diff --git a/scripts/ci-test-costs.json b/scripts/ci-test-costs.json
index 5ea6000a0..d1c76edae 100644
--- a/scripts/ci-test-costs.json
+++ b/scripts/ci-test-costs.json
@@ -361,7 +361,6 @@
     "tests/harness/node-compatibility.test.ts": 3,
     "tests/harness/node-test.test.ts": 14918,
     "tests/harness/npm-fetch-defaults.test.ts": 2062,
-    "tests/harness/npm-static.test.ts": 406491,
     "tests/harness/oci-manifest.test.ts": 3,
     "tests/harness/opaque-class-view.test.ts": 1743,
     "tests/harness/process-argv.test.ts": 1923,
@@ -777,7 +776,6 @@
     "tests/harness/node-compatibility.test.ts": 5,
     "tests/harness/node-test.test.ts": 17953,
     "tests/harness/npm-fetch-defaults.test.ts": 5674,
-    "tests/harness/npm-static.test.ts": 518221,
     "tests/harness/oci-manifest.test.ts": 3,
     "tests/harness/opaque-class-view.test.ts": 2545,
     "tests/harness/process-argv.test.ts": 2583,
diff --git a/scripts/ci-test-costs.mjs b/scripts/ci-test-costs.mjs
index 661d0a859..c227424d0 100644
--- a/scripts/ci-test-costs.mjs
+++ b/scripts/ci-test-costs.mjs
@@ -1,21 +1,27 @@
 #!/usr/bin/env node
-// Refreshes scripts/ci-test-costs.json from the logs of the Linux test shards of green CI runs on main.
+// Refreshes scripts/ci-test-costs.json and tests/harness/npm-static-costs.json from the logs of the Linux test
+// shards of green CI runs on main.
 //
 // Usage: node scripts/ci-test-costs.mjs <job log> ...
 //
 // Each log is a downloaded "test (Linux, plain|san, i/n)" job log. vitest prints every test file it ran with its
-// duration; for each file that runs in the side queue (not the case-sharded corpus, Effect, coverage and cache
-// suites) this stores the median over the logs of its flavor. Files without a measurement keep their current
-// value.
+// duration, and every test slower than its threshold with its duration. ci-test-costs.json stores, for each file
+// that runs whole in the side queue (not the case-sharded corpus, Effect, coverage, cache and npm-static suites),
+// the median over the logs of its flavor; npm-static-costs.json stores the median of each npm-static test. Entries
+// without a new measurement keep their value; files the side queue no longer runs whole are removed.
 import { readFileSync, writeFileSync } from "node:fs";
 import { fileExclusions, isolatedFiles } from "./ci-test-plan.mjs";
 
-const path = new URL("./ci-test-costs.json", import.meta.url);
-const table = JSON.parse(readFileSync(path, "utf8"));
+const filePath = new URL("./ci-test-costs.json", import.meta.url);
+const npmStaticPath = new URL("../tests/harness/npm-static-costs.json", import.meta.url);
+const tables = {
+  files: JSON.parse(readFileSync(filePath, "utf8")),
+  npmStatic: JSON.parse(readFileSync(npmStaticPath, "utf8")),
+};
 const skipped = new Set(
   fileExclusions.filter((file) => !isolatedFiles.some(([, isolated]) => isolated === file)),
 );
-const samples = { plain: {}, san: {} };
+const samples = { files: { plain: {}, san: {} }, npmStatic: { plain: {}, san: {} } };
 for (const log of process.argv.slice(2)) {
   // Logs keep the terminal color codes (ESC [ ... m); drop them before matching.
