diff --git a/.codex/AGENTS.md b/.codex/AGENTS.md
index f06228b..68ca4a2 100644
--- a/.codex/AGENTS.md
+++ b/.codex/AGENTS.md
@@ -2,3 +2,4 @@ after you finish work, commit with a conventional message. only commit the files
 always run `just check` before code commits.
 if you fix anything from `just check`, rerun it and confirm it passes before committing.
 when using gh to edit or create PR descriptions, prefer `--body-file` to preserve newlines.
+always include a "Manual testing" checklist section in PRs.
diff --git a/docs/specification.md b/docs/specification.md
index 3a1d91b..646e5e5 100644
--- a/docs/specification.md
+++ b/docs/specification.md
@@ -353,6 +353,15 @@ Decision (v0.4.0):
 * Users MAY prefix the first non-empty line with `/{engine}` (e.g. `/claude`, `/codex`, or `/pi`) to select the engine for a **new** thread.
   * The bridge MUST strip that directive from the prompt before invoking the runner.
   * If a ResumeToken is resolved from the message or reply, it MUST take precedence and the `/{engine}` directive MUST be ignored.
+* Bridges MAY persist default engine overrides per Telegram scope:
+  * **Topic default**: forum topic (`chat_id + thread_id`)
+  * **Chat default**: chat (`chat_id`)
+* When no ResumeToken is resolved, engine selection MUST follow this precedence:
+  1) explicit `/{engine}` directive
+  2) topic default (if any)
+  3) chat default (if any)
+  4) project default engine (if configured for the resolved context)
+  5) global default engine
 
 ### 8.1 Command menu (Telegram)
 
diff --git a/docs/user-guide.md b/docs/user-guide.md
index d0de121..9cee041 100644
--- a/docs/user-guide.md
+++ b/docs/user-guide.md
@@ -122,6 +122,22 @@ Prefix your message with an engine directive to override the default:
 
 Directives are only parsed at the start of the first non-empty line.
 
+### Default agent per chat or topic
+
+Use `/agent` to view or set a persistent default for the current scope:
+
+```
+/agent
+/agent set claude
+/agent clear
+```
+
+- Inside a forum topic, `/agent set` affects that topic.
+- In normal chats, it affects the whole chat.
+- In group chats, only admins can change defaults.
+
+Precedence (highest to lowest): resume token → `/engine` directive → topic default → chat default → project default → global default.
+
 ### Setting up engines
 
 Takopi shells out to the agent CLIs. Install them and make sure they're on your `PATH`
@@ -257,7 +273,7 @@ takopi chat-id
 
 ## 7. Topics
 
-Topics bind Telegram forum threads to specific project/branch contexts. They also preserve resume tokens, so agents can pick up where they left off.
+Topics bind Telegram forum threads to specific project/branch contexts. They also preserve resume tokens and can store a default agent per topic.
 
 ### Enabling topics
 
@@ -341,7 +357,7 @@ path = "~/dev/takopi"
 chat_id = -1001111111111   # forum-enabled group
 ```
 
-Topic state is stored in `telegram_topics_state.json` next to your config file.
+Topic state is stored in `telegram_topics_state.json` next to your config file. Chat defaults live in `telegram_chat_prefs_state.json`.
 
 ---
 
@@ -496,6 +512,7 @@ worktree_base = "develop"
 | `/cancel` | Reply to the progress message to stop the current run |
 | `/file put <path>` | Upload a document into the repo/worktree |
 | `/file get <path>` | Fetch a file (directories are zipped) |
+| `/agent` | Show/set the default agent for the current scope |
 | `/topic <project> @branch` | Create/bind a topic |
 | `/ctx` | Show current context |
 | `/ctx set <project> @branch` | Update context binding |
diff --git a/src/takopi/telegram/chat_prefs.py b/src/takopi/telegram/chat_prefs.py
new file mode 100644
index 0000000..64937e7
--- /dev/null
+++ b/src/takopi/telegram/chat_prefs.py
@@ -0,0 +1,95 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import msgspec
+
+from ..logging import get_logger
+from .state_store import JsonStateStore
+
+logger = get_logger(__name__)
+
+STATE_VERSION = 1
+STATE_FILENAME = "telegram_chat_prefs_state.json"
+
+
+class _ChatPrefs(msgspec.Struct, forbid_unknown_fields=False):
+    default_engine: str | None = None
+
+
+class _ChatPrefsState(msgspec.Struct, forbid_unknown_fields=False):
+    version: int
+    chats: dict[str, _ChatPrefs] = msgspec.field(default_factory=dict)
+
+
+def resolve_prefs_path(config_path: Path) -> Path:
+    return config_path.with_name(STATE_FILENAME)
+
+
