diff --git a/docs/architecture/proactive-multi-agent-orchestration/spec.md b/docs/architecture/proactive-multi-agent-orchestration/spec.md
index 215161802a..39544fc280 100644
--- a/docs/architecture/proactive-multi-agent-orchestration/spec.md
+++ b/docs/architecture/proactive-multi-agent-orchestration/spec.md
@@ -202,7 +202,13 @@ Host enforcement, not prompt wording, owns delegation consent.
 
 Generation settings and safety state have different lifetimes:
 
-- model and generation settings are frozen when each child turn starts;
+- self-target children inherit the parent Session model and generation settings;
+- cross-agent DeepChat children use the target Agent's default model when configured, otherwise
+  they inherit the parent model. An unavailable configured model fails instead of falling back;
+- switching to a different target model uses that model's generation defaults and the target
+  Agent's system prompt, without copying parent reasoning or token/context overrides;
+- initial model and generation settings are resolved before admission waiting. Agent default
+  changes affect new delegations; follow-up turns retain the existing child Session configuration;
 - permission mode, workdir identity, Session deletion, and capability revocation are checked
   continuously and take effect for active work;
 - the host revalidates safety before authorization, immediately before tool dispatch, and before a
diff --git a/src/main/app/composition.ts b/src/main/app/composition.ts
index fa63e4b4a6..52682c8d70 100644
--- a/src/main/app/composition.ts
+++ b/src/main/app/composition.ts
@@ -175,6 +175,7 @@ import { AgentUnavailableError } from '@/agent/shared/agentCatalogCodec'
 import { resolveAcpAgentAlias } from '@shared/utils/acpAgentAlias'
 import { SessionQuery } from '@/session/query'
 import { SessionAssignmentPolicy } from '@/session/assignmentPolicy'
+import { sanitizeGenerationSettings } from '@/agent/deepchat/runtime/generationSettings'
 import { SessionAssignment } from '@/session/assignment'
 import { SessionDeletion } from '@/session/deletion'
 import { SessionTranscriptMutations } from '@/session/transcriptMutations'
@@ -2268,7 +2269,11 @@ export async function createMainProcessControl(dependencies: {
       getDefaultModel: () => agentSettings.getDefaultModel(),
       getDefaultProjectPath: () => projectService.getDefaultProjectPath(),
       resolveDeepChatAgentConfig: async (agentId) =>
-        await agentSettings.resolveDeepChatAgentConfig(agentId)
+        await agentSettings.resolveDeepChatAgentConfig(agentId),
+      isModelAvailable: (providerId, modelId) =>
+        providerSettings.getProviderById(providerId)?.enable === true &&
+        providerSettings.isKnownModel(providerId, modelId) &&
+        providerSettings.getModelStatus(providerId, modelId)
     }
   )
   const clearNewAgentSessionSkills = skillService.clearNewAgentSessionSkills
@@ -2501,6 +2506,24 @@ export async function createMainProcessControl(dependencies: {
       sessions: {
         ...agentToolDependencies.sessions,
         ...agentToolDependencies.subagents,
+        resolveSubagentExecutionSnapshot: async (input) => {
+          const assignment = await sessionAssignmentPolicy.resolveSubagentAssignment(input)
+          const isAcp = agentManager.resolveBackend(assignment.agentId).descriptor.kind === 'acp'
+          return {
+            providerId: assignment.providerId,
+            modelId: assignment.modelId,
+            toolModeOverride: assignment.toolModeOverride,
+            generationSettings: isAcp
+              ? null
+              : await sanitizeGenerationSettings(
+                  providerSettings,
+                  promptSettings,
+                  assignment.providerId,
+                  assignment.modelId,
+                  assignment.generationSettings ?? {}
+                )
+          }
+        },
         findDelegationChild: async (parentSessionId, delegationId) => {
           const matches = appSessionService
             .list({ includeSubagents: true, parentSessionId })
diff --git a/src/main/orchestration/liveDelegationSafety.ts b/src/main/orchestration/liveDelegationSafety.ts
index b1a8fffe87..0c636e29ec 100644
--- a/src/main/orchestration/liveDelegationSafety.ts
+++ b/src/main/orchestration/liveDelegationSafety.ts
@@ -115,7 +115,8 @@ export class LiveDelegationSafetyCoordinator implements LiveDelegationSafetyPort
       permissionMode: input.parentPermissionMode,
       generationSettings: child.generationSettings ?? undefined,
       disabledAgentTools: child.disabledAgentTools,
-      activeSkills: child.activeSkills
+      activeSkills: child.activeSkills,
+      preserveExecutionConfig: true
     })
     if (assignment.agentId !== child.agentId) {
       throw new Error(`Subagent target changed for child Session ${child.sessionId}.`)
diff --git a/src/main/orchestration/liveDelegationService.ts b/src/main/orchestration/liveDelegationService.ts
index 0003a0df94..7ce37440ee 100644
--- a/src/main/orchestration/liveDelegationService.ts
+++ b/src/main/orchestration/liveDelegationService.ts
@@ -46,6 +46,7 @@ import type { DeepChatTaskContractContext } from '@shared/types/task-contract'
 import { projectTaskEvaluationSummary } from '@/tape/domain/taskEvaluation'
 import type { SessionRuntimeUpdate } from '@/session/runtimeEvents'
 import type { SessionDeletionGatePort } from '@/session/deletionGate'
+import type { SubagentAssignmentInput } from '@/session/contracts'
 import { classifyToolEffect } from '@/tool/effectClassification'
 import type { ToolEffectObservation } from '@/tool/effectObserver'
 import { resolveToolPermissionMode } from '@/tool/permission/permissionMode'
@@ -119,6 +120,9 @@ export interface LiveDelegationAssistantResult {
 
