Apply Patch Format

Apply-Patch Format

The apply_patch extension registers a single apply_patch tool that takes one big Codex-format patch string and applies it atomically. This skill teaches WHEN to reach for it and HOW to shape the patch so the strict parser accepts it on the first try.

Reference: config/pi/extensions/apply-patch.md is the full spec (format, gate composition, env vars, limitations). Read it once before authoring a non-trivial patch.

Decision: apply_patch vs edit vs write

Reach for apply_patch when any of these hold:

• The change spans more than one file and you want all-or-nothing atomicity.
• The change is large enough that emitting it as a sequence of edit calls would burn many turns.
• You need a rename (Move File) — edit cannot rename and a Delete File + Add File pair is not the same thing (the plan explicitly forbids it).
• An opus-class model is driving and the change is non-trivial.

Stay with edit when:

• The change is a one-liner.
• You're a small or fast model. edit + edit-recovery tolerates whitespace paraphrase; apply_patch's parser is strict.
• You don't have the file content in front of you and would have to invent context lines. Re-read first.

Patch shape (the parser is strict)

The format is Codex's own — not unified-diff. Markers must match exactly modulo trailing whitespace.

*** Begin Patch
<one or more ops>
*** End Patch

Op headers

Inside an @@ hunk, every line begins with one of:

• A single space — unchanged context line. Required at least once above and once below each change.
• A minus - — line to remove from the file.
• A plus + — line to add to the file.

A wholly-empty line inside a hunk is treated as a blank context line (the leading space is easy to lose). Anything else inside a hunk is a parse error.

Worked example — multi-file, mixed ops

*** Begin Patch
*** Add File: src/lib.ts
+export const VERSION = '1.0.0';
+
*** Update File: src/index.ts
@@
 import { foo } from './foo';
+import { VERSION } from './lib';
@@
 export function main(): void {
-  console.log('start');
+  console.log(`start v${VERSION}`);
 }
*** Move File: src/util.ts -> src/helpers.ts
@@
 export function noop(): void {}
+
+export function identity<T>(value: T): T {
+  return value;
+}
*** Delete File: src/obsolete.ts
*** End Patch

Author the hunks so the locator hits

The hunk locator (mirrored from edit-recovery's fuzzy pass) runs exact-match first, then whitespace-insensitive. Two things make the hit reliable:

1. Unique context. Include 2-3 lines above and below the change. If the surrounding lines aren't unique in the file, add another anchor line. The locator refuses to guess when matches are ambiguous and returns a recovery block listing every candidate.
2. Verbatim context. Copy context lines from the actual file content, not a paraphrase. Tabs vs spaces, trailing whitespace, smart vs straight quotes — the fuzzy pass handles these, but the more the model deviates, the more likely the wrong region matches.

If you don't have the file content in front of you, read (or grep -n) the target region first. Inventing context from memory is the #1 source of locator failures.

On a hunk-locate failure

apply_patch returns isError: true with two text parts:

1. A summary listing each failing op: apply_patch: 2 op error(s): followed by op[i]: ….
2. The concatenated recovery blocks, one per failing hunk, each showing the line range and ±2 lines of context around every candidate the locator found (or Near line N: snippets when no candidate hit).

The recovery block looks just like edit-recovery's output, so you can apply the same fix recipe: copy the context lines from the snippet verbatim into a fresh hunk and retry. DO NOT re-emit the same patch — the parser will reject the same bytes a second time.

Anti-patterns

• Hand-writing the format from training data. The markers (*** Begin Patch, *** Update File, @@) are exact strings. A paraphrase (** Begin Patch, *** UpdateFile:, @) will hit the parse-error path with a line number.
• Using apply_patch for a single-line edit. edit is shorter, gives tighter feedback, and the recovery story (edit-recovery) is just as good for one-liners.
• Using Delete File + Add File to rename. The plan explicitly mandates Move File for rename + edit combos so the gate sees one op, not two; Delete + Add will also fail the plan-conflict check if the paths overlap.
• Skipping context lines. A hunk with zero (space-prefixed) context is rejected; one with too little context to be unique will hit the ambiguous locator path.
• Patching binary files. Add File body is text only (+-prefixed lines). For binary or large blobs use a separate workflow — base64-encoded blobs are not supported.
• Mixing apply_patch with concurrent edit calls in the same turn. The plan stages against the on-disk snapshot at execute time; an edit between read and commit invalidates the snapshot. Pick one tool per change set.

Quick reference