Wire Protocol

The Mercurial wire protocol is a request-response based protocol with multiple wire representations.

Each request is modeled as a command name, a dictionary of arguments, and optional raw input. Command arguments and their types are intrinsic properties of commands. So is the response type of the command. This means clients can't always send arbitrary arguments to servers and servers can't return multiple response types.

The protocol is synchronous and does not support multiplexing (concurrent commands).

Transport Protocols

HTTP Transport

Commands are issued as HTTP/1.0 or HTTP/1.1 requests. Commands are sent to the base URL of the repository with the command name sent in the "cmd" query string parameter. e.g. "https://example.com/repo?cmd=capabilities". The HTTP method is "GET" or "POST" depending on the command and whether there is a request body.

Command arguments can be sent multiple ways.

The simplest is part of the URL query string using "x-www-form-urlencoded" encoding (see Python's "urllib.urlencode()". However, many servers impose length limitations on the URL. So this mechanism is typically only used if the server doesn't support other mechanisms.

If the server supports the "httpheader" capability, command arguments can be sent in HTTP request headers named "X-HgArg-<N>" where "<N>" is an integer starting at 1. A "x-www-form-urlencoded" representation of the arguments is obtained. This full string is then split into chunks and sent in numbered "X-HgArg-<N>" headers. The maximum length of each HTTP header is defined by the server in the "httpheader" capability value, which defaults to "1024". The server reassembles the encoded arguments string by concatenating the "X-HgArg-<N>" headers then URL decodes them into a dictionary.

The list of "X-HgArg-<N>" headers should be added to the "Vary" request header to instruct caches to take these headers into consideration when caching requests.

If the server supports the "httppostargs" capability, the client may send command arguments in the HTTP request body as part of an HTTP POST request. The command arguments will be URL encoded just like they would for sending them via HTTP headers. However, no splitting is performed: the raw arguments are included in the HTTP request body.

The client sends a "X-HgArgs-Post" header with the string length of the encoded arguments data. Additional data may be included in the HTTP request body immediately following the argument data. The offset of the non-argument data is defined by the "X-HgArgs-Post" header. The "X-HgArgs-Post" header is not required if there is no argument data.

Additional command data can be sent as part of the HTTP request body. The default "Content-Type" when sending data is "application/mercurial-0.1". A "Content-Length" header is currently always sent.

Example HTTP requests:

GET /repo?cmd=capabilities
X-HgArg-1: foo=bar&baz=hello%20world

The request media type should be chosen based on server support. If the "httpmediatype" server capability is present, the client should send the newest mutually supported media type. If this capability is absent, the client must assume the server only supports the "application/mercurial-0.1" media type.

The "Content-Type" HTTP response header identifies the response as coming from Mercurial and can also be used to signal an error has occurred.

The "application/mercurial-*" media types indicate a generic Mercurial data type.

The "application/mercurial-0.1" media type is raw Mercurial data. It is the predecessor of the format below.

The "application/mercurial-0.2" media type is compression framed Mercurial data. The first byte of the payload indicates the length of the compression format identifier that follows. Next are N bytes indicating the compression format. e.g. "zlib". The remaining bytes are compressed according to that compression format. The decompressed data behaves the same as with "application/mercurial-0.1".

The "application/hg-error" media type indicates a generic error occurred. The content of the HTTP response body typically holds text describing the error.

The "application/hg-changegroup" media type indicates a changegroup response type.

Clients also accept the "text/plain" media type. All other media types should cause the client to error.

Behavior of media types is further described in the "Content Negotiation" section below.

Clients should issue a "User-Agent" request header that identifies the client. The server should not use the "User-Agent" for feature detection.

A command returning a "string" response issues a "application/mercurial-0.*" media type and the HTTP response body contains the raw string value (after compression decoding, if used). A "Content-Length" header is typically issued, but not required.

A command returning a "stream" response issues a "application/mercurial-0.*" media type and the HTTP response is typically using *chunked transfer* ("Transfer-Encoding: chunked").

SSH Transport

The SSH transport is a custom text-based protocol suitable for use over any bi-directional stream transport. It is most commonly used with SSH.

A SSH transport server can be started with "hg serve --stdio". The stdin, stderr, and stdout file descriptors of the started process are used to exchange data. When Mercurial connects to a remote server over SSH, it actually starts a "hg serve --stdio" process on the remote server.