   const text = readFileSync(log, "utf8")
@@ -23,21 +29,26 @@ for (const log of process.argv.slice(2)) {
     .replace(/\[[0-9;]*m/g, "");
   const flavor = /SCRIPTC_SAN: 1\b/.test(text) ? "san" : "plain";
   for (const line of text.split("\n")) {
-    const m = /^\S+ {2}✓ (\S+\.test\.ts) \(\d+ tests?[^)]*\) (\d+)ms$/.exec(line);
-    if (!m || skipped.has(m[1])) continue;
-    (samples[flavor][m[1]] ??= []).push(Number(m[2]));
+    let m = /^\S+ {2}✓ (\S+\.test\.ts) \(\d+ tests?[^)]*\) (\d+)ms$/.exec(line);
+    if (m && !skipped.has(m[1])) (samples.files[flavor][m[1]] ??= []).push(Number(m[2]));
+    m = /^\S+ {4}✓ npm-static pilots(?: \(sanitized\))? > (.*?) {2}(\d+)ms$/.exec(line);
+    if (m) (samples.npmStatic[flavor][m[1]] ??= []).push(Number(m[2]));
   }
 }
 const median = (values) => [...values].sort((a, b) => a - b)[Math.floor(values.length / 2)];
 let updated = 0;
-for (const flavor of ["plain", "san"])
-  for (const [file, values] of Object.entries(samples[flavor])) {
-    table[flavor][file] = median(values);
-    updated++;
+for (const [name, table] of Object.entries(tables))
+  for (const flavor of ["plain", "san"]) {
+    for (const [key, values] of Object.entries(samples[name][flavor])) {
+      table[flavor][key] = median(values);
+      updated++;
+    }
+    table[flavor] = Object.fromEntries(
+      Object.entries(table[flavor])
+        .filter(([key]) => name !== "files" || !skipped.has(key))
+        .sort(([a], [b]) => a.localeCompare(b)),
+    );
   }
-for (const flavor of ["plain", "san"])
-  table[flavor] = Object.fromEntries(
-    Object.entries(table[flavor]).sort(([a], [b]) => a.localeCompare(b)),
-  );
-writeFileSync(path, JSON.stringify(table, null, 2) + "\n");
-console.log(`updated ${updated} file costs`);
+writeFileSync(filePath, JSON.stringify(tables.files, null, 2) + "\n");
+writeFileSync(npmStaticPath, JSON.stringify(tables.npmStatic, null, 2) + "\n");
+console.log(`updated ${updated} costs`);
diff --git a/scripts/ci-test-plan.mjs b/scripts/ci-test-plan.mjs
index af716c3c5..d922b53db 100644
--- a/scripts/ci-test-plan.mjs
+++ b/scripts/ci-test-plan.mjs
@@ -19,15 +19,19 @@ export const isolatedFiles = [
   ["self-hosting", "tests/harness/self-hosting.test.ts"],
   ["self-hosting LLVM emitter", "tests/harness/self-hosting-llvm-emitter.test.ts"],
   ["fetch", "tests/harness/fetch.test.ts"],
-  ["npm static", "tests/harness/npm-static.test.ts"],
   ["surface manifest", "tests/harness/surface-manifest.test.ts"],
 ];
+// Suites that split their own tests across the shards (SCRIPTC_TEST_SHARD,
+// skipUnownedTest in tests/harness/shard.ts): every shard runs its share in
+// its own vitest process.
+export const caseShardedSideFiles = [["npm static", "tests/harness/npm-static.test.ts"]];
 export const fileExclusions = [
   ...corpusFiles,
   ...effectFiles,
   coverageFile,
   cacheFile,
   ...isolatedFiles.map(([, file]) => file),
+  ...caseShardedSideFiles.map(([, file]) => file),
   "tests/harness/self-hosting-native-driver.test.ts",
 ];
 
@@ -152,6 +156,8 @@ export function ciTestPlan({ shard, flavor, workers = 4, suite = "full" }) {
     if (owners.get(file) === index)
       side.push({ name, args: ["test", file], env: { ...env, SCRIPTC_TEST_WORKERS: "1" } });
   }
+  for (const [name, file] of caseShardedSideFiles)
+    side.push({ name, args: ["test", file], env: { ...env, SCRIPTC_TEST_WORKERS: "1" } });
   // Frontend coverage and cache invalidation do not consume sanitizer state.
   if (flavor === "plain") {
     side.push({
diff --git a/tests/harness/ci-test-plan.test.ts b/tests/harness/ci-test-plan.test.ts
index 7925d799b..76f7f0bce 100644
--- a/tests/harness/ci-test-plan.test.ts
+++ b/tests/harness/ci-test-plan.test.ts
@@ -8,6 +8,7 @@ import {
   darwinFiles,
   effectFiles,
   fileExclusions,
+  caseShardedSideFiles,
   isolatedFiles,
   testCosts,
 } from "../../scripts/ci-test-plan.mjs";
@@ -31,10 +32,12 @@ test.for([1, 3, 5, 10, 20])(
           !file.includes("coverage") &&
           !file.includes("native-toolchain"),
       )) {
+        // A suite that splits its own tests runs on every shard; the others on one.