+def _chat_key(chat_id: int) -> str:
+    return str(chat_id)
+
+
+def _normalize_text(value: str | None) -> str | None:
+    if value is None:
+        return None
+    value = value.strip()
+    return value or None
+
+
+def _new_state() -> _ChatPrefsState:
+    return _ChatPrefsState(version=STATE_VERSION, chats={})
+
+
+class ChatPrefsStore(JsonStateStore[_ChatPrefsState]):
+    def __init__(self, path: Path) -> None:
+        super().__init__(
+            path,
+            version=STATE_VERSION,
+            state_type=_ChatPrefsState,
+            state_factory=_new_state,
+            log_prefix="telegram.chat_prefs",
+            logger=logger,
+        )
+
+    async def get_default_engine(self, chat_id: int) -> str | None:
+        async with self._lock:
+            self._reload_locked_if_needed()
+            chat = self._get_chat_locked(chat_id)
+            if chat is None:
+                return None
+            return _normalize_text(chat.default_engine)
+
+    async def set_default_engine(self, chat_id: int, engine: str | None) -> None:
+        normalized = _normalize_text(engine)
+        async with self._lock:
+            self._reload_locked_if_needed()
+            if normalized is None:
+                if self._remove_chat_locked(chat_id):
+                    self._save_locked()
+                return
+            chat = self._ensure_chat_locked(chat_id)
+            chat.default_engine = normalized
+            self._save_locked()
+
+    async def clear_default_engine(self, chat_id: int) -> None:
+        await self.set_default_engine(chat_id, None)
+
+    def _get_chat_locked(self, chat_id: int) -> _ChatPrefs | None:
+        return self._state.chats.get(_chat_key(chat_id))
+
+    def _ensure_chat_locked(self, chat_id: int) -> _ChatPrefs:
+        key = _chat_key(chat_id)
+        entry = self._state.chats.get(key)
+        if entry is not None:
+            return entry
+        entry = _ChatPrefs()
+        self._state.chats[key] = entry
+        return entry
+
+    def _remove_chat_locked(self, chat_id: int) -> bool:
+        key = _chat_key(chat_id)
+        if key not in self._state.chats:
+            return False
+        del self._state.chats[key]
+        return True
diff --git a/src/takopi/telegram/commands.py b/src/takopi/telegram/commands.py
index 53a787e..649c1ff 100644
--- a/src/takopi/telegram/commands.py
+++ b/src/takopi/telegram/commands.py
@@ -38,6 +38,7 @@ from ..transport import MessageRef, RenderedMessage, SendOptions
 from ..transport_runtime import ResolvedMessage, TransportRuntime
 from ..utils.paths import reset_run_base_dir, set_run_base_dir
 from .bridge import TelegramBridgeConfig, send_plain
+from .chat_prefs import ChatPrefsStore
 from .chat_sessions import ChatSessionStore
 from .context import (
     _format_context,
@@ -47,6 +48,7 @@ from .context import (
     _usage_ctx_set,
     _usage_topic,
 )
+from .engine_defaults import resolve_engine_for_message
 from .files import (
     default_upload_name,
     default_upload_path,
@@ -79,6 +81,7 @@ __all__ = [
     "FILE_GET_USAGE",
     "FILE_PUT_USAGE",
     "_dispatch_command",
+    "_handle_agent_command",
     "_handle_chat_new_command",
     "_handle_file_command",
     "_handle_file_get",
@@ -97,6 +100,7 @@ __all__ = [
 _MAX_BOT_COMMANDS = 100
 FILE_PUT_USAGE = "usage: `/file put <path>`"
 FILE_GET_USAGE = "usage: `/file get <path>`"
+AGENT_USAGE = "usage: `/agent`, `/agent set <engine>`, or `/agent clear`"
 
 
 def is_cancel_command(text: str) -> bool:
@@ -362,6 +366,29 @@ async def _check_file_permissions(
     return False
 
 
+async def _check_agent_permissions(
+    cfg: TelegramBridgeConfig, msg: TelegramIncomingMessage
+) -> bool:
+    reply = _reply_sender(cfg, msg)
+    sender_id = msg.sender_id
+    if sender_id is None:
+        await reply(text="cannot verify sender for agent defaults.")
+        return False
+    is_private = msg.chat_type == "private"
+    if msg.chat_type is None:
+        is_private = msg.chat_id > 0
+    if is_private:
+        return True
+    member = await cfg.bot.get_chat_member(msg.chat_id, sender_id)
+    if member is None:
+        await reply(text="failed to verify agent permissions.")
+        return False
+    if member.status in {"creator", "administrator"}:
+        return True
+    await reply(text="changing default agents is restricted to group admins.")
