diff --git a/pkg/llm/mock/mock.go b/pkg/llm/mock/mock.go
new file mode 100644
index 0000000..f34f625
--- /dev/null
+++ b/pkg/llm/mock/mock.go
@@ -0,0 +1,121 @@
+package mock
+
+import (
+	"context"
+	"iter"
+	"strings"
+
+	"github.com/VictorVargas/rony-llm-agent/pkg/llm"
+)
+
+// MockLLMClient is a deterministic implementation of llm.LLMClient for testing.
+type MockLLMClient struct {
+	GenerateFunc func(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error)
+	StreamFunc   func(ctx context.Context, req llm.CompletionRequest) iter.Seq2[llm.StreamChunk, error]
+	NameFunc     func() string
+	CapabilitiesFunc func() llm.ProviderCapabilities
+}
+
+func (m *MockLLMClient) Generate(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+	if m.GenerateFunc != nil {
+		return m.GenerateFunc(ctx, req)
+	}
+	return llm.CompletionResponse{
+		Content:    "default response",
+		StopReason: llm.StopReasonEndTurn,
+	}, nil
+}
+
+func (m *MockLLMClient) Stream(ctx context.Context, req llm.CompletionRequest) iter.Seq2[llm.StreamChunk, error] {
+	if m.StreamFunc != nil {
+		return m.StreamFunc(ctx, req)
+	}
+	return func(yield func(llm.StreamChunk, error) bool) {
+		yield(llm.StreamChunk{
+			Delta: "default response",
+		}, nil)
+	}
+}
+
+func (m *MockLLMClient) Name() string {
+	if m.NameFunc != nil {
+		return m.NameFunc()
+	}
+	return "mock"
+}
+
+func (m *MockLLMClient) Capabilities() llm.ProviderCapabilities {
+	if m.CapabilitiesFunc != nil {
+		return m.CapabilitiesFunc()
+	}
+	return llm.ProviderCapabilities{
+		SupportsTools:    true,
+		SupportsVision:   false,
+		SupportsJSON:     true,
+		MaxContextWindow: 128000,
+	}
+}
+
+// New returns a MockLLMClient with default behaviors.
+func New() *MockLLMClient {
+	return &MockLLMClient{}
+}
+
+// NewWithGenerate returns a MockLLMClient that returns the given response.
+func NewWithGenerate(resp llm.CompletionResponse) *MockLLMClient {
+	return &MockLLMClient{
+		GenerateFunc: func(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+			return resp, nil
+		},
+	}
+}
+
+// NewWithStream returns a MockLLMClient that streams the given chunks.
+func NewWithStream(chunks []llm.StreamChunk) *MockLLMClient {
+	return &MockLLMClient{
+		StreamFunc: func(ctx context.Context, req llm.CompletionRequest) iter.Seq2[llm.StreamChunk, error] {
+			return func(yield func(llm.StreamChunk, error) bool) {
+				for _, c := range chunks {
+					if !yield(c, nil) {
+						return
+					}
+				}
+			}
+		},
+	}
+}
+
+// NewWithMatch returns a MockLLMClient that matches input messages against patterns.
+func NewWithMatch(responses []MatchResponse) *MockLLMClient {
+	return &MockLLMClient{
+		GenerateFunc: func(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+			for _, r := range responses {
+				if r.Match == "*" || matchesAny(r.Match, req.Messages) {
+					return llm.CompletionResponse{
+						Content:    r.Response,
+						StopReason: llm.StopReasonEndTurn,
+					}, nil
+				}
+			}
+			return llm.CompletionResponse{
+				Content:    "no match",
+				StopReason: llm.StopReasonEndTurn,
+			}, nil
+		},
+	}
+}
+
+// MatchResponse pairs a pattern with a response string.
+type MatchResponse struct {
+	Match    string // exact string or "*" for wildcard
+	Response string
+}
+
+func matchesAny(pattern string, msgs []llm.Message) bool {
+	for _, m := range msgs {
+		if m.Role == llm.RoleUser && strings.Contains(m.Content, pattern) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/pkg/llm/providers/llamacpp/client.go b/pkg/llm/providers/llamacpp/client.go
new file mode 100644
index 0000000..314803e
--- /dev/null
+++ b/pkg/llm/providers/llamacpp/client.go
@@ -0,0 +1,322 @@
+package llamacpp
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"iter"
+	"strings"
+
+	"github.com/VictorVargas/rony-llm-agent/pkg/llm"
+)
+
+// Config holds the settings needed to create a llama.cpp client.
