fishaudio · Him188 · Aug 6, 2026 · Aug 6, 2026
diff --git a/agents/build/configuration.mdx b/agents/build/configuration.mdx
@@ -32,7 +32,7 @@
 
 ## Voice
 
-The Voice panel selects the voice your agent speaks with (`voice_id`) and its speaking language (`speaking_language`: `en`, `ja`, `zh`, `ko`, `es`, `fr`, `de`). See [Voice & language](/agents/build/voice-language) for picking a voice and how automatic language detection interacts with this setting.
+The Voice panel selects the voice your agent speaks with (`voice_id`) and its speaking language (`speaking_language`: `en`, `ja`, `zh`, `ko`, `es`, `fr`, `de`). See [Voice & language](/agents/build/voice-language) for picking a voice.
 
 ## Conversation settings
 
@@ -68,7 +68,7 @@

 `conversation.timezone` is the default IANA timezone (like `Asia/Shanghai`) the agent uses for dates and times in conversation. Leave it empty for **automatic** — each session follows the caller's device or phone number, falling back to UTC. Set one when your agent serves a single region regardless of who calls. A per-session `timezone` on the [session request](/agents/build/time-timezone) overrides this. See [Time & timezone](/agents/build/time-timezone) for the full resolution order.

 ## Autosave and publishing

 There is no Save button. Each change is written to the agent's draft moments after you stop editing, and the **Saving… / Saved** indicator at the bottom-left of the page shows the current state. If a save fails, the Builder tells you and keeps your pending edits so nothing is lost.


diff --git a/agents/build/system-tools.mdx b/agents/build/system-tools.mdx
@@ -1,24 +1,22 @@
 ---
 title: "System Tools"
-description: "Built-in agent capabilities — language detection, hang up call, and call transfer — enabled per agent"
+description: "Built-in agent capabilities like hanging up the call, enabled per agent"
 icon: "toggle-on"
 ---
 
 System tools are capabilities built into the platform. There is nothing to create or host — each agent simply enables the ones it needs.
 
 | Tool | Key | What it does |
 |---|---|---|
-| Language detection | `language_detection` | Lets the agent detect the language the user is speaking. |
 | Hang up call | `hang_up_call` | Lets the agent end the call itself. |
-| Transfer call | `transfer_call` | Lets the agent hand a live phone call to another number. |
 
 <Note>
 Unlike [webhook tools](/agents/build/webhook-tools), system tools are not workspace resources shared across agents. The on/off state is stored per agent — toggling a system tool on one agent never affects another.
 </Note>
 
 ## Toggle in the Builder
 
-Open your agent's **Tools** page in the Builder. The **System tools** card holds the **Hang up call** switch. The **Language detection** switch lives on the agent's **Configuration** page, next to **Speaking language** — see [Voice & language](/agents/build/voice-language). Flip a switch to enable or disable that capability for this agent.
+Open your agent's **Tools** page in the Builder. The **System tools** card holds the **Hang up call** switch — flip it to enable or disable that capability for this agent.
 
 ## Toggle via the API
 

diff --git a/agents/build/tools.mdx b/agents/build/tools.mdx
@@ -22,9 +22,9 @@

 | Type | Executed by | Use it for |
 |---|---|---|
 | [Webhook](/agents/build/webhook-tools) | Fish Audio — we call your HTTP endpoint during the conversation | Order lookups, CRM reads, bookings — anything your backend can answer |
 | [Client](/agents/build/client-tools) | Your app — the SDK hands the call to code you register | Navigation, UI updates, device actions — anything only the client can do |
-| [System](/agents/build/system-tools) | The platform — no code involved | Language detection, hanging up the call |
+| [System](/agents/build/system-tools) | The platform — no code involved | Hanging up the call |
 
 Webhook and client tools are **custom tools**: you define a name, a description, and arguments, and the agent fills in the argument values when it calls. System tools are ready-made — enable them with a switch on each agent.
 

diff --git a/agents/build/voice-language.mdx b/agents/build/voice-language.mdx
@@ -65,16 +65,7 @@ curl --request PATCH "https://api.fish.audio/v1/agent/agents/$AGENT_ID/config" \
 
 ## Speaking language
 
-**Speaking language** sets the default language for the agent's conversations — one of `en`, `ja`, `zh`, `ko`, `es`, `fr`, or `de` (`voice.speaking_language` on the wire). Whether it is pinned or just a fallback depends on the **Language detection** [system tool](/agents/build/system-tools):
-
-| Language detection | Session behavior |
-|---|---|
-| On | The session runs in automatic language mode — the agent follows the language the caller actually speaks. |
-| Off (default) | Every session uses the configured speaking language. |
-
-<Tip>
-  If your agent must always converse in one specific language, leave Language detection off — the session then sticks to `speaking_language` regardless of what it hears.
-</Tip>
+**Speaking language** sets the language for the agent's conversations — one of `en`, `ja`, `zh`, `ko`, `es`, `fr`, or `de` (`voice.speaking_language` on the wire). Every session converses in this language.
 
