feat(core): add V2 formatter runtime (#39564)

packages/core/src/file-mutation.ts

@@ -5,6 +5,7 @@ import { Context, Effect, Layer, Schema } from "effect"
 import { dirname } from "path"
 import { KeyedMutex } from "./effect/keyed-mutex"
 import { FSUtil } from "@opencode-ai/util/fs-util"
+import { Bom } from "@opencode-ai/util/bom"
 
 export interface Target {
   readonly canonical: string
@@ -108,13 +109,13 @@ const layer = Layer.effect(
     const writeTextPreservingBom = Effect.fn("FileMutation.writeTextPreservingBom")((input: TextWriteInput) =>
       withTargetLock(input.target)(
         Effect.gen(function* () {
-          const next = splitBom(input.content)
+          const next = Bom.split(input.content)
           const current = yield* fs
             .readFile(input.target.canonical)
             .pipe(Effect.catchReason("PlatformError", "NotFound", () => Effect.succeed(undefined)))
           yield* fs.writeWithDirs(
             input.target.canonical,
-            joinBom(next.text, Boolean(current && hasUtf8Bom(current)) || next.bom),
+            Bom.join(next.text, Boolean(current && Bom.has(current)) || next.bom),
           )
           return writeResult(input.target, current !== undefined)
         }),
@@ -172,20 +173,6 @@ const layer = Layer.effect(
   }),
 )
 
-function splitBom(text: string) {
-  const stripped = text.replace(/^\uFEFF+/, "")
-  return { bom: stripped.length !== text.length, text: stripped }
-}
-
-function joinBom(text: string, bom: boolean) {
-  const stripped = splitBom(text).text
-  return bom ? `\uFEFF${stripped}` : stripped
-}
-
-function hasUtf8Bom(content: Uint8Array) {
-  return content[0] === 0xef && content[1] === 0xbb && content[2] === 0xbf
-}
-
 function sameBytes(left: Uint8Array, right: Uint8Array) {
   if (left.length !== right.length) return false
   return left.every((byte, index) => byte === right[index])

packages/core/src/formatter.ts

@@ -0,0 +1,157 @@
+export * as Formatter from "./formatter"
+
+import { Context, Effect, Layer, Schema } from "effect"
+import { ChildProcess } from "effect/unstable/process"
+import path from "path"
+import { makeLocationNode } from "@opencode-ai/util/effect/app-node"
+import { FSUtil } from "@opencode-ai/util/fs-util"
+import { Npm } from "@opencode-ai/util/npm"
+import { AppProcess } from "@opencode-ai/util/process"
+import { Config } from "./config"
+import { Location } from "./location"
+import { make, type Info } from "./formatter/builtins"
+
+export const Status = Schema.Struct({
+  name: Schema.String,
+  extensions: Schema.Array(Schema.String),
+  enabled: Schema.Boolean,
+}).annotate({ identifier: "FormatterStatus" })
+export type Status = typeof Status.Type
+
+export interface Interface {
+  readonly init: () => Effect.Effect<void>
+  readonly status: () => Effect.Effect<Status[]>
+  readonly file: (filepath: string) => Effect.Effect<boolean>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/v2/Formatter") {}
+
+const layer = Layer.effect(
+  Service,
+  Effect.gen(function* () {
+    const config = yield* Config.Service
+    const fs = yield* FSUtil.Service
+    const location = yield* Location.Service
+    const npm = yield* Npm.Service
+    const processes = yield* AppProcess.Service
+    const commands = new Map<string, string[] | false>()
+    let formatters: Info[] = []
+
+    const load = yield* Effect.cached(
+      Effect.gen(function* () {
+        const configured = Config.latest(yield* config.entries(), "formatter")
+        if (!configured) {
+          yield* Effect.logInfo("all formatters are disabled")
+          return
+        }
+
+        const builtIns = make({
+          directory: location.directory,
+          worktree: location.project.directory,
+          fs,
+          npm,
+          processes,
+        })
+        formatters = builtIns
+        if (configured === true) return
+        if (configured.ruff?.disabled || configured.uv?.disabled) {
+          formatters = formatters.filter((formatter) => formatter.name !== "ruff" && formatter.name !== "uv")
+        }
+
+        for (const [name, entry] of Object.entries(configured)) {
+          const index = formatters.findIndex((formatter) => formatter.name === name)
+          if (entry.disabled) {
+            if (index !== -1) formatters.splice(index, 1)
+            continue
+          }
+
+          const builtIn = builtIns.find((formatter) => formatter.name === name)
+          const formatter: Info = {
+            name,
+            extensions: entry.extensions ?? builtIn?.extensions ?? [],
+            environment: { ...builtIn?.environment, ...entry.environment },
+            enabled:
+              builtIn && !entry.command ? builtIn.enabled : Effect.succeed(entry.command ? [...entry.command] : false),
+          }
+          if (index === -1) formatters.push(formatter)
+          else formatters[index] = formatter
+        }
+      }).pipe(Effect.withSpan("Formatter.load")),
+    )
+
+    const command = Effect.fnUntraced(function* (formatter: Info) {
+      const cached = commands.get(formatter.name)
+      if (cached !== undefined) return cached
+      const result = yield* formatter.enabled
+      if (result !== false) commands.set(formatter.name, result)
+      return result
+    })
+
+    const init = Effect.fn("Formatter.init")(function* () {
+      yield* load
+    })
+
+    const status = Effect.fn("Formatter.status")(function* () {
+      yield* load
+      return yield* Effect.forEach(formatters, (formatter) =>
+        command(formatter).pipe(
+          Effect.map((enabled) => ({
+            name: formatter.name,
+            extensions: [...formatter.extensions],
+            enabled: enabled !== false,
+          })),
+        ),
+      )
+    })
+
+    const file = Effect.fn("Formatter.file")(function* (filepath: string) {
+      yield* load
+      const matching = formatters.filter((formatter) =>
+        formatter.extensions.includes(path.extname(filepath)),
+      )
+
+      for (const formatter of matching) {
+        const enabled = yield* command(formatter)
+        if (enabled === false) continue
+        const cmd = enabled.map((argument) => argument.replace("$FILE", filepath))
+        yield* Effect.logInfo("formatting file", { file: filepath, command: cmd })
+        const result = yield* processes
+          .run(
+            ChildProcess.make(cmd[0], cmd.slice(1), {
+              cwd: location.directory,
+              env: formatter.environment,
+              extendEnv: true,
+              stdin: "ignore",
+              stdout: "ignore",
+              stderr: "ignore",
+            }),
+          )
+          .pipe(
+            Effect.catch((error) =>
+              Effect.logError("failed to format file", {
+                file: filepath,
+                command: cmd,
+                error: error.message,
+              }).pipe(Effect.as(undefined)),
+            ),
+          )
+        if (!result) continue
+        if (result.exitCode === 0) return true
+        yield* Effect.logError("formatter exited unsuccessfully", {
+          file: filepath,
+          command: cmd,
+          exitCode: result.exitCode,
+        })
+      }
+      return false
+    })
+
+    return Service.of({ init, status, file })
+  }),
+)
+
+export const node = makeLocationNode({
+  service: Service,
+  layer,
+  deps: [Config.node, FSUtil.node, Location.node, Npm.node, AppProcess.node],
+})

