diff --git a/src/main/acp/acp-dialects/acp-dialect.ts b/src/main/acp/acp-dialects/acp-dialect.ts
index 08b1f272353..1e7786cae45 100644
--- a/src/main/acp/acp-dialects/acp-dialect.ts
+++ b/src/main/acp/acp-dialects/acp-dialect.ts
@@ -100,6 +100,8 @@ export type AcpDialect = {
   notification?(method: string, params: unknown, at: number): AcpDialectNotification | undefined
   contextWindow?(models: unknown): number | undefined
   modelEfforts?(model: ModelInfo): { efforts: AgentSessionOptionChoice[]; defaultEffort?: string }
+  /** The agent clamps an effort another model lacks, so unlisted models' menus are unknown, not empty. */
+  clampsUnlistedEffort?: true
   /** Claimed permission requests are never auto-approved. */
   request?(method: string, params: unknown): AcpRequestPresentation | undefined
   settleRequest?(method: string, params: unknown): AcpRequestSettlement | undefined
diff --git a/src/main/acp/acp-dialects/omp-dialect.ts b/src/main/acp/acp-dialects/omp-dialect.ts
index 7af2b20e536..99ea622010c 100644
--- a/src/main/acp/acp-dialects/omp-dialect.ts
+++ b/src/main/acp/acp-dialects/omp-dialect.ts
@@ -113,6 +113,8 @@ function errorDetails(error: AcpAgentError): string | undefined {
 }
 
 export const OMP_ACP_DIALECT: AcpDialect = {
+  // OMP resolves any thinking level against the selected model (model-controls.ts setThinkingLevel).
+  clampsUnlistedEffort: true,
   normalizeToolUpdate,
   compactionReply,
   authenticationRequired: (error) => {
diff --git a/src/main/acp/acp-structured-options.test.ts b/src/main/acp/acp-structured-options.test.ts
index 2f54e1e7f50..22c011a96db 100644
--- a/src/main/acp/acp-structured-options.test.ts
+++ b/src/main/acp/acp-structured-options.test.ts
@@ -2,6 +2,8 @@ import { readFileSync } from 'node:fs'
 import { describe, expect, it } from 'vitest'
 import { AcpStructuredOptions } from './acp-structured-options'
 import { GROK_ACP_DIALECT } from './acp-dialects/grok-dialect'
+import { OMP_ACP_DIALECT } from './acp-dialects/omp-dialect'
+import { OPENCODE_ACP_DIALECT } from './acp-dialects/opencode-dialect'
 
 const configOptions = [
   {
@@ -30,7 +32,8 @@ const configOptions = [
 
 describe('ACP session options', () => {
   it('keeps the session’s picks out of the catalog facts', () => {
-    const options = new AcpStructuredOptions()
+    // OpenCode refuses an effort the selected model lacks, so another model's empty menu stays strict.
+    const options = new AcpStructuredOptions(OPENCODE_ACP_DIALECT)
     options.adoptSession({ configOptions })
     const { models, current } = options.read()
     // Current values supply an effort menu, never a catalog default.
@@ -49,6 +52,18 @@ describe('ACP session options', () => {
     expect(current).toEqual({ model: 'model-b', effort: 'high', confirmed: ['model', 'effort'] })
   })
 
+  it('leaves another model’s menu unknown for an agent that clamps unlisted efforts', () => {
+    const options = new AcpStructuredOptions(OMP_ACP_DIALECT)
+    options.adoptSession({ configOptions })
+    expect(options.read().models[0]).toEqual({
+      id: 'model-a',
+      label: 'Model A',
+      isDefault: false,
+      efforts: [],
+      effortsKnown: false
+    })
+  })
+
   it('reads each model’s own menu where the agent advertises one', () => {
     const options = new AcpStructuredOptions(GROK_ACP_DIALECT)
     options.adoptSession({
diff --git a/src/main/acp/acp-structured-options.ts b/src/main/acp/acp-structured-options.ts
index c33219483a1..f262044e99d 100644
--- a/src/main/acp/acp-structured-options.ts
+++ b/src/main/acp/acp-structured-options.ts
@@ -51,7 +51,10 @@ function isSelect(option: SessionConfigOption): option is SelectOption {
 
