diff --git a/src/_locales/en/main.json b/src/_locales/en/main.json
index 8ad74e6ef..0397d490d 100644
--- a/src/_locales/en/main.json
+++ b/src/_locales/en/main.json
@@ -124,6 +124,9 @@
   "Override provider temperature": "Override provider temperature",
   "The temperature parameter is not sent. The provider or model default is used.": "The temperature parameter is not sent. The provider or model default is used.",
   "The current model does not accept a custom temperature. The parameter will not be sent.": "The current model does not accept a custom temperature. The parameter will not be sent.",
+  "Extra Request Body (JSON)": "Extra Request Body (JSON)",
+  "Merged into the API request body. Must be a JSON object, other values are ignored.": "Merged into the API request body. Must be a JSON object, other values are ignored.",
+  "Invalid JSON object, this value is ignored.": "Invalid JSON object, this value is ignored.",
   "API Url": "API Url",
   "Provider": "Provider",
   "Others": "Others",
diff --git a/src/_locales/zh-hans/main.json b/src/_locales/zh-hans/main.json
index cfad6d2d1..3748a96ba 100644
--- a/src/_locales/zh-hans/main.json
+++ b/src/_locales/zh-hans/main.json
@@ -118,6 +118,9 @@
   "Override provider temperature": "覆盖提供商的温度参数",
   "The temperature parameter is not sent. The provider or model default is used.": "不会发送温度参数，将使用提供商或模型的默认值。",
   "The current model does not accept a custom temperature. The parameter will not be sent.": "当前模型不接受自定义温度参数，因此不会发送该参数。",
+  "Extra Request Body (JSON)": "额外请求参数 (JSON)",
+  "Merged into the API request body. Must be a JSON object, other values are ignored.": "会合并进 API 请求体，必须是 JSON 对象，其他类型的值会被忽略。",
+  "Invalid JSON object, this value is ignored.": "不是合法的 JSON 对象，该值会被忽略。",
   "API Url": "API地址",
   "Provider": "提供商",
   "Others": "其他",
diff --git a/src/_locales/zh-hant/main.json b/src/_locales/zh-hant/main.json
index 3dd20a229..832f8792f 100644
--- a/src/_locales/zh-hant/main.json
+++ b/src/_locales/zh-hant/main.json
@@ -118,6 +118,9 @@
   "Override provider temperature": "覆寫供應商的溫度參數",
   "The temperature parameter is not sent. The provider or model default is used.": "不會傳送溫度參數，將使用供應商或模型的預設值。",
   "The current model does not accept a custom temperature. The parameter will not be sent.": "目前的模型不接受自訂溫度參數，因此不會傳送這個參數。",
+  "Extra Request Body (JSON)": "額外請求參數 (JSON)",
+  "Merged into the API request body. Must be a JSON object, other values are ignored.": "會合併進 API 請求主體，必須是 JSON 物件，其他類型的值會被忽略。",
+  "Invalid JSON object, this value is ignored.": "不是合法的 JSON 物件，這個值會被忽略。",
   "API Url": "API 網址",
   "Provider": "供應商",
   "Others": "其他",
diff --git a/src/config/index.mjs b/src/config/index.mjs
index 81c956e46..1f1853fda 100644
--- a/src/config/index.mjs
+++ b/src/config/index.mjs
@@ -855,6 +855,7 @@ export const defaultConfig = {
   maxConversationContextLength: 9,
   temperatureOverrideEnabled: false,
   temperature: 1,
+  extraBody: '',
   customChatGptWebApiUrl: 'https://chatgpt.com',
   customChatGptWebApiPath: '/backend-api/conversation',
   customOpenAiApiUrl: 'https://api.openai.com',
diff --git a/src/popup/sections/AdvancedPart.jsx b/src/popup/sections/AdvancedPart.jsx
index 017b9191f..134535198 100644
--- a/src/popup/sections/AdvancedPart.jsx
+++ b/src/popup/sections/AdvancedPart.jsx
@@ -3,6 +3,7 @@ import { parseFloatWithClamp, parseIntWithClamp } from '../../utils/index.mjs'
 import { getModelValue } from '../../utils/model-name-convert.mjs'
 import { isUsingAzureOpenAiApiModel } from '../../config/index.mjs'
