diff --git a/internal/message/content.go b/internal/message/content.go
new file mode 100644
index 0000000..71c7ef6
--- /dev/null
+++ b/internal/message/content.go
@@ -0,0 +1,78 @@
+package message
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// ContentType discriminates the content block union.
+type ContentType int
+
+const (
+	ContentText       ContentType = iota + 1
+	ContentToolCall
+	ContentToolResult
+	ContentThinking
+)
+
+func (ct ContentType) String() string {
+	switch ct {
+	case ContentText:
+		return "text"
+	case ContentToolCall:
+		return "tool_call"
+	case ContentToolResult:
+		return "tool_result"
+	case ContentThinking:
+		return "thinking"
+	default:
+		return fmt.Sprintf("unknown(%d)", ct)
+	}
+}
+
+// Content is a discriminated union. Exactly one payload field is set per Type.
+type Content struct {
+	Type       ContentType
+	Text       string      // ContentText
+	ToolCall   *ToolCall   // ContentToolCall
+	ToolResult *ToolResult // ContentToolResult
+	Thinking   *Thinking   // ContentThinking
+}
+
+// ToolCall represents the model's request to invoke a tool.
+type ToolCall struct {
+	ID        string          `json:"id"`
+	Name      string          `json:"name"`
+	Arguments json.RawMessage `json:"arguments"`
+}
+
+// ToolResult is the output of executing a tool, correlated by ToolCallID.
+type ToolResult struct {
+	ToolCallID string `json:"tool_call_id"`
+	Content    string `json:"content"`
+	IsError    bool   `json:"is_error"`
+}
+
+// Thinking represents a reasoning/thinking trace.
+// Signature must round-trip unchanged (Anthropic requirement).
+type Thinking struct {
+	Text      string `json:"text,omitempty"`
+	Signature string `json:"signature,omitempty"`
+	Redacted  bool   `json:"redacted,omitempty"`
+}
+
+func NewTextContent(text string) Content {
+	return Content{Type: ContentText, Text: text}
+}
+
+func NewToolCallContent(tc ToolCall) Content {
+	return Content{Type: ContentToolCall, ToolCall: &tc}
+}
+
+func NewToolResultContent(tr ToolResult) Content {
+	return Content{Type: ContentToolResult, ToolResult: &tr}
+}
+
+func NewThinkingContent(th Thinking) Content {
+	return Content{Type: ContentThinking, Thinking: &th}
+}
diff --git a/internal/message/content_test.go b/internal/message/content_test.go
new file mode 100644
index 0000000..43fd108
--- /dev/null
+++ b/internal/message/content_test.go
@@ -0,0 +1,174 @@
+package message
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestNewTextContent(t *testing.T) {
+	c := NewTextContent("hello world")
+
+	if c.Type != ContentText {
+		t.Errorf("Type = %v, want %v", c.Type, ContentText)
+	}
+	if c.Text != "hello world" {
+		t.Errorf("Text = %q, want %q", c.Text, "hello world")
+	}
+	if c.ToolCall != nil {
+		t.Error("ToolCall should be nil for text content")
+	}
+	if c.ToolResult != nil {
+		t.Error("ToolResult should be nil for text content")
+	}
+	if c.Thinking != nil {
+		t.Error("Thinking should be nil for text content")
+	}
+}
+
+func TestNewToolCallContent(t *testing.T) {
+	args := json.RawMessage(`{"command":"ls -la"}`)
+	tc := ToolCall{
+		ID:        "tc_001",
+		Name:      "bash",
+		Arguments: args,
+	}
+	c := NewToolCallContent(tc)
+
+	if c.Type != ContentToolCall {
+		t.Errorf("Type = %v, want %v", c.Type, ContentToolCall)
+	}
+	if c.ToolCall == nil {
+		t.Fatal("ToolCall should not be nil")
+	}
+	if c.ToolCall.ID != "tc_001" {
+		t.Errorf("ToolCall.ID = %q, want %q", c.ToolCall.ID, "tc_001")
+	}
+	if c.ToolCall.Name != "bash" {
+		t.Errorf("ToolCall.Name = %q, want %q", c.ToolCall.Name, "bash")
+	}
+	if string(c.ToolCall.Arguments) != `{"command":"ls -la"}` {
+		t.Errorf("ToolCall.Arguments = %s, want %s", c.ToolCall.Arguments, args)
+	}
+	if c.Text != "" {
+		t.Error("Text should be empty for tool call content")
+	}
+}
+
+func TestNewToolResultContent(t *testing.T) {
+	tr := ToolResult{
+		ToolCallID: "tc_001",
+		Content:    "file1.go\nfile2.go",
+		IsError:    false,
+	}
+	c := NewToolResultContent(tr)
+
+	if c.Type != ContentToolResult {
+		t.Errorf("Type = %v, want %v", c.Type, ContentToolResult)
+	}
+	if c.ToolResult == nil {
+		t.Fatal("ToolResult should not be nil")
+	}
+	if c.ToolResult.ToolCallID != "tc_001" {
+		t.Errorf("ToolResult.ToolCallID = %q, want %q", c.ToolResult.ToolCallID, "tc_001")
+	}
+	if c.ToolResult.IsError {
+		t.Error("ToolResult.IsError should be false")
+	}
+}
+
+func TestNewToolResultContent_Error(t *testing.T) {
+	tr := ToolResult{
+		ToolCallID: "tc_002",
+		Content:    "permission denied",
+		IsError:    true,
+	}
+	c := NewToolResultContent(tr)
+
+	if !c.ToolResult.IsError {
+		t.Error("ToolResult.IsError should be true")
+	}
+	if c.ToolResult.Content != "permission denied" {
+		t.Errorf("ToolResult.Content = %q, want %q", c.ToolResult.Content, "permission denied")
+	}
+}
+
+func TestNewThinkingContent(t *testing.T) {
+	th := Thinking{
+		Text:      "Let me think about this...",
+		Signature: "sig_abc123",
+	}
+	c := NewThinkingContent(th)
+
+	if c.Type != ContentThinking {
+		t.Errorf("Type = %v, want %v", c.Type, ContentThinking)
+	}
+	if c.Thinking == nil {
+		t.Fatal("Thinking should not be nil")
+	}
+	if c.Thinking.Text != "Let me think about this..." {
+		t.Errorf("Thinking.Text = %q", c.Thinking.Text)
+	}
+	if c.Thinking.Signature != "sig_abc123" {
+		t.Errorf("Thinking.Signature = %q", c.Thinking.Signature)
+	}
+	if c.Thinking.Redacted {
+		t.Error("Thinking.Redacted should be false")
+	}
+}
+
+func TestNewRedactedThinkingContent(t *testing.T) {
+	th := Thinking{
+		Redacted: true,
+	}
+	c := NewThinkingContent(th)
+
+	if !c.Thinking.Redacted {
+		t.Error("Thinking.Redacted should be true")
+	}
+}
+
+func TestContentType_String(t *testing.T) {
+	tests := []struct {
+		ct   ContentType
+		want string
+	}{
+		{ContentText, "text"},
+		{ContentToolCall, "tool_call"},
+		{ContentToolResult, "tool_result"},
+		{ContentThinking, "thinking"},
+		{ContentType(99), "unknown(99)"},
+	}
+	for _, tt := range tests {
+		if got := tt.ct.String(); got != tt.want {
+			t.Errorf("ContentType(%d).String() = %q, want %q", tt.ct, got, tt.want)
+		}
+	}
+}
+
+func TestToolCall_JSON_RoundTrip(t *testing.T) {
+	original := ToolCall{
+		ID:        "tc_100",
+		Name:      "fs.read",
+		Arguments: json.RawMessage(`{"path":"/tmp/test.go","offset":0}`),
+	}
+
+	data, err := json.Marshal(original)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+
+	var decoded ToolCall
+	if err := json.Unmarshal(data, &decoded); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+
+	if decoded.ID != original.ID {
+		t.Errorf("ID = %q, want %q", decoded.ID, original.ID)
+	}
+	if decoded.Name != original.Name {
+		t.Errorf("Name = %q, want %q", decoded.Name, original.Name)
+	}
+	if string(decoded.Arguments) != string(original.Arguments) {
+		t.Errorf("Arguments = %s, want %s", decoded.Arguments, original.Arguments)
+	}
+}
diff --git a/internal/message/message.go b/internal/message/message.go
new file mode 100644
index 0000000..c160e24
--- /dev/null
+++ b/internal/message/message.go
@@ -0,0 +1,89 @@
+package message
+
+import "strings"
+
+// Role identifies the sender of a message.
