Agent cache does not record or replay custom tool calls

[hoverlover]

Problem

When using agent caching with custom tools (via the tools option in agent()), custom tool executions are not recorded in the cache and are skipped during replay. This causes agent replay to fail when custom tools perform essential actions like form filling with credentials.

Reproduction

1. Create an agent with custom tools (e.g., for filling login credentials)
2. Enable caching via cacheDir option
3. Execute the agent - custom tools work correctly
4. Re-run the same instruction (cache hit) - custom tool calls are skipped

Root Cause

I traced through the source code and found two issues:

1. Custom tools are not recorded during execution

In v3CuaAgentHandler.ts, the custom_tool case just returns success without recording:

case "custom_tool": {
  // Custom tools are handled by the agent client directly
  return { success: true };
}

Compare this to other action types like goto, scroll, wait which all call this.v3.recordAgentReplayStep().

2. Custom tools are not replayed

In AgentCache.ts, the executeAgentReplayStep method's switch statement doesn't handle custom_tool:

switch (step.type) {
  case "act": ...
  case "fillForm": ...
  case "goto": ...
  // ... other cases ...
  default:
    this.logger({ message: `agent cache skipping step type: ${step.type}` });
    return step;  // Custom tools fall through here and are skipped
}

Proposed Solution

1. Add new type for custom tool steps

In types/private/cache.ts:

export interface AgentReplayCustomToolStep {
  type: "custom_tool";
  name: string;
  arguments: Record<string, unknown>;
}

export type AgentReplayStep =
  | AgentReplayActStep
  // ... existing types ...
  | AgentReplayCustomToolStep
  | { type: string; [key: string]: unknown };

2. Record custom tool executions

In v3CuaAgentHandler.ts or where the action is processed after tool execution:

case "custom_tool": {
  if (recording) {
    this.v3.recordAgentReplayStep({
      type: "custom_tool",
      name: action.name as string,
      arguments: action.arguments as Record<string, unknown>,
    });
  }
  return { success: true };
}

3. Add replay support for custom tools

This requires passing the tools object to AgentCache so it can re-execute tools during replay:

private async replayAgentCustomToolStep(
  step: AgentReplayCustomToolStep,
  tools: ToolSet,
): Promise<void> {
  const tool = tools[step.name];
  if (tool) {
    await tool.execute(step.arguments, { toolCallId: `replay_${Date.now()}`, messages: [] });
  }
}

Use Case

I'm building a system that uses custom tools for filling forms with runtime credentials. Without custom tool caching, the cache records button clicks but skips the credential filling, causing login failures during replay.

Willingness to Contribute

I'm happy to submit a PR implementing this fix if the approach looks reasonable to the maintainers.