diff --git a/packages/roach/src/action.ts b/packages/roach/src/action.ts
index 8f0c0a4..36b93a7 100644
--- a/packages/roach/src/action.ts
+++ b/packages/roach/src/action.ts
@@ -51,7 +51,7 @@ function runCommand(command: string, env: NodeJS.ProcessEnv): Promise<number> {
 }
 
 const command = process.env.INPUT_RUN;
-if (!command) throw new Error("The run input is required");
+if (!command) throw new Error("Roach: the run input is required");
 // The runner gives an empty string for an input that the step does not set.
 const token = readEnv("INPUT_TOKEN");
 const file = await readRoachFile();
diff --git a/packages/roach/src/client.ts b/packages/roach/src/client.ts
index 8868c0f..6f1865b 100644
--- a/packages/roach/src/client.ts
+++ b/packages/roach/src/client.ts
@@ -11,6 +11,7 @@ import http from "node:http";
 import https from "node:https";
 import { tmpdir } from "node:os";
 import path from "node:path";
+import { DEFAULT_NO_PROXY } from "./config.ts";
 import {
   CONTROL_PATH,
   type OpenedSession,
@@ -91,7 +92,7 @@ function callControl<T>(
           if (status >= 400) {
             reject(
               new Error(
-                `Roach ${method} ${route} failed with HTTP ${status}: ${text}`,
+                `Roach: ${method} ${route} failed with HTTP ${status}: ${text}`,
               ),
             );
           } else {
@@ -129,7 +130,7 @@ export function connectRoach({
       const url = opened?.url;
       if (typeof url !== "string") {
         throw new Error(
-          `Roach service at ${new URL(controlUrl).origin} is older than this client: it returned no session URL. Deploy the service at ${SESSION_URL_VERSION} or later.`,
+          `Roach: the service at ${new URL(controlUrl).origin} is older than this client: it returned no session URL. Deploy the service at ${SESSION_URL_VERSION} or later.`,
         );
       }
       return {
@@ -168,7 +169,7 @@ export interface RemoteRoach extends RemoteRun, RoachControl {
 export async function startRemoteRun(
   service: { url: string; token?: string | undefined },
   config: RunConfig,
-  { noProxy = "localhost,127.0.0.1,::1" }: { noProxy?: string } = {},
+  { noProxy = DEFAULT_NO_PROXY }: { noProxy?: string } = {},
 ): Promise<RemoteRoach> {
   const run = await callControl<RemoteRun>(
     `${service.url.replace(/\/$/, "")}${CONTROL_PATH}`,
diff --git a/packages/roach/src/config.ts b/packages/roach/src/config.ts
index acc5117..33ceafa 100644
--- a/packages/roach/src/config.ts
+++ b/packages/roach/src/config.ts
@@ -17,6 +17,9 @@ import { VALUE_PATTERNS } from "./values.ts";
 /** The Roach service of Sentry. See "Deploy" in `README.md`. */
 export const DEFAULT_SERVICE_URL = "https://roach-proxy.getsentry.net";
 
+/** The hosts that do not use the proxy when `roach.json` sets no `noProxy`. */
+export const DEFAULT_NO_PROXY = "localhost,127.0.0.1,::1";
+
 /** The name of the config file. */
 const FILE_NAME = "roach.json";
 
@@ -65,6 +68,23 @@ export function readEnv(
   return value === "" ? undefined : value;
 }
 
+/**
+ * Set the variables `values` in `process.env`. Returns a function that
+ * sets each of them back. A variable that was not set before is deleted.
+ */
+export function swapEnv(values: Record<string, string>): () => void {
+  const previous = Object.keys(values).map(
+    (name) => [name, process.env[name]] as const,
+  );
+  Object.assign(process.env, values);
+  return () => {
+    for (const [name, value] of previous) {
+      if (value === undefined) delete process.env[name];
+      else process.env[name] = value;
+    }
+  };
+}
+
 /**
  * Find `roach.json` in `from` or the closest parent directory, and read
  * it. Throws when no directory has one.
@@ -79,10 +99,17 @@ export async function readRoachFile(
       () => false,
     );
     if (found) {
-      return {
-        ...(JSON.parse(await readFile(file, "utf8")) as RoachFile),
-        path: file,
-      };
+      const text = await readFile(file, "utf8");
+      let parsed: RoachFile;
+      try {
+        parsed = JSON.parse(text) as RoachFile;
+      } catch (error) {
+        throw new Error(
+          `Roach: ${file} is not valid JSON: ${(error as Error).message}`,
+          { cause: error },
+        );
+      }
+      return { ...parsed, path: file };
     }
     const parent = path.dirname(directory);
     if (parent === directory) {
diff --git a/packages/roach/src/vitest-setup.ts b/packages/roach/src/vitest-setup.ts
index 26493b7..057d930 100644
--- a/packages/roach/src/vitest-setup.ts
+++ b/packages/roach/src/vitest-setup.ts
@@ -19,6 +19,7 @@ import {
   type ProvidedContext,
 } from "vitest";
 import { connectRoach } from "./client.ts";
+import { swapEnv } from "./config.ts";
 
