diff --git a/index.js b/index.js
index 1561d4d..5b3d5e4 100644
--- a/index.js
+++ b/index.js
@@ -817,7 +817,12 @@ function createResponsesApiResponse(result, model) {
 export function mapFinishReason(finish) {
   if (!finish) return "stop"
   if (finish.includes("length")) return "length"
-  if (finish.includes("tool")) return "tool_calls"
+  // Opencode ends json_schema / StructuredOutput turns with a finish string that
+  // contains "tool". Callers already set finish_reason to "tool_calls" when
+  // proxy-level toolCalls.length > 0. Mapping the internal structured finish to
+  // tool_calls here leaves message.content JSON with finish_reason tool_calls
+  // and an empty tool_calls array — Vercel AI SDK Output.object then throws
+  // AI_NoOutputGeneratedError. Keep "stop" for content-only completions.
   return "stop"
 }
 
@@ -1627,7 +1632,9 @@ export function normalizeAnthropicSystem(system) {
 export function mapFinishReasonToAnthropic(finish) {
   if (!finish) return "end_turn"
   if (finish.includes("length")) return "max_tokens"
-  if (finish.includes("tool")) return "tool_use"
+  // Same as mapFinishReason: do not map Opencode StructuredOutput finish to
+  // tool_use when the proxy already flattened schema JSON into text content.
+  // Real tool_use is still set by createAnthropicResponse when toolCalls.length > 0.
   return "end_turn"
 }
 
diff --git a/index.test.js b/index.test.js
index c27c36e..ac4b1b8 100644
--- a/index.test.js
+++ b/index.test.js
@@ -845,9 +845,13 @@ describe("mapFinishReason", () => {
     assert.equal(mapFinishReason("length"), "length")
   })
 
-  it("returns 'tool_calls' when finish includes 'tool'", () => {
-    assert.equal(mapFinishReason("tool_use"), "tool_calls")
-    assert.equal(mapFinishReason("tool"), "tool_calls")
+  it("returns 'stop' when finish includes 'tool' (StructuredOutput); real tools use toolCalls.length", () => {
+    // Opencode json_schema ends on the internal StructuredOutput tool. Mapping
+    // that finish to tool_calls breaks AI SDK Output.object. Callers force
+    // tool_calls when proxy-level toolCalls.length > 0.
+    assert.equal(mapFinishReason("tool_use"), "stop")
+    assert.equal(mapFinishReason("tool"), "stop")
+    assert.equal(mapFinishReason("tool-calls"), "stop")
   })
 
   it("returns 'stop' for unrecognised values", () => {
@@ -1580,8 +1584,9 @@ describe("mapFinishReasonToAnthropic", () => {
     assert.equal(mapFinishReasonToAnthropic("max_length"), "max_tokens")
   })
 
-  it("returns tool_use when finish includes tool", () => {
-    assert.equal(mapFinishReasonToAnthropic("tool_use"), "tool_use")
+  it("returns end_turn when finish includes tool (StructuredOutput); real tools use toolCalls.length", () => {
+    assert.equal(mapFinishReasonToAnthropic("tool_use"), "end_turn")
+    assert.equal(mapFinishReasonToAnthropic("tool"), "end_turn")
   })
 
   it("returns end_turn for unrecognised values", () => {