diff --git a/pkg/domain/entity/account.go b/pkg/domain/entity/account.go
new file mode 100644
index 0000000..3b88baa
--- /dev/null
+++ b/pkg/domain/entity/account.go
@@ -0,0 +1,22 @@
+package entity
+
+// Account represents an account in the pool
+type Account struct {
+	ConfigDir      string
+	ActiveRequests int
+	LastUsed       int64
+	RateLimitUntil int64
+}
+
+// AccountStat represents account statistics
+type AccountStat struct {
+	ConfigDir       string
+	ActiveRequests  int
+	TotalRequests   int
+	TotalSuccess    int
+	TotalErrors     int
+	TotalRateLimits int
+	TotalLatencyMs  int64
+	IsRateLimited   bool
+	RateLimitUntil  int64
+}
diff --git a/pkg/domain/entity/chunk.go b/pkg/domain/entity/chunk.go
new file mode 100644
index 0000000..5726c9d
--- /dev/null
+++ b/pkg/domain/entity/chunk.go
@@ -0,0 +1,20 @@
+package entity
+
+// ChunkType represents the type of stream chunk
+type ChunkType int
+
+const (
+	ChunkText ChunkType = iota
+	ChunkThinking
+	ChunkToolCall
+	ChunkDone
+)
+
+// StreamChunk represents a chunk in SSE streaming
+type StreamChunk struct {
+	Type     ChunkType
+	Text     string
+	Thinking string
+	ToolCall *ToolCall
+	Done     bool
+}
diff --git a/pkg/domain/entity/message.go b/pkg/domain/entity/message.go
new file mode 100644
index 0000000..d1d25f0
--- /dev/null
+++ b/pkg/domain/entity/message.go
@@ -0,0 +1,33 @@
+package entity
+
+// Message represents a chat message
+type Message struct {
+	Role    string
+	Content interface{}
+}
+
+// Tool represents a tool definition
+type Tool struct {
+	Type     string
+	Function ToolFunction
+}
+
+// ToolFunction represents a tool function definition
+type ToolFunction struct {
+	Name        string
+	Description string
+	Parameters  interface{}
+}
+
+// ToolCall represents a tool call result
+type ToolCall struct {
+	ID        string
+	Name      string
+	Arguments string
+}
+
+// FunctionCall represents a function call
+type FunctionCall struct {
+	Name      string
+	Arguments string
+}
diff --git a/pkg/domain/repository/account.go b/pkg/domain/repository/account.go
new file mode 100644
index 0000000..1850e48
--- /dev/null
+++ b/pkg/domain/repository/account.go
@@ -0,0 +1,27 @@
+package repository
+
+import (
+	"context"
+
+	"cursor-api-proxy/pkg/domain/entity"
+)
+
+// AccountPool defines the interface for account pool management
+type AccountPool interface {
+	GetNextConfigDir() string
+	ReportRequestStart(configDir string)
+	ReportRequestEnd(configDir string)
+	ReportRequestSuccess(configDir string, latencyMs int64)
+	ReportRequestError(configDir string, latencyMs int64)
+	ReportRateLimit(configDir string, penaltyMs int64)
+	GetStats() []entity.AccountStat
+	Count() int
+}
+
+// Provider defines the interface for AI providers
+type Provider interface {
+	Name() string
+	Generate(ctx context.Context, model string, messages []entity.Message,
+		tools []entity.Tool, callback func(entity.StreamChunk)) error
+	Close() error
+}
diff --git a/pkg/domain/types/errors.go b/pkg/domain/types/errors.go
new file mode 100644
index 0000000..c3951d7
--- /dev/null
+++ b/pkg/domain/types/errors.go
@@ -0,0 +1,13 @@
+package types
+
+import "errors"
+
+var (
+	ErrInvalidRequest    = errors.New("invalid request")
+	ErrProviderNotFound   = errors.New("provider not found")
+	ErrAccountExhausted   = errors.New("all accounts exhausted")
+	ErrRateLimited       = errors.New("rate limited")
+	ErrTimeout           = errors.New("request timeout")
+	ErrClientDisconnect   = errors.New("client disconnected")
+	ErrAgentError        = errors.New("agent execution error")
+)
\ No newline at end of file
diff --git a/pkg/domain/types/models.go b/pkg/domain/types/models.go
new file mode 100644
index 0000000..8842664
--- /dev/null
+++ b/pkg/domain/types/models.go
@@ -0,0 +1,30 @@
+package types
+
+// Model mappings for Cursor API
+var AnthropicToCursor = map[string]string{
+	"claude-3-5-sonnet":      "claude-3.5-sonnet",
+	"claude-3-5-sonnet-20241022": "claude-3.5-sonnet",
+	"claude-3-5-haiku":       "claude-3.5-haiku",
+	"claude-3-opus":          "claude-3-opus",
+	"claude-3-sonnet":        "claude-3-sonnet",
+	"claude-3-haiku":         "claude-3-haiku",
+}
+
+// Cursor model aliases
+var CursorModelAliases = []string{
+	"auto",
+	"claude-3.5-sonnet",
+	"claude-3.5-haiku",
+	"claude-3-opus",
+	"gpt-4",
+	"gpt-4o",
+	"gemini-2.0-flash",
+}
+
+// ResolveToCursorModel resolves a model name to Cursor model
+func ResolveToCursorModel(model string) string {
+	if mapped, ok := AnthropicToCursor[model]; ok {
+		return mapped
+	}
+	return model
+}
\ No newline at end of file
diff --git a/pkg/domain/usecase/chat.go b/pkg/domain/usecase/chat.go
new file mode 100644
index 0000000..3fbc138
--- /dev/null
+++ b/pkg/domain/usecase/chat.go
@@ -0,0 +1,47 @@
+package usecase
+
+import (
+	"context"
+
+	"cursor-api-proxy/pkg/domain/entity"
+)
+
+// ChatUsecase defines the interface for chat operations
+type ChatUsecase interface {
+	Execute(ctx context.Context, input ChatInput) (ChatOutput, error)
+	Stream(ctx context.Context, input ChatInput, callback func(entity.StreamChunk)) error
+}
+
+// ChatInput represents the input for chat operations
+type ChatInput struct {
+	Model    string
+	Messages []entity.Message
+	Tools    []entity.Tool
+	Stream   bool
+}
+
+// ChatOutput represents the output from chat operations
+type ChatOutput struct {
+	Content   string
+	Thinking  string
+	ToolCalls []entity.ToolCall
+}
+
+// AgentRunner defines the interface for running AI agents
+type AgentRunner interface {
+	RunSync(ctx context.Context, config interface{}, args []string) (RunResult, error)
+	RunStream(ctx context.Context, config interface{}, args []string, onLine func(string)) (StreamResult, error)
+}
+
+// RunResult represents the result of a synchronous agent run
+type RunResult struct {
+	Code   int
+	Stdout string
+	Stderr string
+}
+
+// StreamResult represents the result of a streaming agent run
+type StreamResult struct {
+	Code   int
+	Stderr string
+}