+type Role string
+
+const (
+	RoleUser      Role = "user"
+	RoleAssistant Role = "assistant"
+	RoleSystem    Role = "system"
+)
+
+// Message represents a single turn in the conversation.
+type Message struct {
+	Role    Role
+	Content []Content
+}
+
+func NewUserText(text string) Message {
+	return Message{
+		Role:    RoleUser,
+		Content: []Content{NewTextContent(text)},
+	}
+}
+
+func NewAssistantText(text string) Message {
+	return Message{
+		Role:    RoleAssistant,
+		Content: []Content{NewTextContent(text)},
+	}
+}
+
+func NewAssistantContent(blocks ...Content) Message {
+	return Message{
+		Role:    RoleAssistant,
+		Content: blocks,
+	}
+}
+
+func NewSystemText(text string) Message {
+	return Message{
+		Role:    RoleSystem,
+		Content: []Content{NewTextContent(text)},
+	}
+}
+
+func NewToolResults(results ...ToolResult) Message {
+	content := make([]Content, len(results))
+	for i, r := range results {
+		content[i] = NewToolResultContent(r)
+	}
+	return Message{
+		Role:    RoleUser,
+		Content: content,
+	}
+}
+
+// HasToolCalls returns true if any content block is a tool call.
+func (m Message) HasToolCalls() bool {
+	for _, c := range m.Content {
+		if c.Type == ContentToolCall {
+			return true
+		}
+	}
+	return false
+}
+
+// ToolCalls extracts all tool call blocks.
+func (m Message) ToolCalls() []ToolCall {
+	var calls []ToolCall
+	for _, c := range m.Content {
+		if c.Type == ContentToolCall && c.ToolCall != nil {
+			calls = append(calls, *c.ToolCall)
+		}
+	}
+	return calls
+}
+
+// TextContent concatenates all text blocks.
+func (m Message) TextContent() string {
+	var b strings.Builder
+	for _, c := range m.Content {
+		if c.Type == ContentText {
+			b.WriteString(c.Text)
+		}
+	}
+	return b.String()
+}
diff --git a/internal/message/message_test.go b/internal/message/message_test.go
new file mode 100644
index 0000000..29ae1b1
--- /dev/null
+++ b/internal/message/message_test.go
@@ -0,0 +1,214 @@
+package message
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestNewUserText(t *testing.T) {
+	m := NewUserText("hello")
+	if m.Role != RoleUser {
+		t.Errorf("Role = %q, want %q", m.Role, RoleUser)
+	}
+	if len(m.Content) != 1 {
+		t.Fatalf("len(Content) = %d, want 1", len(m.Content))
+	}
+	if m.Content[0].Type != ContentText {
+		t.Errorf("Content[0].Type = %v, want %v", m.Content[0].Type, ContentText)
+	}
+	if m.Content[0].Text != "hello" {
+		t.Errorf("Content[0].Text = %q, want %q", m.Content[0].Text, "hello")
+	}
+}
+
+func TestNewAssistantText(t *testing.T) {
+	m := NewAssistantText("response")
+	if m.Role != RoleAssistant {
+		t.Errorf("Role = %q, want %q", m.Role, RoleAssistant)
+	}
+	if m.TextContent() != "response" {
+		t.Errorf("TextContent() = %q, want %q", m.TextContent(), "response")
+	}
+}
+
+func TestNewSystemText(t *testing.T) {
+	m := NewSystemText("you are a helper")
+	if m.Role != RoleSystem {
+		t.Errorf("Role = %q, want %q", m.Role, RoleSystem)
+	}
+}
+
+func TestNewAssistantContent_Mixed(t *testing.T) {
+	m := NewAssistantContent(
+		NewTextContent("I'll run that command."),
+		NewToolCallContent(ToolCall{
+			ID:        "tc_1",
+			Name:      "bash",
+			Arguments: json.RawMessage(`{"command":"ls"}`),
+		}),
+	)
+
+	if m.Role != RoleAssistant {
+		t.Errorf("Role = %q, want %q", m.Role, RoleAssistant)
+	}
+	if len(m.Content) != 2 {
+		t.Fatalf("len(Content) = %d, want 2", len(m.Content))
+	}
+	if m.Content[0].Type != ContentText {
+		t.Errorf("Content[0].Type = %v, want text", m.Content[0].Type)
+	}
+	if m.Content[1].Type != ContentToolCall {
+		t.Errorf("Content[1].Type = %v, want tool_call", m.Content[1].Type)
+	}
+}
+
+func TestNewToolResults(t *testing.T) {
+	m := NewToolResults(
+		ToolResult{ToolCallID: "tc_1", Content: "output1"},
+		ToolResult{ToolCallID: "tc_2", Content: "output2", IsError: true},
+	)
+
+	if m.Role != RoleUser {
+		t.Errorf("Role = %q, want %q", m.Role, RoleUser)
+	}
+	if len(m.Content) != 2 {
+		t.Fatalf("len(Content) = %d, want 2", len(m.Content))
+	}
+	if m.Content[0].ToolResult.ToolCallID != "tc_1" {
+		t.Errorf("Content[0].ToolResult.ToolCallID = %q", m.Content[0].ToolResult.ToolCallID)
+	}
+	if m.Content[1].ToolResult.IsError != true {
