fix: guard Anthropic thinking budgetTokens for unknown output limits

Custom providers without an explicit output limit get limit.output = 0
(sentinel value). The budgetTokens calculation Math.floor(output / 2 - 1)
becomes negative, causing "maxOutputTokens must be >= 1" from the API.

Extract highBudget() to centralize the formula and its validation
(result >= 1024, Anthropic's minimum). Both variants() and the kimi-k2.5
path share this helper, so the guard lives in one place.

Fixes #22253

Author: Jongho Kim

packages/opencode/src/provider/transform.ts

@@ -365,6 +365,14 @@ export namespace ProviderTransform {
   const WIDELY_SUPPORTED_EFFORTS = ["low", "medium", "high"]
   const OPENAI_EFFORTS = ["none", "minimal", ...WIDELY_SUPPORTED_EFFORTS, "xhigh"]
 
+  // Returns safe budgetTokens for Anthropic's 'high' thinking variant, or undefined
+  // when the output limit is unknown (0) or the result would be below Anthropic's
+  // minimum of 1024. Both callers (variants() and kimi-k2.5) share this formula.
+  function highBudget(output: number): number | undefined {
+    const tokens = Math.floor(output / 2 - 1)
+    return tokens >= 1024 ? Math.min(16_000, tokens) : undefined
+  }
+
   export function variants(model: Provider.Model): Record<string, Record<string, any>> {
     if (!model.capabilities.reasoning) return {}
 
@@ -570,11 +578,14 @@ export namespace ProviderTransform {
           )
         }
 
+        const high = highBudget(model.limit.output)
+        if (!high) return {}
+
         return {
           high: {
             thinking: {
               type: "enabled",
-              budgetTokens: Math.min(16_000, Math.floor(model.limit.output / 2 - 1)),
+              budgetTokens: high,
             },
           },
           max: {
@@ -809,9 +820,12 @@ export namespace ProviderTransform {
       (input.model.api.npm === "@ai-sdk/anthropic" || input.model.api.npm === "@ai-sdk/google-vertex/anthropic") &&
       (modelId.includes("k2p5") || modelId.includes("kimi-k2.5") || modelId.includes("kimi-k2p5"))
     ) {
-      result["thinking"] = {
-        type: "enabled",
-        budgetTokens: Math.min(16_000, Math.floor(input.model.limit.output / 2 - 1)),
+      const high = highBudget(input.model.limit.output)
+      if (high) {
+        result["thinking"] = {
+          type: "enabled",
+          budgetTokens: high,
+        }
       }
     }
 

packages/opencode/test/provider/transform.test.ts

@@ -2577,6 +2577,36 @@ describe("ProviderTransform.variants", () => {
         },
       })
     })
+
+    test("returns empty object when limit.output is 0 (unknown)", () => {
+      const model = createMockModel({
+        id: "anthropic/claude-4",
+        providerID: "anthropic",
+        api: {
+          id: "claude-4",
+          url: "https://api.anthropic.com",
+          npm: "@ai-sdk/anthropic",
+        },
+        limit: { context: 200_000, output: 0 },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(result).toEqual({})
+    })
+
+    test("returns empty object when limit.output is too small for valid budgetTokens", () => {
+      const model = createMockModel({
+        id: "anthropic/claude-4",
+        providerID: "anthropic",
+        api: {
+          id: "claude-4",
+          url: "https://api.anthropic.com",
+          npm: "@ai-sdk/anthropic",
+        },
+        limit: { context: 200_000, output: 2049 },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(result).toEqual({})
+    })
   })
 
   describe("@ai-sdk/amazon-bedrock", () => {