coder · ibetitsmike · Jul 14, 2026 · Jul 9, 2026 · Jul 9, 2026 · Jul 9, 2026
diff --git a/coderd/database/dbauthz/dbauthz.go b/coderd/database/dbauthz/dbauthz.go
@@ -3036,6 +3036,13 @@ func (q *querier) GetChatByIDForUpdate(ctx context.Context, id uuid.UUID) (datab
 	return fetch(q.log, q.auth, q.db.GetChatByIDForUpdate)(ctx, id)
 }
 
+func (q *querier) GetChatCompactionModelOverride(ctx context.Context) (string, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
+		return "", err
+	}
+	return q.db.GetChatCompactionModelOverride(ctx)
+}
+
 func (q *querier) GetChatComputerUseProvider(ctx context.Context) (string, error) {
 	// The computer-use provider is a deployment-wide runtime chat setting
 	// read by authenticated chat users and chatd. Feature and experiment
@@ -8707,6 +8714,13 @@ func (q *querier) UpsertChatAutoArchiveDays(ctx context.Context, autoArchiveDays
 	return q.db.UpsertChatAutoArchiveDays(ctx, autoArchiveDays)
 }
 
+func (q *querier) UpsertChatCompactionModelOverride(ctx context.Context, value string) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.UpsertChatCompactionModelOverride(ctx, value)
+}
+
 func (q *querier) UpsertChatComputerUseProvider(ctx context.Context, provider string) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return err

diff --git a/coderd/database/dbauthz/dbauthz_test.go b/coderd/database/dbauthz/dbauthz_test.go
@@ -1209,6 +1209,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatTitleGenerationModelOverride(gomock.Any()).Return("", nil).AnyTimes()
 		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead)
 	}))
+	s.Run("GetChatCompactionModelOverride", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatCompactionModelOverride(gomock.Any()).Return("", nil).AnyTimes()
+		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead)
+	}))
 	s.Run("GetChatPlanModeInstructions", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().GetChatPlanModeInstructions(gomock.Any()).Return("", nil).AnyTimes()
 		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
@@ -1640,6 +1644,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UpsertChatTitleGenerationModelOverride(gomock.Any(), "").Return(nil).AnyTimes()
 		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
 	}))
+	s.Run("UpsertChatCompactionModelOverride", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().UpsertChatCompactionModelOverride(gomock.Any(), "").Return(nil).AnyTimes()
+		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
 	s.Run("UpsertChatPlanModeInstructions", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().UpsertChatPlanModeInstructions(gomock.Any(), "").Return(nil).AnyTimes()
 		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)

diff --git a/coderd/database/dbmetrics/querymetrics.go b/coderd/database/dbmetrics/querymetrics.go
diff --git a/coderd/database/dbmock/dbmock.go b/coderd/database/dbmock/dbmock.go
diff --git a/coderd/database/querier.go b/coderd/database/querier.go
diff --git a/coderd/database/queries.sql.go b/coderd/database/queries.sql.go
diff --git a/coderd/database/queries/siteconfig.sql b/coderd/database/queries/siteconfig.sql
@@ -191,6 +191,14 @@ SELECT
 INSERT INTO site_configs (key, value) VALUES ('agents_chat_title_generation_model_override', $1)
 ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_title_generation_model_override';
 
+-- name: GetChatCompactionModelOverride :one
+SELECT
+	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_compaction_model_override'), '') :: text AS model_config_id;
+
+-- name: UpsertChatCompactionModelOverride :exec
+INSERT INTO site_configs (key, value) VALUES ('agents_chat_compaction_model_override', $1)
+ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_compaction_model_override';
+
 -- name: GetChatDesktopEnabled :one
 SELECT
 	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_desktop_enabled'), false) :: boolean AS enable_desktop;

diff --git a/coderd/exp_chats.go b/coderd/exp_chats.go
@@ -709,6 +709,12 @@ func (api *API) chatModelOverrideSiteConfig(
 			getter: api.Database.GetChatTitleGenerationModelOverride,
 			upsert: api.Database.UpsertChatTitleGenerationModelOverride,
 		}, nil
+	case codersdk.ChatModelOverrideContextCompaction:
+		return chatModelOverrideSiteConfig{
+			label:  "compaction",
+			getter: api.Database.GetChatCompactionModelOverride,
+			upsert: api.Database.UpsertChatCompactionModelOverride,
+		}, nil
 	default:
 		return chatModelOverrideSiteConfig{}, xerrors.Errorf(
 			"unknown chat model override context %q",