+        const everyShard = caseShardedSideFiles.some(([, sharded]) => sharded === file);
         expect(
           sideFiles.filter((selected) => selected === file),
           file,
-        ).toHaveLength(1);
+        ).toHaveLength(everyShard ? count : 1);
       }
       // Each measured file runs on exactly one shard, outside vitest's file shard.
       const isolated = new Set(isolatedFiles.map(([, file]) => file));
diff --git a/tests/harness/npm-static-costs.json b/tests/harness/npm-static-costs.json
new file mode 100644
index 000000000..2f1cdde1e
--- /dev/null
+++ b/tests/harness/npm-static-costs.json
@@ -0,0 +1,176 @@
+{
+  "plain": {
+    "__toESM dependencies compile recursively without explicit selection": 626,
+    "--npm-static=auto degrades a deviant __toESM package with the same note": 496,
+    "--npm-static=auto opts a getter-table bundle in": 442,
+    "--npm-static=auto opts the eligible pilot in": 476,
+    "--npm-static=auto probes runtime JS from the importing file's realm": 1819,
+    "--npm-static=auto refuses a package with no own .d.ts": 314,
+    "a cyclic package tree retains an impure dependency through an unused namespace re-export": 1517,
+    "a deviant __toESM helper refuses the package with a construct-naming note": 532,
+    "a package's computed specifier preserves createRequire fallback": 708,
+    "a package's destructured reassignment preserves createRequire fallback": 769,
+    "a package's early declarator call preserves createRequire fallback": 722,
+    "a package's early hoisted call preserves createRequire fallback": 754,
+    "a package's early let call preserves createRequire fallback": 733,
+    "a package's early shorthand call preserves createRequire fallback": 713,
+    "a package's early var call preserves createRequire fallback": 768,
+    "a package's escaped mutable shorthand preserves createRequire fallback": 701,
+    "a package's escaped require preserves createRequire fallback": 698,
+    "a package's exported mutable alias preserves createRequire fallback": 659,
+    "a package's exported mutable require preserves createRequire fallback": 756,
+    "a package's foreign base preserves createRequire fallback": 718,
+    "a package's literal createRequire calls compile without an engine (llvm)": 2137,
+    "a package's optional resolution preserves createRequire fallback": 738,
+    "a package's reassigned require preserves createRequire fallback": 718,
+    "a package's redeclared require preserves createRequire fallback": 729,
+    "a package's shadowed filename preserves createRequire fallback": 730,
+    "a preflight-refused package reports its native refusal": 608,
+    "a workspace-linked package classifies as an npm import without flags": 342,
+    "a workspace-linked package compiles statically under --npm-static": 1490,
+    "an ineligible untyped package is refused alongside a compiled dependency": 566,
+    "auto preserves safe declaration overloads while compiling the JavaScript body": 2164,
+    "bundled class aliases preserve declared methods and callback fields (llvm)": 5433,
+    "bundled CommonJS function imports compile without an engine (llvm)": 2234,
+    "bundler-emitted CJS 2465-getter-table compiles statically and byte-matches Node": 1713,
+    "bundler-emitted CJS 2466-getter-star compiles statically and byte-matches Node": 1570,
+    "bundler-emitted CJS 2467-star-barrel compiles statically and byte-matches Node": 1587,
+    "bundler-emitted CJS 2468-defineprop-exports compiles statically and byte-matches Node": 1484,
+    "bundler-emitted CJS 2556-toesm-external compiles statically and byte-matches Node": 1645,
+    "commander compiles statically and byte-matches Node": 23670,
+    "commander computed option listener registration compiles statically and byte-matches Node": 7303,
+    "conditional exports keep distinct import and require behavior": 4421,
+    "conflicting public class aliases keep the JavaScript return inference": 598,
+    "createRequire loads an opted-in package through its require entry": 3154,
+    "dualist compiles statically and byte-matches Node": 1896,
+    "effect4-shape-match-metadata.ts preserves native object layout and module initialization": 1990,
+    "effect4-shape-prototype-factory.ts preserves native object layout and module initialization": 1790,
