Copilot comments

Signed-off-by: roytman <roytman@il.ibm.com>

Author: roytman

pkg/epp/framework/plugins/scheduling/profilehandler/headerphase/README.md

@@ -13,8 +13,9 @@ Reads the configured header from the incoming request and looks up the
 `schedulingProfiles` entry with that exact name:
 
 - If a matching profile hasn't run yet, it runs that profile alone.
-- If the header is missing or names a profile that isn't configured, no profile runs and
-  the request fails with an error identifying the header.
+- If the header is missing or names a profile that isn't configured, no profile runs; the
+  request fails with the scheduler's generic "failed to run any scheduler profile" error,
+  and the EPP logs the specific reason (missing header vs. unconfigured value).
 
 This differs from the [disagg profile handler](../disagg/README.md), which decides which
 profiles to run via decider plugins rather than a header.

pkg/epp/framework/plugins/scheduling/profilehandler/headerphase/header_phase_profile_handler.go

@@ -22,6 +22,8 @@ import (
 	"fmt"
 	"strings"
 
+	"sigs.k8s.io/controller-runtime/pkg/log"
+
 	fwkplugin "github.com/llm-d/llm-d-router/pkg/epp/framework/interface/plugin"
 	fwksched "github.com/llm-d/llm-d-router/pkg/epp/framework/interface/scheduling"
 )
@@ -103,10 +105,22 @@ func (h *HeaderPhaseProfileHandler) phaseHeader(request *fwksched.InferenceReque
 	return strings.TrimSpace(request.Headers[h.headerName])
 }
 
+// noMatchError explains why no configured scheduling profile matches phase, the
+// already-trimmed value of the phase header.
+func (h *HeaderPhaseProfileHandler) noMatchError(phase string) error {
+	if phase == "" {
+		return fmt.Errorf("header-phase profile handler: missing %q header", h.headerName)
+	}
+	return fmt.Errorf("header-phase profile handler: no scheduling profile configured for %q header value %q", h.headerName, phase)
+}
+
 // Pick selects the single SchedulingProfile named by the request's phase header. It
 // returns an empty map once that profile has run, or when the header is missing or
-// names a profile that isn't configured (ProcessResults then reports the error).
-func (h *HeaderPhaseProfileHandler) Pick(_ context.Context, request *fwksched.InferenceRequest, profiles map[string]fwksched.SchedulerProfile,
+// names a profile that isn't configured. In the latter case the scheduler's run loop
+// (pkg/epp/scheduling.Scheduler.Schedule) stops without ever calling ProcessResults,
+// since no profile ever ran, so the specific reason is logged here rather than
+// returned from ProcessResults, where it would be unreachable in practice.
+func (h *HeaderPhaseProfileHandler) Pick(ctx context.Context, request *fwksched.InferenceRequest, profiles map[string]fwksched.SchedulerProfile,
 	profileResults map[string]*fwksched.ProfileRunResult) map[string]fwksched.SchedulerProfile {
 	// TODO(#2135): single profile per request; extend to loop over an ordered phase
 	// list parsed from the header for non-deferred multi-profile scheduling.
@@ -117,6 +131,7 @@ func (h *HeaderPhaseProfileHandler) Pick(_ context.Context, request *fwksched.In
 	phase := h.phaseHeader(request)
 	profile, ok := profiles[phase]
 	if !ok {
+		log.FromContext(ctx).Error(h.noMatchError(phase), "no scheduling profile selected for request")
 		return map[string]fwksched.SchedulerProfile{}
 	}
 
@@ -133,11 +148,7 @@ func (h *HeaderPhaseProfileHandler) ProcessResults(_ context.Context, request *f
 	// multi-profile scheduling.
 	switch len(profileResults) {
 	case 0:
-		phase := h.phaseHeader(request)
-		if phase == "" {
-			return nil, fmt.Errorf("header-phase profile handler: missing %q header", h.headerName)
-		}
-		return nil, fmt.Errorf("header-phase profile handler: no scheduling profile configured for %q header value %q", h.headerName, phase)
+		return nil, h.noMatchError(h.phaseHeader(request))
 	case 1:
 		// exactly one profile ran, handled below
 	default:

pkg/epp/framework/plugins/scheduling/profilehandler/headerphase/header_phase_profile_handler_test.go

@@ -106,6 +106,39 @@ func TestHeaderPhaseProfileHandlerFactory(t *testing.T) {
 	}
 }
 
+func TestHeaderPhaseNoMatchError(t *testing.T) {
+	handler := NewHeaderPhaseProfileHandler(defaultHeaderName)
+
+	tests := []struct {
+		name           string
+		phase          string
+		wantErrContain string
+	}{
+		{
+			name:           "empty phase reports missing header",
+			phase:          "",
+			wantErrContain: `missing "epp-phase" header`,
+		},
+		{
+			name:           "non-empty phase reports the unconfigured value",
+			phase:          "prefill",
+			wantErrContain: `no scheduling profile configured for "epp-phase" header value "prefill"`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := handler.noMatchError(tt.phase)
+			if err == nil {
+				t.Fatalf("noMatchError() returned nil, want an error")
+			}
+			if !strings.Contains(err.Error(), tt.wantErrContain) {
+				t.Errorf("noMatchError() = %q, want it to contain %q", err.Error(), tt.wantErrContain)
+			}
+		})
+	}
+}
+
 func TestHeaderPhaseWithName(t *testing.T) {
 	handler := NewHeaderPhaseProfileHandler(defaultHeaderName).WithName("renamed")