 export interface LiveDelegationServiceSessionPort
   extends AgentToolSessionPort, AgentSubagentToolPort {
+  resolveSubagentExecutionSnapshot(
+    input: SubagentAssignmentInput
+  ): Promise<LiveDelegationTurnExecutionSnapshot>
   findDelegationChild(
     parentSessionId: string,
     delegationId: string
@@ -400,8 +404,21 @@ export class LiveDelegationService {
 
     const delegationId = nanoid()
     const turnId = nanoid()
-    const executionSnapshot = createTurnExecutionSnapshot(parent)
+    const parentSnapshot = createTurnExecutionSnapshot(parent)
     const projectDir = await this.resolveParentProjectDir(parent)
+    const executionSnapshot = await this.options.sessions.resolveSubagentExecutionSnapshot({
+      agentId: targetAgentId,
+      parentAgentId: parent.agentId,
+      targetAgentId,
+      projectDir,
+      providerId: parentSnapshot.providerId,
+      modelId: parentSnapshot.modelId,
+      generationSettings: parentSnapshot.generationSettings ?? undefined,
+      toolModeOverride: parentSnapshot.toolModeOverride,
+      permissionMode: parent.permissionMode,
+      disabledAgentTools: parent.disabledAgentTools,
+      activeSkills: parent.activeSkills
+    })
     const created = this.runAuthorizedStartMutation(parent, 'spawn', authorization, () =>
       this.options.repository.create(
         {
@@ -1057,6 +1074,7 @@ export class LiveDelegationService {
           projectDir: safety.projectDir,
           providerId: executionSnapshot.providerId,
           modelId: executionSnapshot.modelId,
+          preserveExecutionConfig: true,
           permissionMode: safety.parent.permissionMode,
           toolModeOverride: executionSnapshot.toolModeOverride,
           generationSettings: executionSnapshot.generationSettings
@@ -2039,7 +2057,11 @@ function createTurnExecutionSnapshot(
     providerId: session.providerId,
     modelId: session.modelId,
     generationSettings: session.generationSettings
-      ? structuredClone(session.generationSettings)
+      ? {
+          ...structuredClone(session.generationSettings),
+          thinkingBudget: session.generationSettings.thinkingBudget,
+          reasoningEffort: session.generationSettings.reasoningEffort
+        }
       : null,
     toolModeOverride: session.toolModeOverride
   }
diff --git a/src/main/session/assignmentPolicy.ts b/src/main/session/assignmentPolicy.ts
index 9ecb6a7ee3..d36cddf9f2 100644
--- a/src/main/session/assignmentPolicy.ts
+++ b/src/main/session/assignmentPolicy.ts
@@ -165,23 +165,35 @@ export class SessionAssignmentPolicy implements SessionAssignmentPolicyPort {
       }
     }
 