 import { canApplyTemperatureOverride } from '../../services/apis/temperature-params.mjs'
+import { parseExtraBody } from '../../services/apis/extra-body-params.mjs'
 import PropTypes from 'prop-types'
 import { Tab, TabList, TabPanel, Tabs } from 'react-tabs'
 import Browser from 'webextension-polyfill'
@@ -22,6 +23,8 @@ function ApiParams({ config, updateConfig }) {
     ? config.customModelName
     : getModelValue(config)
   const temperatureOverrideAvailable = canApplyTemperatureOverride(selectedModel)
+  const extraBodyValue = typeof config.extraBody === 'string' ? config.extraBody : ''
+  const extraBodyInvalid = extraBodyValue.trim() !== '' && !parseExtraBody(extraBodyValue)
 
   return (
     <>
@@ -89,6 +92,21 @@ function ApiParams({ config, updateConfig }) {
           />
         </label>
       )}
+      <label>
+        {t('Extra Request Body (JSON)')}
+        <textarea
+          value={extraBodyValue}
+          placeholder={'{\n  "reasoning_effort": "high"\n}'}
+          onChange={(e) => {
+            updateConfig({ extraBody: e.target.value })
+          }}
+        />
+      </label>
+      <small>
+        {extraBodyInvalid
+          ? t('Invalid JSON object, this value is ignored.')
+          : t('Merged into the API request body. Must be a JSON object, other values are ignored.')}
+      </small>
     </>
   )
 }
diff --git a/src/services/apis/azure-openai-api.mjs b/src/services/apis/azure-openai-api.mjs
index f5e1b4b6d..7eb4850ad 100644
--- a/src/services/apis/azure-openai-api.mjs
+++ b/src/services/apis/azure-openai-api.mjs
@@ -5,6 +5,7 @@ import { fetchSSE } from '../../utils/fetch-sse.mjs'
 import { isEmpty } from 'lodash-es'
 import { getModelValue } from '../../utils/model-name-convert.mjs'
 import { getTemperatureParams } from './temperature-params.mjs'
+import { getExtraBodyParams } from './extra-body-params.mjs'
 
 /**
  * @param {Runtime.Port} port
@@ -42,6 +43,7 @@ export async function generateAnswersWithAzureOpenaiApi(port, question, session)
         max_tokens: config.maxResponseTokenLength,
         // Azure deployment names are opaque aliases, not canonical model identifiers.
         ...getTemperatureParams(config),
+        ...getExtraBodyParams(config),
       }),
       onMessage(message) {
         console.debug('sse message', message)
diff --git a/src/services/apis/claude-api.mjs b/src/services/apis/claude-api.mjs
index 5d298877a..d9a12bd39 100644
--- a/src/services/apis/claude-api.mjs
+++ b/src/services/apis/claude-api.mjs
@@ -5,6 +5,7 @@ import { isEmpty } from 'lodash-es'
 import { getConversationPairs } from '../../utils/get-conversation-pairs.mjs'
 import { getModelValue } from '../../utils/model-name-convert.mjs'
 import { getTemperatureParams } from './temperature-params.mjs'
+import { getExtraBodyParams } from './extra-body-params.mjs'
 
 function getThinkingConfig(model) {
   if (model === 'claude-sonnet-5') return { type: 'disabled' }
@@ -37,6 +38,8 @@ export async function generateAnswersWithClaudeApi(port, question, session) {
   }
   const thinking = getThinkingConfig(model)
   if (thinking) body.thinking = thinking
+  // The user-provided body wins over the built-in defaults above.
+  Object.assign(body, getExtraBodyParams(config))
 
   let answer = ''
   let stopReason = ''
diff --git a/src/services/apis/extra-body-params.mjs b/src/services/apis/extra-body-params.mjs
new file mode 100644
index 000000000..3be95c3e0
--- /dev/null
+++ b/src/services/apis/extra-body-params.mjs
@@ -0,0 +1,35 @@
+/**
+ * Parse the user-provided extra request body.