+		t.Error("Content[1].ToolResult.IsError should be true")
+	}
+}
+
+func TestMessage_HasToolCalls(t *testing.T) {
+	tests := []struct {
+		name string
+		msg  Message
+		want bool
+	}{
+		{
+			name: "text only",
+			msg:  NewUserText("hello"),
+			want: false,
+		},
+		{
+			name: "with tool call",
+			msg: NewAssistantContent(
+				NewTextContent("running..."),
+				NewToolCallContent(ToolCall{ID: "tc_1", Name: "bash"}),
+			),
+			want: true,
+		},
+		{
+			name: "tool results (not calls)",
+			msg:  NewToolResults(ToolResult{ToolCallID: "tc_1", Content: "ok"}),
+			want: false,
+		},
+		{
+			name: "empty message",
+			msg:  Message{Role: RoleAssistant},
+			want: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.msg.HasToolCalls(); got != tt.want {
+				t.Errorf("HasToolCalls() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestMessage_ToolCalls(t *testing.T) {
+	m := NewAssistantContent(
+		NewTextContent("here are two commands"),
+		NewToolCallContent(ToolCall{ID: "tc_1", Name: "bash", Arguments: json.RawMessage(`{"command":"ls"}`)}),
+		NewTextContent("and another"),
+		NewToolCallContent(ToolCall{ID: "tc_2", Name: "fs.read", Arguments: json.RawMessage(`{"path":"go.mod"}`)}),
+	)
+
+	calls := m.ToolCalls()
+	if len(calls) != 2 {
+		t.Fatalf("len(ToolCalls()) = %d, want 2", len(calls))
+	}
+	if calls[0].ID != "tc_1" {
+		t.Errorf("calls[0].ID = %q, want tc_1", calls[0].ID)
+	}
+	if calls[1].Name != "fs.read" {
+		t.Errorf("calls[1].Name = %q, want fs.read", calls[1].Name)
+	}
+}
+
+func TestMessage_ToolCalls_Empty(t *testing.T) {
+	m := NewUserText("no tools here")
+	calls := m.ToolCalls()
+	if len(calls) != 0 {
+		t.Errorf("len(ToolCalls()) = %d, want 0", len(calls))
+	}
+}
+
+func TestMessage_TextContent(t *testing.T) {
+	tests := []struct {
+		name string
+		msg  Message
+		want string
+	}{
+		{
+			name: "single text",
+			msg:  NewUserText("hello"),
+			want: "hello",
+		},
+		{
+			name: "multiple text blocks",
+			msg: NewAssistantContent(
+				NewTextContent("first "),
+				NewToolCallContent(ToolCall{ID: "tc_1", Name: "bash"}),
+				NewTextContent("second"),
+			),
+			want: "first second",
+		},
+		{
+			name: "no text",
+			msg:  NewToolResults(ToolResult{ToolCallID: "tc_1", Content: "output"}),
+			want: "",
+		},
+		{
+			name: "empty message",
+			msg:  Message{Role: RoleAssistant},
+			want: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.msg.TextContent(); got != tt.want {
+				t.Errorf("TextContent() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestResponse_Fields(t *testing.T) {
+	r := Response{
+		Message:    NewAssistantText("done"),
+		StopReason: StopEndTurn,
+		Usage:      Usage{InputTokens: 100, OutputTokens: 50},
+		Model:      "mistral-large-latest",
+	}
+
+	if r.StopReason != StopEndTurn {
+		t.Errorf("StopReason = %q, want %q", r.StopReason, StopEndTurn)
+	}
+	if r.Usage.TotalTokens() != 150 {
+		t.Errorf("Usage.TotalTokens() = %d, want 150", r.Usage.TotalTokens())
+	}
+	if r.Model != "mistral-large-latest" {
+		t.Errorf("Model = %q", r.Model)
+	}
+	if r.Message.TextContent() != "done" {
+		t.Errorf("Message.TextContent() = %q", r.Message.TextContent())
+	}
+}
diff --git a/internal/message/response.go b/internal/message/response.go
new file mode 100644
index 0000000..bb3097c
--- /dev/null
+++ b/internal/message/response.go
@@ -0,0 +1,9 @@
+package message
+
+// Response wraps a completed assistant turn.
+type Response struct {
+	Message    Message
+	StopReason StopReason
+	Usage      Usage
+	Model      string
+}
diff --git a/internal/message/stop.go b/internal/message/stop.go
new file mode 100644
index 0000000..d1624fd
--- /dev/null
+++ b/internal/message/stop.go
@@ -0,0 +1,11 @@
+package message
+
+// StopReason indicates why the model stopped generating.
+type StopReason string
+
+const (
+	StopEndTurn   StopReason = "end_turn"
+	StopMaxTokens StopReason = "max_tokens"
+	StopToolUse   StopReason = "tool_use"
+	StopSequence  StopReason = "stop_sequence"
+)
diff --git a/internal/message/usage.go b/internal/message/usage.go
new file mode 100644
index 0000000..affdc6f
--- /dev/null
+++ b/internal/message/usage.go
@@ -0,0 +1,20 @@
+package message
+
+// Usage tracks token consumption for a single API turn.