-    // Cross-agent child: keep the parent workdir/model and intersect its live authority with the
-    // target Agent policy. A target default may restrict the parent but can never elevate it.
     const agentConfig = await this.config.resolveDeepChatAgentConfig(descriptor.id)
+    const modelPreset = input.preserveExecutionConfig ? null : agentConfig?.defaultModelPreset
+    const providerId = modelPreset ? modelPreset.providerId?.trim() : input.providerId
+    const modelId = modelPreset ? modelPreset.modelId?.trim() : input.modelId
+    if (
+      !providerId ||
+      !modelId ||
+      (modelPreset && !this.config.isModelAvailable(providerId, modelId))
+    ) {
+      throw new Error(`Subagent ${descriptor.id} default model is unavailable.`)
+    }
+    this.assertAcpSessionHasWorkdir(providerId, input.projectDir)
+    const modelChanged = providerId !== input.providerId || modelId !== input.modelId
     const parentGeneration = input.generationSettings ?? {}
-    const generationSettings = this.mergeDefaultGenerationSettings(agentConfig, {
-      ...parentGeneration,
-      systemPrompt:
-        typeof agentConfig?.systemPrompt === 'string'
-          ? agentConfig.systemPrompt
-          : parentGeneration.systemPrompt
-    })
+    const generationSettings = input.preserveExecutionConfig
+      ? input.generationSettings
+      : this.mergeDefaultGenerationSettings(agentConfig, {
+          ...(modelChanged ? {} : parentGeneration),
+          systemPrompt:
+            typeof agentConfig?.systemPrompt === 'string'
+              ? agentConfig.systemPrompt
+              : parentGeneration.systemPrompt
+        })
 
     return {
       agentId: descriptor.id,
       targetAgentId,
-      providerId: input.providerId,
-      modelId: input.modelId,
+      providerId,
+      modelId,
       permissionMode: resolveCrossAgentPermissionMode(
         input.permissionMode,
         agentConfig?.permissionMode
diff --git a/src/main/session/contracts.ts b/src/main/session/contracts.ts
index a120374147..bc05b993ca 100644
--- a/src/main/session/contracts.ts
+++ b/src/main/session/contracts.ts
@@ -396,6 +396,7 @@ export interface SessionAssignmentConfigPort {
   getDefaultModel(): { providerId: string; modelId: string } | null | undefined
   getDefaultProjectPath(): string | null
   resolveDeepChatAgentConfig(agentId: string): Promise<DeepChatAgentConfig | null>
+  isModelAvailable(providerId: string, modelId: string): boolean
 }
 
 export interface CreateAssignmentInput {
@@ -434,6 +435,7 @@ export interface SubagentAssignmentInput {
   disabledAgentTools?: string[]
   activeSkills?: string[]
   toolModeOverride?: ToolModeOverride
+  preserveExecutionConfig?: boolean
 }
 
 export interface ResolvedSubagentAssignment {
@@ -610,6 +612,7 @@ export interface SessionLifecycleSubagentInput {
   activeSkills?: string[]
   toolModeOverride?: ToolModeOverride
   liveDelegationContext?: LiveDelegationSubagentContext
+  preserveExecutionConfig?: boolean
 }
 
 export interface SessionLifecyclePort {
diff --git a/src/main/session/lifecycle.ts b/src/main/session/lifecycle.ts
index 9e0fccefd5..bde5551a12 100644
--- a/src/main/session/lifecycle.ts
+++ b/src/main/session/lifecycle.ts
@@ -390,7 +390,8 @@ export class SessionLifecycle implements SessionLifecyclePort {
       generationSettings: input.generationSettings,
       disabledAgentTools: input.disabledAgentTools,
       activeSkills: input.activeSkills,
-      toolModeOverride: input.toolModeOverride
+      toolModeOverride: input.toolModeOverride,
+      preserveExecutionConfig: input.preserveExecutionConfig
     })
     return await this.dependencies.agentLifecycle.runWithAgentOperation(
       runtimeConfig.agentId,
diff --git a/src/main/tool/runtimePorts.ts b/src/main/tool/runtimePorts.ts
index 68a329c73f..e9d29536e2 100644
--- a/src/main/tool/runtimePorts.ts
+++ b/src/main/tool/runtimePorts.ts
@@ -94,6 +94,7 @@ export interface CreateSubagentSessionInput {
   disabledAgentTools?: string[]
   activeSkills?: string[]
   liveDelegationContext?: LiveDelegationSubagentContext
+  preserveExecutionConfig?: boolean
 }
 