+type Config struct {
+	BaseURL   string // defaults to http://localhost:8080/v1
+	Model     string
+	Timeout   int    // request timeout in seconds (0 = default)
+	TopK      int    // top-k sampling (0 = default)
+	TopP      float32
+	Temperature float32
+}
+
+// Client implements llm.LLMClient for llama.cpp.
+type Client struct {
+	baseURL string
+	http    *http.Client
+}
+
+// New returns a new llama.cpp client.
+func New(cfg Config) (*Client, error) {
+	baseURL := cfg.BaseURL
+	if baseURL == "" {
+		baseURL = "http://localhost:8080/v1"
+	}
+
+	return &Client{
+		baseURL: baseURL,
+		http:    http.DefaultClient,
+	}, nil
+}
+
+func (c *Client) Generate(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+	endpoint := c.baseURL + "/chat/completions"
+
+	payload, err := c.buildRequest(req, false)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("building request: %w", err)
+	}
+
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, payload)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("creating request: %w", err)
+	}
+	httpReq.Header.Set("Content-Type", "application/json")
+
+	resp, err := c.http.Do(httpReq)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(resp.Body)
+		return llm.CompletionResponse{}, fmt.Errorf("API error %d: %s", resp.StatusCode, string(body))
+	}
+
+	var apiResp llamaChatResponse
+	if err := json.NewDecoder(resp.Body).Decode(&apiResp); err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("decoding response: %w", err)
+	}
+
+	return c.toResponse(apiResp), nil
+}
+
+func (c *Client) Stream(ctx context.Context, req llm.CompletionRequest) iter.Seq2[llm.StreamChunk, error] {
+	return func(yield func(llm.StreamChunk, error) bool) {
+		endpoint := c.baseURL + "/chat/completions"
+
+		payload, err := c.buildRequest(req, true)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("building request: %w", err))
+			return
+		}
+
+		httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, payload)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("creating request: %w", err))
+			return
+		}
+		httpReq.Header.Set("Content-Type", "application/json")
+
+		resp, err := c.http.Do(httpReq)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("request failed: %w", err))
+			return
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+			body, _ := io.ReadAll(resp.Body)
+			yield(llm.StreamChunk{}, fmt.Errorf("API error %d: %s", resp.StatusCode, string(body)))
+			return
+		}
+
+		scanner := bufio.NewScanner(resp.Body)
+		for scanner.Scan() {
+			line := scanner.Text()
+			if !strings.HasPrefix(line, "data: ") {
+				continue
+			}
+			data := strings.TrimPrefix(line, "data: ")
+			if data == "[DONE]" {
+				return
+			}
+
+			var event llamaStreamEvent
+			if err := json.Unmarshal([]byte(data), &event); err != nil {
+				yield(llm.StreamChunk{}, fmt.Errorf("decoding event: %w", err))
+				return
+			}
+
+			for _, choice := range event.Choices {
+				chunk := llm.StreamChunk{
+					Delta: choice.Delta.Content,
+				}
+				if choice.FinishReason != "" {
+					chunk.FinishReason = choice.FinishReason
+				}
+				if !yield(chunk, nil) {
+					return
+				}
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("stream error: %w", err))
+		}
+	}
+}
+
+func (c *Client) Name() string {
+	return "llama.cpp"
+}
+
+func (c *Client) Capabilities() llm.ProviderCapabilities {
+	return llm.ProviderCapabilities{
+		SupportsTools:    true,
+		SupportsVision:   false,
+		SupportsJSON:     true,
+		MaxContextWindow: 32768,
+	}
+}
+
+// buildRequest converts an llm.CompletionRequest to the llama.cpp API format.
+func (c *Client) buildRequest(req llm.CompletionRequest, stream bool) (io.Reader, error) {
+	messages := make([]llamaMessage, len(req.Messages))
+	for i, m := range req.Messages {
+		messages[i] = llamaMessage{
+			Role:    string(m.Role),
+			Content: m.Content,
+		}
+	}
+
+	var tools []llamaTool
+	for _, t := range req.Tools {
+		var tool llamaTool
+		if err := json.Unmarshal(t, &tool); err != nil {
+			return nil, fmt.Errorf("parsing tool: %w", err)
+		}
+		tools = append(tools, tool)
+	}
+
+	openReq := llamaChatRequest{
+		Model:    req.Model,
+		Messages: messages,
+		Stream:   stream,
+	}
+	if len(tools) > 0 {
+		openReq.Tools = tools
+	}
+	if req.ToolChoice != nil {
+		openReq.ToolChoice = req.ToolChoice
+	}
+	if req.Temperature != nil {
+		tmp := *req.Temperature
+		openReq.Temperature = tmp
+	}
+	if req.MaxTokens != nil {
+		tmp := *req.MaxTokens
+		openReq.MaxTokens = tmp
+	}
+	if len(req.Stop) > 0 {
+		openReq.Stop = req.Stop
+	}
+
+	data, err := json.Marshal(openReq)
+	if err != nil {
+		return nil, fmt.Errorf("marshaling request: %w", err)
+	}
+	return strings.NewReader(string(data)), nil
+}
+
+// toResponse converts a llama.cpp API response to our CompletionResponse.
