fix: validate and reject duplicate single-value CLI flags #8564

Sandijigs · 2025-10-24T01:14:26Z

Motivation

Duplicating single-value CLI flags (like --checkpointSyncUrl)
by accident gets silently turned into an array, leading to
confusing errors later on. This PR catches duplicate single-value flags early and shows
users exactly what went wrong, instead of letting the error
happen later with a confusing message.

Description

This PR implements validation middleware to detect and
reject duplicate single-value CLI flags with clear error
messages.
Changes made:

 - Added `validateNoDuplicateSingleValueFlags()` function in
  `/packages/utils/src/command.ts` to check if single-value
 options (type: `string`, `number`, `boolean`) have received
  multiple values

 - Integrated validation as middleware in
 `registerCommandToYargs()` to run before command handler
 execution

 - Created test suite in
 `/packages/utils/test/unit/command.test.ts` with 10 test
 cases covering various scenarios

 - Validation explicitly skips array-type options to
 preserve their multi-value functionality

Closes #8178

Steps to test or reproduce

Run the new unit tests:

yarn vitest run packages/utils/test/unit/command.test.ts
Should show 10 tests passing.

Test with actual CLI (reproduces the original bug):bash ./lodestar beacon --checkpointSyncUrl https://example1.com --checkpointSyncUrl https://example2.com Before fix: Gets confusing error later
("HttpClient.urls[0] is empty")
After fix: Gets immediate clear error: Option
--checkpointSyncUrl' is a single-value flag but was provided multiple times
Test that array flags still work: ./lodestar beacon --bootnodes=node1 --bootnodes=node2

gemini-code-assist · 2025-10-24T01:14:38Z

Summary of Changes

Hello @Sandijigs, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where duplicating single-value Command Line Interface (CLI) flags would silently convert them into arrays, leading to confusing and delayed errors. The changes introduce a robust validation mechanism that proactively detects and rejects such duplicate flags, providing immediate and clear feedback to the user. This significantly improves the user experience by preventing obscure runtime errors related to CLI argument parsing.

Highlights

New Validation Function: Introduced validateNoDuplicateSingleValueFlags() in packages/utils/src/command.ts to identify when single-value CLI options (string, number, boolean) are provided multiple times.
Middleware Integration: The new validation function is integrated as middleware within registerCommandToYargs(), ensuring that duplicate single-value flags are caught and rejected with clear error messages before command execution.
Comprehensive Test Suite: A new test file, packages/utils/test/unit/command.test.ts, has been added with 10 unit tests to cover various scenarios, including successful single-value and array flag parsing, and error handling for duplicated single-value flags.
Array Flag Exemption: The validation explicitly bypasses array-type options, preserving their intended multi-value functionality.

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Code Review

This pull request introduces a validation middleware to prevent duplicate single-value CLI flags, which is a great improvement for user experience. The implementation in command.ts is clean and effective, and the new test suite in command.test.ts is comprehensive. I have a few minor suggestions to improve code style in the error message construction and to make the test assertions more efficient.

gemini-code-assist · 2025-10-24T01:15:51Z

packages/utils/src/command.ts

+      throw new Error(
+        `Option '--${optionName}' is a single-value flag but was provided multiple times. ` +
+          `Received values: [${value.join(", ")}]. Please provide only one value.`
+      );


For better readability and to avoid string concatenation, you can use a single template literal for the error message.

throw new Error( `Option '--${optionName}' is a single-value flag but was provided multiple times. Received values: [${value.join(", ")]. Please provide only one value.` );

gemini-code-assist · 2025-10-24T01:15:51Z

packages/utils/test/unit/command.test.ts

+      expect(() => parser.parseSync()).toThrow(
+        "Option '--url' is a single-value flag but was provided multiple times"
+      );
+      expect(() => parser.parseSync()).toThrow("https://example1.com");
+      expect(() => parser.parseSync()).toThrow("https://example2.com");


Calling parser.parseSync() multiple times is inefficient and can be brittle if the function has side effects. It's better to perform a single parse and check the thrown error message for all required substrings. You can combine these assertions into a single toThrow with a more specific error message.

Suggested change

expect(() => parser.parseSync()).toThrow(

"Option '--url' is a single-value flag but was provided multiple times"

);

expect(() => parser.parseSync()).toThrow("https://example1.com");

expect(() => parser.parseSync()).toThrow("https://example2.com");

expect(() => parser.parseSync()).toThrow(

"Option '--url' is a single-value flag but was provided multiple times. Received values: [https://example1.com, https://example2.com]. Please provide only one value."

);

gemini-code-assist · 2025-10-24T01:15:51Z

packages/utils/test/unit/command.test.ts

+      expect(() => parser.parseSync()).toThrow(
+        "Option '--checkpointSyncUrl' is a single-value flag but was provided multiple times"
+      );
+      expect(() => parser.parseSync()).toThrow(
+        "https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io"
+      );


Calling parser.parseSync() multiple times is inefficient. It's better to perform a single parse and check the thrown error message. You can combine these assertions into a single toThrow with a more specific error message that includes the duplicated values.

Suggested change

expect(() => parser.parseSync()).toThrow(

"Option '--checkpointSyncUrl' is a single-value flag but was provided multiple times"

);

expect(() => parser.parseSync()).toThrow(

"https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io"

);

expect(() => parser.parseSync()).toThrow(

"Option '--checkpointSyncUrl' is a single-value flag but was provided multiple times. Received values: [https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io, https://checkpoint-sync.fusaka-devnet-4.ethpandaops.io]. Please provide only one value."

);

nflaig

please make sure your code passes yarn build, yarn lint and yarn check-types

fix: validate and reject duplicate single-value CLI flags

4aa90b5

Sandijigs requested a review from a team as a code owner October 24, 2025 01:14

gemini-code-assist bot reviewed Oct 24, 2025

View reviewed changes

nflaig reviewed Oct 27, 2025

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philknows marked this pull request as draft October 27, 2025 16:33

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