diff --git a/packages/opencode/package.json b/packages/opencode/package.json
index 4bdc3a963cd..a6caca8adfb 100644
--- a/packages/opencode/package.json
+++ b/packages/opencode/package.json
@@ -6,6 +6,7 @@
   "license": "MIT",
   "private": true,
   "scripts": {
+    "prepare": "effect-language-service patch || true",
     "typecheck": "tsgo --noEmit",
     "test": "bun test --timeout 30000",
     "build": "bun run script/build.ts",
diff --git a/packages/opencode/src/filesystem/index.ts b/packages/opencode/src/filesystem/index.ts
new file mode 100644
index 00000000000..d8f7d6053e7
--- /dev/null
+++ b/packages/opencode/src/filesystem/index.ts
@@ -0,0 +1,197 @@
+import { NodeFileSystem } from "@effect/platform-node"
+import { dirname, join, relative, resolve as pathResolve } from "path"
+import { realpathSync } from "fs"
+import { lookup } from "mime-types"
+import { Effect, FileSystem, Layer, Schema, ServiceMap } from "effect"
+import type { PlatformError } from "effect/PlatformError"
+import { Glob } from "../util/glob"
+
+export namespace AppFileSystem {
+  export class FileSystemError extends Schema.TaggedErrorClass<FileSystemError>()("FileSystemError", {
+    method: Schema.String,
+    cause: Schema.optional(Schema.Defect),
+  }) {}
+
+  export type Error = PlatformError | FileSystemError
+
+  export interface Interface extends FileSystem.FileSystem {
+    readonly isDir: (path: string) => Effect.Effect<boolean, Error>
+    readonly isFile: (path: string) => Effect.Effect<boolean, Error>
+    readonly readJson: (path: string) => Effect.Effect<unknown, Error>
+    readonly writeJson: (path: string, data: unknown, mode?: number) => Effect.Effect<void, Error>
+    readonly ensureDir: (path: string) => Effect.Effect<void, Error>
+    readonly writeWithDirs: (path: string, content: string | Uint8Array, mode?: number) => Effect.Effect<void, Error>
+    readonly findUp: (target: string, start: string, stop?: string) => Effect.Effect<string[], Error>
+    readonly up: (options: { targets: string[]; start: string; stop?: string }) => Effect.Effect<string[], Error>
+    readonly globUp: (pattern: string, start: string, stop?: string) => Effect.Effect<string[], Error>
+    readonly glob: (pattern: string, options?: Glob.Options) => Effect.Effect<string[], Error>
+    readonly globMatch: (pattern: string, filepath: string) => boolean
+  }
+
+  export class Service extends ServiceMap.Service<Service, Interface>()("@opencode/FileSystem") {}
+
+  export const layer = Layer.effect(
+    Service,
+    Effect.gen(function* () {
+      const fs = yield* FileSystem.FileSystem
+
+      const isDir = Effect.fn("FileSystem.isDir")(function* (path: string) {
+        const info = yield* fs.stat(path).pipe(Effect.catch(() => Effect.void))
+        return info?.type === "Directory"
+      })
+
+      const isFile = Effect.fn("FileSystem.isFile")(function* (path: string) {
+        const info = yield* fs.stat(path).pipe(Effect.catch(() => Effect.void))
+        return info?.type === "File"
+      })
+
+      const readJson = Effect.fn("FileSystem.readJson")(function* (path: string) {
+        const text = yield* fs.readFileString(path)
+        return JSON.parse(text)
+      })
+
+      const writeJson = Effect.fn("FileSystem.writeJson")(function* (path: string, data: unknown, mode?: number) {
+        const content = JSON.stringify(data, null, 2)
+        yield* fs.writeFileString(path, content)
+        if (mode) yield* fs.chmod(path, mode)
+      })
+
+      const ensureDir = Effect.fn("FileSystem.ensureDir")(function* (path: string) {
+        yield* fs.makeDirectory(path, { recursive: true })
+      })
+
+      const writeWithDirs = Effect.fn("FileSystem.writeWithDirs")(function* (
+        path: string,
+        content: string | Uint8Array,
+        mode?: number,
+      ) {
+        const write = typeof content === "string" ? fs.writeFileString(path, content) : fs.writeFile(path, content)
+
+        yield* write.pipe(
+          Effect.catchIf(
+            (e) => e.reason._tag === "NotFound",
+            () =>
+              Effect.gen(function* () {
+                yield* fs.makeDirectory(dirname(path), { recursive: true })
+                yield* write
+              }),
+          ),
+        )
+        if (mode) yield* fs.chmod(path, mode)
+      })
+
+      const glob = Effect.fn("FileSystem.glob")(function* (pattern: string, options?: Glob.Options) {
+        return yield* Effect.tryPromise({