+func (c *Client) toResponse(resp llamaChatResponse) llm.CompletionResponse {
+	choice := resp.Choices[0]
+	result := llm.CompletionResponse{
+		ID:         resp.ID,
+		Model:      resp.Model,
+		Content:    choice.Message.Content,
+		StopReason: choice.FinishReason,
+	}
+
+	for _, tc := range choice.Message.ToolCalls {
+		result.ToolCalls = append(result.ToolCalls, llm.ToolCall{
+			ID:        tc.ID,
+			Name:      tc.Function.Name,
+			Arguments: json.RawMessage(tc.Function.Arguments),
+		})
+	}
+
+	result.Usage = llm.TokenUsage{
+		InputTokens:  resp.Usage.PromptTokens,
+		OutputTokens: resp.Usage.CompletionTokens,
+		TotalTokens:  resp.Usage.TotalTokens,
+	}
+
+	return result
+}
+
+// llama.cpp API types
+
+type llamaChatRequest struct {
+	Model       string              `json:"model"`
+	Messages    []llamaMessage      `json:"messages"`
+	Tools       []llamaTool         `json:"tools,omitempty"`
+	ToolChoice interface{}         `json:"tool_choice,omitempty"`
+	Temperature float32             `json:"temperature,omitempty"`
+	MaxTokens   int                `json:"max_tokens,omitempty"`
+	TopK        int                `json:"top_k,omitempty"`
+	TopP        float32            `json:"top_p,omitempty"`
+	Stop        []string           `json:"stop,omitempty"`
+	Stream      bool               `json:"stream"`
+}
+
+type llamaMessage struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type llamaTool struct {
+	Type     string          `json:"type"`
+	Function json.RawMessage `json:"function"`
+}
+
+type llamaChatResponse struct {
+	ID      string               `json:"id"`
+	Model   string               `json:"model"`
+	Choices []llamaChoice        `json:"choices"`
+	Usage   llamaUsage           `json:"usage"`
+}
+
+type llamaChoice struct {
+	Index        int                `json:"index"`
+	Message      llamaMessageResult `json:"message"`
+	FinishReason string              `json:"finish_reason"`
+}
+
+type llamaMessageResult struct {
+	Role        string             `json:"role"`
+	Content     string             `json:"content"`
+	ToolCalls   []llamaToolCall    `json:"tool_calls"`
+}
+
+type llamaToolCall struct {
+	ID       string             `json:"id"`
+	Type     string             `json:"type"`
+	Function llamaFunction      `json:"function"`
+}
+
+type llamaFunction struct {
+	Name        string `json:"name"`
+	Arguments   string `json:"arguments"`
+}
+
+type llamaUsage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// Stream event types
+
+type llamaStreamEvent struct {
+	ID      string               `json:"id"`
+	Choices []llamaStreamChoice `json:"choices"`
+}
+
+type llamaStreamChoice struct {
+	Index        int                  `json:"index"`
+	Delta        llamaStreamDelta    `json:"delta"`
+	FinishReason string               `json:"finish_reason"`
+}
+
+type llamaStreamDelta struct {
+	Content  string                `json:"content"`
+	Role     string               `json:"role"`
+	ToolCalls []llamaStreamToolCall `json:"tool_calls"`
+}
+
+type llamaStreamToolCall struct {
+	Index    int                   `json:"index"`
+	ID       string                `json:"id"`
+	Type     string                `json:"type"`
+	Function llamaStreamFunction   `json:"function"`
+}
+
+type llamaStreamFunction struct {
+	Name        string `json:"name"`
+	Arguments   string `json:"arguments"`
+}
diff --git a/pkg/llm/providers/llamacpp/client_test.go b/pkg/llm/providers/llamacpp/client_test.go
new file mode 100644
index 0000000..7b98377
--- /dev/null
+++ b/pkg/llm/providers/llamacpp/client_test.go
@@ -0,0 +1,232 @@
+package llamacpp
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/VictorVargas/rony-llm-agent/pkg/llm"
+)
+
+func TestClient_Name(t *testing.T) {
+	client := &Client{}
+	if client.Name() != "llama.cpp" {
+		t.Errorf("expected 'llama.cpp', got %q", client.Name())
+	}
+}
+
+func TestClient_Capabilities(t *testing.T) {
+	client := &Client{}
+	caps := client.Capabilities()
+	if !caps.SupportsTools {
+		t.Error("expected SupportsTools to be true")
+	}
+	if caps.SupportsVision {
+		t.Error("expected SupportsVision to be false")
+	}
+}
+
+func TestClient_Generate(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(llamaChatResponse{
+			ID:      "llama-123",
+			Model:   "llama3",
+			Choices: []llamaChoice{
+				{
+					Index:        0,
+					FinishReason: "stop",
+					Message: llamaMessageResult{
+						Role:    "assistant",
+						Content: "Hello from llama.cpp!",
+					},
+				},
+			},
+			Usage: llamaUsage{
+				PromptTokens:     10,
+				CompletionTokens: 5,
