diff --git a/apps/web/src/components/settings/ProjectDefaultsSettings.tsx b/apps/web/src/components/settings/ProjectDefaultsSettings.tsx
index 43c4419962a02..b10af2fd1dda4 100644
--- a/apps/web/src/components/settings/ProjectDefaultsSettings.tsx
+++ b/apps/web/src/components/settings/ProjectDefaultsSettings.tsx
@@ -7,13 +7,16 @@ import { createModelSelection } from "@t3tools/shared/model";
 import { resolveProjectSettings } from "@t3tools/shared/projectSettings";
 import { useNavigate } from "@tanstack/react-router";
 
+import { useClientSettings } from "../../hooks/useSettings";
 import {
+  defaultModelSelectionsAreMixed,
   getCustomModelOptionsByInstance,
   resolveEffectiveDefaultModelSelection,
 } from "../../modelSelection";
 import {
   providerInstanceEntriesFor,
   resolveDefaultProviderModelSelection,
+  resolveProjectDefaultProviderModelSelection,
 } from "../../providerInstances";
 import { useEnvironmentProviders, useEnvironments } from "../../state/environments";
 import { EMPTY_SERVER_PROVIDERS } from "../../state/server";
@@ -48,8 +51,9 @@ function isWorktreeSubmodules(value: string | null): value is WorktreeSubmodules
 }
 
 export function ProjectDefaultsSettings({ category }: { category: ProjectSettingsCategory }) {
-  const { scope, target, connectedEnvironments } = useSettingsScope();
+  const { scope, target, targets, connectedEnvironments } = useSettingsScope();
   const settings = useScopedSettings();
+  const clientSettings = useClientSettings();
   const updateSettings = useUpdateScopedSettings();
   const navigate = useNavigate();
   const { environments } = useEnvironments();
@@ -61,10 +65,10 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
     (representative?.serverConfig
       ? providersByEnvironment.get(representative.environmentId)
       : undefined) ?? EMPTY_SERVER_PROVIDERS;
-  const storedSelection = resolveDefaultProviderModelSelection(
-    providers,
-    settings.defaultModelSelection,
-  );
+  const storedSelection =
+    settings.defaultModelSelection === null
+      ? resolveProjectDefaultProviderModelSelection(providers, settings, target?.projectId ?? null)
+      : resolveDefaultProviderModelSelection(providers, settings.defaultModelSelection);
   const selection = storedSelection
     ? resolveEffectiveDefaultModelSelection(settings, providers, storedSelection)
     : null;
@@ -76,7 +80,17 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
     selection?.model,
   );
   const activeEntry = entries.find((entry) => entry.instanceId === selection?.instanceId);
-  const mixedModel = useScopedSettingsMixed(["defaultModelSelection"]);
+  const mixedModel = defaultModelSelectionsAreMixed(
+    clientSettings,
+    targets.map((candidate) => ({
+      projectId: candidate.projectId,
+      settings: candidate.settings,
+      providers: providersByEnvironment.get(candidate.environmentId) ?? EMPTY_SERVER_PROVIDERS,
+    })),
+  );
+  const automaticModel =
+    targets.length > 0 &&
+    targets.every((candidate) => candidate.settings.defaultModelSelection === null);
   const mixedFallback = useScopedSettingsMixed(["fallbackModelSelection"]);
   const fallbackSelection = settings.fallbackModelSelection ?? selection;
   const fallbackModelOptions = getCustomModelOptionsByInstance(
@@ -99,6 +113,9 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
     : null;
 
   const modelDisabledReason = useScopedModelDisabledReason(settings, entries);
+  const effortDisabledReason = selection
+    ? modelDisabledReason(selection.instanceId, selection.model)
+    : null;
 
