diff --git a/open-sse/config/providerRegistry.ts b/open-sse/config/providerRegistry.ts
index 02a19bfc1e..9f56748fd7 100644
--- a/open-sse/config/providerRegistry.ts
+++ b/open-sse/config/providerRegistry.ts
@@ -12,8 +12,21 @@ export interface RegistryModel {
   id: string;
   name: string;
   targetFormat?: string;
+  unsupportedParams?: readonly string[];
 }
 
+// Reasoning models reject temperature, top_p, penalties, logprobs, n.
+// Frozen to prevent accidental mutation (shared across all model entries).
+const REASONING_UNSUPPORTED: readonly string[] = Object.freeze([
+  "temperature",
+  "top_p",
+  "frequency_penalty",
+  "presence_penalty",
+  "logprobs",
+  "top_logprobs",
+  "n",
+]);
+
 export interface RegistryOAuth {
   clientIdEnv?: string;
   clientIdDefault?: string;
@@ -426,8 +439,11 @@ export const REGISTRY: Record<string, RegistryEntry> = {
       { id: "gpt-4o", name: "GPT-4o" },
       { id: "gpt-4o-mini", name: "GPT-4o Mini" },
       { id: "gpt-4-turbo", name: "GPT-4 Turbo" },
-      { id: "o1", name: "O1" },
-      { id: "o1-mini", name: "O1 Mini" },
+      { id: "o1", name: "O1", unsupportedParams: REASONING_UNSUPPORTED },
+      { id: "o1-mini", name: "O1 Mini", unsupportedParams: REASONING_UNSUPPORTED },
+      { id: "o1-pro", name: "O1 Pro", unsupportedParams: REASONING_UNSUPPORTED },
+      { id: "o3", name: "O3", unsupportedParams: REASONING_UNSUPPORTED },
+      { id: "o3-mini", name: "O3 Mini", unsupportedParams: REASONING_UNSUPPORTED },
     ],
   },
 
@@ -1060,6 +1076,43 @@ export function getRegisteredProviders(): string[] {
   return Object.keys(REGISTRY);
 }
 
+// Precomputed map: modelId → unsupportedParams (O(1) lookup instead of O(N×M) scan).
+// Built once at module load from all registry entries.
+const _unsupportedParamsMap = new Map<string, readonly string[]>();
+for (const entry of Object.values(REGISTRY)) {
+  for (const model of entry.models) {
+    if (model.unsupportedParams && !_unsupportedParamsMap.has(model.id)) {
+      _unsupportedParamsMap.set(model.id, model.unsupportedParams);
+    }
+  }
+}
+
+/**
+ * Get unsupported parameters for a specific model.
+ * Uses O(1) precomputed lookup. Also handles prefixed model IDs
+ * (e.g., "openai/o3" → strips prefix and looks up "o3").
+ * Returns empty array if no restrictions are defined.
+ */
+export function getUnsupportedParams(provider: string, modelId: string): readonly string[] {
+  // 1. Check current provider's registry (exact match)
+  const entry = getRegistryEntry(provider);
+  const modelEntry = entry?.models.find((m) => m.id === modelId);
+  if (modelEntry?.unsupportedParams) return modelEntry.unsupportedParams;
+
+  // 2. O(1) lookup in precomputed map (handles cross-provider routing)
+  const cached = _unsupportedParamsMap.get(modelId);
+  if (cached) return cached;
+
+  // 3. Handle prefixed model IDs (e.g., "openai/o3" → "o3")
+  if (modelId.includes("/")) {
+    const bareId = modelId.split("/").pop() || "";
+    const bare = _unsupportedParamsMap.get(bareId);
+    if (bare) return bare;
+  }
+
+  return [];
+}
+
 /**
  * Get provider category: "oauth" or "apikey"
  * Used by the resilience layer to apply different cooldown/backoff profiles.
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 1919add060..820692030a 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -13,6 +13,7 @@ import { refreshWithRetry } from "../services/tokenRefresh.ts";
 import { createRequestLogger } from "../utils/requestLogger.ts";
 import { getModelTargetFormat, PROVIDER_ID_TO_ALIAS } from "../config/providerModels.ts";
 import { resolveModelAlias } from "../services/modelDeprecation.ts";
+import { getUnsupportedParams } from "../config/providerRegistry.ts";
 import { createErrorResult, parseUpstreamError, formatProviderError } from "../utils/error.ts";
 import { HTTP_STATUS } from "../config/constants.ts";
 import { handleBypassRequest } from "../utils/bypassHandler.ts";
@@ -53,7 +54,9 @@ export function shouldUseNativeCodexPassthrough({
 }): boolean {
   if (provider !== "codex") return false;
   if (sourceFormat !== FORMATS.OPENAI_RESPONSES) return false;
-  return String(endpointPath || "").toLowerCase().endsWith("/responses");
+  return String(endpointPath || "")
+    .toLowerCase()
+    .endsWith("/responses");
 }
 
 /**
@@ -287,6 +290,21 @@ export async function handleChatCore({
   // Update model in body
   translatedBody.model = model;
 
+  // Strip unsupported parameters for reasoning models (o1, o3, etc.)
+  const unsupported = getUnsupportedParams(provider, model);
+  if (unsupported.length > 0) {
+    const stripped: string[] = [];
+    for (const param of unsupported) {
+      if (Object.hasOwn(translatedBody, param)) {
+        stripped.push(param);
+        delete translatedBody[param];
+      }
+    }
+    if (stripped.length > 0) {
+      log?.warn?.("PARAMS", `Stripped unsupported params for ${model}: ${stripped.join(", ")}`);
+    }
+  }
+
   // Get executor for this provider
   const executor = getExecutor(provider);