 export interface AgentToolSessionPort {
diff --git a/test/main/orchestration/liveDelegationService.test.ts b/test/main/orchestration/liveDelegationService.test.ts
index baa50893dc..ecf05004fb 100644
--- a/test/main/orchestration/liveDelegationService.test.ts
+++ b/test/main/orchestration/liveDelegationService.test.ts
@@ -7,7 +7,8 @@ import {
   LiveDelegationDetailSchema,
   type LiveDelegationDetail
 } from '@shared/orchestration/liveDelegation'
-import type { ConversationSessionInfo } from '@/tool/runtimePorts'
+import type { ConversationSessionInfo, CreateSubagentSessionInput } from '@/tool/runtimePorts'
+import type { SubagentAssignmentInput } from '@/session/contracts'
 import type { SessionRuntimeUpdate } from '@/session/runtimeEvents'
 import { SessionDeletionGate } from '@/session/deletionGate'
 import { LiveDelegationConsentAuthority } from '@/orchestration/liveDelegationConsent'
@@ -1102,6 +1103,68 @@ describeIfSqlite('LiveDelegationService', () => {
     await vi.waitFor(() => expect(repository.require(spawned.delegation.id).status).toBe('idle'))
   })
 
+  it('applies target default changes only to new delegations, including admission waiting', async () => {
+    let defaults = {
+      providerId: 'anthropic',
+      modelId: 'review-model-a',
+      generationSettings: { systemPrompt: 'Reviewer prompt A', temperature: 0.2 },
+      toolModeOverride: null
+    }
+    harness.sessions.resolveSubagentExecutionSnapshot.mockImplementation(async () =>
+      structuredClone(defaults)
+    )
+    const firstBlocker = await admission.acquire({ ownerId: 'other-session-1' })
+    const secondBlocker = await admission.acquire({ ownerId: 'other-session-2' })
+    const spawned = await service.spawn('parent', {
+      slotId: 'reviewer',
+      title: 'Use the configured reviewer',
+      prompt: 'Review this change.'
+    })
+    defaults = {
+      ...defaults,
+      modelId: 'review-model-b',
+      generationSettings: { systemPrompt: 'Reviewer prompt B', temperature: 0.6 }
+    }
+    firstBlocker.release()
+    secondBlocker.release()
+
+    await vi.waitFor(() => expect(harness.sessions.sendConversationMessage).toHaveBeenCalledOnce())
+    const childId = repository.require(spawned.delegation.id).childSessionId!
+    const child = await harness.sessions.resolveConversationSessionInfo(childId)
+    expect(child).toMatchObject({
+      providerId: 'anthropic',
+      modelId: 'review-model-a',
+      generationSettings: { systemPrompt: 'Reviewer prompt A', temperature: 0.2 }
+    })
+    harness.publishAnswer(childId, 'First review complete.', 200)
+    harness.publish({ sessionId: childId, kind: 'status', updatedAt: 201, status: 'idle' })
+    await vi.waitFor(() => expect(repository.require(spawned.delegation.id).status).toBe('idle'))
+
+    await service.followUp('parent', spawned.delegation.id, 'Check the remaining files.')
+    await vi.waitFor(() =>
+      expect(harness.sessions.sendConversationMessage).toHaveBeenCalledTimes(2)
+    )
+    expect(child).toMatchObject({ modelId: 'review-model-a' })
+    harness.publishAnswer(childId, 'Follow-up review complete.', 300)
+    harness.publish({ sessionId: childId, kind: 'status', updatedAt: 301, status: 'idle' })
+    await vi.waitFor(() => expect(repository.require(spawned.delegation.id).status).toBe('idle'))
+
+    const next = await service.spawn('parent', {
+      slotId: 'reviewer',
+      title: 'Start another review',
+      prompt: 'Review another change.'
+    })
+    await vi.waitFor(() =>
+      expect(harness.sessions.sendConversationMessage).toHaveBeenCalledTimes(3)
+    )
+    const nextChildId = repository.require(next.delegation.id).childSessionId!
+    expect(await harness.sessions.resolveConversationSessionInfo(nextChildId)).toMatchObject({
+      providerId: 'anthropic',
+      modelId: 'review-model-b',
+      generationSettings: { systemPrompt: 'Reviewer prompt B', temperature: 0.6 }
+    })
+  })
+
   it('restores the child execution snapshot before a queued follow-up starts', async () => {
     const spawned = await service.spawn('parent', {
       slotId: 'reviewer',
@@ -2858,16 +2921,26 @@ function createSessionHarness(db: InstanceType<typeof DatabaseCtor>) {
   }
 
