Model rules: who a chat's model may bring into a conversation

Rules read from the main model decide who it is offered as a crowd member,
a friend and on its roster; any-to-any with denies by default, or
none-to-none with allows. The crowd picker names what it holds back and
why, and a model held back only by a person's own rule -- or by anything,
with the new rules.override -- can still be added by hand. Another data
group is now a deny that an explicit rule opens. Admin -> Model rules and
a card in Settings, each with a matrix drawn by the enforcing function.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,39 @@ for 1.0.0 have something to be assembled from.
 
 ## Unreleased
 
+## 1.11.0
+
+Rules for which model may talk to which. This is the second of three releases;
+the third adds helpers on a different model.
+
+- **Model rules.** The new **Admin → Model rules** page sets who a chat's model
+  may bring into the conversation: as a crowd member, as a friend it asks, and
+  on its list of other models. A rule names two models, or *any model* on either
+  side, and allows or forbids. The starting point is either *any model may talk
+  to any other* (the default, so nothing changes) or *no model may talk to
+  another*. The most specific rule wins. The page ends with a table of every
+  model against every other, drawn by the same rules that are enforced.
+- **A rule is read from the chat's own model.** If gpt-oss may not talk to
+  qwen38, then in a gpt-oss chat qwen38 is not on its list of other models, it
+  cannot be asked as a friend, and the crowd picker does not offer it. Members
+  of a crowd are not checked against each other.
+- **The crowd picker names what it holds back, and why.** Models the rules do
+  not offer are listed under *Not offered to this model* with the reason. You
+  can still tick one by hand when the rule holding it back is your own, or when
+  you may override the instance's rules. A member added by hand keeps speaking
+  when the round runs; one the instance later forbids is skipped and shown
+  crossed out, as a member that cannot be reached always was.
+- **Your own rules.** Settings → Models has a card for your own starting point
+  and rules, and the same table for you. Anybody can narrow the instance's
+  rules for themselves. With the new *Override the model rules for themselves*
+  permission (off by default), your rules and starting point win over the
+  instance's, and you can add any model to a crowd by hand.
+- **Another data group is a rule, not a wall.** In 1.10.0 a model from another
+  data group could never join a conversation. Now it is not offered unless a
+  rule explicitly allows it: the instance's, or your own with the override.
+  When it does join, it reads its own group's memories and notes, never the
+  chat's.
+
 ## 1.10.0
 
 Data groups: a provider's models read only the data of the group their

src/lembas/__init__.py

@@ -1,3 +1,3 @@
 """LLeMbas - a Middle-earth themed web UI for OpenAI-compatible LLM endpoints."""
 
-__version__ = "1.10.0"
+__version__ = "1.11.0"

src/lembas/api/admin_rules.py

@@ -0,0 +1,85 @@
+"""Model rules: which model may bring which into a conversation.
+
+The instance's layer. A person's own rules and mode are on their settings page
+(`api/preferences.py`), and `services/talk.py` is where the two are combined.
+
+The page ends in a matrix -- every main model against every other -- drawn by
+`talk.matrix`, which calls the same `decide` that enforces the rules. A preview
+computed any other way would be a second copy of the logic, and a preview that
+disagrees with enforcement is worse than none: it is believed.
+"""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, Form, Request, Response, status
+from fastapi.responses import RedirectResponse
+
+from lembas.api.deps import AdminUser, Db
+from lembas.api.pages import describe_verdict
+from lembas.db.models import ANY_MODEL, EFFECT_ALLOW, EFFECT_DENY
+from lembas.services import chat as chat_service
+from lembas.services import settings_store, talk
+from lembas.web.templating import render
+
+router = APIRouter(prefix="/admin/rules", tags=["admin-rules"])
+
+
+def model_ids(db, user) -> list[str]:
+    """Every model id this person can reach, once each, in the admin's order."""
+    seen: list[str] = []
+    for model in chat_service.available_models(db, user):
+        if model.model_id not in seen:
+            seen.append(model.model_id)
+    return seen
+
+
+@router.get("")
+async def rules_page(request: Request, db: Db, user: AdminUser, saved: str = ""):
+    models = chat_service.available_models(db, user)
+    return render(
+        request,
+        "admin/rules.html",
+        {
+            "mode": settings_store.rules(db)["mode"],
+            "rules": talk.rules_of(db, None),
+            "model_ids": model_ids(db, user),
+            "any_model": ANY_MODEL,
+            "allow": EFFECT_ALLOW,
+            "deny": EFFECT_DENY,
+            "matrix_models": models,
+            # The instance's own view: no person's layer and no override, which
+            # is what somebody without `rules.override` gets unless they narrow.
+            "matrix": talk.matrix(db, None, models),
+            "describe_verdict": describe_verdict,
+            "saved": saved,
+        },
+    )
+
+
+@router.post("/mode")
+async def save_mode(db: Db, user: AdminUser, mode: str = Form(talk.MODE_OPEN)) -> Response:
+    settings_store.update(
+        db,
+        {"mode": talk.MODE_CLOSED if mode == talk.MODE_CLOSED else talk.MODE_OPEN},
+        key=settings_store.RULES,
+    )
+    return RedirectResponse("/admin/rules?saved=1", status_code=status.HTTP_303_SEE_OTHER)
+
+
+@router.post("")
+async def add_rule(
+    db: Db,
+    user: AdminUser,
+    from_model: str = Form(ANY_MODEL),
+    to_model: str = Form(ANY_MODEL),
+    effect: str = Form(EFFECT_DENY),
+    both: bool = Form(False),
+) -> Response:
+    talk.set_rule(db, None, from_model, to_model, effect, both=both)
+    return RedirectResponse("/admin/rules?saved=1", status_code=status.HTTP_303_SEE_OTHER)
+
+
+@router.post("/{rule_id}/delete")
+async def delete_rule(db: Db, user: AdminUser, rule_id: str) -> Response:
+    talk.delete_rule(db, None, rule_id)
+    return RedirectResponse("/admin/rules?saved=1", status_code=status.HTTP_303_SEE_OTHER)

src/lembas/api/chats.py

@@ -1546,15 +1546,15 @@ def _apply_crowd(db: DBSession, chat: Chat, user: User, values: list[str]) -> No
     and a model offered by two connections is two rows with different capabilities.
     """
     from lembas.db.models import CrowdMember
+    from lembas.services import talk
 
     settings = settings_store.crowd(db)
-    # Only models in the chat's own data group: a member is sent the whole
-    # conversation, so one from another group would carry it to that provider.
+    # What this person may add by hand, by the talk rules from the chat's main
+    # model. A member is sent the whole conversation, so one in another data
+    # group comes in only when a rule explicitly lets it.
     reachable = {
         model.model_id: model
-        for model in chat_service.available_models(
-            db, user, data_groups.for_chat(db, chat)
-        )
+        for model in talk.addable(db, user, chat.model_id, data_groups.for_chat(db, chat))
     }
     wanted: list[str] = []
     for value in values:

src/lembas/api/pages.py

@@ -213,6 +213,27 @@ def _scope_context(db: DBSession, user: User, chat: Chat | None) -> dict:
     }
 
 
+def _talk_settings(db: DBSession, user: User) -> dict:
+    """The person's own talk rules, and the matrix as it applies to them."""
+    from lembas.api.admin_rules import model_ids
+    from lembas.db.models import ANY_MODEL, EFFECT_ALLOW, EFFECT_DENY
+    from lembas.services import talk
+
+    models = chat_service.available_models(db, user)
+    return {
+        "talk_mode": talk.user_mode(user),
+        "talk_override": talk.may_override(db, user),
+        "talk_rules": talk.rules_of(db, user),
+        "talk_matrix_models": models,
+        "talk_matrix": talk.matrix(db, user, models),
+        "model_ids": model_ids(db, user),
+        "any_model": ANY_MODEL,
+        "allow": EFFECT_ALLOW,
+        "deny": EFFECT_DENY,
+        "describe_verdict": describe_verdict,
+    }
+
+
 def _data_group_settings(db: DBSession, user: User) -> dict:
     """What the Data tab on /settings shows: which group each connection reads.
 
