A network failure is printed and recorded with its reason (closes #196)

Node's `fetch` rejects every failure below HTTP as `fetch failed` and puts
the reason on `cause`. `describeError` in `src/errors.ts` gives the message
followed by the messages on the cause chain, and for an `AggregateError`,
which Node gives when every address of a host fails, the message of each
address. `run` prints it, and the backup logs and records it. `classify`
reads that text and calls a failure transient when `hasTransportCode`, now
exported by `src/retry.ts`, finds a transport errno on the chain. A failed
ML data fetch keeps its error as the cause. The bounded walk along the
chain moved from `src/retry.ts` to `src/errors.ts`.

A truncated download's message leaves the authentication failure to its
cause.

Model: opus-5-5

README.md

@@ -141,6 +141,18 @@ matters.
 | `2`  | Another backup of the same directory is running. This run sent no request and changed nothing.                                                                                    |
 | `3`  | There is no usable session: none is saved, the saved one is corrupt, or the server no longer accepts it. Run `quak login` as the user the cron job runs as.                       |
 
+`<message>` is the error's message, followed by the message of each error that
+caused it. Under Node, as `yarn quak` runs quak, `fetch` reports every failure
+below HTTP as just `fetch failed` and gives the reason as its cause, so a run
+that could not reach the server prints, for example,
+`quak: fetch failed: connect ECONNREFUSED 1.2.3.4:443`. When the server has
+several addresses and none of them could be reached, the line gives the reason
+for each, separated by `, `. A file whose download failed that way is in
+`failures.json` with that error, as a transient failure. So is each file with no
+ML data cached when the ML data fetch failed that way, its error starting
+`ML data: `. The `~/bin/quak` that `make install` builds runs under bun, and
+prints bun's own wording for such a failure.
+
 After a `1`, the next run fetches each missing original again, fetches the ML
 data that is not cached, and rebuilds the album links. An album quak could not
 decrypt, or a file in `failures.json` because quak could not decrypt it, keeps
@@ -354,7 +366,8 @@ quak/
         exif.ts         EXIF read from an image's bytes with exifreader
         mldata-fetch.ts fetch + decrypt per-file ML data
         filename.ts     safe file names from server metadata
-        errors.ts       error types shared across layers
+        errors.ts       error types shared across layers, and how an error
+                        is described
         retry.ts        retry classifier + exponential backoff with jitter
         thumbnails.ts   detect + regenerate missing thumbnails
         client.ts       high-level Client class assembled from the above

TODO.md

@@ -17,10 +17,11 @@ pre-1.0
 
 # Next Step
 
-Four bugs are open: https://git.eeqj.de/sneak/quak/issues/196,
-https://git.eeqj.de/sneak/quak/issues/197,
-https://git.eeqj.de/sneak/quak/issues/198 and
-https://git.eeqj.de/sneak/quak/issues/199. The check of quak against a live Ente
+Five bugs are open: https://git.eeqj.de/sneak/quak/issues/197,
+https://git.eeqj.de/sneak/quak/issues/198,
+https://git.eeqj.de/sneak/quak/issues/199,
+https://git.eeqj.de/sneak/quak/issues/205 and
+https://git.eeqj.de/sneak/quak/issues/206. The check of quak against a live Ente
 account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
 has, so it waits on him.
 
@@ -29,6 +30,18 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-15: A network failure is printed and recorded with its reason, not as
+  just `fetch failed` (issue 196). `describeError` in `src/errors.ts` gives an
+  error's message followed by the message of each error on its `cause` chain,
+  such as `fetch failed: connect ECONNREFUSED 1.2.3.4:443`, and each address's
+  reason when a host has several and none could be reached. `run` prints that,
+  and the backup logs it and records it in `failures.json`, where `classify` now
+  calls the failure transient when the chain holds an errno that
+  `hasTransportCode` in `src/retry.ts` recognises, for a failed ML data fetch
+  too. The walk along the chain moved from `src/retry.ts` to `src/errors.ts`,
+  which both use. Tests refuse every connection of a backup, then only one
+  file's download, and fail the ML data fetch.
+
 - 2026-10-15: The README says that `make install` needs nix (issue 200), in
   Getting Started and in "Running the backup from cron": `make build-bin` runs
   bun through `nix-shell -p bun`. The `Makefile` is unchanged.

