diff --git a/apps/web/src/components/chat/ChatComposer.tsx b/apps/web/src/components/chat/ChatComposer.tsx
index c54b3386166..5fa79868c88 100644
--- a/apps/web/src/components/chat/ChatComposer.tsx
+++ b/apps/web/src/components/chat/ChatComposer.tsx
@@ -71,6 +71,7 @@ import { ComposerPlanFollowUpBanner } from "./ComposerPlanFollowUpBanner";
 import { resolveComposerMenuActiveItemId } from "./composerMenuHighlight";
 import { searchSlashMenuItems } from "./composerSlashMenuSearch";
 import {
+  getComposerPromptInjectionState,
   getComposerProviderState,
   renderProviderTraitsMenuContent,
   renderProviderTraitsPicker,
@@ -747,18 +748,22 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
     [selectedProviderEntry],
   );
 
+  const composerPromptInjectionState = useMemo(
+    () => getComposerPromptInjectionState(prompt),
+    [prompt],
+  );
   const composerProviderState = useMemo(
     () =>
       getComposerProviderState({
         provider: selectedProvider,
         model: selectedModel,
         models: selectedProviderModels,
-        prompt,
+        promptInjectionState: composerPromptInjectionState,
         modelOptions: composerModelOptions?.[selectedInstanceId],
       }),
     [
       composerModelOptions,
-      prompt,
+      composerPromptInjectionState,
       selectedInstanceId,
       selectedModel,
       selectedProvider,
diff --git a/apps/web/src/components/chat/composerProviderState.test.tsx b/apps/web/src/components/chat/composerProviderState.test.tsx
index 07eec55de2c..b0bd2d20c99 100644
--- a/apps/web/src/components/chat/composerProviderState.test.tsx
+++ b/apps/web/src/components/chat/composerProviderState.test.tsx
@@ -6,6 +6,7 @@ import {
   type ServerProviderModel,
 } from "@t3tools/contracts";
 import {
+  getComposerPromptInjectionState,
   getComposerProviderState,
   renderProviderTraitsMenuContent,
   renderProviderTraitsPicker,
@@ -61,6 +62,16 @@ const ULTRATHINK_FRAME_CLASSES = {
 } as const;
 
 describe("getComposerProviderState", () => {
+  it("derives a stable prompt injection state for ordinary prompt edits", () => {
+    expect(getComposerPromptInjectionState("Investigate this failure")).toBe("none");
+    expect(getComposerPromptInjectionState("Ultrathink:\nInvestigate this failure")).toBe(
+      "ultrathink",
+    );
+    expect(getComposerPromptInjectionState("Ultrathink:\nInvestigate this failure deeply")).toBe(
+      "ultrathink",
+    );
+  });
+
   it("returns descriptor defaults when no selections are provided", () => {
     const state = getComposerProviderState({
       provider: PROVIDER,
@@ -71,7 +82,6 @@ describe("getComposerProviderState", () => {
           { id: "high", label: "High", isDefault: true },
         ]),
       ]),
-      prompt: "",
       modelOptions: undefined,
     });
 
@@ -93,7 +103,6 @@ describe("getComposerProviderState", () => {
         ]),
         booleanDescriptor("fastMode"),
       ]),
-      prompt: "",
       modelOptions: selections(["effort", "low"], ["fastMode", true]),
     });
 
@@ -112,7 +121,6 @@ describe("getComposerProviderState", () => {
         selectDescriptor("effort", [{ id: "high", label: "High", isDefault: true }]),
         booleanDescriptor("fastMode"),
       ]),
-      prompt: "",
       modelOptions: selections(["effort", "high"], ["fastMode", false]),
     });
 
@@ -126,7 +134,6 @@ describe("getComposerProviderState", () => {
       provider: PROVIDER,
       model: MODEL,
       models: modelWith([booleanDescriptor("thinking")]),
-      prompt: "",
       modelOptions: selections(["effort", "max"], ["thinking", false]),
     });
 
@@ -152,7 +159,6 @@ describe("getComposerProviderState", () => {
           { id: "plan", label: "Plan" },
         ]),
       ]),
-      prompt: "",
       modelOptions: selections(["agent", "plan"]),
     });
 
@@ -167,7 +173,6 @@ describe("getComposerProviderState", () => {
       provider: PROVIDER,
       model: MODEL,
       models: modelWith([]),
-      prompt: "",
       modelOptions: selections(["anything", "value"]),
     });
 
@@ -193,7 +198,9 @@ describe("getComposerProviderState", () => {
           ["ultrathink"],
         ),
       ]),
-      prompt: "Ultrathink:\nInvestigate this failure",
+      promptInjectionState: getComposerPromptInjectionState(
+        "Ultrathink:\nInvestigate this failure",
+      ),
       modelOptions: selections(["effort", "medium"]),
     });
 
