diff --git a/core/llm/llms/OpenAI.test.ts b/core/llm/llms/OpenAI.test.ts
index 943e9436903..4a9f5f92f6b 100644
--- a/core/llm/llms/OpenAI.test.ts
+++ b/core/llm/llms/OpenAI.test.ts
@@ -24,6 +24,13 @@ describe("OpenAI", () => {
     expect(openai.isOSeriesOrGpt5PlusModel("gpt-5.4-pro")).toBeTruthy();
     expect(openai.isOSeriesOrGpt5PlusModel("gpt-6")).toBeTruthy();
     expect(openai.isOSeriesOrGpt5PlusModel("gpt-7-turbo")).toBeTruthy();
+
+    // case-insensitive matching (e.g. Azure deployments named in upper case)
+    expect(openai.isOSeriesOrGpt5PlusModel("GPT-5")).toBeTruthy();
+    expect(openai.isOSeriesOrGpt5PlusModel("GPT-5-mini")).toBeTruthy();
+    expect(openai.isOSeriesOrGpt5PlusModel("Gpt-5.4")).toBeTruthy();
+    expect(openai.isOSeriesOrGpt5PlusModel("O1")).toBeTruthy();
+    expect(openai.isOSeriesOrGpt5PlusModel("O3-mini")).toBeTruthy();
   });
   test("should identify incorrect o-series models", () => {
     const openai = new OpenAI({
diff --git a/core/llm/llms/OpenAI.ts b/core/llm/llms/OpenAI.ts
index c65b55dc1a5..c833f792570 100644
--- a/core/llm/llms/OpenAI.ts
+++ b/core/llm/llms/OpenAI.ts
@@ -219,7 +219,9 @@ class OpenAI extends BaseLLM {
   }
 
   public isOSeriesOrGpt5PlusModel(model?: string): boolean {
-    return !!model && (!!model.match(/^o[0-9]+/) || !!model.match(/gpt-[5-9]/));
+    return (
+      !!model && (!!model.match(/^o[0-9]+/i) || !!model.match(/gpt-[5-9]/i))
+    );
   }
 
   private isFireworksAiModel(model?: string): boolean {
diff --git a/packages/openai-adapters/src/apis/OpenAI.ts b/packages/openai-adapters/src/apis/OpenAI.ts
index d0f8d30ca3a..70655171aa7 100644
--- a/packages/openai-adapters/src/apis/OpenAI.ts
+++ b/packages/openai-adapters/src/apis/OpenAI.ts
@@ -67,7 +67,8 @@ export class OpenAIApi implements BaseLlmApi {
     // o-series models - only apply for official OpenAI API
     const isOfficialOpenAIAPI = this.apiBase === "https://api.openai.com/v1/";
     if (isOfficialOpenAIAPI) {
-      if (body.model.startsWith("o") || body.model.includes("gpt-5")) {
+      const modelLower = body.model.toLowerCase();
+      if (modelLower.startsWith("o") || modelLower.includes("gpt-5")) {
         // a) use max_completion_tokens instead of max_tokens
         body.max_completion_tokens = body.max_tokens;
         body.max_tokens = undefined;
@@ -80,7 +81,7 @@ export class OpenAIApi implements BaseLlmApi {
           return message;
         });
       }
-      if (body.tools?.length && !body.model.startsWith("o3")) {
+      if (body.tools?.length && !modelLower.startsWith("o3")) {
         body.parallel_tool_calls = false;
       }
     }