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NAME | SYNOPSIS | DESCRIPTION | MULTI_ACK | MULTI_ACK_DETAILED | NO-DONE | THIN-PACK | SIDE-BAND, SIDE-BAND-64K | OFS-DELTA | AGENT | OBJECT-FORMAT | SYMREF | SHALLOW | DEEPEN-SINCE | DEEPEN-NOT | DEEPEN-RELATIVE | NO-PROGRESS | INCLUDE-TAG | REPORT-STATUS | REPORT-STATUS-V2 | DELETE-REFS | QUIET | ATOMIC | PUSH-OPTIONS | ALLOW-TIP-SHA1-IN-WANT | ALLOW-REACHABLE-SHA1-IN-WANT | PUSH-CERT=<NONCE> | FILTER | SESSION-ID=<SESSION-ID> | GIT | NOTES | COLOPHON |
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GITPROTOCOL-CAPABILITIES(5) Git Manual GITPROTOCOL-CAPABILITIES(5)
gitprotocol-capabilities - Protocol v0 and v1 capabilities
<over-the-wire-protocol>
Note
this document describes capabilities for versions 0 and 1 of
the pack protocol. For version 2, please refer to the
gitprotocol-v2(5) doc.
Servers SHOULD support all capabilities defined in this document.
On the very first line of the initial server response of either
receive-pack and upload-pack the first reference is followed by a
NUL byte and then a list of space delimited server capabilities.
These allow the server to declare what it can and cannot support
to the client.
Client will then send a space separated list of capabilities it
wants to be in effect. The client MUST NOT ask for capabilities
the server did not say it supports.
Server MUST diagnose and abort if capabilities it does not
understand were sent. Server MUST NOT ignore capabilities that
client requested and server advertised. As a consequence of these
rules, server MUST NOT advertise capabilities it does not
understand.
The atomic, report-status, report-status-v2, delete-refs, quiet,
and push-cert capabilities are sent and recognized by the
receive-pack (push to server) process.
The ofs-delta and side-band-64k capabilities are sent and
recognized by both upload-pack and receive-pack protocols. The
agent and session-id capabilities may optionally be sent in both
protocols.
All other capabilities are only recognized by the upload-pack
(fetch from server) process.
The multi_ack capability allows the server to return "ACK obj-id
continue" as soon as it finds a commit that it can use as a common
base, between the client’s wants and the client’s have set.
By sending this early, the server can potentially head off the
client from walking any further down that particular branch of the
client’s repository history. The client may still need to walk
down other branches, sending have lines for those, until the
server has a complete cut across the DAG, or the client has said
"done".
Without multi_ack, a client sends have lines in --date-order until
the server has found a common base. That means the client will
send have lines that are already known by the server to be common,
because they overlap in time with another branch on which the
server hasn’t found a common base yet.
For example suppose the client has commits in caps that the server
doesn’t and the server has commits in lower case that the client
doesn’t, as in the following diagram:
+---- u ---------------------- x
/ +----- y
/ /
a -- b -- c -- d -- E -- F
\
+--- Q -- R -- S
If the client wants x,y and starts out by saying have F,S, the
server doesn’t know what F,S is. Eventually the client says "have
d" and the server sends "ACK d continue" to let the client know to
stop walking down that line (so don’t send c-b-a), but it’s not
done yet, it needs a base for x. The client keeps going with
S-R-Q, until a gets reached, at which point the server has a clear
base and it all ends.
Without multi_ack the client would have sent that c-b-a chain
anyway, interleaved with S-R-Q.
This is an extension of multi_ack that permits the client to
better understand the server’s in-memory state. See
gitprotocol-pack(5), section "Packfile Negotiation" for more
information.
This capability should only be used with the smart HTTP protocol.
If multi_ack_detailed and no-done are both present, then the
sender is free to immediately send a pack following its first "ACK
obj-id ready" message.
Without no-done in the smart HTTP protocol, the server session
would end and the client has to make another trip to send "done"
before the server can send the pack. no-done removes the last
round and thus slightly reduces latency.
A thin pack is one with deltas which reference base objects not
contained within the pack (but are known to exist at the receiving
end). This can reduce the network traffic significantly, but it
requires the receiving end to know how to "thicken" these packs by
adding the missing bases to the pack.
