diff --git a/package.json b/package.json
index f8dc822..d8d3e83 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@missionsquad/rosetta-ai",
-  "version": "1.12.0",
+  "version": "1.12.1",
   "description": "Unified TypeScript SDK for interacting with multiple AI providers (Anthropic, Google, Groq, OpenAI).",
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
diff --git a/src/core/mapping/gpt5.support.ts b/src/core/mapping/gpt5.support.ts
index 2f9fdeb..09e904f 100644
--- a/src/core/mapping/gpt5.support.ts
+++ b/src/core/mapping/gpt5.support.ts
@@ -7,6 +7,7 @@ export interface Gpt5Support {
   fixedReasoningEffort?: ReasoningEffort
   supportsVerbosity: boolean
   supportsSampling: 'never' | 'always' | 'only_with_reasoning_none'
+  supportsFunctionToolsWithReasoning?: boolean
 }
 
 // Chat Completions-supported GPT-5 models verified for this implementation.
@@ -16,62 +17,71 @@ const GPT5_CHAT_COMPLETIONS_SUPPORT: Record<string, Gpt5Support> = {
     allowedReasoningEfforts: ['minimal', 'low', 'medium', 'high'],
     defaultReasoningEffort: 'medium',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5-mini': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['minimal', 'low', 'medium', 'high'],
     defaultReasoningEffort: 'medium',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5-nano': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['minimal', 'low', 'medium', 'high'],
     defaultReasoningEffort: 'medium',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5.1': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['none', 'low', 'medium', 'high'],
     defaultReasoningEffort: 'none',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5.2': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['none', 'low', 'medium', 'high', 'xhigh'],
     defaultReasoningEffort: 'none',
     supportsVerbosity: true,
-    supportsSampling: 'only_with_reasoning_none'
+    supportsSampling: 'only_with_reasoning_none',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5.4': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['none', 'low', 'medium', 'high', 'xhigh'],
     defaultReasoningEffort: 'none',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: false
   },
   'gpt-5-chat-latest': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: [],
     supportsVerbosity: true,
-    supportsSampling: 'always'
+    supportsSampling: 'always',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5.1-chat-latest': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['medium'],
     fixedReasoningEffort: 'medium',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   },
   'gpt-5.2-chat-latest': {
     chatCompletionsSupported: true,
     allowedReasoningEfforts: ['medium'],
     fixedReasoningEffort: 'medium',
     supportsVerbosity: true,
-    supportsSampling: 'never'
+    supportsSampling: 'never',
+    supportsFunctionToolsWithReasoning: true
   }
 }
 
diff --git a/src/core/mapping/openai.mapper.ts b/src/core/mapping/openai.mapper.ts
index 8bdb085..6135d46 100644
--- a/src/core/mapping/openai.mapper.ts
+++ b/src/core/mapping/openai.mapper.ts
@@ -191,6 +191,7 @@ export class OpenAIMapper implements IProviderMapper {
     const isThinking = isThinkingModel(params.model!)
     const isGPT5 = isGPT5Model(params.model!)
     const hasEffectiveLimit = baseMappedParams.maxTokens !== undefined
+    const hasFunctionTools = Array.isArray(tools) && tools.length > 0
 
     if (!isThinking) {
       // Non-thinking OpenAI models must use max_completion_tokens only.
@@ -235,7 +236,11 @@ export class OpenAIMapper implements IProviderMapper {
           }
         }
 
-        if (effectiveEffort !== undefined) {
+        const shouldOmitReasoningForFunctionTools =
+          hasFunctionTools &&
+          support.supportsFunctionToolsWithReasoning === false
+
+        if (effectiveEffort !== undefined && !shouldOmitReasoningForFunctionTools) {
           basePayload.reasoning_effort = effectiveEffort
         } else {
           delete basePayload.reasoning_effort
diff --git a/tests/unit/core/mapping/openai.mapper.spec.ts b/tests/unit/core/mapping/openai.mapper.spec.ts
index f67b33e..edd705f 100644
--- a/tests/unit/core/mapping/openai.mapper.spec.ts
+++ b/tests/unit/core/mapping/openai.mapper.spec.ts
@@ -522,6 +522,28 @@ describe('OpenAI Mapper', () => {
       expect(result.max_completion_tokens).toBe(321)
     })
 
+    it('[Medium] should omit reasoning_effort for gpt-5.4 when function tools are present', () => {
+      const params: GenerateParams = {
+        ...baseParams,
+        model: 'gpt-5.4',
+        messages: [{ role: 'user', content: 'Generate.' }],
+        reasoningEffort: 'high',
+        tools: [
+          {
+            type: 'function',
+            function: {
+              name: 'myFunc',
+              parameters: { type: 'object', properties: {} },
+              zodSchema: z.object({})
+            }
+          }
+        ]
+      }
+      const result = mapper.mapToProviderParams(params) as any
+      expect(result.reasoning_effort).toBeUndefined()
+      expect(result.tools).toHaveLength(1)
+    })
+
     it('[Medium] should map toolChoice required and none', () => {
       const paramsRequired: GenerateParams = { ...baseParams, messages: [], toolChoice: 'required' }
       const paramsNone: GenerateParams = { ...baseParams, messages: [], toolChoice: 'none' }