diff --git a/github/client.go b/github/client.go
new file mode 100644
index 0000000..2dc27ca
--- /dev/null
+++ b/github/client.go
@@ -0,0 +1,260 @@
+// Package github provides a client for the GitHub API.
+// It supports pull request operations, file content retrieval,
+// and review submission for both github.com and GitHub Enterprise.
+package github
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	defaultBaseURL = "https://api.github.com"
+
+	// maxRetryAttempts is the number of times doRequest will attempt a request.
+	maxRetryAttempts = 3
+
+	// maxRetryAfter caps the maximum delay from a Retry-After header to prevent
+	// a server from stalling the client indefinitely.
+	maxRetryAfter = 60 * time.Second
+
+	// maxErrorBodyBytes limits how much of an error response body we read
+	// to protect against malicious servers sending unbounded data.
+	maxErrorBodyBytes = 64 * 1024 // 64 KB
+
+	// maxResponseBodyBytes limits how much of a successful response body we read
+	// for defense-in-depth against servers returning excessively large payloads.
+	maxResponseBodyBytes = 10 * 1024 * 1024 // 10 MB
+)
+
+// APIError represents an HTTP error response from the GitHub API.
+// It carries the status code so callers can distinguish between
+// different failure modes (e.g. 404 vs 500).
+//
+// The Body field stores up to 64 KiB of the raw response for programmatic
+// inspection. Error() truncates to 200 bytes for safe logging, but callers
+// should avoid logging or propagating Body directly in production since it may
+// contain sensitive details from the upstream server.
+type APIError struct {
+	StatusCode int
+	Body       string
+}
+
+func (e *APIError) Error() string {
+	body := e.Body
+	if len(body) > 200 {
+		body = body[:200] + "...(truncated)"
+	}
+	// Sanitize newlines to prevent log injection from upstream response bodies.
+	body = strings.ReplaceAll(body, "\n", " ")
+	body = strings.ReplaceAll(body, "\r", " ")
+	return fmt.Sprintf("HTTP %d: %s", e.StatusCode, body)
+}
+
+// IsNotFound reports whether an error is an API 404 response.
+func IsNotFound(err error) bool {
+	if apiErr, ok := asAPIError(err); ok {
+		return apiErr.StatusCode == http.StatusNotFound
+	}
+	return false
+}
+
+// IsUnauthorized reports whether an error is an API 401 response.
+func IsUnauthorized(err error) bool {
+	if apiErr, ok := asAPIError(err); ok {
+		return apiErr.StatusCode == http.StatusUnauthorized
+	}
+	return false
+}
+
+func asAPIError(err error) (*APIError, bool) {
+	if err == nil {
+		return nil, false
+	}
+	var target *APIError
+	if errors.As(err, &target) {
+		return target, true
+	}
+	return nil, false
+}
+
+// Client interacts with the GitHub API.
+// A Client is safe for concurrent use by multiple goroutines.
+// SetHTTPClient and SetRetryBackoff are intended for test setup only and must
+// be called before any goroutines issue requests; they have no synchronization.
+type Client struct {
+	// TODO: baseURL is populated by NewClient but not yet consumed by doRequest/doGet.
+	// Higher-level exported methods (GetPullRequest, etc.) will use it to
+	// construct request URLs; remove this field if those methods end up
+	// accepting full URLs instead.
+	baseURL string
+	token   string
+	httpClient *http.Client
+
+	// retryBackoff defines the delays between retry attempts for 429 responses.
+	// retryBackoff[i] is the delay before attempt i+1 (after attempt i fails).
+	// If nil, defaults to {1s, 2s}.
+	retryBackoff []time.Duration
+
+	// now returns the current time. Defaults to time.Now.
+	// Override in tests to control HTTP-date Retry-After calculations.
+	now func() time.Time
+}
+
+// NewClient creates a new GitHub API client.
+// If baseURL is empty, it defaults to https://api.github.com.
+// For GitHub Enterprise, pass the API base URL (e.g. https://github.concur.com/api/v3).