packages/core/src/formatter/builtins.ts

@@ -0,0 +1,315 @@
+import { Effect } from "effect"
+import { ChildProcess } from "effect/unstable/process"
+import { FSUtil } from "@opencode-ai/util/fs-util"
+import { Npm } from "@opencode-ai/util/npm"
+import { AppProcess } from "@opencode-ai/util/process"
+import { which } from "../util/which"
+
+export interface Info {
+  readonly name: string
+  readonly environment?: Record<string, string>
+  readonly extensions: readonly string[]
+  readonly enabled: Effect.Effect<string[] | false>
+}
+
+export function make(input: {
+  readonly directory: string
+  readonly worktree: string
+  readonly fs: FSUtil.Interface
+  readonly npm: Npm.Interface
+  readonly processes: AppProcess.Interface
+  readonly experimentalOxfmt?: boolean
+}) {
+  const disabled = false as const
+  const findUp = (target: string) => input.fs.findUp(target, input.directory, input.worktree)
+  const readText = (file: string) => input.fs.readFileString(file).pipe(Effect.orElseSucceed(() => ""))
+  const commandOutput = (command: string[]) =>
+    input.processes
+      .run(
+        ChildProcess.make(command[0], command.slice(1), {
+          cwd: input.directory,
+          extendEnv: true,
+          stdin: "ignore",
+        }),
+      )
+      .pipe(Effect.option)
+
+  const gofmt: Info = {
+    name: "gofmt",
+    extensions: [".go"],
+    enabled: Effect.sync(() => {
+      const match = which("gofmt")
+      return match ? [match, "-w", "$FILE"] : disabled
+    }),
+  }
+
+  const mix: Info = {
+    name: "mix",
+    extensions: [".ex", ".exs", ".eex", ".heex", ".leex", ".neex", ".sface"],
+    enabled: Effect.sync(() => {
+      const match = which("mix")
+      return match ? [match, "format", "$FILE"] : disabled
+    }),
+  }
+
+  const prettier: Info = {
+    name: "prettier",
+    environment: { BUN_BE_BUN: "1" },
+    extensions: [
+      ".js",
+      ".jsx",
+      ".mjs",
+      ".cjs",
+      ".ts",
+      ".tsx",
+      ".mts",
+      ".cts",
+      ".html",
+      ".htm",
+      ".css",
+      ".scss",
+      ".sass",
+      ".less",
+      ".vue",
+      ".svelte",
+      ".json",
+      ".jsonc",
+      ".yaml",
+      ".yml",
+      ".toml",
+      ".xml",
+      ".md",
+      ".mdx",
+      ".graphql",
+      ".gql",
+    ],
+    enabled: Effect.gen(function* () {
+      for (const file of yield* findUp("package.json")) {
+        if (!hasDependency(yield* input.fs.readJson(file), "prettier")) continue
+        const bin = yield* input.npm.which("prettier")
+        if (bin) return [bin, "--write", "$FILE"]
+      }
+      return disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const oxfmt: Info = {
+    name: "oxfmt",
+    environment: { BUN_BE_BUN: "1" },
+    extensions: [".js", ".jsx", ".mjs", ".cjs", ".ts", ".tsx", ".mts", ".cts"],
+    enabled: Effect.gen(function* () {
+      for (const file of yield* findUp("package.json")) {
+        if (!hasDependency(yield* input.fs.readJson(file), "oxfmt")) continue
+        const bin = yield* input.npm.which("oxfmt")
+        if (bin) return [bin, "$FILE"]
+      }
+      return disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const biome: Info = {
+    name: "biome",
+    environment: { BUN_BE_BUN: "1" },
+    extensions: [
+      ".js",
+      ".jsx",
+      ".mjs",
+      ".cjs",
+      ".ts",
+      ".tsx",
+      ".mts",
+      ".cts",
+      ".html",
+      ".htm",
+      ".css",
+      ".scss",
+      ".sass",
+      ".less",
+      ".vue",
+      ".svelte",
+      ".json",
+      ".jsonc",
+      ".yaml",
+      ".yml",
+      ".toml",
+      ".xml",
+      ".md",
+      ".mdx",
+      ".graphql",
+      ".gql",
+    ],
+    enabled: Effect.gen(function* () {
+      const found = yield* Effect.forEach(["biome.json", "biome.jsonc"], findUp, { concurrency: "unbounded" })
+      if (!found.some((items) => items.length > 0)) return disabled
+      const bin = yield* input.npm.which("@biomejs/biome")
+      return bin ? [bin, "format", "--write", "$FILE"] : disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const zig: Info = {
+    name: "zig",
+    extensions: [".zig", ".zon"],
+    enabled: Effect.sync(() => {
+      const match = which("zig")
+      return match ? [match, "fmt", "$FILE"] : disabled
+    }),
+  }
+
+  const clang: Info = {
+    name: "clang-format",
+    extensions: [".c", ".cc", ".cpp", ".cxx", ".c++", ".h", ".hh", ".hpp", ".hxx", ".h++", ".ino", ".C", ".H"],
+    enabled: Effect.gen(function* () {
+      if (!(yield* findUp(".clang-format")).length) return disabled
+      const match = which("clang-format")
+      return match ? [match, "-i", "$FILE"] : disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const ktlint: Info = {
+    name: "ktlint",
+    extensions: [".kt", ".kts"],
+    enabled: Effect.sync(() => {
+      const match = which("ktlint")
+      return match ? [match, "-F", "$FILE"] : disabled
+    }),
+  }
+
+  const ruff: Info = {
+    name: "ruff",
+    extensions: [".py", ".pyi"],
+    enabled: Effect.gen(function* () {
+      if (!which("ruff")) return disabled
+      for (const config of ["pyproject.toml", "ruff.toml", ".ruff.toml"]) {
+        const found = yield* findUp(config)
+        if (!found.length) continue
+        if (config !== "pyproject.toml" || (yield* readText(found[0])).includes("[tool.ruff]")) {
+          return ["ruff", "format", "$FILE"]
+        }
+      }
+      for (const dependency of ["requirements.txt", "pyproject.toml", "Pipfile"]) {
+        const found = yield* findUp(dependency)
+        if (found.length && (yield* readText(found[0])).includes("ruff")) return ["ruff", "format", "$FILE"]
+      }
+      return disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const air: Info = {
+    name: "air",
+    extensions: [".R"],
+    enabled: Effect.gen(function* () {
+      const bin = which("air")
+      if (!bin) return disabled
+      const output = yield* commandOutput([bin, "--help"])
+      if (output._tag === "None" || output.value.exitCode !== 0) return disabled
+      const first = output.value.stdout.toString("utf8").split("\n")[0]
+      return first.includes("R language") && first.includes("formatter") ? [bin, "format", "$FILE"] : disabled
+    }),
+  }
+
+  const uv: Info = {
+    name: "uv",
+    extensions: [".py", ".pyi"],
+    enabled: Effect.gen(function* () {
+      const bin = which("uv")
+      if (!bin) return disabled
+      const output = yield* commandOutput([bin, "format", "--help"])
+      return output._tag === "Some" && output.value.exitCode === 0
+        ? [bin, "format", "--", "$FILE"]
+        : disabled
+    }),
+  }
+
+  const rubocop = executable("rubocop", [".rb", ".rake", ".gemspec", ".ru"], ["--autocorrect", "$FILE"])
+  const standardrb = executable("standardrb", [".rb", ".rake", ".gemspec", ".ru"], ["--fix", "$FILE"])
+  const htmlbeautifier = executable("htmlbeautifier", [".erb", ".html.erb"], ["$FILE"])
+  const dart = executable("dart", [".dart"], ["format", "$FILE"])
+
+  const ocamlformat: Info = {
+    name: "ocamlformat",
+    extensions: [".ml", ".mli"],
+    enabled: Effect.gen(function* () {
+      if (!(yield* findUp(".ocamlformat")).length) return disabled
+      const match = which("ocamlformat")
+      return match ? [match, "-i", "$FILE"] : disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const terraform = executable("terraform", [".tf", ".tfvars"], ["fmt", "$FILE"])
+  const latexindent = executable("latexindent", [".tex"], ["-w", "-s", "$FILE"])
+  const gleam = executable("gleam", [".gleam"], ["format", "$FILE"])
+  const shfmt = executable("shfmt", [".sh", ".bash"], ["-w", "$FILE"])
+  const nixfmt = executable("nixfmt", [".nix"], ["$FILE"])
+  const rustfmt = executable("rustfmt", [".rs"], ["$FILE"])
+
+  const pint: Info = {
+    name: "pint",
+    extensions: [".php"],
+    enabled: Effect.gen(function* () {
+      for (const file of yield* findUp("composer.json")) {
+        const json = yield* input.fs.readJson(file)
+        if (hasRecordKey(json, "require", "laravel/pint") || hasRecordKey(json, "require-dev", "laravel/pint")) {
+          return ["./vendor/bin/pint", "$FILE"]
+        }
+      }
+      return disabled
+    }).pipe(Effect.orElseSucceed(() => disabled)),
+  }
+
+  const ormolu = executable("ormolu", [".hs"], ["-i", "$FILE"])
+  const cljfmt = executable("cljfmt", [".clj", ".cljs", ".cljc", ".edn"], ["fix", "--quiet", "$FILE"])
+  const dfmt = executable("dfmt", [".d"], ["-i", "$FILE"])
+
+  return [
+    gofmt,
+    mix,
+    oxfmt,
+    prettier,
+    biome,
+    zig,
+    clang,
+    ktlint,
+    ruff,
+    air,
+    uv,
+    rubocop,
+    standardrb,
+    htmlbeautifier,
+    dart,
+    ocamlformat,
+    terraform,
+    latexindent,
+    gleam,
+    shfmt,
+    nixfmt,
+    rustfmt,
+    pint,
+    ormolu,
+    cljfmt,
+    dfmt,
+  ] satisfies Info[]
+}
+
+function executable(name: string, extensions: readonly string[], args: string[]): Info {
+  return {
+    name,
+    extensions,
+    enabled: Effect.sync(() => {
+      const match = which(name)
+      return match ? [match, ...args] : false
+    }),
+  }
+}
+
+function hasDependency(input: unknown, dependency: string) {
+  return hasRecordKey(input, "dependencies", dependency) || hasRecordKey(input, "devDependencies", dependency)
+}
+
+function hasRecordKey(input: unknown, field: string, key: string) {
+  if (!isRecord(input)) return false
+  return isRecord(input[field]) && key in input[field]
+}
+
+function isRecord(input: unknown): input is Record<string, unknown> {
+  return Boolean(input && typeof input === "object" && !Array.isArray(input))
+}

