🐛 Bug Report: MCP Tool JSON Parsing Errors on OpenCode (GLM-4.7)

[izzamoe]

Description

Summary

When using MCP generator tools with GLM-4.7 in OpenCode, the model frequently produces invalid JSON arguments for MCP tool calls, causing repeated JSON parsing failed errors. The same MCP server + model works correctly on other clients (e.g. RooCode, KiloCode).

This leads to:

• Tool call failures
• Infinite retry loops
• Model “giving up” mid-task

---

Environment

• Client: OpenCode

• Model: GLM-4.7

• MCP Server: Custom local MCP (generator tools)

• Other clients tested (working):

  • RooCode ✅
  • KiloCode ✅

• OS: Linux

• Transport: MCP stdio / local

---

Affected Tools (examples)

• generator-mcp_get_relations
• generator-mcp_update_database
• generator-mcp_execute_dynamic_api

---

Example Errors

❌ Invalid JSON (unquoted value)

Invalid input for tool generator-mcp_get_relations: JSON parsing failed
Text: {"table_name":"t_purchase_invoice","direction":both}
Error: Unexpected identifier "both"

❌ UUID not quoted

Invalid input for tool generator-mcp_execute_dynamic_api: JSON parsing failed
Text:
{"id":cf56856a-5b7f-47aa-9877-8f45d1d2c1ee,"method":"GET","model":"m_customer","operation":"detail"}
Error: Unexpected identifier "cf56856a"

❌ Repeated failure even after self-correction

{"source":api}
Error: Unexpected identifier "api"

Model attempts to “fix” it but keeps emitting invalid JSON.

---

Expected Behavior

• OpenCode should enforce or validate strict JSON serialization before MCP tool execution
• Model output for tool arguments should be auto-corrected or rejected before sending to MCP
• Same behavior consistency as RooCode / KiloCode

---

Actual Behavior

• Model emits JS-like objects, not valid JSON
• No client-side sanitation or retry with correction
• Tool execution fails repeatedly
• Model eventually halts or degrades output quality

---

Why This Is a Client Bug (Not MCP / Model)

• ✅ Same MCP server works perfectly on other clients
• ✅ Same prompts + GLM-4.7 succeed elsewhere
• ❌ Only OpenCode shows persistent JSON formatting failures

This strongly suggests:

OpenCode does not strictly constrain / validate tool call JSON before execution

---

Suggested Fixes

1. Strict JSON validation layer before MCP call
2. Auto-quoting for enums / strings when schema requires it
3. Reject & re-prompt model when tool args are invalid (instead of executing)
4. Optional: show tool schema-aware JSON fixer

---

Impact

• Makes MCP unusable for complex workflows
• Breaks parallel tool execution
• Causes model instability and task abandonment

---

Reproducibility

• 🔁 Reproducible ~80–90% of the time on OpenCode
• ❌ Not reproducible on RooCode / KiloCode

---

Closing

This issue significantly affects MCP reliability in OpenCode. Please prioritize JSON argument enforcement for tool calls.

Thanks 🙏

Plugins

No response

OpenCode version

1.1.15

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response