+func NewClient(token, baseURL string) *Client {
+	if baseURL == "" {
+		baseURL = defaultBaseURL
+	}
+	return &Client{
+		baseURL: strings.TrimRight(baseURL, "/"),
+		token:   token,
+		httpClient: &http.Client{Timeout: 30 * time.Second},
+		now:     time.Now,
+	}
+}
+
+// SetHTTPClient sets the underlying HTTP client used for requests.
+// This is intended for testing to inject mock transports.
+func (c *Client) SetHTTPClient(hc *http.Client) {
+	c.httpClient = hc
+}
+
+// SetRetryBackoff sets the delays between retry attempts.
+// This is intended for testing to speed up retry tests.
+//
+// Note: if an empty non-nil slice is provided, Retry-After delays parsed from
+// server responses will be computed and capped but not applied (because
+// attempt < len(backoff) is always false). This is acceptable for the
+// test-only use case but callers should be aware of this edge case.
+func (c *Client) SetRetryBackoff(backoff []time.Duration) {
+	c.retryBackoff = backoff
+}
+
+// parseRetryAfter parses a Retry-After header value, supporting both integer
+// seconds (e.g. "120") and HTTP-date format (e.g. "Thu, 01 Dec 2025 16:00:00 GMT")
+// as specified in RFC 7231 §7.1.3.
+//
+// For integer values, it returns the duration directly.
+// For HTTP-date values, it computes the delay as the difference between the
+// parsed time and now. If the date is in the past, it returns 0.
+//
+// Returns (0, false) if the value cannot be parsed as either format.
+func (c *Client) parseRetryAfter(value string) (time.Duration, bool) {
+	value = strings.TrimSpace(value)
+
+	// Try integer seconds first (most common from GitHub).
+	// RFC 7231 allows delta-seconds of 0 to indicate immediate retry.
+	if seconds, err := strconv.Atoi(value); err == nil && seconds >= 0 {
+		return time.Duration(seconds) * time.Second, true
+	}
+
+	// Try HTTP-date format (RFC 7231 §7.1.3).
+	// http.ParseTime handles RFC 1123, RFC 850, and ASCTIME formats.
+	if retryAt, err := http.ParseTime(value); err == nil {
+		delay := retryAt.Sub(c.now())
+		if delay < 0 {
+			delay = 0
+		}
+		return delay, true
+	}
+
+	return 0, false
+}
+
+// doRequest performs an HTTP request with retry on 429 rate limit responses.
+// It respects the Retry-After header when present, supporting both integer
+// seconds and HTTP-date formats (capped at maxRetryAfter).
+func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept string) ([]byte, error) {
+	var backoff []time.Duration
+	if c.retryBackoff != nil {
+		backoff = append([]time.Duration(nil), c.retryBackoff...)
+	} else {
+		backoff = []time.Duration{1 * time.Second, 2 * time.Second}
+	}
+
+	var lastErr error
+	for attempt := 0; attempt < maxRetryAttempts; attempt++ {
+		if attempt > 0 {
+			var delay time.Duration
+			if attempt-1 < len(backoff) {
+				delay = backoff[attempt-1]
+			}
+			if delay > 0 {
+				timer := time.NewTimer(delay)
+				select {
+				case <-timer.C:
+					timer.Stop()
+				case <-ctx.Done():
+					timer.Stop()
+					return nil, ctx.Err()
+				}
+			}
+		}
+
+		req, err := http.NewRequestWithContext(ctx, method, reqURL, nil)
+		if err != nil {
+			return nil, fmt.Errorf("create request: %w", err)
+		}
+		req.Header.Set("Authorization", "Bearer "+c.token)
+		if accept != "" {
+			req.Header.Set("Accept", accept)
+		} else {
+			req.Header.Set("Accept", "application/vnd.github+json")
+		}
+
+		resp, err := c.httpClient.Do(req)
+		if err != nil {
+			return nil, fmt.Errorf("do request: %w", err)
+		}
+
+		if resp.StatusCode >= 200 && resp.StatusCode < 300 {
+			body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodyBytes))
+			resp.Body.Close()
+			if err != nil {
+				return nil, fmt.Errorf("read response body: %w", err)
+			}
+			return body, nil
+		}
+
+		errBody, _ := io.ReadAll(io.LimitReader(resp.Body, maxErrorBodyBytes))
+		resp.Body.Close()
+
+		lastErr = &APIError{StatusCode: resp.StatusCode, Body: string(errBody)}
+
+		// Retry on 429 rate limit
+		if resp.StatusCode == http.StatusTooManyRequests && attempt < maxRetryAttempts-1 {
+			// Check for Retry-After header and override backoff if present.
