diff --git a/src/plugin/chat-params.test.ts b/src/plugin/chat-params.test.ts
index 5886b7204..736e36d21 100644
--- a/src/plugin/chat-params.test.ts
+++ b/src/plugin/chat-params.test.ts
@@ -5,7 +5,7 @@ import { join } from "node:path"
 
 import { createChatParamsHandler, type ChatParamsOutput } from "./chat-params"
 import * as dataPathModule from "../shared/data-path"
-import { writeProviderModelsCache } from "../shared"
+import * as sharedModule from "../shared"
 import {
   clearSessionPromptParams,
   getSessionPromptParams,
@@ -21,13 +21,13 @@ describe("createChatParamsHandler", () => {
     getCacheDirSpy = spyOn(dataPathModule, "getOmoOpenCodeCacheDir").mockReturnValue(
       join(tempCacheRoot, "oh-my-opencode"),
     )
-    writeProviderModelsCache({ connected: [], models: {} })
+    sharedModule.writeProviderModelsCache({ connected: [], models: {} })
   })
 
   afterEach(() => {
     clearSessionPromptParams("ses_chat_params")
     clearSessionPromptParams("ses_chat_params_temperature")
-    writeProviderModelsCache({ connected: [], models: {} })
+    sharedModule.writeProviderModelsCache({ connected: [], models: {} })
     getCacheDirSpy?.mockRestore()
     if (tempCacheRoot) {
       rmSync(tempCacheRoot, { recursive: true, force: true })
@@ -101,7 +101,7 @@ describe("createChatParamsHandler", () => {
 
   test("applies stored prompt params for the session", async () => {
     //#given
-    writeProviderModelsCache({
+    sharedModule.writeProviderModelsCache({
       connected: ["openai"],
       models: {
         openai: [
@@ -253,4 +253,74 @@ describe("createChatParamsHandler", () => {
       options: {},
     })
   })
+
+  test("falls back to default maxOutputTokens when stored and compatibility tokens are non-positive", async () => {
+    //#given
+    const logSpy = spyOn(sharedModule, "log").mockImplementation(() => undefined)
+    setSessionPromptParams("ses_chat_params", {
+      maxOutputTokens: 0,
+    })
+
+    const handler = createChatParamsHandler({
+      anthropicEffort: null,
+    })
+
+    const input = {
+      sessionID: "ses_chat_params",
+      agent: { name: "oracle" },
+      model: { providerID: "custom-provider", modelID: "custom-model" },
+      provider: { id: "custom-provider" },
+      message: {},
+    }
+
+    const output: ChatParamsOutput = {
+      topP: 1,
+      topK: 1,
+      maxOutputTokens: 0,
+      options: {},
+    }
+
+    //#when
+    await handler(input, output)
+
+    //#then
+    expect(output.maxOutputTokens).toBe(4096)
+    expect(logSpy).toHaveBeenCalledWith(
+      "[plugin] maxOutputTokens=0 is non-positive; using safe fallback 4096",
+    )
+
+    logSpy.mockRestore()
+  })
+
+  test("uses safe fallback instead of model max when stored maxOutputTokens is non-positive", async () => {
+    //#given
+    setSessionPromptParams("ses_chat_params", {
+      maxOutputTokens: -1,
+    })
+
+    const handler = createChatParamsHandler({
+      anthropicEffort: null,
+    })
+
+    const input = {
+      sessionID: "ses_chat_params",
+      agent: { name: "oracle" },
+      model: { providerID: "openai", modelID: "gpt-5.4" },
+      provider: { id: "openai" },
+      message: {},
+    }
+
+    const output: ChatParamsOutput = {
+      topP: 1,
+      topK: 1,
+      maxOutputTokens: -1,
+      options: {},
+    }
+
+    //#when
+    await handler(input, output)
+
+    //#then
+    expect(output.maxOutputTokens).toBe(4096)
+  })
 })
diff --git a/src/plugin/chat-params.ts b/src/plugin/chat-params.ts
index 41e4a0200..26f35d03d 100644
--- a/src/plugin/chat-params.ts
+++ b/src/plugin/chat-params.ts
@@ -1,5 +1,7 @@
 import { getSessionPromptParams } from "../shared/session-prompt-params-state"
-import { getModelCapabilities, resolveCompatibleModelSettings } from "../shared"
+import { getModelCapabilities, log, resolveCompatibleModelSettings } from "../shared"
+
+const SAFE_MAX_OUTPUT_TOKENS_FALLBACK = 4096
 