+          try: () => Glob.scan(pattern, options),
+          catch: (cause) => new FileSystemError({ method: "glob", cause }),
+        })
+      })
+
+      const findUp = Effect.fn("FileSystem.findUp")(function* (target: string, start: string, stop?: string) {
+        const result: string[] = []
+        let current = start
+        while (true) {
+          const search = join(current, target)
+          if (yield* fs.exists(search)) result.push(search)
+          if (stop === current) break
+          const parent = dirname(current)
+          if (parent === current) break
+          current = parent
+        }
+        return result
+      })
+
+      const up = Effect.fn("FileSystem.up")(function* (options: { targets: string[]; start: string; stop?: string }) {
+        const result: string[] = []
+        let current = options.start
+        while (true) {
+          for (const target of options.targets) {
+            const search = join(current, target)
+            if (yield* fs.exists(search)) result.push(search)
+          }
+          if (options.stop === current) break
+          const parent = dirname(current)
+          if (parent === current) break
+          current = parent
+        }
+        return result
+      })
+
+      const globUp = Effect.fn("FileSystem.globUp")(function* (pattern: string, start: string, stop?: string) {
+        const result: string[] = []
+        let current = start
+        while (true) {
+          const matches = yield* glob(pattern, { cwd: current, absolute: true, include: "file", dot: true }).pipe(
+            Effect.catch(() => Effect.succeed([] as string[])),
+          )
+          result.push(...matches)
+          if (stop === current) break
+          const parent = dirname(current)
+          if (parent === current) break
+          current = parent
+        }
+        return result
+      })
+
+      return Service.of({
+        ...fs,
+        isDir,
+        isFile,
+        readJson,
+        writeJson,
+        ensureDir,
+        writeWithDirs,
+        findUp,
+        up,
+        globUp,
+        glob,
+        globMatch: Glob.match,
+      })
+    }),
+  )
+
+  export const defaultLayer = layer.pipe(Layer.provide(NodeFileSystem.layer))
+
+  // Pure helpers that don't need Effect (path manipulation, sync operations)
+  export function mimeType(p: string): string {
+    return lookup(p) || "application/octet-stream"
+  }
+
+  export function normalizePath(p: string): string {
+    if (process.platform !== "win32") return p
+    try {
+      return realpathSync.native(p)
+    } catch {
+      return p
+    }
+  }
+
+  export function resolve(p: string): string {
+    const resolved = pathResolve(windowsPath(p))
+    try {
+      return normalizePath(realpathSync(resolved))
+    } catch (e: any) {
+      if (e?.code === "ENOENT") return normalizePath(resolved)
+      throw e
+    }
+  }
+
+  export function windowsPath(p: string): string {
+    if (process.platform !== "win32") return p
+    return p
+      .replace(/^\/([a-zA-Z]):(?:[\\/]|$)/, (_, drive) => `${drive.toUpperCase()}:/`)
+      .replace(/^\/([a-zA-Z])(?:\/|$)/, (_, drive) => `${drive.toUpperCase()}:/`)
+      .replace(/^\/cygdrive\/([a-zA-Z])(?:\/|$)/, (_, drive) => `${drive.toUpperCase()}:/`)
+      .replace(/^\/mnt\/([a-zA-Z])(?:\/|$)/, (_, drive) => `${drive.toUpperCase()}:/`)
+  }
+
+  export function overlaps(a: string, b: string) {
+    const relA = relative(a, b)
+    const relB = relative(b, a)
+    return !relA || !relA.startsWith("..") || !relB || !relB.startsWith("..")
+  }
+
+  export function contains(parent: string, child: string) {
+    return !relative(parent, child).startsWith("..")
+  }
+}
diff --git a/packages/opencode/src/skill/discovery.ts b/packages/opencode/src/skill/discovery.ts
index e5279503d90..e1039750321 100644
--- a/packages/opencode/src/skill/discovery.ts
+++ b/packages/opencode/src/skill/discovery.ts
@@ -1,7 +1,8 @@
-import { NodeFileSystem, NodePath } from "@effect/platform-node"
-import { Effect, FileSystem, Layer, Path, Schema, ServiceMap } from "effect"
+import { NodePath } from "@effect/platform-node"
+import { Effect, Layer, Path, Schema, ServiceMap } from "effect"
 import { FetchHttpClient, HttpClient, HttpClientRequest, HttpClientResponse } from "effect/unstable/http"
 import { withTransientReadRetry } from "@/util/effect-http-client"
+import { AppFileSystem } from "@/filesystem"
 import { Global } from "../global"
 import { Log } from "../util/log"
 
