From 279f0d150f234b4102f07c9e75a17acb97d60dcc Mon Sep 17 00:00:00 2001
From: YeonGyu-Kim <code.yeon.gyu@gmail.com>
Date: Sun, 10 May 2026 15:03:01 +0900
Subject: [PATCH] fix(chat-params): guard non-positive max output tokens

---
 src/plugin/chat-params.test.ts                | 32 +++++++++++++++++++
 src/plugin/chat-params.ts                     | 11 +++++--
 .../model-settings-compatibility.test.ts      | 12 +++++++
 src/shared/model-settings-compatibility.ts    |  4 +++
 4 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/src/plugin/chat-params.test.ts b/src/plugin/chat-params.test.ts
index 5886b7204..de7acb5df 100644
--- a/src/plugin/chat-params.test.ts
+++ b/src/plugin/chat-params.test.ts
@@ -253,4 +253,36 @@ describe("createChatParamsHandler", () => {
       options: {},
     })
   })
+
+  test("falls back to default maxOutputTokens when stored and compatibility tokens are non-positive", async () => {
+    //#given
+    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)
+  })
 })
diff --git a/src/plugin/chat-params.ts b/src/plugin/chat-params.ts
index 41e4a0200..ac665b62b 100644
--- a/src/plugin/chat-params.ts
+++ b/src/plugin/chat-params.ts
@@ -96,7 +96,10 @@ 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 +165,12 @@ 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 capabilitiesLimit = capabilities?.maxOutputTokens
+        output.maxOutputTokens =
+          typeof capabilitiesLimit === "number" && capabilitiesLimit > 0 ? capabilitiesLimit : 4096
       }
     }
 
diff --git a/src/shared/model-settings-compatibility.test.ts b/src/shared/model-settings-compatibility.test.ts
index 725b2a54c..a653f7259 100644
--- a/src/shared/model-settings-compatibility.test.ts
+++ b/src/shared/model-settings-compatibility.test.ts
@@ -553,6 +553,18 @@ 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 c2354038e..4e9145192 100644
--- a/src/shared/model-settings-compatibility.ts
+++ b/src/shared/model-settings-compatibility.ts
@@ -162,6 +162,10 @@ export function resolveCompatibleModelSettings(
   }
 
   let maxTokens = input.desired.maxTokens
+  if (maxTokens !== undefined && maxTokens <= 0) {
+    maxTokens = undefined
+  }
+
   if (
     maxTokens !== undefined &&
     input.capabilities?.maxOutputTokens !== undefined &&