src/backup.ts

@@ -82,6 +82,7 @@ import {
     init,
 } from "./crypto/index.js";
 import { removeLeftoverTempFiles } from "./download/index.js";
+import { describeError } from "./errors.js";
 import { sanitizeFileName, withExtension } from "./filename.js";
 import {
     copyAtomic,
@@ -94,6 +95,7 @@ import type { Undecryptable } from "./library/store.js";
 import { extractImageMetadata } from "./metadata-backup.js";
 import type { MLData } from "./mldata-fetch.js";
 import type { Collection, EnteFile, FileMetadata } from "./model/types.js";
+import { hasTransportCode } from "./retry.js";
 
 export type ProgressCallback = (message: string) => void;
 
@@ -219,11 +221,13 @@ const isPresent = (path: string): boolean => {
 
 // Best-effort classification for the ledger. Retryable server/network problems
 // are transient; refusals and local filesystem/decrypt errors are permanent;
-// anything else is unknown. Both the error code and message are inspected.
+// anything else is unknown. The error code and the error as `describeError`
+// gives it are inspected, and so is each errno on the error's cause chain,
+// where Node's `fetch` puts it.
 const classify = (err: unknown): FailureClass => {
+    if (hasTransportCode(err)) return "transient";
     const e = err as NodeJS.ErrnoException;
-    const text =
-        `${e?.code ?? ""} ${err instanceof Error ? err.message : String(err)}`.toLowerCase();
+    const text = `${e?.code ?? ""} ${describeError(err)}`.toLowerCase();
     if (
         /timeout|timed out|econnreset|econnrefused|econnaborted|network|socket|eai_again|throttl|temporarily|429|500|502|503|504/.test(
             text,
@@ -241,9 +245,6 @@ const classify = (err: unknown): FailureClass => {
     return "unknown";
 };
 
-const errorMessage = (err: unknown): string =>
-    err instanceof Error ? err.message : String(err);
-
 // Ensure `linkPath` is a symlink to `target`, rebuilding a missing, wrong, or
 // non-symlink entry. Throws on failure (a directory in the way, no permission)
 // so the caller records it and moves on rather than aborting the run.
@@ -470,7 +471,7 @@ const imageMetadataFor = async (
         const bytes = await readFile(originalPath);
         return { imageMetadata: extractImageMetadata(bytes) ?? {} };
     } catch (err) {
-        return { imageMetadataError: errorMessage(err) };
+        return { imageMetadataError: describeError(err) };
     }
 };
 
@@ -610,7 +611,7 @@ const runLockedBackup = async (
         // and thumbnail both fail this run must not double its attempt count
         // or appear twice in errors.
         if (failedThisRun.has(fileID)) return;
-        const error = errorMessage(err);
+        const error = describeError(err);
         errors.push({ fileID, title, collection, error });
         const prior = ledger.get(fileID);
         ledger.set(fileID, {
@@ -681,7 +682,7 @@ const runLockedBackup = async (
                 }
             } catch (err) {
                 log(
-                    `FAILED verifying original ${file.metadata.title}: ${errorMessage(err)}`,
+                    `FAILED verifying original ${file.metadata.title}: ${describeError(err)}`,
                 );
                 recordFailure(
                     file.id,
@@ -717,7 +718,9 @@ const runLockedBackup = async (
             }
             downloaded++;
         } catch (err) {
-            log(`FAILED original ${file.metadata.title}: ${errorMessage(err)}`);
+            log(
+                `FAILED original ${file.metadata.title}: ${describeError(err)}`,
+            );
             recordFailure(
                 file.id,
                 file.metadata.title,
@@ -785,12 +788,14 @@ const runLockedBackup = async (
     // instead and is recorded as failed. The next run fetches its ML data
     // again because none is cached.
     if (includeOriginals) {
+        let mlDataFailure: unknown;
         let mlDataError: string | undefined;
         try {
             log("Fetching ML data...");
             await lib.fetchMLData();
         } catch (err) {
-            mlDataError = errorMessage(err);
+            mlDataFailure = err;
+            mlDataError = describeError(err);
             log(`FAILED ML data: ${mlDataError}`);
         }
         for (const file of distinct.values()) {
@@ -814,7 +819,7 @@ const runLockedBackup = async (
                     file.id,
                     file.metadata.title,
                     collectionName.get(file.collectionID) ?? "",
-                    new Error(`ML data: ${mlDataError}`),
+                    new Error("ML data", { cause: mlDataFailure }),
                 );
             } else {
                 writeSidecar(path, file, { mlData }, image);
@@ -846,7 +851,7 @@ const runLockedBackup = async (
             downloadDirectory,
         );
     } catch (err) {
-        log(`FAILED removing old album directories: ${errorMessage(err)}`);
+        log(`FAILED removing old album directories: ${describeError(err)}`);
     }
 