packages/core/src/location-services.ts

@@ -8,6 +8,7 @@ import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { Node } from "@opencode-ai/util/effect/app-node"
 import { Bus } from "./bus"
 import { FileMutation } from "./file-mutation"
+import { Formatter } from "./formatter"
 import { FileSystem } from "./filesystem"
 import { FileSystemSearch } from "./filesystem/search"
 import { Generate } from "./generate"
@@ -73,6 +74,7 @@ const locationServiceNodes = [
   InstructionDiscovery.node,
   LocationMutation.node,
   FileMutation.node,
+  Formatter.node,
   MCP.node,
   Permission.node,
   Tool.node,

packages/core/src/plugin/internal.ts

@@ -16,6 +16,7 @@ import { ConfigSkillPlugin } from "../config/plugin/skill"
 import { ConfigWebSearchPlugin } from "../config/plugin/websearch"
 import { Bus } from "../bus"
 import { FileMutation } from "../file-mutation"
+import { Formatter } from "../formatter"
 import { Form } from "../form"
 import { FileSystem } from "../filesystem"
 import { FSUtil } from "@opencode-ai/util/fs-util"
@@ -68,6 +69,7 @@ const services = Effect.fn("PluginInternal.services")(function* () {
   const config = yield* Config.Service
   const bus = yield* Bus.Service
   const mutation = yield* FileMutation.Service
+  const formatter = yield* Formatter.Service
   const filesystem = yield* FileSystem.Service
   const fs = yield* FSUtil.Service
   const global = yield* Global.Service
@@ -98,6 +100,7 @@ const services = Effect.fn("PluginInternal.services")(function* () {
     Context.make(Config.Service, config),
     Context.make(Bus.Service, bus),
     Context.make(FileMutation.Service, mutation),
+    Context.make(Formatter.Service, formatter),
     Context.make(FileSystem.Service, filesystem),
     Context.make(FSUtil.Service, fs),
     Context.make(Global.Service, global),

packages/core/src/plugin/supervisor.ts

@@ -14,6 +14,7 @@ import { makeLocationNode } from "@opencode-ai/util/effect/app-node"
 import { httpClient } from "@opencode-ai/util/effect/app-node-platform"
 import { Bus } from "../bus"
 import { FileMutation } from "../file-mutation"
+import { Formatter } from "../formatter"
 import { FileSystem } from "../filesystem"
 import { Watcher } from "../filesystem/watcher"
 import { Form } from "../form"
@@ -318,6 +319,7 @@ export const node = makeLocationNode({
     Config.node,
     Bus.node,
     FileMutation.node,
+    Formatter.node,
     FileSystem.node,
     FSUtil.node,
     Global.node,

packages/core/src/tool/plugin/edit.ts

@@ -9,9 +9,11 @@ export * as EditTool from "./edit"
 import type { Context as PluginContext } from "@opencode-ai/plugin/effect/plugin"
 import { ToolFailure } from "@opencode-ai/ai"
 import { FileDiff } from "@opencode-ai/schema/file-diff"
+import { Bom } from "@opencode-ai/util/bom"
 import { createTwoFilesPatch, diffLines } from "diff"
 import { Effect, Schema } from "effect"
 import { FileMutation } from "../../file-mutation"
+import { Formatter } from "../../formatter"
 import { FSUtil } from "@opencode-ai/util/fs-util"
 import { LocationMutation } from "../../location-mutation"
 import { Permission } from "../../permission"
@@ -99,7 +101,6 @@ const findLineOccurrences = (content: string, search: string) => {
 }
 