   const sessions = {
+    resolveSubagentExecutionSnapshot: vi.fn(async (input: SubagentAssignmentInput) => ({
+      providerId: input.providerId,
+      modelId: input.modelId,
+      generationSettings: input.generationSettings
+        ? structuredClone(input.generationSettings)
+        : null,
+      toolModeOverride: input.toolModeOverride ?? null
+    })),
     resolveConversationWorkdir: vi.fn().mockResolvedValue('/repo'),
     resolveConversationSessionInfo: vi.fn(async (sessionId: string) =>
       sessionId === 'parent' ? parent : (children.get(sessionId) ?? null)
     ),
-    createSubagentSession: vi.fn(
-      async (input: { liveDelegationContext?: { delegationId: string } }) => {
-        const delegationId = input.liveDelegationContext?.delegationId ?? 'missing'
-        return addChild(`child-${delegationId}`, delegationId, 'idle')
-      }
-    ),
+    createSubagentSession: vi.fn(async (input: CreateSubagentSessionInput) => {
+      const delegationId = input.liveDelegationContext?.delegationId ?? 'missing'
+      const child = addChild(`child-${delegationId}`, delegationId, 'idle')
+      child.providerId = input.providerId
+      child.modelId = input.modelId
+      child.generationSettings = input.generationSettings ?? null
+      return child
+    }),
     linkSubagentTape: vi.fn(async (input: { childSessionId: string; outcome: string }) => ({
       linkEntry: { sessionId: 'parent', entryId: 10 },
       childSessionId: input.childSessionId,
diff --git a/test/main/session/assignmentPolicy.test.ts b/test/main/session/assignmentPolicy.test.ts
index bbbc498a65..3c38757154 100644
--- a/test/main/session/assignmentPolicy.test.ts
+++ b/test/main/session/assignmentPolicy.test.ts
@@ -38,7 +38,8 @@ function createHarness() {
   const config = {
     getDefaultModel: vi.fn(() => ({ providerId: 'openai', modelId: 'gpt-4' })),
     getDefaultProjectPath: vi.fn(() => '/global-project'),
-    resolveDeepChatAgentConfig: vi.fn(async (agentId: string) => configs.get(agentId) ?? null)
+    resolveDeepChatAgentConfig: vi.fn(async (agentId: string) => configs.get(agentId) ?? null),
+    isModelAvailable: vi.fn(() => true)
   }
   return { policy: new SessionAssignmentPolicy(catalog, config), catalog, config, configs }
 }
@@ -217,7 +218,7 @@ describe('SessionAssignmentPolicy', () => {
     })
   })
 
-  it('leaves cross-agent subagent skills for target catalog validation', async () => {
+  it('uses the target model and prompt while intersecting cross-agent authority', async () => {
     const { policy, configs } = createHarness()
     configs.set('reviewer', {
       defaultModelPreset: { providerId: 'anthropic', modelId: 'claude-review' },
@@ -243,12 +244,11 @@ describe('SessionAssignmentPolicy', () => {
     ).resolves.toEqual({
       agentId: 'reviewer',
       targetAgentId: 'reviewer',
-      providerId: 'openai',
-      modelId: 'gpt-4',
+      providerId: 'anthropic',
+      modelId: 'claude-review',
       permissionMode: 'default',
       generationSettings: {
-        systemPrompt: 'Reviewer prompt',
-        temperature: 0.4
+        systemPrompt: 'Reviewer prompt'
       },
       disabledAgentTools: ['exec', 'read', 'write'],
       activeSkills: ['skill-a', 'skill-b', 'skill-c'],
@@ -256,6 +256,74 @@ describe('SessionAssignmentPolicy', () => {
     })
   })
 