     for (const c of collections) {
@@ -868,7 +873,9 @@ const runLockedBackup = async (
         try {
             removeStaleLinks(colDir, new Set(linkNames), downloadDirectory);
         } catch (err) {
-            log(`FAILED removing old links in ${c.name}: ${errorMessage(err)}`);
+            log(
+                `FAILED removing old links in ${c.name}: ${describeError(err)}`,
+            );
         }
 
         const metaFiles = files.map((f) => ({
@@ -885,7 +892,7 @@ const runLockedBackup = async (
                 );
             } catch (err) {
                 log(
-                    `FAILED symlink ${c.name}/${linkName}: ${errorMessage(err)}`,
+                    `FAILED symlink ${c.name}/${linkName}: ${describeError(err)}`,
                 );
                 recordFailure(
                     link.file.id,

src/cli-run.ts

@@ -2,14 +2,17 @@
 
 import type { Writable } from "node:stream";
 import { ApiError } from "./api/client.js";
+import { describeError } from "./errors.js";
 
 // Run a command and exit with its code once stdout/stderr have drained.
 // Exiting before the drain can truncate piped output, and the library can keep
 // the event loop alive after a command returns, so a plain return could hang.
 // An error the command throws is printed as one `quak: MESSAGE` line, without
-// the stack trace, and exits 1. A 401 from the server means it no longer
-// accepts the saved session: that prints one line saying to log in again and
-// exits 3, as a missing or corrupt session file does.
+// the stack trace, and exits 1. MESSAGE is the error as `describeError` gives
+// it, so it carries the reason Node's `fetch` puts on `cause`. A 401 from the
+// server means it no longer accepts the saved session: that prints one line
+// saying to log in again and exits 3, as a missing or corrupt session file
+// does.
 export const run = async (
     command: Promise<number>,
     stdout: Writable,
@@ -26,9 +29,7 @@ export const run = async (
             );
             code = 3;
         } else {
-            stderr.write(
-                `quak: ${err instanceof Error ? err.message : String(err)}\n`,
-            );
+            stderr.write(`quak: ${describeError(err)}\n`);
             code = 1;
         }
     }

src/download/index.ts

@@ -135,15 +135,17 @@ const streamDecrypt = async (
                     // cannot tell a partial chunk from a corrupt one, so this
                     // is reported as the truncation it almost always is, with
                     // the authentication failure kept as the error's cause.
-                    // Only the pull is guarded: a sink failure on a chunk
-                    // that did authenticate is a disk error, not a
+                    // The message leaves that failure to the cause, which
+                    // `describeError` appends to what `quak backup` prints and
+                    // records. Only the pull is guarded: a sink failure on a
+                    // chunk that did authenticate is a disk error, not a
                     // truncation.
                     let pulled;
                     try {
                         pulled = pullStreamChunk(state, buffer);
                     } catch (err) {
                         throw new TruncatedStreamError(
-                            `download: stream truncated: response body ended with ${buffer.length} trailing bytes that did not authenticate as a final chunk (transfer stopped mid-chunk, or the data is corrupt)`,
+                            `download: stream truncated: response body ended with ${buffer.length} trailing bytes (transfer stopped mid-chunk, or the data is corrupt)`,
                             { cause: err },
                         );
                     }

src/errors.ts

@@ -1,10 +1,13 @@
-// Error types that more than one layer of quak needs to recognise.
+// Error types that more than one layer of quak needs to recognise, and how an
+// error is described in what quak prints and in `failures.json`.
 //
-// They live here rather than beside the code that throws them so that the
+// The types live here rather than beside the code that throws them so that the
 // retry classifier can identify them without importing the HTTP client or the
 // download layer — both of which import the classifier. `ApiError` is
 // re-exported from `src/api/client.ts`, which is where callers have always
-// imported it from and where it still belongs conceptually.
+// imported it from and where it still belongs conceptually. The walk along an
+// error's `cause` chain is here too, because the retry classifier reads the
+// chain for errnos and `describeError` for messages.
 