+				TotalTokens:      15,
+			},
+		})
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		BaseURL: server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	resp, err := client.Generate(context.Background(), llm.CompletionRequest{
+		Model: "llama3",
+		Messages: []llm.Message{
+			{Role: llm.RoleUser, Content: "Hi"},
+		},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if resp.Content != "Hello from llama.cpp!" {
+		t.Errorf("expected 'Hello from llama.cpp!', got %q", resp.Content)
+	}
+	if resp.ID != "llama-123" {
+		t.Errorf("expected 'llama-123', got %q", resp.ID)
+	}
+	if resp.Usage.InputTokens != 10 {
+		t.Errorf("expected 10 input tokens, got %d", resp.Usage.InputTokens)
+	}
+}
+
+func TestClient_Generate_Error(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte("server error"))
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		BaseURL: server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	_, err = client.Generate(context.Background(), llm.CompletionRequest{})
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+}
+
+func TestClient_Generate_ToolCall(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(llamaChatResponse{
+			ID:      "llama-tool-1",
+			Model:   "llama3",
+			Choices: []llamaChoice{
+				{
+					Index:        0,
+					FinishReason: "tool_calls",
+					Message: llamaMessageResult{
+						Role:    "assistant",
+						Content: "",
+						ToolCalls: []llamaToolCall{
+							{
+								ID:   "call-1",
+								Type: "function",
+								Function: llamaFunction{
+									Name:      "calculate",
+									Arguments: `{"a":1,"b":2}`,
+								},
+							},
+						},
+					},
+				},
+			},
+			Usage: llamaUsage{
+				PromptTokens:     20,
+				CompletionTokens: 10,
+				TotalTokens:      30,
+			},
+		})
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		BaseURL: server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	resp, err := client.Generate(context.Background(), llm.CompletionRequest{
+		Model: "llama3",
+		Messages: []llm.Message{
+			{Role: llm.RoleUser, Content: "What is 1 + 2?"},
+		},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(resp.ToolCalls) != 1 {
+		t.Fatalf("expected 1 tool call, got %d", len(resp.ToolCalls))
+	}
+	if resp.ToolCalls[0].Name != "calculate" {
+		t.Errorf("expected 'calculate', got %q", resp.ToolCalls[0].Name)
+	}
+}
+
+func TestClient_Stream(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\"Hello\"},\"finish_reason\":null}]}\n"))
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\" world\"},\"finish_reason\":null}]}\n"))
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\"!\"},\"finish_reason\":\"stop\"}]}\n"))
+		w.Write([]byte("data: [DONE]\n"))
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		BaseURL: server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var chunks []llm.StreamChunk
+	stream := client.Stream(context.Background(), llm.CompletionRequest{})
+	for chunk, err := range stream {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		chunks = append(chunks, chunk)
+	}
+
+	if len(chunks) != 3 {
+		t.Errorf("expected 3 chunks, got %d", len(chunks))
+	}
+	if chunks[0].Delta != "Hello" {
+		t.Errorf("expected 'Hello', got %q", chunks[0].Delta)
+	}
+	if chunks[1].Delta != " world" {
+		t.Errorf("expected ' world', got %q", chunks[1].Delta)
+	}
+	if chunks[2].Delta != "!" {
+		t.Errorf("expected '!', got %q", chunks[2].Delta)
+	}
+}
+
+func TestClient_Stream_FinishReason(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\"done\"},\"finish_reason\":\"stop\"}]}\n"))
+		w.Write([]byte("data: [DONE]\n"))
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		BaseURL: server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var chunks []llm.StreamChunk
+	stream := client.Stream(context.Background(), llm.CompletionRequest{})
+	for chunk, err := range stream {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		chunks = append(chunks, chunk)
+	}
+
+	if len(chunks) != 1 {
+		t.Fatalf("expected 1 chunk, got %d", len(chunks))
+	}
+	if chunks[0].FinishReason != "stop" {
+		t.Errorf("expected 'stop' finish reason, got %q", chunks[0].FinishReason)
+	}
+}
diff --git a/pkg/llm/providers/openai/client.go b/pkg/llm/providers/openai/client.go
new file mode 100644
index 0000000..d3967cf
--- /dev/null
+++ b/pkg/llm/providers/openai/client.go
@@ -0,0 +1,327 @@
+package openai
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"iter"
+	"strings"
+
+	"github.com/VictorVargas/rony-llm-agent/pkg/llm"
+)
+
+// Config holds the settings needed to create an OpenAI client.
