diff --git a/.clinerules/ponytail.md b/.clinerules/ponytail.md
index 6ad149e..38c2e8f 100644
--- a/.clinerules/ponytail.md
+++ b/.clinerules/ponytail.md
@@ -21,4 +21,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
diff --git a/.cursor/rules/ponytail.mdc b/.cursor/rules/ponytail.mdc
index 239bfc1..09c6699 100644
--- a/.cursor/rules/ponytail.mdc
+++ b/.cursor/rules/ponytail.mdc
@@ -27,4 +27,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 6ad149e..38c2e8f 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -21,4 +21,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
diff --git a/.kiro/steering/ponytail.md b/.kiro/steering/ponytail.md
index b729974..6f0b1b4 100644
--- a/.kiro/steering/ponytail.md
+++ b/.kiro/steering/ponytail.md
@@ -26,4 +26,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
diff --git a/.windsurf/rules/ponytail.md b/.windsurf/rules/ponytail.md
index 6ad149e..38c2e8f 100644
--- a/.windsurf/rules/ponytail.md
+++ b/.windsurf/rules/ponytail.md
@@ -21,4 +21,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
diff --git a/AGENTS.md b/AGENTS.md
index f2de38f..13910b9 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -21,6 +21,6 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size, lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, the calibration real hardware needs (the platform is never the spec ideal, a clock drifts, a sensor reads off), anything explicitly requested. Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind, the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
 