 export class ApiError extends Error {
     readonly status: number;
@@ -43,3 +46,68 @@ export class TruncatedStreamError extends Error {
         this.name = "TruncatedStreamError";
     }
 }
+
+// `cause` is an arbitrary user-settable property and nothing prevents it from
+// forming a cycle, so the walk is bounded. Hanging the process would be worse
+// than misclassifying an error or describing only part of it.
+const MAX_CAUSE_DEPTH = 8;
+
+// The error and each error on its `cause` chain, top first, at most
+// `MAX_CAUSE_DEPTH` of them. `complete` is false when the walk stopped at the
+// depth limit with more of the chain still below it.
+export const causeChain = (
+    err: unknown,
+): { chain: object[]; complete: boolean } => {
+    const chain: object[] = [];
+    let current: unknown = err;
+    for (let depth = 0; depth < MAX_CAUSE_DEPTH; depth++) {
+        if (current === null || typeof current !== "object") {
+            return { chain, complete: true };
+        }
+        chain.push(current);
+        const { cause } = current as { cause?: unknown };
+        if (cause === current) return { chain, complete: true };
+        current = cause;
+    }
+    return { chain, complete: current === null || typeof current !== "object" };
+};
+
+// An error as quak prints it and records it in `failures.json`: its message,
+// then the message of each error on its `cause` chain, joined with ": ". Node's
+// `fetch` rejects with just "fetch failed" and puts the reason on `cause`, so
+// this gives "fetch failed: connect ECONNREFUSED 1.2.3.4:443". When the host
+// has several addresses and every one fails, the cause is an `AggregateError`
+// with an empty message and one error per address in `errors`, so an
+// `AggregateError` is given as the messages of its errors, joined with ", ".
+// A cause that is not an object, such as a thrown string, is given as
+// `String(cause)`. Each message is trimmed of the white space around it, as
+// OpenSSL ends its own with a newline. A message the text already holds is
+// left out.
+export const describeError = (err: unknown): string => {
+    if (!(err instanceof Error)) return String(err);
+    const { chain } = causeChain(err);
+    const messages = chain
+        .slice(1)
+        .map((cause) =>
+            cause instanceof AggregateError
+                ? cause.errors
+                      .map((e: unknown) =>
+                          (e instanceof Error ? e.message : String(e)).trim(),
+                      )
+                      .join(", ")
+                : (cause as { message?: unknown }).message,
+        );
+    // `causeChain` stops at a cause that is not an object, so that one is read
+    // from the last error on the chain.
+    const last = chain[chain.length - 1] as { cause?: unknown };
+    if (last.cause !== undefined && typeof last.cause !== "object") {
+        messages.push(String(last.cause));
+    }
+    let text = err.message.trim();
+    for (const message of messages) {
+        if (typeof message !== "string") continue;
+        const trimmed = message.trim();
+        if (!text.includes(trimmed)) text += `: ${trimmed}`;
+    }
+    return text;
+};

src/retry.ts

@@ -10,7 +10,7 @@
 // declining to retry a transient one costs a single file that the next run
 // picks up anyway. When the evidence is ambiguous, fail fast.
 
-import { ApiError, TruncatedStreamError } from "./errors.js";
+import { ApiError, causeChain, TruncatedStreamError } from "./errors.js";
 
 export interface RetryOptions {
     // Total calls, not retries: `attempts: 1` disables retrying.
@@ -89,11 +89,6 @@ const TRANSPORT_CODES = new Set([
 // narrowed. See `isSafeToReplay`.
 const CONNECT_CODES = new Set(["ENOTFOUND", "EAI_AGAIN", "ECONNREFUSED"]);
 
-// `cause` is an arbitrary user-settable property and nothing prevents it from
-// forming a cycle, so the walk is bounded. Hanging the process would be a
-// worse outcome than any misclassification.
-const MAX_CAUSE_DEPTH = 8;
-
 // Collect every `code` in an error's cause chain. undici does not put the
 // errno on the error it throws — it hangs the underlying socket error off
 // `cause`, sometimes more than one level down — so a classifier that only read
@@ -101,20 +96,21 @@ const MAX_CAUSE_DEPTH = 8;
 // connection permanent. `complete` is false when the walk stopped at the
 // depth limit with more of the chain still below it.
 const causeCodes = (err: unknown): { codes: string[]; complete: boolean } => {
+    const { chain, complete } = causeChain(err);
     const codes: string[] = [];
-    let current: unknown = err;
-    for (let depth = 0; depth < MAX_CAUSE_DEPTH; depth++) {
-        if (current === null || typeof current !== "object") {
-            return { codes, complete: true };
-        }
-        const { code, cause } = current as { code?: unknown; cause?: unknown };
+    for (const link of chain) {
+        const { code } = link as { code?: unknown };
         if (typeof code === "string") codes.push(code);
-        if (cause === current) return { codes, complete: true };
-        current = cause;
     }
-    return { codes, complete: current === null || typeof current !== "object" };
+    return { codes, complete };
 };
 