+type Config struct {
+	APIKey    string
+	Model     string
+	BaseURL   string // defaults to https://api.openai.com/v1
+}
+
+// Client implements llm.LLMClient for OpenAI.
+type Client struct {
+	apiKey  string
+	baseURL string
+	http    *http.Client
+}
+
+// New returns a new OpenAI client.
+func New(cfg Config) (*Client, error) {
+	if cfg.APIKey == "" {
+		return nil, fmt.Errorf("openai: API key is required")
+	}
+
+	baseURL := cfg.BaseURL
+	if baseURL == "" {
+		baseURL = "https://api.openai.com/v1"
+	}
+
+	return &Client{
+		apiKey:  cfg.APIKey,
+		baseURL: baseURL,
+		http:    http.DefaultClient,
+	}, nil
+}
+
+func (c *Client) Generate(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+	endpoint := c.baseURL + "/chat/completions"
+
+	payload, err := c.buildRequest(req)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("building request: %w", err)
+	}
+
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, payload)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("creating request: %w", err)
+	}
+	httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
+	httpReq.Header.Set("Content-Type", "application/json")
+
+	resp, err := c.http.Do(httpReq)
+	if err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(resp.Body)
+		return llm.CompletionResponse{}, fmt.Errorf("API error %d: %s", resp.StatusCode, string(body))
+	}
+
+	var apiResp openaiChatResponse
+	if err := json.NewDecoder(resp.Body).Decode(&apiResp); err != nil {
+		return llm.CompletionResponse{}, fmt.Errorf("decoding response: %w", err)
+	}
+
+	return c.toResponse(apiResp), nil
+}
+
+func (c *Client) Stream(ctx context.Context, req llm.CompletionRequest) iter.Seq2[llm.StreamChunk, error] {
+	return func(yield func(llm.StreamChunk, error) bool) {
+		endpoint := c.baseURL + "/chat/completions"
+
+		payload, err := c.buildRequest(req)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("building request: %w", err))
+			return
+		}
+
+		httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, payload)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("creating request: %w", err))
+			return
+		}
+		httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
+		httpReq.Header.Set("Content-Type", "application/json")
+		httpReq.Header.Set("Accept", "text/event-stream")
+
+		resp, err := c.http.Do(httpReq)
+		if err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("request failed: %w", err))
+			return
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+			body, _ := io.ReadAll(resp.Body)
+			yield(llm.StreamChunk{}, fmt.Errorf("API error %d: %s", resp.StatusCode, string(body)))
+			return
+		}
+
+		scanner := bufio.NewScanner(resp.Body)
+		for scanner.Scan() {
+			line := scanner.Text()
+			if !strings.HasPrefix(line, "data: ") {
+				continue
+			}
+			data := strings.TrimPrefix(line, "data: ")
+			if data == "[DONE]" {
+				return
+			}
+
+			var event openaiStreamEvent
+			if err := json.Unmarshal([]byte(data), &event); err != nil {
+				yield(llm.StreamChunk{}, fmt.Errorf("decoding event: %w", err))
+				return
+			}
+
+			for _, choice := range event.Choices {
+				chunk := llm.StreamChunk{
+					Delta: choice.Delta.Content,
+				}
+				if choice.FinishReason != "" {
+					chunk.FinishReason = choice.FinishReason
+				}
+				if !yield(chunk, nil) {
+					return
+				}
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			yield(llm.StreamChunk{}, fmt.Errorf("stream error: %w", err))
+		}
+	}
+}
+
+func (c *Client) Name() string {
+	return "openai"
+}
+
+func (c *Client) Capabilities() llm.ProviderCapabilities {
+	return llm.ProviderCapabilities{
+		SupportsTools:    true,
+		SupportsVision:   true,
+		SupportsJSON:     true,
+		MaxContextWindow: 128000,
+	}
+}
+
+// buildRequest converts an llm.CompletionRequest to the OpenAI API format.
