feat(lmstudio): add generateObject override for OpenAI-compatible APIs

[teknomage8]

Summary

This PR adds proper generateObject support for LM Studio and other OpenAI-compatible APIs (vLLM, LocalAI, etc.) that don't support OpenAI's structured output feature (zodResponseFormat / chat.completions.parse).

Problem

The current LMStudioLLM class extends OpenAILLM without any overrides. When generateObject is called, it uses the parent's implementation which relies on OpenAI's proprietary structured output API:

// This doesn't work with LM Studio or other OpenAI-compatible APIs
const response = await this.openAIClient.chat.completions.parse({
  ...
  response_format: zodResponseFormat(input.schema, 'response'),
});

This results in errors like:

• chat.completions.parse is not a function
• response_format.json_schema is not supported

Solution

Override generateObject in LMStudioLLM to use the standard chat completion endpoint with JSON mode:

1. Primary approach: Use response_format: { type: 'json_object' } which is widely supported
2. Fallback: For models that don't support JSON mode, use prompt engineering to request JSON output
3. Robust parsing: Integrate with the existing @toolsycc/json-repair library to handle malformed JSON

Key Features

• ✅ Works with LM Studio, vLLM, LocalAI, and other OpenAI-compatible APIs
• ✅ Uses your existing repairJson library for robust parsing
• ✅ Automatic fallback for models without JSON mode support
• ✅ Full Zod schema validation on parsed responses
• ✅ Clear error messages for debugging

Testing

Tested with:

• LM Studio (various models including DeepSeek, Qwen)
• Suggestions feature (which uses generateObject)
• Search functionality

Credits

Contributed by The Noble House™ AI Lab (https://thenoblehouse.ai)

---

Summary by cubic

Adds generateObject support for LM Studio and other OpenAI-compatible APIs by overriding LMStudioLLM to use JSON mode and manual parsing. Fixes structured output errors and re-enables features like suggestions and search on LM Studio, vLLM, and LocalAI.

• New Features
  • Override LMStudioLLM.generateObject to use chat.completions with response_format: json_object and include the JSON schema in the prompt.
  • Fallback for servers without JSON mode using a strict JSON-only system prompt.
  • Parse with @toolsycc/json-repair and validate with Zod; clearer errors for bad JSON.

^{Written for commit 7adac48. Summary will update on new commits.}

[cubic-dev-ai]

2 issues found across 1 file

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="src/lib/models/providers/lmstudio/lmstudioLLM.ts">

<violation number="1" location="src/lib/models/providers/lmstudio/lmstudioLLM.ts:33">
P2: Accessing `input.messages[input.messages.length - 1]` will crash if `messages` array is empty. Consider adding a guard clause at the start of the method.</violation>

<violation number="2" location="src/lib/models/providers/lmstudio/lmstudioLLM.ts:75">
P2: Fallback condition `err.status === 400` is too broad. A 400 error can occur for many reasons (invalid model, malformed request, etc.), not just unsupported JSON mode. Consider checking for more specific error messages or codes to avoid masking unrelated errors.</violation>
</file>

---

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_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: Fallback condition err.status === 400 is too broad. A 400 error can occur for many reasons (invalid model, malformed request, etc.), not just unsupported JSON mode. Consider checking for more specific error messages or codes to avoid masking unrelated errors.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/lib/models/providers/lmstudio/lmstudioLLM.ts, line 75:

<comment>Fallback condition `err.status === 400` is too broad. A 400 error can occur for many reasons (invalid model, malformed request, etc.), not just unsupported JSON mode. Consider checking for more specific error messages or codes to avoid masking unrelated errors.</comment>

<file context>
@@ -1,5 +1,139 @@
+      throw new Error(&#39;No response from LM Studio&#39;);
+    } catch (err: any) {
+      // If JSON mode isn&#39;t supported, try without it
+      if (err.message?.includes(&#39;response_format&#39;) || err.status === 400) {
+        return this.generateObjectWithoutJsonMode&lt;T&gt;(input, jsonSchema);
+      }
</file context>