+// Does the error's cause chain hold the errno of a transport failure, one of
+// `TRANSPORT_CODES`? `isRetryable` retries such an error, and `classify` in
+// `src/backup.ts` calls it transient.
+export const hasTransportCode = (err: unknown): boolean =>
+    causeCodes(err).codes.some((code) => TRANSPORT_CODES.has(code));
+
 const isAbort = (err: unknown): boolean => {
     if (err === null || typeof err !== "object") return false;
     const { name } = err as { name?: unknown };
@@ -158,7 +154,7 @@ export const isRetryable = (err: unknown): boolean => {
     // have succeeded; the cost of the imprecision is bounded by the attempt
     // count.
     if (err instanceof TypeError) return true;
-    return causeCodes(err).codes.some((code) => TRANSPORT_CODES.has(code));
+    return hasTransportCode(err);
 };
 
 // Could the first attempt already have taken effect on the server?

test/cli/backup-end-to-end.test.ts

@@ -329,6 +329,41 @@ const downloadOf = (input: RequestInfo | URL): number | undefined => {
         : undefined;
 };
 
+// The error Node gives for a connection to `address` that was refused.
+const refusedAt = (address: string): Error =>
+    Object.assign(new Error(`connect ECONNREFUSED ${address}`), {
+        code: "ECONNREFUSED",
+    });
+
+// What Node's `fetch` rejects with when the server refuses the connection, as
+// for every failure below HTTP: a TypeError that says only "fetch failed",
+// with the reason, and its errno, as its cause. When the server has several
+// addresses and each one refuses, the cause is an AggregateError with an empty
+// message, one error per address and the errno of the first. `reason` is the
+// error as quak prints it and records it in `failures.json`.
+const REFUSED = [
+    {
+        addresses: "its one address",
+        error: (): TypeError =>
+            new TypeError("fetch failed", { cause: refusedAt("1.2.3.4:443") }),
+        reason: "fetch failed: connect ECONNREFUSED 1.2.3.4:443",
+    },
+    {
+        addresses: "each of its two addresses",
+        error: (): TypeError =>
+            new TypeError("fetch failed", {
+                cause: Object.assign(
+                    new AggregateError([
+                        refusedAt("2001:db8::1:443"),
+                        refusedAt("1.2.3.4:443"),
+                    ]),
+                    { code: "ECONNREFUSED" },
+                ),
+            }),
+        reason: "fetch failed: connect ECONNREFUSED 2001:db8::1:443, connect ECONNREFUSED 1.2.3.4:443",
+    },
+];
+
 // The fake server, pushing onto `downloads` the ID of each file whose original
 // is downloaded.
 const recordingDownloads =
@@ -619,6 +654,115 @@ describe("quak backup", () => {
         expect(readdirSync(dir)).toEqual([]);
     });
 
