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What Happened

The FORGE workflow failed on 5 consecutive executions (125-129) at the same early node: Fetch Epic Issue returned HTTP 404. A previous debugging agent spent 1.5 hours applying two “fixes” to nodes that were 20+ nodes deep into the pipeline — nodes that were never reached in any failing execution. The fixes applied:
  1. SplitInBatches v3 → v1 downgrade on Iterate Blocks and Iterate Tasks
  2. A 5,148 → 6,434 character rewrite of Assemble FORGE Prompt
Both were speculative. Neither addressed the 404.

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The Actual Root Cause

The webhook payload sent epic_number but the workflow’s Set Configuration node reads $json.body.epic_issue_number. The epic issue number was silently undefined, cast to NaN, and the GitHub URL became .../issues/NaN — a 404. Time to fix once identified: 30 seconds.

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The Anti-Pattern

Hypothesis fixation: The agent assumed the failure was in complex internal logic (batch processing, template assembly) rather than checking the simplest thing first — whether the input data was correct. In n8n workflows, execution traces show exactly which node failed and its input/output data. Executions 125-129 all showed the same pattern: Set Configuration output epic_issue: NaN, then Fetch Epic Issue returned 404. The root cause was visible in the first execution’s trace data.

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The Rule: Trace From Input Forward

When debugging n8n workflows:
  1. Start at the webhook trigger — log the raw payload
  2. Check Set Configuration outputs match expectations
  3. Walk forward through the execution trace, node by node
  4. The first node that produces unexpected output is the problem
Do not jump to nodes deep in the pipeline until you’ve confirmed the early nodes are producing correct data.

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What Also Got Fixed (Positive Outcomes)

The debugging session also produced a genuine architectural improvement: replacing SplitInBatches feedback loops with vectorized $input.all().map() processing. This makes the workflow simpler (fewer nodes, no loop connections) and avoids n8n’s limitation where $('NodeName').all() only returns current-iteration items inside a SplitInBatches loop.

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Operational Takeaway

Field name mismatches between webhook callers and workflow consumers are the #1 cause of silent n8n failures. Always validate payload field names at the boundary before investigating deeper nodes.