+    "effect4-shape-semantics.ts preserves native object layout and module initialization": 9277,
+    "effect4-shape-string-constructor.ts preserves native object layout and module initialization": 2844,
+    "escape-string-regexp compiles statically and byte-matches Node": 1619,
+    "hoisted CommonJS function exports stay static (auto)": 2139,
+    "hoisted CommonJS function exports stay static (explicit)": 2187,
+    "inferred class methods dispatch through inherited checked receivers (dev)": 1752,
+    "inferred class methods dispatch through inherited checked receivers (release)": 2240,
+    "inferred helpers preserve native array storage (dev)": 1171,
+    "inferred helpers preserve native array storage (release)": 1217,
+    "inferred variadic factories preserve checked callable results": 1231,
+    "ms compiles statically and byte-matches Node": 2306,
+    "ms supports invalid duration strings without runtime fences": 691,
+    "opted-in npm packages retain imports deeper than four JavaScript modules": 1799,
+    "picocolors compiles fully statically and byte-matches both color branches": 3698,
+    "picocolors inherited hasOwnProperty matches Node": 3008,
+    "picocolors inherited valueOf retains the prototype-method fence": 708,
+    "published type-only declarations coexist with a static runtime bundle": 1829,
+    "purebarrel namespace re-exports byte-match Node": 1449,
+    "recursive inferred JavaScript results retain their promised ABI": 1369,
+    "renderer startup 'async-backend-factory.mjs' compiles from shipped JavaScript ('llvm')": 2230,
+    "renderer startup 'checked-renderer-defaults.js' compiles from shipped JavaScript ('llvm')": 2359,
+    "renderer startup 'destructure-inferred-row-defaults.js' compiles from shipped JavaScript ('llvm')": 2313,
+    "renderer startup 'fresh-array-union-layout.js' compiles from shipped JavaScript ('llvm')": 2260,
+    "renderer startup 'frozen-checked-dictionaries.mjs' compiles from shipped JavaScript ('llvm')": 2579,
+    "renderer startup 'object-from-checked-entries.js' compiles from shipped JavaScript ('llvm')": 3447,
+    "renderer startup 'regex-checked-storage.js' compiles from shipped JavaScript ('llvm')": 2648,
+    "renderer startup 'renderer-export-dictionaries.js' compiles from shipped JavaScript ('llvm')": 2405,
+    "renderer startup 'renderer-specializations.js' compiles from shipped JavaScript ('llvm')": 2578,
+    "renderer startup 'set-checked-methods.js' compiles from shipped JavaScript ('llvm')": 2375,
+    "renderer startup 'stored-native-builtins.js' compiles from shipped JavaScript ('llvm')": 2495,
+    "renderer startup 'stored-object-helpers.js' compiles from shipped JavaScript ('llvm')": 2509,
+    "renderer values and callbacks compile from shipped JavaScript (llvm)": 5773,
+    "selectivebarrel-cli.ts preserves native object layout and module initialization": 1772,
+    "slash compiles statically and byte-matches Node": 1646,
+    "statefulbarrel namespace re-exports byte-match Node": 1427,
+    "the crypto utility package compiles fully statically and byte-matches Node": 2554,
+    "the published Effect Function subpath initializes and runs statically": 9097,
+    "the published Effect generator has deterministic native results": 105577,
+    "the published Effect generator logs and completes statically": 112814,
+    "untyped package methods preserve virtual overrides (llvm)": 3065
+  },
+  "san": {
+    "__toESM dependencies compile recursively without explicit selection": 458,
+    "--npm-static=auto degrades a deviant __toESM package with the same note": 370,
+    "--npm-static=auto opts a getter-table bundle in": 317,
+    "--npm-static=auto opts the eligible pilot in": 524,
+    "--npm-static=auto probes runtime JS from the importing file's realm": 2349,
+    "--npm-static=auto refuses a package with no own .d.ts": 424,
+    "a cyclic package tree retains an impure dependency through an unused namespace re-export": 1505,
+    "a deviant __toESM helper refuses the package with a construct-naming note": 427,
+    "a package's computed specifier preserves createRequire fallback": 623,
+    "a package's destructured reassignment preserves createRequire fallback": 585,
