vitest-dev/vitest · error · Error

Cannot define a nested project for a ${browser} browser. The

Error message

Cannot define a nested project for a ${browser} browser. The project name "${name}" was already defined. If you have multiple instances for the same browser, make sure to define a custom "name". All projects should have unique names. Make sure your configuration is correct.

What it means

During browser-instance expansion (resolveProjects.ts:749-757), each instance becomes its own project and its name is added to a `names` set. If two instances produce the same name (e.g. two chromium instances without custom names), the second throws. The message tells you to give each instance a custom `name` when running multiple instances of the same browser engine.

Source

Thrown at packages/vitest/src/node/projects/resolveProjects.ts:750

            `@vitest/coverage-v8 does not work with\n${browserConfig}\n`
            + `\nUse either:\n${correctExample}`
            + `\n\n...or change your coverage provider to:\n${coverageExample}\n`,
          )
        }

        if (globalConfig.inspect || globalConfig.inspectBrk) {
          const inspectOption = `--inspect${globalConfig.inspectBrk ? '-brk' : ''}`

          throw new Error(
            `${inspectOption} does not work with\n${browserConfig}\n`
            + `\nUse either:\n${correctExample}`
            + `\n\n...or disable ${inspectOption}\n`,
          )
        }
      }

      if (names.has(name)) {
        throw new Error(
          [
            `Cannot define a nested project for a ${browser} browser. The project name "${name}" was already defined. `,
            'If you have multiple instances for the same browser, make sure to define a custom "name". ',
            'All projects should have unique names. Make sure your configuration is correct.',
          ].join(''),
        )
      }
      names.add(name)

      const clonedConfig = cloneProjectConfigForBrowserInstance(projectConfig, instance)
      clonedConfig.name = name

      result.push({
        viteConfig, // shared with parent
        projectConfig: clonedConfig,
        ancestors: entry.ancestors,
      })
    })

View on GitHub (pinned to d568f8ce37)

Solutions

1. Give every instance a unique `name`: { browser: 'chromium', name: 'chromium-mobile' }, { browser: 'chromium', name: 'chromium-desktop' }.
2. If you only need one instance per engine, remove the duplicate.
3. Generate names programmatically from the options (e.g. viewport) when building the instances array.

Example fix

// before
test: { browser: { enabled: true, provider: 'playwright', instances: [
  { browser: 'chromium', viewport: { width: 375, height: 667 } },
  { browser: 'chromium', viewport: { width: 1280, height: 720 } },
] } }

// after
test: { browser: { enabled: true, provider: 'playwright', instances: [
  { browser: 'chromium', name: 'mobile', viewport: { width: 375, height: 667 } },
  { browser: 'chromium', name: 'desktop', viewport: { width: 1280, height: 720 } },
] } }

const instanceNames = instances.map(i => i.name ?? i.browser)
const dupes = instanceNames.filter((n, i) => instanceNames.indexOf(n) !== i)
if (dupes.length) throw new Error(`Duplicate browser instance names: ${[...new Set(dupes)].join(', ')}`)

function uniqueInstanceNames(instances: { name?: string; browser: string }[]): boolean {
  const names = instances.map(i => i.name ?? i.browser)
  return new Set(names).size === names.length
}

When it happens

Trigger: browser.instances contains two entries with the same browser engine and no distinguishing `name` (both fall back to the engine name); instances with explicitly identical `name` fields.

Common situations: Wanting to run chromium twice with different viewports/options but forgetting to name them; copy-pasting an instance; defaulting instances from an array of options that omits name.

Related errors

• Project name "${name}" is not unique. All projects should ha
• Project name "${name}"${entryFile ? ` from "${entryFile}"` :
• The browser configuration must have a "browser" property. Th
• The browser configuration must have a "name" property. This
• The instance cannot have a different provider from its paren

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/31c54bfbaace443e.json. Report an issue: GitHub.