 /** Deferred edit behavior and UX integrations remain visible at the model-facing seam. */
-// TODO: Add formatter integration after formatter runtime exists.
 // TODO: Publish watcher/file-edit events after watcher integration exists.
 // TODO: Add snapshots / undo after design exists.
 // TODO: Add LSP notification and diagnostics after LSP runtime exists.
@@ -109,6 +110,7 @@ export const Plugin = {
   effect: Effect.fn("EditTool.Plugin")(function* (ctx: PluginContext) {
     const mutation = yield* LocationMutation.Service
     const files = yield* FileMutation.Service
+    const formatter = yield* Formatter.Service
     const fs = yield* FSUtil.Service
     const permission = yield* Permission.Service
 
@@ -167,9 +169,8 @@ export const Plugin = {
                   if (info.type === "Directory") {
                     return yield* new ToolFailure({ message: `Path is a directory, not a file: ${input.path}` })
                   }
-                  const bytes = yield* fs.readFile(target.canonical)
+                  const original = yield* Bom.readFile(fs, target.canonical)
-                  const bom = bytes[0] === 0xef && bytes[1] === 0xbb && bytes[2] === 0xbf
+                  const source = original.text
-                  const source = new TextDecoder().decode(bom ? bytes.slice(3) : bytes)
                   const ending = source.includes(crlf) ? crlf : "\n"
                   const oldString = input.oldString.replaceAll(crlf, "\n").replaceAll("\n", ending)
                   const newString = input.newString.replaceAll(crlf, "\n").replaceAll("\n", ending)
@@ -201,23 +202,27 @@ export const Plugin = {
                         `${content.slice(0, match.start)}${newString}${content.slice(match.end)}`,
                       source,
                     )
-                  const counts = diffLines(source, replaced).reduce(
+                  const replacementBom = replaced.startsWith("\uFEFF")
+                  const result = yield* files.write({
+                    target,
+                    content: Bom.join(replaced, original.bom || replacementBom),
+                  })
+                  const bom = original.bom || replacementBom
+                  const formatted = (yield* formatter.file(target.canonical))
+                    ? yield* Bom.syncFile(fs, target.canonical, bom)
+                    : (yield* Bom.readFile(fs, target.canonical)).text
+                  const counts = diffLines(source, formatted).reduce(
                     (result, item) => ({
                       additions: result.additions + (item.added ? (item.count ?? 0) : 0),
                       deletions: result.deletions + (item.removed ? (item.count ?? 0) : 0),
                     }),
                     { additions: 0, deletions: 0 },
                   )
-                  const replacementBom = replaced.startsWith("\uFEFF")
-                  const result = yield* files.write({
-                    target,
-                    content: `${bom || replacementBom ? "\uFEFF" : ""}${replacementBom ? replaced.slice(1) : replaced}`,
-                  })
                   return {
                     files: [
                       {
                         file: result.resource,
-                        patch: createTwoFilesPatch(result.resource, result.resource, source, replaced),
+                        patch: createTwoFilesPatch(result.resource, result.resource, source, formatted),
                         status: "modified" as const,
                         ...counts,
                       },

packages/core/src/tool/plugin/patch.ts

@@ -7,7 +7,9 @@ import { createTwoFilesPatch, diffLines } from "diff"
 import { Effect, Schema } from "effect"
 import { PlatformError } from "effect/PlatformError"
 import path from "path"
+import { Bom } from "@opencode-ai/util/bom"
 import { FSUtil } from "@opencode-ai/util/fs-util"
+import { Formatter } from "../../formatter"
 import { Location } from "../../location"
 import { Patch } from "@opencode-ai/util/patch"
 import { Permission } from "../../permission"
@@ -68,6 +70,7 @@ export const Plugin = {
   id: "opencode.tool.patch",
   effect: Effect.fn("PatchTool.Plugin")(function* (ctx: PluginContext) {
     const fs = yield* FSUtil.Service
+    const formatter = yield* Formatter.Service
     const location = yield* Location.Service
     const permission = yield* Permission.Service
 
@@ -129,15 +132,16 @@ export const Plugin = {
                           ...hunk,
                           target,
                           before: "",
-                          after: (hunk.contents.endsWith("\n") || hunk.contents === ""
+                          after: Bom.split(
-                            ? hunk.contents
+                            hunk.contents.endsWith("\n") || hunk.contents === ""
-                            : `${hunk.contents}\n`
+                              ? hunk.contents
-                          ).replace(/^\uFEFF/, ""),
+                              : `${hunk.contents}\n`,
+                          ).text,
                         })
                         return
                       }
                       if (hunk.type === "delete") {
-                        const content = yield* fs.readFile(target.canonical).pipe(
+                        const content = yield* Bom.readFile(fs, target.canonical).pipe(
                           Effect.mapError(
                             (error) =>
                               new ToolFailure({
@@ -145,8 +149,7 @@ export const Plugin = {
                               }),
                           ),
                         )
-                        const original = new TextDecoder("utf-8", { ignoreBOM: true }).decode(content)
+                        prepared.push({ ...hunk, target, before: content.text, after: "" })
-                        prepared.push({ ...hunk, target, before: original.replace(/^\uFEFF/, ""), after: "" })
                         return
                       }
                       const previous = updates.get(target.canonical)
@@ -166,18 +169,17 @@ export const Plugin = {
                               message: `patch verification failed: Failed to read file to update ${target.canonical}: path is a directory`,
                             })
                           }
-                          return new TextDecoder("utf-8", { ignoreBOM: true }).decode(
+                          const content = yield* Bom.readFile(fs, target.canonical).pipe(
-                            yield* fs.readFile(target.canonical).pipe(
+                            Effect.mapError(
-                              Effect.mapError(
+                              (error) =>
-                                (error) =>
+                                new ToolFailure({
-                                  new ToolFailure({
+                                  message: `patch verification failed: Failed to read file to update ${target.canonical}: ${errorMessage(error)}`,
-                                    message: `patch verification failed: Failed to read file to update ${target.canonical}: ${errorMessage(error)}`,
+                                }),
-                                  }),
-                              ),
                             ),
                           )
+                          return Bom.join(content.text, content.bom)
                         }))
-                      const before = original.replace(/^\uFEFF/, "")
+                      const before = Bom.split(original).text
                       const update = yield* Effect.try({
                         try: () => Patch.derive(hunk.path, hunk.chunks, original),
                         catch: (error) =>
@@ -217,7 +219,7 @@ export const Plugin = {
                     )
                   }
 
