diff --git a/pkg/adapter/anthropic/anthropic.go b/pkg/adapter/anthropic/anthropic.go
new file mode 100644
index 0000000..fe3f64e
--- /dev/null
+++ b/pkg/adapter/anthropic/anthropic.go
@@ -0,0 +1,174 @@
+package anthropic
+
+import (
+	"cursor-api-proxy/pkg/adapter/openai"
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+type MessageParam struct {
+	Role    string      `json:"role"`
+	Content interface{} `json:"content"`
+}
+
+type MessagesRequest struct {
+	Model     string        `json:"model"`
+	MaxTokens int           `json:"max_tokens"`
+	Messages  []MessageParam `json:"messages"`
+	System    interface{}   `json:"system"`
+	Stream    bool          `json:"stream"`
+	Tools     []interface{} `json:"tools"`
+}
+
+func systemToText(system interface{}) string {
+	if system == nil {
+		return ""
+	}
+	switch v := system.(type) {
+	case string:
+		return strings.TrimSpace(v)
+	case []interface{}:
+		var parts []string
+		for _, p := range v {
+			if m, ok := p.(map[string]interface{}); ok {
+				if m["type"] == "text" {
+					if t, ok := m["text"].(string); ok {
+						parts = append(parts, t)
+					}
+				}
+			}
+		}
+		return strings.Join(parts, "\n")
+	}
+	return ""
+}
+
+func anthropicBlockToText(p interface{}) string {
+	if p == nil {
+		return ""
+	}
+	switch v := p.(type) {
+	case string:
+		return v
+	case map[string]interface{}:
+		typ, _ := v["type"].(string)
+		switch typ {
+		case "text":
+			if t, ok := v["text"].(string); ok {
+				return t
+			}
+		case "image":
+			if src, ok := v["source"].(map[string]interface{}); ok {
+				srcType, _ := src["type"].(string)
+				switch srcType {
+				case "base64":
+					mt, _ := src["media_type"].(string)
+					if mt == "" {
+						mt = "image"
+					}
+					return "[Image: base64 " + mt + "]"
+				case "url":
+					url, _ := src["url"].(string)
+					return "[Image: " + url + "]"
+				}
+			}
+			return "[Image]"
+		case "document":
+			title, _ := v["title"].(string)
+			if title == "" {
+				if src, ok := v["source"].(map[string]interface{}); ok {
+					title, _ = src["url"].(string)
+				}
+			}
+			if title != "" {
+				return "[Document: " + title + "]"
+			}
+			return "[Document]"
+		case "tool_use":
+			name, _ := v["name"].(string)
+			id, _ := v["id"].(string)
+			input := v["input"]
+			inputJSON, _ := json.Marshal(input)
+			if inputJSON == nil {
+				inputJSON = []byte("{}")
+			}
+			tag := fmt.Sprintf("<tool_call>\n{\"name\": \"%s\", \"arguments\": %s}\n</tool_call>", name, string(inputJSON))
+			if id != "" {
+				tag = fmt.Sprintf("[tool_use_id=%s] ", id) + tag
+			}
+			return tag
+		case "tool_result":
+			toolUseID, _ := v["tool_use_id"].(string)
+			content := v["content"]
+			var contentText string
+			switch c := content.(type) {
+			case string:
+				contentText = c
+			case []interface{}:
+				var parts []string
+				for _, block := range c {
+					if bm, ok := block.(map[string]interface{}); ok {
+						if bm["type"] == "text" {
+							if t, ok := bm["text"].(string); ok {
+								parts = append(parts, t)
+							}
+						}
+					}
+				}
+				contentText = strings.Join(parts, "\n")
+			}
+			label := "Tool result"
+			if toolUseID != "" {
+				label += " [id=" + toolUseID + "]"
+			}
+			return label + ": " + contentText
+		}
+	}
+	return ""
+}
+
+func anthropicContentToText(content interface{}) string {
+	switch v := content.(type) {
+	case string:
+		return v
+	case []interface{}:
+		var parts []string
+		for _, p := range v {
+			if t := anthropicBlockToText(p); t != "" {
+				parts = append(parts, t)
+			}
+		}
+		return strings.Join(parts, " ")
+	}
+	return ""
+}
+
+func BuildPromptFromAnthropicMessages(messages []MessageParam, system interface{}) string {
+	var oaiMessages []interface{}
+
+	systemText := systemToText(system)
+	if systemText != "" {
+		oaiMessages = append(oaiMessages, map[string]interface{}{
+			"role":    "system",
+			"content": systemText,
+		})
+	}
+
+	for _, m := range messages {
+		text := anthropicContentToText(m.Content)
+		if text == "" {
+			continue