+type Usage struct {
+	InputTokens         int64 `json:"input_tokens"`
+	OutputTokens        int64 `json:"output_tokens"`
+	CacheReadTokens     int64 `json:"cache_read_tokens,omitempty"`
+	CacheCreationTokens int64 `json:"cache_creation_tokens,omitempty"`
+}
+
+func (u Usage) TotalTokens() int64 {
+	return u.InputTokens + u.OutputTokens
+}
+
+func (u *Usage) Add(other Usage) {
+	u.InputTokens += other.InputTokens
+	u.OutputTokens += other.OutputTokens
+	u.CacheReadTokens += other.CacheReadTokens
+	u.CacheCreationTokens += other.CacheCreationTokens
+}
diff --git a/internal/message/usage_test.go b/internal/message/usage_test.go
new file mode 100644
index 0000000..01f58b2
--- /dev/null
+++ b/internal/message/usage_test.go
@@ -0,0 +1,70 @@
+package message
+
+import "testing"
+
+func TestUsage_TotalTokens(t *testing.T) {
+	u := Usage{InputTokens: 100, OutputTokens: 50}
+	if got := u.TotalTokens(); got != 150 {
+		t.Errorf("TotalTokens() = %d, want 150", got)
+	}
+}
+
+func TestUsage_TotalTokens_Zero(t *testing.T) {
+	var u Usage
+	if got := u.TotalTokens(); got != 0 {
+		t.Errorf("TotalTokens() = %d, want 0", got)
+	}
+}
+
+func TestUsage_Add(t *testing.T) {
+	u := Usage{
+		InputTokens:         100,
+		OutputTokens:        50,
+		CacheReadTokens:     10,
+		CacheCreationTokens: 5,
+	}
+	other := Usage{
+		InputTokens:         200,
+		OutputTokens:        80,
+		CacheReadTokens:     20,
+		CacheCreationTokens: 15,
+	}
+
+	u.Add(other)
+
+	if u.InputTokens != 300 {
+		t.Errorf("InputTokens = %d, want 300", u.InputTokens)
+	}
+	if u.OutputTokens != 130 {
+		t.Errorf("OutputTokens = %d, want 130", u.OutputTokens)
+	}
+	if u.CacheReadTokens != 30 {
+		t.Errorf("CacheReadTokens = %d, want 30", u.CacheReadTokens)
+	}
+	if u.CacheCreationTokens != 20 {
+		t.Errorf("CacheCreationTokens = %d, want 20", u.CacheCreationTokens)
+	}
+}
+
+func TestUsage_Add_Multiple(t *testing.T) {
+	var total Usage
+	turns := []Usage{
+		{InputTokens: 100, OutputTokens: 50},
+		{InputTokens: 200, OutputTokens: 80},
+		{InputTokens: 150, OutputTokens: 60},
+	}
+
+	for _, turn := range turns {
+		total.Add(turn)
+	}
+
+	if total.InputTokens != 450 {
+		t.Errorf("InputTokens = %d, want 450", total.InputTokens)
+	}
+	if total.OutputTokens != 190 {
+		t.Errorf("OutputTokens = %d, want 190", total.OutputTokens)
+	}
+	if total.TotalTokens() != 640 {
+		t.Errorf("TotalTokens() = %d, want 640", total.TotalTokens())
+	}
+}
diff --git a/internal/provider/errors.go b/internal/provider/errors.go
new file mode 100644
index 0000000..4fb6b3c
--- /dev/null
+++ b/internal/provider/errors.go
@@ -0,0 +1,79 @@
+package provider
+
+import (
+	"fmt"
+	"time"
+)
+
+// ErrorKind classifies provider errors for retry decisions.
+type ErrorKind int
+
+const (
+	ErrTransient  ErrorKind = iota + 1 // 429, 500, 502, 503, 529 — retry with backoff
+	ErrAuth                            // 401, 403 — don't retry
+	ErrBadRequest                      // 400 — don't retry, fix request
+	ErrNotFound                        // 404 — model/endpoint not found
+	ErrOverloaded                      // capacity exhausted — backoff + retry
+)
+
+func (k ErrorKind) String() string {
+	switch k {
+	case ErrTransient:
+		return "transient"
+	case ErrAuth:
+		return "auth"
+	case ErrBadRequest:
+		return "bad_request"
+	case ErrNotFound:
+		return "not_found"
+	case ErrOverloaded:
+		return "overloaded"
+	default:
+		return fmt.Sprintf("unknown(%d)", k)
+	}
+}
+
+// ProviderError wraps an SDK error with classification metadata.
+type ProviderError struct {
+	Kind       ErrorKind
+	Provider   string
+	StatusCode int
+	Message    string
+	Retryable  bool
+	RetryAfter time.Duration // from Retry-After or rate limit headers
+	Err        error         // underlying SDK error
+}
+
+func (e *ProviderError) Error() string {
+	if e.Err != nil {
+		return fmt.Sprintf("%s %s (%d): %s: %v", e.Provider, e.Kind, e.StatusCode, e.Message, e.Err)
+	}
+	return fmt.Sprintf("%s %s (%d): %s", e.Provider, e.Kind, e.StatusCode, e.Message)
+}
+
+func (e *ProviderError) Unwrap() error {
+	return e.Err
+}
+
+// ClassifyHTTPStatus returns the ErrorKind and retryability for an HTTP status code.
+func ClassifyHTTPStatus(status int) (ErrorKind, bool) {
+	switch {
+	case status == 401 || status == 403:
+		return ErrAuth, false
+	case status == 400:
+		return ErrBadRequest, false
+	case status == 404:
+		return ErrNotFound, false
+	case status == 429 || status == 529:
+		return ErrTransient, true
+	case status == 500 || status == 502 || status == 503:
+		return ErrTransient, true
+	case status == 504:
+		return ErrOverloaded, true
+	default:
+		if status >= 500 {
+			return ErrTransient, true
+		}
+		return ErrBadRequest, false
+	}
+}
diff --git a/internal/provider/errors_test.go b/internal/provider/errors_test.go
new file mode 100644
index 0000000..07f7c2c
--- /dev/null
+++ b/internal/provider/errors_test.go
@@ -0,0 +1,118 @@
+package provider
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestProviderError_Error(t *testing.T) {
+	err := &ProviderError{
+		Kind:       ErrTransient,
+		Provider:   "mistral",
+		StatusCode: 429,
+		Message:    "rate limited",
+	}
+	got := err.Error()
+	want := "mistral transient (429): rate limited"
+	if got != want {
+		t.Errorf("Error() = %q, want %q", got, want)
+	}
+}
+
+func TestProviderError_Error_WithWrapped(t *testing.T) {
+	inner := errors.New("connection reset")
+	err := &ProviderError{
+		Kind:       ErrTransient,
+		Provider:   "openai",
+		StatusCode: 502,
+		Message:    "bad gateway",
+		Err:        inner,
+	}
+	got := err.Error()
+	want := "openai transient (502): bad gateway: connection reset"
+	if got != want {
+		t.Errorf("Error() = %q, want %q", got, want)
+	}
+}
+
+func TestProviderError_Unwrap(t *testing.T) {
