fix(core): strip BOM from AI-generated content (Issue #368)

packages/core/src/file-mutation.ts

@@ -112,9 +112,10 @@ const layer = Layer.effect(
           const current = yield* fs
             .readFile(input.target.canonical)
             .pipe(Effect.catchReason("PlatformError", "NotFound", () => Effect.succeed(undefined)))
+          // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
           yield* fs.writeWithDirs(
             input.target.canonical,
-            joinBom(next.text, Boolean(current && hasUtf8Bom(current)) || next.bom),
+            joinBom(next.text, Boolean(current && hasUtf8Bom(current))),
           )
           return writeResult(input.target, current !== undefined)
         }),

packages/core/src/patch.ts

@@ -77,7 +77,8 @@ export function derive(path: string, chunks: ReadonlyArray<UpdateFileChunk>, ori
   for (const [start, remove, insert] of replacements.toReversed()) updated.splice(start, remove, ...insert)
   if (updated.at(-1) !== "") updated.push("")
   const next = splitBom(updated.join("\n"))
-  return { content: next.text, bom: source.bom || next.bom }
+  // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
+  return { content: next.text, bom: source.bom }
 }
 
 export function joinBom(text: string, bom: boolean) {

packages/core/src/tool/edit.ts

@@ -188,11 +188,12 @@ const layer = Layer.effectDiscard(
                   { additions: 0, deletions: 0 },
                 )
                 const next = splitBom(replaced)
+                // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
                 const result = yield* unableToEdit(
                   files.writeIfUnchanged({
                     target,
                     expected: source.content,
-                    content: joinBom(next.text, source.bom || next.bom),
+                    content: joinBom(next.text, source.bom),
                   }),
                 )
                 return {

packages/core/test/file-mutation.test.ts

@@ -71,7 +71,7 @@ describe("FileMutation", () => {
     ),
   )
 
-  it.live("preserves exactly one BOM for text writes and normalizes created text", () =>
+  it.live("preserves BOM from existing files but strips BOM from new content", () =>
     withTmp((directory) =>
       Effect.gen(function* () {
         const preservedPath = path.join(directory, "preserved.txt")
@@ -84,7 +84,27 @@ describe("FileMutation", () => {
         yield* files.writeTextPreservingBom({ target: created, content: "\uFEFF\uFEFF\uFEFFcreated" })
 
         expect(yield* Effect.promise(() => fs.readFile(preservedPath, "utf8"))).toBe("\uFEFFafter")
-        expect(yield* Effect.promise(() => fs.readFile(created.canonical, "utf8"))).toBe("\uFEFFcreated")
+        expect(yield* Effect.promise(() => fs.readFile(created.canonical, "utf8"))).toBe("created")
+      }).pipe(provide(directory)),
+    ),
+  )
+
+  it.live("strips BOM from AI-generated content in new files (Issue #368)", () =>
+    withTmp((directory) =>
+      Effect.gen(function* () {
+        const files = yield* FileMutation.Service
+        const mutation = yield* LocationMutation.Service
+        const testPath = path.join(directory, "ai-generated.txt")
+        const target = yield* mutation.resolve({ path: "ai-generated.txt" })
+
+        // AI generates content with BOM
+        const aiGeneratedContent = "\uFEFF# Test Script\necho 'Hello'\n"
+        yield* files.writeTextPreservingBom({ target, content: aiGeneratedContent })
+
+        // File should not have BOM
+        const result = yield* Effect.promise(() => fs.readFile(testPath, "utf8"))
+        expect(result).toBe("# Test Script\necho 'Hello'\n")
+        expect(result.startsWith("\uFEFF")).toBe(false)
       }).pipe(provide(directory)),
     ),
   )

packages/opencode/src/patch/index.ts

@@ -332,10 +332,11 @@ export function deriveNewContentsFromChunks(
   // Generate unified diff
   const unifiedDiff = generateUnifiedDiff(originalContent.text, newContent)
 
+  // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
   return {
     unified_diff: unifiedDiff,
     content: newContent,
-    bom: originalContent.bom || next.bom,
+    bom: originalContent.bom,
   }
 }
 

packages/opencode/src/tool/edit.ts

@@ -95,7 +95,8 @@ export const EditTool = Tool.define(
                   )
                 }
                 const next = Bom.split(params.newString)
-                const desiredBom = next.bom
+                // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
+                const desiredBom = false
                 contentOld = ""
                 contentNew = next.text
                 diff = trimDiff(createTwoFilesPatch(filePath, filePath, contentOld, contentNew))
@@ -131,7 +132,8 @@ export const EditTool = Tool.define(
               const replacement = convertToLineEnding(normalizeLineEndings(params.newString), ending)
 
               const next = Bom.split(replace(contentOld, old, replacement, params.replaceAll))
-              const desiredBom = source.bom || next.bom
+              // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
+              const desiredBom = source.bom
               contentNew = next.text
 
               diff = trimDiff(

packages/opencode/src/tool/write.ts

@@ -46,7 +46,8 @@ export const WriteTool = Tool.define(
           const exists = yield* fs.existsSafe(filepath)
           const source = exists ? yield* Bom.readFile(fs, filepath) : { bom: false, text: "" }
           const next = Bom.split(params.content)
-          const desiredBom = source.bom || next.bom
+          // ANRCODE_CHANGE {"issue":368,"branch":"anr/368/strip-bom-ai-content","date":"2026-07-30"}
+          const desiredBom = source.bom
           const contentOld = source.text
           const contentNew = next.text