+			// Supports both integer seconds (common) and HTTP-date format (RFC 7231).
+			if ra := resp.Header.Get("Retry-After"); ra != "" {
+				if delay, ok := c.parseRetryAfter(ra); ok {
+					if delay > maxRetryAfter {
+						delay = maxRetryAfter
+					}
+					if attempt < len(backoff) {
+						backoff[attempt] = delay
+					}
+				}
+			}
+			continue
+		}
+
+		// Don't retry other errors
+		return nil, lastErr
+	}
+
+	return nil, lastErr
+}
+
+// doGet is a convenience wrapper for GET requests with the default Accept header.
+func (c *Client) doGet(ctx context.Context, url string) ([]byte, error) {
+	return c.doRequest(ctx, http.MethodGet, url, "")
+}
diff --git a/github/client_test.go b/github/client_test.go
new file mode 100644
index 0000000..8a5e4a3
--- /dev/null
+++ b/github/client_test.go
@@ -0,0 +1,409 @@
+package github
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestNewClient_DefaultBaseURL(t *testing.T) {
+	c := NewClient("tok", "")
+	if c.baseURL != defaultBaseURL {
+		t.Errorf("baseURL = %q, want %q", c.baseURL, defaultBaseURL)
+	}
+}
+
+func TestNewClient_CustomBaseURL(t *testing.T) {
+	c := NewClient("tok", "https://github.concur.com/api/v3/")
+	if c.baseURL != "https://github.concur.com/api/v3" {
+		t.Errorf("baseURL = %q, want trailing slash stripped", c.baseURL)
+	}
+}
+
+func TestDoRequest_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if got := r.Header.Get("Authorization"); got != "Bearer test-token" {
+			t.Errorf("Authorization = %q, want Bearer test-token", got)
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"ok":true}`))
+	}))
+	defer srv.Close()
+
+	c := NewClient("test-token", srv.URL)
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != `{"ok":true}` {
+		t.Errorf("body = %q, want %q", body, `{"ok":true}`)
+	}
+}
+
+func TestDoRequest_429_RetryAfter_IntegerSeconds(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", "0")
+			w.WriteHeader(http.StatusTooManyRequests)
+			w.Write([]byte("rate limited"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("success"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.SetRetryBackoff([]time.Duration{0, 0})
+
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "success" {
+		t.Errorf("body = %q, want %q", body, "success")
+	}
+	if attempts != 2 {
+		t.Errorf("attempts = %d, want 2", attempts)
+	}
+}
+
+func TestDoRequest_429_RetryAfter_HTTPDate(t *testing.T) {
+	// Fix "now" to a known time for deterministic testing.
+	fixedNow := time.Date(2025, 12, 1, 15, 59, 59, 0, time.UTC)
+	retryAt := "Mon, 01 Dec 2025 16:00:00 GMT" // 1 second in the future
+
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", retryAt)
+			w.WriteHeader(http.StatusTooManyRequests)
+			w.Write([]byte("rate limited"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("success"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.now = func() time.Time { return fixedNow }
+	// Initial backoff is 0; the HTTP-date parser will compute 1s and override.
+	c.SetRetryBackoff([]time.Duration{0, 0})
+
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "success" {
+		t.Errorf("body = %q, want %q", body, "success")
+	}
+	if attempts != 2 {
+		t.Errorf("attempts = %d, want 2", attempts)
+	}
+}
+
+func TestDoRequest_429_RetryAfter_HTTPDate_InPast(t *testing.T) {
+	// If the HTTP-date is in the past, delay should be 0 (retry immediately).