@@ -251,6 +272,32 @@ def _data_group_settings(db: DBSession, user: User) -> dict:
     }
 
 
+def describe_verdict(verdict) -> str:
+    """Why a model is not offered, in the reader's language.
+
+    `talk.Verdict` carries a code so this can be said here with `t()`, while a
+    model refused a friend is told the same thing in English.
+    """
+    from lembas.services import talk
+
+    rule = verdict.rule
+    if verdict.why in (talk.WHY_INSTANCE_RULE, talk.WHY_YOUR_RULE) and rule is not None:
+        frm = i18n.t("any model") if rule.from_model == "*" else rule.from_model
+        to = i18n.t("any model") if rule.to_model == "*" else rule.to_model
+        if verdict.why == talk.WHY_INSTANCE_RULE:
+            if rule.effect == "allow":
+                return i18n.t("The instance's rule %(a)s → %(b)s allows it.", a=frm, b=to)
+            return i18n.t("The instance's rule %(a)s → %(b)s forbids it.", a=frm, b=to)
+        if rule.effect == "allow":
+            return i18n.t("Your rule %(a)s → %(b)s allows it.", a=frm, b=to)
+        return i18n.t("Your rule %(a)s → %(b)s forbids it.", a=frm, b=to)
+    return {
+        talk.WHY_GROUP: i18n.t("It is in another data group."),
+        talk.WHY_INSTANCE_CLOSED: i18n.t("The instance lets no model talk to another."),
+        talk.WHY_YOUR_CLOSED: i18n.t("Your setting lets no model talk to another."),
+    }.get(verdict.why, "")
+
+
 def _crowd_context(
     db: DBSession, user: User, chat: Chat | None, models: list, default_model_id: str = ""
 ) -> dict:
@@ -271,24 +318,39 @@ def _crowd_context(
 
     settings = settings_store.crowd(db)
     if not settings["enabled"]:
-        return {"crowd_available": [], "crowd_member_ids": [], "crowd_skipped": []}
+        return {
+            "crowd_available": [],
+            "crowd_held_back": [],
+            "crowd_member_ids": [],
+            "crowd_skipped": [],
+        }
 
     # On the new-chat screen the "own" model is whichever one the picker is
     # showing, so the list excludes it for the same reason it does in a chat:
     # adding it would have it answer twice in a row.
     own = chat.model_id if chat is not None else default_model_id
-    # Only the main model's data group: a member is sent the whole conversation.
-    # On the new-chat screen that is the group of the model the picker shows,
-    # which is the one the chat will be pinned to.
+    # The talk rules, evaluated from the main model -- on the new-chat screen the
+    # one the picker shows, whose data group the chat will be pinned to. Offered
+    # models are the main list; the rest are named below it with the reason, and
+    # may still be ticked by hand where the rules let this person do that.
+    from lembas.services import talk
+
     group = (
         data_groups.for_chat(db, chat)
         if chat is not None
         else (data_groups.for_pair(db, user, own) if own else None)
     )
-    if group is not None:
-        allowed = {m.id for m in chat_service.available_models(db, user, group)}
-        models = [model for model in models if model.id in allowed]
-    others = [model for model in models if model.model_id != own]
+    if own and group is not None:
+        pairs = talk.candidates(db, user, own, group)
+    else:
+        pairs = [(model, talk.Verdict(offered=True, addable=True)) for model in models]
+    pairs = [(model, verdict) for model, verdict in pairs if model.model_id != own]
+    others = [model for model, verdict in pairs if verdict.offered]
+    held_back = [
+        {"model": model, "addable": verdict.addable, "reason": describe_verdict(verdict)}
+        for model, verdict in pairs
+        if not verdict.offered
+    ]
     members = (
         [
             row.model_id
@@ -297,11 +359,12 @@ def _crowd_context(
         if chat is not None
         else []
     )
-    reachable = {model.model_id for model in others}
+    reachable = {model.model_id for model, verdict in pairs if verdict.addable}
     speakers = 1 + len([model_id for model_id in members if model_id in reachable])
     rounds = int(settings["max_rounds"])
     return {
         "crowd_available": others,
+        "crowd_held_back": held_back,
         "crowd_member_ids": [model_id for model_id in members if model_id in reachable],
         "crowd_skipped": (
             crowd_service.unreachable_members(db, chat, user) if chat is not None else []
@@ -1034,6 +1097,7 @@ async def settings_page(
             # template shows the two apart rather than printing the raw key.
             "split_key": personas_service.split_key,
             **_data_group_settings(db, user),
+            **_talk_settings(db, user),
             # Sorted rather than left in set order, because a list of six
             # hundred zones that is not alphabetical is one nobody can use.
             "languages": i18n.LANGUAGES,

src/lembas/api/preferences.py

@@ -371,3 +371,44 @@ async def delete_personal_group(db: Db, user: RequiredUser, group_id: str) -> Re
     except ValueError as exc:
         return RedirectResponse(f"/settings?error={quote(str(exc))}", status_code=303)
     return RedirectResponse("/settings?saved=Data+group+deleted.", status_code=303)
+
+
+# --- Talk rules ----------------------------------------------------------------
+# A person's own layer of who may talk to whom. Anybody may keep one: without
+# `rules.override` it can only narrow what the instance allows, which is theirs
+# to decide; with it, it wins. See services/talk.py.
+@router.post("/talk-mode")
+async def set_talk_mode(db: Db, user: RequiredUser, mode: str = Form("")) -> Response:
+    from lembas.services import talk
+
+    settings_map = {**(user.settings_json or {})}
+    if mode in talk.MODES:
+        settings_map[talk.SETTING_KEY] = mode
+    else:
+        settings_map.pop(talk.SETTING_KEY, None)
+    user.settings_json = settings_map
+    db.commit()
+    return RedirectResponse("/settings?saved=Model+rules+updated.", status_code=303)
+
+
+@router.post("/talk-rules")
+async def add_talk_rule(
+    db: Db,
+    user: RequiredUser,
+    from_model: str = Form("*"),
+    to_model: str = Form("*"),
+    effect: str = Form("deny"),
+    both: bool = Form(False),
+) -> Response:
+    from lembas.services import talk
+
+    talk.set_rule(db, user, from_model, to_model, effect, both=both)
+    return RedirectResponse("/settings?saved=Model+rules+updated.", status_code=303)
+
+
+@router.post("/talk-rules/{rule_id}/delete")
+async def delete_talk_rule(db: Db, user: RequiredUser, rule_id: str) -> Response:
+    from lembas.services import talk
+
+    talk.delete_rule(db, user, rule_id)
+    return RedirectResponse("/settings?saved=Model+rules+updated.", status_code=303)

src/lembas/db/models/__init__.py

@@ -84,6 +84,7 @@ from lembas.db.models.schedule import (
 )
 from lembas.db.models.setting import Setting
 from lembas.db.models.suggestion import Suggestion
+from lembas.db.models.talk import ANY_MODEL, EFFECT_ALLOW, EFFECT_DENY, EFFECTS, TalkRule
 from lembas.db.models.tool import (
     RESPONSE_JSON,
     RESPONSE_MODES,
@@ -170,6 +171,11 @@ __all__ = [
     "Connection",
     "DEFAULT_GROUP",
     "DataGroup",
+    "ANY_MODEL",
+    "EFFECT_ALLOW",
+    "EFFECT_DENY",
+    "EFFECTS",
+    "TalkRule",
     "InDataGroup",
     "CustomTool",
     "CHUNK_DOCUMENT",

src/lembas/db/models/talk.py

@@ -0,0 +1,43 @@
+"""Talk rules: which model may talk to which.
+
+A rule names a *main* model -- the one a chat belongs to -- and a *target*, and
+says allow or deny. It governs who a main model is offered as a crowd member, as
+a friend to ask, and on its roster of peers. Keyed on the models' text ids, never
+a `Model` primary key, for the reason every such reference here is: "Test &
+refresh" recreates the row, and a rule that silently stopped applying after a
+refresh is the worst kind of rule.
+
+`owner_id` NULL is an instance rule, written by an administrator. Set, it is one
+person's own. `*` on either side means any model. See services/talk.py for how
+the two layers and the instance's mode are combined.
+"""
+
+from __future__ import annotations
+
+from sqlalchemy import ForeignKey, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from lembas.db.base import Base, Timestamps, UUIDPrimaryKey
+
+ANY_MODEL = "*"
+EFFECT_ALLOW = "allow"
+EFFECT_DENY = "deny"
+EFFECTS = (EFFECT_ALLOW, EFFECT_DENY)
+
+
+class TalkRule(UUIDPrimaryKey, Timestamps, Base):
+    """One rule: may `from_model` talk to `to_model`, for everybody or one person."""
+
+    __tablename__ = "talk_rules"
+    __table_args__ = (UniqueConstraint("owner_id", "from_model", "to_model"),)
+
+    owner_id: Mapped[str | None] = mapped_column(
+        String(32), ForeignKey("users.id", ondelete="CASCADE"), nullable=True, index=True
+    )
+    from_model: Mapped[str] = mapped_column(String(300), nullable=False)
+    to_model: Mapped[str] = mapped_column(String(300), nullable=False)
+    effect: Mapped[str] = mapped_column(String(8), nullable=False, default=EFFECT_DENY)
+
+    def __repr__(self) -> str:
+        whose = self.owner_id or "instance"
+        return f"<TalkRule {whose} {self.from_model} -> {self.to_model} {self.effect}>"