@@ -212,7 +219,9 @@ describe("getComposerProviderState", () => {
       models: modelWith([
         selectDescriptor("effort", [{ id: "high", label: "High", isDefault: true }]),
       ]),
-      prompt: "Ultrathink:\nInvestigate this failure",
+      promptInjectionState: getComposerPromptInjectionState(
+        "Ultrathink:\nInvestigate this failure",
+      ),
       modelOptions: undefined,
     });
 
diff --git a/apps/web/src/components/chat/composerProviderState.tsx b/apps/web/src/components/chat/composerProviderState.tsx
index b5cc790538d..ed1c4fef8f1 100644
--- a/apps/web/src/components/chat/composerProviderState.tsx
+++ b/apps/web/src/components/chat/composerProviderState.tsx
@@ -21,10 +21,13 @@ export type ComposerProviderStateInput = {
   provider: ProviderDriverKind;
   model: string;
   models: ReadonlyArray<ServerProviderModel>;
-  prompt: string;
+  // Coarse prompt-derived state keeps provider chrome stable across ordinary body edits.
+  promptInjectionState?: ComposerPromptInjectionState;
   modelOptions: ReadonlyArray<ProviderOptionSelection> | null | undefined;
 };
 
+export type ComposerPromptInjectionState = "none" | "ultrathink";
+
 export type ComposerProviderState = {
   provider: ProviderDriverKind;
   promptEffort: string | null;
@@ -46,8 +49,12 @@ type TraitsRenderInput = {
   onPromptChange: (prompt: string) => void;
 };
 
+export function getComposerPromptInjectionState(prompt: string): ComposerPromptInjectionState {
+  return isClaudeUltrathinkPrompt(prompt) ? "ultrathink" : "none";
+}
+
 export function getComposerProviderState(input: ComposerProviderStateInput): ComposerProviderState {
-  const { provider, model, models, prompt, modelOptions } = input;
+  const { provider, model, models, modelOptions, promptInjectionState = "none" } = input;
   const caps = getProviderModelCapabilities(models, model, provider);
   const descriptors = getProviderOptionDescriptors({ caps, selections: modelOptions });
   const primarySelectDescriptor = descriptors.find(
@@ -58,7 +65,7 @@ export function getComposerProviderState(input: ComposerProviderStateInput): Com
   const promptEffort = typeof primaryValue === "string" ? primaryValue : null;
   const ultrathinkActive =
     (primarySelectDescriptor?.promptInjectedValues?.length ?? 0) > 0 &&
-    isClaudeUltrathinkPrompt(prompt);
+    promptInjectionState === "ultrathink";
 
   return {
     provider,
diff --git a/apps/web/src/index.css b/apps/web/src/index.css
index a48a00c9bc7..8ba440e095c 100644
--- a/apps/web/src/index.css
+++ b/apps/web/src/index.css
@@ -707,12 +707,35 @@ label:has(> select#reasoning-effort) select {
     #ec4899 90%,
     #ff6b6b 100%
   );
+  position: relative;
+  background: transparent;
+}
+
+.ultrathink-frame::before {
+  content: "";
+  position: absolute;
+  inset: 0;
+  border-radius: inherit;
+  padding: 2px;
   background-image: var(--ultrathink-spectrum);
   background-size: 220% 220%;
+  pointer-events: none;
   filter: saturate(0.82) brightness(0.92);
+  -webkit-mask:
+    linear-gradient(#000 0 0) content-box,
+    linear-gradient(#000 0 0);
+  -webkit-mask-composite: xor;
+  mask:
+    linear-gradient(#000 0 0) content-box,
+    linear-gradient(#000 0 0);
+  mask-composite: exclude;
   animation: ultrathink-rainbow 10s linear infinite;
 }
 
+.ultrathink-frame > * {
+  position: relative;
+}
+
 .ultrathink-chroma {
   animation: ultrathink-chroma-shift 10s linear infinite;
 }
diff --git a/apps/web/src/modelSelection.ts b/apps/web/src/modelSelection.ts
index 0fcf680b732..7ede9665ac9 100644
--- a/apps/web/src/modelSelection.ts
+++ b/apps/web/src/modelSelection.ts
@@ -303,7 +303,6 @@ export function resolveAppModelSelectionState(
       provider,
       model,
       models: entry.models,
-      prompt: "",
       modelOptions: selectedEntry ? selection.options : undefined,
     });
 
@@ -321,7 +320,6 @@ export function resolveAppModelSelectionState(
     provider,
     model,
     models: getProviderModels(providers, provider),
-    prompt: "",
     modelOptions: keptSelectedProvider ? selection.options : undefined,
   });