diff --git a/open-sse/services/modelCapabilities.ts b/open-sse/services/modelCapabilities.ts
index de86e1fd7a..b3822e9672 100644
--- a/open-sse/services/modelCapabilities.ts
+++ b/open-sse/services/modelCapabilities.ts
@@ -48,3 +48,54 @@ export function supportsToolCalling(modelStr: string): boolean {
 
   return !blocked;
 }
+
+// Models that do NOT support reasoning/thinking parameters.
+// AG (Antigravity) claude-sonnet-4-6 routes through a Google internal API
+// that returns 400 if thinking params are included.
+const REASONING_UNSUPPORTED_PATTERNS = [
+  "antigravity/claude-sonnet-4-6",
+  "antigravity/claude-sonnet-4-5",
+  "antigravity/claude-sonnet-4",
+  "ag/claude-sonnet-4-6",
+  "ag/claude-sonnet-4-5",
+  "ag/claude-sonnet-4",
+];
+
+function getRegistryReasoningFlag(providerIdOrAlias: string, modelId: string): boolean | null {
+  const providerAlias = PROVIDER_ID_TO_ALIAS[providerIdOrAlias] || providerIdOrAlias;
+  const models = PROVIDER_MODELS[providerAlias];
+  if (!Array.isArray(models)) return null;
+  const found = models.find((m) => m?.id === modelId);
+  if (!found) return null;
+  return typeof found.supportsReasoning === "boolean" ? found.supportsReasoning : null;
+}
+
+/**
+ * Returns whether a model supports reasoning/thinking parameters.
+ *
+ * Decision order:
+ * 1) Provider registry metadata (supportsReasoning flag) when available.
+ * 2) Explicit denylist for known unsupported models (e.g. AG Claude Sonnet).
+ * 3) Default true (pass through — safe, provider will ignore if unsupported).
+ */
+export function supportsReasoning(modelStr: string): boolean {
+  const parsed = parseModel(modelStr);
+  const provider = parsed.provider || parsed.providerAlias || "";
+  const model = parsed.model || modelStr;
+
+  if (provider) {
+    const fromRegistry = getRegistryReasoningFlag(provider, model);
+    if (fromRegistry !== null) return fromRegistry;
+  }
+
+  const normalized = String(modelStr || "").toLowerCase();
+  if (!normalized) return true;
+
+  const blocked = REASONING_UNSUPPORTED_PATTERNS.some((pattern) =>
+    normalized === pattern ||
+    normalized.endsWith(`/${pattern}`) ||
+    normalized.includes(pattern)
+  );
+
+  return !blocked;
+}
diff --git a/open-sse/services/thinkingBudget.ts b/open-sse/services/thinkingBudget.ts
index 4028037c97..e47cfbc9fb 100644
--- a/open-sse/services/thinkingBudget.ts
+++ b/open-sse/services/thinkingBudget.ts
@@ -14,6 +14,7 @@ export const ThinkingMode = {
 };
 
 import { capThinkingBudget, getDefaultThinkingBudget } from "@/shared/constants/modelSpecs";
+import { supportsReasoning } from "./modelCapabilities.ts";
 
 // Effort → budget token mapping
 export const EFFORT_BUDGETS = {
@@ -151,6 +152,13 @@ export function applyThinkingBudget(body, config = null) {
   const cfg = config || _config;
   if (!body || typeof body !== "object") return body;
 
+  // Early exit: strip ALL reasoning/thinking params for models that don't support them.
+  // Sending thinking params to unsupported models (e.g. AG claude-sonnet-4-6) causes 400 errors.
+  const modelStr = typeof body.model === "string" ? body.model : "";
+  if (modelStr && !supportsReasoning(modelStr)) {
+    return stripThinkingConfig(body);
+  }
+
   // Pre-processing: convert string thinkingLevel to numeric budget
   let processed = normalizeThinkingLevel(body);