+	inner := errors.New("timeout")
+	err := &ProviderError{
+		Kind: ErrTransient,
+		Err:  inner,
+	}
+	if !errors.Is(err, inner) {
+		t.Error("errors.Is should find inner error")
+	}
+}
+
+func TestProviderError_AsType(t *testing.T) {
+	inner := &ProviderError{
+		Kind:       ErrAuth,
+		Provider:   "anthropic",
+		StatusCode: 401,
+		Message:    "invalid key",
+	}
+	wrapped := fmt.Errorf("api call failed: %w", inner)
+
+	pErr, ok := errors.AsType[*ProviderError](wrapped)
+	if !ok {
+		t.Fatal("errors.AsType should find ProviderError")
+	}
+	if pErr.Kind != ErrAuth {
+		t.Errorf("Kind = %v, want %v", pErr.Kind, ErrAuth)
+	}
+	if pErr.Provider != "anthropic" {
+		t.Errorf("Provider = %q", pErr.Provider)
+	}
+}
+
+func TestClassifyHTTPStatus(t *testing.T) {
+	tests := []struct {
+		status    int
+		wantKind  ErrorKind
+		wantRetry bool
+	}{
+		{200, ErrBadRequest, false}, // shouldn't happen, but safe default
+		{400, ErrBadRequest, false},
+		{401, ErrAuth, false},
+		{403, ErrAuth, false},
+		{404, ErrNotFound, false},
+		{429, ErrTransient, true},
+		{500, ErrTransient, true},
+		{502, ErrTransient, true},
+		{503, ErrTransient, true},
+		{504, ErrOverloaded, true},
+		{529, ErrTransient, true},
+		{599, ErrTransient, true}, // unknown 5xx
+	}
+	for _, tt := range tests {
+		kind, retry := ClassifyHTTPStatus(tt.status)
+		if kind != tt.wantKind {
+			t.Errorf("ClassifyHTTPStatus(%d) kind = %v, want %v", tt.status, kind, tt.wantKind)
+		}
+		if retry != tt.wantRetry {
+			t.Errorf("ClassifyHTTPStatus(%d) retry = %v, want %v", tt.status, retry, tt.wantRetry)
+		}
+	}
+}
+
+func TestErrorKind_String(t *testing.T) {
+	tests := []struct {
+		kind ErrorKind
+		want string
+	}{
+		{ErrTransient, "transient"},
+		{ErrAuth, "auth"},
+		{ErrBadRequest, "bad_request"},
+		{ErrNotFound, "not_found"},
+		{ErrOverloaded, "overloaded"},
+		{ErrorKind(99), "unknown(99)"},
+	}
+	for _, tt := range tests {
+		if got := tt.kind.String(); got != tt.want {
+			t.Errorf("ErrorKind(%d).String() = %q, want %q", tt.kind, got, tt.want)
+		}
+	}
+}
diff --git a/internal/provider/provider.go b/internal/provider/provider.go
new file mode 100644
index 0000000..4dec0fd
--- /dev/null
+++ b/internal/provider/provider.go
@@ -0,0 +1,44 @@
+package provider
+
+import (
+	"context"
+	"encoding/json"
+
+	"somegit.dev/Owlibou/gnoma/internal/message"
+	"somegit.dev/Owlibou/gnoma/internal/stream"
+)
+
+// Request encapsulates everything needed for a single LLM API call.
+type Request struct {
+	Model         string
+	SystemPrompt  string
+	Messages      []message.Message
+	Tools         []ToolDefinition
+	MaxTokens     int64
+	Temperature   *float64
+	TopP          *float64
+	TopK          *int64
+	StopSequences []string
+	Thinking      *ThinkingConfig
+}
+
+// ToolDefinition is the provider-agnostic tool schema.
+type ToolDefinition struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description"`
+	Parameters  json.RawMessage `json:"parameters"` // JSON Schema passthrough
+}
+
+// ThinkingConfig controls extended thinking / reasoning.
+type ThinkingConfig struct {
+	BudgetTokens int64
+}
+
+// Provider is the core abstraction over all LLM backends.
+type Provider interface {
+	// Stream initiates a streaming request and returns an event stream.
+	Stream(ctx context.Context, req Request) (stream.Stream, error)
+
+	// Name returns the provider identifier (e.g., "mistral", "anthropic").
+	Name() string
+}
diff --git a/internal/provider/registry.go b/internal/provider/registry.go
new file mode 100644
index 0000000..b1e5254
--- /dev/null
+++ b/internal/provider/registry.go
@@ -0,0 +1,69 @@
+package provider
+
+import (
+	"fmt"
+	"sync"
+)
+
+// ProviderConfig is the common configuration for any provider.
+type ProviderConfig struct {
+	Name    string
+	APIKey  string
+	BaseURL string         // override for OpenAI-compat endpoints
+	Model   string         // default model for this provider
+	Options map[string]any // provider-specific options
+}
+
+// Factory creates a Provider from configuration.
+type Factory func(cfg ProviderConfig) (Provider, error)
+
+// Registry maps provider names to factory functions.
+type Registry struct {
+	mu        sync.RWMutex
+	factories map[string]Factory
+}
+
+func NewRegistry() *Registry {
+	return &Registry{
+		factories: make(map[string]Factory),
+	}
+}
+
+// Register adds a provider factory. Overwrites if name already exists.
+func (r *Registry) Register(name string, f Factory) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.factories[name] = f
+}
+
+// Create instantiates a provider by name with the given config.
+func (r *Registry) Create(name string, cfg ProviderConfig) (Provider, error) {
+	r.mu.RLock()
+	f, ok := r.factories[name]
+	r.mu.RUnlock()
+
+	if !ok {
+		return nil, fmt.Errorf("unknown provider: %q", name)
+	}
+	cfg.Name = name
+	return f(cfg)
+}
+
+// Has returns true if a factory is registered for the given name.
+func (r *Registry) Has(name string) bool {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	_, ok := r.factories[name]
+	return ok
+}
+
+// Names returns all registered provider names.
+func (r *Registry) Names() []string {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	names := make([]string, 0, len(r.factories))
+	for name := range r.factories {
+		names = append(names, name)
+	}
+	return names
+}
diff --git a/internal/provider/registry_test.go b/internal/provider/registry_test.go
new file mode 100644
index 0000000..525194d
--- /dev/null
+++ b/internal/provider/registry_test.go
@@ -0,0 +1,131 @@
+package provider
+
+import (
+	"context"
+	"errors"
+	"slices"
+	"sort"
+	"testing"
+
+	"somegit.dev/Owlibou/gnoma/internal/stream"
+)
+
+// mockProvider implements Provider for testing.