src/lembas/main.py

@@ -23,6 +23,7 @@ from lembas.api import (
     admin_images,
     admin_models,
     admin_prompts,
+    admin_rules,
     admin_schedules,
     admin_search,
     admin_suggestions,
@@ -231,6 +232,7 @@ def create_app() -> FastAPI:
     app.include_router(admin_agents.router)
     app.include_router(admin_crowd.router)
     app.include_router(admin_data_groups.router)
+    app.include_router(admin_rules.router)
     app.include_router(push.router)
     app.include_router(branding.router)
 

src/lembas/security/permissions.py

@@ -334,6 +334,19 @@ PERMISSION_DEFS: tuple[PermissionDef, ...] = (
     # themselves, and move their own records between groups. Off by default,
     # because it moves what a provider can read -- and an instance that never
     # looks should keep the arrangement its administrator made.
+    # Talk rules: which model may bring which into a conversation. The
+    # instance's rules apply to everybody; a person may always narrow them for
+    # themselves, and with this they may also widen them -- their own rules and
+    # mode then win, and they may add any model to a crowd by hand. Off by
+    # default, because an instance rule is usually there for a reason.
+    PermissionDef(
+        "rules.override",
+        "Override the model rules for themselves",
+        "Let this person's own rules about which model may talk to which win over "
+        "the instance's, and let them add any model to a crowd by hand.",
+        False,
+        "Chat",
+    ),
     PermissionDef(
         "data.manage",
         "Manage their own data groups",

src/lembas/services/chat.py

@@ -906,7 +906,14 @@ def roster_models(
     would be both a leak and a dead end, since asking it anything is refused by
     the same check.
     """
-    return [model for model in available_models(db, user, group) if model.model_id != exclude]
+    if group is not None:
+        # Who the main model may talk to, by the talk rules -- a different data
+        # group counting as a deny that only an explicit rule opens. `exclude`
+        # is the main model at every call site that passes a group.
+        from lembas.services import talk
+
+        return talk.offered(db, user, exclude, group)
+    return [model for model in available_models(db, user) if model.model_id != exclude]
 
 
 def roster_block(

src/lembas/services/crowd.py

@@ -273,17 +273,18 @@ def member_speakers(db: DBSession, chat: Chat, user=None) -> list:
     Deduplicated against the main model: adding the chat's own model to the crowd
     would have it answer twice in a row, which is not what anybody meant by it.
 
-    And narrowed to the chat's data group: a member from another group would be
-    handed this conversation, which is exactly what groups exist to prevent.
+    And narrowed by the talk rules, evaluated from the main model -- which is
+    also where a different data group counts as a deny that only a rule opens.
     """
     from lembas.services import chat as chat_service
-    from lembas.services import data_groups
+    from lembas.services import data_groups, talk
 
+    # `addable`, not `offered`: a member somebody added by hand is exactly one the
+    # rules would not have offered, and skipping it when the round runs would
+    # quietly undo their choice.
     reachable = {
         model.model_id: model
-        for model in chat_service.roster_models(
-            db, user, exclude="", group=data_groups.for_chat(db, chat)
-        )
+        for model in talk.addable(db, user, chat.model_id, data_groups.for_chat(db, chat))
     }
     speakers = [chat_service.Speaker(chat.model_id, chat.connection_id)]
     seen = {chat.model_id}
@@ -297,14 +298,11 @@ def member_speakers(db: DBSession, chat: Chat, user=None) -> list:
 
 def unreachable_members(db: DBSession, chat: Chat, user=None) -> list[str]:
     """Members that will be skipped, so a screen can say so rather than lie."""
-    from lembas.services import chat as chat_service
-    from lembas.services import data_groups
+    from lembas.services import data_groups, talk
 
     reachable = {
         model.model_id
-        for model in chat_service.roster_models(
-            db, user, exclude="", group=data_groups.for_chat(db, chat)
-        )
+        for model in talk.addable(db, user, chat.model_id, data_groups.for_chat(db, chat))
     }
     return [
         member.model_id

src/lembas/services/settings_store.py

@@ -33,6 +33,7 @@ IMAGES = "images"
 SCHEDULES = "schedules"
 SUBAGENTS = "subagents"
 CROWD = "crowd"
+RULES = "rules"
 BRANDING = "branding"
 EXTRACTION = "extraction"
 
@@ -349,6 +350,16 @@ def _schedules_defaults() -> dict[str, Any]:
     }
 
 
+def _rules_defaults() -> dict[str, Any]:
+    """Who may talk to whom, instance-wide. See services/talk.py.
+
+    `open` is any model to any model, with deny rules; `closed` is none to none,
+    with allow rules. Open by default, so an instance that never looks behaves
+    exactly as it did before rules existed.
+    """
+    return {"mode": "open"}
+
+
 def _crowd_defaults() -> dict[str, Any]:
     """Several models answering one turn, in order, then again in reverse.
 
@@ -431,6 +442,7 @@ _DEFAULTS: dict[str, Any] = {
     SCHEDULES: _schedules_defaults,
     SUBAGENTS: _subagents_defaults,
     CROWD: _crowd_defaults,
+    RULES: _rules_defaults,
     # Whose instance this is. The defaults live in `services/branding.py`
     # beside the code that reads them, because every one of them is paired with
     # a label and a hint for the admin page and splitting the three across two
@@ -726,6 +738,13 @@ def crowd(db: DBSession) -> dict[str, Any]:
     return values
 
 
+def rules(db: DBSession) -> dict[str, Any]:
+    """The talk-rules group, with the mode clamped to the two that exist."""
+    values = get_group(db, RULES)
+    values["mode"] = "closed" if values.get("mode") == "closed" else "open"
+    return values
+
+
 def images_ready(db: DBSession) -> bool:
     """Whether image generation can actually happen.
 

