feat(run): controlled scenario system_prompt (fixes uncontrolled-prompt A/B bias)#18
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…cross arms)
copeca inherited the agent CLI's full default system prompt (which promotes
the native Read/Grep/Glob tools), so any tool-under-test that competes with
native tools was systematically under-adopted — the system prompt was an
UNCONTROLLED variable, breaking the "hold everything fixed, vary one thing"
guarantee. Empirically: the same tilth+sonnet+version that tilth's own
(lean-prompt) harness measured cheaper, copeca measured costlier, because its
agent used tilth ~30% vs tilth's ~100%.
Add Scenario.system_prompt: a fixed prompt threaded to EVERY arm (baseline +
experimental) via the runner's --system-prompt, REPLACING the CLI default so
the prompt is a controlled, held-constant condition. "{cwd}" is substituted
with the per-run worktree path. None keeps the CLI's own default (prior
behavior, fully back-compatible).
- config/models.py: Scenario.system_prompt field (documented)
- orchestration/run.py: thread scenario.system_prompt -> run_single ->
build_command, with {cwd} substitution
- tests: system_prompt reaches build_command on every arm; None => None
Verified offline: baseline and experimental arms build byte-identical
commands except the experimental's --mcp-config.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Why
copeca's A/B promise is "hold everything fixed, vary one thing." But it inherited the agent CLI's full default system prompt (which promotes the native Read/Grep/Glob tools) — an uncontrolled variable. Any tool-under-test that competes with native tools was systematically under-adopted.
Empirically: the same tilth + Sonnet + version that tilth's own lean-prompt harness measured cheaper, copeca measured costlier, purely because copeca's agent used tilth ~30% vs tilth's ~100%. Matching the system prompt flips adoption.
What
Add
Scenario.system_prompt— a fixed prompt threaded to every arm (baseline + experimental) via the runner's--system-prompt, replacing the CLI default so the prompt is a controlled, held-constant condition.config/models.py:Scenario.system_promptfield (documented).orchestration/run.py: threadscenario.system_prompt→run_single→build_command, with{cwd}→ per-run worktree substitution.tests: system prompt reachesbuild_commandon every arm;None⇒None(CLI default kept — fully back-compatible).Validity
Verified offline that
baselineand experimental arms build byte-identical commands except the experimental's--mcp-config. CI:ruff format --check+ruff check+ full pytest.🤖 Generated with Claude Code