test_sim reports SUCCEEDED with 0 tests when per-call projectPath is ignored

Start at the test_sim tool and its handling of the per-call projectPath alongside session_set_defaults. Reproduce the report with separate worktree paths and an -only-testing filter, then verify that the requested project is used or that a zero-test filtered run fails instead of returning SUCCEEDED.

Summary

test_sim appears to ignore a per-call projectPath and use the session default instead. When the two differ, it reported SUCCEEDED having run zero tests.

What happened

I had several agents working concurrently in separate git worktrees of one repo. Session defaults were set once, pointing projectPath at the main checkout:

session_set_defaults
  projectPath: <repo>/src/PinPoint.xcodeproj
  scheme: PinPoint
  simulatorName: iPhone 17 Pro

An agent working in <repo>/.claude/worktrees/agent-XXXX/ called test_sim with its own worktree's projectPath and an -only-testing filter for a suite that exists only in that worktree. The call returned SUCCEEDED with 0 tests passed and 0 failed.

The suite it was asked to run did not exist in the project it actually built, so there was nothing to run and nothing to fail. The agent's only clue was the zero count; the status field said success.

Why this is worth fixing above its size

A test tool that returns SUCCEEDED without running the requested tests is the worst available failure mode — it is indistinguishable from a pass at the point where a caller decides whether work is done. In an agent workflow nothing reads the count; it reads the status. Two of my agents would have reported tickets complete on the strength of it. Both worked around it by driving xcodebuild directly with their own -derivedDataPath, which is the right answer but only because they noticed.

Suggested

Either honour a per-call projectPath over the session default, or fail loudly when one is passed and cannot be honoured. Failing that, a zero-test run should not report as SUCCEEDED — an -only-testing filter matching nothing is an error, not a pass.

Environment

Multiple concurrent git worktrees of one project, one shared MCP session, iOS simulator destination. Repointing the shared session defaults was not an option, since a concurrent agent was relying on them.