UNIX - Until loop condition expects integer from a string?

Question

So, I have some code which is supposed to check whether a pre-check sequence has completed before I begin patching a server. I'm doing this by using an until loop and as the condition, I have a flag set to true. When the flag is set to false that means that the pre-checks are done and I can now proceed out of the until loop.

The problem is I get stuck in an infinite loop and I get an error message saying [: true: integer expression expected

I continually call a URL to conduct my checks, and it returns a file which I save. An example extract from that file would be:

inProgress":true,"status":"IN_PROGRESS","preCheckMessages":[],"precheckResultItems":[]}

The part I need to extract from this is the value from the inProgress field (which could be 'true' or 'false'). I then use this value to compare to my flag. So I extract the value from the inProgress field.

The section of code causing me an issue is as follows:

    inProgress="true"

    until [ "$inProgress" -eq "false" ]
    do
            long_URL_goes_here > jobIdCheck.txt
            inProgress=$(grep -o 'inProgress.*$' jobIdCheck.txt)

            word="inProgress\":"
            tempVar=${inProgress##${word}}

            inProgress=${tempVar%%,*}
            echo inProgress value = ${inProgress}

    done

echo "Pre-checks complete. Ready to apply patch"

This is where I hit the infinite loop with the error message saying [: true: integer expression expected. (I never see the "Pre-checks complete message")

But I don't understand why it would be expecting an integer? Surely the string comparison should suffice?

Is the fact that the value true in the inProgress field ISN'T encapsulated in double quotes important?

Any help will be greatly received.