@@ -2,6 +2,17 @@ import { describe, expect, test } from "bun:test"
 import { Patch } from "@opencode-ai/core/patch"
 
 describe("Patch", () => {
+  test.each([
+    ["", "added\n"],
+    ["\n", "\nadded\n"],
+    ["before", "before\nadded\n"],
+    ["before\n", "before\nadded\n"],
+    ["before\n\n", "before\n\nadded\n"],
+    ["before\n\n\n", "before\n\n\nadded\n"],
+  ])("appends pure additions after all content lines in %j", (original, expected) => {
+    expect(Patch.derive("update.txt", [{ oldLines: [], newLines: ["added"] }], original).content).toBe(expected)
+  })
+
   test.each(["\n", "\n\n", "\n\n\n"])("preserves an unchanged trailing %j", (ending) => {
     expect(Patch.derive("update.txt", [{ oldLines: ["old"], newLines: ["new"] }], `\uFEFFold${ending}`)).toEqual({
       content: `new${ending}`,
 
@@ -360,11 +360,8 @@ function computeReplacements(
 
     // Handle pure addition (no old lines)
     if (chunk.old_lines.length === 0) {
-      const insertionIdx =
-        originalLines.length > 0 && originalLines[originalLines.length - 1] === ""
-          ? originalLines.length - 1
-          : originalLines.length
-      replacements.push([insertionIdx, 0, chunk.new_lines])
+      // The split sentinel was already removed; any remaining blank lines are content.
+      replacements.push([originalLines.length, 0, chunk.new_lines])
       continue
     }
 
 
@@ -140,6 +140,27 @@ PATCH`
   })
 
   describe("applyPatch", () => {
+    for (const [original, expected] of [
+      ["", "added\n"],
+      ["\n", "\nadded\n"],
+      ["before", "before\nadded\n"],
+      ["before\n", "before\nadded\n"],
+      ["before\n\n", "before\n\nadded\n"],
+      ["before\n\n\n", "before\n\n\nadded\n"],
+    ]) {
+      it.live(`appends pure additions after all content lines in ${JSON.stringify(original)}`, () =>
+        Effect.gen(function* () {
+          const target = path.join(tempDir, "append.txt")
+          yield* Effect.promise(() => fs.writeFile(target, original))
+          const result = yield* Patch.applyPatch(
+            `*** Begin Patch\n*** Update File: ${target}\n@@\n+added\n*** End Patch`,
+          )
+          expect(result.modified).toEqual([target])
+          expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe(expected)
+        }),
+      )
+    }
+
     it.live("preserves trailing blank lines across repeated updates", () =>
       Effect.gen(function* () {
         const target = path.join(tempDir, "trailing.txt")