-                  const patchFiles = prepared.map(patchFile)
+                  const patchFiles = prepared.map((change) => patchFile(change))
                   yield* permission.assert({
                     action: "edit",
                     resources: [...new Set(targets.map((target) => target.resource))],
@@ -295,7 +297,31 @@ export const Plugin = {
                       }),
                     { discard: true },
                   )
-                  return { applied, files: patchFiles }
+                  const formatted = new Map<string, string>()
+                  yield* Effect.forEach(
+                    [...new Set(applied.filter((item) => item.type !== "delete").map((item) => item.target))],
+                    (target) =>
+                      Effect.gen(function* () {
+                        const current = yield* Bom.readFile(fs, target).pipe(
+                          Effect.mapError((error) => fail(`Failed to read ${target}`, error)),
+                        )
+                        formatted.set(
+                          target,
+                          (yield* formatter.file(target))
+                            ? yield* Bom.syncFile(fs, target, current.bom).pipe(
+                                Effect.mapError((error) => fail(`Failed to sync ${target}`, error)),
+                              )
+                            : current.text,
+                        )
+                      }),
+                    { discard: true },
+                  )
+                  const files = yield* Effect.forEach(prepared, (change) => {
+                    if (change.type === "delete") return Effect.succeed(patchFile(change))
+                    const target = change.type === "update" && change.moveTarget ? change.moveTarget : change.target
+                    return Effect.succeed(patchFile(change, formatted.get(target.canonical)))
+                  })
+                  return { applied, files }
                 }).pipe(
                   Effect.map((output) => ({
                     output,
@@ -337,15 +363,15 @@ function errorMessage(error: unknown) {
   return error instanceof Error ? error.message : String(error)
 }
 
-function patchFile(change: Prepared): typeof FileDiff.Info.Type {
+function patchFile(change: Prepared, after = change.after): typeof FileDiff.Info.Type {
   const target = (change.type === "update" ? change.moveTarget : undefined)?.resource ?? change.target.resource
   const patch = trimDiff(
-    createTwoFilesPatch(change.target.canonical, change.target.canonical, change.before, change.after),
+    createTwoFilesPatch(change.target.canonical, change.target.canonical, change.before, after),
   )
   const counts =
     change.type === "delete"
       ? { additions: 0, deletions: change.before.split("\n").length }
-      : diffLines(change.before, change.after).reduce(
+      : diffLines(change.before, after).reduce(
           (result, item) => ({
             additions: result.additions + (item.added ? (item.count ?? 0) : 0),
             deletions: result.deletions + (item.removed ? (item.count ?? 0) : 0),

packages/core/src/tool/plugin/write.ts

@@ -9,7 +9,10 @@ export * as WriteTool from "./write"
 import type { Context as PluginContext } from "@opencode-ai/plugin/effect/plugin"
 import { ToolFailure } from "@opencode-ai/ai"
 import { Effect, Schema } from "effect"
+import { Bom } from "@opencode-ai/util/bom"
+import { FSUtil } from "@opencode-ai/util/fs-util"
 import { FileMutation } from "../../file-mutation"
+import { Formatter } from "../../formatter"
 import { LocationMutation } from "../../location-mutation"
 import { Permission } from "../../permission"
 
@@ -36,7 +39,6 @@ export const toModelOutput = (output: Output) =>
   `${output.existed ? "Wrote" : "Created"} file successfully: ${output.resource}`
 
 /** Deferred write UX integrations remain visible at the model-facing seam. */
-// TODO: Add formatter integration after formatter runtime exists.
 // TODO: Publish watcher/file-edit events after watcher integration exists.
 // TODO: Add snapshots / undo after design exists.
 // TODO: Add LSP notification and diagnostics after LSP runtime exists.
@@ -46,6 +48,8 @@ export const Plugin = {
   effect: Effect.fn("WriteTool.Plugin")(function* (ctx: PluginContext) {
     const mutation = yield* LocationMutation.Service
     const files = yield* FileMutation.Service
+    const formatter = yield* Formatter.Service
+    const fs = yield* FSUtil.Service
     const permission = yield* Permission.Service
 
     yield* ctx.tool
@@ -82,7 +86,10 @@ export const Plugin = {
                     agent: context.agent,
                     source,
                   })
-                  return yield* files.writeTextPreservingBom({ target, content: input.content })
+                  const result = yield* files.writeTextPreservingBom({ target, content: input.content })
+                  const bom = (yield* Bom.readFile(fs, target.canonical)).bom
+                  if (yield* formatter.file(target.canonical)) yield* Bom.syncFile(fs, target.canonical, bom)
+                  return result
                 }).pipe(
                   Effect.map((output) => ({ output, content: toModelOutput(output) })),
                   Effect.mapError((error) => new ToolFailure({ message: `Unable to write ${input.path}`, error })),

