What are Git commit trailers, and how do Signed-off-by and Co-authored-by get added?

## What a trailer is A commit message is free text, but teams constantly want structured facts attached to a commit: who reviewed it, who co-wrote it, which ticket it belongs to, that someone certified its origin. Git's answer is the **trailer**: a line of the form `Token: value` in the last paragraph of the message. ``` Fix retry accounting in the payment client The loop ran one attempt too many. Signed-off-by: Ada Lovelace <[email protected]> Co-authored-by: Grace Hopper <[email protected]> Refs: PROJ-142 ``` The shape is deliberately email-header-like, because Git's culture grew around mailed patches and this block survives `git format-patch` and `git am` intact. ## The placement rule Only the **last** paragraph is scanned. A `Reviewed-by:` line in the middle of a prose body is not a trailer — it is prose that happens to contain a colon. The block must be separated from the rest of the body by a blank line, and it should consist of trailer lines (a continuation line indented under a trailer is allowed). Getting this wrong is the usual reason a tool "does not see" a trailer that is visibly present in the message. ## Signed-off-by `git commit -s` appends `Signed-off-by: <name> <email>` using your committer identity from `user.name` and `user.email`. Mechanically that is all it is: a text line, not a cryptographic signature. Do not confuse it with `git commit -S`, which creates a real cryptographic signature over the commit object — a different mechanism entirely, verified with different tooling. A sign-off asserts something about the author's claim regarding the change; the specific policy meaning depends on the project that asks for it. ## Co-authored-by `Co-authored-by: Name <email>` records that more than one person wrote a commit — useful for pairing, since a commit object has room for exactly one author field. Git stores and parses the line like any other trailer; whether it produces visible credit anywhere else depends on the tools reading the history. Add it manually in the editor, with `git commit --trailer "Co-authored-by: Grace Hopper <[email protected]>"` in recent Git, or through `git interpret-trailers`. ## git interpret-trailers This is the plumbing command for manipulating the trailer block, and its value is that it knows the placement rules so you do not have to do string surgery: - `git interpret-trailers --trailer "Reviewed-by: Ada <[email protected]>" --in-place <file>` adds a trailer to a message file, creating the block if needed. - `--only-trailers` prints just the trailer block from a message on stdin. - `--parse` normalises the output into one trailer per line, convenient for scripts. - `--if-exists` and `--if-missing` control what happens when the token is already present — add another, replace, or do nothing. Configuration lives under `trailer.*`: `trailer.separators` defines which characters count as the separator, and per-token settings such as `trailer.<token>.key` (a canonical spelling to insert) and `trailer.<token>.ifExists` let you define shorthand tokens for a repository. Because it reads a message on stdin or in a file, it is the natural tool for a `commit-msg` hook that needs to inject a trailer automatically. ## Reading trailers back The pretty-format placeholder `%(trailers)` prints the block; it takes arguments, so `git log --format='%(trailers:key=Co-authored-by,valueonly)'` extracts just the values for one token. That is how you answer questions like "which commits credit this person as a co-author" without regex-scraping whole messages — the parser respects the placement rule, a grep does not. ## Why they matter Trailers are the sanctioned way to attach facts to a commit without inventing a parallel database. They travel with the commit through `format-patch`, `am`, `cherry-pick` and rebase, because they are just message text. They are also cheap to abuse: a trailer block of fifteen tokens on every commit becomes noise, and a trailer nobody reads is dead weight. Add tokens that someone or something actually consumes. ## Interview framing Define the shape and the last-paragraph rule, name `-s` as the built-in for sign-off, separate it clearly from `-S` cryptographic signing, and mention `git interpret-trailers` as the correct tool rather than hand-editing. That combination shows you know trailers are a parsed structure, not a stylistic habit.