+type mockProvider struct {
+	name string
+}
+
+func (m *mockProvider) Stream(_ context.Context, _ Request) (stream.Stream, error) {
+	return nil, nil
+}
+
+func (m *mockProvider) Name() string {
+	return m.name
+}
+
+func TestRegistry_RegisterAndCreate(t *testing.T) {
+	r := NewRegistry()
+
+	r.Register("mock", func(cfg ProviderConfig) (Provider, error) {
+		return &mockProvider{name: cfg.Name}, nil
+	})
+
+	p, err := r.Create("mock", ProviderConfig{})
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if p.Name() != "mock" {
+		t.Errorf("Name() = %q, want %q", p.Name(), "mock")
+	}
+}
+
+func TestRegistry_Create_Unknown(t *testing.T) {
+	r := NewRegistry()
+
+	_, err := r.Create("nonexistent", ProviderConfig{})
+	if err == nil {
+		t.Fatal("expected error for unknown provider")
+	}
+	want := `unknown provider: "nonexistent"`
+	if err.Error() != want {
+		t.Errorf("error = %q, want %q", err.Error(), want)
+	}
+}
+
+func TestRegistry_Create_FactoryError(t *testing.T) {
+	r := NewRegistry()
+	r.Register("broken", func(cfg ProviderConfig) (Provider, error) {
+		return nil, errors.New("missing api key")
+	})
+
+	_, err := r.Create("broken", ProviderConfig{})
+	if err == nil {
+		t.Fatal("expected error from factory")
+	}
+	if err.Error() != "missing api key" {
+		t.Errorf("error = %q", err.Error())
+	}
+}
+
+func TestRegistry_Create_SetsName(t *testing.T) {
+	r := NewRegistry()
+
+	var receivedName string
+	r.Register("test", func(cfg ProviderConfig) (Provider, error) {
+		receivedName = cfg.Name
+		return &mockProvider{name: cfg.Name}, nil
+	})
+
+	_, _ = r.Create("test", ProviderConfig{APIKey: "sk-123"})
+	if receivedName != "test" {
+		t.Errorf("factory received Name = %q, want %q", receivedName, "test")
+	}
+}
+
+func TestRegistry_Has(t *testing.T) {
+	r := NewRegistry()
+	r.Register("exists", func(cfg ProviderConfig) (Provider, error) {
+		return nil, nil
+	})
+
+	if !r.Has("exists") {
+		t.Error("Has(exists) = false, want true")
+	}
+	if r.Has("nope") {
+		t.Error("Has(nope) = true, want false")
+	}
+}
+
+func TestRegistry_Names(t *testing.T) {
+	r := NewRegistry()
+	r.Register("alpha", func(cfg ProviderConfig) (Provider, error) { return nil, nil })
+	r.Register("beta", func(cfg ProviderConfig) (Provider, error) { return nil, nil })
+	r.Register("gamma", func(cfg ProviderConfig) (Provider, error) { return nil, nil })
+
+	names := r.Names()
+	sort.Strings(names)
+
+	want := []string{"alpha", "beta", "gamma"}
+	if !slices.Equal(names, want) {
+		t.Errorf("Names() = %v, want %v", names, want)
+	}
+}
+
+func TestRegistry_Register_Overwrite(t *testing.T) {
+	r := NewRegistry()
+
+	r.Register("dup", func(cfg ProviderConfig) (Provider, error) {
+		return &mockProvider{name: "old"}, nil
+	})
+	r.Register("dup", func(cfg ProviderConfig) (Provider, error) {
+		return &mockProvider{name: "new"}, nil
+	})
+
+	p, err := r.Create("dup", ProviderConfig{})
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if p.Name() != "new" {
+		t.Errorf("Name() = %q, want %q (overwritten factory)", p.Name(), "new")
+	}
+}
diff --git a/internal/stream/accumulator.go b/internal/stream/accumulator.go
new file mode 100644
index 0000000..155926c
--- /dev/null
+++ b/internal/stream/accumulator.go
@@ -0,0 +1,143 @@
+package stream
+
+import (
+	"strings"
+
+	"somegit.dev/Owlibou/gnoma/internal/message"
+)
+
+// Accumulator assembles a message.Response from a sequence of Events.
+// Provider adapters translate SDK events into stream.Events;
+// the Accumulator — shared, tested once — builds the final Response.
+type Accumulator struct {
+	content []message.Content
+	usage   message.Usage
+
+	// Active text block being built
+	textBuf *strings.Builder
+
+	// Active thinking block being built
+	thinkBuf *strings.Builder
+
+	// Tool calls in progress, keyed by ToolCallID
+	toolCalls map[string]*toolCallAccum
+	// Ordered tool call IDs to preserve emission order
+	toolCallOrder []string
+}
+
+type toolCallAccum struct {
+	id      string
+	name    string
+	argsBuf strings.Builder
+	args    []byte // final complete args (from Done event)
+}
+
+func NewAccumulator() *Accumulator {
+	return &Accumulator{
+		toolCalls: make(map[string]*toolCallAccum),
+	}
+}
+
+// Apply processes a single event, updating the accumulator's state.
+func (a *Accumulator) Apply(e Event) {
+	switch e.Type {
+	case EventTextDelta:
+		a.flushThinking()
+		if a.textBuf == nil {
+			a.textBuf = &strings.Builder{}
+		}
+		a.textBuf.WriteString(e.Text)
+
+	case EventThinkingDelta:
+		a.flushText()
+		if a.thinkBuf == nil {
+			a.thinkBuf = &strings.Builder{}
+		}
+		a.thinkBuf.WriteString(e.Text)
+
+	case EventToolCallStart:
+		a.flushText()
+		a.flushThinking()
+		tc := &toolCallAccum{id: e.ToolCallID, name: e.ToolCallName}
+		a.toolCalls[e.ToolCallID] = tc
+		a.toolCallOrder = append(a.toolCallOrder, e.ToolCallID)
+
+	case EventToolCallDelta:
+		if tc, ok := a.toolCalls[e.ToolCallID]; ok {
+			tc.argsBuf.WriteString(e.ArgDelta)
+		}
+
+	case EventToolCallDone:
+		if tc, ok := a.toolCalls[e.ToolCallID]; ok {
+			if e.Args != nil {
+				// Done event carries authoritative complete args
+				tc.args = e.Args
+			} else {
+				// Fall back to accumulated deltas
+				tc.args = []byte(tc.argsBuf.String())
+			}
+		}
+
+	case EventUsage:
+		if e.Usage != nil {
+			a.usage.Add(*e.Usage)
+		}
+
+	case EventError:
+		// Errors are handled by the stream consumer, not accumulated
+	}
+}
+
+// Response builds the final message.Response from all accumulated events.