src/lembas/services/talk.py

@@ -0,0 +1,354 @@
+"""Who may talk to whom: the rules behind the crowd, `ask_friend` and the roster.
+
+Always evaluated **from the chat's main model**. If the main model may not talk
+to a target, the target is not *offered* -- not listed on its roster, not named
+as a friend it may ask, not in the crowd picker's main list. Members of a crowd
+are not checked against each other: the rule is about who a conversation's own
+model brings in, and a member loses `friend` and `subagent` anyway.
+
+Two answers, because the owner asked for two things:
+
+* **offered** -- what happens on its own: the roster, a friend a model names, the
+  picker's main list.
+* **addable** -- what a person may do by hand in the crowd picker. A rule a
+  person wrote for themselves is soft for them; an instance rule is hard, unless
+  they hold `rules.override`.
+
+`decide` is pure -- plain values in, a `Verdict` out -- so every combination is
+tested without a database, and the admin page's matrix is drawn by the same
+function that enforces the rules, which is what makes the matrix trustworthy.
+
+**A different data group is an implicit deny.** A crowd member or a friend is
+sent the conversation, so a model in another group joins only when a rule says
+so explicitly: the administrator's, or a person's own when they hold the
+override. It then reads its own group's stores, never the chat's.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session as DBSession
+
+from lembas.db.models import (
+    ANY_MODEL,
+    EFFECT_ALLOW,
+    EFFECT_DENY,
+    EFFECTS,
+    Model,
+    TalkRule,
+    User,
+)
+
+MODE_OPEN = "open"
+MODE_CLOSED = "closed"
+MODES = (MODE_OPEN, MODE_CLOSED)
+
+# Where a person's own mode is kept in `settings_json`. Empty follows the instance.
+SETTING_KEY = "talk_mode"
+
+# Lets a person's own rules and mode win over the instance's, for them alone.
+PERMISSION = "rules.override"
+
+
+@dataclass(frozen=True)
+class Rule:
+    from_model: str
+    to_model: str
+    effect: str
+
+
+# Why something is not offered. A code rather than a sentence, so a screen can
+# say it in the reader's language (`api/admin_rules.py:describe`) while a model
+# refused a friend is told it in English (`Verdict.reason`).
+WHY_INSTANCE_RULE = "instance_rule"
+WHY_YOUR_RULE = "your_rule"
+WHY_GROUP = "group"
+WHY_INSTANCE_CLOSED = "instance_closed"
+WHY_YOUR_CLOSED = "your_closed"
+
+
+@dataclass(frozen=True)
+class Verdict:
+    offered: bool
+    addable: bool
+    why: str = ""
+    rule: Rule | None = None
+
+    @property
+    def reason(self) -> str:
+        """The reason in English, for a model -- or "" when it is offered."""
+        if self.offered or not self.why:
+            return ""
+        if self.why in (WHY_INSTANCE_RULE, WHY_YOUR_RULE) and self.rule is not None:
+            whose = "the instance's" if self.why == WHY_INSTANCE_RULE else "your"
+            return _named(self.rule, whose)
+        return {
+            WHY_GROUP: "it is in another data group",
+            WHY_INSTANCE_CLOSED: "the instance allows no model to talk to another",
+            WHY_YOUR_CLOSED: "your setting allows no model to talk to another",
+        }.get(self.why, "")
+
+
+def match(rules: list[Rule], main: str, target: str) -> Rule | None:
+    """The most specific rule for a pair: exact, then `main -> *`, `* -> target`, `* -> *`."""
+    for wanted in ((main, target), (main, ANY_MODEL), (ANY_MODEL, target), (ANY_MODEL, ANY_MODEL)):
+        for rule in rules:
+            if (rule.from_model, rule.to_model) == wanted:
+                return rule
+    return None
+
+
+def _named(rule: Rule, whose: str) -> str:
+    frm = "any model" if rule.from_model == ANY_MODEL else rule.from_model
+    to = "any model" if rule.to_model == ANY_MODEL else rule.to_model
+    verb = "allows" if rule.effect == EFFECT_ALLOW else "forbids"
+    return f"{whose} rule {frm} → {to} {verb} it"
+
+
+def decide(
+    *,
+    instance_mode: str,
+    instance_rule: Rule | None,
+    user_mode: str = "",
+    user_rule: Rule | None = None,
+    override: bool = False,
+    same_group: bool = True,
+) -> Verdict:
+    """Whether a main model may talk to a target, offered and by hand.
+
+    The instance's verdict is its most specific rule, or failing that its mode
+    -- with a different data group counting as a deny that only an explicit
+    allow opens.
+
+    Without the override a person can only narrow: their own rule or their
+    `closed` mode can take something off what is offered, and since those are
+    theirs, they may still add it by hand. With the override, their explicit
+    rule wins outright, then their mode, then the instance's verdict; and they
+    may add anything by hand, because doing so is their explicit decision.
+    """
+    if instance_rule is not None:
+        instance_ok = instance_rule.effect == EFFECT_ALLOW
+        instance_why: tuple[str, Rule | None] = (WHY_INSTANCE_RULE, instance_rule)
+    elif not same_group:
+        instance_ok, instance_why = False, (WHY_GROUP, None)
+    else:
+        instance_ok = instance_mode != MODE_CLOSED
+        instance_why = (WHY_INSTANCE_CLOSED, None)
+
+    if not override:
+        if user_rule is not None:
+            user_ok = user_rule.effect == EFFECT_ALLOW
+            user_why: tuple[str, Rule | None] = (WHY_YOUR_RULE, user_rule)
+        elif user_mode == MODE_CLOSED:
+            user_ok, user_why = False, (WHY_YOUR_CLOSED, None)
+        else:
+            user_ok, user_why = True, ("", None)
+        offered = instance_ok and user_ok
+        why, rule = ("", None) if offered else (instance_why if not instance_ok else user_why)
+        return Verdict(offered=offered, addable=instance_ok, why=why, rule=rule)
+
+    if user_rule is not None:
+        ok = user_rule.effect == EFFECT_ALLOW
+        why, rule = (WHY_YOUR_RULE, user_rule)
+    elif user_mode == MODE_CLOSED:
+        ok, (why, rule) = False, (WHY_YOUR_CLOSED, None)
+    elif user_mode == MODE_OPEN:
+        allowed = instance_rule is not None and instance_rule.effect == EFFECT_ALLOW
+        ok = same_group or allowed
+        why, rule = (WHY_GROUP, None)
+    else:
+        ok = instance_ok
+        why, rule = instance_why
+    if ok:
+        why, rule = "", None
+    return Verdict(offered=ok, addable=True, why=why, rule=rule)
+
+
+# --- Loading what `decide` needs ---------------------------------------------------
+def _rules(db: DBSession, owner_id: str | None) -> list[Rule]:
+    rows = db.scalars(
+        select(TalkRule).where(
+            TalkRule.owner_id.is_(None) if owner_id is None else TalkRule.owner_id == owner_id
+        )
+    )
+    return [Rule(r.from_model, r.to_model, r.effect) for r in rows]
+
+
+def user_mode(user: User | None) -> str:
+    if user is None:
+        return ""
+    value = str((user.settings_json or {}).get(SETTING_KEY) or "")
+    return value if value in MODES else ""
+
+
+def may_override(db: DBSession, user: User | None) -> bool:
+    from lembas.security import permissions
+
+    return user is not None and permissions.has(db, user, PERMISSION)
+
+
+class Judge:
+    """Everything `decide` needs for one person, loaded once per request.
+
+    The model lists ask about every model they show; loading the rules and the
+    connection groups per question would be a query per row of every picker.
+    `user=None` is the instance's own view, for the admin page's matrix.
+    """
+
+    def __init__(self, db: DBSession, user: User | None) -> None:
+        from lembas.services import data_groups, settings_store
+
+        self.instance_mode = settings_store.rules(db)["mode"]
+        self.instance_rules = _rules(db, None)
+        self.user_rules = _rules(db, user.id) if user is not None else []
+        self.user_mode = user_mode(user)
+        self.override = may_override(db, user)
+        self.groups = data_groups.connection_groups(db, user)
+        self.default = data_groups.DEFAULT_GROUP
+
+    def group_of(self, model: Model) -> str:
+        return self.groups.get(model.connection_id, self.default)
+
+    def verdict(self, main_model: str, main_group: str, target: Model) -> Verdict:
+        return decide(
+            instance_mode=self.instance_mode,
+            instance_rule=match(self.instance_rules, main_model, target.model_id),
+            user_mode=self.user_mode,
+            user_rule=match(self.user_rules, main_model, target.model_id),
+            override=self.override,
+            same_group=self.group_of(target) == main_group,
+        )
+
+
+def candidates(
+    db: DBSession, user: User | None, main_model: str, main_group: str
+) -> list[tuple[Model, Verdict]]:
+    """Every model this person can reach other than the main one, with its verdict."""
+    from lembas.services import chat as chat_service
+
+    judge = Judge(db, user)
+    return [
+        (model, judge.verdict(main_model, main_group, model))
+        for model in chat_service.available_models(db, user)
+        if model.model_id != main_model
+    ]
+
+
+def offered(db: DBSession, user: User | None, main_model: str, main_group: str) -> list[Model]:
+    """The models a main model is offered on its own: the roster, a friend, the picker."""
+    found = candidates(db, user, main_model, main_group)
+    return [model for model, verdict in found if verdict.offered]
+
+
+def addable(db: DBSession, user: User | None, main_model: str, main_group: str) -> list[Model]:
+    """The models a person may add to a crowd by hand."""
+    found = candidates(db, user, main_model, main_group)
+    return [model for model, verdict in found if verdict.addable]
+
+
+# --- Changing rules --------------------------------------------------------------------
+def rules_of(db: DBSession, owner: User | None) -> list[TalkRule]:
+    return list(
+        db.scalars(
+            select(TalkRule)
+            .where(
+                TalkRule.owner_id.is_(None)
+                if owner is None
+                else TalkRule.owner_id == owner.id
+            )
+            .order_by(TalkRule.from_model, TalkRule.to_model)
+        )
+    )
+
+
+def set_rule(
+    db: DBSession,
+    owner: User | None,
+    from_model: str,
+    to_model: str,
+    effect: str,
+    *,
+    both: bool = False,
+) -> None:
+    """Write one rule, or a pair in both directions. Replaces one for the same pair."""
+    from_model = (from_model or "").strip()[:300] or ANY_MODEL
+    to_model = (to_model or "").strip()[:300] or ANY_MODEL
+    if effect not in EFFECTS:
+        effect = EFFECT_DENY
+    pairs = [(from_model, to_model)]
+    if both and from_model != to_model:
+        pairs.append((to_model, from_model))
+    for frm, to in pairs:
+        existing = db.scalar(
+            select(TalkRule).where(
+                (TalkRule.owner_id.is_(None) if owner is None else TalkRule.owner_id == owner.id),
+                TalkRule.from_model == frm,
+                TalkRule.to_model == to,
+            )
+        )
+        if existing is not None:
+            existing.effect = effect
+        else:
+            db.add(
+                TalkRule(
+                    owner_id=owner.id if owner is not None else None,
+                    from_model=frm,
+                    to_model=to,
+                    effect=effect,
+                )
+            )
+    db.commit()
+
+
+def delete_rule(db: DBSession, owner: User | None, rule_id: str) -> bool:
+    """Remove one rule, only from the layer it belongs to."""
+    rule = db.get(TalkRule, rule_id)
+    if rule is None or rule.owner_id != (owner.id if owner is not None else None):
+        return False
+    db.delete(rule)
+    db.commit()
+    return True
+
+
+def matrix(db: DBSession, user: User | None, models: list[Model]) -> list[dict]:
+    """Main x target, drawn by the same `decide` that enforces the rules.
+
+    Evaluated as if the main model's chat were in the main model's own group,
+    which is what a chat started on it is.
+    """
+    judge = Judge(db, user)
+    rows = []
+    for main in models:
+        group = judge.group_of(main)
+        cells = []
+        for target in models:
+            if target.model_id == main.model_id:
+                cells.append(None)
+                continue
+            cells.append(judge.verdict(main.model_id, group, target))
+        rows.append({"main": main, "cells": cells})
+    return rows
+
+
+__all__ = [
+    "MODES",
+    "MODE_CLOSED",
+    "MODE_OPEN",
+    "PERMISSION",
+    "Judge",
+    "Rule",
+    "Verdict",
+    "addable",
+    "candidates",
+    "decide",
+    "delete_rule",
+    "match",
+    "matrix",
+    "may_override",
+    "offered",
+    "rules_of",
+    "set_rule",
+    "user_mode",
+]