+		}
+		role := m.Role
+		if role != "user" && role != "assistant" {
+			role = "user"
+		}
+		oaiMessages = append(oaiMessages, map[string]interface{}{
+			"role":    role,
+			"content": text,
+		})
+	}
+
+	return openai.BuildPromptFromMessages(oaiMessages)
+}
diff --git a/pkg/adapter/anthropic/anthropic_test.go b/pkg/adapter/anthropic/anthropic_test.go
new file mode 100644
index 0000000..1945a9d
--- /dev/null
+++ b/pkg/adapter/anthropic/anthropic_test.go
@@ -0,0 +1,109 @@
+package anthropic_test
+
+import (
+	"cursor-api-proxy/pkg/adapter/anthropic"
+	"strings"
+	"testing"
+)
+
+func TestBuildPromptFromAnthropicMessages_Simple(t *testing.T) {
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: "Hello"},
+		{Role: "assistant", Content: "Hi there"},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, nil)
+	if !strings.Contains(prompt, "Hello") {
+		t.Errorf("prompt missing user message: %q", prompt)
+	}
+	if !strings.Contains(prompt, "Hi there") {
+		t.Errorf("prompt missing assistant message: %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_WithSystem(t *testing.T) {
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: "ping"},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, "You are a helpful bot.")
+	if !strings.Contains(prompt, "You are a helpful bot.") {
+		t.Errorf("prompt missing system: %q", prompt)
+	}
+	if !strings.Contains(prompt, "ping") {
+		t.Errorf("prompt missing user: %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_SystemArray(t *testing.T) {
+	system := []interface{}{
+		map[string]interface{}{"type": "text", "text": "Part A"},
+		map[string]interface{}{"type": "text", "text": "Part B"},
+	}
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: "test"},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, system)
+	if !strings.Contains(prompt, "Part A") {
+		t.Errorf("prompt missing Part A: %q", prompt)
+	}
+	if !strings.Contains(prompt, "Part B") {
+		t.Errorf("prompt missing Part B: %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_ContentBlocks(t *testing.T) {
+	content := []interface{}{
+		map[string]interface{}{"type": "text", "text": "block one"},
+		map[string]interface{}{"type": "text", "text": "block two"},
+	}
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: content},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, nil)
+	if !strings.Contains(prompt, "block one") {
+		t.Errorf("prompt missing 'block one': %q", prompt)
+	}
+	if !strings.Contains(prompt, "block two") {
+		t.Errorf("prompt missing 'block two': %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_ImageBlock(t *testing.T) {
+	content := []interface{}{
+		map[string]interface{}{
+			"type": "image",
+			"source": map[string]interface{}{
+				"type":       "base64",
+				"media_type": "image/png",
+				"data":       "abc123",
+			},
+		},
+	}
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: content},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, nil)
+	if !strings.Contains(prompt, "[Image") {
+		t.Errorf("prompt missing [Image]: %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_EmptyContentSkipped(t *testing.T) {
+	messages := []anthropic.MessageParam{
+		{Role: "user", Content: ""},
+		{Role: "assistant", Content: "response"},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, nil)
+	if !strings.Contains(prompt, "response") {
+		t.Errorf("prompt missing 'response': %q", prompt)
+	}
+}
+
+func TestBuildPromptFromAnthropicMessages_UnknownRoleBecomesUser(t *testing.T) {
+	messages := []anthropic.MessageParam{
+		{Role: "system", Content: "system-as-user"},
+	}
+	prompt := anthropic.BuildPromptFromAnthropicMessages(messages, nil)
+	if !strings.Contains(prompt, "system-as-user") {
+		t.Errorf("prompt missing 'system-as-user': %q", prompt)
+	}
+}
diff --git a/pkg/adapter/openai/openai.go b/pkg/adapter/openai/openai.go
new file mode 100644
index 0000000..86919a6
--- /dev/null
+++ b/pkg/adapter/openai/openai.go
@@ -0,0 +1,243 @@
+package openai
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+type ChatCompletionRequest struct {