 <Note>
   The voice model and the speaking language are independent settings: picking a voice does not change the language, and vice versa. Choose a voice that sounds natural in the language you configure.
@@ -89,7 +80,7 @@ Both settings can also be replaced for a single session — send `overrides.voic
     System prompt, first message, and conversation settings.
   </Card>
   <Card title="System tools" icon="wrench" href="/agents/build/system-tools">
-    Language detection and other built-in capabilities.
+    Built-in capabilities like hanging up the call.
   </Card>
   <Card title="Preview calls" icon="phone" href="/agents/test/preview-calls">
     Talk to your draft agent and hear the voice live.

diff --git a/agents/deploy/authenticated-sessions.mdx b/agents/deploy/authenticated-sessions.mdx
@@ -18,7 +18,7 @@
 <Note>
  No backend, and anyone may talk to the agent? A [public
  agent](/agents/deploy/public-agents) lets the SDK create sessions with just an
  `agentId` — no token involved, gated by an origin allowlist and rate limits.
 </Note>

 ## Create a token on your backend
@@ -109,7 +109,7 @@

 <Note>
  `overrides`, `dynamic_variables`, `language`, `tool_events`, `timezone`, and
  `world_context` belong in your backend's creation request — the SDK forwards
  these options only in [public agent](/agents/deploy/public-agents) mode.
 </Note>