   const setModel = (value: ModelSelection | null) => {
     const reason = value ? modelDisabledReason(value.instanceId, value.model) : null;
@@ -119,13 +136,15 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
       description={
         isProjectScope
           ? "Model for new threads in this project."
-          : !unavailable &&
-              !mixedModel &&
-              modelSource !== "project" &&
-              settings.defaultModelSelection === null
+          : !unavailable && modelSource !== "project" && automaticModel
             ? "Default model for new threads, selected automatically. Projects can override it."
             : "Default model for new threads. Projects can override it."
       }
+      status={
+        mixedModel
+          ? `${automaticModel ? "Automatic defaults differ." : "Defaults differ."} Select one environment or choose a shared model to edit effort.`
+          : effortDisabledReason
+      }
       resetAction={
         settings.defaultModelSelection !== null ? (
           <SettingResetButton label="default model" onClick={() => setModel(null)} />
@@ -142,7 +161,9 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
               instanceEntries={entries}
               modelOptionsByInstance={modelOptions}
               triggerClassName={SETTINGS_PICKER_TRIGGER_CLASSNAME}
-              {...(mixedModel ? { triggerLabel: "Mixed" } : {})}
+              {...(mixedModel
+                ? { triggerLabel: automaticModel ? "Automatic (mixed)" : "Mixed" }
+                : {})}
               getModelDisabledReason={modelDisabledReason}
               onOpenProviderSetup={(instanceId) => {
                 if (representative)
@@ -155,7 +176,7 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
                 setModel(createModelSelection(instanceId, model))
               }
             />
-            {!mixedModel ? (
+            {!mixedModel && !effortDisabledReason ? (
               <TraitsPicker
                 provider={activeEntry.driverKind}
                 models={activeEntry.models}
@@ -171,7 +192,9 @@ export function ProjectDefaultsSettings({ category }: { category: ProjectSetting
             ) : null}
           </div>
         ) : (
-          <span className="text-sm text-muted-foreground">No providers available</span>
+          <span className="text-sm text-muted-foreground">
+            {mixedModel ? "Mixed" : "No providers available"}
+          </span>
         )
       }
     />
diff --git a/apps/web/src/modelSelection.test.ts b/apps/web/src/modelSelection.test.ts
index b5e9be7d9e011..430188a7c99c0 100644
--- a/apps/web/src/modelSelection.test.ts
+++ b/apps/web/src/modelSelection.test.ts
@@ -1,11 +1,17 @@
 import {
   ANTIGRAVITY_DEFAULT_MODEL,
   DEFAULT_TEXT_GENERATION_MODEL,
+  DEFAULT_SERVER_SETTINGS,
+  ProjectId,
   ProviderDriverKind,
   ProviderInstanceId,
   type ServerProvider,
 } from "@t3tools/contracts";
-import { DEFAULT_UNIFIED_SETTINGS, type UnifiedSettings } from "@t3tools/contracts/settings";
+import {
+  DEFAULT_CLIENT_SETTINGS,
+  DEFAULT_UNIFIED_SETTINGS,
+  type UnifiedSettings,
+} from "@t3tools/contracts/settings";
 import { describe, expect, it } from "vite-plus/test";
 import { createModelSelection } from "@t3tools/shared/model";
 import { deriveEffectiveComposerModelState } from "./composerDraftStore";
@@ -17,6 +23,7 @@ import {
   providerInstanceEntriesFor,
 } from "./providerInstances";
 import {
+  defaultModelSelectionsAreMixed,
   getAppModelOptionsForInstance,
   resolveAppModelSelectionForInstance,
   resolveAppModelSelectionState,
@@ -73,6 +80,142 @@ function settingsWithProviderInstances(): UnifiedSettings {
   };
 }
 
