man > GITPROTOCOL-CAPABILITIES(5)

GITPROTOCOL-CAPABILITIES(5)                  Git Manual                  GITPROTOCOL-CAPABILITIES(5)

NAME
       gitprotocol-capabilities - Protocol v0 and v1 capabilities

SYNOPSIS
       <over-the-wire-protocol>


DESCRIPTION
           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.

MULTI_ACK
       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.

MULTI_ACK_DETAILED
       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.

NO-DONE
       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.

THIN-PACK
       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.

SIDE-BAND, SIDE-BAND-64K
       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.

OFS-DELTA
       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.

AGENT
       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.

OBJECT-FORMAT
       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.

SYMREF
       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.

SHALLOW
       This capability adds "deepen", "shallow" and "unshallow" commands to the
       fetch-pack/upload-pack protocol so clients can request shallow clones.

DEEPEN-SINCE
       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".

DEEPEN-NOT
       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".

DEEPEN-RELATIVE
       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.

NO-PROGRESS
       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.

INCLUDE-TAG
       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.

REPORT-STATUS
       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.

REPORT-STATUS-V2
       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.

DELETE-REFS
       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.

QUIET
       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).

ATOMIC
       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.

PUSH-OPTIONS
       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.

ALLOW-TIP-SHA1-IN-WANT
       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.

ALLOW-REACHABLE-SHA1-IN-WANT
       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.

PUSH-CERT=<NONCE>
       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.

FILTER
       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.

SESSION-ID=<SESSION ID>
       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.

GIT
       Part of the git(1) suite

NOTES
        1. api-trace2
           file:///usr/share/doc/git/html/technical/api-trace2.html

Git 2.43.0                                   07/02/2025                  GITPROTOCOL-CAPABILITIES(5)