diff --git a/internal/acp/handlers/handlers.go b/internal/acp/handlers/handlers.go
index 2ad8a80..3851160 100644
--- a/internal/acp/handlers/handlers.go
+++ b/internal/acp/handlers/handlers.go
@@ -38,9 +38,16 @@ type FsError struct {
 	Message string
 }
 
-// FsHandler is the placeholder for fs/read_text_file + fs/write_text_file.
-// Fields and methods are added in Task D4.
-type FsHandler struct{}
+// FsHandler combines fs/* method handlers so Relay can hold one reference.
+type FsHandler struct {
+	Read  *FsReadHandler
+	Write *FsWriteHandler
+}
+
+// NewFsHandler constructs a composite FsHandler with default sub-handlers.
+func NewFsHandler() *FsHandler {
+	return &FsHandler{Read: NewFsReadHandler(), Write: NewFsWriteHandler()}
+}
 
 // PermissionHandler is the placeholder for session/request_permission.
 // Fields and methods are added in Task D5.
diff --git a/internal/acp/handlers/server_fs_read.go b/internal/acp/handlers/server_fs_read.go
new file mode 100644
index 0000000..a5de71d
--- /dev/null
+++ b/internal/acp/handlers/server_fs_read.go
@@ -0,0 +1,108 @@
+package handlers
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// FsReadHandler handles fs/read_text_file (spec §5.4).
+type FsReadHandler struct{}
+
+// NewFsReadHandler constructs an FsReadHandler.
+func NewFsReadHandler() *FsReadHandler { return &FsReadHandler{} }
+
+// fsReadParams is the ACP-protocol input for fs/read_text_file.
+type fsReadParams struct {
+	SessionID string `json:"sessionId"`
+	Path      string `json:"path"`
+	Line      *int   `json:"line,omitempty"`
+	Limit     *int   `json:"limit,omitempty"`
+}
+
+type fsReadResult struct {
+	Content string `json:"content"`
+}
+
+// Handle reads a file inside sess.CwdPath, returning content (with optional
+// line/limit slicing). Returns FsError for path-escape or read failure.
+func (h *FsReadHandler) Handle(_ context.Context, sess SessionContext, paramsJSON json.RawMessage) FsResult {
+	var p fsReadParams
+	if err := json.Unmarshal(paramsJSON, &p); err != nil {
+		return FsResult{Err: &FsError{Code: -32602, Message: "invalid params: " + err.Error()}}
+	}
+	abs, err := resolveSafePath(sess.CwdPath, p.Path)
+	if err != nil {
+		return FsResult{Err: &FsError{Code: -32001, Message: err.Error()}}
+	}
+	data, err := os.ReadFile(abs)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return FsResult{Err: &FsError{Code: -32603, Message: "file not found"}}
+		}
+		return FsResult{Err: &FsError{Code: -32603, Message: "read: " + err.Error()}}
+	}
+	content := string(data)
+	if p.Line != nil || p.Limit != nil {
+		content = sliceLines(content, p.Line, p.Limit)
+	}
+	out, _ := json.Marshal(fsReadResult{Content: content})
+	return FsResult{OK: out}
+}
+
+// sliceLines returns lines[line:line+limit] joined by "\n".
+// line is 0-based; limit caps the line count.
+func sliceLines(s string, line, limit *int) string {
+	lines := strings.Split(s, "\n")
+	start := 0
+	if line != nil && *line > 0 {
+		start = *line
+	}
+	if start > len(lines) {
+		start = len(lines)
+	}
+	end := len(lines)
+	if limit != nil && *limit >= 0 {
+		if start+*limit < end {
+			end = start + *limit
+		}
+	}
+	return strings.Join(lines[start:end], "\n")
+}
+
+// resolveSafePath duplicates the logic from internal/acp.ResolveSafePath to
+// avoid an import cycle (handlers is a sub-package and acp imports handlers).
+func resolveSafePath(cwd, requested string) (string, error) {
+	if !filepath.IsAbs(cwd) {
+		return "", fmt.Errorf("cwd must be absolute")
+	}
+	cleanCwd := filepath.Clean(cwd)
+	var abs string
+	if filepath.IsAbs(requested) {
+		abs = filepath.Clean(requested)
+	} else {
+		abs = filepath.Clean(filepath.Join(cleanCwd, requested))
+	}
+	if !hasPathPrefix(abs, cleanCwd) {
+		return "", fmt.Errorf("path outside worktree")
+	}
+	if real, err := filepath.EvalSymlinks(abs); err == nil {
+		if !hasPathPrefix(real, cleanCwd) {
+			return "", fmt.Errorf("path resolves outside worktree via symlink")
+		}
+	}
+	return abs, nil
+}
+
+func hasPathPrefix(abs, base string) bool {
+	if abs == base {
+		return true
+	}
+	sep := string(filepath.Separator)
+	return strings.HasPrefix(abs, base+sep)
+}
diff --git a/internal/acp/handlers/server_fs_read_test.go b/internal/acp/handlers/server_fs_read_test.go
new file mode 100644
index 0000000..4498fd2
--- /dev/null
+++ b/internal/acp/handlers/server_fs_read_test.go
@@ -0,0 +1,89 @@
+package handlers_test
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/yan1h/agent-coding-workflow/internal/acp/handlers"
+)
+
+func TestFsRead_HappyPath(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(tmp, "a.txt"), []byte("hello\n"), 0o644))
+
+	h := handlers.NewFsReadHandler()
+	sess := handlers.SessionContext{SessionID: uuid.New(), CwdPath: tmp}
+	params := mustJSON(t, map[string]any{"sessionId": "x", "path": filepath.Join(tmp, "a.txt")})