src/lembas/web/i18n/sk.py

@@ -1859,3 +1859,50 @@ MESSAGES.update(
         'Image review': 'Posudzovanie obrázkov',
     }
 )
+
+# --- Model rules (1.11.0) ------------------------------------------------------
+MESSAGES.update(
+    {
+        'Model rules': 'Pravidlá modelov',
+        "Which model may bring which into a conversation: as a crowd member, as a friend it asks, and on the list of other models it is told about. A rule is read from the chat's own model. A model in another data group is not offered unless a rule allows it.": 'Ktorý model môže priviesť ktorý do konverzácie: ako člena skupiny, ako priateľa, ktorého sa pýta, a na zozname ďalších modelov, o ktorých sa dozvie. Pravidlo sa číta od vlastného modelu konverzácie. Model v inej dátovej oblasti sa neponúka, kým to pravidlo nepovolí.',
+        'The starting point': 'Východisko',
+        'Any model may talk to any other, except where a rule forbids it': 'Každý model môže hovoriť s každým, okrem prípadov, keď to pravidlo zakazuje',
+        'No model may talk to another, except where a rule allows it': 'Žiadny model nesmie hovoriť s iným, okrem prípadov, keď to pravidlo povoľuje',
+        'People can always narrow this for themselves. Widening it for themselves needs the permission to override the model rules.': 'Ľudia si to pre seba môžu vždy zúžiť. Rozšíriť si to pre seba môžu len s oprávnením prekonať pravidlá modelov.',
+        'Rules': 'Pravidlá',
+        'Not offered to this model': 'Tomuto modelu sa neponúkajú',
+        'You may still add it yourself.': 'Môžete ho aj tak pridať sami.',
+        'any model': 'ľubovoľný model',
+        'may talk': 'smie hovoriť',
+        'may not talk': 'nesmie hovoriť',
+        'Delete this rule': 'Zmazať toto pravidlo',
+        'No rules yet.': 'Zatiaľ žiadne pravidlá.',
+        'This model': 'Tento model',
+        "The chat's own model.": 'Vlastný model konverzácie.',
+        'May': 'Smie',
+        'may not talk to': 'nesmie hovoriť s',
+        'may talk to': 'smie hovoriť s',
+        'That model': 'Tamten model',
+        'A crowd member, a friend it asks, a model on its roster.': 'Člen skupiny, priateľ, ktorého sa pýta, model na jeho zozname.',
+        'Both directions': 'Oboma smermi',
+        'Add rule': 'Pridať pravidlo',
+        'Who may talk to whom': 'Kto smie hovoriť s kým',
+        "Rows are the chat's own model, columns the model it would bring in. Drawn by the same rules that are enforced, so what this shows is what happens.": 'Riadky sú vlastný model konverzácie, stĺpce model, ktorý by priviedol. Kreslí to tie isté pravidlá, ktoré sa uplatňujú, takže čo tu vidíte, to sa aj stane.',
+        'itself': 'on sám',
+        'Offered': 'Ponúka sa',
+        'Who your models may talk to': 'S kým smú hovoriť vaše modely',
+        "Your rules and your setting win over the instance's, for you. You can also add any model to a crowd by hand.": 'Vaše pravidlá a vaše nastavenie majú pre vás prednosť pred pravidlami inštancie. Do skupiny môžete ručne pridať aj ľubovoľný model.',
+        "Your rules can only narrow the instance's. A model your own rule holds back can still be added to a crowd by hand; one the instance holds back cannot.": 'Vaše pravidlá môžu pravidlá inštancie len zúžiť. Model, ktorý zadrží vaše vlastné pravidlo, môžete do skupiny aj tak pridať ručne; model, ktorý zadrží inštancia, nie.',
+        'Your starting point': 'Vaše východisko',
+        'Follow the instance': 'Podľa inštancie',
+        'Any model may talk to any other': 'Každý model môže hovoriť s každým',
+        'No model may talk to another': 'Žiadny model nesmie hovoriť s iným',
+        "The instance's rule %(a)s → %(b)s allows it.": 'Pravidlo inštancie %(a)s → %(b)s to povoľuje.',
+        "The instance's rule %(a)s → %(b)s forbids it.": 'Pravidlo inštancie %(a)s → %(b)s to zakazuje.',
+        'Your rule %(a)s → %(b)s allows it.': 'Vaše pravidlo %(a)s → %(b)s to povoľuje.',
+        'Your rule %(a)s → %(b)s forbids it.': 'Vaše pravidlo %(a)s → %(b)s to zakazuje.',
+        'It is in another data group.': 'Je v inej dátovej oblasti.',
+        'The instance lets no model talk to another.': 'Inštancia nedovoľuje žiadnemu modelu hovoriť s iným.',
+        'Your setting lets no model talk to another.': 'Vaše nastavenie nedovoľuje žiadnemu modelu hovoriť s iným.',
+    }
+)

src/lembas/web/static/css/app.css

@@ -1954,3 +1954,47 @@ body.is-resizing .canvas__body { pointer-events: none; }
 }
 a.card, .card--action { transition: transform var(--dur-2) var(--ease-out),
                                     box-shadow var(--dur-2) var(--ease-out); }
+
+/* --- Who may talk to whom --------------------------------------------------
+   The talk-rules matrix: one row per main model, one column per model it would
+   bring in. Its own scroller, because a dozen model ids across is wider than a
+   phone and the page must never scroll sideways. */
+.talk-matrix-scroll {
+  overflow-x: auto;
+  max-width: 100%;
+}
+.talk-matrix {
+  border-collapse: collapse;
+  font-size: var(--text-xs);
+}
+.talk-matrix th,
+.talk-matrix td {
+  padding: var(--sp-1) var(--sp-2);
+  border: var(--border-w) solid var(--border);
+  text-align: center;
+  white-space: nowrap;
+}
+.talk-matrix thead th { color: var(--ink-muted); font-weight: 500; }
+.talk-matrix tbody th { text-align: left; color: var(--ink-muted); font-weight: 500; }
+.talk-matrix__yes { color: var(--leaf); }
+.talk-matrix__no { color: var(--danger); }
+.talk-matrix__self { color: var(--ink-faint); }
+
+/* The add-a-rule form: three fields on a shared track so the label, the
+   control and the hint of each line up whatever the text does -- the tree's
+   subgrid rule. Stacks below a phone's width. */
+.talk-rule-form {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(min(100%, 12rem), 1fr));
+  grid-template-rows: auto auto auto;
+  column-gap: var(--sp-3);
+  min-width: 0;
+  margin-top: var(--sp-4);
+}
+.talk-rule-form > .field {
+  display: grid;
+  grid-template-rows: subgrid;
+  grid-row: span 3;
+  min-width: 0;
+}
+.talk-rule-form > .btn-row { grid-column: 1 / -1; }

src/lembas/web/templates/admin/_layout.html

@@ -63,6 +63,10 @@
           {{ icon("users", "icon--sm") }}
           <span class="nav-item__label">{{ t("A crowd") }}</span>
         </a>
+        <a class="nav-item {{ 'is-active' if section == 'rules' }}" href="/admin/rules">
+          {{ icon("users", "icon--sm") }}
+          <span class="nav-item__label">{{ t("Model rules") }}</span>
+        </a>
         <a class="nav-item {{ 'is-active' if section == 'audio' }}" href="/admin/audio">
           {{ icon("speaker", "icon--sm") }}
           <span class="nav-item__label">{{ t("Audio") }}</span>

