GITPROTOCOL-CAPABILITIES(5)

📦
version 2.56.0 · 25 versions
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💖 NAME 💖

okay so this is gitprotocol-capabilities — the manual for Protocol v0 and v1 capabilities, bestie. No cap, that's the whole name.

✨ SYNOPSIS ✨

<over-the-wire-protocol>

🎀 DESCRIPTION 🎀

🦋 quick note before we start: this doc is describing capabilities for versions 0 and 1 of the pack protocol. For version 2, go read the gitprotocol-v2(5) doc instead, iykyk.

Servers SHOULD support all the capabilities defined in this document. Like, all of them. Be the overachiever.

So here's the vibe: on the very first line of the initial server response of either receive-pack or upload-pack, the first reference is followed by a NUL byte and then a list of space delimited server capabilities. That's the server's way of announcing to the client what it can and cannot support. Main character energy, honestly.

Then the client sends back its own space separated list of capabilities it wants to be in effect. And the client MUST NOT ask for capabilities the server did not say it supports. Rude and also illegal 💀

The server MUST diagnose and abort if capabilities it does not understand were sent. And the server MUST NOT ignore capabilities that the client requested and the server advertised. As a consequence of these rules (follow the logic with me here, it's giving airtight): the server MUST NOT advertise capabilities it does not understand. Periodt.

Now for the ✨who-does-what✨ breakdown:

  • 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 lets the server return ACK obj-id continue the second it finds a commit it can use as a common base — you know, somewhere between the client's wants and the client's have set.

By sending that early, the server can potentially head off the client from walking any further down that particular branch of the client's repository history. Which is lowkey a huge save. 🫶 But 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. Which 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. Awkward. 🙈

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

So 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. Slay. ✨

Without multi_ack, the client would have sent that c-b-a chain anyway, interleaved with S-R-Q. Messy.

🌸 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. Big caveat, don't forget it.

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. We love a shortcut that's actually correct. 💅

⭐ THIN-PACK ⭐

So 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.

Here's the deal for each side:

  • 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. Do not lie to the server bestie.

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. It's giving legacy code with lore. 📜

🌈 SIDE-BAND, SIDE-BAND-64K 🌈

This capability means the server can send, and the client can understand, multiplexed progress reports and error info interleaved with the packfile itself. Two things at once, multitasking icon.

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. Considerate queen behavior. 👑

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 math is mathing. 🧮

⚠️ the client MUST send only one of side-band and side-band-64k. The server MUST diagnose it as an error if the 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. Short and iconic.

🧚 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). Read that twice, it's a trap. 🪤

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. It's a fun fact, not an API. 💅

💜 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. Default behavior, love that for us.

🎀 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.

So helpful. So thoughtful. 🫶

🐚 SHALLOW 🐚

This capability adds the "deepen", "shallow" and "unshallow" commands to the fetch-pack/upload-pack protocol so clients can request shallow clones. That's it, that's the whole thing.

🌟 DEEPEN-SINCE 🌟

This capability adds the "deepen-since" command to the 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 the equivalent of doing "rev-list --max-age=<timestamp>" on the server side.

"deepen-since" cannot be used with "deepen".

🔥 DEEPEN-NOT 🔥

This capability adds the "deepen-not" command to the 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 the 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 the "deepen" command is changed. The "depth" argument is the depth from the current shallow boundary, instead of the depth from remote refs. Shifted the whole frame of reference, honestly kind of a serve.

💀 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." Iconic boundary-setting. 🙅‍♀️

However, the sideband channel 3 is still used for error responses. So errors still get through — you can't mute those. 😭

🎀 INCLUDE-TAG 🎀

Okay bestie, this one is so good — it's literally about sending annotated tags along when we're already sending the objects they point to. 💖 So picture this: if we pack an object to the client, and a tag object points exactly at that object, we pack the tag object too. That's it, that's the vibe. In general this lets a client get all its new annotated tags when it fetches a branch — in a single network connection. One trip. Iconic. ✨

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. So like, don't overthink it, it's about what tags you want, not what the server shouted about.

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. Rude? Yeah. 😭 In such cases the client SHOULD issue a subsequent fetch to acquire the tags that include-tag would have otherwise given the client. Like, go back and ask again, bestie, it's fine.

The server SHOULD send include-tag, if it supports it, regardless of whether or not there are tags available. Advertise it anyway, king. 👑

💅 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. Snitching, but make it protocol. 😌

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. And if any of those were not successful, it will send back an error message. See gitprotocol-pack(5) for example messages. Go look, they're cute.

✨ 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. And while that's happening, the capability report-status cannot report for such case — it just doesn't have the range, poor thing. 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. So it's a one-way little heads up, no cap. 💀

🤫 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). Matching energy, basically. 🫶

⚛️ 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. All-or-nothing, main character energy, periodt. 👑

📦 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. Timing is everything, bestie. ⏰

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. So yeah, the name is a little vintage, don't come for her. 🙈

🌈 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. Yes it's nearly the same paragraph as the one above, no we are not skipping it, every capability gets her moment. 💖

🖊️ 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. Like, do not just show up with your certificate uninvited. 💀

🍓 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. Less is more, lowkey. ✨

🧚 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. Mutual, cute, slay. 🫶

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.

No cap: that's an implementation detail, not a promise. Don't build your whole personality on it. 💅

👑 GIT 👑

Part of the git(1) suite. Obviously. 💜

📝 NOTES 📝

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

And that's the whole page, bestie — you ate. 🍓 This one's Git 2.56.0, dated 2026-09-27, and it's GITPROTOCOL-CAPABILITIES(5) in all her glory. Go forth and negotiate some capabilities. 💖✨

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