diff --git a/src/server/routes/proxy/upstreamEndpoint.test.ts b/src/server/routes/proxy/upstreamEndpoint.test.ts
index 01eb370..60cffc6 100644
--- a/src/server/routes/proxy/upstreamEndpoint.test.ts
+++ b/src/server/routes/proxy/upstreamEndpoint.test.ts
@@ -614,6 +614,71 @@ describe('buildUpstreamEndpointRequest', () => {
     expect(request.headers['Content-Type']).toBe('application/json');
   });
 
+  it('strips unsupported openai parameters like frequency_penalty for Gemini models on chat endpoints', () => {
+    const request = buildUpstreamEndpointRequest({
+      endpoint: 'chat',
+      modelName: 'gemini-1.5-pro',
+      stream: false,
+      tokenValue: 'sk-test',
+      sitePlatform: 'gemini',
+      openaiBody: {
+        model: 'gemini-1.5-pro',
+        messages: [{ role: 'user', content: 'hello' }],
+        frequency_penalty: 0.5,
+        presence_penalty: 0.2,
+        logit_bias: { '100': 1 },
+        logprobs: true,
+        top_logprobs: 2,
+        store: true,
+        temperature: 0.8,
+        top_p: 1.0,
+      },
+      downstreamFormat: 'openai',
+    });
+
+    expect(request.path).toBe('/v1beta/openai/chat/completions');
+    expect(request.body.frequency_penalty).toBeUndefined();
+    expect(request.body.presence_penalty).toBeUndefined();
+    expect(request.body.logit_bias).toBeUndefined();
+    expect(request.body.logprobs).toBeUndefined();
+    expect(request.body.top_logprobs).toBeUndefined();
+    expect(request.body.store).toBeUndefined();
+    expect(request.body.temperature).toBe(0.8);
+    expect(request.body.top_p).toBe(1.0);
+  });
+
+  it('strips unsupported openai parameters like frequency_penalty for Gemini models on responses endpoints', () => {
+    const request = buildUpstreamEndpointRequest({
+      endpoint: 'responses',
+      modelName: 'gemini-1.5-pro',
+      stream: false,
+      tokenValue: 'sk-test',
+      sitePlatform: 'gemini',
+      openaiBody: {
+        model: 'gemini-1.5-pro',
+        messages: [{ role: 'user', content: 'hello' }],
+        frequency_penalty: 0.5,
+        presence_penalty: 0.2,
+        logit_bias: { '100': 1 },
+        logprobs: true,
+        top_logprobs: 2,
+        store: true,
+        temperature: 0.8,
+        top_p: 1.0,
+      },
+      downstreamFormat: 'openai',
+    });
+
+    expect(request.path).toBe('/v1beta/openai/responses');
+    expect(request.body.frequency_penalty).toBeUndefined();
+    expect(request.body.presence_penalty).toBeUndefined();
+    expect(request.body.logit_bias).toBeUndefined();
+    expect(request.body.logprobs).toBeUndefined();
+    expect(request.body.top_logprobs).toBeUndefined();
+    expect(request.body.store).toBeUndefined();
+    expect(request.body.temperature).toBe(0.8);
+  });
+
   it('preserves structured responses content blocks instead of flattening them', () => {
     const request = buildUpstreamEndpointRequest({
       endpoint: 'responses',
diff --git a/src/server/routes/proxy/upstreamEndpoint.ts b/src/server/routes/proxy/upstreamEndpoint.ts
index 5698bd9..29f0298 100644
--- a/src/server/routes/proxy/upstreamEndpoint.ts
+++ b/src/server/routes/proxy/upstreamEndpoint.ts
@@ -437,6 +437,7 @@ export function buildUpstreamEndpointRequest(input: {
 } {
   const sitePlatform = normalizePlatformName(input.sitePlatform);
   const isClaudeUpstream = sitePlatform === 'claude';
+  const isGeminiUpstream = sitePlatform === 'gemini';
 
   const resolveGeminiEndpointPath = (endpoint: UpstreamEndpoint): string => {
     const normalizedSiteUrl = asTrimmedString(input.siteUrl).toLowerCase();
@@ -452,7 +453,7 @@ export function buildUpstreamEndpointRequest(input: {
   };
 
   const resolveEndpointPath = (endpoint: UpstreamEndpoint): string => {
-    if (sitePlatform === 'gemini') {
+    if (isGeminiUpstream) {
       return resolveGeminiEndpointPath(endpoint);
     }
 
@@ -480,6 +481,21 @@ export function buildUpstreamEndpointRequest(input: {
     commonHeaders.Authorization = `Bearer ${input.tokenValue}`;
   }
 
+  const openaiBody = { ...input.openaiBody };
+  if (isGeminiUpstream) {
+    const blacklist = [
+      'frequency_penalty',
+      'presence_penalty',
+      'logit_bias',
+      'logprobs',
+      'top_logprobs',
+      'store',
+    ];
+    for (const key of blacklist) {
+      delete openaiBody[key];
+    }
+  }
+
   if (input.endpoint === 'messages') {
     const claudeHeaders = input.downstreamFormat === 'claude'
       ? extractClaudePassthroughHeaders(input.downstreamHeaders)
@@ -514,7 +530,7 @@ export function buildUpstreamEndpointRequest(input: {
     const sanitizedBody = nativeClaudeBody
       ?? normalizedClaudeBody
       ?? sanitizeAnthropicMessagesBody(
-        convertOpenAiBodyToAnthropicMessagesBody(input.openaiBody, input.modelName, input.stream),
+        convertOpenAiBodyToAnthropicMessagesBody(openaiBody, input.modelName, input.stream),
       );
 
     const headers = ensureStreamAcceptHeader({
@@ -542,7 +558,7 @@ export function buildUpstreamEndpointRequest(input: {
           model: input.modelName,
           stream: input.stream,
         }
-        : convertOpenAiBodyToResponsesBodyViaTransformer(input.openaiBody, input.modelName, input.stream)
+        : convertOpenAiBodyToResponsesBodyViaTransformer(openaiBody, input.modelName, input.stream)
     );
     const body = sanitizeResponsesBodyForProxyViaTransformer(rawBody, input.modelName, input.stream);
 
@@ -563,7 +579,7 @@ export function buildUpstreamEndpointRequest(input: {
     path: resolveEndpointPath('chat'),
     headers,
     body: {
-      ...input.openaiBody,
+      ...openaiBody,
       model: input.modelName,
       stream: input.stream,
     },