@@ -24,12 +25,12 @@ export namespace Discovery {
 
   export class Service extends ServiceMap.Service<Service, Interface>()("@opencode/SkillDiscovery") {}
 
-  export const layer: Layer.Layer<Service, never, FileSystem.FileSystem | Path.Path | HttpClient.HttpClient> =
+  export const layer: Layer.Layer<Service, never, AppFileSystem.Service | Path.Path | HttpClient.HttpClient> =
     Layer.effect(
       Service,
       Effect.gen(function* () {
         const log = Log.create({ service: "skill-discovery" })
-        const fs = yield* FileSystem.FileSystem
+        const fs = yield* AppFileSystem.Service
         const path = yield* Path.Path
         const http = HttpClient.filterStatusOk(withTransientReadRetry(yield* HttpClient.HttpClient))
         const cache = path.join(Global.Path.cache, "skills")
@@ -40,11 +41,7 @@ export namespace Discovery {
           return yield* HttpClientRequest.get(url).pipe(
             http.execute,
             Effect.flatMap((res) => res.arrayBuffer),
-            Effect.flatMap((body) =>
-              fs
-                .makeDirectory(path.dirname(dest), { recursive: true })
-                .pipe(Effect.flatMap(() => fs.writeFile(dest, new Uint8Array(body)))),
-            ),
+            Effect.flatMap((body) => fs.writeWithDirs(dest, new Uint8Array(body))),
             Effect.as(true),
             Effect.catch((err) =>
               Effect.sync(() => {
@@ -113,7 +110,7 @@ export namespace Discovery {
 
   export const defaultLayer: Layer.Layer<Service> = layer.pipe(
     Layer.provide(FetchHttpClient.layer),
-    Layer.provide(NodeFileSystem.layer),
+    Layer.provide(AppFileSystem.defaultLayer),
     Layer.provide(NodePath.layer),
   )
 }
diff --git a/packages/opencode/src/snapshot/index.ts b/packages/opencode/src/snapshot/index.ts
index 9f0eef56bc6..887bce33416 100644
--- a/packages/opencode/src/snapshot/index.ts
+++ b/packages/opencode/src/snapshot/index.ts
@@ -1,10 +1,11 @@
 import { NodeChildProcessSpawner, NodeFileSystem, NodePath } from "@effect/platform-node"
-import { Cause, Duration, Effect, FileSystem, Layer, Schedule, ServiceMap, Stream } from "effect"
+import { Cause, Duration, Effect, Layer, Schedule, ServiceMap, Stream } from "effect"
 import { ChildProcess, ChildProcessSpawner } from "effect/unstable/process"
 import path from "path"
 import z from "zod"
 import { InstanceContext } from "@/effect/instance-context"
 import { runPromiseInstance } from "@/effect/runtime"
+import { AppFileSystem } from "@/filesystem"
 import { Config } from "../config/config"
 import { Global } from "../global"
 import { Log } from "../util/log"
@@ -85,12 +86,12 @@ export namespace Snapshot {
   export const layer: Layer.Layer<
     Service,
     never,
-    InstanceContext | FileSystem.FileSystem | ChildProcessSpawner.ChildProcessSpawner
+    InstanceContext | AppFileSystem.Service | ChildProcessSpawner.ChildProcessSpawner
   > = Layer.effect(
     Service,
     Effect.gen(function* () {
       const ctx = yield* InstanceContext
-      const fs = yield* FileSystem.FileSystem
+      const fs = yield* AppFileSystem.Service
       const spawner = yield* ChildProcessSpawner.ChildProcessSpawner
       const directory = ctx.directory
       const worktree = ctx.worktree
@@ -124,9 +125,8 @@ export namespace Snapshot {
         ),
       )
 
