fix(chat-params): cap invalid token fallback by model output limit

Keep the conservative non-positive maxOutputTokens fallback while preventing 4096 from exceeding tiny model limits by using min(4096, capabilities max).

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/chat-params.test.ts

@@ -323,4 +323,48 @@ describe("createChatParamsHandler", () => {
     //#then
     expect(output.maxOutputTokens).toBe(4096)
   })
+
+  test("uses model max when non-positive stored maxOutputTokens fallback exceeds model limit", async () => {
+    //#given
+    sharedModule.writeProviderModelsCache({
+      connected: ["custom-provider"],
+      models: {
+        "custom-provider": [
+          {
+            id: "tiny-model",
+            name: "tiny-model",
+            limit: { output: 512 },
+          },
+        ],
+      },
+    })
+    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: "tiny-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(512)
+  })
 })

src/plugin/chat-params.ts

@@ -3,6 +3,14 @@ import { getModelCapabilities, log, resolveCompatibleModelSettings } from "../sh
 
 const SAFE_MAX_OUTPUT_TOKENS_FALLBACK = 4096
 
+function resolveSafeMaxOutputTokensFallback(capabilitiesMaxOutputTokens: number | undefined): number {
+  if (typeof capabilitiesMaxOutputTokens !== "number" || capabilitiesMaxOutputTokens <= 0) {
+    return SAFE_MAX_OUTPUT_TOKENS_FALLBACK
+  }
+
+  return Math.min(SAFE_MAX_OUTPUT_TOKENS_FALLBACK, capabilitiesMaxOutputTokens)
+}
+
 export type ChatParamsInput = {
   sessionID: string
   agent: { name?: string }
@@ -173,10 +181,10 @@ export function createChatParamsHandler(args: {
         const originalMaxOutputTokens = typeof output.maxOutputTokens === "number"
           ? output.maxOutputTokens
           : compatibility.maxTokens
-        output.maxOutputTokens = SAFE_MAX_OUTPUT_TOKENS_FALLBACK
+        output.maxOutputTokens = resolveSafeMaxOutputTokensFallback(capabilities?.maxOutputTokens)
         if (typeof originalMaxOutputTokens === "number" && originalMaxOutputTokens <= 0) {
           log(
-            `[plugin] maxOutputTokens=${originalMaxOutputTokens} is non-positive; using safe fallback ${SAFE_MAX_OUTPUT_TOKENS_FALLBACK}`,
+            `[plugin] maxOutputTokens=${originalMaxOutputTokens} is non-positive; using safe fallback ${output.maxOutputTokens}`,
           )
         }
       }