diff --git a/packages/amico-run/test/coordination-ledger.contract.test.ts b/packages/amico-run/test/coordination-ledger.contract.test.ts
index f127e3d2..98629cb5 100644
--- a/packages/amico-run/test/coordination-ledger.contract.test.ts
+++ b/packages/amico-run/test/coordination-ledger.contract.test.ts
@@ -1,10 +1,50 @@
 // Contract suite for coordination ledger (spec #318) — runs against both cloud and sqlite ref
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, beforeAll, afterAll } from "vitest";
+import { existsSync, mkdtempSync, readFileSync, rmSync } from "node:fs";
+import { homedir, tmpdir } from "node:os";
+import { join } from "node:path";
 import { coordinationService, SqliteCoordinationService, workId } from "../src/coordination_ledger.js";
 import { degradedStamp } from "../src/coordination_ledger.js";
 
 const wid = (s: string) => workId({ structure_hash: s, goal: "CZ", N: 100, T: 30 });
 
+// ── isolation bridge (#642) ─────────────────────────────────────────────────
+// Claim appends made by this suite must land in a per-run tmp partition and
+// NEVER in the production claims ledger (~/.amico/ledger/claims.jsonl) — the
+// Prova simulated-ledger-partition pattern from docs/ledger.md, via the
+// $AMICO_CLAIMS_FILE seam the service reads per append. Before the bridge, the
+// suite constructed the service bare with the env unset, so every contract run
+// appended fixture rows to the real ledger: 176 rows before ops archived it as
+// claims.jsonl.archive-20260830-test-pollution (#642).
+const PRODUCTION_CLAIMS = join(homedir(), ".amico", "ledger", "claims.jsonl");
+// Snapshot at module load — before any test in this file has run.
+const productionSnapshot = existsSync(PRODUCTION_CLAIMS)
+  ? readFileSync(PRODUCTION_CLAIMS).toString("base64")
+  : null;
+const ISO_ROOT = mkdtempSync(join(tmpdir(), "amico-claims-iso-"));
+const ISOLATED_CLAIMS = join(ISO_ROOT, "claims.jsonl");
+
+const productionNow = (): string | null =>
+  existsSync(PRODUCTION_CLAIMS) ? readFileSync(PRODUCTION_CLAIMS).toString("base64") : null;
+
+// The bridge itself: route EVERY service construction in this file at the
+// isolated partition. The env is the seam claimsFile() reads per append, so
+// bare constructions and future ones are covered equally; production callers
+// never set it and keep their default path.
+const prevClaimsEnv = process.env.AMICO_CLAIMS_FILE;
+beforeAll(() => {
+  process.env.AMICO_CLAIMS_FILE = ISOLATED_CLAIMS;
+});
+afterAll(() => {
+  // Suite-wide guard, order-robust: after every test has run, the production
+  // ledger is still pristine (byte-identical, or still absent — CI and local).
+  expect(productionNow()).toBe(productionSnapshot);
+  // Never leak process state across suites (vitest reuses forked workers).
+  if (prevClaimsEnv === undefined) delete process.env.AMICO_CLAIMS_FILE;
+  else process.env.AMICO_CLAIMS_FILE = prevClaimsEnv;
+  rmSync(ISO_ROOT, { recursive: true, force: true });
+});
+
 describe("coordination ledger — claim serialization (spec §3)", () => {
   it("simultaneous claims serialize by receipt order; loser gets holder", async () => {
     const svc = new SqliteCoordinationService();
@@ -52,3 +92,16 @@ describe("coordination ledger — claim serialization (spec §3)", () => {
     expect(list.some(s => s.user === "alice" && s.host === "h1")).toBe(true);
   });
 });
+
+describe("coordination ledger — test isolation (#642)", () => {
+  it("claim appends land in the per-run tmp partition, never the production claims ledger", async () => {
+    const svc = new SqliteCoordinationService();
+    const r = await svc.preflight({ work_id: wid("iso-guard"), agent_id: "iso", user: "iso", org: "iso", host: "iso" });
+    expect(r.ok).toBe(true);
+    // the durable append still works — the claim line landed in the isolated partition
+    const written = existsSync(ISOLATED_CLAIMS) ? readFileSync(ISOLATED_CLAIMS, "utf8") : "";
+    expect(written).toContain(wid("iso-guard"));
+    // and the production ledger is byte-identical to its pre-suite snapshot (or still absent)
+    expect(productionNow()).toBe(productionSnapshot);
+  });
+});
diff --git a/packages/amico-run/test/setup.ts b/packages/amico-run/test/setup.ts
index 4a2596a8..cc6efffe 100644
--- a/packages/amico-run/test/setup.ts
+++ b/packages/amico-run/test/setup.ts
@@ -21,3 +21,13 @@ process.env.AMICO_CRITIC_BIN = "/nonexistent/amico-test-guard/no-real-model-call
 if (!process.env.AMICO_LEDGER) {
   process.env.AMICO_LEDGER = "/nonexistent/amico-test-guard/ledger.jsonl";
 }
+
+// And for the coordination claims ledger ($AMICO_CLAIMS_FILE → ~/.amico/ledger/claims.jsonl):
+// the coordination-ledger contract suite constructed its service bare, and every run appended
+// fixture rows to the real claims ledger — 176 rows before ops archived it 2026-08-30 as
+// claims.jsonl.archive-20260830-test-pollution (#642; no in-repo action needed beyond this
+// guard). The suite now routes its writes through its own per-run tmp partition; this backstop
+// fails closed for the next suite that forgets.
+if (!process.env.AMICO_CLAIMS_FILE) {
+  process.env.AMICO_CLAIMS_FILE = "/nonexistent/amico-test-guard/claims.jsonl";
+}