+    // The server refuses every connection, at its one address or at each of
+    // several. quak tries each request again, here without waiting, and then
+    // the run stops at the refresh it starts with. The line it prints gives
+    // the reason Node's `fetch` put on the error's cause, so a cron log tells
+    // a refused connection from a name that did not resolve, and the exit
+    // code is 1.
+    it.each(REFUSED)(
+        "prints why the refresh could not reach the server at $addresses, and exits 1",
+        async ({ error, reason }) => {
+            const dir = mkdtempSync(join(root, "backup-"));
+            const fetch: typeof globalThis.fetch = async () => {
+                throw error();
+            };
+
+            const { code, stderr } = await backup(dir, {}, fetch);
+
+            expect(code).toBe(1);
+            expect(stderr).toBe(
+                "Starting backup...\n" +
+                    "Refreshing library...\n" +
+                    `quak: ${reason}\n`,
+            );
+        },
+    );
+
+    // The files CDN refuses every connection for toast.mp4, at its one
+    // address or at each of several. Once its retries are spent, the file
+    // goes into `failures.json` with the reason, as a transient failure,
+    // since a later run may well reach the CDN, and the lines the backup
+    // prints for it give the same reason. The rest of the account is backed
+    // up, and the exit code is 1.
+    it.each(REFUSED)(
+        "records why a download could not reach the server at $addresses in failures.json, as transient, and exits 1",
+        async ({ error, reason }) => {
+            let attempts = 0;
+            const fetch: typeof globalThis.fetch = async (input, init) => {
+                if (downloadOf(input) === 200) {
+                    attempts++;
+                    throw error();
+                }
+                return server(input, init);
+            };
+            const dir = mkdtempSync(join(root, "backup-"));
+
+            const { code, stderr } = await backup(dir, {}, fetch);
+
+            expect(code).toBe(1);
+            expect(attempts).toBe(UNATTENDED_RETRY_OPTIONS.attempts);
+            const failures = readJSON(join(dir, "failures.json"));
+            expect(Object.keys(failures.files)).toEqual(["200"]);
+            expect(failures.files["200"]).toMatchObject({
+                fileID: 200,
+                title: "toast.mp4",
+                classification: "transient",
+                attempts: 1,
+                error: reason,
+            });
+            expect(stderr).toContain(`FAILED original toast.mp4: ${reason}\n`);
+            expect(stderr).toContain(
+                `[Friend's Wedding] toast.mp4 (id 200): ${reason}\n`,
+            );
+            expect(readFileSync(join(dir, DAY, "2024-03-01.100.jpg"))).toEqual(
+                Buffer.from(JPEG_WITH_EXIF),
+            );
+            expect(existsSync(join(dir, DAY, "2024-03-01.200.mp4"))).toBe(
+                false,
+            );
+        },
+    );
+
+    // The name of Ente's API server does not resolve when quak fetches the ML
+    // data, after every original has been backed up. No file has ML data
+    // cached yet, so each goes into `failures.json` with the reason, as a
+    // transient failure, as a download that could not reach the server does,
+    // and the exit code is 1.
+    it("records why the ML data fetch could not reach the server in failures.json, as transient, and exits 1", async () => {
+        const fetch: typeof globalThis.fetch = async (input, init) => {
+            if (new URL(String(input)).pathname === "/files/data/fetch") {
+                throw new TypeError("fetch failed", {
+                    cause: Object.assign(
+                        new Error("getaddrinfo ENOTFOUND api.ente.io"),
+                        { code: "ENOTFOUND" },
+                    ),
+                });
+            }
+            return server(input, init);
+        };
+        const dir = mkdtempSync(join(root, "backup-"));
+
+        const { code, stderr } = await backup(dir, {}, fetch);
+
+        expect(code).toBe(1);
+        const error =
+            "ML data: fetch failed: getaddrinfo ENOTFOUND api.ente.io";
+        expect(stderr).toContain(`FAILED ${error}\n`);
+        const failures = readJSON(join(dir, "failures.json"));
+        expect(
+            Object.values(failures.files).map((f) => [
+                f.fileID,
+                f.classification,
+                f.error,
+            ]),
+        ).toEqual([
+            [100, "transient", error],
+            [200, "transient", error],
+            [300, "transient", error],
+        ]);
+    });
+
     // Something changed beach.jpg on disk after it was backed up. A run with
     // --verify hashes every stored original, finds that one no longer matches
     // the hash Ente recorded, says so, and downloads it again; it put back the

test/cli/run.test.ts

@@ -48,6 +48,78 @@ describe("run", () => {
         );
     });
 