Commands are issued by sending the command name followed by a trailing newline "\n" to the server. e.g. "capabilities\n".

Command arguments are sent in the following format:

<argument> <length>\n<value>

That is, the argument string name followed by a space followed by the integer length of the value (expressed as a string) followed by a newline ("\n") followed by the raw argument value.

Dictionary arguments are encoded differently:

<argument> <# elements>\n
<key1> <length1>\n<value1>
<key2> <length2>\n<value2>
...

Non-argument data is sent immediately after the final argument value. It is encoded in chunks:

<length>\n<data>

Each command declares a list of supported arguments and their types. If a client sends an unknown argument to the server, the server should abort immediately. The special argument "*" in a command's definition indicates that all argument names are allowed.

The definition of supported arguments and types is initially made when a new command is implemented. The client and server must initially independently agree on the arguments and their types. This initial set of arguments can be supplemented through the presence of *capabilities* advertised by the server.

Each command has a defined expected response type.

A "string" response type is a length framed value. The response consists of the string encoded integer length of a value followed by a newline ("\n") followed by the value. Empty values are allowed (and are represented as "0\n").

A "stream" response type consists of raw bytes of data. There is no framing.

A generic error response type is also supported. It consists of a an error message written to "stderr" followed by "\n-\n". In addition, "\n" is written to "stdout".

If the server receives an unknown command, it will send an empty "string" response.

The server terminates if it receives an empty command (a "\n" character).

Capabilities

Servers advertise supported wire protocol features. This allows clients to probe for server features before blindly calling a command or passing a specific argument.

The server's features are exposed via a *capabilities* string. This is a space-delimited string of tokens/features. Some features are single words like "lookup" or "batch". Others are complicated key-value pairs advertising sub-features. e.g. "httpheader=2048". When complex, non-word values are used, each feature name can define its own encoding of sub-values. Comma-delimited and "x-www-form-urlencoded" values are common.

The following document capabilities defined by the canonical Mercurial server implementation.

batch

Whether the server supports the "batch" command.

This capability/command was introduced in Mercurial 1.9 (released July 2011).

branchmap

Whether the server supports the "branchmap" command.

This capability/command was introduced in Mercurial 1.3 (released July 2009).

bundle2-exp

Precursor to "bundle2" capability that was used before bundle2 was a stable feature.

This capability was introduced in Mercurial 3.0 behind an experimental flag. This capability should not be observed in the wild.

bundle2

Indicates whether the server supports the "bundle2" data exchange format.

The value of the capability is a URL quoted, newline ("\n") delimited list of keys or key-value pairs.

A key is simply a URL encoded string.

A key-value pair is a URL encoded key separated from a URL encoded value by an "=". If the value is a list, elements are delimited by a "," after URL encoding.

For example, say we have the values:

{'HG20': [], 'changegroup': ['01', '02'], 'digests': ['sha1', 'sha512']}

We would first construct a string:

HG20\nchangegroup=01,02\ndigests=sha1,sha512

We would then URL quote this string:

HG20%0Achangegroup%3D01%2C02%0Adigests%3Dsha1%2Csha512

This capability was introduced in Mercurial 3.4 (released May 2015).

changegroupsubset

Whether the server supports the "changegroupsubset" command.

This capability was introduced in Mercurial 0.9.2 (released December 2006).

This capability was introduced at the same time as the "lookup" capability/command.

compression

Declares support for negotiating compression formats.

Presence of this capability indicates the server supports dynamic selection of compression formats based on the client request.

Servers advertising this capability are required to support the "application/mercurial-0.2" media type in response to commands returning streams. Servers may support this media type on any command.

The value of the capability is a comma-delimited list of strings declaring supported compression formats. The order of the compression formats is in server-preferred order, most preferred first.

The identifiers used by the official Mercurial distribution are:

bzip2

bzip2

none

uncompressed / raw data

zlib

zlib (no gzip header)

zstd

zstd

This capability was introduced in Mercurial 4.1 (released February 2017).

getbundle

Whether the server supports the "getbundle" command.

This capability was introduced in Mercurial 1.9 (released July 2011).

httpheader

Whether the server supports receiving command arguments via HTTP request headers.

