[BUG] Invalid JSON Argument while calling tools

[vinkolar]

Problem (one or two sentences)

Started getting this error about "Roo tried to use with an invalid JSON argument" from today morning.

Example output

Date/time: 2026-01-23T17:35:02.130Z
Extension version: 3.42.0
Provider: XXXXX
Model: XXXXXX

Roo tried to use <tool> with an invalid JSON argument. Retrying...

It tries two times in the same way/error, and it succeeds the third time with

I'm experiencing a persistent issue with the MCP tool interface. Let me try using the direct MCP function call from the <mcp-server> server:

Context (who is affected and when)

Happens when we invoke MCPs
Note: Roo also started warning about too many tools. Not sure if this is related

Too many tools enabled
You have <N> tools enabled via <M> MCP servers. Such a high number can confuse the model and lead to errors. Try to keep it below 60.

Reproduction steps

1. Environment Setup: macOS Tahoe Version 26.2 (25C56)
2. Roo-Code Extension version: 3.42.0
3. Happens when we call any MCP tool

Expected result

There is no invalid JSON error while calling MCP tool (no retries either)

Actual result

2 retries while calling tool and then succeeds after defaulting to direct MCP function call

Variations tried (optional)

Tried 6 various MCP servers

App Version

3.42.0

API Provider (optional)

Amazon Bedrock

Model Used (optional)

Claude Sonnet 4.5

Roo Code Task Links (optional)

No response

Relevant logs or errors (optional)

[roomote]

I've analyzed this issue and here's my proposed implementation plan:

Root Cause Analysis

The error "Roo tried to use <tool> with an invalid JSON argument" originates from UseMcpToolTool.validateParams in src/core/tools/UseMcpToolTool.ts (lines 110-124).

The validation currently requires params.arguments to be a JavaScript object:

if (params.arguments !== undefined) {
  if (typeof params.arguments !== "object" || params.arguments === null || Array.isArray(params.arguments)) {
    // Error: invalid JSON argument
    ...
  }
}

However, some models (like Claude Sonnet 4.5 via Amazon Bedrock) may send the arguments parameter as a JSON-encoded string instead of an object. For example:

• Model sends: "arguments": "{\"key\": \"value\"}" (string)
• Expected: "arguments": {"key": "value"} (object)

This type mismatch causes the validation to fail, triggering the "invalid JSON argument" error and retry logic.

Proposed Fix

Modify UseMcpToolTool.validateParams to handle the case where params.arguments is a valid JSON string:

1. If params.arguments is already an object, use it directly (current behavior)
2. If params.arguments is a string, attempt to parse it as JSON:
   • If parsing succeeds and results in an object, use the parsed object
   • If parsing fails or results in a non-object, trigger the existing error

This approach maintains backward compatibility while adding robustness for models that stringify their tool arguments.

Implementation Details

1. Modify src/core/tools/UseMcpToolTool.ts: Update validateParams to parse string arguments
2. Add tests: Add test cases for string argument handling in src/core/tools/__tests__/useMcpToolTool.spec.ts

This fix is targeted and minimally invasive, addressing only the argument type coercion without changing the broader tool execution flow.

Please mention @roomote in a comment saying that you approve this plan, and I'll proceed with the implementation.

[hannesrudolph]

Which MCP server?
Does it happen every time?
Has rolling back a version changed anything?

[vinkolar]

Which MCP server?

This is happening across all MCP servers. What I am using is proprietary MCP servers

Does it happen every time?

Yes..deterministically 2 fails (retries) and 1 direct MCP (success)

Has rolling back a version changed anything?

Have not tried it yet