diff --git a/proto/trogonai/policy/model_ceilings/state/v1alpha1/state.proto b/proto/trogonai/policy/model_ceilings/state/v1alpha1/state.proto
new file mode 100644
index 000000000..95137cc0b
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/state/v1alpha1/state.proto
@@ -0,0 +1,25 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.state.v1alpha1;
+
+import "buf/validate/validate.proto";
+import "google/protobuf/empty.proto";
+import "trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto";
+import "trogonai/policy/subjects/v1alpha1/subject.proto";
+
+// Policy fold only. Replay rejects unknown events and a subject change. Clear
+// retains the history.
+message State {
+  oneof state {
+    option (buf.validate.oneof).required = true;
+
+    google.protobuf.Empty missing = 1;
+    ModelCeilingHistory history = 2;
+  }
+}
+
+message ModelCeilingHistory {
+  trogonai.policy.subjects.v1alpha1.PolicySubject subject = 1 [features.field_presence = LEGACY_REQUIRED];
+  // Absence means cleared, not a zero cap.
+  trogonai.policy.model_ceilings.v1alpha1.ModelCeilingPolicy policy = 2;
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto
new file mode 100644
index 000000000..41226a355
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto
@@ -0,0 +1,41 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "buf/validate/validate.proto";
+
+// Exact match on the catalog model identifier of ModelVersionRef, any version of
+// it. A version arm is an additive change for when a ceiling needs to pin
+// releases. Names, families, and patterns are not arms because a late-bound name
+// would widen a ceiling without a policy change.
+message ModelMatcher {
+  oneof matcher {
+    option (buf.validate.oneof).required = true;
+
+    string model_id = 1 [(buf.validate.field).string.min_len = 1];
+  }
+}
+
+// A ceiling bounds which catalog models the subject's Sessions may select,
+// including auxiliary roles. A Session's every model selection must match some
+// matcher. An empty ceiling is invalid: use ClearModelCeiling to remove the
+// ceiling and a hold to stop work.
+//
+// Absence means no ceiling from this subject, never a zero cap, and never
+// bypasses connection-level model authorization under ADR#0032: the
+// connection's allowed models authorize, and a ceiling only narrows. Policy
+// never substitutes a model; a selection outside the ceiling fails admission
+// (ADR#0032 Decision 6).
+//
+// Ceilings follow the delegation lineage downward exactly like holds and
+// budgets, so an Agent limited to one model cannot reach another by delegating.
+// Evaluation is a conjunction over every ceiling on the chain.
+message ModelCeilingPolicy {
+  option (buf.validate.message).cel = {
+    id: "model_ceiling_policy.models_unique"
+    message: "each model identifier may appear at most once"
+    expression: "this.within.map(m, m.model_id).unique()"
+  };
+
+  repeated ModelMatcher within = 1 [(buf.validate.field).repeated.min_items = 1];
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/clear_model_ceiling.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/clear_model_ceiling.proto
new file mode 100644
index 000000000..305bbb20f
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/clear_model_ceiling.proto
@@ -0,0 +1,14 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "trogonai/policy/subjects/v1alpha1/subject.proto";
+
+// Removes only this subject's ceiling; ceilings elsewhere on a Session's chain
+// still apply. Clearing a missing history rejects with ModelCeilingNotSet. An
+// already cleared history succeeds without an event after the concurrency
+// check. Dispatch requires live administration authority and a nonempty Decider
+// command_id plus the original positive expected_revision; retries preserve both.
+message ClearModelCeiling {
+  trogonai.policy.subjects.v1alpha1.PolicySubject subject = 1 [features.field_presence = LEGACY_REQUIRED];
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/events.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/events.proto
new file mode 100644
index 000000000..a1c5fed06
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/events.proto
@@ -0,0 +1,18 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "buf/validate/validate.proto";
+import "trogonai/policy/model_ceilings/v1alpha1/model_ceiling_cleared.proto";
+import "trogonai/policy/model_ceilings/v1alpha1/model_ceiling_set.proto";
+
+// An unset or unknown event rejects replay; skipping a ceiling change could
+// admit a model selection the current ceiling forbids, or refuse one it allows.
+message ModelCeilingEvent {
+  oneof event {
+    option (buf.validate.oneof).required = true;
+
+    ModelCeilingSet model_ceiling_set = 1;
+    ModelCeilingCleared model_ceiling_cleared = 2;
+  }
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/faults.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/faults.proto
new file mode 100644
index 000000000..0d47ab881
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/faults.proto
@@ -0,0 +1,39 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "trogon/error/v1alpha1/options.proto";
+
+// Schema-only templates for the policy.model_ceilings module's DeciderService
+// refusals. These messages are never encoded. Administration authorization and
+// model selection admission keep their own failure contracts; this fold only
+// owns ceiling mutations.