+    // Node's `fetch` rejects every failure below HTTP with a TypeError that
+    // says only "fetch failed", and hangs the reason, with its errno, off
+    // `cause`. The printed line carries the reason, so a cron log tells a
+    // refused connection from a name that did not resolve.
+    it("prints the reason a fetch failed, from the error's cause", async () => {
+        const refused = new TypeError("fetch failed", {
+            cause: Object.assign(
+                new Error("connect ECONNREFUSED 1.2.3.4:443"),
+                { code: "ECONNREFUSED" },
+            ),
+        });
+        const result = await runToExit(Promise.reject(refused));
+        expect(result).toEqual({
+            code: 1,
+            stdout: "",
+            stderr: "quak: fetch failed: connect ECONNREFUSED 1.2.3.4:443\n",
+        });
+    });
+
+    // When the host has several addresses and every one refuses, Node's
+    // `fetch` gives as the cause an AggregateError with an empty message and
+    // one error per address. The printed line gives each address's reason.
+    it("prints the reason for each address a fetch could not reach", async () => {
+        const refusedAt = (address: string): Error =>
+            Object.assign(new Error(`connect ECONNREFUSED ${address}`), {
+                code: "ECONNREFUSED",
+            });
+        const refused = new TypeError("fetch failed", {
+            cause: Object.assign(
+                new AggregateError([
+                    refusedAt("2001:db8::1:443"),
+                    refusedAt("1.2.3.4:443"),
+                ]),
+                { code: "ECONNREFUSED" },
+            ),
+        });
+        const result = await runToExit(Promise.reject(refused));
+        expect(result).toEqual({
+            code: 1,
+            stdout: "",
+            stderr: "quak: fetch failed: connect ECONNREFUSED 2001:db8::1:443, connect ECONNREFUSED 1.2.3.4:443\n",
+        });
+    });
+
+    // An error that already quotes its cause's message is printed as it is.
+    it("does not print a cause's message twice", async () => {
+        const reset = new Error("read ECONNRESET");
+        const wrapped = new Error(`download: file 7: ${reset.message}`, {
+            cause: reset,
+        });
+        const result = await runToExit(Promise.reject(wrapped));
+        expect(result.stderr).toBe("quak: download: file 7: read ECONNRESET\n");
+    });
+
+    // OpenSSL ends its message for a failed TLS connection, such as one to a
+    // server that answers in plain HTTP, with a newline. The printed line is
+    // still one line.
+    it("prints a cause's message without the white space around it", async () => {
+        const wrongVersion = new TypeError("fetch failed", {
+            cause: Object.assign(
+                new Error(
+                    "C0B73A55A47F0000:error:0A00010B:SSL routines:ssl3_get_record:wrong version number:../deps/openssl/openssl/ssl/record/ssl3_record.c:354:\n",
+                ),
+                { code: "ERR_SSL_WRONG_VERSION_NUMBER" },
+            ),
+        });
+        const result = await runToExit(Promise.reject(wrongVersion));
+        expect(result.stderr).toBe(
+            "quak: fetch failed: C0B73A55A47F0000:error:0A00010B:SSL routines:ssl3_get_record:wrong version number:../deps/openssl/openssl/ssl/record/ssl3_record.c:354:\n",
+        );
+    });
+
     it("prints a thrown value that is not an Error", async () => {
         const result = await runToExit(Promise.reject("offline"));
         expect(result).toEqual({
@@ -57,6 +129,14 @@ describe("run", () => {
         });
     });
 
+    // A cause that is not an Error, such as a thrown string, is printed as a
+    // thrown value that is not an Error is.
+    it("prints a cause that is not an Error", async () => {
+        const wrapped = new Error("ML data", { cause: "offline" });
+        const result = await runToExit(Promise.reject(wrapped));
+        expect(result.stderr).toBe("quak: ML data: offline\n");
+    });
+
     it("on a 401 from the server says to run quak login, on one line, and exits 3", async () => {
         const result = await runToExit(
             Promise.reject(new ApiError("unauthorized", 401)),

test/download/download.test.ts

@@ -74,7 +74,11 @@ import {
 } from "vitest";
 import { init, toBase64, STREAM_CHUNK_SIZE } from "../../src/crypto/index.js";
 import { ApiClient } from "../../src/api/client.js";
-import { ApiError, TruncatedStreamError } from "../../src/errors.js";
+import {
+    ApiError,
+    describeError,
+    TruncatedStreamError,
+} from "../../src/errors.js";
 import type { RetryOptions } from "../../src/retry.js";
 import {
     downloadFile,
@@ -834,6 +838,11 @@ describe.each(entryPoints)(
             expect(((err as Error).cause as Error).message).toMatch(
                 /authentication failed/i,
             );
+            // As quak prints it and records it in `failures.json`, the error
+            // states the authentication failure once, from its cause.
+            expect(describeError(err)).toMatch(
+                /^download: stream truncated: response body ended with \d+ trailing bytes \(transfer stopped mid-chunk, or the data is corrupt\): secretstream chunk authentication failed$/,
+            );
 
             // And, as with every other failure, the destination is untouched
             // and no staged temp file survives.