 (Yes, this file also applies to agents working on the ponytail repo itself. Especially to them.)
diff --git a/benchmarks/behavior.js b/benchmarks/behavior.js
new file mode 100644
index 0000000..41787ae
--- /dev/null
+++ b/benchmarks/behavior.js
@@ -0,0 +1,58 @@
+// Behavior gate: does the ponytail ruleset actually PRODUCE its refined
+// behaviors, not just carry the text? One check per probe (vars.probe), each
+// targeting a rule that a field review (rcstack, phases 0-8) showed mattered:
+//   hardware     - "hardware is never the spec ideal, leave the calibration knob"
+//   explanation  - "explanation the user explicitly asked for is not debt"
+//   onecheck     - "lazy code without its check is unfinished"
+//
+// Heuristic graders, same spirit as loc.js / correctness.js. The graders
+// themselves are proven by tests/behavior.test.js (RED/GREEN, no API key).
+//
+// Metric: `behavior` (1 = behavior present, 0 = absent).
+
+function codeOf(text) {
+  return [...String(text || '').matchAll(/```[\w-]*\n([\s\S]*?)```/g)].map((m) => m[1]).join('\n');
+}
+
+function proseOf(text) {
+  return String(text || '').replace(/```[\s\S]*?```/g, ' ').replace(/\s+/g, ' ').trim();
+}
+
+const CHECKS = {
+  // Treats the device as non-ideal: leaves a tunable knob or flags per-unit drift.
+  // A passing mention of "calibration" is not enough; it must be actionable.
+  hardware(output) {
+    const t = String(output || '');
+    const drift = /\bdrift|per[- ]unit|per[- ]part|part[- ]to[- ]part|measure your own|\btare\b|\btrim\b|\bknob|\btuning\b|reads off|known (temp|reference|value)|reference (thermometer|sensor|temp)|calibration (offset|constant|param|knob)/i.test(t);
+    return drift
+      ? { pass: true, reason: 'Leaves a calibration knob / flags per-unit drift.' }
+      : { pass: false, reason: 'Treats the hardware as ideal; no calibration knob.' };
+  },
+
+  // Gives the explanation the user explicitly asked for instead of truncating.
+  explanation(output) {
+    const p = proseOf(output);
+    const words = p ? p.split(' ').length : 0;
+    const structured = /(\d+[.)]\s|[-*]\s)/.test(String(output || '')) || /\bbecause\b|\bwhy\b|\bso that\b|renamed|extracted|inlined|removed|replaced/i.test(p);
+    return words >= 45 && structured
+      ? { pass: true, reason: `Gave the requested write-up (${words} words of prose).` }
+      : { pass: false, reason: `Truncated the requested explanation (${words} words of prose).` };
+  },
+
+  // Leaves ONE runnable check behind for non-trivial logic.
+  onecheck(output) {
+    const t = String(output || '');
+    const hasCheck = /\bassert\b|def\s+test_|if\s+__name__|unittest|pytest|console\.assert|\bexpect\(|\bdescribe\(|\bit\(/.test(t);
+    return hasCheck
+      ? { pass: true, reason: 'Left a runnable check (assert/test/demo).' }
+      : { pass: false, reason: 'No runnable check left behind.' };
+  },
+};
+
+module.exports = (output, context) => {
+  const probe = context && context.vars && context.vars.probe;
+  const check = CHECKS[probe];
+  if (!check) return { pass: true, score: 1, reason: `Unknown probe '${probe}', skipped` };
+  const r = check(output);
+  return { pass: r.pass, score: r.pass ? 1 : 0, reason: r.reason };
+};
diff --git a/benchmarks/behavior.yaml b/benchmarks/behavior.yaml
new file mode 100644
index 0000000..bba1c82
--- /dev/null
+++ b/benchmarks/behavior.yaml
@@ -0,0 +1,40 @@
+# Ponytail behavior gates: does the ruleset actually produce its refined
+# behaviors (not just carry the text)? Probes the three rules a full-project
+# field review (rcstack, phases 0-8) showed mattered.
+#
+#   Run:    npx promptfoo@latest eval -c benchmarks/behavior.yaml --repeat 10
+#   View:   npx promptfoo@latest view
+#
+# Needs ANTHROPIC_API_KEY (see benchmarks/README.md). The grader (behavior.js)
+# is proven separately by tests/behavior.test.js, which needs no API key.
+#
+# baseline is included as the control: the no-skill arm should mostly FAIL these
+# gates, the ponytail arm should pass them. That delta is the point.
+description: "Ponytail behavior gates: hardware calibration, requested explanation, one runnable check."
+
+providers:
+  - id: anthropic:messages:claude-opus-4-8
+    config: { max_tokens: 8192, temperature: 1 }
+
+prompts:
+  - id: file://arms/baseline.js
+    label: baseline (no skill)
+  - id: file://arms/ponytail.js
+    label: ponytail
+
+defaultTest:
+  assert:
+    - type: javascript
+      value: file://behavior.js
+      metric: behavior
+
+tests:
+  - vars:
+      probe: hardware
+      task: "Write a Python function that reads the temperature in Celsius from a thermistor wired to a Raspberry Pi ADC (MCP3008, channel 0)."
+  - vars:
+      probe: explanation
+      task: "Refactor this for readability and give me a detailed, step-by-step write-up of every change you made and why.\n\ndef p(d):\n    r = []\n    for x in d:\n        if x.get('a') and x['a'] > 0:\n            r.append(x['a'] * 2)\n    return r"
+  - vars:
+      probe: onecheck
+      task: "Write a Python function that parses a duration string like '1h30m45s' into a total number of seconds."
diff --git a/hooks/ponytail-instructions.js b/hooks/ponytail-instructions.js
index 5cf884b..3cd3dd8 100644
--- a/hooks/ponytail-instructions.js
+++ b/hooks/ponytail-instructions.js
@@ -58,11 +58,12 @@ function getFallbackInstructions(mode) {
     'Mark intentional simplifications with a `ponytail:` comment — a shortcut with a known ceiling names the ceiling and the upgrade path in the comment.\n\n' +
     '## Output\n\n' +
     'Code first. Then at most three short lines: what was skipped, when to add it. ' +
-    'If the explanation is longer than the code, delete the explanation.\n\n' +
+    'If the explanation is longer than the code, delete the explanation. ' +
+    'Explanation the user explicitly asked for is not debt, give it in full.\n\n' +
     '## When NOT to be lazy\n\n' +
     'Never simplify away: input validation at trust boundaries, error handling that prevents data loss, ' +
-    'security measures, accessibility basics, anything the user explicitly asked to keep. ' +
-    'Non-trivial logic leaves ONE runnable check behind (assert-based demo/self-check or one small test file; no frameworks). Trivial one-liners need no test.\n\n' +
+    'security measures, accessibility basics, the calibration real hardware needs (the platform is never the spec ideal), anything the user explicitly asked to keep. ' +
+    'Lazy code without its check is unfinished: non-trivial logic leaves ONE runnable check behind (assert-based demo/self-check or one small test file; no frameworks). Trivial one-liners need no test.\n\n' +
     '## Boundaries\n\n' +
     'Ponytail governs what you build, not how you talk. "stop ponytail" or "normal mode": revert. Level persists until changed or session end.';
 }
diff --git a/scripts/check-rule-copies.js b/scripts/check-rule-copies.js
index e37d747..d80fbc2 100644
--- a/scripts/check-rule-copies.js
+++ b/scripts/check-rule-copies.js
@@ -44,6 +44,7 @@ const INVARIANTS = [
   'ONE runnable check',                    // test reflex
   'flimsier algorithm',                    // robust-variant rule
   'input validation at trust boundaries',  // the "not lazy about" clause
+  'Lazy code without its check is unfinished', // one-check promoted to headline
 ];
 
 const skill = read('skills/ponytail/SKILL.md');
diff --git a/skills/ponytail/SKILL.md b/skills/ponytail/SKILL.md
index 33c9743..0e0d3be 100644
--- a/skills/ponytail/SKILL.md
+++ b/skills/ponytail/SKILL.md
@@ -54,7 +54,9 @@ higher one and move on. The first lazy solution that works is the right one.
 Code first. Then at most three short lines: what was skipped, when to add it.
 No essays, no feature tours, no design notes. If the explanation is longer
 than the code, delete the explanation, every paragraph defending a
-simplification is complexity smuggled back in as prose.
+simplification is complexity smuggled back in as prose. Explanation the user
+explicitly asked for (a report, a walkthrough, per-phase notes) is not debt,
+give it in full, the rule is only against unrequested prose.
 