+message ModelCeilingCommandInvalidError {
+  option (trogon.error.v1alpha1.message).template = {
+    domain: "policy.model_ceilings"
+    reason: "MODEL_CEILING_COMMAND_INVALID"
+    message: "The command violates the model ceiling policy contract."
+    code: FAILED_PRECONDITION
+    visibility: VISIBILITY_PUBLIC
+  };
+}
+
+message ModelCeilingNotSetError {
+  option (trogon.error.v1alpha1.message).template = {
+    domain: "policy.model_ceilings"
+    reason: "MODEL_CEILING_NOT_SET"
+    message: "No model ceiling history exists for this subject."
+    code: FAILED_PRECONDITION
+    visibility: VISIBILITY_PUBLIC
+  };
+}
+
+message ModelCeilingSubjectMismatchError {
+  option (trogon.error.v1alpha1.message).template = {
+    domain: "policy.model_ceilings"
+    reason: "MODEL_CEILING_SUBJECT_MISMATCH"
+    message: "The command names a different subject from this model ceiling history."
+    code: FAILED_PRECONDITION
+    visibility: VISIBILITY_PUBLIC
+  };
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_cleared.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_cleared.proto
new file mode 100644
index 000000000..67e6b05a0
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_cleared.proto
@@ -0,0 +1,11 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "trogonai/policy/subjects/v1alpha1/subject.proto";
+
+// Removes the subject's ceiling and retains its history. A later set reuses the
+// same stream.
+message ModelCeilingCleared {
+  trogonai.policy.subjects.v1alpha1.PolicySubject subject = 1 [features.field_presence = LEGACY_REQUIRED];
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_set.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_set.proto
new file mode 100644
index 000000000..c20f26e38
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/model_ceiling_set.proto
@@ -0,0 +1,12 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto";
+import "trogonai/policy/subjects/v1alpha1/subject.proto";
+
+// Records the ceiling in force from this event, replacing any earlier one.
+message ModelCeilingSet {
+  trogonai.policy.subjects.v1alpha1.PolicySubject subject = 1 [features.field_presence = LEGACY_REQUIRED];
+  ModelCeilingPolicy policy = 2 [features.field_presence = LEGACY_REQUIRED];
+}
diff --git a/proto/trogonai/policy/model_ceilings/v1alpha1/set_model_ceiling.proto b/proto/trogonai/policy/model_ceilings/v1alpha1/set_model_ceiling.proto
new file mode 100644
index 000000000..d5d9da2b5
--- /dev/null
+++ b/proto/trogonai/policy/model_ceilings/v1alpha1/set_model_ceiling.proto
@@ -0,0 +1,18 @@
+edition = "2024";
+
+package trogonai.policy.model_ceilings.v1alpha1;
+
+import "trogonai/policy/model_ceilings/v1alpha1/ceiling_policy.proto";
+import "trogonai/policy/subjects/v1alpha1/subject.proto";
+
+// Sets or replaces the subject's ceiling. There is no Initialize command and no
+// provisioning participation, because absence needs nothing set up. On a missing
+// history it creates the stream: NoStream requires a nonempty Decider command_id
+// and no expected_revision. Later commands carry the original positive
+// expected_revision; retries preserve both. Dispatch verifies the subject exists
+// and live administration authority. Equal matchers succeed without an event
+// after the concurrency check; reordering matchers alone is not a change.
+message SetModelCeiling {
+  trogonai.policy.subjects.v1alpha1.PolicySubject subject = 1 [features.field_presence = LEGACY_REQUIRED];
+  ModelCeilingPolicy policy = 2 [features.field_presence = LEGACY_REQUIRED];
+}
diff --git a/proto/trogonai/policy/subjects/v1alpha1/rule_ref.proto b/proto/trogonai/policy/subjects/v1alpha1/rule_ref.proto
index 09777b0ca..405ab29d3 100644
--- a/proto/trogonai/policy/subjects/v1alpha1/rule_ref.proto
+++ b/proto/trogonai/policy/subjects/v1alpha1/rule_ref.proto
@@ -15,6 +15,7 @@ enum PolicyRuleKind {
   POLICY_RULE_KIND_AGENT_BUDGET = 1;
   POLICY_RULE_KIND_AGENT_RATE_LIMIT = 2;
   POLICY_RULE_KIND_HOLD = 3;
+  POLICY_RULE_KIND_MODEL_CEILING = 4;
 }
 
 message PolicyRuleRef {