diff --git a/test/fixtures/skill-md-size-allowlist.json b/test/fixtures/skill-md-size-allowlist.json
new file mode 100644
index 000000000..88b8e9d99
--- /dev/null
+++ b/test/fixtures/skill-md-size-allowlist.json
@@ -0,0 +1,95 @@
+{
+  "_comment": "Generated SKILL.md files over the limit, pinned at their exact character count (#3199). test/skill-md-size-guard.test.ts fails when a pinned skill grows past its pin or shrinks below it: lower the pin when a skill shrinks and delete it once the skill is within the limit. Raising a pin is an owner decision. The shared-preamble hoist that would retire most pins is deferred: owner Garry, revisit 2026-11-15.",
+  "limit": 100000,
+  "pins": {
+    "agy/gstack-autoplan": 130280,
+    "agy/gstack-design-consultation": 108034,
+    "agy/gstack-design-review": 143383,
+    "agy/gstack-land-and-deploy": 116595,
+    "agy/gstack-plan-design-review": 139639,
+    "agy/gstack-plan-devex-review": 132068,
+    "agy/gstack-plan-eng-review": 152679,
+    "agy/gstack-review": 124266,
+    "claude/design-review": 136823,
+    "codex/gstack-autoplan": 130239,
+    "codex/gstack-design-consultation": 109236,
+    "codex/gstack-design-review": 144817,
+    "codex/gstack-land-and-deploy": 121172,
+    "codex/gstack-office-hours": 102128,
+    "codex/gstack-plan-design-review": 142026,
+    "codex/gstack-plan-devex-review": 134891,
+    "codex/gstack-plan-eng-review": 155159,
+    "codex/gstack-review": 125071,
+    "copilot/gstack-autoplan": 131413,
+    "copilot/gstack-codex": 100103,
+    "copilot/gstack-design-consultation": 109386,
+    "copilot/gstack-design-review": 144688,
+    "copilot/gstack-land-and-deploy": 117865,
+    "copilot/gstack-office-hours": 100416,
+    "copilot/gstack-plan-design-review": 140510,
+    "copilot/gstack-plan-devex-review": 133220,
+    "copilot/gstack-plan-eng-review": 153404,
+    "copilot/gstack-review": 125537,
+    "cursor/gstack-autoplan": 129054,
+    "cursor/gstack-design-consultation": 107072,
+    "cursor/gstack-design-review": 142362,
+    "cursor/gstack-land-and-deploy": 115534,
+    "cursor/gstack-plan-design-review": 138594,
+    "cursor/gstack-plan-devex-review": 130899,
+    "cursor/gstack-plan-eng-review": 151533,
+    "cursor/gstack-review": 140664,
+    "factory/gstack-autoplan": 131338,
+    "factory/gstack-codex": 100523,
+    "factory/gstack-design-consultation": 108520,
+    "factory/gstack-design-review": 144152,
+    "factory/gstack-land-and-deploy": 119717,
+    "factory/gstack-office-hours": 101295,
+    "factory/gstack-plan-design-review": 141375,
+    "factory/gstack-plan-devex-review": 134268,
+    "factory/gstack-plan-eng-review": 154647,
+    "factory/gstack-review": 144443,
+    "gbrain/gstack-autoplan": 129591,
+    "gbrain/gstack-design-review": 132730,
+    "gbrain/gstack-land-and-deploy": 116056,
+    "gbrain/gstack-plan-design-review": 127514,
+    "gbrain/gstack-plan-devex-review": 111470,
+    "gbrain/gstack-plan-eng-review": 134298,
+    "hermes/gstack-autoplan": 129681,
+    "hermes/gstack-design-review": 132837,
+    "hermes/gstack-land-and-deploy": 116158,
+    "hermes/gstack-plan-design-review": 127623,
+    "hermes/gstack-plan-devex-review": 111556,
+    "hermes/gstack-plan-eng-review": 134383,
+    "kiro/gstack-autoplan": 128816,
+    "kiro/gstack-design-consultation": 106914,
+    "kiro/gstack-design-review": 142176,
+    "kiro/gstack-land-and-deploy": 115424,
+    "kiro/gstack-plan-design-review": 138434,
+    "kiro/gstack-plan-devex-review": 130739,
+    "kiro/gstack-plan-eng-review": 151383,
+    "kiro/gstack-review": 140476,
+    "openclaw/gstack-autoplan": 129296,
+    "openclaw/gstack-design-review": 131535,
+    "openclaw/gstack-land-and-deploy": 115644,
+    "openclaw/gstack-plan-design-review": 126365,
+    "openclaw/gstack-plan-devex-review": 110309,
+    "openclaw/gstack-plan-eng-review": 132695,
+    "opencode/gstack-autoplan": 131416,
+    "opencode/gstack-design-consultation": 109207,
+    "opencode/gstack-design-review": 144560,
+    "opencode/gstack-land-and-deploy": 117642,
+    "opencode/gstack-office-hours": 100250,