+func (c *Client) buildRequest(req llm.CompletionRequest) (io.Reader, error) {
+	// Convert messages to OpenAI format
+	messages := make([]openaiMessage, len(req.Messages))
+	for i, m := range req.Messages {
+		messages[i] = openaiMessage{
+			Role:    string(m.Role),
+			Content: m.Content,
+		}
+	}
+
+	tools := make([]openaiTool, len(req.Tools))
+	for i, t := range req.Tools {
+		var tool openaiTool
+		if err := json.Unmarshal(t, &tool); err != nil {
+			return nil, fmt.Errorf("parsing tool %d: %w", i, err)
+		}
+		tools[i] = tool
+	}
+
+	openaiReq := openaiChatRequest{
+		Model:    req.Model,
+		Messages: messages,
+		Stream:   false,
+	}
+	if len(tools) > 0 {
+		openaiReq.Tools = tools
+	}
+	if req.ToolChoice != nil {
+		openaiReq.ToolChoice = req.ToolChoice
+	}
+	if req.Temperature != nil {
+		tmp := *req.Temperature
+		openaiReq.Temperature = &tmp
+	}
+	if req.MaxTokens != nil {
+		max := *req.MaxTokens
+		openaiReq.MaxTokens = &max
+	}
+	if len(req.Stop) > 0 {
+		openaiReq.Stop = req.Stop
+	}
+
+	data, err := json.Marshal(openaiReq)
+	if err != nil {
+		return nil, fmt.Errorf("marshaling request: %w", err)
+	}
+	return strings.NewReader(string(data)), nil
+}
+
+// toResponse converts an OpenAI API response to our CompletionResponse.
+func (c *Client) toResponse(resp openaiChatResponse) llm.CompletionResponse {
+	choice := resp.Choices[0]
+	result := llm.CompletionResponse{
+		ID:         resp.ID,
+		Model:      resp.Model,
+		Content:    choice.Message.Content,
+		StopReason: choice.FinishReason,
+	}
+
+	for _, tc := range choice.Message.ToolCalls {
+		result.ToolCalls = append(result.ToolCalls, llm.ToolCall{
+			ID:        tc.ID,
+			Name:      tc.Function.Name,
+			Arguments: json.RawMessage(tc.Function.Arguments),
+		})
+	}
+
+	result.Usage = llm.TokenUsage{
+		InputTokens:  resp.Usage.PromptTokens,
+		OutputTokens: resp.Usage.CompletionTokens,
+		TotalTokens:  resp.Usage.TotalTokens,
+	}
+
+	return result
+}
+
+// OpenAI API types
+
+type openaiChatRequest struct {
+	Model       string              `json:"model"`
+	Messages    []openaiMessage     `json:"messages"`
+	Tools       []openaiTool        `json:"tools,omitempty"`
+	ToolChoice interface{}         `json:"tool_choice,omitempty"`
+	Temperature *float32            `json:"temperature,omitempty"`
+	MaxTokens   *int               `json:"max_tokens,omitempty"`
+	Stop        []string           `json:"stop,omitempty"`
+	Stream      bool               `json:"stream"`
+}
+
+type openaiMessage struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type openaiTool struct {
+	Type     string           `json:"type"`
+	Function json.RawMessage  `json:"function"`
+}
+
+type openaiChatResponse struct {
+	ID      string                  `json:"id"`
+	Model   string                  `json:"model"`
+	Choices []openaiChoice         `json:"choices"`
+	Usage   openaiUsage             `json:"usage"`
+}
+
+type openaiChoice struct {
+	Index        int                 `json:"index"`
+	Message      openaiMessageResult `json:"message"`
+	FinishReason string               `json:"finish_reason"`
+}
+
+type openaiMessageResult struct {
+	Role        string               `json:"role"`
+	Content     string               `json:"content"`
+	ToolCalls   []openaiToolCall     `json:"tool_calls"`
+}
+
+type openaiToolCall struct {
+	ID       string               `json:"id"`
+	Type     string               `json:"type"`
+	Function openaiFunction       `json:"function"`
+}
+
+type openaiFunction struct {
+	Name        string `json:"name"`
+	Arguments   string `json:"arguments"`
+}
+
+type openaiUsage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// Stream event types
+
+type openaiStreamEvent struct {
+	ID      string                  `json:"id"`
+	Choices []openaiStreamChoice   `json:"choices"`
+}
+
+type openaiStreamChoice struct {
+	Index        int                    `json:"index"`
+	Delta        openaiStreamDelta     `json:"delta"`
+	FinishReason string                `json:"finish_reason"`
+}
+
