diff --git a/src/javascript/recovery/RecoveryFiles.test.ts b/src/javascript/recovery/RecoveryFiles.test.ts
index 574699abd5..10c2c5d689 100644
--- a/src/javascript/recovery/RecoveryFiles.test.ts
+++ b/src/javascript/recovery/RecoveryFiles.test.ts
@@ -1,9 +1,12 @@
 import { execFile } from "node:child_process";
-import { writeFile } from "node:fs/promises";
+import { mkdir, stat, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 import { promisify } from "node:util";
 import { expect, it, vi } from "vitest";
-import { AnalysisOutputError } from "../../domain/analysisErrorCore.js";
+import {
+  AnalysisInputError,
+  AnalysisOutputError,
+} from "../../domain/analysisErrorCore.js";
 import { OwnedFileHandle } from "../../filesystem/OwnedFileHandle.js";
 import {
   openRegularFile,
@@ -11,7 +14,7 @@ import {
 } from "../../filesystem/RegularFile.js";
 import { readWithoutFifoWriter } from "../../../tests/fixtures/fifoInput.js";
 import { createTestTempDirectory } from "../../../tests/fixtures/temporaryDirectory.js";
-import { readRecoveryFile } from "./RecoveryFiles.js";
+import { readRecoveryFile, snapshotRecoveryInputs } from "./RecoveryFiles.js";
 
 vi.mock("../../filesystem/RegularFile.js", async (importOriginal) => {
   const actual =
@@ -19,6 +22,49 @@ vi.mock("../../filesystem/RegularFile.js", async (importOriginal) => {
   return { ...actual, openRegularFile: vi.fn(actual.openRegularFile) };
 });
 
+// Fault injection for the snapshot write; other calls use the real filesystem.
+const snapshotFault = vi.hoisted(() => ({
+  mkdir: undefined as Error | undefined,
+  writeFile: undefined as Error | undefined,
+}));
+
+vi.mock("node:fs/promises", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs/promises")>();
+  return {
+    ...actual,
+    mkdir: async (...args: Parameters<typeof actual.mkdir>) => {
+      if (snapshotFault.mkdir !== undefined) throw snapshotFault.mkdir;
+      return actual.mkdir(...args);
+    },
+    writeFile: async (...args: Parameters<typeof actual.writeFile>) => {
+      if (snapshotFault.writeFile !== undefined) throw snapshotFault.writeFile;
+      return actual.writeFile(...args);
+    },
+  };
+});
+
+const filesystemError = (code: string): NodeJS.ErrnoException =>
+  Object.assign(new Error(`${code}: injected snapshot failure`), { code });
+
+const snapshotWithFault = async (
+  fault: Partial<typeof snapshotFault>,
+): Promise<unknown> => {
+  const root = await createTestTempDirectory("rea-recovery-fault-");
+  const selected = join(root, "bundle.js");
+  await writeFile(selected, "module.exports = 1;\n");
+  const snapshots = join(root, "snapshots");
+  await mkdir(snapshots);
+  Object.assign(snapshotFault, fault);
+  try {
+    return await snapshotRecoveryInputs([selected], snapshots).catch(
+      (cause: unknown) => cause,
+    );
+  } finally {
+    snapshotFault.mkdir = undefined;
+    snapshotFault.writeFile = undefined;
+  }
+};
+
 it.skipIf(process.platform === "win32")(
   "rejects a named pipe without waiting for a writer",
   async () => {
@@ -53,3 +99,78 @@ it("keeps a failed admission close owned when cancellation is also requested", a
     readRecoveryFile("/selected.js", 1024, controller.signal),
   ).rejects.toBe(failure);
 });
+
+it("reports one file selected under two letter cases as an input error", async (context) => {
+  const root = await createTestTempDirectory("rea-recovery-case-");
+  const lower = join(root, "bundle.js");
+  const upper = join(root, "BUNDLE.js");
+  await writeFile(lower, "module.exports = 1;\n");
+  // Only a case-insensitive filesystem, such as the default Windows and macOS
+  // ones, resolves both spellings to the selected file.
