garmin · Lijah99 · Apr 8, 2026 · Apr 8, 2026
diff --git a/README.md b/README.md
@@ -58,7 +58,7 @@ A file must pass all three of these tests to be considered a valid FIT file. See
 #### Read Method
 The Read method decodes all messages from the input stream and returns an object containing a list of errors encountered during the decoding and a dictionary of decoded messages grouped by message type. Any exceptions encountered during decoding will be caught by the Read method and added to the list of errors.
 
-The Read method accepts an optional options object that can be used to customize how field data is represented in the decoded messages. All options are enabled by default. Disabling options may speed up file decoding. Options may also be enabled or disable based on how the decoded data will be used.
+The Read method accepts an optional options object that can be used to customize how field data is represented in the decoded messages. All options are enabled by default. Disabling options may speed up file decoding. Options may also be enabled or disabled based on how the decoded data will be used.
 
 ```py
 messages, errors = read(
@@ -72,7 +72,7 @@ messages, errors = read(
             mesg_listener = None)
 ```
 #### mesg_listener
-Optional callback function that can be used to inspect or manipulate messages after they are fully decoded and all the options have been applied. The message is mutable and we be returned from the Read method in the messages dictionary.
+Optional callback function that can be used to inspect or manipulate messages after they are fully decoded and all the options have been applied. The message is mutable and will be returned from the Read method in the messages dictionary.
 
 Example mesg_listener callback that tracks the field names across all Record messages.
 
@@ -107,7 +107,7 @@ When true the scale and offset values as defined in the FIT Profile are applied
 When false the raw field value is used.
 ```py
 {
-  'altitude': 10435 ## raw value store in file
+  'altitude': 10435 ## raw value stored in file
 }
 ```
 #### enable_crc_check: true | false
@@ -164,7 +164,7 @@ When true FIT Epoch values are converted to Python datetime objects.
 ```py
 { 'time_created': {Python datetime object} }
 ```
-When false the FIT Epoch value  is used.
+When false the FIT Epoch value is used.
 ```py
 { 'time_created': 995749880 }
 ```
@@ -210,8 +210,78 @@ The FIT_EPOCH_S value can be used to convert FIT Epoch values to Python datetime
 ```py
 python_date = datetime.datetime.fromtimestamp(fitDateTime + FIT_EPOCH_S, datetime.UTC)
 ```
+### BASE_TYPE_TO_FIELD_TYPE Constant
+`BASE_TYPE_TO_FIELD_TYPE` is a dictionary that maps FIT base type values to their corresponding field type name strings as defined in the FIT Profile.
+```py
+field_type_string = BASE_TYPE_TO_FIELD_TYPE[base_type_value]
+# e.g. BASE_TYPE_TO_FIELD_TYPE[BASE_TYPE['UINT32']] == 'uint32'
+```
+### FIELD_TYPE_TO_BASE_TYPE Constant
+`FIELD_TYPE_TO_BASE_TYPE` is the inverse mapping of `BASE_TYPE_TO_FIELD_TYPE`. It maps FIT field type name strings to their corresponding base type values as defined in the FIT Profile.
+```py
+base_type_value = FIELD_TYPE_TO_BASE_TYPE[field_type_string]
+# e.g. FIELD_TYPE_TO_BASE_TYPE['uint32'] == BASE_TYPE['UINT32']
+```
 ### convert_timestamp_to_datetime Method
 A convenience method for converting FIT Epoch values to Python Datetime objects.
 ```py
 python_date = convert_timestamp_to_datetime(fit_datetime)
-```
+```
+### convert_datetime_to_timestamp Method
+A convenience method for converting Python Datetime objects to FIT Epoch values.