+    return False
+
+
 async def _prepare_file_put_plan(
     cfg: TelegramBridgeConfig,
     msg: TelegramIncomingMessage,
@@ -1034,6 +1061,125 @@ async def _handle_ctx_command(
     )
 
 
+async def _handle_agent_command(
+    cfg: TelegramBridgeConfig,
+    msg: TelegramIncomingMessage,
+    args_text: str,
+    ambient_context: RunContext | None,
+    topic_store: TopicStateStore | None,
+    chat_prefs: ChatPrefsStore | None,
+    *,
+    resolved_scope: str | None = None,
+    scope_chat_ids: frozenset[int] | None = None,
+) -> None:
+    reply = _reply_sender(cfg, msg)
+    tkey = (
+        _topic_key(msg, cfg, scope_chat_ids=scope_chat_ids)
+        if topic_store is not None
+        else None
+    )
+    tokens = split_command_args(args_text)
+    action = tokens[0].lower() if tokens else "show"
+
+    if action in {"show", ""}:
+        try:
+            resolved = cfg.runtime.resolve_message(
+                text="",
+                reply_text=msg.reply_to_text,
+                ambient_context=ambient_context,
+                chat_id=msg.chat_id,
+            )
+        except DirectiveError as exc:
+            await reply(text=f"error:\n{exc}")
+            return
+        selection = await resolve_engine_for_message(
+            runtime=cfg.runtime,
+            context=resolved.context,
+            explicit_engine=None,
+            chat_id=msg.chat_id,
+            topic_key=tkey,
+            topic_store=topic_store,
+            chat_prefs=chat_prefs,
+        )
+        source_labels = {
+            "directive": "directive",
+            "topic_default": "topic default",
+            "chat_default": "chat default",
+            "project_default": "project default",
+            "global_default": "global default",
+        }
+        agent_line = f"agent: {selection.engine} ({source_labels[selection.source]})"
+        topic_default = selection.topic_default or "none"
+        if tkey is None:
+            topic_default = "none"
+        if chat_prefs is None:
+            chat_default = "unavailable"
+        else:
+            chat_default = selection.chat_default or "none"
+        project_default = (
+            selection.project_default
+            if selection.project_default is not None
+            else "none"
+        )
+        defaults_line = (
+            "defaults: "
+            f"topic: {topic_default}, "
+            f"chat: {chat_default}, "
+            f"project: {project_default}, "
+            f"global: {cfg.runtime.default_engine}"
+        )
+        available = ", ".join(cfg.runtime.engine_ids)
+        available_line = f"available: {available}"
+        await reply(text="\n\n".join([agent_line, defaults_line, available_line]))
+        return
+
+    if action == "set":
+        if len(tokens) < 2:
+            await reply(text=AGENT_USAGE)
+            return
+        if not await _check_agent_permissions(cfg, msg):
+            return
+        engine = tokens[1].strip().lower()
+        if engine not in cfg.runtime.engine_ids:
+            available = ", ".join(cfg.runtime.engine_ids)
+            await reply(
+                text=f"unknown engine `{engine}`.\navailable agents: `{available}`",
+            )
+            return
+        if tkey is not None:
+            if topic_store is None:
+                await reply(text="topic defaults are unavailable.")
+                return
+            await topic_store.set_default_engine(tkey[0], tkey[1], engine)
+            await reply(text=f"topic default agent set to `{engine}`")
+            return
+        if chat_prefs is None:
+            await reply(text="chat defaults are unavailable (no config path).")
+            return
+        await chat_prefs.set_default_engine(msg.chat_id, engine)
+        await reply(text=f"chat default agent set to `{engine}`")
+        return
+
+    if action == "clear":
+        if not await _check_agent_permissions(cfg, msg):
+            return
+        if tkey is not None:
+            if topic_store is None:
+                await reply(text="topic defaults are unavailable.")
+                return
+            await topic_store.clear_default_engine(tkey[0], tkey[1])
+            await reply(text="topic default agent cleared.")
+            return
+        if chat_prefs is None:
+            await reply(text="chat defaults are unavailable (no config path).")
+            return
+        await chat_prefs.clear_default_engine(msg.chat_id)
+        await reply(text="chat default agent cleared.")
