Socket Mode: ack() with text starting with '{' is never sent (JSONDecodeError) #1594

Reproducible in:

The slack_bolt version

slack_bolt==1.30.0
slack_sdk==3.45.0

Python runtime version

Python 3.12.5

OS info

macOS 27.0

Steps to reproduce:

In Socket Mode, call ack() with a plain text string that starts with { but is not valid JSON.

import os
from slack_bolt import App
from slack_bolt.adapter.socket_mode import SocketModeHandler

app = App(token=os.environ["SLACK_BOT_TOKEN"])

@app.command("/hello")
def hello(ack):
    ack("{curly braces are fun}")

SocketModeHandler(app, os.environ["SLACK_APP_TOKEN"]).start()

1. Start the app in Socket Mode.
2. Run /hello in Slack.

Expected result:

Slack receives the ack and shows the text {curly braces are fun} as the response.

Actual result:

The ack is never sent. Slack shows "failed with the error operation_timeout" and the app logs a json.JSONDecodeError.

The cause is in slack_bolt/adapter/socket_mode/internals.py (send_response) and async_internals.py (send_async_response). They decide the ack body is JSON by checking whether the text starts with {, and then call json.loads() on it without handling a parse failure:

if content_type.startswith("application/json"):
    payload = json.loads(bolt_resp.body)

BoltResponse sets Content-Type: application/json for any string body that starts with {, so the same problem happens with ack("{", ...), ack("{foo"), or any text that happens to begin with that character. The non-Socket-Mode HTTP path does not have this issue because the body is sent through as-is.

I have a fix ready in #1593 that falls back to sending the string as {"text": body} whenever it cannot be parsed as a JSON object. Opening this issue at the maintainers' request so it can be tracked.

Requirements

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