diff --git a/packages/mcp-server/src/internal/agents/openai-provider.ts b/packages/mcp-server/src/internal/agents/openai-provider.ts
index 88b60a41..b9147ffe 100644
--- a/packages/mcp-server/src/internal/agents/openai-provider.ts
+++ b/packages/mcp-server/src/internal/agents/openai-provider.ts
@@ -21,34 +21,47 @@ export function setOpenAIBaseUrl(baseUrl: string | undefined): void {
   configuredBaseUrl = baseUrl;
 }
 
+/**
+ * Check if a model supports reasoning effort parameter (o1 series only)
+ */
+function supportsReasoningEffort(modelName: string): boolean {
+  return modelName.startsWith("o1-") || modelName.startsWith("o3-");
+}
+
 /**
  * Retrieve an OpenAI language model configured from environment variables and explicit config.
  *
  * Configuration:
  * - OPENAI_MODEL: Model to use (default: "gpt-5") - env var OK
- * - OPENAI_REASONING_EFFORT: Reasoning effort for o1 models: "low", "medium", "high", or "" to disable (default: "low") - env var OK
+ * - OPENAI_REASONING_EFFORT: Reasoning effort for o1 models: "low", "medium", "high", or "" to disable (default: "low" for o1 models) - env var OK
  * - Base URL: Must be set via setOpenAIBaseUrl() - NOT from env vars (security risk)
  */
 export function getOpenAIModel(model?: string): LanguageModelV1 {
   const defaultModel = process.env.OPENAI_MODEL || DEFAULT_OPENAI_MODEL;
+  const selectedModel = model ?? defaultModel;
 
   // Handle reasoning effort: empty string explicitly disables it, undefined uses default
   const envReasoningEffort = process.env.OPENAI_REASONING_EFFORT;
   let reasoningEffort: ReasoningEffort | undefined;
 
+  // Validate reasoning effort if it's set (regardless of model)
   if (envReasoningEffort === "") {
     // Empty string explicitly disables reasoning effort
     reasoningEffort = undefined;
   } else if (envReasoningEffort === undefined) {
-    // Not set - use default
-    reasoningEffort = DEFAULT_REASONING_EFFORT;
+    // Not set - use default only for o1 models
+    if (supportsReasoningEffort(selectedModel)) {
+      reasoningEffort = DEFAULT_REASONING_EFFORT;
+    }
   } else if (
     VALID_REASONING_EFFORTS.includes(envReasoningEffort as ReasoningEffort)
   ) {
-    // Valid value
-    reasoningEffort = envReasoningEffort as ReasoningEffort;
+    // Valid value - only use it for o1 models
+    if (supportsReasoningEffort(selectedModel)) {
+      reasoningEffort = envReasoningEffort as ReasoningEffort;
+    }
   } else {
-    // Invalid value - provide helpful error with all valid options
+    // Invalid value - always throw error to help users catch configuration mistakes
     const validValues = VALID_REASONING_EFFORTS.map((v) => `"${v}"`).join(", ");
     throw new Error(
       `Invalid OPENAI_REASONING_EFFORT value: "${envReasoningEffort}". Must be one of: ${validValues}, or "" (empty string to disable). Default is "${DEFAULT_REASONING_EFFORT}".`,
@@ -62,7 +75,7 @@ export function getOpenAIModel(model?: string): LanguageModelV1 {
     },
   });
 
-  return factory(model ?? defaultModel, {
+  return factory(selectedModel, {
     ...(reasoningEffort && { reasoningEffort }),
   });
 }