fix: make error hints match the error class

internal/clierr/clierr.go

@@ -21,10 +21,13 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"net"
+	"net/http"
 	"syscall"
 
 	"github.com/a2aproject/a2a-go/v2/a2a"
+	"github.com/a2aproject/a2a-go/v2/a2aclient/agentcard"
 )
 
 // Code is a symbolic error identifier.
@@ -111,11 +114,26 @@ func CardResolution(err error) *Error {
 		Code:    CodeCardInvalid,
 		Message: msg,
 		Exit:    4,
-		Hint:    "check the --agent-card reference (host, full card URL, or local file path)",
+		Hint:    cardInvalidHint(err),
 		err:     err,
 	}
 }
 
+// cardInvalidHint picks remediation for a card that was reached but not usable,
+// based on the HTTP status of the card request when there is one.
+func cardInvalidHint(err error) string {
+	var status *agentcard.ErrStatusNotOK
+	if errors.As(err, &status) {
+		switch {
+		case status.StatusCode == http.StatusUnauthorized || status.StatusCode == http.StatusForbidden:
+			return "pass credentials with --auth, --svc-param or --credential-helper"
+		case status.StatusCode >= http.StatusInternalServerError:
+			return "the agent failed to serve its card, check its logs or retry"
+		}
+	}
+	return "check the --agent-card reference (host, full card URL, or local file path)"
+}
+
 // MarshalJSON implements [json.Marshaler].
 func (e *Error) MarshalJSON() ([]byte, error) {
 	type body struct {
@@ -186,10 +204,12 @@ func Classify(err error) *Error {
 
 // isIOError reports whether err is a transport-level failure — a network error,
 // a broken stream, or a reset/closed connection — identified by type and
-// sentinel rather than by matching message text.
+// sentinel rather than by matching message text. A local file error is not one,
+// even though the syscall.Errno it wraps satisfies net.Error.
 func isIOError(err error) bool {
 	var netErr net.Error
-	if errors.As(err, &netErr) {
+	var pathErr *fs.PathError
+	if errors.As(err, &netErr) && !errors.As(err, &pathErr) {
 		return true
 	}
 	return errors.Is(err, io.ErrUnexpectedEOF) ||

internal/clierr/clierr_test.go

@@ -18,11 +18,13 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io/fs"
 	"net"
 	"syscall"
 	"testing"
 
 	"github.com/a2aproject/a2a-go/v2/a2a"
+	"github.com/a2aproject/a2a-go/v2/a2aclient/agentcard"
 )
 
 func TestClassify(t *testing.T) {
@@ -85,6 +87,18 @@ func TestClassify(t *testing.T) {
 			wantCode: CodeIO,
 			wantExit: 3,
 		},
+		{
+			name:     "local file error is not io, exit 1",
+			err:      fmt.Errorf("failed to save file parts: %w", &fs.PathError{Op: "open", Path: "out/x.txt", Err: syscall.EACCES}),
+			wantCode: CodeInternal,
+			wantExit: 1,
+		},
+		{
+			name:     "broken pipe on a file is still io",
+			err:      fmt.Errorf("writing to plugin: %w", &fs.PathError{Op: "write", Path: "|1", Err: syscall.EPIPE}),
+			wantCode: CodeIO,
+			wantExit: 3,
+		},
 		{
 			name:     "unclassified error is internal, exit 1",
 			err:      errors.New("something odd happened"),
@@ -158,6 +172,27 @@ func TestCardResolution(t *testing.T) {
 			wantExit: 4,
 			wantHint: "check the --agent-card reference (host, full card URL, or local file path)",
 		},
+		{
+			name:     "unauthorized card request asks for credentials",
+			err:      &agentcard.ErrStatusNotOK{StatusCode: 401, Status: "401 Unauthorized"},
+			wantCode: CodeCardInvalid,
+			wantExit: 4,
+			wantHint: "pass credentials with --auth, --svc-param or --credential-helper",
+		},
+		{
+			name:     "forbidden card request asks for credentials",
+			err:      &agentcard.ErrStatusNotOK{StatusCode: 403, Status: "403 Forbidden"},
+			wantCode: CodeCardInvalid,
+			wantExit: 4,
+			wantHint: "pass credentials with --auth, --svc-param or --credential-helper",
+		},
+		{
+			name:     "server error on card request points at the agent",
+			err:      &agentcard.ErrStatusNotOK{StatusCode: 500, Status: "500 Internal Server Error"},
+			wantCode: CodeCardInvalid,
+			wantExit: 4,
+			wantHint: "the agent failed to serve its card, check its logs or retry",
+		},
 		{
 			name:     "deadline exceeded is a timeout",
 			err:      context.DeadlineExceeded,