ONNX ModelProto, field by field

The whole file is one ModelProto, the top-level message of ONNX’s schema onnx.proto (onnx 1.23, IR version 14, syntax = "proto2", package onnx). This page lists every field of it and of the smaller messages it holds directly. Tables are in the order of the schema, which is not always the order of the numbers. For the bytes of tags, varints and lengths, see the Protocol Buffers reference.

ModelProto

The model: its IR version, the operator sets it imports, the program that made it, and the graph.

Field	Name	Type	Wire type	Meaning
1	`ir_version`	int64	VARINT	The IR version of the format the model follows: 1 to 14. Required.
8	`opset_import`	repeated OperatorSetIdProto	LEN, one per set	The operator sets the model uses, each a domain and a version. At least one, for the default domain, from IR 3.
2	`producer_name`	string	LEN	The tool that wrote the model, such as `pytorch` or `tf2onnx`.
3	`producer_version`	string	LEN	That tool’s version.
4	`domain`	string	LEN	A reverse domain name for the model, such as `com.example`.
5	`model_version`	int64	VARINT	The model’s own version number.
6	`doc_string`	string	LEN	Documentation; Markdown is allowed.
7	`graph`	GraphProto	LEN	The graph that runs the model: nodes, inputs, outputs and weights.
14	`metadata_props`	repeated StringStringEntryProto	LEN, one per entry	Named metadata, key and value strings.
20	`training_info`	repeated TrainingInfoProto	LEN, one per entry	How to initialise and train the model (IR 7).
25	`functions`	repeated FunctionProto	LEN, one per function	Functions local to the model (IR 8).
26	`configuration`	repeated DeviceConfigurationProto	LEN, one per configuration	Multi-device configurations (IR 11).

opset_import is field 8 but comes second in the schema. Writers usually put fields in number order, so it is often the last field of the file, after the graph.

08 08 12 07 70 79 74 6F 72 63 68 1A 05 32 2E 38 2E 30 3A E7 D5 D7 FD 03 …

The start of a 1 GB model examined for this site: ir_version 8, producer_name pytorch, producer_version 2.8.0, then the graph, 1,068,886,759 bytes

OperatorSetIdProto

One operator set: a domain and its version. An operator such as Conv means what that version of its set says it means.

Field	Name	Type	Wire type	Meaning
1	`domain`	string	LEN	The operator set’s domain. Empty or absent is the default ONNX domain, `ai.onnx`; `ai.onnx.ml` is ONNX’s machine learning set; others belong to runtimes or models.
2	`version`	int64	VARINT	The operator set’s version, such as 17.

StringStringEntryProto

A key and a value, both strings: ONNX’s stand-in for a map. It is used for metadata on the model, graph, nodes and tensors, for external data, and for the bindings of training information.

Field	Name	Type	Wire type	Meaning
1	`key`	string	LEN	The key.
2	`value`	string	LEN	The value.

TrainingInfoProto

Training information (IR 7): an initialisation graph, an algorithm graph, and the bindings between their outputs and the model’s initialisers.

Field	Name	Type	Wire type	Meaning
1	`initialization`	GraphProto	LEN	A graph that computes initial values.
2	`algorithm`	GraphProto	LEN	One step of the training algorithm.
3	`initialization_binding`	repeated StringStringEntryProto	LEN, one per entry	Which initialiser each output of the initialisation sets.
4	`update_binding`	repeated StringStringEntryProto	LEN, one per entry	Which initialiser each output of the algorithm updates.

FunctionProto

A function local to the model (IR 8): a named subgraph of nodes with its own inputs, outputs and operator sets. A node calls it by giving its name as op_type and its domain as domain.

Field	Name	Type	Wire type	Meaning
1	`name`	string	LEN	The function’s name, used as an `op_type`.
4	`input`	repeated string	LEN, one per name	Input names.
5	`output`	repeated string	LEN, one per name	Output names.
6	`attribute`	repeated string	LEN, one per name	Names of attributes without defaults.
11	`attribute_proto`	repeated AttributeProto	LEN, one per attribute	Attributes with default values (IR 9).
7	`node`	repeated NodeProto	LEN, one per node	The nodes of the function body.
8	`doc_string`	string	LEN	Documentation.
9	`opset_import`	repeated OperatorSetIdProto	LEN, one per set	The operator sets the body uses.
10	`domain`	string	LEN	The function’s domain.
13	`overload`	string	LEN	An overload identifier, so that several functions can share a name (IR 10).
12	`value_info`	repeated ValueInfoProto	LEN, one per value	Types of values inside the function.
14	`metadata_props`	repeated StringStringEntryProto	LEN, one per entry	Named metadata.

Fields 2 and 3 are not used.

DeviceConfigurationProto

A multi-device configuration (IR 11): a name, a device count and the device names. Nodes refer to it by name in their device configurations.

Field	Name	Type	Wire type	Meaning
1	`name`	string	LEN	The configuration’s name.
2	`num_devices`	int32	VARINT	How many devices.
3	`device`	repeated string	LEN, one per device	Device names.