 export type ChatParamsInput = {
   sessionID: string
@@ -96,7 +98,10 @@ export function createChatParamsHandler(args: {
       if (storedPromptParams.topP !== undefined) {
         output.topP = storedPromptParams.topP
       }
-      if (storedPromptParams.maxOutputTokens !== undefined) {
+      if (
+        typeof storedPromptParams.maxOutputTokens === "number" &&
+        storedPromptParams.maxOutputTokens > 0
+      ) {
         (output as Record<string, unknown>).maxOutputTokens = storedPromptParams.maxOutputTokens
       }
       if (storedPromptParams.options) {
@@ -162,10 +167,18 @@ export function createChatParamsHandler(args: {
     }
 
     if ("maxTokens" in compatibility) {
-      if (compatibility.maxTokens !== undefined) {
+      if (compatibility.maxTokens !== undefined && compatibility.maxTokens > 0) {
         output.maxOutputTokens = compatibility.maxTokens
       } else {
-        delete output.maxOutputTokens
+        const originalMaxOutputTokens = typeof output.maxOutputTokens === "number"
+          ? output.maxOutputTokens
+          : compatibility.maxTokens
+        output.maxOutputTokens = SAFE_MAX_OUTPUT_TOKENS_FALLBACK
+        if (typeof originalMaxOutputTokens === "number" && originalMaxOutputTokens <= 0) {
+          log(
+            `[plugin] maxOutputTokens=${originalMaxOutputTokens} is non-positive; using safe fallback ${SAFE_MAX_OUTPUT_TOKENS_FALLBACK}`,
+          )
+        }
       }
     }
 
diff --git a/src/shared/model-settings-compatibility.test.ts b/src/shared/model-settings-compatibility.test.ts
index 9d92b2c7d..fd3568755 100644
--- a/src/shared/model-settings-compatibility.test.ts
+++ b/src/shared/model-settings-compatibility.test.ts
@@ -615,6 +615,18 @@ describe("resolveCompatibleModelSettings", () => {
     expect(result.changes).toEqual([])
   })
 
+  test("#given desired.maxTokens is 0 #then maxTokens is dropped", () => {
+    const result = resolveCompatibleModelSettings({
+      providerID: "openai",
+      modelID: "gpt-5.4",
+      desired: { maxTokens: 0 },
+      capabilities: { maxOutputTokens: 128_000 },
+    })
+
+    expect(result.maxTokens).toBeUndefined()
+    expect(result.changes).toEqual([])
+  })
+
   // Passthrough: undefined desired values produce no changes
   test("no-op when desired settings are empty", () => {
     const result = resolveCompatibleModelSettings({
diff --git a/src/shared/model-settings-compatibility.ts b/src/shared/model-settings-compatibility.ts
index 414638fef..c8997d669 100644
--- a/src/shared/model-settings-compatibility.ts
+++ b/src/shared/model-settings-compatibility.ts
@@ -175,6 +175,10 @@ export function resolveCompatibleModelSettings(
   }
 
   let maxTokens = input.desired.maxTokens
+  if (maxTokens !== undefined && maxTokens <= 0) {
+    maxTokens = undefined
+  }
+
   if (
     maxTokens !== undefined &&
     input.capabilities?.maxOutputTokens !== undefined &&