 export class AcpStructuredOptions {
   constructor(
-    private readonly dialect: Pick<AcpDialect, 'modelEfforts' | 'withheldCommands'> = {}
+    private readonly dialect: Pick<
+      AcpDialect,
+      'modelEfforts' | 'clampsUnlistedEffort' | 'withheldCommands'
+    > = {}
   ) {}
 
   private configOptions: SessionConfigOption[] = []
@@ -158,7 +161,13 @@ export class AcpStructuredOptions {
     const models: AgentSessionModelOption[] = listed.map((model) => {
       const info = this.models?.availableModels.find((entry) => entry.modelId === model.id)
       const advertised = info ? this.dialect.modelEfforts?.(info) : undefined
-      const sessionMenu = { efforts: model.id === currentModel ? sessionEfforts : [] }
+      // A clamping agent's session menu says nothing about another model's menu: unknown, not none.
+      const sessionMenu =
+        model.id === currentModel
+          ? { efforts: sessionEfforts }
+          : this.dialect.clampsUnlistedEffort && effortOption && !advertised
+            ? { efforts: [], effortsKnown: false as const }
+            : { efforts: [] }
       return {
         ...model,
         isDefault: false,
diff --git a/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.test.ts b/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.test.ts
index c1553272f9d..d194ae27741 100644
--- a/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.test.ts
+++ b/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.test.ts
@@ -39,3 +39,26 @@ describe('the effort menu of a model whose configured effort one listing lacks',
     expect(merged).not.toHaveProperty('defaultEffort')
   })
 })
+
+const unknownChat = (at: number): AgentModelCatalogListing => ({
+  ...listing('live-session', at, []),
+  models: [
+    { id: 'openai/gpt-6', label: 'GPT-6', isDefault: false, efforts: [], effortsKnown: false }
+  ]
+})
+
+describe('a model whose newer chat listing does not know its efforts', () => {
+  it('takes the menu the older listing supplies', () => {
+    const merged = model(listing('probe', 1, menu('off', 'high')), unknownChat(2))
+    expect(merged?.efforts).toEqual(menu('off', 'high'))
+    expect(merged).not.toHaveProperty('effortsKnown')
+  })
+
+  it('keeps an older listing’s explicit empty menu', () => {
+    expect(model(listing('probe', 1, []), unknownChat(2))).not.toHaveProperty('effortsKnown')
+  })
+
+  it('stays unknown when no listing knows its menu', () => {
+    expect(model(unknownChat(1), unknownChat(2))).toMatchObject({ effortsKnown: false })
+  })
+})
diff --git a/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.ts b/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.ts
index ee66c39ee5a..eac0b909ff6 100644
--- a/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.ts
+++ b/src/main/native-chat/agent-model-catalog/agent-model-catalog-entry.ts
@@ -139,7 +139,8 @@ function mergedModels(
   return (newer?.models ?? []).map((model) => {
     const listed = discovered?.models.find((entry) => entry.id === model.id)
     const reported = live?.models.find((entry) => entry.id === model.id)
-    const olderEfforts = older?.models.find((entry) => entry.id === model.id)?.efforts ?? []
+    const olderModel = older?.models.find((entry) => entry.id === model.id)
+    const olderEfforts = olderModel?.efforts ?? []
     const runsConfigured =
       configured?.modelId === model.id || configured?.sameModelIds?.includes(model.id) === true
     const configuredEffort = runsConfigured ? configured?.effort : undefined
@@ -152,9 +153,13 @@ function mergedModels(
     const defaultEffort = [configuredEffort, listed?.defaultEffort, reported?.defaultEffort].find(
       (effort) => effort !== undefined && efforts.some((choice) => choice.value === effort)
     )
-    const { defaultEffort: _own, ...rest } = model
+    const { defaultEffort: _own, effortsKnown, ...rest } = model
     return {
       ...rest,
+      // Efforts stay unknown only when neither listing knows this model's menu.
+      ...(effortsKnown === false && (!olderModel || olderModel.effortsKnown === false)
+        ? { effortsKnown }
+        : {}),
       // Without discovery/configuration, live flags are the only available default evidence.
       isDefault: configured
         ? model.id === configured.modelId