diff --git a/configconverter/converters.py b/configconverter/converters.py
new file mode 100644
index 0000000..e7529bc
--- /dev/null
+++ b/configconverter/converters.py
@@ -0,0 +1,180 @@
+"""Core conversion logic for config-converter-cli."""
+
+import io
+import json
+from typing import Any, Dict, Optional
+
+import tomlkit
+from ruamel.yaml import YAML
+
+from configconverter.exceptions import (
+    InvalidFormatError,
+    ParseError,
+    UnsupportedConversionError,
+)
+
+
+def _toml_to_dict(obj: Any) -> Any:
+    """Convert TOMLDocument to dict recursively."""
+    if hasattr(obj, 'unwrap'):
+        return _toml_to_dict(obj.unwrap())
+    elif isinstance(obj, tomlkit.items.Array):
+        return [_toml_to_dict(item) for item in obj]
+    elif isinstance(obj, dict):
+        return {key: _toml_to_dict(value) for key, value in obj.items()}
+    return obj
+
+
+class Converter:
+    """Bidirectional converter between JSON, YAML, and TOML formats."""
+
+    SUPPORTED_FORMATS = ("json", "yaml", "toml")
+
+    def __init__(self):
+        self.yaml_parser = YAML()
+        self.yaml_parser.preserve_quotes = True
+        self.yaml_parser.allow_duplicate_keys = False
+
+    def detect_format(self, content: str) -> str:
+        """Detect the format of the input content."""
+        content = content.strip()
+
+        if not content:
+            raise InvalidFormatError("Empty content")
+
+        first_char = content[0]
+
+        if first_char in ("{", "["):
+            if self._is_valid_json(content):
+                return "json"
+
+        try:
+            parsed = tomlkit.parse(content)
+            if parsed:
+                return "toml"
+        except Exception:
+            pass
+
+        try:
+            self.yaml_parser.load(content)
+            return "yaml"
+        except Exception:
+            pass
+
+        raise InvalidFormatError("Could not determine input format")
+
+    def _is_valid_json(self, content: str) -> bool:
+        """Check if content is valid JSON."""
+        try:
+            json.loads(content)
+            return True
+        except json.JSONDecodeError:
+            return False
+
+    def convert(self, content: str, from_format: str, to_format: str) -> str:
+        """Convert content from one format to another."""
+        from_format = from_format.lower()
+        if from_format == "auto":
+            from_format = self.detect_format(content)
+
+        to_format = to_format.lower()
+
+        if from_format not in self.SUPPORTED_FORMATS:
+            raise UnsupportedConversionError(f"Unsupported source format: {from_format}")
+        if to_format not in self.SUPPORTED_FORMATS:
+            raise UnsupportedConversionError(f"Unsupported target format: {to_format}")
+
+        data = self._parse(content, from_format)
+
+        return self._serialize(data, to_format)
+
+    def _parse(self, content: str, format: str) -> Any:
+        """Parse content based on format."""
+        if format == "json":
+            return self._parse_json(content)
+        elif format == "yaml":
+            return self._parse_yaml(content)
+        elif format == "toml":
+            return self._parse_toml(content)
+        else:
+            raise UnsupportedConversionError(f"Unsupported format: {format}")
+
+    def _parse_json(self, content: str) -> Any:
+        """Parse JSON content."""
+        try:
+            return json.loads(content)
+        except json.JSONDecodeError as e:
+            raise ParseError(
+                f"Invalid JSON syntax: {e.msg}",
+                line_number=e.lineno,
+                column=e.colno,
+                context=e.doc,
+            )
+
+    def _parse_yaml(self, content: str) -> Any:
+        """Parse YAML content."""
+        try:
+            return self.yaml_parser.load(content)
+        except Exception as e:
+            error_msg = str(e)
+            line_info = ""
+            if hasattr(e, "problem_mark") and e.problem_mark:
+                line_info = f" (line {e.problem_mark.line + 1}, column {e.problem_mark.column + 1})"
+            raise ParseError(f"Invalid YAML syntax: {error_msg}{line_info}")
+
+    def _parse_toml(self, content: str) -> Any:
+        """Parse TOML content."""
+        try:
+            parsed = tomlkit.parse(content)
+            return _toml_to_dict(parsed)
+        except tomlkit.exceptions.ParseError as e:
+            line_info = ""
+            if e.line is not None:
+                line_info = f" (line {e.line})"
+            raise ParseError(f"Invalid TOML syntax: {e}{line_info}")
+
+    def _serialize(self, data: Any, format: str) -> str:
+        """Serialize data to the specified format."""
+        if format == "json":
+            return self._to_json(data)
+        elif format == "yaml":
+            return self._to_yaml(data)
+        elif format == "toml":
+            return self._to_toml(data)
+        else:
+            raise UnsupportedConversionError(f"Unsupported format: {format}")
+
+    def _to_json(self, data: Any, indent: int = 2) -> str:
+        """Convert data to JSON string."""
+        return json.dumps(data, indent=indent, ensure_ascii=False)
+
+    def _to_yaml(self, data: Any) -> str:
+        """Convert data to YAML string."""
+        stream = io.StringIO()
+        self.yaml_parser.dump(data, stream)
+        return stream.getvalue()
+
+    def _to_toml(self, data: Any) -> str:
+        """Convert data to TOML string."""
+        return tomlkit.dumps(data)
+
+    def serialize_with_indent(
+        self, data: Any, format: str, indent: int = 2, compact: bool = False
+    ) -> str:
+        """Serialize data with custom indentation."""
+        if format == "json":
+            if compact:
+                return json.dumps(data, separators=(',', ':'), ensure_ascii=False)
+            return json.dumps(data, indent=indent, ensure_ascii=False)
+        elif format == "yaml":
+            stream = io.StringIO()
+            self.yaml_parser.indent(mapping=indent, sequence=indent, offset=2)
+            self.yaml_parser.dump(data, stream)
+            return stream.getvalue()
+        elif format == "toml":
+            return tomlkit.dumps(data)
+        else:
+            raise UnsupportedConversionError(f"Unsupported format: {format}")
+
+
+converter = Converter()