The value of the capability is an integer describing the max header length that clients should send. Clients should ignore any content after a comma in the value, as this is reserved for future use.

This capability was introduced in Mercurial 1.9 (released July 2011).

httpmediatype

Indicates which HTTP media types ("Content-Type" header) the server is capable of receiving and sending.

The value of the capability is a comma-delimited list of strings identifying support for media type and transmission direction. The following strings may be present:

0.1rx

Indicates server support for receiving "application/mercurial-0.1" media types.

0.1tx

Indicates server support for sending "application/mercurial-0.1" media types.

0.2rx

Indicates server support for receiving "application/mercurial-0.2" media types.

0.2tx

Indicates server support for sending "application/mercurial-0.2" media types.

minrx=X

Minimum media type version the server is capable of receiving. Value is a string like "0.2".

This capability can be used by servers to limit connections from legacy clients not using the latest supported media type. However, only clients with knowledge of this capability will know to consult this value. This capability is present so the client may issue a more user-friendly error when the server has locked out a legacy client.

mintx=X

Minimum media type version the server is capable of sending. Value is a string like "0.1".

Servers advertising support for the "application/mercurial-0.2" media type should also advertise the "compression" capability.

This capability was introduced in Mercurial 4.1 (released February 2017).

httppostargs

**Experimental**

Indicates that the server supports and prefers clients send command arguments via a HTTP POST request as part of the request body.

This capability was introduced in Mercurial 3.8 (released May 2016).

known

Whether the server supports the "known" command.

This capability/command was introduced in Mercurial 1.9 (released July 2011).

lookup

Whether the server supports the "lookup" command.

This capability was introduced in Mercurial 0.9.2 (released December 2006).

This capability was introduced at the same time as the "changegroupsubset" capability/command.

pushkey

Whether the server supports the "pushkey" and "listkeys" commands.

This capability was introduced in Mercurial 1.6 (released July 2010).

standardbundle

**Unsupported**

This capability was introduced during the Mercurial 0.9.2 development cycle in 2006. It was never present in a release, as it was replaced by the "unbundle" capability. This capability should not be encountered in the wild.

stream-preferred

If present the server prefers that clients clone using the streaming clone protocol ("hg clone --stream") rather than the standard changegroup/bundle based protocol.

This capability was introduced in Mercurial 2.2 (released May 2012).

streamreqs

Indicates whether the server supports *streaming clones* and the *requirements* that clients must support to receive it.

If present, the server supports the "stream_out" command, which transmits raw revlogs from the repository instead of changegroups. This provides a faster cloning mechanism at the expense of more bandwidth used.

The value of this capability is a comma-delimited list of repo format *requirements*. These are requirements that impact the reading of data in the ".hg/store" directory. An example value is "streamreqs=generaldelta,revlogv1" indicating the server repo requires the "revlogv1" and "generaldelta" requirements.

If the only format requirement is "revlogv1", the server may expose the "stream" capability instead of the "streamreqs" capability.

This capability was introduced in Mercurial 1.7 (released November 2010).

stream

Whether the server supports *streaming clones* from "revlogv1" repos.

If present, the server supports the "stream_out" command, which transmits raw revlogs from the repository instead of changegroups. This provides a faster cloning mechanism at the expense of more bandwidth used.

This capability was introduced in Mercurial 0.9.1 (released July 2006).

When initially introduced, the value of the capability was the numeric revlog revision. e.g. "stream=1". This indicates the changegroup is using "revlogv1". This simple integer value wasn't powerful enough, so the "streamreqs" capability was invented to handle cases where the repo requirements have more than just "revlogv1". Newer servers omit the "=1" since it was the only value supported and the value of "1" can be implied by clients.

unbundlehash

Whether the "unbundle" commands supports receiving a hash of all the heads instead of a list.

For more, see the documentation for the "unbundle" command.

This capability was introduced in Mercurial 1.9 (released July 2011).

unbundle

Whether the server supports pushing via the "unbundle" command.

This capability/command has been present since Mercurial 0.9.1 (released July 2006).

Mercurial 0.9.2 (released December 2006) added values to the capability indicating which bundle types the server supports receiving. This value is a comma-delimited list. e.g. "HG10GZ,HG10BZ,HG10UN". The order of values reflects the priority/preference of that type, where the first value is the most preferred type.

