diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index d522e4061..9bf2f2bcf 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -15,36 +15,8 @@ import {
   resolveInheritedPromptTools,
   createInternalAgentTextPart,
 } from "../../shared"
+import { applySessionPromptParams } from "../../shared/session-prompt-params-helpers"
 import { setSessionTools } from "../../shared/session-tools-store"
-import { setSessionPromptParams } from "../../shared/session-prompt-params-state"
-
-type PromptParamsModel = {
-  reasoningEffort?: string
-  thinking?: { type: "enabled" | "disabled"; budgetTokens?: number }
-  maxTokens?: number
-  temperature?: number
-  top_p?: number
-}
-
-function applySessionPromptParams(sessionID: string, model: PromptParamsModel): void {
-  const promptOptions: Record<string, unknown> = {
-    ...(model.reasoningEffort ? { reasoningEffort: model.reasoningEffort } : {}),
-    ...(model.thinking ? { thinking: model.thinking } : {}),
-    ...(model.maxTokens !== undefined ? { maxTokens: model.maxTokens } : {}),
-  }
-
-  if (
-    model.temperature !== undefined ||
-    model.top_p !== undefined ||
-    Object.keys(promptOptions).length > 0
-  ) {
-    setSessionPromptParams(sessionID, {
-      ...(model.temperature !== undefined ? { temperature: model.temperature } : {}),
-      ...(model.top_p !== undefined ? { topP: model.top_p } : {}),
-      ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
-    })
-  }
-}
 import { SessionCategoryRegistry } from "../../shared/session-category-registry"
 import { ConcurrencyManager } from "./concurrency"
 import type { BackgroundTaskConfig, TmuxConfig } from "../../config/schema"
diff --git a/src/features/background-agent/spawner.ts b/src/features/background-agent/spawner.ts
index 13139c29d..e8fc49e32 100644
--- a/src/features/background-agent/spawner.ts
+++ b/src/features/background-agent/spawner.ts
@@ -2,7 +2,7 @@ import type { BackgroundTask, LaunchInput, ResumeInput } from "./types"
 import type { OpencodeClient, OnSubagentSessionCreated, QueueItem } from "./constants"
 import { TMUX_CALLBACK_DELAY_MS } from "./constants"
 import { log, getAgentToolRestrictions, promptWithModelSuggestionRetry, createInternalAgentTextPart } from "../../shared"
-import { setSessionPromptParams } from "../../shared/session-prompt-params-state"
+import { applySessionPromptParams } from "../../shared/session-prompt-params-helpers"
 import { subagentSessions } from "../claude-code-session-state"
 import { getTaskToastManager } from "../task-toast-manager"
 import { isInsideTmux } from "../../shared/tmux"
@@ -136,25 +136,7 @@ export async function startTask(
     : undefined
   const launchVariant = input.model?.variant
 
-  if (input.model) {
-    const promptOptions: Record<string, unknown> = {
-      ...(input.model.reasoningEffort ? { reasoningEffort: input.model.reasoningEffort } : {}),
-      ...(input.model.thinking ? { thinking: input.model.thinking } : {}),
-      ...(input.model.maxTokens !== undefined ? { maxTokens: input.model.maxTokens } : {}),
-    }
-
-    if (
-      input.model.temperature !== undefined ||
-      input.model.top_p !== undefined ||
-      Object.keys(promptOptions).length > 0
-    ) {
-      setSessionPromptParams(sessionID, {
-        ...(input.model.temperature !== undefined ? { temperature: input.model.temperature } : {}),
-        ...(input.model.top_p !== undefined ? { topP: input.model.top_p } : {}),
-        ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
-      })
-    }
-  }
+  applySessionPromptParams(sessionID, input.model)
 
   promptWithModelSuggestionRetry(client, {
     path: { id: sessionID },
@@ -244,25 +226,7 @@ export async function resumeTask(
     : undefined
   const resumeVariant = task.model?.variant
 
-  if (task.model) {
-    const promptOptions: Record<string, unknown> = {
-      ...(task.model.reasoningEffort ? { reasoningEffort: task.model.reasoningEffort } : {}),
-      ...(task.model.thinking ? { thinking: task.model.thinking } : {}),
-      ...(task.model.maxTokens !== undefined ? { maxTokens: task.model.maxTokens } : {}),
-    }
-
-    if (
-      task.model.temperature !== undefined ||
-      task.model.top_p !== undefined ||
-      Object.keys(promptOptions).length > 0
-    ) {
-      setSessionPromptParams(task.sessionID, {
-        ...(task.model.temperature !== undefined ? { temperature: task.model.temperature } : {}),
-        ...(task.model.top_p !== undefined ? { topP: task.model.top_p } : {}),
-        ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
-      })
-    }
-  }
+  applySessionPromptParams(task.sessionID, task.model)
 
   client.session.promptAsync({
     path: { id: task.sessionID },
diff --git a/src/plugin/chat-params.ts b/src/plugin/chat-params.ts
index b048c04c8..d265b57d3 100644
--- a/src/plugin/chat-params.ts
+++ b/src/plugin/chat-params.ts
@@ -156,7 +156,9 @@ export function createChatParamsHandler(args: {
       providerID: normalizedInput.model.providerID,
       modelID: normalizedInput.model.modelID,
       desired: {
-        variant: normalizedInput.message.variant,
+        variant: typeof normalizedInput.message.variant === "string"
+          ? normalizedInput.message.variant
+          : undefined,
         reasoningEffort: typeof output.options.reasoningEffort === "string"
           ? output.options.reasoningEffort
           : undefined,
diff --git a/src/shared/model-settings-compatibility.test.ts b/src/shared/model-settings-compatibility.test.ts
index 84026406d..55fc2a872 100644
--- a/src/shared/model-settings-compatibility.test.ts
+++ b/src/shared/model-settings-compatibility.test.ts
@@ -210,19 +210,19 @@ describe("resolveCompatibleModelSettings", () => {
     })
   })
 
