fix(codex): never advertise null service_tiers in models manifest

intersectUpstreamModelMetadata falls back to null for every Codex tool
capability field whose value differs across peer accounts. Since
e6d191a83 added service_tiers to codexToolCapabilityFields, a model served
by accounts with different (or partially declared) tier lists ends up with
"service_tiers": null in the generated Codex manifest. Codex decodes
service_tiers as a required array and rejects the whole manifest, so the
client loses model metadata for every model.

Fall back to [] for conflicting service_tiers (do not advertise any tier
that is not shared by all peers), and normalize an explicit null to []
when applying capabilities.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/internal/service/openai_codex_model_metadata.go

@@ -21,6 +21,11 @@ func applyCodexToolCapabilities(dst, src map[string]json.RawMessage, overwrite b
 		if len(value) == 0 {
 			continue
 		}
+		// Codex decodes service_tiers as a required array and rejects the whole
+		// models manifest on null, so an undeclared tier list is advertised as [].
+		if field == "service_tiers" && bytes.Equal(value, []byte("null")) {
+			value = []byte("[]")
+		}
 		// These Codex fields are nullable booleans or strings, never arbitrary objects.
 		if !bytes.Equal(value, []byte("null")) {
 			if field == "service_tiers" {
@@ -280,6 +285,9 @@ func intersectUpstreamModelMetadata(modelID string, candidates []UpstreamModelMe
 			fallback := json.RawMessage("null")
 			if field == "supports_search_tool" || field == "use_responses_lite" {
 				fallback = json.RawMessage("false")
+			} else if field == "service_tiers" {
+				// Conflicting peers must not advertise any tier, and Codex requires an array.
+				fallback = json.RawMessage("[]")
 			}
 			result.CodexToolCapabilities[field] = fallback
 		}

backend/internal/service/openai_codex_model_metadata_test.go

@@ -108,6 +108,43 @@ func TestAstraCodexToolCapabilitiesUseAccountScopeAndSharedDeclarations(t *testi
 	require.Equal(t, "3000", model["comp_hash"])
 }
 
+// Scenario: Codex rejects the whole models manifest when service_tiers is null,
+// so conflicting or null tier declarations must be advertised as an empty array.
+func TestCodexToolCapabilitiesNeverAdvertiseNullServiceTiers(t *testing.T) {
+	newAccount := func(id int64, tiers string) Account {
+		account := Account{ID: id, Platform: PlatformOpenAI, Type: AccountTypeAPIKey, Credentials: map[string]any{
+			"base_url": "https://relay.example/v1", "model_mapping": map[string]any{"gpt-6-sol": "gpt-6-sol"},
+		}}
+		if tiers != "" {
+			account.SetUpstreamModelMetadataSnapshot(UpstreamModelMetadataSnapshot{Models: map[string]UpstreamModelMetadata{
+				"gpt-6-sol": {CodexToolCapabilities: map[string]json.RawMessage{"service_tiers": json.RawMessage(tiers)}},
+			}})
+		}
+		return account
+	}
+	for _, tt := range []struct {
+		name     string
+		accounts []Account
+	}{
+		{"conflicting peer tiers", []Account{newAccount(1, `[{"id":"priority"}]`), newAccount(2, `[]`)}},
+		{"undeclared peer tiers", []Account{newAccount(1, `[{"id":"priority"}]`), newAccount(2, "")}},
+		{"explicit null tiers", []Account{newAccount(1, "null")}},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			body, err := buildCodexModelsManifestForAccounts(PlatformOpenAI, []string{"gpt-6-sol"}, tt.accounts, nil, nil, true)
+			require.NoError(t, err)
+			model := decodeCodexManifestModels(t, body)[0]
+			require.Equal(t, []any{}, model["service_tiers"])
+		})
+	}
+
+	merged := intersectUpstreamModelMetadata("gpt-6-sol", []UpstreamModelMetadata{
+		{CodexToolCapabilities: map[string]json.RawMessage{"service_tiers": json.RawMessage(`[{"id":"priority"}]`)}},
+		{CodexToolCapabilities: map[string]json.RawMessage{"service_tiers": json.RawMessage(`[]`)}},
+	})
+	require.JSONEq(t, `[]`, string(merged.CodexToolCapabilities["service_tiers"]))
+}
+
 func TestAstraCodexToolCapabilitiesPreserveLiveNullAndFalse(t *testing.T) {
 	account := &Account{Platform: PlatformOpenAI, Type: AccountTypeAPIKey,
 		Credentials: map[string]any{"base_url": "https://api.openai.com/v1"}}