PreToolUse hooks: input mutation via `updatedInput`

[simeq]

I thought about connecting rtk to Kimi Code CLI, but the current PreToolUse hook protocol makes it less efficient to do. To change a call, a hook can only block it with permissionDecision: "deny" and hope that the model tries again with the right command. It would be great to have a way to change the tool input and still let execution go on.

The idea

Instead of blocking and suggesting a fix:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "deny",
    "permissionDecisionReason": "Use `rtk git status` instead"
  }
}

I'd like to return the rewritten input directly:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "updatedInput": {
      "command": "rtk git status"
    }
  }
}

The tool runs with the mutated parameters. No block, no retry loop, no wasted tokens.

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Claude Code supports this. Their hook protocol has updatedInput alongside permissionDecision — the decision controls permission flow, while updatedInput independently mutates tool parameters.

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Would love to hear if this fits; I'm happy to create PR if needed 😄

[Necmttn]

If Kimi adds this, I would pin more than the JSON shape. The dangerous failure mode is a hook appearing to rewrite a tool while the host still executes the original input.

Acceptance criteria I would use:

• the hook result records both original input and effective input in the tool audit trail;
• multiple PreToolUse hooks have deterministic precedence, especially deny vs allow-with-update;
• an invalid updatedInput for the target tool fails closed with a visible hook error instead of falling back to the original call;
• the model-visible transcript makes it clear which input actually ran;
• tests cover at least shell command args and one structured/MCP-style tool input.

That keeps updatedInput useful for routing wrappers like rtk without creating a silent bypass when a rewrite is ignored or only partially applied.

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