+type openaiStreamDelta struct {
+	Content  string                `json:"content"`
+	Role     string                `json:"role"`
+	ToolCalls []openaiStreamToolCall `json:"tool_calls"`
+}
+
+type openaiStreamToolCall struct {
+	Index    int                   `json:"index"`
+	ID       string                `json:"id"`
+	Type     string                `json:"type"`
+	Function openaiStreamFunction `json:"function"`
+}
+
+type openaiStreamFunction struct {
+	Name        string `json:"name"`
+	Arguments   string `json:"arguments"`
+}
diff --git a/pkg/llm/providers/openai/client_test.go b/pkg/llm/providers/openai/client_test.go
new file mode 100644
index 0000000..dd838f6
--- /dev/null
+++ b/pkg/llm/providers/openai/client_test.go
@@ -0,0 +1,141 @@
+package openai
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/VictorVargas/rony-llm-agent/pkg/llm"
+)
+
+func TestClient_Name(t *testing.T) {
+	client := &Client{}
+	if client.Name() != "openai" {
+		t.Errorf("expected 'openai', got %q", client.Name())
+	}
+}
+
+func TestClient_Capabilities(t *testing.T) {
+	client := &Client{}
+	caps := client.Capabilities()
+	if !caps.SupportsTools {
+		t.Error("expected SupportsTools to be true")
+	}
+	if !caps.SupportsVision {
+		t.Error("expected SupportsVision to be true")
+	}
+}
+
+func TestClient_Generate(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode(openaiChatResponse{
+			ID:      "test-123",
+			Model:   "gpt-4",
+			Choices: []openaiChoice{
+				{
+					Index:        0,
+					FinishReason: "stop",
+					Message: openaiMessageResult{
+						Role:    "assistant",
+						Content: "Hello!",
+					},
+				},
+			},
+			Usage: openaiUsage{
+				PromptTokens:     10,
+				CompletionTokens: 5,
+				TotalTokens:      15,
+			},
+		})
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		APIKey:    "test-key",
+		BaseURL:   server.URL,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	resp, err := client.Generate(context.Background(), llm.CompletionRequest{
+		Model: "gpt-4",
+		Messages: []llm.Message{
+			{Role: llm.RoleUser, Content: "Hi"},
+		},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if resp.Content != "Hello!" {
+		t.Errorf("expected 'Hello!', got %q", resp.Content)
+	}
+	if resp.ID != "test-123" {
+		t.Errorf("expected 'test-123', got %q", resp.ID)
+	}
+	if resp.Usage.InputTokens != 10 {
+		t.Errorf("expected 10 input tokens, got %d", resp.Usage.InputTokens)
+	}
+}
+
+func TestClient_Generate_Error(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		w.Write([]byte("invalid api key"))
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		APIKey:    "bad-key",
+		BaseURL:   server.URL,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	_, err = client.Generate(context.Background(), llm.CompletionRequest{})
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+}
+
+func TestClient_Stream(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\"Hello\"},\"finish_reason\":null}]}\n"))
+		w.Write([]byte("data: {\"choices\":[{\"delta\":{\"content\":\" world\"},\"finish_reason\":null}]}\n"))
+		w.Write([]byte("data: [DONE]\n"))
+	}))
+	defer server.Close()
+
+	client, err := New(Config{
+		APIKey:    "test-key",
+		BaseURL:   server.URL + "/v1",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var chunks []llm.StreamChunk
+	stream := client.Stream(context.Background(), llm.CompletionRequest{})
+	for chunk, err := range stream {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		chunks = append(chunks, chunk)
+	}
+
+	if len(chunks) != 2 {
+		t.Errorf("expected 2 chunks, got %d", len(chunks))
+	}
+	if chunks[0].Delta != "Hello" {
+		t.Errorf("expected 'Hello', got %q", chunks[0].Delta)
+	}
+	if chunks[1].Delta != " world" {
+		t.Errorf("expected ' world', got %q", chunks[1].Delta)
+	}
+}
diff --git a/pkg/llm/types.go b/pkg/llm/types.go
new file mode 100644
index 0000000..f7d9fa8
--- /dev/null
+++ b/pkg/llm/types.go
@@ -0,0 +1,120 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+	"iter"
+)
+
+// Role represents the role of a message participant.
