apply_patch: pure-addition chunk ignores its `@@` context and silently inserts at end of file, reporting Success

[evanklem]

What version of Codex CLI is running?

codex-cli 0.142.5

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What issue are you seeing?

When an *** Update File hunk contains a chunk that consists only of + (added) lines anchored by a @@ context marker, a pure insertion, with no surrounding context lines and no - lines apply_patch:

1. successfully locates the @@ context line in the file,
2. then discards that position and appends the new lines at the end of the file instead,
3. and reports Success. Updated the following files: M <file> with exit code 0.

The inserted code lands in a different scope than the patch requested (e.g. inside the last class in the file instead of the class named in the @@ line). Since the file still parses, the misplaced method can go unnoticed.

If the @@ line doesn't exist in the file, apply_patch correctly fails with Failed to find context ... so the anchor is validated, just not used.

What steps can reproduce the bug?

In a normal session, this triggers whenever the model emits a pure-addition chunk (an @@ anchor followed by only + lines) e.g. when asked to add a method to a named class. Since whether the model repeats context lines is nondeterministic per turn, the steps below invoke the apply_patch applet directly via --codex-run-as-apply-patch, which is the same entry point codex core uses when the model calls apply_patch (arg0/src/lib.rs, CODEX_CORE_APPLY_PATCH_ARG1), with a fixed payload:

cd "$(mktemp -d)"

cat > sample.py <<'PYEOF'
class Alpha:
    def method_a(self):
        return 1

class Beta:
    def method_b(self):
        return 2

class Gamma:
    def method_c(self):
        return 3
PYEOF

codex --codex-run-as-apply-patch "$(cat <<'EOF'
*** Begin Patch
*** Update File: sample.py
@@ class Alpha:
+    def method_new(self):
+        return 99
*** End Patch
EOF
)"

cat sample.py

Observed exit code 0, and method_new lands inside Gamma, not Alpha:

Success. Updated the following files:
M sample.py

class Alpha:
    def method_a(self):
        return 1

class Beta:
    def method_b(self):
        return 2

class Gamma:
    def method_c(self):
        return 3
    def method_new(self):
        return 99

What is the expected behavior?

Either of:

1. The added lines are inserted immediately after the located @@ class Alpha: context line, matching how every other chunk type respects the seek position after a context match, or
2. If pure-addition chunks are intentionally unsupported, apply_patch rejects the chunk with an error instead of applying it at the wrong location.

Additional information

Root cause is in codex-rs/apply-patch/src/lib.rs, compute_replacements():

• The @@ context is found via seek_sequence, and line_index is advanced to just past it (~lines 727–740).
• The next branch handles empty old_lines by unconditionally appending at EOF, never reading line_index:

if chunk.old_lines.is_empty() {
    // Pure addition (no old lines). We'll add them at the end or just
    // before the final empty line if one exists.
    let insertion_idx = if original_lines.last().is_some_and(String::is_empty) {
        original_lines.len() - 1
    } else {
        original_lines.len()
    };
    replacements.push((insertion_idx, 0, chunk.new_lines.clone()));
    continue;
}

The EOF fallback is only correct for the anchorless pure-addition case (no @@ at all). When chunk.change_context is Some(..) and the seek succeeded, the insertion should use line_index:

let insertion_idx = if chunk.change_context.is_some() {
    line_index
} else if original_lines.last().is_some_and(String::is_empty) {
    original_lines.len() - 1
} else {
    original_lines.len()
};

The parser explicitly allows this chunk shape (UpdateFileChunk { change_context: Some(..), old_lines: vec![], .. } — parser.rs doc comment: "old_lines must occur strictly after change_context"). This chunk shape is emitted whenever the model adds a method/function to a named scope without repeating context lines.

Distinct from #30505 (indentation corruption on fuzzy match).

[github-actions]

Potential duplicates detected. Please review them and close your issue if it is a duplicate.

• apply_patch: fuzzy seek_sequence matching silently corrupts context line indentation #30505

Powered by Codex Action

[krutftw]

I tested this against current main (da4c8ca57) and it looks separate from #30505.

#30505 is the fuzzy-context path: seek_sequence() accepts a whitespace-normalized match, then the replacement writes the patch's context lines back into the file.

This one does not need a fuzzy match. The @@ class Alpha: anchor is found, line_index is moved to the line after it, and then the old_lines.is_empty() branch ignores that position and uses the EOF fallback.

I put the small patch and regression test here: https://github.com/krutftw/codex/tree/codex/fix-anchored-pure-addition

Validation:

• cargo test -p codex-apply-patch pure_addition_chunk -- --nocapture
• cargo test -p codex-apply-patch
• cargo fmt -p codex-apply-patch --check
• git diff --check

The behavior I tested is:

• anchored pure additions insert immediately after the matched @@ line
• anchorless pure additions keep the existing EOF fallback

I did not open a PR because the contributing docs say external PRs are invitation-only, but happy to open one if the team wants it.