Handshake Protocol

While not explicitly required, it is common for clients to perform a *handshake* when connecting to a server. The handshake accomplishes 2 things:

• Obtaining capabilities and other server features
• Flushing extra server output (e.g. SSH servers may print extra text when connecting that may confuse the wire protocol)

This isn't a traditional *handshake* as far as network protocols go because there is no persistent state as a result of the handshake: the handshake is simply the issuing of commands and commands are stateless.

The canonical clients perform a capabilities lookup at connection establishment time. This is because clients must assume a server only supports the features of the original Mercurial server implementation until proven otherwise (from advertised capabilities). Nearly every server running today supports features that weren't present in the original Mercurial server implementation. Rather than wait for a client to perform functionality that needs to consult capabilities, it issues the lookup at connection start to avoid any delay later.

For HTTP servers, the client sends a "capabilities" command request as soon as the connection is established. The server responds with a capabilities string, which the client parses.

For SSH servers, the client sends the "hello" command (no arguments) and a "between" command with the "pairs" argument having the value "0000000000000000000000000000000000000000-0000000000000000000000000000000000000000".

The "between" command has been supported since the original Mercurial server. Requesting the empty range will return a "\n" string response, which will be encoded as "1\n\n" (value length of "1" followed by a newline followed by the value, which happens to be a newline).

The "hello" command was later introduced. Servers supporting it will issue a response to that command before sending the "1\n\n" response to the "between" command. Servers not supporting "hello" will send an empty response ("0\n").

In addition to the expected output from the "hello" and "between" commands, servers may also send other output, such as *message of the day (MOTD)* announcements. Clients assume servers will send this output before the Mercurial server replies to the client-issued commands. So any server output not conforming to the expected command responses is assumed to be not related to Mercurial and can be ignored.

Content Negotiation

The wire protocol has some mechanisms to help peers determine what content types and encoding the other side will accept. Historically, these mechanisms have been built into commands themselves because most commands only send a well-defined response type and only certain commands needed to support functionality like compression.

Currently, only the HTTP transport supports content negotiation at the protocol layer.

HTTP requests advertise supported response formats via the "X-HgProto-<N>" request header, where "<N>" is an integer starting at 1 allowing the logical value to span multiple headers. This value consists of a list of space-delimited parameters. Each parameter denotes a feature or capability.

The following parameters are defined:

0.1

Indicates the client supports receiving "application/mercurial-0.1" responses.

0.2

Indicates the client supports receiving "application/mercurial-0.2" responses.

comp

Indicates compression formats the client can decode. Value is a list of comma delimited strings identifying compression formats ordered from most preferential to least preferential. e.g. "comp=zstd,zlib,none".

This parameter does not have an effect if only the "0.1" parameter is defined, as support for "application/mercurial-0.2" or greater is required to use arbitrary compression formats.

If this parameter is not advertised, the server interprets this as equivalent to "zlib,none".

Clients may choose to only send this header if the "httpmediatype" server capability is present, as currently all server-side features consulting this header require the client to opt in to new protocol features advertised via the "httpmediatype" capability.

A server that doesn't receive an "X-HgProto-<N>" header should infer a value of "0.1". This is compatible with legacy clients.

A server receiving a request indicating support for multiple media type versions may respond with any of the supported media types. Not all servers may support all media types on all commands.

Commands

This section contains a list of all wire protocol commands implemented by the canonical Mercurial server.

batch

Issue multiple commands while sending a single command request. The purpose of this command is to allow a client to issue multiple commands while avoiding multiple round trips to the server therefore enabling commands to complete quicker.

The command accepts a "cmds" argument that contains a list of commands to execute.

The value of "cmds" is a ";" delimited list of strings. Each string has the form "<command> <arguments>". That is, the command name followed by a space followed by an argument string.

The argument string is a "," delimited list of "<key>=<value>" values corresponding to command arguments. Both the argument name and value are escaped using a special substitution map:

: -> :c
, -> :o
; -> :s
= -> :e

The response type for this command is "string". The value contains a ";" delimited list of responses for each requested command. Each value in this list is escaped using the same substitution map used for arguments.