+    "a package's early declarator call preserves createRequire fallback": 559,
+    "a package's early hoisted call preserves createRequire fallback": 531,
+    "a package's early let call preserves createRequire fallback": 599,
+    "a package's early shorthand call preserves createRequire fallback": 545,
+    "a package's early var call preserves createRequire fallback": 586,
+    "a package's escaped mutable shorthand preserves createRequire fallback": 596,
+    "a package's escaped require preserves createRequire fallback": 566,
+    "a package's exported mutable alias preserves createRequire fallback": 565,
+    "a package's exported mutable require preserves createRequire fallback": 515,
+    "a package's foreign base preserves createRequire fallback": 551,
+    "a package's literal createRequire calls compile without an engine (llvm)": 2051,
+    "a package's optional resolution preserves createRequire fallback": 605,
+    "a package's reassigned require preserves createRequire fallback": 535,
+    "a package's redeclared require preserves createRequire fallback": 644,
+    "a package's shadowed filename preserves createRequire fallback": 626,
+    "a preflight-refused package reports its native refusal": 659,
+    "a workspace-linked package classifies as an npm import without flags": 340,
+    "a workspace-linked package compiles statically under --npm-static": 1446,
+    "an ineligible untyped package is refused alongside a compiled dependency": 552,
+    "auto preserves safe declaration overloads while compiling the JavaScript body": 2370,
+    "bundled class aliases preserve declared methods and callback fields (llvm)": 4728,
+    "bundled CommonJS function imports compile without an engine (llvm)": 2088,
+    "bundler-emitted CJS 2465-getter-table compiles statically and byte-matches Node": 1490,
+    "bundler-emitted CJS 2466-getter-star compiles statically and byte-matches Node": 1325,
+    "bundler-emitted CJS 2467-star-barrel compiles statically and byte-matches Node": 1370,
+    "bundler-emitted CJS 2468-defineprop-exports compiles statically and byte-matches Node": 1361,
+    "bundler-emitted CJS 2556-toesm-external compiles statically and byte-matches Node": 1608,
+    "commander compiles statically and byte-matches Node": 32614,
+    "commander computed option listener registration compiles statically and byte-matches Node": 7719,
+    "conditional exports keep distinct import and require behavior": 5017,
+    "conflicting public class aliases keep the JavaScript return inference": 482,
+    "createRequire loads an opted-in package through its require entry": 2581,
+    "dualist compiles statically and byte-matches Node": 1864,
+    "effect4-shape-match-metadata.ts preserves native object layout and module initialization": 2106,
+    "effect4-shape-prototype-factory.ts preserves native object layout and module initialization": 1612,
+    "effect4-shape-semantics.ts preserves native object layout and module initialization": 23336,
+    "effect4-shape-string-constructor.ts preserves native object layout and module initialization": 2806,
+    "escape-string-regexp compiles statically and byte-matches Node": 1816,
+    "hoisted CommonJS function exports stay static (auto)": 1955,
+    "hoisted CommonJS function exports stay static (explicit)": 1983,
+    "inferred class methods dispatch through inherited checked receivers (dev)": 1954,
+    "inferred class methods dispatch through inherited checked receivers (release)": 2111,
+    "inferred helpers preserve native array storage (dev)": 1189,
+    "inferred helpers preserve native array storage (release)": 1079,
+    "inferred variadic factories preserve checked callable results": 1157,
+    "ms compiles statically and byte-matches Node": 2387,
+    "ms supports invalid duration strings without runtime fences": 669,
+    "opted-in npm packages retain imports deeper than four JavaScript modules": 2301,
+    "picocolors compiles fully statically and byte-matches both color branches": 2954,
+    "picocolors inherited hasOwnProperty matches Node": 2682,
+    "picocolors inherited valueOf retains the prototype-method fence": 535,
+    "published type-only declarations coexist with a static runtime bundle": 1684,
+    "purebarrel namespace re-exports byte-match Node": 1308,