+  it('inherits the parent model and overrides when the target has no default model', async () => {
+    const { policy, configs } = createHarness()
+    configs.set('reviewer', { systemPrompt: 'Reviewer prompt' })
+
+    await expect(
+      policy.resolveSubagentAssignment({
+        agentId: 'reviewer',
+        parentAgentId: 'deepchat',
+        projectDir: '/repo',
+        providerId: 'openai',
+        modelId: 'gpt-4',
+        generationSettings: { systemPrompt: 'Parent prompt', temperature: 0.4 }
+      })
+    ).resolves.toMatchObject({
+      providerId: 'openai',
+      modelId: 'gpt-4',
+      generationSettings: { systemPrompt: 'Reviewer prompt', temperature: 0.4 }
+    })
+  })
+
+  it.each([
+    { providerId: 'anthropic', modelId: 'unavailable' },
+    { providerId: '', modelId: 'claude-review' },
+    { providerId: 'anthropic', modelId: '' }
+  ])('rejects an unavailable target model without falling back: %j', async (defaultModelPreset) => {
+    const { policy, configs, config } = createHarness()
+    configs.set('reviewer', { defaultModelPreset })
+    config.isModelAvailable.mockReturnValue(false)
+
+    await expect(
+      policy.resolveSubagentAssignment({
+        agentId: 'reviewer',
+        parentAgentId: 'deepchat',
+        projectDir: '/repo',
+        providerId: 'openai',
+        modelId: 'gpt-4'
+      })
+    ).rejects.toThrow('Subagent reviewer default model is unavailable.')
+  })
+
+  it('preserves frozen execution settings while applying the current target permission policy', async () => {
+    const { policy, configs, config } = createHarness()
+    configs.set('reviewer', {
+      defaultModelPreset: { providerId: 'anthropic', modelId: 'changed-model' },
+      systemPrompt: 'Changed prompt',
+      permissionMode: 'default'
+    })
+    config.isModelAvailable.mockReturnValue(false)
+
+    await expect(
+      policy.resolveSubagentAssignment({
+        agentId: 'reviewer',
+        parentAgentId: 'deepchat',
+        projectDir: '/repo',
+        providerId: 'openai',
+        modelId: 'frozen-model',
+        permissionMode: 'full_access',
+        generationSettings: { systemPrompt: 'Frozen prompt', temperature: 0.4 },
+        preserveExecutionConfig: true
+      })
+    ).resolves.toMatchObject({
+      providerId: 'openai',
+      modelId: 'frozen-model',
+      generationSettings: { systemPrompt: 'Frozen prompt', temperature: 0.4 },
+      permissionMode: 'default'
+    })
+  })
+
   it('never elevates parent authority for cross-agent subagents', async () => {
     const { policy, configs } = createHarness()
 
diff --git a/test/main/session/sessionFixture.ts b/test/main/session/sessionFixture.ts
index 78c624c21d..2e8a4b2783 100644
--- a/test/main/session/sessionFixture.ts
+++ b/test/main/session/sessionFixture.ts
@@ -60,7 +60,11 @@ export const createSessionFixture = (input: {
       resolveDeepChatAgentConfig: async (agentId) => {
         if (typeof input.providerSettings.resolveDeepChatAgentConfig !== 'function') return null
         return await input.providerSettings.resolveDeepChatAgentConfig(agentId)
-      }
+      },
+      isModelAvailable: (providerId, modelId) =>
+        input.providerSettings.getProviderById(providerId)?.enable === true &&
+        input.providerSettings.isKnownModel(providerId, modelId) &&
+        input.providerSettings.getModelStatus(providerId, modelId)
     }
   )
   const deletion = new SessionDeletion({