+    "opencode/gstack-plan-design-review": 140364,
+    "opencode/gstack-plan-devex-review": 133120,
+    "opencode/gstack-plan-eng-review": 153285,
+    "opencode/gstack-review": 142899,
+    "slate/gstack-autoplan": 128935,
+    "slate/gstack-design-consultation": 106993,
+    "slate/gstack-design-review": 142269,
+    "slate/gstack-land-and-deploy": 115479,
+    "slate/gstack-plan-design-review": 138514,
+    "slate/gstack-plan-devex-review": 130819,
+    "slate/gstack-plan-eng-review": 151458,
+    "slate/gstack-review": 140570
+  }
+}
diff --git a/test/helpers/free-fixtures-data.ts b/test/helpers/free-fixtures-data.ts
index 06d53089a..a3be17eba 100644
--- a/test/helpers/free-fixtures-data.ts
+++ b/test/helpers/free-fixtures-data.ts
@@ -169,6 +169,7 @@ export const FREE_FIXTURES: Readonly<Record<string, readonly string[]>> = {
   'test/fixtures/shared-libs-plan-callers-verbless-scope-37186854666.json': ['test/shared-libs-plan-actor.test.ts'],
   'test/fixtures/shared-libs/**': ['test/shared-libs-checker-interface-evidence.test.ts', 'test/shared-libs-fixture.test.ts', 'test/shared-libs-plan-actor.test.ts', 'test/shared-libs-revalidation-prompt.test.ts', 'test/shared-libs-review-start-evidence.test.ts', 'test/shared-libs-source-reads.test.ts', 'test/shared-libs-stage-actor.test.ts'],
   'test/fixtures/ship-triage/**': ['test/ship-triage-labels.test.ts'],
+  'test/fixtures/skill-md-size-allowlist.json': ['test/skill-md-size-guard.test.ts'],
   'test/fixtures/skillify-narration/**': ['test/skill-fixture.test.ts'],
   'test/fixtures/test-value-seam-evidence.json': ['test/test-value-bar.test.ts'],
   'test/fixtures/third-party-actions-recovery-public.json': ['test/third-party-actions-recording.test.ts', 'test/third-party-actions.test.ts'],
diff --git a/test/skill-md-size-guard.test.ts b/test/skill-md-size-guard.test.ts
new file mode 100644
index 000000000..75e2f1afb
--- /dev/null
+++ b/test/skill-md-size-guard.test.ts
@@ -0,0 +1,118 @@
+/**
+ * Generated SKILL.md size guard (#3199). Several agent runtimes and skill
+ * loaders cap one SKILL.md at 100,000 characters; Hermes' skill editor refuses
+ * every edit to a larger skill without naming the size. This free test renders
+ * every host and fails when any generated SKILL.md is over the limit.
+ *
+ * Skills already over the limit are pinned at their exact size in
+ * test/fixtures/skill-md-size-allowlist.json (a ratchet): growth past a pin
+ * fails, and a shrink below a pin fails until the pin is lowered. The shared
+ * preamble hoist that would retire most pins is deferred (owner Garry,
+ * revisit 2026-11-15).
+ */
+import { afterAll, describe, expect, test } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { ALL_HOST_CONFIGS } from '../hosts/index';
+
+const ROOT = path.resolve(import.meta.dir, '..');
+const ALLOWLIST_FILE = 'test/fixtures/skill-md-size-allowlist.json';
+
+interface Allowlist {
+  limit: number;
+  pins: Record<string, number>;
+}
+
+function countChars(text: string): number {
+  let count = 0;
+  for (const _ of text) count++;
+  return count;
+}
+
+function sizeViolations(sizes: Record<string, number>, allowlist: Allowlist): string[] {
+  const { limit, pins } = allowlist;
+  const violations: string[] = [];
+  for (const [key, actual] of Object.entries(sizes).sort(([a], [b]) => a.localeCompare(b))) {
+    const [host, skill] = key.split('/');
+    const pin = pins[key];
+    const where = `host ${host}, skill ${skill}: ${actual} chars, limit ${limit}`;
+    if (pin === undefined) {
+      if (actual > limit) violations.push(`${where}, not pinned. Shrink it to ${limit} chars or fewer.`);
+    } else if (actual > pin) {
+      violations.push(`${where}, pinned at ${pin}: grew by ${actual - pin}. Shrink it back to ${pin} chars or fewer.`);
+    } else if (actual < pin) {
+      violations.push(actual > limit
+        ? `${where}, pinned at ${pin}: shrank by ${pin - actual}. Lower the pin "${key}" to ${actual} in ${ALLOWLIST_FILE}.`
+        : `${where}, pinned at ${pin}: now within the limit. Delete the pin "${key}" from ${ALLOWLIST_FILE}.`);
+    }
+  }