 const provided = inject("roach");
 if (provided === undefined) {
@@ -36,12 +37,10 @@ function useRun(run: NonNullable<ProvidedContext["roach"]>): void {
     // The full name starts with the file, so tests of two files never share
     // a session.
     const session = await control.startSession(task.fullName);
-    const previous = {
-      HTTP_PROXY: process.env.HTTP_PROXY,
-      HTTPS_PROXY: process.env.HTTPS_PROXY,
-    };
-    process.env.HTTP_PROXY = session.url;
-    process.env.HTTPS_PROXY = session.url;
+    const restoreEnv = swapEnv({
+      HTTP_PROXY: session.url,
+      HTTPS_PROXY: session.url,
+    });
     const restoreProxy = http.setGlobalProxyFromEnv({
       HTTP_PROXY: session.url,
       HTTPS_PROXY: session.url,
@@ -49,10 +48,7 @@ function useRun(run: NonNullable<ProvidedContext["roach"]>): void {
     });
     onTestFinished(async ({ task: finished }) => {
       restoreProxy();
-      for (const [name, value] of Object.entries(previous)) {
-        if (value === undefined) delete process.env[name];
-        else process.env[name] = value;
-      }
+      restoreEnv();
       const { missed } = await session.end(finished.result?.state === "pass");
       // `replay` mode failed these requests, so nothing went live. The test
       // must fail, also when it recovered from the error.
diff --git a/packages/roach/src/vitest.ts b/packages/roach/src/vitest.ts
index ab3ec72..61408b3 100644
--- a/packages/roach/src/vitest.ts
+++ b/packages/roach/src/vitest.ts
@@ -19,10 +19,12 @@ import tls from "node:tls";
 import type { TestProject } from "vitest/node";
 import { startRemoteRun } from "./client.ts";
 import {
+  DEFAULT_NO_PROXY,
   DEFAULT_SERVICE_URL,
   readEnv,
   readRoachFile,
   runConfigOf,
+  swapEnv,
 } from "./config.ts";
 import { writeRunReport } from "./report.ts";
 import type { RemoteRun } from "./types.ts";
@@ -54,9 +56,9 @@ export async function startRoach(
   }
   // Only a run needs it, so `off` mode works on older Node too.
   if (typeof http.setGlobalProxyFromEnv !== "function") {
-    throw new Error("Roach needs Node 24.14 or later for vitest");
+    throw new Error("Roach: vitest needs Node 24.14 or later");
   }
-  const noProxy = file.noProxy ?? "localhost,127.0.0.1,::1";
+  const noProxy = file.noProxy ?? DEFAULT_NO_PROXY;
   const run = await startRemoteRun(
     { url: readEnv("ROACH_URL") ?? DEFAULT_SERVICE_URL, token },
     config,
@@ -66,11 +68,8 @@ export async function startRoach(
   if (process.env.GITHUB_ACTIONS) {
     process.stdout.write(`::add-mask::${run.token}\n`);
   }
-  const previousEnv = Object.keys(run.env).map(
-    (name) => [name, process.env[name]] as const,
-  );
   // Test workers and their child processes start later and inherit these.
-  Object.assign(process.env, run.env);
+  const restoreEnv = swapEnv(run.env);
   // This process started before them, so it reads neither the proxy nor
   // the CA file. Set both here.
   const previousCa = tls.getCACertificates("default");
@@ -95,10 +94,7 @@ export async function startRoach(
     } finally {
       restoreProxy();
       tls.setDefaultCACertificates(previousCa);
-      for (const [name, value] of previousEnv) {
-        if (value === undefined) delete process.env[name];
-        else process.env[name] = value;
-      }
+      restoreEnv();
     }
   };
 }
diff --git a/packages/service/tests/service.test.ts b/packages/service/tests/service.test.ts
index 4af2ee9..ece17c8 100644
--- a/packages/service/tests/service.test.ts
+++ b/packages/service/tests/service.test.ts
@@ -698,7 +698,7 @@ describe("roach service", () => {
         token: "run-token",
       }).startSession("test"),
     ).rejects.toThrow(
-      `Roach service at ${oldUrl} is older than this client: it returned no session URL. Deploy the service at 0.2.0 or later.`,
+      `Roach: the service at ${oldUrl} is older than this client: it returned no session URL. Deploy the service at 0.2.0 or later.`,
     );
   });