diff --git a/.beads/issues.jsonl b/.beads/issues.jsonl
index 5d3ed06..f7cf06e 100644
--- a/.beads/issues.jsonl
+++ b/.beads/issues.jsonl
@@ -18,7 +18,7 @@
 {"id":"checkvist-api-cb8","title":"Extended Features","description":"Phase 3: Implement P1 (should-have) features including client-side filtering and builder patterns for fluent interfaces.","status":"open","priority":1,"issue_type":"epic","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:55.624242123+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:55.624242123+01:00"}
 {"id":"checkvist-api-e9p","title":"Implement data models","description":"Create models.go with all data structures:\n- Checklist struct (ID, Name, Public, Archived, ReadOnly, TaskCount, TaskCompleted, Tags, TagsAsText, UpdatedAt)\n- Task struct (ID, ChecklistID, ParentID, Content, Status, Position, Priority, Tags, TagsAsText, DueDateRaw, DueDate, AssigneeIDs, CommentsCount, UpdateLine, UpdatedAt, CreatedAt, Children, Notes)\n- TaskStatus enum (StatusOpen=0, StatusClosed=1, StatusInvalidated=2)\n- Note struct (ID, TaskID, Comment, UpdatedAt, CreatedAt)\n- Tags type (map[string]bool)\n- User struct (ID, Username, Email)\n- DueDate struct with constructors (DueAt, DueString, DueInDays) and constants (DueToday, DueTomorrow, DueNextWeek, DueNextMonth)","status":"closed","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:06.900391036+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T13:22:22.273934664+01:00","closed_at":"2026-01-14T13:22:22.273934664+01:00","close_reason":"Closed","dependencies":[{"issue_id":"checkvist-api-e9p","depends_on_id":"checkvist-api-5wr","type":"blocks","created_at":"2026-01-14T12:32:46.433908937+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-lpn","title":"Implement authentication with auto token renewal","description":"Add to client.go:\n- Authenticate(ctx context.Context) error - explicit login\n- refreshToken(ctx context.Context) error - token renewal\n- ensureAuthenticated(ctx context.Context) error - auto-auth before requests\n- CurrentUser(ctx context.Context) (*User, error) - get logged in user\n- Token management: store token and expiry, auto-refresh before expiry\n- Thread-safe token access using mutex\n- Support for optional 2FA token\nAPI endpoints:\n- POST /auth/login.json?version=2 (login)\n- POST /auth/refresh_token.json?version=2 (refresh)\n- GET /auth/curr_user.json (current user)\nToken sent via X-Client-Token header","status":"open","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:08.358878117+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:31:08.358878117+01:00","dependencies":[{"issue_id":"checkvist-api-lpn","depends_on_id":"checkvist-api-ymg","type":"blocks","created_at":"2026-01-14T12:32:47.656124681+01:00","created_by":"Oliver Jakoubek"}]}
-{"id":"checkvist-api-mnh","title":"Implement error types and sentinel errors","description":"Create errors.go with:\n- APIError struct (StatusCode, Message, RequestID, Err) with Error() and Unwrap() methods\n- Sentinel errors: ErrUnauthorized, ErrNotFound, ErrRateLimited, ErrBadRequest, ErrServerError\n- Helper function to create APIError from HTTP response","status":"open","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:07.619359293+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:31:07.619359293+01:00","dependencies":[{"issue_id":"checkvist-api-mnh","depends_on_id":"checkvist-api-5wr","type":"blocks","created_at":"2026-01-14T12:32:46.754134799+01:00","created_by":"Oliver Jakoubek"}]}
+{"id":"checkvist-api-mnh","title":"Implement error types and sentinel errors","description":"Create errors.go with:\n- APIError struct (StatusCode, Message, RequestID, Err) with Error() and Unwrap() methods\n- Sentinel errors: ErrUnauthorized, ErrNotFound, ErrRateLimited, ErrBadRequest, ErrServerError\n- Helper function to create APIError from HTTP response","status":"closed","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:07.619359293+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T13:23:43.296883265+01:00","closed_at":"2026-01-14T13:23:43.296883265+01:00","close_reason":"Closed","dependencies":[{"issue_id":"checkvist-api-mnh","depends_on_id":"checkvist-api-5wr","type":"blocks","created_at":"2026-01-14T12:32:46.754134799+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-nrk","title":"Create README with quickstart","description":"Create README.md (in English) with:\n- Project description\n- Installation: go get code.beautifulmachines.dev/jakoubek/checkvist-api\n- Quick start example (init client, list checklists, create task)\n- API overview (Checklists, Tasks, Notes, Filters)\n- Builder pattern examples\n- Error handling examples\n- Configuration options\n- Link to GoDoc\n- License (MIT)","status":"open","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:38.724338606+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:31:38.724338606+01:00","dependencies":[{"issue_id":"checkvist-api-nrk","depends_on_id":"checkvist-api-c2k","type":"blocks","created_at":"2026-01-14T12:33:15.785698203+01:00","created_by":"Oliver Jakoubek"},{"issue_id":"checkvist-api-nrk","depends_on_id":"checkvist-api-rl9","type":"blocks","created_at":"2026-01-14T12:33:16.125115134+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-rl9","title":"Implement