packages/core/test/formatter.test.ts

@@ -0,0 +1,199 @@
+import fs from "fs/promises"
+import path from "path"
+import { describe, expect } from "bun:test"
+import { Effect, Layer, Schema, Stream } from "effect"
+import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
+import { AbsolutePath } from "@opencode-ai/core/schema"
+import { Npm } from "@opencode-ai/util/npm"
+import { Config } from "../src/config"
+import { Formatter } from "../src/formatter"
+import { Location } from "../src/location"
+import { location } from "./fixture/location"
+import { tmpdir } from "./fixture/tmpdir"
+import { testEffect } from "./lib/effect"
+
+const it = testEffect(Layer.empty)
+type ConfigInput = typeof Config.Info.Encoded
+
+function formatterLayer(directory: string, configured?: ConfigInput["formatter"]) {
+  const entries =
+    configured === undefined
+      ? []
+      : [
+          new Config.Document({
+            type: "document",
+            info: Schema.decodeUnknownSync(Config.Info)({ formatter: configured }),
+          }),
+        ]
+  return AppNodeBuilder.build(Formatter.node, [
+    [
+      Config.node,
+      Layer.succeed(
+        Config.Service,
+        Config.Service.of({
+          entries: () => Effect.succeed(entries),
+          changes: () => Stream.empty,
+        }),
+      ),
+    ],
+    [
+      Location.node,
+      Layer.succeed(
+        Location.Service,
+        Location.Service.of(location({ directory: AbsolutePath.make(directory) })),
+      ),
+    ],
+    [Npm.node, Layer.mock(Npm.Service, { which: () => Effect.succeed(undefined) })],
+  ])
+}
+
+function withTemp<A, E, R>(body: (directory: string) => Effect.Effect<A, E, R>) {
+  return Effect.acquireUseRelease(
+    Effect.promise(() => tmpdir()),
+    (tmp) => body(tmp.path),
+    (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+  )
+}
+
+describe("Formatter", () => {
+  it.live("status() returns empty list when no formatters are configured", () =>
+    withTemp((directory) =>
+      Formatter.Service.use((formatter) => formatter.status()).pipe(Effect.provide(formatterLayer(directory))),
+    ),
+  )
+
+  it.live("status() returns built-in formatters when formatter is true", () =>
+    withTemp((directory) =>
+      Formatter.Service.use((formatter) =>
+        Effect.gen(function* () {
+          const statuses = yield* formatter.status()
+          const gofmt = statuses.find((item) => item.name === "gofmt")
+          expect(gofmt).toBeDefined()
+          expect(gofmt?.extensions).toContain(".go")
+        }),
+      ).pipe(Effect.provide(formatterLayer(directory, true))),
+    ),
+  )
+
+  it.live("status() keeps built-in formatters when config object is provided", () =>
+    withTemp((directory) =>
+      Formatter.Service.use((formatter) =>
+        Effect.gen(function* () {
+          const statuses = yield* formatter.status()
+          expect(statuses.find((item) => item.name === "gofmt")?.extensions).toContain(".go")
+          expect(statuses.find((item) => item.name === "mix")).toBeDefined()
+        }),
+      ).pipe(Effect.provide(formatterLayer(directory, { gofmt: {} }))),
+    ),
+  )
+
+  it.live("status() excludes formatters marked as disabled in config", () =>
+    withTemp((directory) =>
+      Formatter.Service.use((formatter) =>
+        Effect.gen(function* () {
+          const statuses = yield* formatter.status()
+          expect(statuses.find((item) => item.name === "gofmt")).toBeUndefined()
+          expect(statuses.find((item) => item.name === "mix")).toBeDefined()
+        }),
+      ).pipe(Effect.provide(formatterLayer(directory, { gofmt: { disabled: true } }))),
+    ),
+  )
+
+  it.live("service initializes without error", () =>
+    withTemp((directory) =>
+      Formatter.Service.use((formatter) => formatter.init()).pipe(Effect.provide(formatterLayer(directory))),
+    ),
+  )
+
+  it.live("file() returns false when no formatter runs", () =>
+    withTemp((directory) =>
+      Effect.gen(function* () {
+        const file = path.join(directory, "test.txt")
+        yield* Effect.promise(() => fs.writeFile(file, "x"))
+        expect(yield* Formatter.Service.use((formatter) => formatter.file(file))).toBe(false)
+      }).pipe(Effect.provide(formatterLayer(directory, false))),
+    ),
+  )
+
+  it.live("status() initializes formatter state per directory", () =>
+    Effect.acquireUseRelease(
+      Effect.promise(() => Promise.all([tmpdir(), tmpdir()])),
+      ([off, on]) =>
+        Effect.gen(function* () {
+          const disabled = yield* Formatter.Service.use((formatter) => formatter.status()).pipe(
+            Effect.provide(formatterLayer(off.path, false)),
+          )
+          const enabled = yield* Formatter.Service.use((formatter) => formatter.status()).pipe(
+            Effect.provide(formatterLayer(on.path, true)),
+          )
+          expect(disabled).toEqual([])
+          expect(enabled.find((item) => item.name === "gofmt")).toBeDefined()
+        }),
+      (directories) =>
+        Effect.promise(() => Promise.all(directories.map((tmp) => tmp[Symbol.asyncDispose]())).then(() => undefined)),
+    ),
+  )
+
+  it.live("stops after the first matching formatter succeeds", () =>
+    withTemp((directory) =>
+      Effect.gen(function* () {
+        const file = path.join(directory, "test.seq")
+        yield* Effect.promise(() => fs.writeFile(file, "x"))
+        expect(yield* Formatter.Service.use((formatter) => formatter.file(file))).toBe(true)
+        expect(yield* Effect.promise(() => fs.readFile(file, "utf8"))).toBe("xA")
+      }).pipe(
+        Effect.provide(
+          formatterLayer(directory, {
+            first: {
+              command: [
+                process.execPath,
+                "-e",
+                "const fs = require('fs'); const file = process.argv.at(-1); fs.appendFileSync(file, 'A')",
+                "$FILE",
+              ],
+              extensions: [".seq"],
+            },
+            second: {
+              command: [
+                process.execPath,
+                "-e",
+                "const fs = require('fs'); const file = process.argv.at(-1); fs.appendFileSync(file, 'B')",
+                "$FILE",
+              ],
+              extensions: [".seq"],
+            },
+          }),
+        ),
+      ),
+    ),
+  )
+
+  it.live("tries the next matching formatter when the first fails", () =>
+    withTemp((directory) =>
+      Effect.gen(function* () {
+        const file = path.join(directory, "test.fallback")
+        yield* Effect.promise(() => fs.writeFile(file, "x"))
+        expect(yield* Formatter.Service.use((formatter) => formatter.file(file))).toBe(true)
+        expect(yield* Effect.promise(() => fs.readFile(file, "utf8"))).toBe("xB")
+      }).pipe(
+        Effect.provide(
+          formatterLayer(directory, {
+            first: {
+              command: [process.execPath, "-e", "process.exit(1)", "$FILE"],
+              extensions: [".fallback"],
+            },
+            second: {
+              command: [
+                process.execPath,
+                "-e",
+                "const fs = require('fs'); const file = process.argv.at(-1); fs.appendFileSync(file, 'B')",
+                "$FILE",
+              ],
+              extensions: [".fallback"],
+            },
+          }),
+        ),
+      ),
+    ),
+  )
+})

packages/core/test/tool-edit.test.ts

@@ -5,6 +5,7 @@ import { Effect, Layer } from "effect"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { FileMutation } from "@opencode-ai/core/file-mutation"
+import { Formatter } from "@opencode-ai/core/formatter"
 import { FSUtil } from "@opencode-ai/util/fs-util"
 import { Location } from "@opencode-ai/core/location"
 import { LocationMutation } from "@opencode-ai/core/location-mutation"
@@ -22,7 +23,7 @@ import { toolIdentity, executeTool, registerToolPlugin, toolDefinitions } from "
 const editToolNode = makeLocationNode({
   name: "test/edit-tool-plugin",
   layer: Layer.effectDiscard(registerToolPlugin(EditTool.Plugin)),
-  deps: [Tool.node, LocationMutation.node, FileMutation.node, FSUtil.node, Permission.node],
+  deps: [Tool.node, LocationMutation.node, FileMutation.node, Formatter.node, FSUtil.node, Permission.node],
 })
 
 const sessionID = Session.ID.make("ses_edit_tool_test")
@@ -31,6 +32,7 @@ const writes: string[] = []
 let reads = 0
 let denyAction: string | undefined
 let afterRead = (_target: string, _content: Uint8Array): Effect.Effect<void> => Effect.void
+let formatFile = (_target: string): Effect.Effect<boolean> => Effect.succeed(false)
 
 const permission = Layer.succeed(
   Permission.Service,
@@ -57,12 +59,17 @@ const permission = Layer.succeed(
   }),
 )
 
