diff --git a/src/cliJsonInput.ts b/src/cliJsonInput.ts
index 8bb2d105a5..9081b875e7 100644
--- a/src/cliJsonInput.ts
+++ b/src/cliJsonInput.ts
@@ -128,10 +128,26 @@ const jsonFileError = (
         cause === undefined ? undefined : { cause },
         reason === "invalid-json"
           ? [{ path: [], reason: "invalid_format", expected: "JSON" }]
-          : [],
+          : [readFailureIssue(path, cause)],
       ),
     ),
     ...(path === undefined ? {} : { input_path: path }),
     input_reason: reason,
   },
 });
+
+/** Name the system error so the caller can tell a typo from a permission denial. */
+const readFailureIssue = (path: string | undefined, cause: unknown) => {
+  const code =
+    typeof cause === "object" &&
+    cause !== null &&
+    "code" in cause &&
+    typeof cause.code === "string"
+      ? ` (${cause.code})`
+      : "";
+  return {
+    path: [],
+    reason: "invalid_value" as const,
+    message: `The JSON input file could not be read${code}: ${path ?? ""}`,
+  };
+};
diff --git a/tests/boundary/filesystem/cliJsonInput.test.ts b/tests/boundary/filesystem/cliJsonInput.test.ts
index 406f14b667..1095840e0e 100644
--- a/tests/boundary/filesystem/cliJsonInput.test.ts
+++ b/tests/boundary/filesystem/cliJsonInput.test.ts
@@ -1,7 +1,7 @@
-import { writeFile } from "node:fs/promises";
+import { chmod, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, onTestFinished } from "vitest";
 
 import { parseCliJsonInput } from "../../../src/cliJsonInput.js";
 import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
@@ -50,4 +50,52 @@ describe("CLI JSON input", () => {
       error: { input_reason: "read-failed" },
     });
   });
+
+  it.each([
+    ["a missing file", "ENOENT"],
+    ["a directory", "EISDIR"],
+  ] as const)(
+    "lists the system cause when %s cannot be read",
+    async (kind, code) => {
+      const root = await createTestTempDirectory("rea-json-input-cause-");
+      const path = kind === "a directory" ? root : join(root, "missing.json");
+      expect(await parseCliJsonInput(path, "test-input")).toMatchObject({
+        ok: false,
+        error: {
+          code: "invalid_request",
+          input_path: path,
+          input_reason: "read-failed",
+          details: {
+            issues: [
+              {
+                path: [],
+                reason: "invalid_value",
+                message: `The JSON input file could not be read (${code}): ${path}`,
+              },
+            ],
+          },
+        },
+      });
+    },
+  );
+
+  it.skipIf(process.platform === "win32" || process.getuid?.() === 0)(
+    "lists a permission denial as the cause of an unreadable file",
+    async () => {
+      const root = await createTestTempDirectory("rea-json-input-denied-");
+      const path = join(root, "input.json");
+      await writeFile(path, "{}");
+      await chmod(path, 0o000);
+      onTestFinished(() => chmod(path, 0o600));
+      expect(await parseCliJsonInput(path, "test-input")).toMatchObject({
+        ok: false,
+        error: {
+          input_reason: "read-failed",
+          details: {
+            issues: [{ message: expect.stringContaining("(EACCES)") }],
+          },
+        },
+      });
+    },
+  );
 });