+      // Snapshot-specific error handling on top of AppFileSystem
       const exists = (file: string) => fs.exists(file).pipe(Effect.orDie)
-      const mkdir = (dir: string) => fs.makeDirectory(dir, { recursive: true }).pipe(Effect.orDie)
-      const write = (file: string, text: string) => fs.writeFileString(file, text).pipe(Effect.orDie)
       const read = (file: string) => fs.readFileString(file).pipe(Effect.catch(() => Effect.succeed("")))
       const remove = (file: string) => fs.remove(file).pipe(Effect.catch(() => Effect.void))
 
@@ -148,12 +148,12 @@ export namespace Snapshot {
       const sync = Effect.fnUntraced(function* () {
         const file = yield* excludes()
         const target = path.join(gitdir, "info", "exclude")
-        yield* mkdir(path.join(gitdir, "info"))
+        yield* fs.ensureDir(path.join(gitdir, "info")).pipe(Effect.orDie)
         if (!file) {
-          yield* write(target, "")
+          yield* fs.writeFileString(target, "").pipe(Effect.orDie)
           return
         }
-        yield* write(target, yield* read(file))
+        yield* fs.writeFileString(target, yield* read(file)).pipe(Effect.orDie)
       })
 
       const add = Effect.fnUntraced(function* () {
@@ -178,7 +178,7 @@ export namespace Snapshot {
       const track = Effect.fn("Snapshot.track")(function* () {
         if (!(yield* enabled())) return
         const existed = yield* exists(gitdir)
-        yield* mkdir(gitdir)
+        yield* fs.ensureDir(gitdir).pipe(Effect.orDie)
         if (!existed) {
           yield* git(["init"], {
             env: { GIT_DIR: gitdir, GIT_WORK_TREE: worktree },
@@ -342,7 +342,8 @@ export namespace Snapshot {
 
   export const defaultLayer = layer.pipe(
     Layer.provide(NodeChildProcessSpawner.layer),
-    Layer.provide(NodeFileSystem.layer),
+    Layer.provide(AppFileSystem.defaultLayer),
+    Layer.provide(NodeFileSystem.layer), // needed by NodeChildProcessSpawner
     Layer.provide(NodePath.layer),
   )
 }
diff --git a/packages/opencode/src/tool/truncate-effect.ts b/packages/opencode/src/tool/truncate-effect.ts
index 60b9d0fa843..4431c18f839 100644
--- a/packages/opencode/src/tool/truncate-effect.ts
+++ b/packages/opencode/src/tool/truncate-effect.ts
@@ -1,7 +1,8 @@
-import { NodeFileSystem, NodePath } from "@effect/platform-node"
-import { Cause, Duration, Effect, FileSystem, Layer, Schedule, ServiceMap } from "effect"
+import { NodePath } from "@effect/platform-node"
+import { Cause, Duration, Effect, Layer, Schedule, ServiceMap } from "effect"
 import path from "path"
 import type { Agent } from "../agent/agent"
+import { AppFileSystem } from "@/filesystem"
 import { PermissionNext } from "../permission"
 import { Identifier } from "../id/id"
 import { Log } from "../util/log"
@@ -44,7 +45,7 @@ export namespace TruncateEffect {
   export const layer = Layer.effect(
     Service,
     Effect.gen(function* () {
-      const fs = yield* FileSystem.FileSystem
+      const fs = yield* AppFileSystem.Service
 
       const cleanup = Effect.fn("Truncate.cleanup")(function* () {
         const cutoff = Identifier.timestamp(Identifier.create("tool", false, Date.now() - Duration.toMillis(RETENTION)))
@@ -101,7 +102,7 @@ export namespace TruncateEffect {
         const preview = out.join("\n")
         const file = path.join(TRUNCATION_DIR, ToolID.ascending())
 
-        yield* fs.makeDirectory(TRUNCATION_DIR, { recursive: true }).pipe(Effect.orDie)
+        yield* fs.ensureDir(TRUNCATION_DIR).pipe(Effect.orDie)
         yield* fs.writeFileString(file, text).pipe(Effect.orDie)
 
         const hint = hasTaskTool(agent)
@@ -132,5 +133,5 @@ export namespace TruncateEffect {
     }),
   )
 