The upload-pack server advertises thin-pack when it can generate
and send a thin pack. A client requests the thin-pack capability
when it understands how to "thicken" it, notifying the server that
it can receive such a pack. A client MUST NOT request the
thin-pack capability if it cannot turn a thin pack into a
self-contained pack.
Receive-pack, on the other hand, is assumed by default to be able
to handle thin packs, but can ask the client not to use the
feature by advertising the no-thin capability. A client MUST NOT
send a thin pack if the server advertises the no-thin capability.
The reasons for this asymmetry are historical. The receive-pack
program did not exist until after the invention of thin packs, so
historically the reference implementation of receive-pack always
understood thin packs. Adding no-thin later allowed receive-pack
to disable the feature in a backwards-compatible manner.
This capability means that the server can send, and the client can
understand, multiplexed progress reports and error info
interleaved with the packfile itself.
These two options are mutually exclusive. A modern client always
favors side-band-64k.
Either mode indicates that the packfile data will be streamed
broken up into packets of up to either 1000 bytes in the case of
side_band, or 65520 bytes in the case of side_band_64k. Each
packet is made up of a leading 4-byte pkt-line length of how much
data is in the packet, followed by a 1-byte stream code, followed
by the actual data.
The stream code can be one of:
1 - pack data
2 - progress messages
3 - fatal error message just before stream aborts
The "side-band-64k" capability came about as a way for newer
clients that can handle much larger packets to request packets
that are actually crammed nearly full, while maintaining backward
compatibility for the older clients.
Further, with side-band and its up to 1000-byte messages, it’s
actually 999 bytes of payload and 1 byte for the stream code. With
side-band-64k, same deal, you have up to 65519 bytes of data and 1
byte for the stream code.
The client MUST send only one of "side-band" and "side- band-64k".
The server MUST diagnose it as an error if client requests both.
The server can send, and the client can understand, PACKv2 with
delta referring to its base by position in pack rather than by an
obj-id. That is, they can send/read OBJ_OFS_DELTA (aka type 6) in
a packfile.
The server may optionally send a capability of the form agent=X to
notify the client that the server is running version X. The client
may optionally return its own agent string by responding with an
agent=Y capability (but it MUST NOT do so if the server did not
mention the agent capability). The X and Y strings may contain any
printable ASCII characters except space (i.e., the byte range 32 <
x < 127), and are typically of the form "package/version" (e.g.,
"git/1.8.3.1"). The agent strings are purely informative for
statistics and debugging purposes, and MUST NOT be used to
programmatically assume the presence or absence of particular
features.
This capability, which takes a hash algorithm as an argument,
indicates that the server supports the given hash algorithms. It
may be sent multiple times; if so, the first one given is the one
used in the ref advertisement.
When provided by the client, this indicates that it intends to use
the given hash algorithm to communicate. The algorithm provided
must be one that the server supports.
If this capability is not provided, it is assumed that the only
supported algorithm is SHA-1.
This parameterized capability is used to inform the receiver which
symbolic ref points to which ref; for example,
"symref=HEAD:refs/heads/master" tells the receiver that HEAD
points to master. This capability can be repeated to represent
multiple symrefs.
Servers SHOULD include this capability for the HEAD symref if it
is one of the refs being sent.
Clients MAY use the parameters from this capability to select the
proper initial branch when cloning a repository.
This capability adds "deepen", "shallow" and "unshallow" commands
to the fetch-pack/upload-pack protocol so clients can request
shallow clones.
This capability adds "deepen-since" command to
fetch-pack/upload-pack protocol so the client can request shallow
clones that are cut at a specific time, instead of depth.
Internally it’s equivalent of doing "rev-list
--max-age=<timestamp>" on the server side. "deepen-since" cannot
be used with "deepen".
This capability adds "deepen-not" command to
fetch-pack/upload-pack protocol so the client can request shallow
clones that are cut at a specific revision, instead of depth.
Internally it’s equivalent of doing "rev-list --not <rev>" on the
server side. "deepen-not" cannot be used with "deepen", but can be
used with "deepen-since".
If this capability is requested by the client, the semantics of
"deepen" command is changed. The "depth" argument is the depth
from the current shallow boundary, instead of the depth from
remote refs.
