- Create patterns/ directory with README, manifest, and 10 initial patterns covering Ollama JSON fallback, API escaping, deprecation, PTY auth, queue-poll, LLM-as-parser, credential rotation, reverse proxy binding, human approval gates, and transient retry. - Wire pattern loading into architecture/pipeline.js based on task tags. - Update architecture/orchestrator.js to load patterns and surface them in the system prompt. - Update MEMORY.md, ARCHITECTURE.md, and CONTEXT.md to document the registry and record the decision.
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Pattern: Ollama Structured Output Fallback
Symptom
A workflow calls a local Ollama model (e.g., gemma3:4b, small quantized models) with a structured-output or JSON schema request, but the returned text is not valid JSON, misses required fields, wraps the JSON in prose, or otherwise fails schema validation.
Affected Projects
- Cyber Tips Newsletter pipeline
- Proxmox VE snapshot summarizer
- AI video generation pipeline (ComfyUI + WAN 2.1 payloads)
- Forex trading analysis workflow
Root Cause
Small local instruction-tuned models have weaker schema adherence than frontier APIs. They may produce JSON-like text that does not strictly conform to the requested schema, especially under complex prompts or when asked to combine generation with strict formatting.
Standard Fix
Add a downstream JavaScript Code node (or equivalent parser) in n8n that:
- Attempts a strict
JSON.parse()first. - On failure, applies regex-based JSON extraction to pull the first
{...}or[...]block from the text. - Optionally sanitizes common issues (trailing commas, unescaped newlines, code fences).
- Falls back to a safe default or error flag if extraction still fails.
This is a deliberate workaround, not a substitute for fixing the model. Document it as such wherever applied.
When to Apply
- Any new n8n workflow that uses a local Ollama model for structured extraction, classification, or JSON generation.
- Any integration where the downstream node requires strict JSON and the LLM is under ~8B parameters or known to drift.
Verification
- Test the fallback with intentionally malformed LLM output.
- Confirm downstream nodes receive valid parsed JSON.
- Log fallback events so model quality can be monitored separately.
Related Patterns
json-escaping-downstream-apillm-as-parser-fallback