+  for (const key of Object.keys(pins).sort()) {
+    if (!(key in sizes)) violations.push(`pin "${key}" names no generated SKILL.md. Delete it from ${ALLOWLIST_FILE}.`);
+  }
+  return violations;
+}
+
+const SIZE_FIX = 'Fix: move skill-specific prose into sections/*.md that the skill reads on demand (on non-Claude hosts, also add the skill '
+  + 'to CARVED_ON_EVERY_HOST in scripts/resolvers/sections.ts so its sections stop being inlined), or cut duplicated text; '
+  + 'then run `bun run gen:skill-docs` and rerun this test. A pin may only go down: when a pinned skill shrinks, '
+  + `lower its pin in ${ALLOWLIST_FILE} to the new size, and delete it once the skill is within the limit. `
+  + 'Raising a pin is an owner decision. The shared-preamble hoist (#3199) is deferred: owner Garry, revisit 2026-11-15.';
+
+function renderAllHosts(outDir: string): Record<string, number> {
+  const render = Bun.spawnSync(
+    ['bun', 'run', 'scripts/gen-skill-docs.ts', '--host', 'all', '--out-dir', outDir],
+    { cwd: ROOT, stdout: 'pipe', stderr: 'pipe', timeout: 180_000 },
+  );
+  if (render.exitCode !== 0) {
+    throw new Error(`gen-skill-docs --host all --out-dir failed (exit ${render.exitCode}):\n${render.stderr.toString()}`);
+  }
+  const sizes: Record<string, number> = {};
+  for (const host of ALL_HOST_CONFIGS) {
+    // Claude's tracked tree is what ships: an out-dir Claude render repoints
+    // section paths into the render, so it is not byte-identical.
+    const skillsRoot = host.name === 'claude' ? ROOT : path.join(outDir, host.hostSubdir, 'skills');
+    const skillDirs = fs.readdirSync(skillsRoot, { withFileTypes: true })
+      .filter(entry => entry.isDirectory() && !entry.name.startsWith('.'))
+      .map(entry => entry.name);
+    if (host.name === 'claude') skillDirs.push('.');
+    for (const dir of skillDirs) {
+      const file = path.join(skillsRoot, dir, 'SKILL.md');
+      if (!fs.existsSync(file)) continue;
+      if (host.name === 'claude' && !fs.existsSync(`${file}.tmpl`)) continue;
+      sizes[`${host.name}/${dir === '.' ? 'gstack' : dir}`] = countChars(fs.readFileSync(file, 'utf-8'));
+    }
+  }
+  return sizes;
+}
+
+describe('generated SKILL.md size guard (#3199)', () => {
+  const outDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-size-guard-'));
+  afterAll(() => fs.rmSync(outDir, { recursive: true, force: true }));
+
+  test('the ratchet flags over-limit, grown, shrunk, within-limit and stale entries', () => {
+    const allowlist = { limit: 100, pins: { 'h/grew': 150, 'h/shrank': 150, 'h/under': 150, 'h/same': 150, 'h/gone': 150 } };
+    const violations = sizeViolations({ 'h/new': 101, 'h/ok': 100, 'h/grew': 151, 'h/shrank': 149, 'h/under': 99, 'h/same': 150 }, allowlist);
+    expect(violations).toEqual([
+      'host h, skill grew: 151 chars, limit 100, pinned at 150: grew by 1. Shrink it back to 150 chars or fewer.',
+      'host h, skill new: 101 chars, limit 100, not pinned. Shrink it to 100 chars or fewer.',
+      `host h, skill shrank: 149 chars, limit 100, pinned at 150: shrank by 1. Lower the pin "h/shrank" to 149 in ${ALLOWLIST_FILE}.`,
+      `host h, skill under: 99 chars, limit 100, pinned at 150: now within the limit. Delete the pin "h/under" from ${ALLOWLIST_FILE}.`,
+      `pin "h/gone" names no generated SKILL.md. Delete it from ${ALLOWLIST_FILE}.`,
+    ]);
+    expect(countChars('é—x')).toBe(3);
+  });
+
+  test('every host\'s generated SKILL.md is within 100,000 chars or at its exact pin', () => {
+    const allowlist: Allowlist = JSON.parse(fs.readFileSync(path.join(ROOT, ALLOWLIST_FILE), 'utf-8'));
+    expect(allowlist.limit).toBe(100_000);
+    const sizes = renderAllHosts(outDir);
+    for (const host of ALL_HOST_CONFIGS) {
+      expect(Object.keys(sizes).some(key => key.startsWith(`${host.name}/`)), `no rendered skills for ${host.name}`).toBe(true);
+    }
+    const violations = sizeViolations(sizes, allowlist);
+    if (violations.length > 0) {
+      throw new Error(`${violations.length} generated SKILL.md size violation(s):\n  ${violations.join('\n  ')}\n${SIZE_FIX}`);
+    }
+  });
+});