+```py
+fit_datetime = convert_datetime_to_timestamp(python_date)
+```
+
+## Encoder
+### Usage
+```py
+from datetime import datetime, timezone
+
+from garmin_fit_sdk import Encoder, Profile
+
+encoder = Encoder()
+
+# Pass the MesgNum and message data as separate parameters to the onMesg() method
+encoder.on_mesg(Profile['mesg_num']['FILE_ID'], {
+    'manufacturer': 'development',
+    'product': 1,
+    'time_created': datetime.now(tz=timezone.utc),
+    'type': 'activity',
+})
+
+# The writeMesg() method expects the mesgNum to be included in the message data
+# Internally, writeMesg() calls onMesg()
+encoder.write_mesg({
+    'mesg_num': Profile['mesg_num']['FILE_ID'],
+    'manufacturer': 'development',
+    'product': 1,
+    'time_created': datetime.now(tz=timezone.utc),
+    'type': 'activity',
+})
+
+# Unknown values in the message will be ignored by the Encoder
+encoder.on_mesg(Profile['mesg_num']['FILE_ID'], {
+    'manufacturer': 'development',
+    'product': 1,
+    'time_created': datetime.now(tz=timezone.utc),
+    'type': 'activity',
+    'customField': 12345, # This value will be ignored by the Encoder
+})
+
+# Subfield values in the message will be ignored by the Encoder
+encoder.on_mesg(Profile['mesg_num']['FILE_ID'], {
+    'manufacturer': 'development',
+    'product': 4440, # This is the main product field, which is a uint16
+    'garmin_product': 'edge_1050', # This value will be ignored by the Encoder, use the main field value instead
+    'time_created': datetime.now(tz=timezone.utc),
+    'type': 'activity',
+})
+
+uint8_array = encoder.close()
+
+# Write the bytes to a file
+with open('example.fit', 'wb') as f:
+    f.write(uint8_array)
+```
+See the [Encode Activity Recipe](https://github.com/garmin/fit-python-sdk/blob/main/tests/test_encode_activity_recipe.py) for a complete example of encoding a FIT Activity file using the FIT Python SDK.
diff --git a/garmin_fit_sdk/__init__.py b/garmin_fit_sdk/__init__.py
@@ -7,19 +7,20 @@
 # Transfer (FIT) Protocol License.
 ###########################################################################################
 # ****WARNING****  This file is auto-generated!  Do NOT edit this file.
-# Profile Version = 21.195.0Release
-# Tag = production/release/21.195.0-0-g569e7e5
+# Profile Version = 21.200.0Release
+# Tag = production/release/21.200.0-0-g28b5705d
 ############################################################################################
 
 
 from garmin_fit_sdk.accumulator import Accumulator
 from garmin_fit_sdk.bitstream import BitStream
 from garmin_fit_sdk.crc_calculator import CrcCalculator
 from garmin_fit_sdk.decoder import Decoder
+from garmin_fit_sdk.encoder import Encoder
 from garmin_fit_sdk.fit import BASE_TYPE, BASE_TYPE_DEFINITIONS
 from garmin_fit_sdk.hr_mesg_utils import expand_heart_rates
 from garmin_fit_sdk.profile import Profile
 from garmin_fit_sdk.stream import Stream
-from garmin_fit_sdk.util import FIT_EPOCH_S, convert_timestamp_to_datetime
+from garmin_fit_sdk.util import FIT_EPOCH_S, convert_datetime_to_timestamp, convert_timestamp_to_datetime, BASE_TYPE_TO_FIELD_TYPE, FIELD_TYPE_TO_BASE_TYPE
 
-__version__ = '21.195.0'
+__version__ = '21.200.0'
diff --git a/garmin_fit_sdk/accumulator.py b/garmin_fit_sdk/accumulator.py
@@ -7,8 +7,8 @@
 # Transfer (FIT) Protocol License.
 ###########################################################################################
 # ****WARNING****  This file is auto-generated!  Do NOT edit this file.
-# Profile Version = 21.195.0Release
-# Tag = production/release/21.195.0-0-g569e7e5
+# Profile Version = 21.200.0Release
+# Tag = production/release/21.200.0-0-g28b5705d
 ############################################################################################
 
 