+    "recursive inferred JavaScript results retain their promised ABI": 1628,
+    "renderer startup 'async-backend-factory.mjs' compiles from shipped JavaScript ('llvm')": 2063,
+    "renderer startup 'checked-renderer-defaults.js' compiles from shipped JavaScript ('llvm')": 2539,
+    "renderer startup 'destructure-inferred-row-defaults.js' compiles from shipped JavaScript ('llvm')": 2301,
+    "renderer startup 'fresh-array-union-layout.js' compiles from shipped JavaScript ('llvm')": 2117,
+    "renderer startup 'frozen-checked-dictionaries.mjs' compiles from shipped JavaScript ('llvm')": 2414,
+    "renderer startup 'object-from-checked-entries.js' compiles from shipped JavaScript ('llvm')": 2968,
+    "renderer startup 'regex-checked-storage.js' compiles from shipped JavaScript ('llvm')": 2997,
+    "renderer startup 'renderer-export-dictionaries.js' compiles from shipped JavaScript ('llvm')": 2440,
+    "renderer startup 'renderer-specializations.js' compiles from shipped JavaScript ('llvm')": 2435,
+    "renderer startup 'set-checked-methods.js' compiles from shipped JavaScript ('llvm')": 2176,
+    "renderer startup 'stored-native-builtins.js' compiles from shipped JavaScript ('llvm')": 2283,
+    "renderer startup 'stored-object-helpers.js' compiles from shipped JavaScript ('llvm')": 2164,
+    "renderer values and callbacks compile from shipped JavaScript (llvm)": 4668,
+    "selectivebarrel-cli.ts preserves native object layout and module initialization": 1725,
+    "slash compiles statically and byte-matches Node": 2173,
+    "statefulbarrel namespace re-exports byte-match Node": 1246,
+    "the crypto utility package compiles fully statically and byte-matches Node": 3064,
+    "the published Effect Function subpath initializes and runs statically": 8280,
+    "the published Effect generator has deterministic native results": 136502,
+    "the published Effect generator logs and completes statically": 181378,
+    "untyped package methods preserve virtual overrides (llvm)": 2730
+  }
+}
diff --git a/tests/harness/npm-static.test.ts b/tests/harness/npm-static.test.ts
index b6e451314..8de3886c8 100644
--- a/tests/harness/npm-static.test.ts
+++ b/tests/harness/npm-static.test.ts
@@ -12,8 +12,9 @@ import {
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { promisify } from "node:util";
-import { describe, expect, test } from "vitest";
+import { beforeEach, describe, expect, test } from "vitest";
 import { analyze, compile } from "@scriptc/compiler";
+import { skipUnownedTest } from "./shard.js";
 
 const execFileAsync = promisify(execFile);
 const repoRoot = join(import.meta.dirname, "../..");
@@ -21,6 +22,13 @@ const fixturesRoot = join(repoRoot, "tests/fixtures");
 const pilotRoot = join(fixturesRoot, "npm-static");
 const cacheDir = join(repoRoot, "node_modules/.cache/scriptc-tests");
 const sanitize = process.env["SCRIPTC_SAN"] === "1";
+// Measured milliseconds per test on a CI runner (scripts/ci-test-costs.mjs).
+const testCosts = (
+  JSON.parse(readFileSync(new URL("./npm-static-costs.json", import.meta.url), "utf8")) as {
+    plain: Record<string, number>;
+    san: Record<string, number>;
+  }
+)[sanitize ? "san" : "plain"];
 
 interface RunResult {
   stdout: Buffer;
@@ -99,6 +107,9 @@ async function buildStatic(entry: string, npmStatic: string[] | "auto"): Promise
 }
 
 describe(`npm-static pilots${sanitize ? " (sanitized)" : ""}`, () => {
+  // CI runs this file on every Linux shard; each runs its share of the tests.
+  beforeEach((context) => skipUnownedTest(context, testCosts));
+
   test.each([
     ["effect4-shape-string-constructor.ts", "effect4-shapes"],
     ["effect4-shape-prototype-factory.ts", "effect4-shapes"],
diff --git a/tests/harness/shard.test.ts b/tests/harness/shard.test.ts
index ea1c10e4d..e645f2cac 100644
--- a/tests/harness/shard.test.ts
+++ b/tests/harness/shard.test.ts
@@ -13,6 +13,7 @@ import {
   shardOf,
   shardSelect,
   shardSuffix,
+  skipUnownedTest,
 } from "./shard.js";
 import corpusCosts from "./corpus-costs.json";
 import fixtureCosts from "./effect4-costs.json";
@@ -208,6 +209,41 @@ describe("balancedShardSelect", () => {
   });
 });
 
+test("skipUnownedTest runs each test of a suite on exactly one shard, longest apart", () => {
+  // Two tests share a title, as test.each titles without parameters do.