+  if (
+    !(await stat(upper).then(
+      () => true,
+      () => false,
+    ))
+  )
+    context.skip();
+  const snapshots = join(root, "snapshots");
+  await mkdir(snapshots);
+
+  const failure = await snapshotRecoveryInputs([lower, upper], snapshots).catch(
+    (cause: unknown) => cause,
+  );
+
+  expect(failure).toBeInstanceOf(AnalysisInputError);
+  expect((failure as AnalysisInputError).issues).toEqual([
+    expect.objectContaining({
+      path: ["additional_paths"],
+      message: expect.stringContaining(upper),
+    }),
+  ]);
+});
+
+it("reports one file selected twice as an input error", async () => {
+  const root = await createTestTempDirectory("rea-recovery-twice-");
+  const selected = join(root, "bundle.js");
+  await writeFile(selected, "module.exports = 1;\n");
+  const snapshots = join(root, "snapshots");
+  await mkdir(snapshots);
+
+  const failure = await snapshotRecoveryInputs(
+    [selected, selected],
+    snapshots,
+  ).catch((cause: unknown) => cause);
+
+  expect(failure).toBeInstanceOf(AnalysisInputError);
+  expect((failure as AnalysisInputError).issues).toEqual([
+    expect.objectContaining({
+      path: ["additional_paths"],
+      message: expect.stringContaining("collides with another selected input"),
+    }),
+  ]);
+  // The filesystem's own report stays attached for diagnosis.
+  expect((failure as AnalysisInputError).cause).toMatchObject({
+    code: "EEXIST",
+  });
+});
+
+it("reports a file and directory name clash as an input error", async () => {
+  const clash = filesystemError("ENOTDIR");
+
+  const failure = await snapshotWithFault({ mkdir: clash });
+
+  expect(failure).toBeInstanceOf(AnalysisInputError);
+  expect((failure as AnalysisInputError).issues).toEqual([
+    expect.objectContaining({ path: ["additional_paths"] }),
+  ]);
+  expect((failure as AnalysisInputError).cause).toBe(clash);
+});
+
+it("keeps an unrelated snapshot write failure unchanged", async () => {
+  const fullDisk = filesystemError("ENOSPC");
+
+  await expect(snapshotWithFault({ writeFile: fullDisk })).resolves.toBe(
+    fullDisk,
+  );
+});
diff --git a/src/javascript/recovery/RecoveryFiles.ts b/src/javascript/recovery/RecoveryFiles.ts
index cfc4c7ef32..28571a7345 100644
--- a/src/javascript/recovery/RecoveryFiles.ts
+++ b/src/javascript/recovery/RecoveryFiles.ts
@@ -194,8 +194,25 @@ const snapshotRecoveryInput = async (
       },
     ]);
   }
-  await mkdir(dirname(snapshotPath), { recursive: true, mode: 0o700 });
-  await writeFile(snapshotPath, bytes, { flag: "wx", mode: 0o600 });
+  try {
+    await mkdir(dirname(snapshotPath), { recursive: true, mode: 0o700 });
+    await writeFile(snapshotPath, bytes, { flag: "wx", mode: 0o600 });
+  } catch (cause: unknown) {
+    // The snapshot directory starts empty, so a collision means another
+    // selected input already took this name: the same file under another
+    // spelling, or a name that differs only in a way this filesystem ignores.
+    if (
+      cause instanceof Error &&
+      "code" in cause &&
+      (cause.code === "EEXIST" || cause.code === "ENOTDIR")
+    )
+      throw recoveryInputFailure(
+        "additional_paths",
+        `Selected input ${path} collides with another selected input at ${relativePath} on this filesystem. Select each file once.`,
+        cause,
+      );
+    throw cause;
+  }
   return {
     path,
     snapshot_path: snapshotPath,