+func (a *Accumulator) Response(stopReason message.StopReason, model string) message.Response {
+	a.flushText()
+	a.flushThinking()
+	a.flushToolCalls()
+
+	return message.Response{
+		Message: message.Message{
+			Role:    message.RoleAssistant,
+			Content: a.content,
+		},
+		StopReason: stopReason,
+		Usage:      a.usage,
+		Model:      model,
+	}
+}
+
+func (a *Accumulator) flushText() {
+	if a.textBuf != nil && a.textBuf.Len() > 0 {
+		a.content = append(a.content, message.NewTextContent(a.textBuf.String()))
+		a.textBuf = nil
+	}
+}
+
+func (a *Accumulator) flushThinking() {
+	if a.thinkBuf != nil && a.thinkBuf.Len() > 0 {
+		a.content = append(a.content, message.NewThinkingContent(message.Thinking{
+			Text: a.thinkBuf.String(),
+		}))
+		a.thinkBuf = nil
+	}
+}
+
+func (a *Accumulator) flushToolCalls() {
+	for _, id := range a.toolCallOrder {
+		tc, ok := a.toolCalls[id]
+		if !ok {
+			continue
+		}
+		args := tc.args
+		if args == nil {
+			// Fallback: use accumulated deltas even if Done never arrived
+			args = []byte(tc.argsBuf.String())
+		}
+		a.content = append(a.content, message.NewToolCallContent(message.ToolCall{
+			ID:        tc.id,
+			Name:      tc.name,
+			Arguments: args,
+		}))
+	}
+	a.toolCalls = make(map[string]*toolCallAccum)
+	a.toolCallOrder = nil
+}
diff --git a/internal/stream/accumulator_test.go b/internal/stream/accumulator_test.go
new file mode 100644
index 0000000..423b5b5
--- /dev/null
+++ b/internal/stream/accumulator_test.go
@@ -0,0 +1,225 @@
+package stream
+
+import (
+	"encoding/json"
+	"testing"
+
+	"somegit.dev/Owlibou/gnoma/internal/message"
+)
+
+func TestAccumulator_TextOnly(t *testing.T) {
+	acc := NewAccumulator()
+
+	acc.Apply(Event{Type: EventTextDelta, Text: "Hello "})
+	acc.Apply(Event{Type: EventTextDelta, Text: "world!"})
+	acc.Apply(Event{Type: EventUsage, Usage: &message.Usage{InputTokens: 10, OutputTokens: 5}})
+
+	resp := acc.Response(message.StopEndTurn, "mistral-large")
+
+	if resp.StopReason != message.StopEndTurn {
+		t.Errorf("StopReason = %q, want %q", resp.StopReason, message.StopEndTurn)
+	}
+	if resp.Model != "mistral-large" {
+		t.Errorf("Model = %q, want %q", resp.Model, "mistral-large")
+	}
+	if resp.Message.Role != message.RoleAssistant {
+		t.Errorf("Role = %q, want %q", resp.Message.Role, message.RoleAssistant)
+	}
+	if resp.Message.TextContent() != "Hello world!" {
+		t.Errorf("TextContent() = %q, want %q", resp.Message.TextContent(), "Hello world!")
+	}
+	if resp.Usage.InputTokens != 10 {
+		t.Errorf("Usage.InputTokens = %d, want 10", resp.Usage.InputTokens)
+	}
+}
+
+func TestAccumulator_SingleToolCall(t *testing.T) {
+	acc := NewAccumulator()
+
+	acc.Apply(Event{Type: EventTextDelta, Text: "I'll run that."})
+	acc.Apply(Event{
+		Type:         EventToolCallStart,
+		ToolCallID:   "tc_1",
+		ToolCallName: "bash",
+	})
+	acc.Apply(Event{
+		Type:       EventToolCallDelta,
+		ToolCallID: "tc_1",
+		ArgDelta:   `{"comma`,
+	})
+	acc.Apply(Event{
+		Type:       EventToolCallDelta,
+		ToolCallID: "tc_1",
+		ArgDelta:   `nd":"ls"}`,
+	})
+	acc.Apply(Event{
+		Type:       EventToolCallDone,
+		ToolCallID: "tc_1",
+		Args:       json.RawMessage(`{"command":"ls"}`),
+	})
+
+	resp := acc.Response(message.StopToolUse, "mistral-large")
+
+	if resp.StopReason != message.StopToolUse {
+		t.Errorf("StopReason = %q, want %q", resp.StopReason, message.StopToolUse)
+	}
+
+	content := resp.Message.Content
+	if len(content) != 2 {
+		t.Fatalf("len(Content) = %d, want 2", len(content))
+	}
+
+	// First block: text
+	if content[0].Type != message.ContentText {
+		t.Errorf("Content[0].Type = %v, want text", content[0].Type)
+	}
+	if content[0].Text != "I'll run that." {
+		t.Errorf("Content[0].Text = %q", content[0].Text)
+	}
+
+	// Second block: tool call
+	if content[1].Type != message.ContentToolCall {
+		t.Errorf("Content[1].Type = %v, want tool_call", content[1].Type)
+	}
+	tc := content[1].ToolCall
+	if tc.ID != "tc_1" {
+		t.Errorf("ToolCall.ID = %q, want tc_1", tc.ID)
+	}
+	if tc.Name != "bash" {
+		t.Errorf("ToolCall.Name = %q, want bash", tc.Name)
+	}
+	if string(tc.Arguments) != `{"command":"ls"}` {
+		t.Errorf("ToolCall.Arguments = %s", tc.Arguments)
+	}
+}
+
+func TestAccumulator_MultipleToolCalls(t *testing.T) {
+	acc := NewAccumulator()
+
+	// Tool call 1
+	acc.Apply(Event{Type: EventToolCallStart, ToolCallID: "tc_1", ToolCallName: "bash"})
+	acc.Apply(Event{Type: EventToolCallDone, ToolCallID: "tc_1", Args: json.RawMessage(`{"command":"ls"}`)})
+
+	// Tool call 2
+	acc.Apply(Event{Type: EventToolCallStart, ToolCallID: "tc_2", ToolCallName: "fs.read"})
+	acc.Apply(Event{Type: EventToolCallDone, ToolCallID: "tc_2", Args: json.RawMessage(`{"path":"go.mod"}`)})
+
+	resp := acc.Response(message.StopToolUse, "")
+	calls := resp.Message.ToolCalls()
+	if len(calls) != 2 {
+		t.Fatalf("len(ToolCalls()) = %d, want 2", len(calls))
+	}
+	if calls[0].Name != "bash" {
+		t.Errorf("calls[0].Name = %q", calls[0].Name)
+	}
+	if calls[1].Name != "fs.read" {
+		t.Errorf("calls[1].Name = %q", calls[1].Name)
+	}
+}
+
+func TestAccumulator_ThinkingBlocks(t *testing.T) {
+	acc := NewAccumulator()
+
+	acc.Apply(Event{Type: EventThinkingDelta, Text: "Let me think"})
+	acc.Apply(Event{Type: EventThinkingDelta, Text: " about this..."})
+	acc.Apply(Event{Type: EventTextDelta, Text: "Here's my answer."})
+
+	resp := acc.Response(message.StopEndTurn, "")
+	content := resp.Message.Content
+
+	if len(content) != 2 {
+		t.Fatalf("len(Content) = %d, want 2", len(content))
+	}
+
+	// Thinking block first
