dagger/dagger · error · RuntimeException

Cannot fetch informations from process session

Error message

Cannot fetch informations from process session

What it means

startCliSession spawns the dagger CLI and waits for it to emit session JSON (port and session_token) on stdout/stderr. If the process exits or never prints parseable session information, the SDK throws 'Cannot fetch informations from process session'.

Source

Thrown at sdk/php/src/Connection/ProcessSessionConnection.php:121

                    $lines = explode("\n", (string) $output);
                    $validLines = array_filter($lines, function ($line) {
                        $this->logger->debug($line);
                        json_decode(trim($line));

                        return JSON_ERROR_NONE === json_last_error();
                    });
                    $sessionInformation = json_decode(array_shift($validLines));
                    $this->logger->info("Started Dagger session on port {$sessionInformation->port}");

                    return true;
                }
            }

            return false;
        });

        if (null === $sessionInformation) {
            throw new RuntimeException('Cannot fetch informations from process session');
        }

        $port = $sessionInformation->port;
        $token = $sessionInformation->session_token;

        $this->client = new Client('http://127.0.0.1:' . $port . '/query', [
            'Authorization' => 'Basic ' . base64_encode($token . ':'),
        ]);

        $this->sessionProcess = $process;

        return $this->client;
    }

    /**
     * @internal
     */
    public function getSessionProcess(): ?Process

View on GitHub (pinned to 82ba2681db)

Solutions

Run 'dagger version' and update the CLI to match the SDK version
Verify the container runtime (docker/podman) is installed and running
Run the dagger command manually to see its actual error output
Check PATH resolves to the intended dagger binary (which dagger)

Example fix

// before
$dagger = Dagger::connect(); // hangs/throws: old CLI emits no session json
// after
// terminal: dagger version -> update
// curl -L https://dl.dagger.io/dagger/install.sh | DAGGER_VERSION=x.y.z sh
$dagger = Dagger::connect();

Defensive patterns

Strategy: try-catch

Validate before calling

exec('dagger version 2>&1', $out, $code);
if ($code !== 0) { throw new \RuntimeException('dagger CLI not runnable: '.implode("\n", $out)); }

Try / catch

try {
    $client = Dagger::connect();
} catch (\RuntimeException $e) {
    if (str_contains($e->getMessage(), 'Cannot fetch informations from process session')) {
        // inspect dagger CLI output manually / update CLI
    }
    throw $e;
}

Prevention

Keep dagger CLI version aligned with the SDK version
Ensure docker/podman engine is running before connecting
Run `dagger` manually once to surface hidden startup errors

When it happens

Trigger: The dagger CLI process fails to start or crashes before printing session info (bad binary, unsupported flags, engine startup failure), so the callback never returns session data and $sessionInformation stays null.

Common situations: Dagger CLI version too old for the SDK's expected session JSON output; engine startup failure (e.g. docker not running); the CLI crashing due to missing dependencies.

Related errors

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/58049e857f43ef57. Report an issue: GitHub.