@@ -118,12 +118,12 @@
 | Field               | Type              | Description                                                                                                                                                                                                                                                                               |
 | ------------------- | ----------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `agent_id`          | string, required  | The agent to talk to. It must have a [published version](/agents/deploy/versions-publishing).                                                                                                                                                                                             |
 | `name`              | string, optional  | Display name for this session in the console's Conversations list, up to 128 characters. Omit it to show the session's start time instead. API-key requests only — keyless (public) creation rejects it with `400`.                                                                       |
 | `overrides`         | object, optional  | Replace parts of the published configuration for this session — see [Overrides](#overrides).                                                                                                                                                                                              |
 | `dynamic_variables` | object, optional  | Up to 50 entries of string, number, or boolean values, substituted into `{{placeholders}}`. See [Dynamic variables](/agents/build/dynamic-variables).                                                                                                                                     |
 | `tool_events`       | boolean, optional | Stream tool lifecycle events (`toolCallStarted` / `toolCallCompleted` / `toolCallFailed`) to the client. Default `true`; set `false` to keep tool inputs and outputs off the client.                                                                                                      |
 | `end_user_id`       | string, optional  | Your identifier for the end user, for attribution in [conversation history](/agents/monitor/conversation-history).                                                                                                                                                                        |
 | `metadata`          | object, optional  | Your own key-value namespace. Stored and returned verbatim on session queries and webhooks — never read or interpreted by the platform.                                                                                                                                                   |
 | `record_audio`      | boolean, optional | Whether to record this session's audio. Overrides the agent's [recording setting](/agents/monitor/conversation-history#what-gets-stored) for this session only — it never changes the agent; omit it to use the agent's configuration.                                                    |
 | `timezone`          | string, optional  | IANA timezone (like `Asia/Shanghai`) for the agent's sense of local time. Invalid names are rejected with `422`. See [Time & timezone](/agents/build/time-timezone).                                                                                                                      |
 | `client_timezone`   | string, optional  | The end user's browser timezone, filled automatically by the SDK in public-agent mode. A hint, not a demand: it applies only when neither `timezone` nor the agent's configured timezone is set, and invalid values are ignored. See the [resolution order](/agents/build/time-timezone). |
@@ -141,14 +141,14 @@
 | `first_message_prompt` | string, up to 10,000 characters          | Instructions the agent generates its opener from, replacing the configured first-message behavior. `{{placeholders}}` render inside it. Mutually exclusive with `first_message` — sending both is `422`.          |
 | `system_prompt`        | string, up to 8,000 characters           | Replaces the configured system prompt entirely. `{{placeholders}}` render inside it.                                                                                                                              |
 | `voice_id`             | string                                   | The voice the agent speaks with — any [voice model id](/agents/build/voice-language#use-any-voice-model) from the Voice Library. Voices bias pronunciation toward their own language, so pair it with `language`. |
-| `language`             | `en`, `ja`, `zh`, `ko`, `es`, `fr`, `de` | Pins the conversation language, taking precedence over the configured [speaking language](/agents/build/voice-language#speaking-language) and automatic language detection.                                       |
+| `language`             | `en`, `ja`, `zh`, `ko`, `es`, `fr`, `de` | Pins the conversation language, taking precedence over the configured [speaking language](/agents/build/voice-language#speaking-language).                                       |
 
 ```json
 {
  "agent_id": "YOUR_AGENT_ID",
  "overrides": {
    "first_message": "Welcome back, {{name}} — picking up where we left off.",
    "voice_id": "802e3bc2b27e49c2995d23ef70e6ac89",
    "language": "ja"
  }
 }
@@ -215,7 +215,7 @@

 | Status | Meaning                                                                                                                                       |
 | ------ | --------------------------------------------------------------------------------------------------------------------------------------------- |
 | `400`  | An `overrides` field is not enabled for this agent, or a keyless request sent an override [public sessions don't accept](#overrides).         |
 | `401`  | Invalid API key. A request with no `Authorization` header at all is treated as a public-agent request instead.                                |
 | `402`  | Quota exceeded.                                                                                                                               |
 | `403`  | Public-agent request rejected: the agent is not public, or the page's `Origin` is not on the allow-list.                                      |

diff --git a/api-reference/openapi.json b/api-reference/openapi.json
@@ -671,7 +671,7 @@
       },
       "post": {
         "summary": "Create Agent Session",
-        "description": "Start a conversation session with an agent and receive a join token for the\nsession transport (currently LiveKit WebRTC). Authenticate with an API key to\nstart sessions with any agent in your team; without credentials only agents\npublished as public are reachable, and the request `Origin` must match the\nagent's allowed origins.\n\n`language` (and `overrides.language`) accepts `en`, `ja`, `zh`, `ko`, `es`,\n`fr`, `de`; anything else is 422. Omit it to use the agent's configured\nbehavior (including automatic language detection when enabled).",
+        "description": "Start a conversation session with an agent and receive a join token for the\nsession transport (currently LiveKit WebRTC). Authenticate with an API key to\nstart sessions with any agent in your team; without credentials only agents\npublished as public are reachable, and the request `Origin` must match the\nagent's allowed origins.\n\n`language` (and `overrides.language`) accepts `en`, `ja`, `zh`, `ko`, `es`,\n`fr`, `de`; anything else is 422. Omit it to use the agent's configured\nspeaking language.",
         "security": [
           {
             "BearerAuth": []
@@ -11704,18 +11704,6 @@
       "PublicSystemToolsPatch": {
         "additionalProperties": false,
         "properties": {
-          "language_detection": {
-            "anyOf": [
-              {
-                "type": "boolean"
-              },
-              {
-                "type": "null"
-              }
-            ],
-            "default": null,
-            "title": "Language Detection"
-          },
           "hang_up_call": {
             "anyOf": [
               {
@@ -12058,11 +12046,6 @@
       },
       "AgentSystemToolsConfig": {
         "properties": {
-          "language_detection": {
-            "default": false,
-            "title": "Language Detection",
-            "type": "boolean"
-          },
           "hang_up_call": {
             "default": false,
             "title": "Hang Up Call",

diff --git a/llms.txt b/llms.txt
@@ -65,7 +65,7 @@
 - [Tools Overview](https://docs.fish.audio/agents/build/tools.md): Webhook, client, and system tools for voice agents.
 - [Webhook Tools](https://docs.fish.audio/agents/build/webhook-tools.md): Let the agent call your HTTP endpoints, with templating and testing.
 - [Client Tools](https://docs.fish.audio/agents/build/client-tools.md): Tools your app executes in the browser through the SDK.
-- [System Tools](https://docs.fish.audio/agents/build/system-tools.md): Built-in language detection and hang-up capabilities.
+- [System Tools](https://docs.fish.audio/agents/build/system-tools.md): Built-in capabilities like hanging up the call.
 - [Dynamic Variables & Overrides](https://docs.fish.audio/agents/build/dynamic-variables.md): Personalize each session with variables and whitelisted overrides.
 - [Versions & Publishing](https://docs.fish.audio/agents/deploy/versions-publishing.md): Draft, publish, clone, and restore agent configurations.
 - [Deployment Overview](https://docs.fish.audio/agents/deploy/overview.md): Pick a deployment channel — widget, public agents, authenticated sessions, or a phone number.