If an error occurs, the generic error response may be sent.

between

(Legacy command used for discovery in old clients)

Obtain nodes between pairs of nodes.

The "pairs" arguments contains a space-delimited list of "-" delimited hex node pairs. e.g.:

a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896-6dc58916e7c070f678682bfe404d2e2d68291a18

Return type is a "string". Value consists of lines corresponding to each requested range. Each line contains a space-delimited list of hex nodes. A newline "\n" terminates each line, including the last one.

branchmap

Obtain heads in named branches.

Accepts no arguments. Return type is a "string".

Return value contains lines with URL encoded branch names followed by a space followed by a space-delimited list of hex nodes of heads on that branch. e.g.:

default a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896 6dc58916e7c070f678682bfe404d2e2d68291a18
stable baae3bf31522f41dd5e6d7377d0edd8d1cf3fccc

There is no trailing newline.

branches

(Legacy command used for discovery in old clients. Clients with "getbundle" use the "known" and "heads" commands instead.)

Obtain ancestor changesets of specific nodes back to a branch point.

Despite the name, this command has nothing to do with Mercurial named branches. Instead, it is related to DAG branches.

The command accepts a "nodes" argument, which is a string of space-delimited hex nodes.

For each node requested, the server will find the first ancestor node that is a DAG root or is a merge.

Return type is a "string". Return value contains lines with result data for each requested node. Each line contains space-delimited nodes followed by a newline ("\n"). The 4 nodes reported on each line correspond to the requested node, the ancestor node found, and its 2 parent nodes (which may be the null node).

capabilities

Obtain the capabilities string for the repo.

Unlike the "hello" command, the capabilities string is not prefixed. There is no trailing newline.

This command does not accept any arguments. Return type is a "string".

changegroup

(Legacy command: use "getbundle" instead)

Obtain a changegroup version 1 with data for changesets that are descendants of client-specified changesets.

The "roots" arguments contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all changesets between the requested root/base nodes and the repo's head nodes at the time of the request.

The return type is a "stream".

changegroupsubset

(Legacy command: use "getbundle" instead)

Obtain a changegroup version 1 with data for changesetsets between client specified base and head nodes.

The "bases" argument contains a list of space-delimited hex nodes. The "heads" argument contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all changesets between the requested base and head nodes at the time of the request.

The return type is a "stream".

clonebundles

Obtains a manifest of bundle URLs available to seed clones.

Each returned line contains a URL followed by metadata. See the documentation in the "clonebundles" extension for more.

The return type is a "string".

getbundle

Obtain a bundle containing repository data.

This command accepts the following arguments:

heads

List of space-delimited hex nodes of heads to retrieve.

common

List of space-delimited hex nodes that the client has in common with the server.

obsmarkers

Boolean indicating whether to include obsolescence markers as part of the response. Only works with bundle2.

bundlecaps

Comma-delimited set of strings defining client bundle capabilities.

listkeys

Comma-delimited list of strings of "pushkey" namespaces. For each namespace listed, a bundle2 part will be included with the content of that namespace.

cg

Boolean indicating whether changegroup data is requested.

cbattempted

Boolean indicating whether the client attempted to use the *clone bundles* feature before performing this request.

bookmarks

Boolean indicating whether bookmark data is requested.

phases

Boolean indicating whether phases data is requested.

The return type on success is a "stream" where the value is bundle. On the HTTP transport, the response is zlib compressed.

If an error occurs, a generic error response can be sent.

Unless the client sends a false value for the "cg" argument, the returned bundle contains a changegroup with the nodes between the specified "common" and "heads" nodes. Depending on the command arguments, the type and content of the returned bundle can vary significantly.

The default behavior is for the server to send a raw changegroup version "01" response.

If the "bundlecaps" provided by the client contain a value beginning with "HG2", a bundle2 will be returned. The bundle2 data may contain additional repository data, such as "pushkey" namespace values.

heads

Returns a list of space-delimited hex nodes of repository heads followed by a newline. e.g. "a9eeb3adc7ddb5006c088e9eda61791c777cbf7c 31f91a3da534dc849f0d6bfc00a395a97cf218a1\n"

This command does not accept any arguments. The return type is a "string".

hello

Returns lines describing interesting things about the server in an RFC-822 like format.