The client was started with "git clone -q" or something similar,
and doesn’t want that side band 2. Basically the client just says
"I do not wish to receive stream 2 on sideband, so do not send it
to me, and if you did, I will drop it on the floor anyway".
However, the sideband channel 3 is still used for error responses.
The include-tag capability is about sending annotated tags if we
are sending objects they point to. If we pack an object to the
client, and a tag object points exactly at that object, we pack
the tag object too. In general this allows a client to get all new
annotated tags when it fetches a branch, in a single network
connection.
Clients MAY always send include-tag, hardcoding it into a request
when the server advertises this capability. The decision for a
client to request include-tag only has to do with the client’s
desires for tag data, whether or not a server had advertised
objects in the refs/tags/* namespace.
Servers MUST pack the tags if their referent is packed and the
client has requested include-tags.
Clients MUST be prepared for the case where a server has ignored
include-tag and has not actually sent tags in the pack. In such
cases the client SHOULD issue a subsequent fetch to acquire the
tags that include-tag would have otherwise given the client.
The server SHOULD send include-tag, if it supports it, regardless
of whether or not there are tags available.
The receive-pack process can receive a report-status capability,
which tells it that the client wants a report of what happened
after a packfile upload and reference update. If the pushing
client requests this capability, after unpacking and updating
references the server will respond with whether the packfile
unpacked successfully and if each reference was updated
successfully. If any of those were not successful, it will send
back an error message. See gitprotocol-pack(5) for example
messages.
Capability report-status-v2 extends capability report-status by
adding new "option" directives in order to support reference
rewritten by the "proc-receive" hook. The "proc-receive" hook may
handle a command for a pseudo-reference which may create or update
a reference with different name, new-oid, and old-oid. While the
capability report-status cannot report for such case. See
gitprotocol-pack(5) for details.
If the server sends back the delete-refs capability, it means that
it is capable of accepting a zero-id value as the target value of
a reference update. It is not sent back by the client, it simply
informs the client that it can be sent zero-id values to delete
references.
If the receive-pack server advertises the quiet capability, it is
capable of silencing human-readable progress output which
otherwise may be shown when processing the received pack. A
send-pack client should respond with the quiet capability to
suppress server-side progress reporting if the local progress
reporting is also being suppressed (e.g., via push -q, or if
stderr does not go to a tty).
If the server sends the atomic capability it is capable of
accepting atomic pushes. If the pushing client requests this
capability, the server will update the refs in one atomic
transaction. Either all refs are updated or none.
If the server sends the push-options capability it is able to
accept push options after the update commands have been sent, but
before the packfile is streamed. If the pushing client requests
this capability, the server will pass the options to the pre- and
post- receive hooks that process this push request.
If the upload-pack server advertises this capability, fetch-pack
may send "want" lines with object names that exist at the server
but are not advertised by upload-pack. For historical reasons, the
name of this capability contains "sha1". Object names are always
given using the object format negotiated through the object-format
capability.
If the upload-pack server advertises this capability, fetch-pack
may send "want" lines with object names that exist at the server
but are not advertised by upload-pack. For historical reasons, the
name of this capability contains "sha1". Object names are always
given using the object format negotiated through the object-format
capability.
The receive-pack server that advertises this capability is willing
to accept a signed push certificate, and asks the <nonce> to be
included in the push certificate. A send-pack client MUST NOT send
a push-cert packet unless the receive-pack server advertises this
capability.
If the upload-pack server advertises the filter capability,
fetch-pack may send "filter" commands to request a partial clone
or partial fetch and request that the server omit various objects
from the packfile.
The server may advertise a session ID that can be used to identify
this process across multiple requests. The client may advertise
its own session ID back to the server as well.
Session IDs should be unique to a given process. They must fit
within a packet-line, and must not contain non-printable or
whitespace characters. The current implementation uses trace2
session IDs (see api-trace2[1] for details), but this may change
and users of the session ID should not rely on this fact.
Part of the git(1) suite
1. api-trace2
file:///home/mtk/share/doc/git-doc/technical/api-trace2.html
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-07.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Git 2.51.0.rc1 2025-08-07 GITPROTOCOL-CAPABILITIES(5)
Pages that refer to this page: git(1), gitprotocol-http(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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