"swarming.py trigger" receives the number of shards it should trigger as a command line argument. When it sees that it should trigger more than one shard, it sets up per-shard environment variables that indicate to the test harness which shard is being run:
https://cs.chromium.org/chromium/infra/luci/client/swarming.py?l=241

swarming trigger scripts manually call "swarming.py trigger" for each shard, but don't (and currently can't) tell "swarming.py" trigger that each invocation is intended as "shard I of N".

This means that these environment variables aren't set. The target invocation runs all tests in each shard.

The collect step is failing when using a swarming trigger script to spawn a sharded test, and I think it's because there are collisions in the output JSON files during the merge.

I suggest adding a command line argument to "swarming.py trigger" for use only by trigger scripts like --trigger-only-shard=[NUM]. Then trigger scripts could pass through the --shards argument to "swarming.py trigger" as well as add this new argument. If specified, this new argument would cause trigger_task_shards in swarming.py to only trigger one shard, but modified in the way all shards are currently modified. It would be a small change:

  if trigger_only_shard is not None:
    requests = [convert(trigger_only_shard)]
  else:
    requests = [convert(index) for index in xrange(shards)]

See this CL, attempting to add the first trigger script to the Chromium repository:
  https://chromium-review.googlesource.com/833505

and this failing try job from Patch Set 10:
  https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/79372

webgl_conformance_tests, which has two shards, fails.

Compare to this successful run on the same tryserver:

https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/79610

Note that both shards from the failing run are running all of the tests. I'm not 100% sure whether that's the only reason that invalid JSON is being generated during the collect step, but the GTEST_SHARD_INDEX and GTEST_TOTAL_SHARDS environment variables are definitely missing from the shards triggered by the swarming trigger script.