+  const names = [...Array.from({ length: 39 }, (_, index) => `case ${index}`), "case 0"];
+  // Most tests measured at a second, two long ones, and the last five unmeasured.
+  const costs: Record<string, number> = Object.fromEntries(
+    names.slice(0, 35).map((name) => [name, 1_000]),
+  );
+  costs["case 3"] = 200_000;
+  costs["case 7"] = 150_000;
+  // One suite per process, as in a CI shard: the owners are computed once per suite.
+  const runs = (spec: { index: number; count: number } | undefined) => {
+    const suite = { tasks: [] as { name: string }[] };
+    suite.tasks = names.map((name) => ({ type: "test", name, suite }));
+    return suite.tasks
+      .filter((task) => {
+        let skipped = false;
+        skipUnownedTest(
+          { task, skip: () => (skipped = true) } as unknown as Parameters<
+            typeof skipUnownedTest
+          >[0],
+          costs,
+          spec,
+        );
+        return !skipped;
+      })
+      .map((task) => task.name);
+  };
+  expect(runs(undefined)).toEqual(names);
+  const shards = Array.from({ length: 5 }, (_, index) => runs({ index: index + 1, count: 5 }));
+  expect(shards.flat().toSorted()).toEqual(names.toSorted());
+  // The two long tests hold shards of their own.
+  expect(shards.find((shard) => shard.includes("case 3"))).toEqual(["case 3"]);
+  expect(shards.find((shard) => shard.includes("case 7"))).toEqual(["case 7"]);
+});
+
 describe("corpusSelect", () => {
   const programs = ["ts", "js", "mjs", "cjs"]
     .flatMap((ext) => [
diff --git a/tests/harness/shard.ts b/tests/harness/shard.ts
index 460a78df3..4c3b9c244 100644
--- a/tests/harness/shard.ts
+++ b/tests/harness/shard.ts
@@ -17,6 +17,7 @@
  */
 import { createHash } from "node:crypto";
 import { readFileSync } from "node:fs";
+import type { Suite, Test, TestContext } from "vitest";
 
 export interface ShardSpec {
   /** 1-based shard index. */
@@ -122,3 +123,44 @@ export function corpusSelect<T>(
 export function shardSuffix(spec: ShardSpec | undefined = parseShardSpec()): string {
   return spec === undefined ? "" : `, shard ${spec.index}/${spec.count}`;
 }
+
+const ownedTests = new WeakMap<Suite, Set<Test>>();
+
+/** For a suite of independent tests, called from beforeEach: skips the tests
+ * this shard does not own. The suite's tests are balanced across the shards
+ * by their measured cost (`costs`: milliseconds by test name; a test without
+ * a measurement weighs as much as the median measured one), so every shard
+ * runs a similar share and the union of the shards runs each test once. */
+export function skipUnownedTest(
+  context: TestContext,
+  costs: Readonly<Record<string, number>>,
+  spec: ShardSpec | undefined = parseShardSpec(),
+): void {
+  const suite = context.task.suite;
+  if (spec === undefined || spec.count === 1 || suite === undefined) return;
+  let owned = ownedTests.get(suite);
+  if (owned === undefined) {
+    const known = Object.values(costs).sort((a, b) => a - b);
+    const median = known.length === 0 ? 1 : known[Math.floor(known.length / 2)]!;
+    const tests = suite.tasks.filter((task): task is Test => task.type === "test");
+    // test.each titles without their parameters repeat; later repeats get "#2", "#3"... as their key.
+    const seen = new Map<string, number>();
+    const keys = new Map(
+      tests.map((test) => {
+        const count = (seen.get(test.name) ?? 0) + 1;
+        seen.set(test.name, count);
+        return [test, count === 1 ? test.name : `${test.name} #${count}`];
+      }),
+    );
+    owned = new Set(
+      balancedShardSelect(
+        tests,
+        (test) => keys.get(test)!,
+        (test) => Math.max(1, costs[test.name] ?? median),
+        spec,
+      ),
+    );
+    ownedTests.set(suite, owned);
+  }
+  if (!owned.has(context.task)) context.skip();
+}