diff --git a/internal/infra/errs/errs.go b/internal/infra/errs/errs.go
new file mode 100644
index 0000000..a498fd6
--- /dev/null
+++ b/internal/infra/errs/errs.go
@@ -0,0 +1,111 @@
+// Package errs defines the application's error type (AppError) and helpers
+// to map domain error codes to HTTP status codes. Service layers wrap
+// underlying errors with errs.Wrap / errs.New; transport layers use errs.As
+// + errs.HTTPStatus to translate them into Google API style responses
+// ({code, message, fields, request_id}).
+package errs
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+// Code is a stable, machine-readable identifier for a class of error.
+// Codes are part of the public API contract surfaced by HTTP responses.
+type Code string
+
+// Canonical error codes. Keep this list in sync with HTTPStatus and any
+// downstream consumers (e.g., HTTP transport, OpenAPI schemas).
+const (
+	CodeInvalidInput Code = "invalid_input"
+	CodeUnauthorized Code = "unauthorized"
+	CodeForbidden    Code = "forbidden"
+	CodeNotFound     Code = "not_found"
+	CodeConflict     Code = "conflict"
+	CodeRateLimited  Code = "rate_limited"
+	CodeUpstream     Code = "upstream_error"
+	CodeUnavailable  Code = "unavailable"
+	CodeInternal     Code = "internal"
+)
+
+// AppError is the application's structured error type. It carries a stable
+// Code, a human-readable Message, an optional underlying Cause, and an
+// optional Fields map for per-field validation details.
+type AppError struct {
+	Code    Code
+	Message string
+	Cause   error
+	Fields  map[string]any
+}
+
+// Error implements the error interface. When a Cause is present the format
+// is "<message>: <cause>"; otherwise just the message is returned.
+func (e *AppError) Error() string {
+	if e.Cause != nil {
+		return fmt.Sprintf("%s: %s", e.Message, e.Cause.Error())
+	}
+	return e.Message
+}
+
+// Unwrap exposes the underlying cause so errors.Is / errors.As can traverse
+// the chain.
+func (e *AppError) Unwrap() error { return e.Cause }
+
+// WithFields attaches per-field validation details and returns the receiver
+// for chaining.
+func (e *AppError) WithFields(f map[string]any) *AppError {
+	e.Fields = f
+	return e
+}
+
+// New constructs a new AppError with the given code and message.
+func New(code Code, msg string) *AppError {
+	return &AppError{Code: code, Message: msg}
+}
+
+// Wrap wraps an existing error with an AppError carrying the given code and
+// message. The original error is preserved as Cause and remains discoverable
+// via errors.Is / errors.As.
+func Wrap(cause error, code Code, msg string) *AppError {
+	return &AppError{Code: code, Message: msg, Cause: cause}
+}
+
+// As is a convenience wrapper around errors.As that extracts the first
+// *AppError in err's chain. It returns (nil, false) when err is nil or no
+// AppError is found.
+func As(err error) (*AppError, bool) {
+	if err == nil {
+		return nil, false
+	}
+	var ae *AppError
+	if errors.As(err, &ae) {
+		return ae, true
+	}
+	return nil, false
+}
+
+// HTTPStatus maps an error Code to its canonical HTTP status code. Unknown
+// codes default to 500 Internal Server Error.
+func HTTPStatus(code Code) int {
+	switch code {
+	case CodeInvalidInput:
+		return http.StatusBadRequest
+	case CodeUnauthorized:
+		return http.StatusUnauthorized
+	case CodeForbidden:
+		return http.StatusForbidden
+	case CodeNotFound:
+		return http.StatusNotFound
+	case CodeConflict:
+		return http.StatusConflict
+	case CodeRateLimited:
+		return http.StatusTooManyRequests
+	case CodeUpstream:
+		return http.StatusBadGateway
+	case CodeUnavailable:
+		return http.StatusServiceUnavailable
+	default:
+		return http.StatusInternalServerError
+	}
+}
diff --git a/internal/infra/errs/errs_test.go b/internal/infra/errs/errs_test.go
new file mode 100644
index 0000000..27f2837
--- /dev/null
+++ b/internal/infra/errs/errs_test.go
@@ -0,0 +1,45 @@
+package errs
+
+import (
+	"errors"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNew_BasicShape(t *testing.T) {
+	e := New(CodeInvalidInput, "字段缺失")
+	require.Equal(t, CodeInvalidInput, e.Code)
+	require.Equal(t, "字段缺失", e.Message)
+	require.Nil(t, e.Cause)
+}
+
+func TestWithFields(t *testing.T) {
+	e := New(CodeInvalidInput, "x").WithFields(map[string]any{"name": "required"})
+	require.Equal(t, "required", e.Fields["name"])
+}
+
+func TestWrap_PreservesCause(t *testing.T) {
+	cause := errors.New("boom")
+	e := Wrap(cause, CodeInternal, "wrapped")
+	require.ErrorIs(t, e, cause)
+	require.Equal(t, "wrapped: boom", e.Error())
+}
+
+func TestHTTPStatus(t *testing.T) {
+	cases := map[Code]int{
+		CodeInvalidInput: http.StatusBadRequest,
+		CodeUnauthorized: http.StatusUnauthorized,
+		CodeForbidden:    http.StatusForbidden,
+		CodeNotFound:     http.StatusNotFound,
+		CodeConflict:     http.StatusConflict,
+		CodeRateLimited:  http.StatusTooManyRequests,
+		CodeUpstream:     http.StatusBadGateway,
+		CodeUnavailable:  http.StatusServiceUnavailable,
+		CodeInternal:     http.StatusInternalServerError,
+	}
+	for code, want := range cases {
+		require.Equal(t, want, HTTPStatus(code), "code=%s", code)
+	}
+}