i++) {
-    if (str_ncmp_ci(haystack.c_str() + i, needle.c_str(), needle_len)) {
-      return i;
-    }
-  }
-
-  return std::string::npos;
-}
+size_t str_find_case_insensitive(const std::string &haystack, const std::string &needle, size_t pos = 0);
 
 // Extract a parameter value from a header line
 // Handles both quoted and unquoted values
-inline std::string extract_header_param(const std::string &header, const std::string &param) {
-  size_t search_pos = 0;
-
-  while (search_pos < header.length()) {
-    // Look for param name
-    size_t pos = str_find_case_insensitive(header, param, search_pos);
-    if (pos == std::string::npos) {
-      return "";
-    }
-
-    // Check if this is a word boundary (not part of another parameter)
-    if (pos > 0 && header[pos - 1] != ' ' && header[pos - 1] != ';' && header[pos - 1] != '\t') {
-      search_pos = pos + 1;
-      continue;
-    }
-
-    // Move past param name
-    pos += param.length();
-
-    // Skip whitespace and find '='
-    while (pos < header.length() && (header[pos] == ' ' || header[pos] == '\t')) {
-      pos++;
-    }
-
-    if (pos >= header.length() || header[pos] != '=') {
-      search_pos = pos;
-      continue;
-    }
-
-    pos++;  // Skip '='
-
-    // Skip whitespace after '='
-    while (pos < header.length() && (header[pos] == ' ' || header[pos] == '\t')) {
-      pos++;
-    }
-
-    if (pos >= header.length()) {
-      return "";
-    }
-
-    // Check if value is quoted
-    if (header[pos] == '"') {
-      pos++;
-      size_t end = header.find('"', pos);
-      if (end != std::string::npos) {
-        return header.substr(pos, end - pos);
-      }
-      // Malformed - no closing quote
-      return "";
-    }
-
-    // Unquoted value - find the end (semicolon, comma, or end of string)
-    size_t end = pos;
-    while (end < header.length() && header[end] != ';' && header[end] != ',' && header[end] != ' ' &&
-           header[end] != '\t') {
-      end++;
-    }
-
-    return header.substr(pos, end - pos);
-  }
-
-  return "";
-}
+std::string extract_header_param(const std::string &header, const std::string &param);
 
 // Case-insensitive string search (like strstr but case-insensitive)
-inline const char *stristr(const char *haystack, const char *needle) {
-  if (!haystack || !needle) {
-    return nullptr;
-  }
-
-  size_t needle_len = strlen(needle);
-  if (needle_len == 0) {
-    return haystack;
-  }
-
-  for (const char *p = haystack; *p; p++) {
-    if (str_ncmp_ci(p, needle, needle_len)) {
-      return p;
-    }
-  }
-
-  return nullptr;
-}
+const char *stristr(const char *haystack, const char *needle);
 
 // Parse boundary from Content-Type header
 // Returns true if boundary found, false otherwise
 // boundary_start and boundary_len will point to the boundary value
-inline bool parse_multipart_boundary(const char *content_type, const char **boundary_start, size_t *boundary_len) {
-  if (!content_type) {
-    return false;
-  }
-
-  // Check for multipart/form-data (case-insensitive)
-  if (!stristr(content_type, "multipart/form-data")) {
-    return false;
-  }
-
-  // Look for boundary parameter
-  const char *b = stristr(content_type, "boundary=");
-  if (!b) {
-    return false;
-  }
-
-  const char *start = b + 9;  // Skip "boundary="
-
-  // Skip whitespace
-  while (*start == ' ' || *start == '\t') {
-    start++;
-  }
-
-  if (!*start) {
-    return false;
-  }
-
-  // Find end of boundary
-  const char *end = start;
-  if (*end == '"') {
-    // Quoted boundary
-    start++;
-    end++;
-    while (*end && *end != '"') {
-      end++;
-    }
-    *boundary_len = end - start;
-  } else {
-    // Unquoted boundary
-    while (*end && *end != ' ' && *end != ';' && *end != '\r' && *end != '\n' && *end != '\t') {
-      end++;
-    }
-    *boundary_len = end - start;
-  }
-
-  if (*boundary_len == 0) {
-    return false;
-  }
-
-  *boundary_start = start;
-  return true;
-}
+bool parse_multipart_boundary(const char *content_type, const char **boundary_start, size_t *boundary_len);
 
 // Check if content type is form-urlencoded (case-insensitive)
-inline bool is_form_urlencoded(const char *content_type) {
-  if (!content_type) {
-    return false;
-  }
-
-  return stristr(content_type, "application/x-www-form-urlencoded") != nullptr;
-}
+bool is_form_urlencoded(const char *content_type);
 
 // Trim whitespace from both ends of a string
-inline std::string str_trim(const std::string &str) {
-  size_t start = str.find_first_not_of(" \t\r\n");
-  if (start == std::string::npos) {
-    return "";
-  }
-  size_t end = str.find_last_not_of(" \t\r\n");
-  return str.substr(start, end - start + 1);
-}
+std::string str_trim(const std::string &str);
 
 }  // namespace web_server_idf
 }  // namespace esphome