Currently, the only line defines the server capabilities. It has the form:

capabilities: <value>

See above for more about the capabilities string.

SSH clients typically issue this command as soon as a connection is established.

This command does not accept any arguments. The return type is a "string".

listkeys

List values in a specified "pushkey" namespace.

The "namespace" argument defines the pushkey namespace to operate on.

The return type is a "string". The value is an encoded dictionary of keys.

Key-value pairs are delimited by newlines ("\n"). Within each line, keys and values are separated by a tab ("\t"). Keys and values are both strings.

lookup

Try to resolve a value to a known repository revision.

The "key" argument is converted from bytes to an "encoding.localstr" instance then passed into "localrepository.__getitem__" in an attempt to resolve it.

The return type is a "string".

Upon successful resolution, returns "1 <hex node>\n". On failure, returns "0 <error string>\n". e.g.:

1 273ce12ad8f155317b2c078ec75a4eba507f1fba\n

0 unknown revision 'foo'\n

known

Determine whether multiple nodes are known.

The "nodes" argument is a list of space-delimited hex nodes to check for existence.

The return type is "string".

Returns a string consisting of "0"s and "1"s indicating whether nodes are known. If the Nth node specified in the "nodes" argument is known, a "1" will be returned at byte offset N. If the node isn't known, "0" will be present at byte offset N.

There is no trailing newline.

pushkey

Set a value using the "pushkey" protocol.

Accepts arguments "namespace", "key", "old", and "new", which correspond to the pushkey namespace to operate on, the key within that namespace to change, the old value (which may be empty), and the new value. All arguments are string types.

The return type is a "string". The value depends on the transport protocol.

The SSH transport sends a string encoded integer followed by a newline ("\n") which indicates operation result. The server may send additional output on the "stderr" stream that should be displayed to the user.

The HTTP transport sends a string encoded integer followed by a newline followed by additional server output that should be displayed to the user. This may include output from hooks, etc.

The integer result varies by namespace. "0" means an error has occurred and there should be additional output to display to the user.

stream_out

Obtain *streaming clone* data.

The return type is either a "string" or a "stream", depending on whether the request was fulfilled properly.

A return value of "1\n" indicates the server is not configured to serve this data. If this is seen by the client, they may not have verified the "stream" capability is set before making the request.

A return value of "2\n" indicates the server was unable to lock the repository to generate data.

All other responses are a "stream" of bytes. The first line of this data contains 2 space-delimited integers corresponding to the path count and payload size, respectively:

<path count> <payload size>\n

The "<payload size>" is the total size of path data: it does not include the size of the per-path header lines.

Following that header are "<path count>" entries. Each entry consists of a line with metadata followed by raw revlog data. The line consists of:

<store path>\0<size>\n

The "<store path>" is the encoded store path of the data that follows. "<size>" is the amount of data for this store path/revlog that follows the newline.

There is no trailer to indicate end of data. Instead, the client should stop reading after "<path count>" entries are consumed.

unbundle

Send a bundle containing data (usually changegroup data) to the server.

Accepts the argument "heads", which is a space-delimited list of hex nodes corresponding to server repository heads observed by the client. This is used to detect race conditions and abort push operations before a server performs too much work or a client transfers too much data.

The request payload consists of a bundle to be applied to the repository, similarly to as if 'hg unbundle' were called.

In most scenarios, a special "push response" type is returned. This type contains an integer describing the change in heads as a result of the operation. A value of "0" indicates nothing changed. "1" means the number of heads remained the same. Values "2" and larger indicate the number of added heads minus 1. e.g. "3" means 2 heads were added. Negative values indicate the number of fewer heads, also off by 1. e.g. "-2" means there is 1 fewer head.

The encoding of the "push response" type varies by transport.

For the SSH transport, this type is composed of 2 "string" responses: an empty response ("0\n") followed by the integer result value. e.g. "1\n2". So the full response might be "0\n1\n2".

For the HTTP transport, the response is a "string" type composed of an integer result value followed by a newline ("\n") followed by string content holding server output that should be displayed on the client (output hooks, etc).

In some cases, the server may respond with a "bundle2" bundle. In this case, the response type is "stream". For the HTTP transport, the response is zlib compressed.

The server may also respond with a generic error type, which contains a string indicating the failure.