diff --git a/.changeset/fix-deduplicate-openai-output.md b/.changeset/fix-deduplicate-openai-output.md
new file mode 100644
index 00000000000..9d694a62b3b
--- /dev/null
+++ b/.changeset/fix-deduplicate-openai-output.md
@@ -0,0 +1,5 @@
+---
+"@effect/ai-openai": patch
+---
+
+fix(ai-openai): deduplicate response.output items to prevent invalid JSON concatenation
diff --git a/packages/ai/openai/src/OpenAiLanguageModel.ts b/packages/ai/openai/src/OpenAiLanguageModel.ts
index c4fef761467..e9b56820874 100644
--- a/packages/ai/openai/src/OpenAiLanguageModel.ts
+++ b/packages/ai/openai/src/OpenAiLanguageModel.ts
@@ -567,7 +567,16 @@ const makeResponse: (
       timestamp: DateTime.formatIso(DateTime.unsafeFromDate(createdAt))
     })
 
+    // Deduplicate output items by ID to handle OpenAI Responses API bug
+    // where duplicate OutputMessage items appear in response.output
+    const seenOutputIds = new Set<string>()
     for (const part of response.output) {
+      if (part.id && seenOutputIds.has(part.id)) {
+        continue
+      }
+      if (part.id) {
+        seenOutputIds.add(part.id)
+      }
       switch (part.type) {
         case "message": {
           for (const contentPart of part.content) {