2026-10-07-agentic.md

Agentic benchmark: Ponytail 5 vs Ponytail vs no skill

2026-10-07. Opus 5.5, default effort, Claude Code CLI 2.1.288. Headless Claude Code sessions, 39 tasks, 5 runs each.

Ponytail 5 replaces the current Ponytail (v4.13.0). Before it shipped, every change to the rules had to beat the current version in a paired run, one change at a time. This file is the final check: Ponytail 5, Ponytail (v4.13.0) and the same agent without any skill, measured in one run so that time of day and model drift hit all three the same way.

Setup

• Agent: a headless Claude Code session per task (claude -p), Opus 5.5, default effort. The skill is loaded with --plugin-dir, the same way a user gets it. The no-skill arm loads nothing. For every arm I checked the session transcript of a finished cell to prove what was injected. This run covers Claude Code; Ponytail also ships for Codex, Cursor, Copilot and more.
• Tasks (39):
  • 12 feature tickets in a real FastAPI + React repo (full-stack-fastapi-template), scored on the git diff.
  • 18 surgical tasks with hidden checks: bug fixes, refunds, SQL with untrusted input, path traversal, forged tokens, rate limiting, a privacy case (one email to many subscribers without exposing their addresses), renames and format changes that must reach every caller.
  • 6 "build me a small app" requests and 3 "show me an example" requests, where the agent picks the scope.
• Runs: 5 per task and arm, 585 sessions. The three arms alternate task by task.
• Isolation: every session runs in a fresh temp folder outside any git repo, with CLAUDE.md loading and auto memory switched off, and only the plugin under test. (An earlier version of the harness ran some tasks inside the Ponytail repo. Claude Code then showed the model the repo's recent commit messages. That is fixed, and none of the numbers here come from that setup.)
• No shell, by design: Bash is disabled for every arm. Agents write code and tests; the benchmark runs them afterwards (hidden checks, injected bugs). This measures what the agent writes, not what it can debug.

Statistics. For each task I take the median of the 5 runs, divide arm by baseline, and report the geometric mean of those ratios over the tasks, with a 95% bootstrap interval. "Better on X of Y tasks" is a sign test over the tasks.

Results

vs no skill	Ponytail (v4.13)	Ponytail 5
Lines of code	-48%	-53% [-63..-41]
Code information (gzip of the delivered code)	-34%	-41% [-51..-30]
Output tokens	-43%	-45% [-54..-35]
Cost	-16% [-27..-5]	-26% [-36..-16]
Time	-38%	-41% [-49..-32]
Turns	-21%	-22%
Cheaper on	25 of 38 tasks	30 of 38 tasks

Ponytail 5 vs Ponytail (v4.13)	change	tasks better / worse
Cost	-10.8% [-14..-8]	35 / 4
Code information (gzip)	-10.6% [-15..-6]	26 / 7
Lines of code	-8.6% [-15..-3]	20 / 7
Time	-4.0% [-9..+1]	23 / 16
Output tokens	-2.7% [-8..+3]	21 / 18
Reply length	+0.1%	20 / 19

Mean cost per session: no skill $0.200, Ponytail $0.156, Ponytail 5 $0.142 (list price).

Correctness and safety

Only the 18 surgical tasks have hidden checks that run the code (right result and no security or privacy hole). On those:

	no skill	Ponytail (v4.13)	Ponytail 5
Hidden checks passed	86/90 (95.6%)	86/90 (95.6%)	87/90 (96.7%)

On the six security tasks of the earlier safety tier (safe-path, sql-user, auth-token, rate-limit, csv-sum, cache) all three arms passed 30 of 30 runs.

Less code did not cost correctness or safety. It did not add any either: the three arms are level. The privacy task is hard for all of them (2 of 5 runs passed in every arm).

The other 21 tasks are there to measure code size. Their "pass" only means the code was delivered or compiles, so I do not count them as a quality result. (Over all 39 tasks no skill passes 186/195, mostly lost to 5 timeouts on one task. That gap says nothing about code quality.)