+const formatter = Layer.mock(Formatter.Service, {
+  file: (target) => formatFile(target),
+})
+
 const reset = () => {
   assertions.length = 0
   writes.length = 0
   reads = 0
   denyAction = undefined
   afterRead = () => Effect.void
+  formatFile = () => Effect.succeed(false)
 }
 
 const filesystem = Layer.effect(
@@ -109,6 +116,7 @@ const withTool = <A, E, R>(directory: string, body: (registry: Tool.Interface) =
         [
           [FSUtil.node, filesystem],
           [Location.node, activeLocation],
+          [Formatter.node, formatter],
           [Permission.node, permission],
         ],
       ),
@@ -181,6 +189,39 @@ describe("EditTool", () => {
     ),
   )
 
+  it.live("returns the diff for final formatted content", () =>
+    Effect.acquireUseRelease(
+      Effect.promise(() => tmpdir()),
+      (tmp) => {
+        reset()
+        const target = path.join(tmp.path, "formatted.txt")
+        formatFile = (file) =>
+          Effect.promise(async () => {
+            await fs.writeFile(file, (await fs.readFile(file, "utf8")).replace("after", "AFTER"))
+            return true
+          })
+        return Effect.promise(() => fs.writeFile(target, "before\n")).pipe(
+          Effect.andThen(
+            withTool(tmp.path, (registry) =>
+              Effect.gen(function* () {
+                const settled = yield* executeTool(
+                  registry,
+                  call({ path: "formatted.txt", oldString: "before", newString: "after" }),
+                )
+                expect(settled.status).toBe("completed")
+                if (settled.status !== "completed") return
+                expect(settled.output.files[0]?.patch).toContain("-before\n+AFTER")
+                expect(settled.metadata?.files?.[0]?.patch).toContain("-before\n+AFTER")
+                expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("AFTER\n")
+              }),
+            ),
+          ),
+        )
+      },
+      (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+    ),
+  )
+
   it.live("accepts an absolute file path inside the active Location", () =>
     Effect.acquireUseRelease(
       Effect.promise(() => tmpdir()),
@@ -574,6 +615,11 @@ describe("EditTool", () => {
       (tmp) => {
         reset()
         const target = path.join(tmp.path, "windows.txt")
+        formatFile = (file) =>
+          Effect.promise(async () => {
+            await fs.writeFile(file, (await fs.readFile(file, "utf8")).replace(/^\uFEFF/, ""))
+            return true
+          })
         return Effect.promise(() => fs.writeFile(target, "\uFEFFbefore\r\nrest\r\n")).pipe(
           Effect.andThen(
             withTool(tmp.path, (registry) =>

packages/core/test/tool-patch.test.ts

@@ -6,6 +6,7 @@ import { systemError } from "effect/PlatformError"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { FSUtil } from "@opencode-ai/util/fs-util"
+import { Formatter } from "@opencode-ai/core/formatter"
 import { Location } from "@opencode-ai/core/location"
 import { Permission } from "@opencode-ai/core/permission"
 import { AbsolutePath } from "@opencode-ai/core/schema"
@@ -21,7 +22,7 @@ import { toolIdentity, executeTool, registerToolPlugin, toolDefinitions } from "
 const patchToolNode = makeLocationNode({
   name: "test/patch-tool-plugin",
   layer: Layer.effectDiscard(registerToolPlugin(PatchTool.Plugin)),
-  deps: [Tool.node, FSUtil.node, Location.node, Permission.node],
+  deps: [Tool.node, Formatter.node, FSUtil.node, Location.node, Permission.node],
 })
 
 const sessionID = Session.ID.make("ses_patch_tool_test")
@@ -33,6 +34,7 @@ let failWriteTarget: string | undefined
 let readsBeforeEditApproval = 0
 let editApproved = false
 let afterEditApproval = (): Effect.Effect<void> => Effect.void
+let formatFile = (_target: string): Effect.Effect<boolean> => Effect.succeed(false)
 
 const permission = Layer.succeed(
   Permission.Service,
@@ -63,6 +65,10 @@ const permission = Layer.succeed(
   }),
 )
 
+const formatter = Layer.mock(Formatter.Service, {
+  file: (target) => formatFile(target),
+})
+
 const reset = () => {
   assertions.length = 0
   denyAction = undefined
@@ -72,6 +78,7 @@ const reset = () => {
   readsBeforeEditApproval = 0
   editApproved = false
   afterEditApproval = () => Effect.void
+  formatFile = () => Effect.succeed(false)
 }
 
 const filesystem = Layer.effect(
@@ -135,6 +142,7 @@ const withTool = <A, E, R>(
       AppNodeBuilder.build(LayerNode.group([Tool.node, patchToolNode]), [
         [FSUtil.node, filesystem],
         [Location.node, activeLocation],
+        [Formatter.node, formatter],
         [Permission.node, permission],
       ]),
     ),
@@ -254,6 +262,28 @@ describe("PatchTool", () => {
     ),
   )
 
+  it.live("returns file diffs for final formatted content", () =>
+    withTempTool((directory, registry) => {
+      const target = path.join(directory, "formatted.txt")
+      formatFile = (file) =>
+        Effect.promise(async () => {
+          await fs.writeFile(file, (await fs.readFile(file, "utf8")).replace("created", "FORMATTED"))
+          return true
+        })
+      return Effect.gen(function* () {
+        const settled = yield* executeTool(
+          registry,
+          call("*** Begin Patch\n*** Add File: formatted.txt\n+created\n*** End Patch"),
+        )
+        expect(settled.status).toBe("completed")
+        if (settled.status !== "completed") return
+        expect(settled.output.files[0]?.patch).toContain("+FORMATTED")
+        expect(settled.metadata?.files?.[0]?.patch).toContain("+FORMATTED")
+        expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("FORMATTED\n")
+      })
+    }),
+  )
+
   it.live("moves and updates a file", () =>
     Effect.acquireUseRelease(
       Effect.promise(() => tmpdir()),
@@ -552,6 +582,11 @@ describe("PatchTool", () => {
         const bom = "\uFEFF"
         const target = path.join(directory, "example.cs")
         yield* Effect.promise(() => fs.writeFile(target, `${bom}using System;\n\nclass Test {}\n`))
+        formatFile = (file) =>
+          Effect.promise(async () => {
+            await fs.writeFile(file, (await fs.readFile(file, "utf8")).replace(/^\uFEFF/, ""))
+            return true
+          })
         const settled = yield* executeTool(
           registry,
           call("*** Begin Patch\n*** Update File: example.cs\n@@\n class Test {}\n+class Next {}\n*** End Patch"),