+	Model       string          `json:"model"`
+	Messages    []interface{}   `json:"messages"`
+	Stream      bool            `json:"stream"`
+	Tools       []interface{}   `json:"tools"`
+	ToolChoice  interface{}     `json:"tool_choice"`
+	Functions   []interface{}   `json:"functions"`
+	FunctionCall interface{}    `json:"function_call"`
+}
+
+func NormalizeModelID(raw string) string {
+	trimmed := strings.TrimSpace(raw)
+	if trimmed == "" {
+		return ""
+	}
+	parts := strings.Split(trimmed, "/")
+	last := parts[len(parts)-1]
+	if last == "" {
+		return ""
+	}
+	return last
+}
+
+func imageURLToText(imageURL interface{}) string {
+	if imageURL == nil {
+		return "[Image]"
+	}
+	var url string
+	switch v := imageURL.(type) {
+	case string:
+		url = v
+	case map[string]interface{}:
+		if u, ok := v["url"].(string); ok {
+			url = u
+		}
+	}
+	if url == "" {
+		return "[Image]"
+	}
+	if strings.HasPrefix(url, "data:") {
+		end := strings.Index(url, ";")
+		mime := "image"
+		if end > 5 {
+			mime = url[5:end]
+		}
+		return "[Image: base64 " + mime + "]"
+	}
+	return "[Image: " + url + "]"
+}
+
+func MessageContentToText(content interface{}) string {
+	if content == nil {
+		return ""
+	}
+	switch v := content.(type) {
+	case string:
+		return v
+	case []interface{}:
+		var parts []string
+		for _, p := range v {
+			if p == nil {
+				continue
+			}
+			switch part := p.(type) {
+			case string:
+				parts = append(parts, part)
+			case map[string]interface{}:
+				typ, _ := part["type"].(string)
+				switch typ {
+				case "text":
+					if t, ok := part["text"].(string); ok {
+						parts = append(parts, t)
+					}
+				case "image_url":
+					parts = append(parts, imageURLToText(part["image_url"]))
+				case "image":
+					src := part["source"]
+					if src == nil {
+						src = part["url"]
+					}
+					parts = append(parts, imageURLToText(src))
+				}
+			}
+		}
+		return strings.Join(parts, " ")
+	}
+	return ""
+}
+
+func ToolsToSystemText(tools []interface{}, functions []interface{}) string {
+	var defs []interface{}
+
+	for _, t := range tools {
+		if m, ok := t.(map[string]interface{}); ok {
+			if m["type"] == "function" {
+				if fn := m["function"]; fn != nil {
+					defs = append(defs, fn)
+				}
+			} else {
+				defs = append(defs, t)
+			}
+		}
+	}
+	defs = append(defs, functions...)
+
+	if len(defs) == 0 {
+		return ""
+	}
+
+	var lines []string
+	lines = append(lines, "Available tools (respond with a JSON object to call one):", "")
+
+	for _, raw := range defs {
+		fn, ok := raw.(map[string]interface{})
+		if !ok {
+			continue
+		}
+		name, _ := fn["name"].(string)
+		desc, _ := fn["description"].(string)
+		params := "{}"
+		if p := fn["parameters"]; p != nil {
+			if b, err := json.MarshalIndent(p, "", "  "); err == nil {
+				params = string(b)
+			}
+		} else if p := fn["input_schema"]; p != nil {
+			if b, err := json.MarshalIndent(p, "", "  "); err == nil {
+				params = string(b)
+			}
+		}
+		lines = append(lines, "Function: "+name+"\nDescription: "+desc+"\nParameters: "+params)
+	}
+
+	lines = append(lines, "",
+		"When you want to call a tool, use this EXACT format:",
+		"",
+		"<tool_call>",
+		`{"name": "function_name", "arguments": {"param1": "value1"}}`,
+		"</tool_call>",
+		"",
+		"Rules:",
+		"- Write your reasoning BEFORE the tool call",
+		"- You may make multiple tool calls by using multiple <tool_call> blocks",
+		"- STOP writing after the last </tool_call> tag",
+		"- If no tool is needed, respond normally without <tool_call> tags",
+	)
+
+	return strings.Join(lines, "\n")
+}
+
+type SimpleMessage struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+func BuildPromptFromMessages(messages []interface{}) string {
+	var systemParts []string
+	var convo []string
+
+	for _, raw := range messages {
+		m, ok := raw.(map[string]interface{})
+		if !ok {
+			continue
+		}
+		role, _ := m["role"].(string)
+		text := MessageContentToText(m["content"])
+
+		switch role {
+		case "system", "developer":
+			if text != "" {