Whether the shorter code on these 21 tasks still does the whole job is checked by the completeness judge (benchmarks/agentic/complete.py, Opus 5.5 as judge, blind, self-test passed). It reads what each run delivered and scores it from 0 (stub) to 3 (fully implements the task).

	no skill	Ponytail 5
Fully implements the task (3 of 3)	100/100	105/105

The 5 no-skill runs killed at the timeout delivered nothing and are not judged. The judge scored one Ponytail 5 run 1 for a missing col import; the import is on line 5 of that file (the judge only saw the diff), so it is counted as complete.

Tests

Ponytail's rule: logic with a branch, a loop, a parser, money or security, or a whole new script, leaves one small test or an assert self-check behind. Trivial changes need none. I sorted the tasks by that rule before measuring (22 need a test, 11 do not, 6 borderline ones count for neither), then checked what each agent left behind. To see whether a test is worth anything, I injected small bugs (a flipped comparison, a dropped if, an off-by-one) into the code the agent wrote and ran the agent's own test against each one. The test and mutation tools ship with the user-runnable benchmark.

	no skill	Ponytail (v4.13)	Ponytail 5
A test where the logic needs one	68%	95%	98%
Tests that run (90 cells)	53	84	86
Injected bugs caught, over all cells	46%	64%	66%
Injected bugs caught, where a test exists	77%	69%	69%
A test where none was needed	36%	45%	45%

No skill writes the most thorough single tests, but often writes none. 20 of the 25 Ponytail 5 cells in the last row are two to four assert lines inside the learning examples, where they show the result; the other 5 add an endpoint test to the repo's existing test suite.

Replies

A blind judge (Sonnet 5.5, order shuffled per pair, gated by a self-test) picked the reply that serves a busy developer better: true to the diff, tells a reviewer everything they must know, easy to read once.

	in the real run (each reply describes its own code)	same code (both replies describe one diff)
Ponytail 5 vs Ponytail (v4.13)	110 : 67 (18 ties, p=0.002)	131 : 89 (12 ties, p=0.006)
Ponytail 5 vs no skill	82 : 106 (7 ties, p=0.09)	99 : 118 (15 ties, p=0.22)

Ponytail 5 replies are 30% shorter than no skill. Against no skill the judge found no significant difference, with a slight lean towards the longer no-skill replies. Against Ponytail (v4.13) it clearly prefers Ponytail 5. The main reason in the judge's notes: Ponytail 5 ends with what it skipped or did not check and the risks a reviewer should know.

Per task

Medians of 5 runs. none = no skill, PT = Ponytail (v4.13), PT5 = Ponytail 5. Lines are the delivered source lines (comments included, tests excluded).

