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fix: parse JSON-string MCP tool arguments before type check #255

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fix: parse JSON-string MCP tool arguments before type check #255

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9 changes: 8 additions & 1 deletion src/core/tools/UseMcpToolTool.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -111,7 +111,14 @@ export class UseMcpToolTool extends BaseTool<"use_mcp_tool"> {
return { isValid: false }
}

// Native-only: arguments are already a structured object.
// Some LLMs emit arguments as JSON-encoded strings rather than objects.
// Parse them early so the type check below sees the unwrapped object.
if (typeof params.arguments === "string") {
try {
params.arguments = JSON.parse(params.arguments)
} catch {}
}

let parsedArguments: Record<string, unknown> | undefined
if (params.arguments !== undefined) {
if (typeof params.arguments !== "object" || params.arguments === null || Array.isArray(params.arguments)) {
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