-  export const defaultLayer = layer.pipe(Layer.provide(NodeFileSystem.layer), Layer.provide(NodePath.layer))
+  export const defaultLayer = layer.pipe(Layer.provide(AppFileSystem.defaultLayer), Layer.provide(NodePath.layer))
 }
diff --git a/packages/opencode/test/filesystem/filesystem.test.ts b/packages/opencode/test/filesystem/filesystem.test.ts
new file mode 100644
index 00000000000..ca73b3336bc
--- /dev/null
+++ b/packages/opencode/test/filesystem/filesystem.test.ts
@@ -0,0 +1,319 @@
+import { describe, test, expect } from "bun:test"
+import { Effect, Layer } from "effect"
+import { NodeFileSystem } from "@effect/platform-node"
+import { AppFileSystem } from "../../src/filesystem"
+import { testEffect } from "../lib/effect"
+import path from "path"
+
+const live = AppFileSystem.layer.pipe(Layer.provide(NodeFileSystem.layer))
+const { effect: it } = testEffect(live)
+
+describe("AppFileSystem", () => {
+  describe("isDir", () => {
+    it(
+      "returns true for directories",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        expect(yield* fs.isDir(tmp)).toBe(true)
+      }),
+    )
+
+    it(
+      "returns false for files",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "test.txt")
+        yield* fs.writeFileString(file, "hello")
+        expect(yield* fs.isDir(file)).toBe(false)
+      }),
+    )
+
+    it(
+      "returns false for non-existent paths",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        expect(yield* fs.isDir("/tmp/nonexistent-" + Math.random())).toBe(false)
+      }),
+    )
+  })
+
+  describe("isFile", () => {
+    it(
+      "returns true for files",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "test.txt")
+        yield* fs.writeFileString(file, "hello")
+        expect(yield* fs.isFile(file)).toBe(true)
+      }),
+    )
+
+    it(
+      "returns false for directories",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        expect(yield* fs.isFile(tmp)).toBe(false)
+      }),
+    )
+  })
+
+  describe("readJson / writeJson", () => {
+    it(
+      "round-trips JSON data",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "data.json")
+        const data = { name: "test", count: 42, nested: { ok: true } }
+
+        yield* fs.writeJson(file, data)
+        const result = yield* fs.readJson(file)
+
+        expect(result).toEqual(data)
+      }),
+    )
+  })
+
+  describe("ensureDir", () => {
+    it(
+      "creates nested directories",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const nested = path.join(tmp, "a", "b", "c")
+
+        yield* fs.ensureDir(nested)
+
+        const info = yield* fs.stat(nested)
+        expect(info.type).toBe("Directory")
+      }),
+    )
+
+    it(
+      "is idempotent",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const dir = path.join(tmp, "existing")
+        yield* fs.makeDirectory(dir)
+
+        yield* fs.ensureDir(dir)
+
+        const info = yield* fs.stat(dir)
+        expect(info.type).toBe("Directory")
+      }),
+    )
+  })
+
+  describe("writeWithDirs", () => {
+    it(
+      "creates parent directories if missing",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "deep", "nested", "file.txt")
+
+        yield* fs.writeWithDirs(file, "hello")
+
+        expect(yield* fs.readFileString(file)).toBe("hello")
+      }),
+    )
+
+    it(
+      "writes directly when parent exists",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "direct.txt")
+
+        yield* fs.writeWithDirs(file, "world")
+
+        expect(yield* fs.readFileString(file)).toBe("world")
+      }),
+    )
+
+    it(
+      "writes Uint8Array content",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "binary.bin")
+        const content = new Uint8Array([0x00, 0x01, 0x02, 0x03])
+
+        yield* fs.writeWithDirs(file, content)
+
+        const result = yield* fs.readFile(file)
+        expect(new Uint8Array(result)).toEqual(content)
+      }),
+    )
+  })
+
+  describe("findUp", () => {
+    it(
+      "finds target in start directory",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "target.txt"), "found")
+
+        const result = yield* fs.findUp("target.txt", tmp)
+        expect(result).toEqual([path.join(tmp, "target.txt")])
+      }),
+    )
+
+    it(
+      "finds target in parent directories",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "marker"), "root")
