windmill-labs/windmill · error

Failed to parse SSE data: ${parseErr}

Error message

Failed to parse SSE data: ${parseErr}

What it means

In `wmill app dev`'s job streaming: a line read from the job's SSE stream failed JSON.parse (or the event decode). Streaming continues with the next line, so at worst one job update is missed — the completed message would end the stream via reader.cancel().

Source

Thrown at cli/src/commands/app/dev.ts:2066

                }),
              );
            }

            // Check if job is completed
            if (update.completed) {
              ws.send(
                JSON.stringify({
                  type: "streamJobRes",
                  reqId,
                  error: false,
                  result: update.only_result,
                }),
              );
              reader.cancel();
              return;
            }
          } catch (parseErr) {
            log.warn(`Failed to parse SSE data: ${parseErr}`);
          }
        }
      }
    }
  } finally {
    reader.releaseLock();
  }
}

View on GitHub (pinned to e474e8803c)

Solutions

1. Check for proxy/encoding corruption between the backend and the CLI (unescaped HTML, truncated chunks)
2. Update CLI if the SSE event format is newer than it understands
3. Retry the run — transient partial frames usually don't recur

When it happens

Trigger: Thrown at cli/src/commands/app/dev.ts:2066 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/8448a4b100c5e6ed. Report an issue: GitHub.