+ * @param {unknown} raw JSON text from the advanced settings textarea
+ * @returns {Record<string, unknown> | null} the parsed object, or null when unusable
+ */
+export function parseExtraBody(raw) {
+  if (typeof raw !== 'string' || !raw.trim()) return null
+  let parsed
+  try {
+    parsed = JSON.parse(raw)
+  } catch {
+    return null
+  }
+  if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed)) return null
+  return parsed
+}
+
+/**
+ * Keys every request builder owns. A custom body may add parameters the UI does
+ * not expose, but replacing these would desync the request from the conversation,
+ * model and settings the user picked, and `stream` must stay on for SSE parsing.
+ */
+const RESERVED_KEYS = ['stream', 'model', 'messages', 'prompt', 'temperature']
+
+/**
+ * Extra fields merged into the API request body.
+ * @param {UserConfig} config
+ * @returns {Record<string, unknown>}
+ */
+export function getExtraBodyParams(config) {
+  const extraBody = parseExtraBody(config?.extraBody)
+  if (!extraBody) return {}
+  for (const key of RESERVED_KEYS) delete extraBody[key]
+  return extraBody
+}
diff --git a/src/services/apis/openai-compatible-core.mjs b/src/services/apis/openai-compatible-core.mjs
index 02dfe4a81..724d34a3f 100644
--- a/src/services/apis/openai-compatible-core.mjs
+++ b/src/services/apis/openai-compatible-core.mjs
@@ -4,6 +4,7 @@ import { isEmpty } from 'lodash-es'
 import { getCompletionPromptBase, pushRecord, setAbortController } from './shared.mjs'
 import { getChatCompletionsTokenParams } from './openai-token-params.mjs'
 import { getTemperatureParams } from './temperature-params.mjs'
+import { getExtraBodyParams } from './extra-body-params.mjs'
 
 function buildHeaders(apiKey, extraHeaders = {}) {
   const headers = {
@@ -76,11 +77,15 @@ export async function generateAnswersWithOpenAICompatible({
   session.conversationRecords = conversationRecords
   const safeExtraBody = { ...extraBody }
   delete safeExtraBody.temperature
+  // Merged last so the Advanced setting wins over built-in values, except for
+  // the token-limit key: only the one the request shape expects may be sent.
+  const configuredExtraBody = getExtraBodyParams(config)
   if (endpointType === 'completion') {
     const prompt =
       (await getCompletionPromptBase()) +
       getConversationPairs(conversationRecords.slice(-config.maxConversationContextLength), true) +
       `Human: ${question}\nAI: `
+    delete configuredExtraBody.max_completion_tokens
     requestBody = {
       prompt,
       model,
@@ -89,6 +94,7 @@ export async function generateAnswersWithOpenAICompatible({
       ...getTemperatureParams(config, model),
       stop: '\nHuman',
       ...safeExtraBody,
+      ...configuredExtraBody,
     }
   } else {
     const messages = getConversationPairs(
@@ -104,6 +110,7 @@ export async function generateAnswersWithOpenAICompatible({
     const conflictingTokenParamKey =
       'max_completion_tokens' in tokenParams ? 'max_tokens' : 'max_completion_tokens'
     delete safeExtraBody[conflictingTokenParamKey]
+    delete configuredExtraBody[conflictingTokenParamKey]
     requestBody = {
       messages,
       model,
@@ -111,6 +118,7 @@ export async function generateAnswersWithOpenAICompatible({
       ...tokenParams,
       ...getTemperatureParams(config, model),
       ...safeExtraBody,
+      ...configuredExtraBody,
     }
   }
 
diff --git a/tests/unit/services/extra-body-params.test.mjs b/tests/unit/services/extra-body-params.test.mjs
new file mode 100644
index 000000000..e7ebe9352
--- /dev/null
+++ b/tests/unit/services/extra-body-params.test.mjs
@@ -0,0 +1,48 @@
+import assert from 'node:assert/strict'
+import { test } from 'node:test'
+import {
+  getExtraBodyParams,
+  parseExtraBody,
+} from '../../../src/services/apis/extra-body-params.mjs'
+
+test('parseExtraBody accepts a JSON object', () => {
+  assert.deepEqual(parseExtraBody('{"thinking":{"type":"enabled","budget_tokens":2048}}'), {
+    thinking: { type: 'enabled', budget_tokens: 2048 },
+  })
+})
+
+test('parseExtraBody rejects anything that is not a JSON object', () => {
+  for (const raw of ['', '   ', 'not json', '[1,2]', '"text"', '42', 'null', null, undefined, {}]) {
+    assert.equal(parseExtraBody(raw), null, `expected null for ${JSON.stringify(raw)}`)
+  }
+})
+
+test('getExtraBodyParams is empty without a usable config value', () => {
+  assert.deepEqual(getExtraBodyParams(undefined), {})
+  assert.deepEqual(getExtraBodyParams({}), {})
+  assert.deepEqual(getExtraBodyParams({ extraBody: '{oops' }), {})
+})
+
+test('getExtraBodyParams forwards user fields and keeps stream under extension control', () => {
+  const config = { extraBody: '{"reasoning_effort":"high","stream":false}' }
+
+  assert.deepEqual(getExtraBodyParams(config), { reasoning_effort: 'high' })
+  // The parsed object is rebuilt per call, so repeated reads stay stripped.