+        return
+
+    await reply(text=AGENT_USAGE)
+
+
 async def _handle_new_command(
     cfg: TelegramBridgeConfig,
     msg: TelegramIncomingMessage,
@@ -1369,6 +1515,7 @@ class _TelegramCommandExecutor(CommandExecutor):
         thread_id: int | None,
         show_resume_line: bool,
         stateful_mode: bool,
+        default_engine_override: EngineId | None,
     ) -> None:
         self._exec_cfg = exec_cfg
         self._runtime = runtime
@@ -1380,6 +1527,7 @@ class _TelegramCommandExecutor(CommandExecutor):
         self._thread_id = thread_id
         self._show_resume_line = show_resume_line
         self._stateful_mode = stateful_mode
+        self._default_engine_override = default_engine_override
         self._reply_ref = MessageRef(
             channel_id=chat_id,
             message_id=user_msg_id,
@@ -1398,6 +1546,15 @@ class _TelegramCommandExecutor(CommandExecutor):
             context=context,
         )
 
+    def _apply_default_engine(self, request: RunRequest) -> RunRequest:
+        if request.engine is not None or self._default_engine_override is None:
+            return request
+        return RunRequest(
+            prompt=request.prompt,
+            engine=self._default_engine_override,
+            context=request.context,
+        )
+
     async def send(
         self,
         message: RenderedMessage | str,
@@ -1425,6 +1582,7 @@ class _TelegramCommandExecutor(CommandExecutor):
         self, request: RunRequest, *, mode: RunMode = "emit"
     ) -> RunResult:
         request = self._apply_default_context(request)
+        request = self._apply_default_engine(request)
         effective_show_resume_line = _should_show_resume_line(
             show_resume_line=self._show_resume_line,
             stateful_mode=self._stateful_mode,
@@ -1511,6 +1669,7 @@ async def _dispatch_command(
     scheduler: ThreadScheduler,
     on_thread_known: Callable[[ResumeToken, anyio.Event], Awaitable[None]] | None,
     stateful_mode: bool,
+    default_engine_override: EngineId | None,
 ) -> None:
     allowlist = cfg.runtime.allowlist
     chat_id = msg.chat_id
@@ -1535,6 +1694,7 @@ async def _dispatch_command(
         thread_id=msg.thread_id,
         show_resume_line=cfg.show_resume_line,
         stateful_mode=stateful_mode,
+        default_engine_override=default_engine_override,
     )
     message_ref = MessageRef(
         channel_id=chat_id,
diff --git a/src/takopi/telegram/engine_defaults.py b/src/takopi/telegram/engine_defaults.py
new file mode 100644
index 0000000..7a3eac9
--- /dev/null
+++ b/src/takopi/telegram/engine_defaults.py
@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Literal
+
+from ..context import RunContext
+from ..model import EngineId
+from ..transport_runtime import TransportRuntime
+from .chat_prefs import ChatPrefsStore
+from .topic_state import TopicStateStore
+
+EngineSource = Literal[
+    "directive",
+    "topic_default",
+    "chat_default",
+    "project_default",
+    "global_default",
+]
+
+
+@dataclass(frozen=True, slots=True)
+class EngineResolution:
+    engine: EngineId
+    source: EngineSource
+    topic_default: EngineId | None
+    chat_default: EngineId | None
+    project_default: EngineId | None
+
+
+async def resolve_engine_for_message(
+    *,
+    runtime: TransportRuntime,
+    context: RunContext | None,
+    explicit_engine: EngineId | None,
+    chat_id: int,
+    topic_key: tuple[int, int] | None,
+    topic_store: TopicStateStore | None,
+    chat_prefs: ChatPrefsStore | None,
+) -> EngineResolution:
+    topic_default = None
+    if topic_store is not None and topic_key is not None:
+        topic_default = await topic_store.get_default_engine(*topic_key)
+    chat_default = None
+    if chat_prefs is not None:
+        chat_default = await chat_prefs.get_default_engine(chat_id)
+    project_default = runtime.project_default_engine(context)
+
+    if explicit_engine is not None:
+        return EngineResolution(
+            engine=explicit_engine,
+            source="directive",
+            topic_default=topic_default,
+            chat_default=chat_default,
+            project_default=project_default,
+        )
+    if topic_default is not None:
+        return EngineResolution(
+            engine=topic_default,
+            source="topic_default",
+            topic_default=topic_default,
+            chat_default=chat_default,
+            project_default=project_default,
+        )
+    if chat_default is not None:
+        return EngineResolution(
+            engine=chat_default,
+            source="chat_default",
+            topic_default=topic_default,
+            chat_default=chat_default,
+            project_default=project_default,
+        )
+    if project_default is not None:
+        return EngineResolution(
+            engine=project_default,
+            source="project_default",
+            topic_default=topic_default,
+            chat_default=chat_default,
+            project_default=project_default,
+        )
+    return EngineResolution(
+        engine=runtime.default_engine,
+        source="global_default",
+        topic_default=topic_default,
+        chat_default=chat_default,
+        project_default=project_default,
+    )