+describe("scoped default model selections", () => {
+  // regression: #27997 equal automatic defaults can resolve to different providers
+  it("compares automatic defaults using each environment's models", () => {
+    const codex = {
+      projectId: null,
+      settings: DEFAULT_UNIFIED_SETTINGS,
+      providers: [provider({ instanceId: "codex", models: ["gpt-6-astra"] })],
+    };
+    const claude = {
+      projectId: null,
+      settings: DEFAULT_UNIFIED_SETTINGS,
+      providers: [
+        provider({
+          instanceId: "claudeAgent",
+          provider: ProviderDriverKind.make("claudeAgent"),
+          models: ["claude-sonnet-4-6"],
+        }),
+      ],
+    };
+
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [codex, claude])).toBe(true);
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [claude, codex])).toBe(true);
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [codex, codex])).toBe(false);
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [claude])).toBe(false);
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [])).toBe(false);
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [codex, { ...codex, providers: [] }]),
+    ).toBe(true);
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [
+        codex,
+        { ...codex, providers: [provider({ instanceId: "codex", models: ["gpt-6-sol"] })] },
+      ]),
+    ).toBe(true);
+
+    const projectId = ProjectId.make("claude-only");
+    const project = {
+      projectId,
+      settings: {
+        ...DEFAULT_SERVER_SETTINGS,
+        projectSettingsOverrides: {
+          [projectId]: {
+            providerInstances: { [ProviderInstanceId.make("codex")]: { enabled: false } },
+          },
+        },
+      },
+      providers: [...codex.providers, ...claude.providers],
+    };
+    const remoteProject = { ...claude, projectId: ProjectId.make("remote-project") };
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [project, remoteProject])).toBe(
+      false,
+    );
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [remoteProject, project])).toBe(
+      false,
+    );
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [
+        project,
+        { ...project, projectId: ProjectId.make("other-project") },
+      ]),
+    ).toBe(true);
+
+    const configured = {
+      projectId: null,
+      settings: {
+        ...DEFAULT_SERVER_SETTINGS,
+        providerModelPreferences: {
+          [ProviderInstanceId.make("codex")]: {
+            hiddenModels: ["gpt-6-astra"],
+            modelOrder: [],
+          },
+        },
+      },
+      providers: [provider({ instanceId: "codex", models: ["gpt-6-astra", "gpt-6-sol"] })],
+    };
+    const inherited = { ...configured, settings: DEFAULT_SERVER_SETTINGS };
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [configured, inherited])).toBe(
+      true,
+    );
+    expect(defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [inherited, configured])).toBe(
+      true,
+    );
+    const clientSettings = {
+      ...DEFAULT_CLIENT_SETTINGS,
+      providerModelPreferences: configured.settings.providerModelPreferences,
+    };
+    expect(defaultModelSelectionsAreMixed(clientSettings, [configured, inherited])).toBe(false);
+    expect(defaultModelSelectionsAreMixed(clientSettings, [inherited, configured])).toBe(false);
+    expect(
+      defaultModelSelectionsAreMixed(clientSettings, [
+        { ...inherited, settings: { ...DEFAULT_SERVER_SETTINGS, providerModelPreferences: {} } },
+        inherited,
+      ]),
+    ).toBe(true);
+  });
+
+  it("keeps automatic, explicit, and different saved options distinct", () => {
+    const providers = [provider({ instanceId: "codex", models: ["gpt-6-astra"] })];
+    const selected = {
+      projectId: null,
+      providers,
+      settings: {
+        ...DEFAULT_UNIFIED_SETTINGS,
+        defaultModelSelection: createModelSelection(
+          ProviderInstanceId.make("codex"),
+          "gpt-6-astra",
+        ),
+      },
+    };
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [selected, { ...selected }]),
+    ).toBe(false);
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [
+        selected,
+        { projectId: null, providers, settings: DEFAULT_UNIFIED_SETTINGS },
+      ]),
+    ).toBe(true);
+    expect(
+      defaultModelSelectionsAreMixed(DEFAULT_CLIENT_SETTINGS, [
+        selected,
+        {
+          ...selected,
+          settings: {
+            ...selected.settings,
+            defaultModelSelection: {
+              ...selected.settings.defaultModelSelection,
+              options: [{ id: "reasoningEffort", value: "xhigh" }],
+            },
+          },
+        },
+      ]),
+    ).toBe(true);
+  });
+});
+
 describe("instance-scoped model selection", () => {
   it("keeps custom models on the provider instance that declared them", () => {
     const providers = [
diff --git a/apps/web/src/modelSelection.ts b/apps/web/src/modelSelection.ts
index db52028c397e0..c94de128de97e 100644
--- a/apps/web/src/modelSelection.ts
+++ b/apps/web/src/modelSelection.ts
@@ -4,9 +4,11 @@ import {
   DEFAULT_TEXT_GENERATION_MODEL_BY_PROVIDER,
   defaultInstanceIdForDriver,
   type ModelSelection,
+  type ProjectId,
   ProviderDriverKind,
   ProviderInstanceId,
   type ServerProvider,
+  type ServerSettings,
 } from "@t3tools/contracts";
 import {
   type CustomModelDefinition,
@@ -16,8 +18,10 @@ import {
   resolveSelectableModel,
 } from "@t3tools/shared/model";
 import { getComposerProviderState } from "./components/chat/composerProviderState";
-import { UnifiedSettings } from "@t3tools/contracts/settings";
+import { type ClientSettings, UnifiedSettings } from "@t3tools/contracts/settings";
+import { resolveModelPreferences } from "@t3tools/shared/serverSettings";
 import * as Arr from "effect/Array";
+import * as Equal from "effect/Equal";
 import * as Result from "effect/Result";
 import {
   getDefaultServerModel,
@@ -29,6 +33,8 @@ import {
   type ProviderInstanceEntry,
   deriveProviderInstanceEntries,
   NO_PROVIDER_MODEL_SELECTION,
+  resolveDefaultProviderModelSelection,
+  resolveProjectDefaultProviderModelSelection,
 } from "./providerInstances";
 import { sortModelsForProviderInstance } from "./modelOrdering";
 import { resolveModelShortName } from "@t3tools/client-runtime/provider-clients";
@@ -308,6 +314,38 @@ export function resolveEffectiveDefaultModelSelection(
   return model === null || model === selection.model ? selection : { ...selection, model };
 }
 
+export function defaultModelSelectionsAreMixed(
+  clientSettings: ClientSettings,
+  targets: ReadonlyArray<{
+    projectId: ProjectId | null;
+    settings: ServerSettings;
+    providers: ReadonlyArray<ServerProvider>;
+  }>,
+): boolean {
+  const selections = targets.map(({ projectId, settings, providers }) => {
+    const stored = settings.defaultModelSelection;
+    const selection =
+      stored === null
+        ? resolveProjectDefaultProviderModelSelection(providers, settings, projectId)
+        : resolveDefaultProviderModelSelection(providers, stored);
+    return {
+      stored,
+      effective: selection
+        ? resolveEffectiveDefaultModelSelection(
+            {
+              ...clientSettings,
+              ...settings,
+              ...resolveModelPreferences(settings, clientSettings),
+            },
+            providers,
+            selection,
+          )
+        : null,
+    };
+  });
+  return selections.some((selection) => !Equal.equals(selection, selections[0]));
+}
+
 export function getCustomModelOptionsByInstance(
   settings: UnifiedSettings,
   providers: ReadonlyArray<ServerProvider>,