-  test("downgrades unsupported GPT reasoningEffort to nearest lower level", () => {
+  test("drops reasoningEffort for standard GPT models (gpt-4.1)", () => {
     const result = resolveCompatibleModelSettings({
       providerID: "openai",
       modelID: "gpt-4.1",
-      desired: { reasoningEffort: "xhigh" },
+      desired: { reasoningEffort: "high" },
     })
 
-    expect(result.reasoningEffort).toBe("high")
+    expect(result.reasoningEffort).toBeUndefined()
     expect(result.changes).toEqual([
       {
         field: "reasoningEffort",
-        from: "xhigh",
-        to: "high",
+        from: "high",
+        to: undefined,
         reason: "unsupported-by-model-family",
       },
     ])
@@ -459,6 +459,57 @@ describe("resolveCompatibleModelSettings", () => {
     })
   })
 
+  // Reasoning effort downgrade within families that support it
+  test("o-series downgrades xhigh reasoningEffort to high", () => {
+    const result = resolveCompatibleModelSettings({
+      providerID: "openai",
+      modelID: "o3-mini",
+      desired: { reasoningEffort: "xhigh" },
+    })
+
+    expect(result.reasoningEffort).toBe("high")
+    expect(result.changes).toEqual([
+      {
+        field: "reasoningEffort",
+        from: "xhigh",
+        to: "high",
+        reason: "unsupported-by-model-family",
+      },
+    ])
+  })
+
+  test("GPT-5 keeps xhigh but would downgrade a hypothetical beyond-max level", () => {
+    // GPT-5 supports up to "xhigh" — verify the ladder works by requesting
+    // a value that IS in the ladder but NOT in the family's allowed list.
+    // Since "xhigh" is the max for GPT-5 reasoningEffort, we verify it stays.
+    const result = resolveCompatibleModelSettings({
+      providerID: "openai",
+      modelID: "gpt-5.4",
+      desired: { reasoningEffort: "xhigh" },
+    })
+
+    expect(result.reasoningEffort).toBe("xhigh")
+    expect(result.changes).toEqual([])
+  })
+
+  test("o-series downgrades unsupported variant to high", () => {
+    const result = resolveCompatibleModelSettings({
+      providerID: "openai",
+      modelID: "o3-mini",
+      desired: { variant: "max" },
+    })
+
+    expect(result.variant).toBe("high")
+    expect(result.changes).toEqual([
+      {
+        field: "variant",
+        from: "max",
+        to: "high",
+        reason: "unsupported-by-model-family",
+      },
+    ])
+  })
+
   // Passthrough: undefined desired values produce no changes
   test("no-op when desired settings are empty", () => {
     const result = resolveCompatibleModelSettings({
diff --git a/src/tools/delegate-task/sync-prompt-sender.ts b/src/tools/delegate-task/sync-prompt-sender.ts
index 65773c92a..502bb6dec 100644
--- a/src/tools/delegate-task/sync-prompt-sender.ts
+++ b/src/tools/delegate-task/sync-prompt-sender.ts
@@ -7,8 +7,8 @@ import {
 } from "../../shared/model-suggestion-retry"
 import { formatDetailedError } from "./error-formatting"
 import { getAgentToolRestrictions } from "../../shared/agent-tool-restrictions"
+import { applySessionPromptParams } from "../../shared/session-prompt-params-helpers"
 import { setSessionTools } from "../../shared/session-tools-store"
-import { setSessionPromptParams } from "../../shared/session-prompt-params-state"
 import { createInternalAgentTextPart } from "../../shared/internal-initiator-marker"
 
 type SendSyncPromptDeps = {
@@ -54,25 +54,7 @@ export async function sendSyncPrompt(
   }
   setSessionTools(input.sessionID, tools)
 
-  if (input.categoryModel) {
-    const promptOptions: Record<string, unknown> = {
-      ...(input.categoryModel.reasoningEffort ? { reasoningEffort: input.categoryModel.reasoningEffort } : {}),
-      ...(input.categoryModel.thinking ? { thinking: input.categoryModel.thinking } : {}),
-      ...(input.categoryModel.maxTokens !== undefined ? { maxTokens: input.categoryModel.maxTokens } : {}),
-    }
-
-    if (
-      input.categoryModel.temperature !== undefined ||
-      input.categoryModel.top_p !== undefined ||
-      Object.keys(promptOptions).length > 0
-    ) {
-      setSessionPromptParams(input.sessionID, {
-        ...(input.categoryModel.temperature !== undefined ? { temperature: input.categoryModel.temperature } : {}),
-        ...(input.categoryModel.top_p !== undefined ? { topP: input.categoryModel.top_p } : {}),
-        ...(Object.keys(promptOptions).length > 0 ? { options: promptOptions } : {}),
-      })
-    }
-  }
+  applySessionPromptParams(input.sessionID, input.categoryModel)
 
   const promptArgs = {
     path: { id: input.sessionID },