diff --git a/coderd/exp_chats_test.go b/coderd/exp_chats_test.go
@@ -12319,6 +12319,16 @@ func TestChatModelOverrides(t *testing.T) {
 				return db.UpsertChatTitleGenerationModelOverride(dbauthz.AsSystemRestricted(ctx), value)
 			},
 		},
+		{
+			name:    "Compaction",
+			context: codersdk.ChatModelOverrideContextCompaction,
+			dbGet: func(ctx context.Context, db database.Store) (string, error) {
+				return db.GetChatCompactionModelOverride(dbauthz.AsSystemRestricted(ctx))
+			},
+			dbUpsert: func(ctx context.Context, db database.Store, value string) error {
+				return db.UpsertChatCompactionModelOverride(dbauthz.AsSystemRestricted(ctx), value)
+			},
+		},
 	}
 
 	for _, setting := range settings {
@@ -12528,7 +12538,7 @@ func TestChatModelOverrides(t *testing.T) {
 		require.Equal(t, "Invalid chat model override context.", sdkErr.Message)
 		require.Equal(
 			t,
-			`Expected one of general, explore, title_generation. Got "not-a-context".`,
+			`Expected one of general, explore, title_generation, compaction. Got "not-a-context".`,
 			sdkErr.Detail,
 		)
 
@@ -12537,7 +12547,7 @@ func TestChatModelOverrides(t *testing.T) {
 		require.Equal(t, "Invalid chat model override context.", sdkErr.Message)
 		require.Equal(
 			t,
-			`Expected one of general, explore, title_generation. Got "not-a-context".`,
+			`Expected one of general, explore, title_generation, compaction. Got "not-a-context".`,
 			sdkErr.Detail,
 		)
 	})

diff --git a/coderd/x/chatd/ARCHITECTURE.md b/coderd/x/chatd/ARCHITECTURE.md
@@ -831,6 +831,19 @@ Model configs may carry a `reasoning_effort` config (`{default, max}`) inside `c
 
 During generation preparation, the effective effort is resolved as the chat's `last_reasoning_effort` if set, else the config's `default`; clamped to the config's `max` on the global scale `none < minimal < low < medium < high < xhigh < max`; and passed through to the provider. The provider verifies whether the configured value is valid for that model at runtime. If the model config has no `reasoning_effort`, any user-selected value is ignored. The resolved value is injected into the provider-native options with `chatprovider.ApplyReasoningEffort` after provider option conversion.
 
+#### Compaction model selection
+
+Compaction is an auxiliary LLM call: when the conversation approaches the context limit, the generation goroutine asks a model to summarize the history, commits the summary as a compressed boundary, and continues the turn on the chat model.
+
+By default the summary is generated with the chat model. Admins can override the compaction model deployment-wide via the `compaction` context of the chat model override API (`/api/experimental/chats/config/model-override/{context}`, stored in the `agents_chat_compaction_model_override` site config). The override affects only the summary call; thresholds, compressed-message storage, and the post-compaction assistant generation keep using the chat model.
+
+Details that follow from the override:
+
+- Context limits: the compaction trigger uses the stricter of the chat model's and the compaction model's context limits, because the history must also fit the summarizer's window. The post-compaction "still over limit" check stays against the chat model's limit, since continuation runs on the chat model.
+- Failure semantics: an unset override uses the chat model. Stale or malformed stored references (deleted or disabled config or provider, missing credentials, non-UUID value) fall back to the chat model with a log. A usable override that fails at use (route or client construction, provider call failure) fails the generation visibly through the normal error path; there is no silent fallback. The override model client is constructed inside the compact generation action, not at prepare time, so a broken override cannot fail turns that finish without compacting (including turns over the threshold whose last assistant step already completed).
+- Prompt safety: the prompt is built and sanitized for the chat model, so when the override points at a different provider the compaction copy of the prompt is re-sanitized: provider-executed tool history is flattened into plain text parts (keeping its content while dropping the provider-specific wire shape), file parts the compaction model rejects are replaced with text placeholders, and Anthropic provider-tool sanitization is re-run for the compaction provider. The assistant generation prompt is never mutated.
+- Observability: compaction metrics and chat debug runs record the provider and model that actually generated the summary. This includes the "still over limit" terminal error, which is recorded before the override client is built: prepare-time resolution keeps the override's provider/model identity so that error lands on the same metric series as the compact action's own events.
+
 #### Interrupt goroutine
 
 The interrupt goroutine is responsible for handling interrupts. It is spawned when the event indicates the core state machine is in `I0` or `I1` (status is `interrupting`).