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Reading history: log, show, blame

Everything here is derived from the changeset DAG on demand. Nothing is stored, so it can never disagree with the history it describes.

ag log, ag show

ag log -n 20                    # newest first
ag log --path src/auth          # only changesets that touched src/auth or anything beneath it
ag show <rev>                   # message, parents, and what changed

ag show lists each change as A (added), M (modified), D (deleted) or R old -> new (NN% similar) for a rename: a path the changeset deleted paired with a similar path it created (the same rename detection the merge uses; renames are display, not history — the stored changeset is still a delete and a put). With --copies a file the changeset created as a copy of another file is listed as C old -> new (NN% similar) (JSON: "op": "C"): identical content, or at least 50 % of the bytes in common with one of at most 200 candidate files (how). Copies are off by default and are never used by a merge.

A merge changeset whose directory rename was followed carries a Directory-Rename: line in its message, and ag preview <proposal> prints the same as a note: before anything is integrated (directory renames).

ag blame

ag blame src/lib.rs                  # the workspace head (or the default stream)
ag blame main:src/lib.rs             # a specific revision: [REV:]PATH
ag blame -L 40,60 src/lib.rs         # only lines 40 to 60
ag blame --json src/lib.rs           # machine-readable
ag blame --copies src/copy.rs        # follow a file that was created as a copy of another

 02c169c6bdf3 user:alice                2026-10-17T16:35:45.911Z src/old.rs 1) use std::fmt;
 02c169c6bdf3 user:alice                2026-10-17T16:35:45.911Z src/old.rs 2) pub fn parse() {}
 4cc4173d2ef1 agent:refactor-bot#run_91 2026-10-17T16:35:45.965Z src/lib.rs 3) pub fn render() {}

Each line shows the changeset that introduced it, its author and time, the path the file had in that changeset (shown when a rename is involved — here the first two lines predate the move from src/old.rs), and the line's text. --json adds the original line number and the changeset's summary.

How the answer is built:

  • Lines pass from a changeset to its parents when the diff (the same deterministic Myers implementation the merge uses) shows them unchanged; a line is blamed on the first changeset where it appears.
  • A changeset that did not touch the path is skipped without reading the file.
  • Merges give each line to the parent that already had it. A merge is blamed only for lines it had to write itself — a conflict resolution — not for lines either side brought.
  • Renames are followed: when a changeset creates the path and deletes a similar file, blame continues in the old file. For merge parents other than the first only the same path is considered.
  • Copies are followed with --copies: when a changeset creates the path and no deleted file matches, blame compares it with the files of the first parent (identical content, or at least 50 % of the bytes in common with one of at most 200 candidates: files the changeset modified, then the new file's directory, then path order). Lines the copy took over are blamed on whoever wrote them in the source file, and the path column shows the source's path; the rest are blamed on the changeset that made the copy. Without the flag a copy is a new file. Over a server (ag remote blame, MCP) copies are not followed yet.
  • A line that moved within a file is a delete plus an insert in the diff, so it is blamed on the changeset that moved it. Moving lines between files is not detected.

Limits, always reported: files over 4 MiB, binary files and files with more than 100 000 lines are refused; after --max-commits changesets (default 100 000) the walk stops and the remaining lines are marked ^ ("boundary": true, "truncated": true) — they were found no later than the changeset shown, and may be older.

Blame needs only the changeset graph and the blobs of the revisions that touched the file, so it works on unpublished snapshots in a workspace as well.

Against a server

ag remote blame main:src/lib.rs      # over gRPC
curl -H "Authorization: Bearer $TOKEN" \
     "$SERVER/v1/repos/acme/web/blame?path=src/lib.rs&rev=main&first_line=40&last_line=60"

The API is history.blame (AGWP reference); it needs the reader role. An empty rev means the repository's default stream.