DurableAgent rejects MCP dynamic tools that have no validate function on inputSchema #1576

Bug

DurableAgent.executeTool() in @workflow/ai@4.1.0-beta.59 incorrectly rejects tool calls for MCP dynamic tools whose inputSchema (created via jsonSchema() from @ai-sdk/provider-utils) has no validate function.

Reproduction

Use @ai-sdk/mcp to create MCP tools (which produces dynamicTool with jsonSchema() wrapper — no validate fn), then pass them to a DurableAgent. When the model calls any of these tools, the agent throws:

Error: Invalid input for tool "toolName": undefined

Root cause

In durable-agent.js L677–693:

const input = await schema.validate?.(JSON.parse(toolCall.input || '{}'));
if (!input?.success) {
    throw new Error(`Invalid input for tool "${toolCall.toolName}": ${input?.error?.message}`);
}

1. schema.validate is undefined for MCP dynamic tools (the jsonSchema() helper from @ai-sdk/provider-utils does not set a validate function)
2. schema.validate?.() returns undefined via optional chaining
3. !undefined?.success → !undefined → true — enters the error branch
4. undefined?.error?.message → undefined — produces the ": undefined" suffix

The AI SDK's own safeValidateTypes handles this correctly by short-circuiting:

if (actualSchema.validate == null) {
    return { success: true, value, rawValue: value };
}

Suggested fix

Add a nullish guard before the validation check:

const input = await schema.validate?.(JSON.parse(toolCall.input || '{}'));
if (input !== undefined && !input.success) {
    // validation ran and failed
    throw new Error(...);
}
parsedInput = input?.value ?? JSON.parse(toolCall.input || '{}');

Or, match the AI SDK pattern:

const parsed = JSON.parse(toolCall.input || '{}');
if (typeof schema.validate === 'function') {
    const input = await schema.validate(parsed);
    if (!input.success) {
        throw new Error(`Invalid input for tool "${toolCall.toolName}": ${input.error?.message}`);
    }
    parsedInput = input.value;
} else {
    parsedInput = parsed;
}

Environment

• @workflow/ai: 4.1.0-beta.59
• ai: 6.0.142
• @ai-sdk/mcp: 1.0.30
• @ai-sdk/provider-utils: 4.0.21
• @modelcontextprotocol/sdk: 1.29.0

Workaround

Patch a passthrough validate onto MCP tool schemas before passing them to DurableAgent:

if (schema && typeof schema.validate !== "function") {
    schema.validate = (value: unknown) => ({ success: true, value });
}