[cubic-dev-ai]

P2: Accessing input.messages[input.messages.length - 1] will crash if messages array is empty. Consider adding a guard clause at the start of the method.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/lib/models/providers/lmstudio/lmstudioLLM.ts, line 33:

<comment>Accessing `input.messages[input.messages.length - 1]` will crash if `messages` array is empty. Consider adding a guard clause at the start of the method.</comment>

<file context>
@@ -1,5 +1,139 @@
+    const messagesWithJsonInstruction = [
+      ...input.messages.slice(0, -1), // All messages except the last
+      {
+        ...input.messages[input.messages.length - 1],
+        content: `${input.messages[input.messages.length - 1].content}\n\nRespond with a valid JSON object matching this schema:\n${JSON.stringify(jsonSchema, null, 2)}`,
+      },
</file context>

[teknomage8]

@cubic-dev-ai Thank you for the thorough review! Both issues have been addressed in commit 15d51c5:

Issue 1: Empty Array Vulnerability (Line 33)

Fixed with: New validateAndGetLastMessage() private method that validates the messages array before access:

private validateAndGetLastMessage(
  messages: GenerateObjectInput['messages'],
): GenerateObjectInput['messages'][number] {
  if (!messages || messages.length === 0) {
    throw new Error(
      'LMStudioLLM: Cannot generate object with empty messages array. ' +
        'At least one message is required.',
    );
  }
  return messages[messages.length - 1];
}

This is now called at the start of both generateObject() and generateObjectWithoutJsonMode().

Issue 2: Overly Broad Error Handling (Line 75)

Fixed with: New isJsonModeUnsupportedError() private method that uses pattern matching instead of status codes:

private static readonly JSON_MODE_UNSUPPORTED_PATTERNS = [
  'response_format',
  'json_object', 
  'json_mode',
  'structured output',
  'not supported',
  'invalid.*format',
] as const;

private isJsonModeUnsupportedError(err: unknown): boolean {
  // Pattern matching against error messages, not status codes
  // Only falls back when error specifically indicates JSON mode unsupported
}

This ensures we only trigger the fallback for actual JSON mode incompatibility, not unrelated 400 errors.

Additional Improvements

• Extracted parseJsonResponse() helper to eliminate code duplication
• Added comprehensive JSDoc documentation
• Improved error messages with raw content preview for debugging
• Stronger TypeScript typing throughout

Please re-review when ready. 🙏

[cubic-dev-ai]

@cubic-dev-ai Thank you for the thorough review! Both issues have been addressed in commit 15d51c5:

Issue 1: Empty Array Vulnerability (Line 33)

Fixed with: New validateAndGetLastMessage() private method that validates the messages array before access:
...

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[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="src/lib/models/providers/lmstudio/lmstudioLLM.ts">

<violation number="1" location="src/lib/models/providers/lmstudio/lmstudioLLM.ts:33">
P1: Accessing `input.messages[input.messages.length - 1]` without checking if `messages` is empty will cause a TypeError if an empty array is passed. Consider adding a guard check or validating that messages has at least one element.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P1: Accessing input.messages[input.messages.length - 1] without checking if messages is empty will cause a TypeError if an empty array is passed. Consider adding a guard check or validating that messages has at least one element.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/lib/models/providers/lmstudio/lmstudioLLM.ts, line 33:

<comment>Accessing `input.messages[input.messages.length - 1]` without checking if `messages` is empty will cause a TypeError if an empty array is passed. Consider adding a guard check or validating that messages has at least one element.</comment>

<file context>
@@ -1,5 +1,139 @@
+    const messagesWithJsonInstruction = [
+      ...input.messages.slice(0, -1), // All messages except the last
+      {
+        ...input.messages[input.messages.length - 1],
+        content: `${input.messages[input.messages.length - 1].content}\n\nRespond with a valid JSON object matching this schema:\n${JSON.stringify(jsonSchema, null, 2)}`,
+      },
</file context>