[BUG] Scorer save fails with "toJSON" error when using mastra dev with getPlatformProxy

[owk-owk130]

Describe the Bug

Thank you for the great framework!

When running Mastra Studio via mastra dev with getPlatformProxy from wrangler, scorer execution completes successfully but saving results to mastra_scorers table fails with an RPC serialization error.

Scorer calculation succeeds but save operation fails with:

{
  "message": "The RPC receiver does not implement the method \"toJSON\".",
  "domain": "SCORER",
  "category": "USER",
  "code": "MASTRA_SCORER_FAILED_TO_RUN_HOOK",
  "details": {
    "scorerId": "knowledge-accuracy-scorer",
    "entityId": "nep-chan",
    "entityType": "AGENT"
  },
  "cause": {
    "message": "The RPC receiver does not implement the method \"toJSON\".",
    "domain": "STORAGE",
    "category": "THIRD_PARTY",
    "code": "MASTRA_STORAGE_LIBSQL_SAVE_SCORE_FAILED",
    "details": {},
    "cause": {
      "message": "The RPC receiver does not implement the method \"toJSON\".",
      "name": "TypeError"
    }
  }
}

Code Example

// mastra/index.ts
import { Mastra } from "@mastra/core/mastra";
import { LibSQLStore } from "@mastra/libsql";
import { getPlatformProxy } from "wrangler";

const { env: cloudflareEnv } = await getPlatformProxy<CloudflareBindings>({
remoteBindings: true,
});

export const mastra = new Mastra({
agents: { myAgent },
scorers: { myScorer },
storage: new LibSQLStore({
  id: "mastra-storage",
  url: "file:mastra.db",
}),
server: {
  middleware: [
    async (c, next) => {
      c.get("requestContext").set("env", cloudflareEnv);
      await next();
    },
  ],
},
});

// scorers/my-scorer.ts
import { createScorer } from "@mastra/core/evals";
import { z } from "zod";

export const myScorer = createScorer({
id: "my-scorer",
name: "My Scorer",
description: "Example scorer",
type: "agent",
judge: {
  model: "google/gemini-2.5-flash",
  instructions: "Evaluate the response.",
},
})
.preprocess(({ run }) => ({ answer: run.output }))
.analyze({
  description: "Analyze response",
  outputSchema: z.object({ score: z.number() }),
  createPrompt: ({ results }) => `Evaluate: ${results.preprocessStepResult.answer}`,
})
.generateScore(({ results }) => results?.analyzeStepResult?.score ?? 0)
.generateReason(({ score }) => `Score: ${score}`);

Notes

• Tested with both LibSQLStore and D1Store - same error occurs
• Normal storage operations (threads, messages, etc.) work correctly
• Only scorer save operation fails

Steps To Reproduce

1. Create a Mastra project with Cloudflare Workers integration using getPlatformProxy
2. Define custom scorers using createScorer
3. Attach scorers to an Agent
4. Run mastra dev and execute the agent
5. Observe scorer save failure in logs

Link to Minimal Reproducible Example

Reproduction requires Cloudflare Workers environment with getPlatformProxy. I can create a minimal reproduction repository if needed.

Expected Behavior

Scorer results should be saved to mastra_scorers table and visible in Mastra Studio.

Environment Information

- System:
  - OS: macOS 15.7.3
  - CPU: arm64 Apple M2 Pro
- Binaries:
  - Node: 24.1.0
  - pnpm: 10.28.1
- npmPackages:
  - @mastra/core: 1.0.4
  - @mastra/libsql: 1.0.0
  - wrangler: 4.53.0
- LLM Provider: Google Gemini

Verification

• I have searched the existing issues to make sure this is not a duplicate
• I have included sufficient information for the team to reproduce and understand the issue

[daneatmastra]

Thank you for reporting this issue! We have labeled it for trio-tnt and we will look into it as soon as possible.

If you're experiencing an error, please provide a minimal reproducible example whenever possible to help us resolve it quickly.

[owk-owk130]

@daneatmastra
Thank you for the quick response!

Here is a minimal reproduction repository:
https://github.com/owk-owk130/mastra-scorer-bug

The README includes steps to reproduce the issue.