Parsing JSON record-per-line with jq?

I've got a tool that outputs a JSON record on each line, and I'd like to process it with jq.

The output looks something like this:

{"ts":"2017-08-15T21:20:47.029Z","id":"123","elapsed_ms":10}{"ts":"2017-08-15T21:20:47.044Z","id":"456","elapsed_ms":13}

When I pass this to jq as follows:

./tool | jq 'group_by(.id)'

...it outputs an error:

jq: error (at <stdin>:1): Cannot index string with string "id"

How do I get jq to handle JSON-record-per-line data?