diff --git a/garmin_fit_sdk/bitstream.py b/garmin_fit_sdk/bitstream.py
@@ -7,8 +7,8 @@
 # Transfer (FIT) Protocol License.
 ###########################################################################################
 # ****WARNING****  This file is auto-generated!  Do NOT edit this file.
-# Profile Version = 21.195.0Release
-# Tag = production/release/21.195.0-0-g569e7e5
+# Profile Version = 21.200.0Release
+# Tag = production/release/21.200.0-0-g28b5705d
 ############################################################################################
 
 

diff --git a/garmin_fit_sdk/crc_calculator.py b/garmin_fit_sdk/crc_calculator.py
@@ -7,8 +7,8 @@
 # Transfer (FIT) Protocol License.
 ###########################################################################################
 # ****WARNING****  This file is auto-generated!  Do NOT edit this file.
-# Profile Version = 21.195.0Release
-# Tag = production/release/21.195.0-0-g569e7e5
+# Profile Version = 21.200.0Release
+# Tag = production/release/21.200.0-0-g28b5705d
 ############################################################################################
 
 

diff --git a/garmin_fit_sdk/decoder.py b/garmin_fit_sdk/decoder.py
@@ -7,8 +7,8 @@
 # Transfer (FIT) Protocol License.
 ###########################################################################################
 # ****WARNING****  This file is auto-generated!  Do NOT edit this file.
-# Profile Version = 21.195.0Release
-# Tag = production/release/21.195.0-0-g569e7e5
+# Profile Version = 21.200.0Release
+# Tag = production/release/21.200.0-0-g28b5705d
 ############################################################################################
 
 
@@ -23,10 +23,7 @@
 
 _CRCSIZE = 2
 _COMPRESSED_HEADER_MASK = 0x80
-_MESG_DEFINITION_MASK = 0x40
 _MESG_HEADER_MASK = 0x00
-_LOCAL_MESG_NUM_MASK = 0x0F
-_DEV_DATA_MASK = 0x20
 
 _HEADER_WITH_CRC_SIZE = 14
 _HEADER_WITHOUT_CRC_SIZE = 12
@@ -60,6 +57,8 @@ def __init__(self, stream: Stream):
         self._decode_mode = DecodeMode.NORMAL
 
         self._mesg_listener = None
+        self._mesg_definition_listener = None
+        self._field_description_listener = None
         self._apply_scale_and_offset = True
         self._convert_timestamps_to_datetimes = True
         self._convert_types_to_strings = True
@@ -121,6 +120,8 @@ def read(self, apply_scale_and_offset = True,
                 expand_components = True,
                 merge_heart_rates = True,
                 mesg_listener = None,
+                mesg_definition_listener = None,
+                field_description_listener = None,
                 decode_mode = DecodeMode.NORMAL):
         '''Reads the entire contents of the fit file and returns the decoded messages'''
         self._apply_scale_and_offset = apply_scale_and_offset
@@ -131,6 +132,8 @@ def read(self, apply_scale_and_offset = True,
         self._expand_components = expand_components
         self._merge_heart_rates = merge_heart_rates
         self._mesg_listener = mesg_listener
+        self._mesg_definition_listener = mesg_definition_listener
+        self._field_description_listener = field_description_listener
         self._decode_mode = decode_mode
 
         self._local_mesg_defs = {}
@@ -185,10 +188,10 @@ def __decode_next_record(self):
         if record_header & _COMPRESSED_HEADER_MASK == _COMPRESSED_HEADER_MASK:
             self.__decode_compressed_timestamp_message()
 
-        if record_header & _MESG_DEFINITION_MASK == _MESG_HEADER_MASK:
+        if record_header & FIT.MESG_DEFINITION_MASK == _MESG_HEADER_MASK:
             self.__decode_message()
 
-        if record_header & _MESG_DEFINITION_MASK == _MESG_DEFINITION_MASK:
+        if record_header & FIT.MESG_DEFINITION_MASK == FIT.MESG_DEFINITION_MASK:
             self.__decode_mesg_def()
 