+	fixedNow := time.Date(2025, 12, 1, 17, 0, 0, 0, time.UTC)
+	retryAt := "Mon, 01 Dec 2025 16:00:00 GMT" // 1 hour in the past
+
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", retryAt)
+			w.WriteHeader(http.StatusTooManyRequests)
+			w.Write([]byte("rate limited"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("success"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.now = func() time.Time { return fixedNow }
+	c.SetRetryBackoff([]time.Duration{0, 0})
+
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "success" {
+		t.Errorf("body = %q, want %q", body, "success")
+	}
+}
+
+func TestDoRequest_429_NoRetryAfter_UsesDefaultBackoff(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.WriteHeader(http.StatusTooManyRequests)
+			w.Write([]byte("rate limited"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("success"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.SetRetryBackoff([]time.Duration{0, 0})
+
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "success" {
+		t.Errorf("body = %q, want %q", body, "success")
+	}
+	if attempts != 2 {
+		t.Errorf("attempts = %d, want 2", attempts)
+	}
+}
+
+func TestDoRequest_429_InvalidRetryAfter_UsesDefaultBackoff(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		if attempts == 1 {
+			w.Header().Set("Retry-After", "not-a-number-or-date")
+			w.WriteHeader(http.StatusTooManyRequests)
+			w.Write([]byte("rate limited"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("success"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.SetRetryBackoff([]time.Duration{0, 0})
+
+	body, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if string(body) != "success" {
+		t.Errorf("body = %q, want %q", body, "success")
+	}
+}
+
+func TestDoRequest_404_NoRetry(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		w.WriteHeader(http.StatusNotFound)
+		w.Write([]byte("not found"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	_, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !IsNotFound(err) {
+		t.Errorf("expected IsNotFound, got %v", err)
+	}
+	if attempts != 1 {
+		t.Errorf("attempts = %d, want 1 (no retry on 404)", attempts)
+	}
+}
+
+func TestDoRequest_401_NoRetry(t *testing.T) {
+	attempts := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		attempts++
+		w.WriteHeader(http.StatusUnauthorized)
+		w.Write([]byte("unauthorized"))
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	_, err := c.doGet(context.Background(), srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !IsUnauthorized(err) {
+		t.Errorf("expected IsUnauthorized, got %v", err)
+	}
+	if attempts != 1 {
+		t.Errorf("attempts = %d, want 1 (no retry on 401)", attempts)
+	}
+}
+
+func TestDoRequest_ContextCanceled(t *testing.T) {
+	// This test exercises the timer-cancel path in the retry select:
+	//   select { case <-timer.C; case <-ctx.Done() }
+	// The server returns 429 with a long Retry-After, and we cancel the
+	// context shortly after the first response so that cancellation races
+	// against the timer rather than preventing the initial HTTP round-trip.
+	requestReceived := make(chan struct{}, 1)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		select {
+		case requestReceived <- struct{}{}:
+		default:
+		}
+		w.Header().Set("Retry-After", "10")
+		w.WriteHeader(http.StatusTooManyRequests)
+	}))
+	defer srv.Close()
+
+	c := NewClient("tok", srv.URL)
+	c.SetRetryBackoff([]time.Duration{10 * time.Second, 10 * time.Second})
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// Cancel the context after the first request completes, while the
+	// client is blocked in the retry timer select.
+	go func() {
+		<-requestReceived
+		// Small delay to ensure we're inside the timer select.
