vitest-dev/vitest · error · Error

Session " " not found.

Error message

Session "${sessionId}" not found.

What it means

ensureCDPHandler looks up a browser session by sessionId in vitest._browserSessions. Sessions are created when a browser test run starts and removed when it ends; a missing session means the ID is unknown, already cleaned up, or never created.

Solutions

1. Refresh or re-establish the CDP connection so it uses the current run's sessionId.
2. Ensure the CDP client does not cache session IDs across test runs.
3. If you are driving CDP programmatically, fetch the live sessionId from the runner state before each call.
4. Check for premature session teardown (e.g. browser closing before CDP work finishes) and serialize teardown after CDP cleanup.

Example fix

// before
const handler = await parent.ensureCDPHandler(staleSessionId, rpcId)
// after - re-read the live session id
const live = vitest._browserSessions.getSession(currentSessionId)!
const handler = await parent.ensureCDPHandler(live.id, rpcId)

function sessionExists(getSession: (id: string) => unknown, id: string): boolean {
  return !!getSession(id)
}

When it happens

Trigger: A CDP debugging client (or browser UI) sends a request referencing a sessionId that has no matching BrowserSession: stale ID after a run finished, typo/malformed ID, a session from a different Vitest process, or a race where the session was torn down before the CDP request arrived.

Common situations: Leaving the browser devtools UI open across a test re-run; reconnecting a CDP client with an old session ID after the server restarted; a flaky teardown that removes the session before in-flight CDP traffic lands.

Related errors

• Browser provider is not defined for the project
• Page " " not found in browser.
• Browser is not initialized
• Cannot spawn child server without a parent dev server.
• Cannot take a screenshot without a test path

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/36619e40dd8f91fd. Report an issue: GitHub.

Appendix: source

Thrown at packages/browser/src/node/projectParent.ts:199

    options: StackTraceParserOptions = {},
  ): ParsedStack[] {
    return parseStacktrace(trace, {
      ...this.stackTraceOptions,
      ...options,
    })
  }

  public readonly cdps: Map<string, BrowserServerCDPHandler> = new Map()
  private cdpSessionsPromises = new Map<string, Promise<CDPSession>>()

  async ensureCDPHandler(sessionId: string, rpcId: string): Promise<BrowserServerCDPHandler> {
    const cachedHandler = this.cdps.get(rpcId)
    if (cachedHandler) {
      return cachedHandler
    }
    const browserSession = this.vitest._browserSessions.getSession(sessionId)
    if (!browserSession) {
      throw new Error(`Session "${sessionId}" not found.`)
    }

    const browser = browserSession.project.browser!
    const provider = browser.provider
    if (!provider) {
      throw new Error(`Browser provider is not defined for the project "${browserSession.project.name}".`)
    }
    if (!provider.getCDPSession) {
      throw new Error(`CDP is not supported by the provider "${provider.name}".`)
    }

    const session = await this.cdpSessionsPromises.get(rpcId) ?? await (async () => {
      const promise = provider.getCDPSession!(sessionId).finally(() => {
        this.cdpSessionsPromises.delete(rpcId)
      })
      this.cdpSessionsPromises.set(rpcId, promise)
      return promise
    })()

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