src/lembas/web/templates/admin/rules.html

@@ -0,0 +1,38 @@
+{% extends "admin/_layout.html" %}
+{% from "_macros.html" import icon %}
+{% set section = "rules" %}
+
+{% block title %}{{ t("Model rules") }} - {{ brand.name }}{% endblock %}
+{% block heading %}{{ t("Model rules") }}{% endblock %}
+
+{% block admin_content %}
+<p class="admin-lede">{{ t("Which model may bring which into a conversation: as a crowd member, as a friend it asks, and on the list of other models it is told about. A rule is read from the chat's own model. A model in another data group is not offered unless a rule allows it.") }}</p>
+
+{% if saved %}
+<div class="alert alert--success">{{ icon("check", "icon--sm") }} <span>{{ t("Settings saved.") }}</span></div>
+{% endif %}
+
+<section class="card">
+  <h2 class="card__title">{{ t("The starting point") }}</h2>
+  <form method="post" action="/admin/rules/mode">
+    <div class="field">
+      <label class="checkbox">
+        <input type="radio" name="mode" value="open" {{ 'checked' if mode == 'open' }}>
+        <span>{{ t("Any model may talk to any other, except where a rule forbids it") }}</span>
+      </label>
+      <label class="checkbox">
+        <input type="radio" name="mode" value="closed" {{ 'checked' if mode == 'closed' }}>
+        <span>{{ t("No model may talk to another, except where a rule allows it") }}</span>
+      </label>
+      <p class="field__hint">{{ t("People can always narrow this for themselves. Widening it for themselves needs the permission to override the model rules.") }}</p>
+    </div>
+    <div class="btn-row"><button class="btn btn--primary" type="submit">{{ t("Save") }}</button></div>
+  </form>
+</section>
+
+<section class="card">
+  <h2 class="card__title">{{ t("Rules") }}</h2>
+  {% set rules_action = "/admin/rules" %}
+  {% include "partials/_talk_rules.html" %}
+</section>
+{% endblock %}

src/lembas/web/templates/chat/_composer.html

@@ -314,7 +314,7 @@
             because a browser submits only the ticked boxes and `start_chat`
             needs to know which ones were not.
           #}
-          {% if crowd_available %}
+          {% if crowd_available or crowd_held_back %}
           <div class="picker picker--up" data-picker>
             <button class="btn btn--icon composer__btn" type="button" data-picker-toggle
                     aria-haspopup="menu" aria-expanded="false"
@@ -366,6 +366,38 @@
                 </span>
               </label>
               {% endfor %}
+              {#
+                Models the talk rules do not offer to this chat's model, named
+                with the reason rather than left out. Tickable where the person
+                may still add them by hand -- their own rule, or the override --
+                and shown disabled where they may not, so the reason is visible
+                either way. Same field and same verb as the list above.
+              #}
+              {% if crowd_held_back %}
+              <p class="picker__group">{{ t("Not offered to this model") }}</p>
+              {% for row in crowd_held_back %}
+              <label class="picker__option picker__option--toggle">
+                {% if chat %}
+                <input type="checkbox" name="crowd_model_ids" value="{{ row.model.model_id }}"
+                       {{ 'checked' if row.model.model_id in crowd_member_ids }}
+                       {{ 'disabled' if not row.addable }}
+                       form="crowd-form"
+                       hx-patch="/api/chats/{{ chat.id }}" hx-swap="none">
+                {% else %}
+                <input type="checkbox" name="crowd_model_ids" value="{{ row.model.model_id }}"
+                       {{ 'checked' if row.model.model_id in crowd_member_ids }}
+                       {{ 'disabled' if not row.addable }}>
+                {% endif %}
+                <span class="picker__option-body">
+                  <span class="picker__option-name">{{ row.model.label }}</span>
+                  <span class="picker__option-note">
+                    {{ row.reason }}
+                    {% if row.addable %}{{ t("You may still add it yourself.") }}{% endif %}
+                  </span>
+                </span>
+              </label>
+              {% endfor %}
+              {% endif %}
               {% if crowd_member_ids %}
               {# One sentence and not three, with the numbers as placeholders: a
                  translation puts the parts in its own order, and two of these

src/lembas/web/templates/partials/_talk_rules.html

@@ -0,0 +1,104 @@
+{#
+  The rules list, the add form and the matrix, shared by the admin page and a
+  person's own settings. The caller sets `rules_action` (where the add form and
+  the deletes post), and passes `rules`, `model_ids`, `matrix`, `matrix_models`,
+  `any_model`, `allow` and `deny`.
+
+  The delete buttons reach one empty form each by `form=`, declared outside the
+  add form, so no form is ever nested inside another -- a nested <form>
+  truncates its parent, the 1.3.0 bug.
+#}
+{% from "_macros.html" import icon %}
+
+{% if rules %}
+<ul class="model-list">
+  {% for rule in rules %}
+  <li class="model-list__item">
+    <div style="min-width: 0">
+      <strong class="mono">{{ t("any model") if rule.from_model == any_model else rule.from_model }}</strong>
+      →
+      <strong class="mono">{{ t("any model") if rule.to_model == any_model else rule.to_model }}</strong>
+    </div>
+    <div class="btn-row">
+      {% if rule.effect == allow %}
+      <span class="badge badge--leaf">{{ t("may talk") }}</span>
+      {% else %}
+      <span class="badge badge--danger">{{ t("may not talk") }}</span>
+      {% endif %}
+      <form id="delete-rule-{{ rule.id }}" method="post" action="{{ rules_action }}/{{ rule.id }}/delete"></form>
+      <button class="btn btn--icon btn--sm btn--danger" type="submit" form="delete-rule-{{ rule.id }}"
+              aria-label="{{ t('Delete this rule') }}" title="{{ t('Delete this rule') }}">
+        {{ icon("trash", "icon--sm") }}
+      </button>
+    </div>
+  </li>
+  {% endfor %}
+</ul>
+{% else %}
+<p class="field__hint">{{ t("No rules yet.") }}</p>
+{% endif %}
+
+<form method="post" action="{{ rules_action }}" class="talk-rule-form">
+  <div class="field">
+    <label class="field__label" for="rule-from">{{ t("This model") }}</label>
+    <select class="select" id="rule-from" name="from_model">
+      <option value="{{ any_model }}">{{ t("any model") }}</option>
+      {% for model_id in model_ids %}<option value="{{ model_id }}">{{ model_id }}</option>{% endfor %}
+    </select>
+    <p class="field__hint">{{ t("The chat's own model.") }}</p>
+  </div>
+  <div class="field">
+    <label class="field__label" for="rule-effect">{{ t("May") }}</label>
+    <select class="select" id="rule-effect" name="effect">
+      <option value="{{ deny }}">{{ t("may not talk to") }}</option>
+      <option value="{{ allow }}">{{ t("may talk to") }}</option>
+    </select>
+    <p class="field__hint"></p>
+  </div>
+  <div class="field">
+    <label class="field__label" for="rule-to">{{ t("That model") }}</label>
+    <select class="select" id="rule-to" name="to_model">
+      <option value="{{ any_model }}">{{ t("any model") }}</option>
+      {% for model_id in model_ids %}<option value="{{ model_id }}">{{ model_id }}</option>{% endfor %}
+    </select>
+    <p class="field__hint">{{ t("A crowd member, a friend it asks, a model on its roster.") }}</p>
+  </div>
+  <div class="btn-row">
+    <label class="checkbox">
+      <input type="checkbox" name="both" value="true">
+      <span>{{ t("Both directions") }}</span>
+    </label>
+    <button class="btn btn--primary" type="submit">{{ icon("plus", "icon--sm") }} {{ t("Add rule") }}</button>
+  </div>
+</form>
+
+{% if matrix_models|length > 1 %}
+<h3 class="card__title" style="margin-top: var(--sp-6)">{{ t("Who may talk to whom") }}</h3>
+<p class="field__hint">{{ t("Rows are the chat's own model, columns the model it would bring in. Drawn by the same rules that are enforced, so what this shows is what happens.") }}</p>
+<div class="talk-matrix-scroll">
+  <table class="talk-matrix">
+    <thead>
+      <tr>
+        <th scope="col"></th>
+        {% for model in matrix_models %}<th scope="col" class="mono">{{ model.model_id }}</th>{% endfor %}
+      </tr>
+    </thead>
+    <tbody>
+      {% for row in matrix %}
+      <tr>
+        <th scope="row" class="mono">{{ row.main.model_id }}</th>
+        {% for cell in row.cells %}
+        {% if cell is none %}
+        <td class="talk-matrix__self" aria-label="{{ t('itself') }}">·</td>
+        {% elif cell.offered %}
+        <td class="talk-matrix__yes" title="{{ t('Offered') }}">✓</td>
+        {% else %}
+        <td class="talk-matrix__no" title="{{ describe_verdict(cell) }}">✗</td>
+        {% endif %}
+        {% endfor %}
+      </tr>
+      {% endfor %}
+    </tbody>
+  </table>
+</div>
+{% endif %}