+	if content[0].Type != message.ContentThinking {
+		t.Errorf("Content[0].Type = %v, want thinking", content[0].Type)
+	}
+	if content[0].Thinking.Text != "Let me think about this..." {
+		t.Errorf("Thinking.Text = %q", content[0].Thinking.Text)
+	}
+
+	// Then text
+	if content[1].Type != message.ContentText {
+		t.Errorf("Content[1].Type = %v, want text", content[1].Type)
+	}
+}
+
+func TestAccumulator_ToolCallDelta_Assembly(t *testing.T) {
+	acc := NewAccumulator()
+
+	acc.Apply(Event{Type: EventToolCallStart, ToolCallID: "tc_1", ToolCallName: "bash"})
+	// Simulate fragmented JSON
+	acc.Apply(Event{Type: EventToolCallDelta, ToolCallID: "tc_1", ArgDelta: `{"`})
+	acc.Apply(Event{Type: EventToolCallDelta, ToolCallID: "tc_1", ArgDelta: `comman`})
+	acc.Apply(Event{Type: EventToolCallDelta, ToolCallID: "tc_1", ArgDelta: `d":"`})
+	acc.Apply(Event{Type: EventToolCallDelta, ToolCallID: "tc_1", ArgDelta: `echo hi`})
+	acc.Apply(Event{Type: EventToolCallDelta, ToolCallID: "tc_1", ArgDelta: `"}`})
+
+	// Done event carries the complete args (authoritative)
+	acc.Apply(Event{
+		Type:       EventToolCallDone,
+		ToolCallID: "tc_1",
+		Args:       json.RawMessage(`{"command":"echo hi"}`),
+	})
+
+	resp := acc.Response(message.StopToolUse, "")
+	calls := resp.Message.ToolCalls()
+	if len(calls) != 1 {
+		t.Fatalf("len(ToolCalls()) = %d, want 1", len(calls))
+	}
+	if string(calls[0].Arguments) != `{"command":"echo hi"}` {
+		t.Errorf("Arguments = %s", calls[0].Arguments)
+	}
+}
+
+func TestAccumulator_EmptyStream(t *testing.T) {
+	acc := NewAccumulator()
+	resp := acc.Response(message.StopEndTurn, "model-x")
+
+	if resp.Model != "model-x" {
+		t.Errorf("Model = %q", resp.Model)
+	}
+	if len(resp.Message.Content) != 0 {
+		t.Errorf("len(Content) = %d, want 0", len(resp.Message.Content))
+	}
+}
+
+func TestAccumulator_UsageCumulative(t *testing.T) {
+	acc := NewAccumulator()
+
+	acc.Apply(Event{Type: EventUsage, Usage: &message.Usage{InputTokens: 100, OutputTokens: 30}})
+	acc.Apply(Event{Type: EventTextDelta, Text: "hi"})
+	acc.Apply(Event{Type: EventUsage, Usage: &message.Usage{InputTokens: 0, OutputTokens: 20}})
+
+	resp := acc.Response(message.StopEndTurn, "")
+	// Last usage wins (providers typically send cumulative, not incremental)
+	// But our Add() is additive — so 100+0=100 input, 30+20=50 output
+	if resp.Usage.InputTokens != 100 {
+		t.Errorf("InputTokens = %d, want 100", resp.Usage.InputTokens)
+	}
+	if resp.Usage.OutputTokens != 50 {
+		t.Errorf("OutputTokens = %d, want 50", resp.Usage.OutputTokens)
+	}
+}
+
+func TestEventType_String(t *testing.T) {
+	tests := []struct {
+		et   EventType
+		want string
+	}{
+		{EventTextDelta, "text_delta"},
+		{EventThinkingDelta, "thinking_delta"},
+		{EventToolCallStart, "tool_call_start"},
+		{EventToolCallDelta, "tool_call_delta"},
+		{EventToolCallDone, "tool_call_done"},
+		{EventUsage, "usage"},
+		{EventError, "error"},
+		{EventType(99), "unknown(99)"},
+	}
+	for _, tt := range tests {
+		if got := tt.et.String(); got != tt.want {
+			t.Errorf("EventType(%d).String() = %q, want %q", tt.et, got, tt.want)
+		}
+	}
+}
diff --git a/internal/stream/event.go b/internal/stream/event.go
new file mode 100644
index 0000000..83609ad
--- /dev/null
+++ b/internal/stream/event.go
@@ -0,0 +1,70 @@
+package stream
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"somegit.dev/Owlibou/gnoma/internal/message"
+)
+
+// EventType discriminates streaming events.
+type EventType int
+
+const (
+	EventTextDelta     EventType = iota + 1
+	EventThinkingDelta
+	EventToolCallStart
+	EventToolCallDelta
+	EventToolCallDone
+	EventUsage
+	EventError
+)
+
+func (et EventType) String() string {
+	switch et {
+	case EventTextDelta:
+		return "text_delta"
+	case EventThinkingDelta:
+		return "thinking_delta"
+	case EventToolCallStart:
+		return "tool_call_start"
+	case EventToolCallDelta:
+		return "tool_call_delta"
+	case EventToolCallDone:
+		return "tool_call_done"
+	case EventUsage:
+		return "usage"
+	case EventError:
+		return "error"
+	default:
+		return fmt.Sprintf("unknown(%d)", et)
+	}
+}
+
+// Event is a single streaming event from a provider.
+type Event struct {
+	Type EventType
+
+	// TextDelta, ThinkingDelta
+	Text string
+
+	// ToolCallStart: ID + Name set
+	// ToolCallDelta: ID + ArgDelta set
+	// ToolCallDone:  ID + Args set (complete JSON)
+	ToolCallID   string
+	ToolCallName string
+	ArgDelta     string          // partial JSON fragment
+	Args         json.RawMessage // complete arguments (on Done)
+
+	// Usage
+	Usage *message.Usage
+
+	// Error
+	Err error
+
+	// StopReason — set on the final event of a stream
+	StopReason message.StopReason
+
+	// Model — set on first event if available
+	Model string
+}
diff --git a/internal/stream/stream.go b/internal/stream/stream.go
new file mode 100644
index 0000000..7411245
--- /dev/null
+++ b/internal/stream/stream.go
@@ -0,0 +1,25 @@
+package stream
+
+// Stream is the unified pull-based iterator over provider events.
+// All provider implementations produce this interface.
+//
+// Usage:
+//
+//	for s.Next() {
+//	    event := s.Current()
+//	    process(event)
+//	}
+//	if err := s.Err(); err != nil {
+//	    handle(err)
+//	}
+//	s.Close()
+type Stream interface {
+	// Next advances to the next event. Returns false when exhausted or errored.
+	Next() bool
+	// Current returns the most recent event. Only valid after Next() returns true.
+	Current() Event
+	// Err returns the first error encountered, or nil.
+	Err() error
+	// Close releases resources. Must be called after consumption.
+	Close() error
+}