+  assert.deepEqual(getExtraBodyParams(config), { reasoning_effort: 'high' })
+})
+
+test('getExtraBodyParams strips every key the request builders own', () => {
+  const config = {
+    extraBody: JSON.stringify({
+      reasoning_effort: 'high',
+      top_p: 0.9,
+      stream: false,
+      model: 'gpt-4o',
+      messages: [{ role: 'user', content: 'tampered' }],
+      prompt: 'tampered',
+      temperature: 0.1,
+    }),
+  }
+
+  assert.deepEqual(getExtraBodyParams(config), { reasoning_effort: 'high', top_p: 0.9 })
+})
diff --git a/tests/unit/services/extra-body-request.test.mjs b/tests/unit/services/extra-body-request.test.mjs
new file mode 100644
index 000000000..b04357163
--- /dev/null
+++ b/tests/unit/services/extra-body-request.test.mjs
@@ -0,0 +1,171 @@
+import assert from 'node:assert/strict'
+import { beforeEach, test } from 'node:test'
+import { generateAnswersWithOpenAICompatible } from '../../../src/services/apis/openai-compatible-core.mjs'
+import { generateAnswersWithClaudeApi } from '../../../src/services/apis/claude-api.mjs'
+import { generateAnswersWithAzureOpenaiApi } from '../../../src/services/apis/azure-openai-api.mjs'
+import { createFakePort } from '../helpers/port.mjs'
+import { createMockSseResponse } from '../helpers/sse-response.mjs'
+
+const CHAT_CHUNKS = [
+  'data: {"choices":[{"delta":{"content":"hi"},"finish_reason":"stop"}]}\n\n',
+  'data: [DONE]\n\n',
+]
+
+const CLAUDE_CHUNKS = [
+  'data: {"type":"content_block_delta","delta":{"type":"text_delta","text":"hi"}}\n\n',
+  'data: {"type":"message_delta","delta":{"stop_reason":"end_turn"}}\n\n',
+  'data: {"type":"message_stop"}\n\n',
+]
+
+beforeEach(() => {
+  globalThis.__TEST_BROWSER_SHIM__.clearStorage()
+})
+
+async function captureRequestBody(t, chunks, run) {
+  let requestBody
+  t.mock.method(console, 'debug', () => {})
+  t.mock.method(globalThis, 'fetch', async (url, options) => {
+    requestBody = JSON.parse(options.body)
+    return createMockSseResponse(chunks)
+  })
+  await run()
+  return requestBody
+}
+
+function openAiCompatibleRequest({ endpointType = 'chat', model = 'gpt-5', extraBody }) {
+  return {
+    port: createFakePort(),
+    question: 'hi',
+    session: { conversationRecords: [] },
+    endpointType,
+    requestUrl: `https://example.com/v1/${
+      endpointType === 'chat' ? 'chat/completions' : 'completions'
+    }`,
+    model,
+    apiKey: 'key',
+    provider: 'openai',
+    config: {
+      maxConversationContextLength: 9,
+      maxResponseTokenLength: 1000,
+      temperatureOverrideEnabled: false,
+      temperature: 1,
+      extraBody,
+    },
+  }
+}
+
+test('OpenAI-compatible requests send the extra body without losing the SSE stream', async (t) => {
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithOpenAICompatible(
+      openAiCompatibleRequest({ extraBody: '{"reasoning_effort":"high","stream":false}' }),
+    ),
+  )
+
+  assert.equal(requestBody.reasoning_effort, 'high')
+  assert.equal(requestBody.stream, true)
+  assert.equal(requestBody.model, 'gpt-5')
+})
+
+test('OpenAI-compatible chat requests keep only the token key the model family uses', async (t) => {
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithOpenAICompatible(
+      openAiCompatibleRequest({ extraBody: '{"max_tokens":123,"max_completion_tokens":456}' }),
+    ),
+  )
+
+  // gpt-5 sends max_completion_tokens, so that is where the override lands.