     def __decode_mesg_def(self):
@@ -197,7 +200,7 @@ def __decode_mesg_def(self):
         struct_format_string = ''
         mesg_def = {}
         mesg_def["record_header"] = record_header
-        mesg_def["local_mesg_num"] = record_header & _LOCAL_MESG_NUM_MASK
+        mesg_def["local_mesg_num"] = record_header & FIT.LOCAL_MESG_NUM_MASK
         mesg_def["reserved"] = self._stream.read_byte()
 
         mesg_def["architecture"] = self._stream.read_byte()
@@ -236,7 +239,7 @@ def __decode_mesg_def(self):
             mesg_def["field_definitions"].append(field_definition)
             mesg_def["message_size"] += field_definition["size"]
 
-        if record_header & _DEV_DATA_MASK == _DEV_DATA_MASK:
+        if record_header & FIT.DEV_DATA_MASK == FIT.DEV_DATA_MASK:
             num_dev_fields = self._stream.read_byte()
 
             for i in range(num_dev_fields):
@@ -250,6 +253,9 @@ def __decode_mesg_def(self):
                 mesg_def["developer_field_defs"].append(developer_field_definition)
                 mesg_def["developer_data_size"] += developer_field_definition["size"]
 
+        if self._mesg_definition_listener is not None:
+            self._mesg_definition_listener({**mesg_def})
+
         if mesg_def["global_mesg_num"] in Profile['messages']:
             message_profile = Profile['messages'][mesg_def["global_mesg_num"]]
         else:
@@ -272,7 +278,7 @@ def __decode_mesg_def(self):
     def __decode_message(self):
         record_header = self._stream.read_byte()
 
-        local_mesg_num = record_header & _LOCAL_MESG_NUM_MASK
+        local_mesg_num = record_header & FIT.LOCAL_MESG_NUM_MASK
         if local_mesg_num in self._local_mesg_defs:
             mesg_def = self._local_mesg_defs[local_mesg_num]
         else:
@@ -330,6 +336,13 @@ def __decode_message(self):
         if self._mesg_listener is not None:
             self._mesg_listener(mesg_def['global_mesg_num'], message)
 
+        if mesg_def['global_mesg_num'] == Profile['mesg_num']['FIELD_DESCRIPTION'] and self._field_description_listener is not None:
+            developer_data_id_mesg = next(
+                (m for m in self._messages.get('developer_data_id_mesgs', [])
+                 if m.get('developer_data_index') == message.get('developer_data_index')),
+                {})
+            self._field_description_listener(message.get('key'), {**developer_data_id_mesg}, {**message})
+
     def __decode_compressed_timestamp_message(self):
         self.__raise_error("Compressed timestamp messages are not currently supported")
 
@@ -632,7 +645,10 @@ def __add_field_description_to_profile(self, message):
         if message is None or message['developer_data_index'] is None or message['developer_data_index']['raw_field_value'] == 0xFF:
             return
 
-        if self._developer_data_defs[message['developer_data_index']['raw_field_value']] is None:
+        developer_data_index = message['developer_data_index']['raw_field_value']
+        developer_data_def = self._developer_data_defs.get(developer_data_index)
+
+        if developer_data_def is None:
             return
 
         if message["fit_base_type_id"] is not None:
@@ -641,7 +657,7 @@ def __add_field_description_to_profile(self, message):
         else:
             base_type_code = None
 
-        self._developer_data_defs[message['developer_data_index']['raw_field_value']]['fields'].append({
+        developer_data_def['fields'].append({
             'developer_data_index': message['developer_data_index']['raw_field_value'],
             'field_definition_number': message['field_definition_number']['raw_field_value'],
             'fit_base_type_id': message['fit_base_type_id']['raw_field_value'] & FIT.BASE_TYPE_MASK if 'fit_base_type_id' in message else None,