diff --git a/packages/cli-conformance/src/checks/tarball.ts b/packages/cli-conformance/src/checks/tarball.ts
index ffdfa48a..8ce15c41 100644
--- a/packages/cli-conformance/src/checks/tarball.ts
+++ b/packages/cli-conformance/src/checks/tarball.ts
@@ -44,7 +44,8 @@ export interface TarballIo {
     readonly rootTarball: string;
     /** Version-qualified name → absolute `file:` tarball path. */
     readonly overrides: Readonly<Record<string, string>>;
-  }): Promise<{ ok: true } | { ok: false; output: string }>;
+    readonly timeoutMs: number;
+  }): Promise<{ ok: true } | { ok: false; timedOut: boolean; output: string }>;
   readInstalledManifest(
     sandboxDir: string,
     name: string,
@@ -97,6 +98,13 @@ export interface TarballInput {
   readonly exceptions: readonly PinException[];
   readonly sandboxDir: string;
   readonly binTimeoutMs?: number;
+  /**
+   * npm does not fail on a peer range no published version satisfies;
+   * it backtracks for hours (2026-09-11: `@effect/*@4.0.0-rc.114` shipped
+   * ahead of `effect`, and every install of the shells ran until CI was
+   * cancelled). The cap turns that into a finding with npm's output.
+   */
+  readonly installTimeoutMs?: number;
   /**
    * Which channel these tarballs are for. Check 4 measures the packed
    * manifests against it; a dev publish is allowed its dev builds.
@@ -326,11 +334,28 @@ async function sandboxFindings(
     }
   };
   visit(root.manifest);
+  const timeoutMs = input.installTimeoutMs ?? 300_000;
   const install = await io.installSandbox({
     sandboxDir,
     rootTarball: root.tarball,
     overrides,
+    timeoutMs,
   });
+  if (!install.ok && install.timedOut) {
+    return [
+      finding(
+        "install-timed-out",
+        packageName,
+        `npm install did not finish within ${timeoutMs / 1000}s and was killed`,
+        "npm is backtracking rather than failing: a dependency in the tree " +
+          "resolves to a version whose peer range no published version " +
+          "satisfies, and npm retries the resolution instead of reporting " +
+          "ERESOLVE. Find the package npm names in the ERESOLVE warnings " +
+          "below and pin it exactly in the package that pulls it in.\n\n" +
+          install.output,
+      ),
+    ];
+  }
   if (!install.ok) {
     return [
       finding(
diff --git a/packages/cli-conformance/src/findings.ts b/packages/cli-conformance/src/findings.ts
index 9211b51e..e1dfc3b4 100644
--- a/packages/cli-conformance/src/findings.ts
+++ b/packages/cli-conformance/src/findings.ts
@@ -27,6 +27,8 @@ export type FindingKind =
   | "validator-malformed"
   | "pack-failed"
   | "install-failed"
+  /** npm never returned: the resolver is backtracking, not failing. */
+  | "install-timed-out"
   | "bin-failed"
   /** The shell and a family it mounts disagree about the engine version. */
   | "engine-pin-mismatch"
diff --git a/packages/cli-conformance/src/tarball-io.ts b/packages/cli-conformance/src/tarball-io.ts
index 7f97472c..59b5f91a 100644
--- a/packages/cli-conformance/src/tarball-io.ts
+++ b/packages/cli-conformance/src/tarball-io.ts
@@ -87,7 +87,7 @@ export function realTarballIo(
       return files;
     },
 
-    async installSandbox({ sandboxDir, rootTarball, overrides }) {
+    async installSandbox({ sandboxDir, rootTarball, overrides, timeoutMs }) {
       const dir = resolve(sandboxDir);
       rmSync(dir, { recursive: true, force: true });
       mkdirSync(dir, { recursive: true });
@@ -115,11 +115,18 @@ export function realTarballIo(
             cwd: dir,
             env: { ...process.env, COREPACK_ENABLE_STRICT: "0" },
             maxBuffer: 64 * 1024 * 1024,
+            timeout: timeoutMs,
+            killSignal: "SIGKILL",
           },
         );
         return { ok: true as const };
       } catch (error) {
-        return { ok: false as const, output: installErrorOutput(error) };
+        const timedOut = (error as { killed?: boolean }).killed === true;
+        return {
+          ok: false as const,
+          timedOut,
+          output: installErrorOutput(error),
+        };
       }
     },
 
diff --git a/packages/cli-conformance/tests/tarball.test.ts b/packages/cli-conformance/tests/tarball.test.ts
index e805b26c..6f1b41d4 100644
--- a/packages/cli-conformance/tests/tarball.test.ts
+++ b/packages/cli-conformance/tests/tarball.test.ts
@@ -387,6 +387,7 @@ describe("checkTarball", () => {
       installSandbox: () =>
         Promise.resolve({
           ok: false as const,
+          timedOut: false,
           output: "ETIMEDOUT registry.npmjs.org",
         }),
     });
@@ -396,6 +397,38 @@ describe("checkTarball", () => {
     expect(installs[0]?.detail).toContain("ETIMEDOUT");
   });
 
+  test("an install npm never finishes is its own finding, names the cap, and starts no bin", async () => {
+    const started: string[] = [];
+    const io = fakeIo({
+      installSandbox: ({ timeoutMs }) =>
+        Promise.resolve({
+          ok: false as const,
+          timedOut: true,
+          output: `killed after ${timeoutMs}ms\nnpm warn ERESOLVE overriding peer dependency\nnpm warn While resolving: @effect/sql-d1@4.0.0-rc.114`,
+        }),
+      startBin: ({ binName }) => {
+        started.push(binName);
+        return Promise.resolve({
+          exitCode: 0,
+          stdout: "",
+          stderr: "",
+          timedOut: false,
+        });
+      },
+    });
+    const findings = await checkTarball(
+      { ...input(), installTimeoutMs: 120_000 },
+      io,
+    );
+    const timeouts = findings.filter((f) => f.kind === "install-timed-out");
+    expect(timeouts).toHaveLength(1);
+    expect(timeouts[0]?.summary).toContain("120s");
+    expect(timeouts[0]?.detail).toContain("backtracking");
+    expect(timeouts[0]?.detail).toContain("@effect/sql-d1@4.0.0-rc.114");
+    expect(findings.filter((f) => f.kind === "install-failed")).toHaveLength(0);
+    expect(started).toHaveLength(0);
+  });
+
   test("3b: every declared bin is started; a non-zero exit names the bin", async () => {
     const started: string[] = [];
     const io = fakeIo({