+        const child = path.join(tmp, "a", "b")
+        yield* fs.makeDirectory(child, { recursive: true })
+
+        const result = yield* fs.findUp("marker", child, tmp)
+        expect(result).toEqual([path.join(tmp, "marker")])
+      }),
+    )
+
+    it(
+      "returns empty array when not found",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const result = yield* fs.findUp("nonexistent", tmp, tmp)
+        expect(result).toEqual([])
+      }),
+    )
+  })
+
+  describe("up", () => {
+    it(
+      "finds multiple targets walking up",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "a.txt"), "a")
+        yield* fs.writeFileString(path.join(tmp, "b.txt"), "b")
+        const child = path.join(tmp, "sub")
+        yield* fs.makeDirectory(child)
+        yield* fs.writeFileString(path.join(child, "a.txt"), "a-child")
+
+        const result = yield* fs.up({ targets: ["a.txt", "b.txt"], start: child, stop: tmp })
+
+        expect(result).toContain(path.join(child, "a.txt"))
+        expect(result).toContain(path.join(tmp, "a.txt"))
+        expect(result).toContain(path.join(tmp, "b.txt"))
+      }),
+    )
+  })
+
+  describe("glob", () => {
+    it(
+      "finds files matching pattern",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "a.ts"), "a")
+        yield* fs.writeFileString(path.join(tmp, "b.ts"), "b")
+        yield* fs.writeFileString(path.join(tmp, "c.json"), "c")
+
+        const result = yield* fs.glob("*.ts", { cwd: tmp })
+        expect(result.sort()).toEqual(["a.ts", "b.ts"])
+      }),
+    )
+
+    it(
+      "supports absolute paths",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "file.txt"), "hello")
+
+        const result = yield* fs.glob("*.txt", { cwd: tmp, absolute: true })
+        expect(result).toEqual([path.join(tmp, "file.txt")])
+      }),
+    )
+  })
+
+  describe("globMatch", () => {
+    it(
+      "matches patterns",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        expect(fs.globMatch("*.ts", "foo.ts")).toBe(true)
+        expect(fs.globMatch("*.ts", "foo.json")).toBe(false)
+        expect(fs.globMatch("src/**", "src/a/b.ts")).toBe(true)
+      }),
+    )
+  })
+
+  describe("globUp", () => {
+    it(
+      "finds files walking up directories",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        yield* fs.writeFileString(path.join(tmp, "root.md"), "root")
+        const child = path.join(tmp, "a", "b")
+        yield* fs.makeDirectory(child, { recursive: true })
+        yield* fs.writeFileString(path.join(child, "leaf.md"), "leaf")
+
+        const result = yield* fs.globUp("*.md", child, tmp)
+        expect(result).toContain(path.join(child, "leaf.md"))
+        expect(result).toContain(path.join(tmp, "root.md"))
+      }),
+    )
+  })
+
+  describe("built-in passthrough", () => {
+    it(
+      "exists works",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "exists.txt")
+        yield* fs.writeFileString(file, "yes")
+
+        expect(yield* fs.exists(file)).toBe(true)
+        expect(yield* fs.exists(file + ".nope")).toBe(false)
+      }),
+    )
+
+    it(
+      "remove works",
+      Effect.gen(function* () {
+        const fs = yield* AppFileSystem.Service
+        const tmp = yield* fs.makeTempDirectoryScoped()
+        const file = path.join(tmp, "delete-me.txt")
+        yield* fs.writeFileString(file, "bye")
+
+        yield* fs.remove(file)
+
+        expect(yield* fs.exists(file)).toBe(false)
+      }),
+    )
+  })
+
+  describe("pure helpers", () => {
+    test("mimeType returns correct types", () => {
+      expect(AppFileSystem.mimeType("file.json")).toBe("application/json")
+      expect(AppFileSystem.mimeType("image.png")).toBe("image/png")
+      expect(AppFileSystem.mimeType("unknown.qzx")).toBe("application/octet-stream")
+    })
+
+    test("contains checks path containment", () => {
+      expect(AppFileSystem.contains("/a/b", "/a/b/c")).toBe(true)
+      expect(AppFileSystem.contains("/a/b", "/a/c")).toBe(false)
+    })
+
+    test("overlaps detects overlapping paths", () => {
+      expect(AppFileSystem.overlaps("/a/b", "/a/b/c")).toBe(true)
+      expect(AppFileSystem.overlaps("/a/b/c", "/a/b")).toBe(true)
+      expect(AppFileSystem.overlaps("/a", "/b")).toBe(false)
+    })
+  })
+})