diff --git a/README.md b/README.md
index dc95e78..d21b7b7 100644
--- a/README.md
+++ b/README.md
@@ -149,6 +149,7 @@ Edit `.env.local` and configure your chosen provider:
 -   Set `AI_PROVIDER` to your chosen provider (bedrock, openai, anthropic, google, azure, ollama, openrouter, deepseek)
 -   Set `AI_MODEL` to the specific model you want to use
 -   Add the required API keys for your provider
+-   `TEMPERATURE`: Optional temperature setting (e.g., `0` for deterministic output). Leave unset for models that don't support it (e.g., reasoning models).
 -   `ACCESS_CODE_LIST`: Optional access password(s), can be comma-separated for multiple passwords.
 
 > Warning: If you do not set `ACCESS_CODE_LIST`, anyone can access your deployed site directly, which may lead to rapid depletion of your token. It is recommended to set this option.
diff --git a/README_CN.md b/README_CN.md
index 5ba3033..d3c7091 100644
--- a/README_CN.md
+++ b/README_CN.md
@@ -149,6 +149,7 @@ cp env.example .env.local
 -   将 `AI_PROVIDER` 设置为您选择的提供商（bedrock, openai, anthropic, google, azure, ollama, openrouter, deepseek）
 -   将 `AI_MODEL` 设置为您要使用的特定模型
 -   添加您的提供商所需的API密钥
+-   `TEMPERATURE`：可选的温度设置（例如 `0` 表示确定性输出）。对于不支持此参数的模型（如推理模型），请不要设置。
 -   `ACCESS_CODE_LIST` 访问密码，可选，可以使用逗号隔开多个密码。
 
 > 警告：如果不填写 `ACCESS_CODE_LIST`，则任何人都可以直接使用你部署后的网站，可能会导致你的 token 被急速消耗完毕，建议填写此选项。
diff --git a/README_JA.md b/README_JA.md
index a80dd6f..fcb82a3 100644
--- a/README_JA.md
+++ b/README_JA.md
@@ -149,6 +149,7 @@ cp env.example .env.local
 -   `AI_PROVIDER`を選択したプロバイダーに設定（bedrock, openai, anthropic, google, azure, ollama, openrouter, deepseek）
 -   `AI_MODEL`を使用する特定のモデルに設定
 -   プロバイダーに必要なAPIキーを追加
+-   `TEMPERATURE`：オプションの温度設定（例：`0`で決定論的な出力）。温度をサポートしないモデル（推論モデルなど）では設定しないでください。
 -   `ACCESS_CODE_LIST` アクセスパスワード（オプション）。カンマ区切りで複数のパスワードを指定できます。
 
 > 警告：`ACCESS_CODE_LIST`を設定しない場合、誰でもデプロイされたサイトに直接アクセスできるため、トークンが急速に消費される可能性があります。このオプションを設定することをお勧めします。
diff --git a/app/api/chat/route.ts b/app/api/chat/route.ts
index 9fa01d5..42be311 100644
--- a/app/api/chat/route.ts
+++ b/app/api/chat/route.ts
@@ -418,7 +418,9 @@ IMPORTANT: Keep edits concise:
                 }),
             },
         },
-        temperature: 0,
+        ...(process.env.TEMPERATURE !== undefined && {
+            temperature: parseFloat(process.env.TEMPERATURE),
+        }),
     })
 
     return result.toUIMessageStreamResponse()
diff --git a/docs/ai-providers.md b/docs/ai-providers.md
index aabae67..0b2157a 100644
--- a/docs/ai-providers.md
+++ b/docs/ai-providers.md
@@ -133,6 +133,20 @@ This task requires exceptionally strong model capabilities, as it involves gener
 
 **Note on Ollama**: While Ollama is supported as a provider, it's generally not practical for this use case unless you're running high-capability models like DeepSeek R1 or Qwen3-235B locally.
 
+## Temperature Setting
+
+You can optionally configure the temperature via environment variable:
+
+```bash
+TEMPERATURE=0  # More deterministic output (recommended for diagrams)
+```
+
+**Important**: Leave `TEMPERATURE` unset for models that don't support temperature settings, such as:
+- GPT-5.1 and other reasoning models
+- Some specialized models
+
+When unset, the model uses its default behavior.
+
 ## Recommendations
 
 -   **Best experience**: Use models with vision support (GPT-4o, Claude, Gemini) for image-to-diagram features
diff --git a/env.example b/env.example
index 7514843..c782158 100644
--- a/env.example
+++ b/env.example
@@ -48,5 +48,10 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # LANGFUSE_SECRET_KEY=sk-lf-...
 # LANGFUSE_BASEURL=https://cloud.langfuse.com  # EU region, use https://us.cloud.langfuse.com for US
 
+# Temperature (Optional)
+# Controls randomness in AI responses. Lower = more deterministic.
+# Leave unset for models that don't support temperature (e.g., GPT-5.1 reasoning models)
+# TEMPERATURE=0
+
 # Access Control (Optional)
 # ACCESS_CODE_LIST=your-secret-code,another-code