+				systemParts = append(systemParts, text)
+			}
+		case "user":
+			if text != "" {
+				convo = append(convo, "User: "+text)
+			}
+		case "assistant":
+			toolCalls, _ := m["tool_calls"].([]interface{})
+			if len(toolCalls) > 0 {
+				var parts []string
+				if text != "" {
+					parts = append(parts, text)
+				}
+				for _, tc := range toolCalls {
+					tcMap, ok := tc.(map[string]interface{})
+					if !ok {
+						continue
+					}
+					fn, _ := tcMap["function"].(map[string]interface{})
+					if fn == nil {
+						continue
+					}
+					name, _ := fn["name"].(string)
+					args, _ := fn["arguments"].(string)
+					if args == "" {
+						args = "{}"
+					}
+					parts = append(parts, fmt.Sprintf("<tool_call>\n{\"name\": \"%s\", \"arguments\": %s}\n</tool_call>", name, args))
+				}
+				if len(parts) > 0 {
+					convo = append(convo, "Assistant: "+strings.Join(parts, "\n"))
+				}
+			} else if text != "" {
+				convo = append(convo, "Assistant: "+text)
+			}
+		case "tool", "function":
+			name, _ := m["name"].(string)
+			toolCallID, _ := m["tool_call_id"].(string)
+			label := "Tool result"
+			if name != "" {
+				label = "Tool result (" + name + ")"
+			}
+			if toolCallID != "" {
+				label += " [id=" + toolCallID + "]"
+			}
+			if text != "" {
+				convo = append(convo, label+": "+text)
+			}
+		}
+	}
+
+	system := ""
+	if len(systemParts) > 0 {
+		system = "System:\n" + strings.Join(systemParts, "\n\n") + "\n\n"
+	}
+	transcript := strings.Join(convo, "\n\n")
+	return system + transcript + "\n\nAssistant:"
+}
+
+func BuildPromptFromSimpleMessages(messages []SimpleMessage) string {
+	ifaces := make([]interface{}, len(messages))
+	for i, m := range messages {
+		ifaces[i] = map[string]interface{}{"role": m.Role, "content": m.Content}
+	}
+	return BuildPromptFromMessages(ifaces)
+}
diff --git a/pkg/adapter/openai/openai_test.go b/pkg/adapter/openai/openai_test.go
new file mode 100644
index 0000000..04ede1b
--- /dev/null
+++ b/pkg/adapter/openai/openai_test.go
@@ -0,0 +1,80 @@
+package openai
+
+import "testing"
+
+func TestNormalizeModelID(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"gpt-4", "gpt-4"},
+		{"openai/gpt-4", "gpt-4"},
+		{"anthropic/claude-3", "claude-3"},
+		{"", ""},
+		{"  ", ""},
+		{"a/b/c", "c"},
+	}
+	for _, tc := range tests {
+		got := NormalizeModelID(tc.input)
+		if got != tc.want {
+			t.Errorf("NormalizeModelID(%q) = %q, want %q", tc.input, got, tc.want)
+		}
+	}
+}
+
+func TestBuildPromptFromMessages(t *testing.T) {
+	messages := []interface{}{
+		map[string]interface{}{"role": "system", "content": "You are helpful."},
+		map[string]interface{}{"role": "user", "content": "Hello"},
+		map[string]interface{}{"role": "assistant", "content": "Hi there"},
+	}
+	got := BuildPromptFromMessages(messages)
+	if got == "" {
+		t.Fatal("expected non-empty prompt")
+	}
+	containsSystem := false
+	containsUser := false
+	containsAssistant := false
+	for i := 0; i < len(got)-10; i++ {
+		if got[i:i+6] == "System" {
+			containsSystem = true
+		}
+		if got[i:i+4] == "User" {
+			containsUser = true
+		}
+		if got[i:i+9] == "Assistant" {
+			containsAssistant = true
+		}
+	}
+	if !containsSystem || !containsUser || !containsAssistant {
+		t.Errorf("prompt missing sections: system=%v user=%v assistant=%v\n%s",
+			containsSystem, containsUser, containsAssistant, got)
+	}
+}
+
+func TestToolsToSystemText(t *testing.T) {
+	tools := []interface{}{
+		map[string]interface{}{
+			"type": "function",
+			"function": map[string]interface{}{
+				"name":        "get_weather",
+				"description": "Get weather",
+				"parameters":  map[string]interface{}{"type": "object"},
+			},
+		},
+	}
+	got := ToolsToSystemText(tools, nil)
+	if got == "" {
+		t.Fatal("expected non-empty tools text")
+	}
+	if len(got) < 10 {
+		t.Errorf("tools text too short: %q", got)
+	}
+}
+
+func TestToolsToSystemTextEmpty(t *testing.T) {
+	got := ToolsToSystemText(nil, nil)
+	if got != "" {
+		t.Errorf("expected empty string for no tools, got %q", got)
+	}
+}