+type Role string
+
+const (
+	RoleSystem   Role = "system"
+	RoleUser     Role = "user"
+	RoleAssistant Role = "assistant"
+	RoleTool     Role = "tool"
+)
+
+// Message is a single message in a conversation.
+type Message struct {
+	Role        Role    `json:"role"`
+	Content     string  `json:"content"`
+	ToolCallID  string  `json:"tool_call_id,omitempty"`
+	Name        string  `json:"name,omitempty"`
+}
+
+// ProviderCapabilities describes what a model supports.
+type ProviderCapabilities struct {
+	SupportsTools    bool `json:"supports_tools"`
+	SupportsVision   bool `json:"supports_vision"`
+	SupportsJSON     bool `json:"supports_json"`
+	MaxContextWindow int  `json:"max_context_window"`
+}
+
+// TokenUsage tracks token consumption for a response.
+type TokenUsage struct {
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+}
+
+// ToolChoice controls how the model chooses which tools to call.
+type ToolChoice string
+
+const (
+	ToolChoiceAuto    ToolChoice = "auto"
+	ToolChoiceNone    ToolChoice = "none"
+	ToolChoiceRequired ToolChoice = "required"
+)
+
+// ToolRef is used when ToolChoice must invoke a specific tool by name.
+type ToolRef struct {
+	Name string `json:"name"`
+}
+
+func (t *ToolRef) MarshalJSON() ([]byte, error) {
+	type Alias ToolRef
+	return json.Marshal(&struct {
+		*Alias
+		Type string `json:"type"`
+	}{
+		Alias: (*Alias)(t),
+		Type:  "function",
+	})
+}
+
+// CompletionRequest is sent to an LLM provider.
+type CompletionRequest struct {
+	Model       string              `json:"model"`
+	Messages    []Message          `json:"messages"`
+	Tools       []json.RawMessage  `json:"tools,omitempty"`
+	ToolChoice interface{}        `json:"tool_choice,omitempty"` // ToolChoice, ToolRef, or null
+	Temperature *float32           `json:"temperature,omitempty"`
+	MaxTokens   *int              `json:"max_tokens,omitempty"`
+	Stop        []string          `json:"stop,omitempty"`
+	Metadata    map[string]string `json:"metadata,omitempty"`
+}
+
+// CompletionResponse is returned from an LLM provider.
+type CompletionResponse struct {
+	ID         string     `json:"id"`
+	Model      string     `json:"model"`
+	Content    string     `json:"content"`
+	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
+	StopReason string     `json:"stop_reason"`
+	Usage      TokenUsage `json:"usage,omitempty"`
+}
+
+// StopReason values.
+const (
+	StopReasonEndTurn   = "end_turn"
+	StopReasonToolUse  = "tool_use"
+	StopReasonMaxTokens = "max_tokens"
+	StopReasonStopSeq  = "stop_sequence"
+)
+
+// ToolCall represents a function invocation requested by the model.
+type ToolCall struct {
+	ID        string          `json:"id"`
+	Name      string          `json:"name"`
+	Arguments json.RawMessage `json:"arguments"`
+	Thought   string          `json:"thought,omitempty"`
+}
+
+// StreamChunk is emitted by the iterator returned from Stream().
+type StreamChunk struct {
+	Delta       string     `json:"delta"`
+	ToolCalls   []ToolCall `json:"tool_calls,omitempty"`
+	FinishReason string    `json:"finish_reason,omitempty"`
+	Usage       TokenUsage `json:"usage,omitempty"` // only present on final chunk
+}
+
+// LLMClient is the interface that every provider implements.
+// Products must never call a provider SDK directly; they go through this interface.
+type LLMClient interface {
+	Generate(ctx context.Context, req CompletionRequest) (CompletionResponse, error)
+	Stream(ctx context.Context, req CompletionRequest) iter.Seq2[StreamChunk, error]
+	Name() string
+	Capabilities() ProviderCapabilities
+}