src/lembas/web/templates/settings.html

@@ -185,6 +185,37 @@
               </ul>
             </div>
             {% endif %}
+            {#
+              Who your models may talk to. Anybody may narrow the instance's rules
+              for themselves; widening them takes `rules.override`, and the text
+              below says which of the two this person has, so a rule that does
+              nothing is not a mystery.
+            #}
+            <div class="card">
+              <h2 class="card__title">{{ t("Who your models may talk to") }}</h2>
+              <p class="card__lede">
+                {% if talk_override %}
+                {{ t("Your rules and your setting win over the instance's, for you. You can also add any model to a crowd by hand.") }}
+                {% else %}
+                {{ t("Your rules can only narrow the instance's. A model your own rule holds back can still be added to a crowd by hand; one the instance holds back cannot.") }}
+                {% endif %}
+              </p>
+              <form method="post" action="/api/preferences/talk-mode" class="row"
+                    style="gap: var(--sp-2); margin-bottom: var(--sp-4)">
+                <label class="visually-hidden" for="talk-mode">{{ t("Your starting point") }}</label>
+                <select class="select" id="talk-mode" name="mode" style="flex: 1; min-width: 0">
+                  <option value="" {{ 'selected' if not talk_mode }}>{{ t("Follow the instance") }}</option>
+                  <option value="open" {{ 'selected' if talk_mode == 'open' }}>{{ t("Any model may talk to any other") }}</option>
+                  <option value="closed" {{ 'selected' if talk_mode == 'closed' }}>{{ t("No model may talk to another") }}</option>
+                </select>
+                <button class="btn" type="submit">{{ t("Save") }}</button>
+              </form>
+              {% set rules_action = "/api/preferences/talk-rules" %}
+              {% set rules = talk_rules %}
+              {% set matrix = talk_matrix %}
+              {% set matrix_models = talk_matrix_models %}
+              {% include "partials/_talk_rules.html" %}
+            </div>
           </section>
 
           {# --- Appearance --- #}

tests/test_data_group_chat_pin.py

@@ -144,7 +144,11 @@ def test_a_chat_whose_model_moved_is_refused_rather_than_sent(db, owner, setup):
 
 # --- Other models reaching the conversation ------------------------------------------
 def test_a_crowd_member_from_another_group_is_refused(client, db, owner, setup):
+    """For somebody without `rules.override`: a different group is a deny only a
+    rule opens. (An administrator holds every permission, so the owner is demoted.)"""
     settings_store.update(db, {"enabled": True}, key=settings_store.CROWD)
+    owner.role = "user"
+    db.commit()
     local, _ = setup
     chat = _chat(db, owner, connection=local)
     client.patch(f"/api/chats/{chat.id}", data={"crowd_model_ids": ["local-b", "cloud-model"]})
@@ -153,6 +157,8 @@ def test_a_crowd_member_from_another_group_is_refused(client, db, owner, setup):
 
 
 def test_a_crowd_member_that_left_the_group_does_not_speak(db, owner, setup):
+    owner.role = "user"
+    db.commit()
     local, cloud = setup
     chat = _chat(db, owner, connection=local)
     db.add(CrowdMember(chat_id=chat.id, model_id="cloud-model", connection_id=cloud.id))

tests/test_talk_decide.py

@@ -0,0 +1,121 @@
+"""`talk.decide`, the one function behind every talk rule, as a table.
+
+Pure -- plain values in, a verdict out -- so every combination is asserted here
+with no database, and the admin matrix, which calls the same function, cannot
+disagree with what is enforced.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from lembas.services import talk
+from lembas.services.talk import Rule, decide, match
+
+ALLOW = Rule("main", "target", "allow")
+DENY = Rule("main", "target", "deny")
+
+
+def _v(**kwargs):
+    kwargs.setdefault("instance_mode", talk.MODE_OPEN)
+    kwargs.setdefault("instance_rule", None)
+    return decide(**kwargs)
+
+
+# --- The instance's layer ------------------------------------------------------------
+@pytest.mark.parametrize(
+    ("mode", "rule", "offered"),
+    [
+        (talk.MODE_OPEN, None, True),
+        (talk.MODE_OPEN, DENY, False),
+        (talk.MODE_OPEN, ALLOW, True),
+        (talk.MODE_CLOSED, None, False),
+        (talk.MODE_CLOSED, ALLOW, True),
+        (talk.MODE_CLOSED, DENY, False),
+    ],
+)
+def test_the_instance_mode_and_its_rules(mode, rule, offered):
+    verdict = _v(instance_mode=mode, instance_rule=rule)
+    assert verdict.offered is offered
+    assert verdict.addable is offered
+
+
+def test_another_data_group_is_a_deny_only_an_explicit_allow_opens():
+    assert _v(same_group=False).offered is False
+    assert _v(same_group=False).why == talk.WHY_GROUP
+    assert _v(same_group=False, instance_rule=ALLOW).offered is True
+    # An open mode is not an explicit allow.
+    assert _v(same_group=False, instance_mode=talk.MODE_OPEN).offered is False
+
+
+# --- A person without the override can only narrow ---------------------------------
+def test_a_persons_own_deny_takes_it_off_what_is_offered_but_not_off_what_is_addable():
+    verdict = _v(user_rule=DENY)
+    assert verdict.offered is False
+    assert verdict.addable is True
+    assert verdict.why == talk.WHY_YOUR_RULE
+
+
+def test_a_persons_closed_mode_narrows_too():
+    verdict = _v(user_mode=talk.MODE_CLOSED)
+    assert (verdict.offered, verdict.addable, verdict.why) == (False, True, talk.WHY_YOUR_CLOSED)
+
+
+def test_without_the_override_a_persons_allow_cannot_widen():
+    verdict = _v(instance_rule=DENY, user_rule=ALLOW)
+    assert (verdict.offered, verdict.addable) == (False, False)
+    assert verdict.why == talk.WHY_INSTANCE_RULE
+
+
+def test_without_the_override_open_mode_cannot_widen_a_closed_instance():
+    verdict = _v(instance_mode=talk.MODE_CLOSED, user_mode=talk.MODE_OPEN)
+    assert (verdict.offered, verdict.addable) == (False, False)
+
+
+# --- With the override, the person's layer wins ---------------------------------------
+def test_with_the_override_a_persons_allow_beats_the_instances_deny():
+    verdict = _v(instance_rule=DENY, user_rule=ALLOW, override=True)
+    assert (verdict.offered, verdict.addable) == (True, True)
+
+
+def test_with_the_override_a_persons_rule_opens_another_group():
+    assert _v(same_group=False, user_rule=ALLOW, override=True).offered is True
+
+
+def test_with_the_override_open_mode_widens_a_closed_instance_but_not_across_groups():
+    assert _v(instance_mode=talk.MODE_CLOSED, user_mode=talk.MODE_OPEN, override=True).offered
+    across = _v(same_group=False, user_mode=talk.MODE_OPEN, override=True)
+    assert across.offered is False and across.why == talk.WHY_GROUP
+    # ...unless some rule explicitly allows it.
+    assert _v(
+        same_group=False, user_mode=talk.MODE_OPEN, override=True, instance_rule=ALLOW
+    ).offered
+
+
+def test_with_the_override_and_no_layer_of_their_own_the_instance_decides():
+    assert _v(instance_rule=DENY, override=True).offered is False
+
+
+def test_with_the_override_anything_may_be_added_by_hand():
+    assert _v(instance_rule=DENY, override=True).addable is True
+    assert _v(same_group=False, override=True).addable is True
+
+
+# --- Specificity -------------------------------------------------------------------------
+def test_the_most_specific_rule_wins():
+    rules = [
+        Rule("*", "*", "deny"),
+        Rule("*", "b", "allow"),
+        Rule("a", "*", "deny"),
+        Rule("a", "b", "allow"),
+    ]
+    assert match(rules, "a", "b").effect == "allow"
+    assert match(rules, "a", "c") == Rule("a", "*", "deny")
+    assert match(rules, "x", "b") == Rule("*", "b", "allow")
+    assert match(rules, "x", "y") == Rule("*", "*", "deny")
+    assert match([], "a", "b") is None
+
+
+def test_a_verdict_says_why_in_english_for_a_model():
+    assert "forbids" in _v(instance_rule=DENY).reason
+    assert _v().reason == ""