task	type	lines: none / PT / PT5	cost $: none / PT / PT5	hidden check passed: none / PT / PT5
auth-token	hidden check	18 / 17 / 16	0.081 / 0.117 / 0.105	5/5 / 5/5 / 5/5
cache	hidden check	11 / 11 / 11	0.096 / 0.097 / 0.083	5/5 / 5/5 / 5/5
conv-seam	hidden check	12 / 14 / 14	0.122 / 0.149 / 0.139	5/5 / 5/5 / 5/5
conv-service	hidden check	7 / 4 / 4	0.099 / 0.110 / 0.089	5/5 / 5/5 / 5/5
conv-ui	hidden check	17 / 17 / 17	0.095 / 0.119 / 0.099	5/5 / 4/5 / 5/5
critic-email	hidden check	33 / 14 / 12	0.127 / 0.125 / 0.115	5/5 / 5/5 / 5/5
csv-sum	hidden check	58 / 24 / 17	0.133 / 0.126 / 0.111	5/5 / 5/5 / 5/5
rate-limit	hidden check	44 / 27 / 26	0.147 / 0.145 / 0.144	5/5 / 5/5 / 5/5
reuse-money	hidden check	9 / 9 / 9	0.083 / 0.113 / 0.096	5/5 / 5/5 / 5/5
reuse-slug	hidden check	20 / 19 / 21	0.091 / 0.118 / 0.103	5/5 / 5/5 / 5/5
safe-path	hidden check	16 / 13 / 13	0.117 / 0.113 / 0.106	5/5 / 5/5 / 5/5
scope-format	hidden check	21 / 15 / 15	0.137 / 0.153 / 0.147	5/5 / 5/5 / 5/5
scope-recipients	hidden check	13 / 6 / 4	0.154 / 0.173 / 0.159	2/5 / 2/5 / 2/5
scope-rename	hidden check	3 / 3 / 3	0.133 / 0.163 / 0.128	4/5 / 5/5 / 5/5
sql-user	hidden check	9 / 7 / 7	0.093 / 0.099 / 0.089	5/5 / 5/5 / 5/5
todo-null	hidden check	203 / 84 / 77	0.166 / 0.163 / 0.125	5/5 / 5/5 / 5/5
trace-amount	hidden check	9 / 9 / 9	0.096 / 0.120 / 0.106	5/5 / 5/5 / 5/5
trace-transfer	hidden check	27 / 21 / 20	0.135 / 0.133 / 0.126	5/5 / 5/5 / 5/5
open-dataclass	learning example	84 / 62 / 51	0.110 / 0.118 / 0.106	n/a
open-decorators	learning example	79 / 45 / 20	0.107 / 0.104 / 0.087	n/a
open-mandelbrot	learning example	129 / 43 / 32	0.237 / 0.114 / 0.113	n/a
tmpl-be-archive	repo feature	99 / 75 / 59	0.353 / 0.319 / 0.269	n/a
tmpl-be-bulkdelete	repo feature	35 / 22 / 21	0.338 / 0.208 / 0.230	n/a
tmpl-be-count	repo feature	25 / 11 / 12	0.280 / 0.172 / 0.145	n/a
tmpl-be-csv	repo feature	48 / 33 / 38	0.270 / 0.226 / 0.218	n/a
tmpl-be-duplicate	repo feature	20 / 21 / 21	0.208 / 0.207 / 0.181	n/a
tmpl-be-search	repo feature	31 / 29 / 29	0.244 / 0.221 / 0.221	n/a
tmpl-fe-colorpicker	repo feature	135 / 55 / 26	0.199 / 0.143 / 0.111	n/a
tmpl-fe-command	repo feature	319 / 169 / 131	0.431 / 0.327 / 0.269	n/a
tmpl-fe-datepicker	repo feature	335 / 11 / 10	0.427 / 0.140 / 0.099	n/a
tmpl-fe-dropzone	repo feature	247 / 65 / 62	0.265 / 0.173 / 0.134	n/a
tmpl-fe-rating	repo feature	136 / 70 / 60	0.183 / 0.152 / 0.124	n/a
tmpl-fe-wizard	repo feature	311 / 95 / 90	0.678 / 0.236 / 0.214	n/a
vibe-csvstats	small app	124 / 45 / 47	0.150 / 0.115 / 0.102	n/a
vibe-langgraph	small app	148 / 28 / 29	0.557 / 0.103 / 0.095	n/a
vibe-md2html	small app	0 / 106 / 139	timeout / 0.287 / 0.316	n/a
vibe-password	small app	224 / 41 / 38	0.175 / 0.117 / 0.097	n/a
vibe-shortener	small app	285 / 83 / 79	0.240 / 0.128 / 0.136	n/a
vibe-todo	small app	197 / 44 / 45	0.157 / 0.101 / 0.106	n/a

Reproduce

See benchmarks/agentic/README.md. Short version: clone the template at cd83fc1, run python run.py --selftest (no API), then

python run.py --task <the 39 task ids from the table above, comma separated> \
  --arms baseline,ponytail,ponytail2 --models opus --runs 5

with PONYTAIL_PLUGIN_DIR pointing at Ponytail (v4.13) and PONYTAIL2_PLUGIN_DIR at Ponytail 5. Every workspace is kept under runs/<stamp>/, so any metric can be recomputed offline with --rescore.