diff --git a/scripts/rea.mjs b/scripts/rea.mjs
index 3b1ef6c4a5..d3380c9c2a 100755
--- a/scripts/rea.mjs
+++ b/scripts/rea.mjs
@@ -58,7 +58,8 @@ if (!(await compiledRuntimeExists(runtimeFiles))) {
   process.stdout.write(result.output);
   process.exitCode = result.exitCode;
 } else {
-  const { createCli } = await import("../dist/cli-runtime/cli.js");
+  const { cliConfigurationError, createCli } =
+    await import("../dist/cli-runtime/cli.js");
   const { createStreamedCliJsonOutput } =
     await import("../dist/cli-runtime/streamedJsonOutput.js");
   const {
@@ -67,7 +68,11 @@ if (!(await compiledRuntimeExists(runtimeFiles))) {
     validateCliOutputArguments,
   } = await import("../dist/cli-runtime/cliOutput.js");
   const outputArguments = validateCliOutputArguments(args);
-  if (!outputArguments.ok) {
+  const configurationError = cliConfigurationError(process.env);
+  if (configurationError !== undefined) {
+    process.stderr.write(`${configurationError}\n`);
+    process.exitCode = 1;
+  } else if (!outputArguments.ok) {
     process.stdout.write(renderCliOutputArgumentError(outputArguments));
     process.exitCode = 1;
   } else {
diff --git a/src/cli.ts b/src/cli.ts
index 1cbe3b896d..5438fd472d 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -8,6 +8,7 @@ import { registerBinaryDiagnosticsCommands } from "./cli/binaryDiagnosticsComman
 import { Cli } from "incur";
 
 import { createLogger, parseLogLevel } from "./logger.js";
+import { projectAnalysisError } from "./domain/analysisErrorProjection.js";
 import { PRODUCT_IDENTITY } from "./identity.js";
 import { npxRegistrationCommand } from "./application/ClientRegistrationIdentity.js";
 import { registerSetupCommands } from "./cli/setupCommands.js";
@@ -41,6 +42,16 @@ import {
   type CliResultOutput,
 } from "./cli/streamedJsonOutput.js";
 
+/** Describe CLI settings that are invalid before any command runs, as MCP does. */
+export const cliConfigurationError = (
+  selectedEnvironment: Readonly<Record<string, string | undefined>>,
+): string | undefined => {
+  const value = selectedEnvironment.REA_LOG_LEVEL;
+  if (value === undefined) return undefined;
+  const level = parseLogLevel(value);
+  return level.ok ? undefined : projectAnalysisError(level.error).message;
+};
+
 /**
  * Build the one-shot Incur CLI without starting Hopper at import time.
  * Analysis commands acquire and close their own sessions; bare `mcp` and
@@ -54,11 +65,14 @@ export const createCli = (
 ): CliInstance => {
   const environment = snapshotEnvironment(selectedEnvironment);
   const analysis = createDirectAnalysis(environment);
+  // The executable reports an invalid level through cliConfigurationError.
+  const level =
+    environment.REA_LOG_LEVEL === undefined
+      ? undefined
+      : parseLogLevel(environment.REA_LOG_LEVEL);
   const logger = createLogger(
     "cli",
-    environment.REA_LOG_LEVEL === undefined
-      ? "silent"
-      : parseLogLevel(environment.REA_LOG_LEVEL),
+    level?.ok === true ? level.value : "silent",
   );
   const cli = Cli.create(PRODUCT_IDENTITY.cliBinary, {
     version: PRODUCT_IDENTITY.packageVersion,
diff --git a/src/logger.ts b/src/logger.ts
index e43c7f97cc..05fc3822fe 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -1,6 +1,9 @@
 import pino, { type Logger } from "pino";
 import { z } from "zod";
 
+import { ConfigurationError } from "./domain/configurationErrors.js";
+import { err, ok, type Result } from "./domain/result.js";
+
 /** Supported structured log levels at the process boundary. */
 export type LogLevel = z.infer<typeof logLevelSchema>;
 
@@ -9,8 +12,21 @@ const logLevelSchema = z
   .default("info");
 
 /** Parse the optional REA_LOG_LEVEL environment variable. */
-export const parseLogLevel = (value: unknown): LogLevel =>
-  logLevelSchema.parse(value);
+export const parseLogLevel = (
+  value: unknown,
+): Result<LogLevel, ConfigurationError> => {
+  const parsed = logLevelSchema.safeParse(value);
+  if (parsed.success) return ok(parsed.data);
+  return err(
+    new ConfigurationError("Invalid REA environment configuration", {
+      cause: parsed.error,
+      settings: parsed.error.issues.map(({ message }) => ({
+        setting: "REA_LOG_LEVEL",
+        constraint: message,
+      })),
+    }),
+  );
+};
 
 /** Create a JSON logger on stderr so protocol and CLI result stdout stay valid. */
 export const createLogger = (mode: "mcp" | "cli", level: LogLevel): Logger =>
diff --git a/tests/boundary/cli/cliLogLevelConfiguration.test.ts b/tests/boundary/cli/cliLogLevelConfiguration.test.ts
new file mode 100644
index 0000000000..cc09a3b944
--- /dev/null
+++ b/tests/boundary/cli/cliLogLevelConfiguration.test.ts
@@ -0,0 +1,21 @@
+import { expect } from "vitest";
+
+import { cliTest } from "../../support/cli/cliFixture.js";
+
+for (const level of ["loud", "INFO"])
+  cliTest(
+    `reports REA_LOG_LEVEL=${level} as invalid configuration`,
+    async ({ cli }) => {
+      for (const arguments_ of [["doctor", "--json"], ["--help"]]) {
+        const result = await cli.run({
+          arguments: arguments_,
+          environment: { REA_LOG_LEVEL: level },
+        });
+        expect(result.exitCode).toBe(1);
+        expect(result.stdout).toBe("");
+        expect(result.stderr).toBe(
+          'REA configuration is invalid: REA_LOG_LEVEL: Invalid option: expected one of "trace"|"debug"|"info"|"warn"|"error"|"fatal"|"silent".\n',
+        );
+      }
+    },
+  );
diff --git a/tests/boundary/cli/dispatcher.test.ts b/tests/boundary/cli/dispatcher.test.ts
index 583bd601dd..efd6ca4144 100644
--- a/tests/boundary/cli/dispatcher.test.ts
+++ b/tests/boundary/cli/dispatcher.test.ts
@@ -25,7 +25,7 @@ describe("executable dispatcher", () => {
         ),
         workspace.write(
           "dist/cli-runtime/cli.js",
-          'export const createCli = () => ({ serve: (args) => process.stdout.write(JSON.stringify({ adapter: "cli", args })) });\n',
+          'export const cliConfigurationError = () => undefined; export const createCli = () => ({ serve: (args) => process.stdout.write(JSON.stringify({ adapter: "cli", args })) });\n',
         ),
         workspace.write(
           "dist/cli-runtime/cliOutput.js",