diff --git a/src/takopi/telegram/loop.py b/src/takopi/telegram/loop.py
index 6f4baed..624ee42 100644
--- a/src/takopi/telegram/loop.py
+++ b/src/takopi/telegram/loop.py
@@ -23,6 +23,7 @@ from .bridge import CANCEL_CALLBACK_DATA, TelegramBridgeConfig, send_plain
 from .commands import (
     FILE_PUT_USAGE,
     _dispatch_command,
+    _handle_agent_command,
     _handle_chat_new_command,
     _handle_ctx_command,
     _handle_file_command,
@@ -50,7 +51,9 @@ from .topics import (
     _validate_topics_setup,
 )
 from .client import poll_incoming
+from .chat_prefs import ChatPrefsStore, resolve_prefs_path
 from .chat_sessions import ChatSessionStore, resolve_sessions_path
+from .engine_defaults import resolve_engine_for_message
 from .topic_state import TopicStateStore, resolve_state_path
 from .types import (
     TelegramCallbackQuery,
@@ -110,6 +113,7 @@ def _dispatch_builtin_command(
     args_text: str,
     ambient_context: RunContext | None,
     topic_store: TopicStateStore | None,
+    chat_prefs: ChatPrefsStore | None,
     resolved_scope: str | None,
     scope_chat_ids: frozenset[int],
     reply: Callable[..., Awaitable[None]],
@@ -165,6 +169,19 @@ def _dispatch_builtin_command(
             }
         )
 
+    if command_id == "agent":
+        handlers["agent"] = partial(
+            _handle_agent_command,
+            cfg,
+            msg,
+            args_text,
+            ambient_context,
+            topic_store,
+            chat_prefs,
+            resolved_scope=resolved_scope,
+            scope_chat_ids=scope_chat_ids,
+        )
+
     handler = handlers.get(command_id)
     if handler is None:
         return False
@@ -311,6 +328,7 @@ async def run_main_loop(
     )
     topic_store: TopicStateStore | None = None
     chat_session_store: ChatSessionStore | None = None
+    chat_prefs: ChatPrefsStore | None = None
     media_groups: dict[tuple[int, str], _MediaGroupState] = {}
     resolved_topics_scope: str | None = None
     topics_chat_ids: frozenset[int] = frozenset()
@@ -333,6 +351,12 @@ async def run_main_loop(
 
     try:
         config_path = cfg.runtime.config_path
+        if config_path is not None:
+            chat_prefs = ChatPrefsStore(resolve_prefs_path(config_path))
+            logger.info(
+                "chat_prefs.enabled",
+                state_path=str(resolve_prefs_path(config_path)),
+            )
         if cfg.session_mode == "chat":
             if config_path is None:
                 raise ConfigError(
@@ -552,6 +576,23 @@ async def run_main_loop(
                     return None
                 return resolved
 
+            async def resolve_engine_defaults(
+                *,
+                explicit_engine: EngineId | None,
+                context: RunContext | None,
+                chat_id: int,
+                topic_key: tuple[int, int] | None,
+            ):
+                return await resolve_engine_for_message(
+                    runtime=cfg.runtime,
+                    context=context,
+                    explicit_engine=explicit_engine,
+                    chat_id=chat_id,
+                    topic_key=topic_key,
+                    topic_store=topic_store,
+                    chat_prefs=chat_prefs,
+                )
+
             async def run_prompt_from_upload(
                 msg: TelegramIncomingMessage,
                 prompt_text: str,
@@ -570,7 +611,6 @@ async def run_main_loop(
                     else None
                 )
                 resume_token = resolved.resume_token
-                engine_override = resolved.engine_override
                 context = resolved.context
                 chat_session_key = _chat_session_key(msg, store=chat_session_store)
                 topic_key = (
@@ -578,6 +618,13 @@ async def run_main_loop(
                     if topic_store is not None
                     else None
                 )
+                engine_resolution = await resolve_engine_defaults(
+                    explicit_engine=resolved.engine_override,
+                    context=context,
+                    chat_id=chat_id,
+                    topic_key=topic_key,
+                )
+                engine_override = engine_resolution.engine
                 if resume_token is None and reply_id is not None:
                     running_task = running_tasks.get(
                         MessageRef(channel_id=chat_id, message_id=reply_id)
@@ -600,10 +647,7 @@ async def run_main_loop(
                     and topic_store is not None
                     and topic_key is not None
                 ):
-                    engine_for_session = cfg.runtime.resolve_engine(
-                        engine_override=engine_override,
-                        context=context,
-                    )
+                    engine_for_session = engine_resolution.engine
                     stored = await topic_store.get_session_resume(
                         topic_key[0], topic_key[1], engine_for_session
                     )
@@ -614,10 +658,7 @@ async def run_main_loop(
                     and chat_session_store is not None
                     and chat_session_key is not None
                 ):
-                    engine_for_session = cfg.runtime.resolve_engine(
-                        engine_override=engine_override,