packages/core/test/tool-write.test.ts

@@ -3,6 +3,7 @@ import path from "path"
 import { describe, expect } from "bun:test"
 import { Effect, Layer } from "effect"
 import { FileMutation } from "@opencode-ai/core/file-mutation"
+import { Formatter } from "@opencode-ai/core/formatter"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { FSUtil } from "@opencode-ai/util/fs-util"
@@ -22,12 +23,13 @@ import { toolIdentity, executeTool, registerToolPlugin, toolDefinitions } from "
 const writeToolNode = makeLocationNode({
   name: "test/write-tool-plugin",
   layer: Layer.effectDiscard(registerToolPlugin(WriteTool.Plugin)),
-  deps: [Tool.node, LocationMutation.node, FileMutation.node, Permission.node],
+  deps: [Tool.node, LocationMutation.node, FileMutation.node, Formatter.node, FSUtil.node, Permission.node],
 })
 
 const sessionID = Session.ID.make("ses_write_tool_test")
 const assertions: Permission.AssertInput[] = []
 const writes: string[] = []
+let formatFile = (_target: string): Effect.Effect<boolean> => Effect.succeed(false)
 let denyAction: string | undefined
 
 const permission = Layer.succeed(
@@ -55,9 +57,14 @@ const permission = Layer.succeed(
   }),
 )
 
+const formatter = Layer.mock(Formatter.Service, {
+  file: (target) => formatFile(target),
+})
+
 const reset = () => {
   assertions.length = 0
   writes.length = 0
+  formatFile = () => Effect.succeed(false)
   denyAction = undefined
 }
 
@@ -93,6 +100,7 @@ const withTool = <A, E, R>(directory: string, body: (registry: Tool.Interface) =
         [
           [FSUtil.node, filesystem],
           [Location.node, activeLocation],
+          [Formatter.node, formatter],
           [Permission.node, permission],
         ],
       ),
@@ -140,6 +148,30 @@ describe("WriteTool", () => {
     ),
   )
 
+  it.live("formats the committed file", () =>
+    Effect.acquireUseRelease(
+      Effect.promise(() => tmpdir()),
+      (tmp) => {
+        reset()
+        const target = path.join(tmp.path, "formatted.txt")
+        formatFile = (file) =>
+          Effect.promise(async () => {
+            await fs.writeFile(file, (await fs.readFile(file, "utf8")).toUpperCase())
+            return true
+          })
+        return withTool(tmp.path, (registry) =>
+          Effect.gen(function* () {
+            expect(yield* executeTool(registry, call({ path: "formatted.txt", content: "format me" }))).toMatchObject({
+              status: "completed",
+            })
+            expect(yield* Effect.promise(() => fs.readFile(target, "utf8"))).toBe("FORMAT ME")
+          }),
+        )
+      },
+      (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+    ),
+  )
+
   it.live("overwrites a relative existing file and reports that it wrote the file", () =>
     Effect.acquireUseRelease(
       Effect.promise(() => tmpdir()),
@@ -174,6 +206,11 @@ describe("WriteTool", () => {
         reset()
         const preserved = path.join(tmp.path, "preserved.txt")
         const deduplicated = path.join(tmp.path, "deduplicated.txt")
+        formatFile = (target) =>
+          Effect.promise(async () => {
+            await fs.writeFile(target, `\uFEFF\uFEFF\uFEFF${(await fs.readFile(target, "utf8")).replace(/^\uFEFF+/, "")}`)
+            return true
+          })
         return Effect.promise(() =>
           Promise.all([fs.writeFile(preserved, "\uFEFFbefore"), fs.writeFile(deduplicated, "\uFEFFbefore")]),
         ).pipe(

packages/util/src/bom.ts

@@ -0,0 +1,38 @@
+export * as Bom from "./bom.js"
+
+import { Effect } from "effect"
+import { FSUtil } from "./fs-util.js"
+
+const code = 0xfeff
+const value = String.fromCharCode(code)
+
+export function split(text: string) {
+  const stripped = text.replace(/^\uFEFF+/, "")
+  return { bom: stripped.length !== text.length, text: stripped }
+}
+
+export function join(text: string, bom: boolean) {
+  const stripped = split(text).text
+  return bom ? value + stripped : stripped
+}
+
+export function has(content: Uint8Array) {
+  return content[0] === 0xef && content[1] === 0xbb && content[2] === 0xbf
+}
+
+export const readFile = Effect.fn("Bom.readFile")(function* (fs: FSUtil.Interface, filepath: string) {
+  return split(decode(yield* fs.readFile(filepath)))
+})
+
+export const syncFile = Effect.fn("Bom.syncFile")(function* (fs: FSUtil.Interface, filepath: string, bom: boolean) {
+  const decoded = decode(yield* fs.readFile(filepath))
+  const current = split(decoded)
+  const canonical = join(current.text, bom)
+  if (decoded === canonical) return current.text
+  yield* fs.writeWithDirs(filepath, canonical)
+  return current.text
+})
+
+function decode(content: Uint8Array) {
+  return new TextDecoder("utf-8", { ignoreBOM: true }).decode(content)
+}

packages/util/src/patch.ts

@@ -1,6 +1,7 @@
 export * as Patch from "./patch.js"
 
 import { Result, Schema } from "effect"
+import { Bom } from "./bom.js"
 
 export class BoundaryError extends Schema.TaggedErrorClass<BoundaryError>()("Patch.BoundaryError", {
   boundary: Schema.Literals(["first", "last"]),
@@ -125,20 +126,19 @@ export function parse(patchText: string): Result.Result<ReadonlyArray<Hunk>, Par
 }
 
 export function derive(path: string, chunks: ReadonlyArray<UpdateFileChunk>, original: string): FileUpdate {
-  const source = splitBom(original)
+  const source = Bom.split(original)
   const lines = source.text.split("\n")
   if (lines.at(-1) === "") lines.pop()
   const replacements = computeReplacements(lines, path, chunks)
   const updated = [...lines]
   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"))
+  const next = Bom.split(updated.join("\n"))
   return { content: next.text, bom: source.bom || next.bom }
 }
 
 export function joinBom(text: string, bom: boolean) {
-  const stripped = splitBom(text).text
+  return Bom.join(text, bom)
-  return bom ? `\uFEFF${stripped}` : stripped
 }
 
 function parseAdd(
@@ -379,6 +379,4 @@ const normalize = (value: string) =>
     .replace(/[“”„‟]/g, '"')
     .replace(/[‐‑‒–—―−]/g, "-")
     .replace(/[\u00A0\u2002-\u200A\u202F\u205F\u3000]/g, " ")
-const splitBom = (text: string) =>
-  text.startsWith("\uFEFF") ? { bom: true, text: text.slice(1) } : { bom: false, text }
 const stripHeredoc = (input: string) => input.match(/^(?:cat\s+)?<<(['"]?)(\w+)\1\s*\n([\s\S]*?)\n\2\s*$/)?.[3] ?? input