Task operations","description":"Create tasks.go with TaskService:\n- client.Tasks(checklistID) returns TaskService\n- List(ctx) ([]Task, error) - GET /checklists/{id}/tasks.json\n- Get(ctx, taskID) (*Task, error) - GET /checklists/{id}/tasks/{task_id}.json (includes parent hierarchy)\n- Create(ctx, builder *TaskBuilder) (*Task, error) - POST /checklists/{id}/tasks.json\n- Update(ctx, taskID, opts) (*Task, error) - PUT /checklists/{id}/tasks/{task_id}.json\n- Delete(ctx, taskID) error - DELETE /checklists/{id}/tasks/{task_id}.json\n- Close(ctx, taskID) (*Task, error) - POST /checklists/{id}/tasks/{task_id}/close.json\n- Reopen(ctx, taskID) (*Task, error) - POST /checklists/{id}/tasks/{task_id}/reopen.json\n- Invalidate(ctx, taskID) (*Task, error) - POST /checklists/{id}/tasks/{task_id}/invalidate.json\nParse DueDate from DueDateRaw when retrieving tasks.","status":"open","priority":0,"issue_type":"feature","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:53.90629838+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:53.90629838+01:00","dependencies":[{"issue_id":"checkvist-api-rl9","depends_on_id":"checkvist-api-8u6","type":"blocks","created_at":"2026-01-14T12:32:55.247292478+01:00","created_by":"Oliver Jakoubek"},{"issue_id":"checkvist-api-rl9","depends_on_id":"checkvist-api-lpn","type":"blocks","created_at":"2026-01-14T12:32:55.536931433+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-tjk","title":"Implement TaskBuilder fluent interface","description":"Enhance task creation with builder pattern:\n- NewTask(content string) *TaskBuilder\n- WithTags(tags ...string) *TaskBuilder\n- WithDueDate(due DueDate) *TaskBuilder\n- WithPriority(p int) *TaskBuilder\n- WithParent(parentID int64) *TaskBuilder\n- WithPosition(pos int) *TaskBuilder\n- Build() returns parameters for API call\nTaskBuilder should be chainable and return itself for fluent usage:\n  checkvist.NewTask(\"Content\").WithTags(\"tag1\").WithDueDate(checkvist.DueTomorrow).WithPriority(1)","status":"open","priority":1,"issue_type":"feature","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:55.929907579+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:55.929907579+01:00","dependencies":[{"issue_id":"checkvist-api-tjk","depends_on_id":"checkvist-api-rl9","type":"blocks","created_at":"2026-01-14T12:33:02.20339206+01:00","created_by":"Oliver Jakoubek"}]}
diff --git a/errors.go b/errors.go
index 7854a73..36e7b12 100644
--- a/errors.go
+++ b/errors.go
@@ -1,3 +1,81 @@
 package checkvist
 
+import (
+	"errors"
+	"fmt"
+	"net/http"
+)
+
 // errors.go contains the APIError type and sentinel errors for common API error conditions.
+
+// Sentinel errors for common API error conditions.
+// Use errors.Is() to check for these errors.
+var (
+	// ErrUnauthorized is returned when authentication fails (HTTP 401).
+	ErrUnauthorized = errors.New("unauthorized: invalid credentials or expired token")
+	// ErrNotFound is returned when a resource is not found (HTTP 404).
+	ErrNotFound = errors.New("not found: the requested resource does not exist")
+	// ErrRateLimited is returned when the API rate limit is exceeded (HTTP 429).
+	ErrRateLimited = errors.New("rate limited: too many requests")
+	// ErrBadRequest is returned for invalid request parameters (HTTP 400).
+	ErrBadRequest = errors.New("bad request: invalid parameters")
+	// ErrServerError is returned for server-side errors (HTTP 5xx).
+	ErrServerError = errors.New("server error: the server encountered an error")
+)
+
+// APIError represents an error returned by the Checkvist API.
+type APIError struct {
+	// StatusCode is the HTTP status code returned by the API.
+	StatusCode int
+	// Message is a human-readable error message.
+	Message string
+	// RequestID is the unique identifier for the request, if available.
+	RequestID string
+	// Err is the underlying sentinel error, if applicable.
+	Err error
+}
+
+// Error implements the error interface.
+func (e *APIError) Error() string {
+	if e.RequestID != "" {
+		return fmt.Sprintf("checkvist API error (status %d, request %s): %s", e.StatusCode, e.RequestID, e.Message)
+	}
+	return fmt.Sprintf("checkvist API error (status %d): %s", e.StatusCode, e.Message)
+}
+
+// Unwrap returns the underlying error for use with errors.Is() and errors.As().
+func (e *APIError) Unwrap() error {
+	return e.Err
+}
+
+// NewAPIError creates an APIError from an HTTP response.
+// It automatically maps the status code to the appropriate sentinel error.
+func NewAPIError(resp *http.Response, message string) *APIError {
+	if message == "" {
+		message = http.StatusText(resp.StatusCode)
+	}
+
+	apiErr := &APIError{
+		StatusCode: resp.StatusCode,
+		Message:    message,
+		RequestID:  resp.Header.Get("X-Request-Id"),
+	}
+
+	// Map status codes to sentinel errors
+	switch resp.StatusCode {
+	case http.StatusUnauthorized:
+		apiErr.Err = ErrUnauthorized
+	case http.StatusNotFound:
+		apiErr.Err = ErrNotFound
+	case http.StatusTooManyRequests:
+		apiErr.Err = ErrRateLimited
+	case http.StatusBadRequest:
+		apiErr.Err = ErrBadRequest
+	default:
+		if resp.StatusCode >= 500 {
+			apiErr.Err = ErrServerError
+		}
+	}
+
+	return apiErr
+}