-                        context=context,
-                    )
+                    engine_for_session = engine_resolution.engine
                     stored = await chat_session_store.get_session_resume(
                         chat_session_key[0],
                         chat_session_key[1],
@@ -815,6 +856,7 @@ async def run_main_loop(
                     args_text=args_text,
                     ambient_context=ambient_context,
                     topic_store=topic_store,
+                    chat_prefs=chat_prefs,
                     resolved_scope=resolved_topics_scope,
                     scope_chat_ids=topics_chat_ids,
                     reply=reply,
@@ -853,6 +895,18 @@ async def run_main_loop(
                     if command_id not in command_ids:
                         refresh_commands()
                     if command_id in command_ids:
+                        engine_resolution = await resolve_engine_defaults(
+                            explicit_engine=None,
+                            context=ambient_context,
+                            chat_id=chat_id,
+                            topic_key=topic_key,
+                        )
+                        default_engine_override = (
+                            engine_resolution.engine
+                            if engine_resolution.source
+                            in {"directive", "topic_default", "chat_default"}
+                            else None
+                        )
                         tg.start_soon(
                             _dispatch_command,
                             cfg,
@@ -868,6 +922,7 @@ async def run_main_loop(
                                 chat_session_key,
                             ),
                             stateful_mode,
+                            default_engine_override,
                         )
                         continue
 
@@ -885,8 +940,14 @@ async def run_main_loop(
 
                 text = resolved.prompt
                 resume_token = resolved.resume_token
-                engine_override = resolved.engine_override
                 context = resolved.context
+                engine_resolution = await resolve_engine_defaults(
+                    explicit_engine=resolved.engine_override,
+                    context=context,
+                    chat_id=chat_id,
+                    topic_key=topic_key,
+                )
+                engine_override = engine_resolution.engine
                 if (
                     topic_store is not None
                     and topic_key is not None
@@ -936,10 +997,7 @@ async def run_main_loop(
                     and topic_store is not None
                     and topic_key is not None
                 ):
-                    engine_for_session = cfg.runtime.resolve_engine(
-                        engine_override=engine_override,
-                        context=context,
-                    )
+                    engine_for_session = engine_resolution.engine
                     stored = await topic_store.get_session_resume(
                         topic_key[0], topic_key[1], engine_for_session
                     )
@@ -950,10 +1008,7 @@ async def run_main_loop(
                     and chat_session_store is not None
                     and chat_session_key is not None
                 ):
-                    engine_for_session = cfg.runtime.resolve_engine(
-                        engine_override=engine_override,
-                        context=context,
-                    )
+                    engine_for_session = engine_resolution.engine
                     stored = await chat_session_store.get_session_resume(
                         chat_session_key[0], chat_session_key[1], engine_for_session
                     )
diff --git a/src/takopi/telegram/topic_state.py b/src/takopi/telegram/topic_state.py
index 8cebd58..4e7da76 100644
--- a/src/takopi/telegram/topic_state.py
+++ b/src/takopi/telegram/topic_state.py
@@ -23,6 +23,7 @@ class TopicThreadSnapshot:
     context: RunContext | None
     sessions: dict[str, str]
     topic_title: str | None
+    default_engine: str | None
 
 
 class _ContextState(msgspec.Struct, forbid_unknown_fields=False):
@@ -38,6 +39,7 @@ class _ThreadState(msgspec.Struct, forbid_unknown_fields=False):
     context: _ContextState | None = None
     sessions: dict[str, _SessionState] = msgspec.field(default_factory=dict)
     topic_title: str | None = None
+    default_engine: str | None = None
 
 
 class _TopicState(msgspec.Struct, forbid_unknown_fields=False):
@@ -151,6 +153,27 @@ class TopicStateStore(JsonStateStore[_TopicState]):
                 return None
             return ResumeToken(engine=engine, value=entry.resume)
 
+    async def get_default_engine(self, chat_id: int, thread_id: int) -> str | None:
+        async with self._lock:
+            self._reload_locked_if_needed()
+            thread = self._get_thread_locked(chat_id, thread_id)
+            if thread is None:
+                return None
+            return _normalize_text(thread.default_engine)
+
+    async def set_default_engine(
+        self, chat_id: int, thread_id: int, engine: str | None
+    ) -> None:
+        normalized = _normalize_text(engine)
+        async with self._lock:
+            self._reload_locked_if_needed()
+            thread = self._ensure_thread_locked(chat_id, thread_id)
+            thread.default_engine = normalized
+            self._save_locked()
+
+    async def clear_default_engine(self, chat_id: int, thread_id: int) -> None:
+        await self.set_default_engine(chat_id, thread_id, None)
+
     async def set_session_resume(
         self, chat_id: int, thread_id: int, token: ResumeToken
     ) -> None:
@@ -205,6 +228,7 @@ class TopicStateStore(JsonStateStore[_TopicState]):
             context=_context_from_state(thread.context),
             sessions=sessions,
             topic_title=thread.topic_title,
+            default_engine=_normalize_text(thread.default_engine),
         )
 
     def _get_thread_locked(self, chat_id: int, thread_id: int) -> _ThreadState | None:
diff --git a/src/takopi/transport_runtime.py b/src/takopi/transport_runtime.py
index e3bc83c..af1abf7 100644
--- a/src/takopi/transport_runtime.py
+++ b/src/takopi/transport_runtime.py
@@ -198,7 +198,6 @@ class TransportRuntime:
         )
         engine_override = self._resolve_engine_override(
             directives_engine=directives.engine,
-            context=context,
         )
 
         return ResolvedMessage(
@@ -209,6 +208,14 @@ class TransportRuntime:
             context_source=context_source,
         )
 
+    def project_default_engine(self, context: RunContext | None) -> EngineId | None:
+        if context is None or context.project is None:
+            return None
+        project = self._projects.projects.get(context.project)
+        if project is None:
+            return None
+        return project.default_engine
+
     def _resolve_context(
         self,
         *,
@@ -253,19 +260,9 @@ class TransportRuntime:
         self,
         *,
         directives_engine: EngineId | None,
-        context: RunContext | None,
     ) -> EngineId | None:
         if directives_engine is not None:
             return directives_engine
-        if context is None:
-            return None
-        project = (
-            self._projects.projects.get(context.project)
-            if context.project is not None
-            else None
-        )
-        if project is not None and project.default_engine is not None:
-            return project.default_engine
         return None
 
     @property
diff --git a/tests/test_telegram_bridge.py b/tests/test_telegram_bridge.py
index eb84931..5731763 100644
--- a/tests/test_telegram_bridge.py
+++ b/tests/test_telegram_bridge.py
@@ -1445,6 +1445,86 @@ async def test_run_main_loop_persists_topic_sessions_in_project_scope(
     assert stored == ResumeToken(engine=CODEX_ENGINE, value=resume_value)
 
 
+@pytest.mark.anyio
+async def test_run_main_loop_auto_resumes_topic_default_engine(
+    tmp_path: Path,
+) -> None:
+    state_path = tmp_path / "takopi.toml"
+    topic_path = resolve_state_path(state_path)
+    store = TopicStateStore(topic_path)
+    await store.set_session_resume(
+        123, 77, ResumeToken(engine=CODEX_ENGINE, value="resume-codex")
+    )
+    await store.set_session_resume(
+        123, 77, ResumeToken(engine=EngineId("claude"), value="resume-claude")
+    )
+    await store.set_default_engine(123, 77, "claude")
+
+    transport = _FakeTransport()
+    bot = _FakeBot()
+    codex_runner = ScriptRunner([Return(answer="ok")], engine=CODEX_ENGINE)
+    claude_runner = ScriptRunner([Return(answer="ok")], engine=EngineId("claude"))
+    router = AutoRouter(
+        entries=[
+            RunnerEntry(engine=codex_runner.engine, runner=codex_runner),
+            RunnerEntry(engine=claude_runner.engine, runner=claude_runner),
+        ],
+        default_engine=codex_runner.engine,
+    )
+    projects = ProjectsConfig(
+        projects={
+            "proj": ProjectConfig(
+                alias="proj",
+                path=tmp_path,
+                worktrees_dir=Path(".worktrees"),
+                chat_id=123,
+            )
+        },
+        default_project=None,
+        chat_map={123: "proj"},
+    )
+    runtime = TransportRuntime(
+        router=router,
+        projects=projects,
+        config_path=state_path,
+    )
+    cfg = TelegramBridgeConfig(
+        bot=bot,
+        runtime=runtime,
+        chat_id=123,
+        startup_msg="",
+        exec_cfg=ExecBridgeConfig(
+            transport=transport,
+            presenter=MarkdownPresenter(),
+            final_notify=True,
+        ),
+        topics=TelegramTopicsSettings(
+            enabled=True,
+            scope="main",
+        ),
+    )
+
+    async def poller(_cfg: TelegramBridgeConfig):
+        yield TelegramIncomingMessage(
+            transport="telegram",
+            chat_id=123,
+            message_id=1,
+            text="hello",
+            reply_to_message_id=None,
+            reply_to_text=None,
+            sender_id=123,
+            thread_id=77,
+        )
+
+    await run_main_loop(cfg, poller)
+
+    assert codex_runner.calls == []
+    assert len(claude_runner.calls) == 1
+    assert claude_runner.calls[0][1] == ResumeToken(
+        engine=EngineId("claude"), value="resume-claude"
+    )
+
+
 @pytest.mark.anyio
 async def test_run_main_loop_auto_resumes_chat_sessions(tmp_path: Path) -> None:
     resume_value = "resume-123"
diff --git a/tests/test_telegram_chat_prefs.py b/tests/test_telegram_chat_prefs.py
new file mode 100644
index 0000000..328c5e1
--- /dev/null
+++ b/tests/test_telegram_chat_prefs.py
@@ -0,0 +1,20 @@
+import pytest
+
+from takopi.telegram.chat_prefs import ChatPrefsStore
+
+
+@pytest.mark.anyio
+async def test_chat_prefs_store_roundtrip(tmp_path) -> None:
+    path = tmp_path / "telegram_chat_prefs_state.json"
+    store = ChatPrefsStore(path)
+    await store.set_default_engine(123, "codex")
+    await store.set_default_engine(123, "codex")
+    await store.clear_default_engine(456)
+
+    assert await store.get_default_engine(123) == "codex"
+
+    store2 = ChatPrefsStore(path)
+    assert await store2.get_default_engine(123) == "codex"
+
+    await store2.clear_default_engine(123)
+    assert await store2.get_default_engine(123) is None
diff --git a/tests/test_telegram_engine_defaults.py b/tests/test_telegram_engine_defaults.py
new file mode 100644
index 0000000..f4d48ce
--- /dev/null
+++ b/tests/test_telegram_engine_defaults.py
@@ -0,0 +1,78 @@
+from pathlib import Path
+
+import pytest
+
+from takopi.config import ProjectConfig, ProjectsConfig
+from takopi.context import RunContext
+from takopi.model import EngineId
+from takopi.router import AutoRouter, RunnerEntry
+from takopi.runners.mock import Return, ScriptRunner
+from takopi.telegram.chat_prefs import ChatPrefsStore
+from takopi.telegram.engine_defaults import resolve_engine_for_message
+from takopi.telegram.topic_state import TopicStateStore
+from takopi.transport_runtime import TransportRuntime
+
+
+@pytest.mark.anyio
+async def test_resolve_engine_for_message_sources(tmp_path) -> None:
+    codex = ScriptRunner([Return(answer="ok")], engine=EngineId("codex"))
+    pi = ScriptRunner([Return(answer="ok")], engine=EngineId("pi"))
+    router = AutoRouter(
+        entries=[
+            RunnerEntry(engine=codex.engine, runner=codex),
+            RunnerEntry(engine=pi.engine, runner=pi),
+        ],
+        default_engine=codex.engine,
+    )
+    project = ProjectConfig(
+        alias="proj",
+        path=tmp_path,
+        worktrees_dir=Path(".worktrees"),
+        default_engine=pi.engine,
+    )
+    runtime = TransportRuntime(
+        router=router,
+        projects=ProjectsConfig(projects={"proj": project}, default_project=None),
+    )
+    chat_prefs = ChatPrefsStore(tmp_path / "telegram_chat_prefs_state.json")
+    topic_store = TopicStateStore(tmp_path / "telegram_topics_state.json")
+    await chat_prefs.set_default_engine(1, "pi")
+    await topic_store.set_default_engine(1, 10, "codex")
+
+    resolved = await resolve_engine_for_message(
+        runtime=runtime,
+        context=RunContext(project="proj"),
+        explicit_engine=EngineId("codex"),
+        chat_id=1,
+        topic_key=(1, 10),
+        topic_store=topic_store,
+        chat_prefs=chat_prefs,
+    )
+    assert resolved.source == "directive"
+    assert resolved.engine == "codex"
+
+    await topic_store.clear_default_engine(1, 10)
+    resolved = await resolve_engine_for_message(
+        runtime=runtime,
+        context=RunContext(project="proj"),
+        explicit_engine=None,
+        chat_id=1,
+        topic_key=(1, 10),
+        topic_store=topic_store,
+        chat_prefs=chat_prefs,
+    )
+    assert resolved.source == "chat_default"
+    assert resolved.engine == "pi"
+
+    await chat_prefs.clear_default_engine(1)
+    resolved = await resolve_engine_for_message(
+        runtime=runtime,
+        context=RunContext(project="proj"),
+        explicit_engine=None,
+        chat_id=1,
+        topic_key=(1, 10),
+        topic_store=topic_store,
+        chat_prefs=chat_prefs,
+    )
+    assert resolved.source == "project_default"
+    assert resolved.engine == "pi"
diff --git a/tests/test_telegram_topic_state.py b/tests/test_telegram_topic_state.py
index 3c839d7..b16cb8b 100644
--- a/tests/test_telegram_topic_state.py
+++ b/tests/test_telegram_topic_state.py
@@ -11,18 +11,21 @@ async def test_topic_state_store_roundtrip(tmp_path) -> None:
     store = TopicStateStore(path)
     context = RunContext(project="proj", branch="feat/topic")
     await store.set_context(1, 10, context)
+    await store.set_default_engine(1, 10, "claude")
     await store.set_session_resume(1, 10, ResumeToken(engine="codex", value="abc123"))
 
     snapshot = await store.get_thread(1, 10)
     assert snapshot is not None
     assert snapshot.context == context
     assert snapshot.sessions == {"codex": "abc123"}
+    assert snapshot.default_engine == "claude"
 
     store2 = TopicStateStore(path)
     snapshot2 = await store2.get_thread(1, 10)
     assert snapshot2 is not None
     assert snapshot2.context == context
     assert snapshot2.sessions == {"codex": "abc123"}
+    assert snapshot2.default_engine == "claude"
 
 
 @pytest.mark.anyio
@@ -47,3 +50,7 @@ async def test_topic_state_store_clear_and_find(tmp_path) -> None:
     snapshot = await store.get_thread(2, 20)
     assert snapshot is not None
     assert snapshot.context is None
+    await store.clear_default_engine(2, 20)
+    snapshot = await store.get_thread(2, 20)
+    assert snapshot is not None
+    assert snapshot.default_engine is None