+		time.Sleep(50 * time.Millisecond)
+		cancel()
+	}()
+
+	_, err := c.doGet(ctx, srv.URL+"/test")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("err = %v, want context.Canceled", err)
+	}
+}
+
+func TestParseRetryAfter_IntegerSeconds(t *testing.T) {
+	c := NewClient("tok", "")
+	delay, ok := c.parseRetryAfter("42")
+	if !ok {
+		t.Fatal("expected ok=true")
+	}
+	if delay != 42*time.Second {
+		t.Errorf("delay = %v, want 42s", delay)
+	}
+}
+
+func TestParseRetryAfter_ZeroSeconds(t *testing.T) {
+	c := NewClient("tok", "")
+	delay, ok := c.parseRetryAfter("0")
+	if !ok {
+		t.Fatal("expected ok=true for zero seconds (RFC 7231 allows immediate retry)")
+	}
+	if delay != 0 {
+		t.Errorf("delay = %v, want 0", delay)
+	}
+}
+
+func TestParseRetryAfter_NegativeSeconds(t *testing.T) {
+	c := NewClient("tok", "")
+	_, ok := c.parseRetryAfter("-5")
+	if ok {
+		t.Error("expected ok=false for negative seconds")
+	}
+}
+
+func TestParseRetryAfter_HTTPDate_Future(t *testing.T) {
+	fixedNow := time.Date(2025, 12, 1, 15, 59, 50, 0, time.UTC)
+	c := NewClient("tok", "")
+	c.now = func() time.Time { return fixedNow }
+
+	delay, ok := c.parseRetryAfter("Mon, 01 Dec 2025 16:00:00 GMT")
+	if !ok {
+		t.Fatal("expected ok=true")
+	}
+	// Should be 10 seconds in the future.
+	if delay != 10*time.Second {
+		t.Errorf("delay = %v, want 10s", delay)
+	}
+}
+
+func TestParseRetryAfter_HTTPDate_Past(t *testing.T) {
+	fixedNow := time.Date(2025, 12, 1, 17, 0, 0, 0, time.UTC)
+	c := NewClient("tok", "")
+	c.now = func() time.Time { return fixedNow }
+
+	delay, ok := c.parseRetryAfter("Mon, 01 Dec 2025 16:00:00 GMT")
+	if !ok {
+		t.Fatal("expected ok=true")
+	}
+	if delay != 0 {
+		t.Errorf("delay = %v, want 0 (past date)", delay)
+	}
+}
+
+func TestParseRetryAfter_RFC850_Format(t *testing.T) {
+	fixedNow := time.Date(2025, 12, 1, 15, 59, 50, 0, time.UTC)
+	c := NewClient("tok", "")
+	c.now = func() time.Time { return fixedNow }
+
+	// RFC 850 format
+	delay, ok := c.parseRetryAfter("Monday, 01-Dec-25 16:00:00 GMT")
+	if !ok {
+		t.Fatal("expected ok=true for RFC 850 format")
+	}
+	if delay != 10*time.Second {
+		t.Errorf("delay = %v, want 10s", delay)
+	}
+}
+
+func TestParseRetryAfter_Invalid(t *testing.T) {
+	c := NewClient("tok", "")
+	_, ok := c.parseRetryAfter("not-valid")
+	if ok {
+		t.Error("expected ok=false for invalid value")
+	}
+}
+
+func TestParseRetryAfter_EmptyString(t *testing.T) {
+	c := NewClient("tok", "")
+	_, ok := c.parseRetryAfter("")
+	if ok {
+		t.Error("expected ok=false for empty string")
+	}
+}
+
+func TestParseRetryAfter_MaxCap(t *testing.T) {
+	// Verify that parseRetryAfter returns the raw value (capping is done by caller).
+	c := NewClient("tok", "")
+	delay, ok := c.parseRetryAfter("3600")
+	if !ok {
+		t.Fatal("expected ok=true")
+	}
+	if delay != 3600*time.Second {
+		t.Errorf("delay = %v, want 3600s (caller is responsible for capping)", delay)
+	}
+}
+
+func TestAPIError_Error_Truncation(t *testing.T) {
+	longBody := make([]byte, 300)
+	for i := range longBody {
+		longBody[i] = 'x'
+	}
+	apiErr := &APIError{StatusCode: 500, Body: string(longBody)}
+	msg := apiErr.Error()
+	if len(msg) > 250 {
+		// "HTTP 500: " (10) + 200 + "...(truncated)" (14) = 224
+		t.Errorf("error message too long: %d chars", len(msg))
+	}
+}
+
+func TestAPIError_Error_NewlineSanitized(t *testing.T) {
+	apiErr := &APIError{StatusCode: 400, Body: "line1\nline2\rline3"}
+	msg := apiErr.Error()
+	for _, c := range msg {
+		if c == '\n' || c == '\r' {
+			t.Errorf("error message contains unsanitized newline: %q", msg)
+			break
+		}
+	}
+}