diff --git a/backend/internal/service/openai_gateway_grok.go b/backend/internal/service/openai_gateway_grok.go
index 502f99f707..379b586136 100644
--- a/backend/internal/service/openai_gateway_grok.go
+++ b/backend/internal/service/openai_gateway_grok.go
@@ -154,6 +154,10 @@ func patchGrokResponsesBody(body []byte, upstreamModel string) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	out, err = sanitizeGrokResponsesModelCapabilities(out, upstreamModel)
+	if err != nil {
+		return nil, err
+	}
 	for _, unsupportedField := range []string{"prompt_cache_retention", "safety_identifier"} {
 		if gjson.GetBytes(out, unsupportedField).Exists() {
 			out, err = sjson.DeleteBytes(out, unsupportedField)
@@ -187,6 +191,38 @@ func patchGrokResponsesBody(body []byte, upstreamModel string) ([]byte, error) {
 	return out, nil
 }
 
+func sanitizeGrokResponsesModelCapabilities(body []byte, upstreamModel string) ([]byte, error) {
+	if !grokModelRejectsReasoningEffort(upstreamModel) {
+		return body, nil
+	}
+
+	out := body
+	for _, field := range []string{"reasoning", "reasoning_effort", "reasoningEffort"} {
+		if !gjson.GetBytes(out, field).Exists() {
+			continue
+		}
+		var err error
+		out, err = sjson.DeleteBytes(out, field)
+		if err != nil {
+			return nil, fmt.Errorf("remove unsupported Grok Composer %s: %w", field, err)
+		}
+	}
+	return out, nil
+}
+
+func grokModelRejectsReasoningEffort(model string) bool {
+	model = strings.TrimSpace(strings.ToLower(model))
+	if slash := strings.LastIndex(model, "/"); slash >= 0 {
+		model = strings.TrimSpace(model[slash+1:])
+	}
+	switch model {
+	case "grok-composer", "grok-composer-2.5-fast", "composer-2.5":
+		return true
+	default:
+		return false
+	}
+}
+
 var grokResponsesUnsupportedRecursiveFields = map[string]struct{}{
 	"external_web_access": {},
 }
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index 6a0942eb72..13020b1c28 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -42,6 +42,50 @@ func TestPatchGrokResponsesBodySetsMappedModelAndDropsUnsupportedFields(t *testi
 	require.Equal(t, "high", gjson.GetBytes(patched, "reasoning.effort").String())
 }
 
+func TestPatchGrokResponsesBodySanitizesComposerReasoningParameters(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name          string
+		upstreamModel string
+		wantReasoning bool
+	}{
+		{name: "composer fast", upstreamModel: "grok-composer-2.5-fast"},
+		{name: "composer shorthand", upstreamModel: "grok-composer"},
+		{name: "composer legacy alias", upstreamModel: "composer-2.5"},
+		{name: "provider-prefixed composer", upstreamModel: "xai/grok-composer-2.5-fast"},
+		{name: "grok 4.5", upstreamModel: "grok-4.5", wantReasoning: true},
+	}
+
+	body := []byte(`{
+		"model": "grok",
+		"input": "hello",
+		"reasoning": {"effort": "medium", "summary": "auto"},
+		"reasoning_effort": "medium",
+		"reasoningEffort": "medium"
+	}`)
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			patched, err := patchGrokResponsesBody(body, tt.upstreamModel)
+			require.NoError(t, err)
+			require.True(t, json.Valid(patched))
+			require.Equal(t, tt.upstreamModel, gjson.GetBytes(patched, "model").String())
+
+			if tt.wantReasoning {
+				require.Equal(t, "medium", gjson.GetBytes(patched, "reasoning.effort").String())
+				require.Equal(t, "medium", gjson.GetBytes(patched, "reasoning_effort").String())
+				require.Equal(t, "medium", gjson.GetBytes(patched, "reasoningEffort").String())
+				return
+			}
+
+			require.False(t, gjson.GetBytes(patched, "reasoning").Exists())
+			require.False(t, gjson.GetBytes(patched, "reasoning_effort").Exists())
+			require.False(t, gjson.GetBytes(patched, "reasoningEffort").Exists())
+		})
+	}
+}
+
 func TestExtractGrokResponsesReasoningEffortSupportsOpenAICompatibleField(t *testing.T) {
 	t.Parallel()