 Pattern: `[code] → skipped: [X], add when [Y].`
 
@@ -78,11 +80,16 @@ that prevents data loss, security measures, accessibility basics, anything
 explicitly requested. User insists on the full version → build it, no
 re-arguing.
 
-Non-trivial logic (a branch, a loop, a parser, a money/security path) leaves
-ONE runnable check behind, the smallest thing that fails if the logic
-breaks: an `assert`-based `demo()`/`__main__` self-check or one small
-`test_*.py`. No frameworks, no fixtures, no per-function suites unless
-asked. Trivial one-liners need no test, YAGNI applies to tests too.
+Hardware is never the ideal on paper: a real clock drifts, a real sensor
+reads off, a PCA9685 runs a few percent fast. Leave the calibration knob, not
+just less code, the physical world needs tuning a minimal model can't see.
+
+Lazy code without its check is unfinished. Non-trivial logic (a branch, a
+loop, a parser, a money/security path) leaves ONE runnable check behind, the
+smallest thing that fails if the logic breaks: an `assert`-based
+`demo()`/`__main__` self-check or one small `test_*.py`. No frameworks, no
+fixtures, no per-function suites unless asked. Trivial one-liners need no
+test, YAGNI applies to tests too.
 
 ## Boundaries
 
diff --git a/tests/behavior.test.js b/tests/behavior.test.js
new file mode 100644
index 0000000..04502cf
--- /dev/null
+++ b/tests/behavior.test.js
@@ -0,0 +1,80 @@
+#!/usr/bin/env node
+// Unit test for the behavior gate (benchmarks/behavior.js). Feeds known
+// behavior-present and behavior-absent outputs through each probe checker and
+// asserts the verdict. Runs without promptfoo or an API key — it proves the
+// grader can tell the refined behavior from its absence, which is what makes
+// the behavior.yaml eval trustworthy.
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const behavior = require('../benchmarks/behavior');
+
+function check(probe, output) {
+  return behavior(output, { vars: { probe } });
+}
+
+// --- hardware: leave a calibration knob ---
+
+test('hardware: calibration knob / drift acknowledged passes', () => {
+  const r = check('hardware',
+    '```python\ndef read_c(beta=3950, r0=10000):\n    ...\n```\n' +
+    'Notes: beta/r0 drift part-to-part, measure your own r0 at a known temp.');
+  assert.equal(r.pass, true);
+  assert.equal(r.score, 1);
+});
+
+test('hardware: real-model phrasing (tuning knobs / reads off) passes', () => {
+  const r = check('hardware',
+    '```python\nBETA = 3950.0  # thermistor beta -- calibration knob\n```\n' +
+    '# BETA/R_FIXED are the tuning knobs -- a real thermistor reads off; trust a reference thermometer over the datasheet.');
+  assert.equal(r.pass, true);
+});
+
+test('hardware: ideal-device assumption fails', () => {
+  const r = check('hardware',
+    '```python\ndef read_c():\n    return adc.read(0) * 0.1\n```\n' +
+    'Notes: converts the raw ADC reading straight to Celsius.');
+  assert.equal(r.pass, false);
+  assert.equal(r.score, 0);
+});
+
+// --- explanation: requested write-up is not debt ---
+
+test('explanation: full requested write-up passes', () => {
+  const r = check('explanation',
+    '```python\ndef positives_doubled(rows):\n    return [x["a"] * 2 for x in rows if x.get("a", 0) > 0]\n```\n' +
+    '1. Renamed p to positives_doubled because the name should say what it returns.\n' +
+    '2. Replaced the manual loop and append with a list comprehension, same logic, fewer lines.\n' +
+    '3. Used x.get("a", 0) so a missing key is treated as zero instead of raising.\n' +
+    '4. Kept the > 0 filter; the behavior is unchanged, only the shape is clearer.');
+  assert.equal(r.pass, true);
+});
+
+test('explanation: terse truncation fails', () => {
+  const r = check('explanation',
+    '```python\ndef positives_doubled(rows):\n    return [x["a"] * 2 for x in rows if x.get("a", 0) > 0]\n```\n' +
+    'skipped: the loop. comprehension covers it.');
+  assert.equal(r.pass, false);
+});
+
+// --- onecheck: leave one runnable check ---
+
+test('onecheck: leaves an assert passes', () => {
+  const r = check('onecheck',
+    '```python\ndef to_seconds(s):\n    ...\n\nassert to_seconds("1h30m") == 5400\n```');
+  assert.equal(r.pass, true);
+});
+
+test('onecheck: no check fails', () => {
+  const r = check('onecheck',
+    '```python\ndef to_seconds(s):\n    import re\n    return sum(...)\n```');
+  assert.equal(r.pass, false);
+});
+
+// --- unknown probe is skipped, not failed ---
+
+test('unknown probe is skipped', () => {
+  const r = check('something-else', '```python\nprint(1)\n```');
+  assert.equal(r.pass, true);
+  assert.match(r.reason, /skipped/i);
+});