diff --git a/.changeset/fix-output-object-json-mode.md b/.changeset/fix-output-object-json-mode.md
new file mode 100644
index 000000000000..4950dc332875
--- /dev/null
+++ b/.changeset/fix-output-object-json-mode.md
@@ -0,0 +1,5 @@
+---
+'@ai-sdk/openai-compatible': patch
+---
+
+fix(openai-compatible): inject JSON schema instruction when structuredOutputs is disabled. Improves reliability of generateText + Output.object() with OpenAI-compatible providers that don't support json_schema response format.
diff --git a/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.test.ts b/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.test.ts
index ee71b886e191..460327f6ed47 100644
--- a/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.test.ts
+++ b/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.test.ts
@@ -835,7 +835,7 @@ describe('doGenerate', () => {
       `);
     });
 
-    it('should forward json response format as "json_object" and omit schema when structuredOutputs are disabled', async () => {
+    it('should forward json response format as "json_object" and inject schema instruction when structuredOutputs are disabled', async () => {
       prepareJsonResponse({ content: '{"value":"Spark"}' });
 
       const model = new OpenAICompatibleChatLanguageModel('gpt-4o-2024-08-06', {
@@ -862,6 +862,12 @@ describe('doGenerate', () => {
       expect(await server.calls[0].requestBodyJson).toMatchInlineSnapshot(`
         {
           "messages": [
+            {
+              "content": "JSON schema:
+        {"type":"object","properties":{"value":{"type":"string"}},"required":["value"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}
+        You MUST answer with a JSON object that matches the JSON schema above.",
+              "role": "system",
+            },
             {
               "content": "Hello",
               "role": "user",
diff --git a/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.ts b/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.ts
index 9a803cb1173e..3cb7bfeaf35d 100644
--- a/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.ts
+++ b/packages/openai-compatible/src/chat/openai-compatible-chat-language-model.ts
@@ -18,6 +18,7 @@ import {
   createJsonResponseHandler,
   FetchFunction,
   generateId,
+  injectJsonInstructionIntoMessages,
   isParsableJson,
   parseProviderOptions,
   ParseResult,
@@ -162,11 +163,15 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
       warnings.push({ type: 'unsupported', feature: 'topK' });
     }
 
-    if (
+    // When structured outputs are not supported but a schema is provided,
+    // inject the JSON schema as a system instruction so the model knows
+    // what structure to produce (since the schema is dropped from response_format).
+    const shouldInjectSchemaInstruction =
       responseFormat?.type === 'json' &&
       responseFormat.schema != null &&
-      !this.supportsStructuredOutputs
-    ) {
+      !this.supportsStructuredOutputs;
+
+    if (shouldInjectSchemaInstruction) {
       warnings.push({
         type: 'unsupported',
         feature: 'responseFormat',
@@ -175,6 +180,13 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
       });
     }
 
+    const messagesPrompt = shouldInjectSchemaInstruction
+      ? injectJsonInstructionIntoMessages({
+          messages: prompt,
+          schema: responseFormat.schema,
+        })
+      : prompt;
+
     const {
       tools: openaiTools,
       toolChoice: openaiToolChoice,
@@ -231,7 +243,7 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
         verbosity: compatibleOptions.textVerbosity,
 
         // messages:
-        messages: convertToOpenAICompatibleChatMessages(prompt),
+        messages: convertToOpenAICompatibleChatMessages(messagesPrompt),
 
         // tools:
         tools: openaiTools,