+
+	res := h.Handle(context.Background(), sess, params)
+	require.Nil(t, res.Err)
+	require.NotNil(t, res.OK)
+	var out struct {
+		Content string `json:"content"`
+	}
+	require.NoError(t, json.Unmarshal(res.OK, &out))
+	assert.Equal(t, "hello\n", out.Content)
+}
+
+func TestFsRead_PathOutsideWorktree(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	h := handlers.NewFsReadHandler()
+	sess := handlers.SessionContext{CwdPath: tmp}
+	outside := filepath.Join(filepath.VolumeName(tmp)+string(filepath.Separator), "etc", "passwd")
+	params := mustJSON(t, map[string]any{"sessionId": "x", "path": outside})
+
+	res := h.Handle(context.Background(), sess, params)
+	require.NotNil(t, res.Err)
+	assert.Equal(t, -32001, res.Err.Code)
+}
+
+func TestFsRead_FileNotFound(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	h := handlers.NewFsReadHandler()
+	sess := handlers.SessionContext{CwdPath: tmp}
+	params := mustJSON(t, map[string]any{"sessionId": "x", "path": filepath.Join(tmp, "nope.txt")})
+
+	res := h.Handle(context.Background(), sess, params)
+	require.NotNil(t, res.Err)
+	assert.Equal(t, -32603, res.Err.Code)
+}
+
+func TestFsRead_LineLimit(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(tmp, "f.txt"),
+		[]byte("L0\nL1\nL2\nL3\n"), 0o644))
+
+	h := handlers.NewFsReadHandler()
+	sess := handlers.SessionContext{CwdPath: tmp}
+	line := 1
+	limit := 2
+	params := mustJSON(t, map[string]any{
+		"sessionId": "x", "path": filepath.Join(tmp, "f.txt"),
+		"line": line, "limit": limit,
+	})
+	res := h.Handle(context.Background(), sess, params)
+	require.Nil(t, res.Err)
+	var out struct {
+		Content string `json:"content"`
+	}
+	_ = json.Unmarshal(res.OK, &out)
+	assert.Equal(t, "L1\nL2", out.Content)
+}
+
+func mustJSON(t *testing.T, v any) json.RawMessage {
+	t.Helper()
+	b, err := json.Marshal(v)
+	require.NoError(t, err)
+	return b
+}
diff --git a/internal/acp/handlers/server_fs_write.go b/internal/acp/handlers/server_fs_write.go
new file mode 100644
index 0000000..0cf2e42
--- /dev/null
+++ b/internal/acp/handlers/server_fs_write.go
@@ -0,0 +1,40 @@
+package handlers
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// FsWriteHandler handles fs/write_text_file (spec §5.4).
+type FsWriteHandler struct{}
+
+// NewFsWriteHandler constructs an FsWriteHandler.
+func NewFsWriteHandler() *FsWriteHandler { return &FsWriteHandler{} }
+
+type fsWriteParams struct {
+	SessionID string `json:"sessionId"`
+	Path      string `json:"path"`
+	Content   string `json:"content"`
+}
+
+// Handle writes a file inside sess.CwdPath, creating parent dirs as needed.
+// Returns FsError for path-escape or filesystem failure.
+func (h *FsWriteHandler) Handle(_ context.Context, sess SessionContext, paramsJSON json.RawMessage) FsResult {
+	var p fsWriteParams
+	if err := json.Unmarshal(paramsJSON, &p); err != nil {
+		return FsResult{Err: &FsError{Code: -32602, Message: "invalid params: " + err.Error()}}
+	}
+	abs, err := resolveSafePath(sess.CwdPath, p.Path)
+	if err != nil {
+		return FsResult{Err: &FsError{Code: -32001, Message: err.Error()}}
+	}
+	if err := os.MkdirAll(filepath.Dir(abs), 0o755); err != nil {
+		return FsResult{Err: &FsError{Code: -32603, Message: "mkdir: " + err.Error()}}
+	}
+	if err := os.WriteFile(abs, []byte(p.Content), 0o644); err != nil {
+		return FsResult{Err: &FsError{Code: -32603, Message: "write: " + err.Error()}}
+	}
+	return FsResult{OK: json.RawMessage(`{}`)}
+}
diff --git a/internal/acp/handlers/server_fs_write_test.go b/internal/acp/handlers/server_fs_write_test.go
new file mode 100644
index 0000000..8d3668d
--- /dev/null
+++ b/internal/acp/handlers/server_fs_write_test.go
@@ -0,0 +1,47 @@
+package handlers_test
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/yan1h/agent-coding-workflow/internal/acp/handlers"
+)
+
+func TestFsWrite_HappyPath(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	h := handlers.NewFsWriteHandler()
+	sess := handlers.SessionContext{SessionID: uuid.New(), CwdPath: tmp}
+
+	target := filepath.Join(tmp, "newdir", "x.txt")
+	params := mustJSON(t, map[string]any{
+		"sessionId": "x", "path": target, "content": "hello",
+	})
+	res := h.Handle(context.Background(), sess, params)
+	require.Nil(t, res.Err)
+
+	got, err := os.ReadFile(target)
+	require.NoError(t, err)
+	assert.Equal(t, "hello", string(got))
+}
+
+func TestFsWrite_OutsideWorktree(t *testing.T) {
+	t.Parallel()
+	tmp := t.TempDir()
+	h := handlers.NewFsWriteHandler()
+	sess := handlers.SessionContext{CwdPath: tmp}
+
+	outside := filepath.Join(filepath.VolumeName(tmp)+string(filepath.Separator), "tmp", "escape.txt")
+	params := mustJSON(t, map[string]any{
+		"sessionId": "x", "path": outside, "content": "evil",
+	})
+	res := h.Handle(context.Background(), sess, params)
+	require.NotNil(t, res.Err)
+	assert.Equal(t, -32001, res.Err.Code)
+}