tests/test_talk_rules.py

@@ -0,0 +1,203 @@
+"""Talk rules end to end: stored, edited, and obeyed by the roster, a friend and a crowd.
+
+The table of what `decide` answers is `test_talk_decide.py`. This file is where
+those answers reach something: the roster a model is told, the friend it may
+ask, the crowd picker's two lists, a member added by hand that must still speak,
+and a rule that lets a model from another data group in.
+"""
+
+from __future__ import annotations
+
+import pytest
+from sqlalchemy import select
+
+from lembas.db.models import (
+    DEFAULT_GROUP,
+    Chat,
+    Connection,
+    CrowdMember,
+    DataGroup,
+    Model,
+    TalkRule,
+    User,
+)
+from lembas.services import chat as chat_service
+from lembas.services import crowd, settings_store, talk
+from lembas.services import subagent as subagent_service
+from lembas.services.crypto import encrypt
+
+
+@pytest.fixture
+def owner(db, registered) -> User:
+    """The first account, demoted: an administrator holds `rules.override`."""
+    user = db.scalars(select(User).order_by(User.created_at)).first()
+    user.role = "user"
+    db.commit()
+    return user
+
+
+@pytest.fixture
+def setup(db, owner):
+    settings_store.update(db, {"enabled": True}, key=settings_store.CROWD)
+    db.add(DataGroup(id="hosted", name="Hosted"))
+    local = Connection(name="Local", base_url="http://127.0.0.1:1", api_key_encrypted=encrypt(""))
+    cloud = Connection(
+        name="Cloud",
+        base_url="http://127.0.0.1:2",
+        api_key_encrypted=encrypt(""),
+        data_group_id="hosted",
+    )
+    db.add_all([local, cloud])
+    db.flush()
+    for position, (connection, name) in enumerate(
+        [(local, "gpt-oss"), (local, "qwen38"), (local, "bonsai"), (cloud, "deepseek")]
+    ):
+        db.add(Model(connection_id=connection.id, model_id=name, position=position))
+    chat = Chat(
+        user_id=owner.id,
+        model_id="gpt-oss",
+        connection_id=local.id,
+        data_group_id=DEFAULT_GROUP,
+        title="t",
+    )
+    db.add(chat)
+    db.commit()
+    return chat
+
+
+def _ids(models) -> list[str]:
+    return [m.model_id for m in models]
+
+
+def _grant_override(db):
+    settings_store.update(db, {"default_permissions": {talk.PERMISSION: True}})
+
+
+# --- The roster and the friend ------------------------------------------------------
+def test_an_instance_deny_takes_a_model_off_the_roster(db, owner, setup):
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    roster = chat_service.roster_block(db, owner, exclude="gpt-oss", group=DEFAULT_GROUP)
+    assert "qwen38" not in roster and "bonsai" in roster
+
+
+def test_a_friend_the_rules_forbid_is_refused_with_the_reason(db, owner, setup):
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    friend, refusal = subagent_service._resolve_friend(
+        db, owner, "qwen38", asking="gpt-oss", group=DEFAULT_GROUP
+    )
+    assert friend is None
+    listed = refusal.split("These are the ones you can:")[-1]
+    assert "qwen38" not in listed
+
+
+def test_the_rule_is_read_from_the_main_model_only(db, owner, setup):
+    """bonsai may not talk to qwen38 -- which says nothing about gpt-oss's chats."""
+    talk.set_rule(db, None, "bonsai", "qwen38", "deny")
+    assert "qwen38" in _ids(talk.offered(db, owner, "gpt-oss", DEFAULT_GROUP))
+    assert "qwen38" not in _ids(talk.offered(db, owner, "bonsai", DEFAULT_GROUP))
+
+
+def test_a_closed_instance_offers_only_what_is_allowed(db, owner, setup):
+    settings_store.update(db, {"mode": "closed"}, key=settings_store.RULES)
+    talk.set_rule(db, None, "gpt-oss", "bonsai", "allow")
+    assert _ids(talk.offered(db, owner, "gpt-oss", DEFAULT_GROUP)) == ["bonsai"]
+
+
+# --- Other data groups -----------------------------------------------------------------
+def test_another_group_is_not_offered_until_a_rule_allows_it(db, owner, setup):
+    assert "deepseek" not in _ids(talk.offered(db, owner, "gpt-oss", DEFAULT_GROUP))
+    talk.set_rule(db, None, "gpt-oss", "deepseek", "allow")
+    assert "deepseek" in _ids(talk.offered(db, owner, "gpt-oss", DEFAULT_GROUP))
+
+
+def test_a_member_from_another_group_reads_its_own_groups_data(db, owner, setup):
+    from lembas.services import data_groups
+
+    speaker = chat_service.Speaker("deepseek", None)
+    assert data_groups.for_speaker(db, owner, setup, speaker) == "hosted"
+
+
+# --- The crowd: offered, and by hand -------------------------------------------------------
+def test_the_picker_names_what_it_holds_back_and_why(client, db, owner, setup):
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    talk.set_rule(db, owner, "gpt-oss", "bonsai", "deny")
+    page = client.get(f"/chat/{setup.id}").text
+    held = page.split("Not offered to this model")[1]
+    assert "qwen38" in held and "bonsai" in held
+    assert "The instance's rule" in held or "The instance's rule" in held
+    assert "You may still add it yourself." in held
+
+
+def test_a_persons_own_rule_is_soft_for_them(client, db, owner, setup):
+    talk.set_rule(db, owner, "gpt-oss", "bonsai", "deny")
+    client.patch(f"/api/chats/{setup.id}", data={"crowd_model_ids": ["bonsai"]})
+    assert [m.model_id for m in db.scalars(select(CrowdMember))] == ["bonsai"]
+
+
+def test_an_instance_rule_is_hard_without_the_override(client, db, owner, setup):
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    client.patch(f"/api/chats/{setup.id}", data={"crowd_model_ids": ["qwen38"]})
+    assert list(db.scalars(select(CrowdMember))) == []
+
+
+def test_with_the_override_an_instance_rule_can_be_passed_by_hand(client, db, owner, setup):
+    _grant_override(db)
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    client.patch(f"/api/chats/{setup.id}", data={"crowd_model_ids": ["qwen38"]})
+    assert [m.model_id for m in db.scalars(select(CrowdMember))] == ["qwen38"]
+
+
+def test_a_member_added_by_hand_still_speaks(db, owner, setup):
+    """Send time asks `addable`, not `offered`, or the choice would be undone."""
+    talk.set_rule(db, owner, "gpt-oss", "bonsai", "deny")
+    db.add(CrowdMember(chat_id=setup.id, model_id="bonsai"))
+    db.commit()
+    assert _ids(crowd.member_speakers(db, setup, owner)) == ["gpt-oss", "bonsai"]
+
+
+def test_a_member_the_instance_later_forbids_is_skipped_and_named(db, owner, setup):
+    db.add(CrowdMember(chat_id=setup.id, model_id="qwen38"))
+    db.commit()
+    talk.set_rule(db, None, "gpt-oss", "qwen38", "deny")
+    assert _ids(crowd.member_speakers(db, setup, owner)) == ["gpt-oss"]
+    assert crowd.unreachable_members(db, setup, owner) == ["qwen38"]
+
+
+# --- Storing and editing ------------------------------------------------------------------------
+def test_both_directions_writes_two_rules_and_a_second_write_replaces(db, setup):
+    talk.set_rule(db, None, "a", "b", "deny", both=True)
+    assert {(r.from_model, r.to_model) for r in db.scalars(select(TalkRule))} == {
+        ("a", "b"),
+        ("b", "a"),
+    }
+    talk.set_rule(db, None, "a", "b", "allow")
+    assert db.scalar(select(TalkRule).where(TalkRule.from_model == "a")).effect == "allow"
+
+
+def test_a_person_cannot_delete_an_instance_rule(db, owner, setup):
+    talk.set_rule(db, None, "a", "b", "deny")
+    rule = db.scalar(select(TalkRule))
+    assert talk.delete_rule(db, owner, rule.id) is False
+    assert talk.delete_rule(db, None, rule.id) is True
+
+
+def test_the_admin_page_adds_a_rule_and_draws_the_matrix(client, db, registered, setup):
+    # The fixture demoted the first account; the page is an administrator's.
+    user = db.scalars(select(User).order_by(User.created_at)).first()
+    user.role = "admin"
+    db.commit()
+    client.post("/admin/rules", data={"from_model": "gpt-oss", "to_model": "bonsai",
+                                      "effect": "deny"})
+    page = client.get("/admin/rules").text
+    assert "Who may talk to whom" in page
+    assert page.count("talk-matrix__no") >= 1
+
+
+def test_the_settings_card_sets_a_persons_mode_and_rules(client, db, owner, setup):
+    client.post("/api/preferences/talk-mode", data={"mode": "closed"})
+    client.post("/api/preferences/talk-rules", data={"from_model": "gpt-oss",
+                                                     "to_model": "bonsai", "effect": "allow"})
+    db.expire_all()
+    user = db.get(User, owner.id)
+    assert talk.user_mode(user) == "closed"
+    assert _ids(talk.offered(db, user, "gpt-oss", DEFAULT_GROUP)) == ["bonsai"]