diff --git a/backend/internal/pkg/apicompat/anthropic_to_responses.go b/backend/internal/pkg/apicompat/anthropic_to_responses.go
index 23acc0bea8..008121e0f9 100644
--- a/backend/internal/pkg/apicompat/anthropic_to_responses.go
+++ b/backend/internal/pkg/apicompat/anthropic_to_responses.go
@@ -469,10 +469,38 @@ func boolPtr(v bool) *bool {
 
 // isReasoningModel reports whether model is a reasoning model that does not
 // support sampling parameters (temperature, top_p) via the Responses API.
-// All gpt-5.x models are reasoning-only; the Responses API returns
-// "Unsupported parameter: temperature" if these fields are present.
+// GPT-5 and every later generation are reasoning-only; the Responses API
+// returns "Unsupported parameter: temperature" if these fields are present.
+//
+// Keyed on the generation number instead of a "gpt-5" prefix: pinning the
+// prefix meant each new family (gpt-6-astra and whatever follows) silently
+// fell through to the sampling branch and failed upstream on every compat
+// request until someone edited this line.
 func isReasoningModel(model string) bool {
-	return strings.HasPrefix(model, "gpt-5") || openai.IsGPT6SolOrLunaModelSpelling(model)
+	major, ok := openAIModelGeneration(model)
+	return (ok && major >= 5) || openai.IsGPT6SolOrLunaModelSpelling(model)
+}
+
+// openAIModelGeneration extracts N from a "gpt-N[.M][-suffix]" model id.
+// ok is false for non-GPT ids and for GPT families that carry no numeric
+// generation (gpt-image-1, gpt-audio, ...).
+func openAIModelGeneration(model string) (int, bool) {
+	rest, ok := strings.CutPrefix(strings.ToLower(strings.TrimSpace(model)), "gpt-")
+	if !ok {
+		return 0, false
+	}
+	major, digits := 0, 0
+	for _, r := range rest {
+		if r < '0' || r > '9' {
+			break
+		}
+		major = major*10 + int(r-'0')
+		digits++
+	}
+	if digits == 0 {
+		return 0, false
+	}
+	return major, true
 }
 
 // normalizeToolParameters ensures the tool parameter schema is valid for
diff --git a/backend/internal/pkg/apicompat/reasoning_generation_test.go b/backend/internal/pkg/apicompat/reasoning_generation_test.go
new file mode 100644
index 0000000000..8347be8fdd
--- /dev/null
+++ b/backend/internal/pkg/apicompat/reasoning_generation_test.go
@@ -0,0 +1,78 @@
+package apicompat
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// GPT-5 and every later generation are reasoning-only: forwarding temperature or
+// top_p makes the Responses API reject the request with "Unsupported parameter".
+// The check is keyed on the generation number so a new family (gpt-6-astra and
+// whatever follows) does not silently fall into the sampling branch.
+func TestIsReasoningModelCoversLaterGenerations(t *testing.T) {
+	for _, tc := range []struct {
+		model string
+		want  bool
+	}{
+		{"gpt-6-astra", true},
+		{"gpt-6", true},
+		{"gpt-7-whatever", true},
+		{"gpt-6-sol", true},
+		{"gpt-5.5", true},
+		{"gpt-5.2", true},
+		{"gpt-5", true},
+		{"GPT-6-Astra", true},
+		{"  gpt-6-astra  ", true},
+		{"gpt-4o", false},
+		{"gpt-4.1", false},
+		{"gpt-image-1", false},
+		{"gpt-audio", false},
+		{"claude-opus-4-6", false},
+		{"gemini-3.1-pro", false},
+		{"", false},
+	} {
+		if got := isReasoningModel(tc.model); got != tc.want {
+			t.Errorf("isReasoningModel(%q) = %v, want %v", tc.model, got, tc.want)
+		}
+	}
+}
+
+func TestOpenAIModelGeneration(t *testing.T) {
+	for _, tc := range []struct {
+		model     string
+		wantMajor int
+		wantOK    bool
+	}{
+		{"gpt-6-astra", 6, true},
+		{"gpt-5.5", 5, true},
+		{"gpt-4o", 4, true},
+		{"gpt-10-future", 10, true},
+		{"gpt-image-1", 0, false},
+		{"claude-opus-4-6", 0, false},
+		{"", 0, false},
+	} {
+		major, ok := openAIModelGeneration(tc.model)
+		if major != tc.wantMajor || ok != tc.wantOK {
+			t.Errorf("openAIModelGeneration(%q) = (%d, %v), want (%d, %v)",
+				tc.model, major, ok, tc.wantMajor, tc.wantOK)
+		}
+	}
+}
+
+func TestAnthropicToResponses_TemperatureStrippedForGPT6Astra(t *testing.T) {
+	temp := 0.7
+	req := &AnthropicRequest{
+		Model:       "gpt-6-astra",
+		MaxTokens:   1024,
+		Messages:    []AnthropicMessage{{Role: "user", Content: json.RawMessage(`"Hello"`)}},
+		Temperature: &temp,
+		TopP:        &temp,
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+	require.Nil(t, resp.Temperature, "gpt-6-astra is reasoning-only: temperature must be stripped")
+	require.Nil(t, resp.TopP, "gpt-6-astra is reasoning-only: top_p must be stripped")
+}