fix: Preserve trailing newlines instead of adding them unconditionally

packages/opencode/src/patch/index.ts

@@ -313,19 +313,20 @@ export function deriveNewContentsFromChunks(
   originalText: string,
 ): ApplyPatchFileUpdate {
   const originalContent = Bom.split(originalText)
+  const originalHadTrailingNewline = originalContent.text.endsWith("\n")
 
-  let originalLines = originalContent.text.split("\n")
+  const originalLines = originalContent.text.split("\n")
 
   // Drop trailing empty element for consistent line counting
-  if (originalLines.length > 0 && originalLines[originalLines.length - 1] === "") {
+  if (originalHadTrailingNewline || originalContent.text === "") {
     originalLines.pop()
   }
 
   const replacements = computeReplacements(originalLines, filePath, chunks)
-  let newLines = applyReplacements(originalLines, replacements)
+  const newLines = applyReplacements(originalLines, replacements)
 
-  // Ensure trailing newline
-  if (newLines.length === 0 || newLines[newLines.length - 1] !== "") {
+  // Preserve the original file's trailing newline state instead of adding one unconditionally.
+  if (originalHadTrailingNewline && newLines.length > 0 && newLines[newLines.length - 1] !== "") {
     newLines.push("")
   }
 

packages/opencode/src/tool/apply_patch.ts

@@ -76,9 +76,7 @@ export const ApplyPatchTool = Tool.define(
         switch (hunk.type) {
           case "add": {
             const oldContent = ""
-            const newContent =
-              hunk.contents.length === 0 || hunk.contents.endsWith("\n") ? hunk.contents : `${hunk.contents}\n`
-            const next = Bom.split(newContent)
+            const next = Bom.split(hunk.contents)
             const diff = trimDiff(createTwoFilesPatch(filePath, filePath, oldContent, next.text))
 
             let additions = 0

packages/opencode/test/patch/patch.test.ts

@@ -331,11 +331,11 @@ PATCH`
         expect(result.modified).toHaveLength(1)
 
         const content = yield* Effect.promise(() => fs.readFile(emptyFile, "utf-8"))
-        expect(content).toBe("First line\n")
+        expect(content).toBe("First line")
       }),
     )
 
-    it.live("should handle files with no trailing newline", () =>
+    it.live("should preserve files with no trailing newline", () =>
       Effect.gen(function* () {
         const filePath = path.join(tempDir, "no-newline.txt")
         yield* Effect.promise(() => fs.writeFile(filePath, "no newline"))
@@ -351,7 +351,27 @@ PATCH`
         expect(result.modified).toHaveLength(1)
 
         const content = yield* Effect.promise(() => fs.readFile(filePath, "utf-8"))
-        expect(content).toBe("has newline now\n")
+        expect(content).toBe("has newline now")
+      }),
+    )
+
+    it.live("should preserve files with a trailing newline", () =>
+      Effect.gen(function* () {
+        const filePath = path.join(tempDir, "with-newline.txt")
+        yield* Effect.promise(() => fs.writeFile(filePath, "has newline\n"))
+
+        const patchText = `*** Begin Patch
+*** Update File: ${filePath}
+@@
+-has newline
++still has newline
+*** End Patch`
+
+        const result = yield* Patch.applyPatch(patchText)
+        expect(result.modified).toHaveLength(1)
+
+        const content = yield* Effect.promise(() => fs.readFile(filePath, "utf-8"))
+        expect(content).toBe("still has newline\n")
       }),
     )
 

packages/opencode/test/tool/apply_patch.test.ts

@@ -152,7 +152,7 @@ describe("tool.apply_patch freeform", () => {
         expect(updateFile?.patch).toContain("-line2")
         expect(updateFile?.patch).toContain("+changed")
 
-        expect(yield* readText(path.join(test.directory, "nested", "new.txt"))).toBe("created\n")
+        expect(yield* readText(path.join(test.directory, "nested", "new.txt"))).toBe("created")
         expect(yield* readText(modifyPath)).toBe("line1\nchanged\n")
         yield* expectReadFailure(deletePath)
       }),
@@ -244,7 +244,7 @@ describe("tool.apply_patch freeform", () => {
     }),
   )
 
-  it.instance("appends trailing newline on update", () =>
+  it.instance("preserves missing trailing newline on update", () =>
     Effect.gen(function* () {
       const test = yield* TestInstance
       const { ctx } = makeCtx()
@@ -257,8 +257,24 @@ describe("tool.apply_patch freeform", () => {
       yield* execute({ patchText }, ctx)
 
       const contents = yield* readText(target)
-      expect(contents.endsWith("\n")).toBe(true)
-      expect(contents).toBe("first line\nsecond line\n")
+      expect(contents.endsWith("\n")).toBe(false)
+      expect(contents).toBe("first line\nsecond line")
+    }),
+  )
+
+  it.instance("keeps trailing newline when original file has one", () =>
+    Effect.gen(function* () {
+      const test = yield* TestInstance
+      const { ctx } = makeCtx()
+      const target = path.join(test.directory, "with_newline.txt")
+      yield* writeText(target, "line at end\n")
+
+      const patchText =
+        "*** Begin Patch\n*** Update File: with_newline.txt\n@@\n-line at end\n+updated line\n*** End Patch"
+
+      yield* execute({ patchText }, ctx)
+
+      expect(yield* readText(target)).toBe("updated line\n")
     }),
   )
 
@@ -312,7 +328,19 @@ describe("tool.apply_patch freeform", () => {
       const patchText = "*** Begin Patch\n*** Add File: duplicate.txt\n+new content\n*** End Patch"
 
       yield* execute({ patchText }, ctx)
-      expect(yield* readText(target)).toBe("new content\n")
+      expect(yield* readText(target)).toBe("new content")
+    }),
+  )
+
+  it.instance("adds file with explicit trailing newline", () =>
+    Effect.gen(function* () {
+      const test = yield* TestInstance
+      const { ctx } = makeCtx()
+
+      const patchText = "*** Begin Patch\n*** Add File: explicit_newline.txt\n+new content\n+\n*** End Patch"
+
+      yield* execute({ patchText }, ctx)
+      expect(yield* readText(path.join(test.directory, "explicit_newline.txt"))).toBe("new content\n")
     }),
   )
 
@@ -457,7 +485,7 @@ describe("tool.apply_patch freeform", () => {
 EOF`
 
       yield* execute({ patchText }, ctx)
-      expect(yield* readText(path.join(test.directory, "heredoc_test.txt"))).toBe("heredoc content\n")
+      expect(yield* readText(path.join(test.directory, "heredoc_test.txt"))).toBe("heredoc content")
     }),
   )
 
@@ -473,7 +501,7 @@ EOF`
 EOF`
 
       yield* execute({ patchText }, ctx)
-      expect(yield* readText(path.join(test.directory, "heredoc_no_cat.txt"))).toBe("no cat prefix\n")
+      expect(yield* readText(path.join(test.directory, "heredoc_no_cat.txt"))).toBe("no cat prefix")
     }),
   )