+  assert.equal(requestBody.max_completion_tokens, 456)
+  assert.equal('max_tokens' in requestBody, false)
+})
+
+test('OpenAI-compatible chat requests drop the unused token key for max_tokens models', async (t) => {
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithOpenAICompatible(
+      openAiCompatibleRequest({
+        model: 'gpt-4.1',
+        extraBody: '{"max_tokens":123,"max_completion_tokens":456}',
+      }),
+    ),
+  )
+
+  assert.equal(requestBody.max_tokens, 123)
+  assert.equal('max_completion_tokens' in requestBody, false)
+})
+
+test('OpenAI-compatible completion requests stay on max_tokens', async (t) => {
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithOpenAICompatible(
+      openAiCompatibleRequest({
+        endpointType: 'completion',
+        model: 'gpt-3.5-turbo-instruct',
+        extraBody: '{"max_tokens":123,"max_completion_tokens":456}',
+      }),
+    ),
+  )
+
+  assert.equal(requestBody.max_tokens, 123)
+  assert.equal('max_completion_tokens' in requestBody, false)
+})
+
+test('OpenAI-compatible requests keep the conversation and model under extension control', async (t) => {
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithOpenAICompatible(
+      openAiCompatibleRequest({
+        extraBody: '{"model":"gpt-4o","messages":[{"role":"user","content":"tampered"}]}',
+      }),
+    ),
+  )
+
+  assert.equal(requestBody.model, 'gpt-5')
+  assert.deepEqual(requestBody.messages, [{ role: 'user', content: 'hi' }])
+})
+
+test('Azure OpenAI requests send the extra body', async (t) => {
+  globalThis.__TEST_BROWSER_SHIM__.replaceStorage({
+    azureApiKey: 'key',
+    azureEndpoint: 'https://example.openai.azure.com',
+    azureDeploymentName: 'deployment',
+    extraBody: '{"reasoning_effort":"high"}',
+  })
+
+  const requestBody = await captureRequestBody(t, CHAT_CHUNKS, () =>
+    generateAnswersWithAzureOpenaiApi(createFakePort(), 'hi', { conversationRecords: [] }),
+  )
+
+  assert.equal(requestBody.reasoning_effort, 'high')
+})
+
+test('Claude requests let the extra body override the built-in thinking default', async (t) => {
+  globalThis.__TEST_BROWSER_SHIM__.replaceStorage({
+    anthropicApiKey: 'key',
+    extraBody: '{"thinking":{"type":"enabled","budget_tokens":2048}}',
+  })
+
+  const requestBody = await captureRequestBody(t, CLAUDE_CHUNKS, () =>
+    generateAnswersWithClaudeApi(createFakePort(), 'hi', {
+      // This model normally gets thinking forced off.
+      modelName: 'claudeSonnet5Api',
+      conversationRecords: [],
+    }),
+  )
+
+  assert.equal(requestBody.model, 'claude-sonnet-5')
+  assert.deepEqual(requestBody.thinking, { type: 'enabled', budget_tokens: 2048 })
+})
+
+test('Claude requests keep the model and conversation under extension control', async (t) => {
+  globalThis.__TEST_BROWSER_SHIM__.replaceStorage({
+    anthropicApiKey: 'key',
+    extraBody: '{"model":"claude-opus-9","messages":[{"role":"user","content":"tampered"}]}',
+  })
+
+  const requestBody = await captureRequestBody(t, CLAUDE_CHUNKS, () =>
+    generateAnswersWithClaudeApi(createFakePort(), 'hi', {
+      modelName: 'claudeSonnet5Api',
+      conversationRecords: [],
+    }),
+  )
+
+  assert.equal(requestBody.model, 'claude-sonnet-5')
+  assert.deepEqual(requestBody.messages, [{ role: 'user', content: 'hi' }])
+})