Backend Troubleshooting

When the AI: OK / Degraded / Offline pill in the top bar reports a problem, work through this page from top to bottom. The first three sections (Health States, Diagnostic Flow, Switching Backend) apply to every backend. The Per-Backend Error Signatures section covers the specific failure shapes you are most likely to hit.

Health States Recap

The plugin polls the active backend every five seconds. The polled state is rendered as a colored pill plus tooltip:

Pill	Internal state	What you should do
AI: OK	Healthy	Nothing — backend is responding.
AI: Degraded	Degraded	Read the tooltip. Usually transient (slow first token, soft rate-limit) — retry the in-flight request before changing anything.
AI: Offline	Offline / Unavailable	Read the tooltip. This means the next request will either fail fast (HTTP) or never start (CLI). Fix or switch backend.

Each state transition lands as an entry in the AI Request Logger. Use the Type filter set to ERROR / RETRY to see the recent failure timeline alongside backend metadata.

Diagnostic Flow

1. Read the tooltip. The pill tooltip carries the underlying error message from the health probe. That message is almost always enough to identify the root cause.

2. Look at the AI Request Logger. Filter by trace ID for the most recent prompt. The ERROR entry usually carries the upstream status code or process exit code.

3. Trigger a one-off chat prompt. A single "hello" is the cheapest reproduction. If the chat fails with the same error, the issue is the backend itself, not a particular scanner pipeline.

4. Check the backend-specific section below.

5. Switch backends. If the failure is upstream (API outage, rate-limit) and you have a configured alternative, change Preferred Backend under Settings → AI Backend rather than waiting it out.

When the Circuit Breaker Has Tripped

HTTP backends are wrapped in a circuit breaker that opens after 5 consecutive failures. While open, the next probe fails fast with <backend> backend is temporarily unavailable (circuit open) and the AI Request Logger shows synthetic errors instead of new requests.

• The breaker stays open for 30 seconds, then allows a single half-open probe. Success closes it; failure reopens it.

• The breaker resets immediately when you switch backends — the circuit is per-backend.

• If you genuinely fixed the upstream issue and do not want to wait, switch backend and switch back to force a fresh probe.

Per-Backend Error Signatures

HTTP backends (Ollama, LM Studio, NVIDIA NIM, Perplexity, Generic OpenAI-compatible)

Symptom	Likely cause	Fix
Connection refused on probe	Local server not running (Ollama, LM Studio) or wrong URL	Start the server (ollama serve, lms server start) or verify the URL field.
401 Unauthorized	API key missing, wrong, or expired	Re-paste the API Key; check it starts with the expected prefix (pplx-…, nvapi-…, sk-…).
404 Not Found on the chat endpoint	Wrong base URL or wrong path. Common with Perplexity if pointed via Generic OpenAI-compatible (which expects /v1/chat/completions).	Use the dedicated Perplexity backend, not Generic. Verify the URL does not double-up /v1/.
400 Bad Request mentioning response_format	Backend does not support JSON mode but a request forced it	Use a backend that supports JSON mode for scanner workflows, or accept text-mode parsing on Perplexity.
model_not_found / invalid_model	Model identifier typo	Check the model name matches the provider's catalog exactly (case-sensitive).
Slow first token, then Degraded recovers to OK	Cold-start latency on shared infra (NVIDIA NIM, Perplexity)	Normal. Repeat the request; subsequent ones are fast.
Persistent Degraded with retries in the logger	Provider rate limiting (soft 429s wrapped as retryable)	The plugin retries 6 times with stepped backoff. If retries do not clear it, switch to a different model or backend.
<backend> backend is temporarily unavailable (circuit open)	5 consecutive failures tripped the breaker	See "When the Circuit Breaker Has Tripped" above.

Burp AI (built-in, Burp Pro only)

Symptom	Likely cause	Fix
AI: Offline with tooltip Burp AI is not enabled	Settings → Burp AI → Use AI for extensions is off	Toggle it on inside Burp Suite. The plugin picks it up on the next health cycle (within 5 s) — no restart.
AI: Offline and Burp Community	Backend is Pro-only	Switch to any non-Burp-AI backend.
Quota errors surfaced as ERROR entries	Burp Pro AI credits exhausted	Top up credits via PortSwigger, or switch backend.
Backend missing from the Preferred Backend dropdown	Older Montoya API without ai() surface	The plugin auto-hides Burp AI when the API surface is absent. Upgrade Burp Suite.

CLI backends (Gemini, Claude, Codex, Copilot, OpenCode)

Symptom	Likely cause	Fix
command not found in health output	CLI not on the PATH inherited by Burp	Pass an absolute path in the corresponding CLI Command setting, or relaunch Burp from a shell where the CLI works.
CreateProcess error=193 (Windows)	npm-installed CLI shim isn't directly executable from Java	Point to the .cmd shim (e.g., C:\\Users\\<you>\\AppData\\Roaming\\npm\\claude.cmd). See Backends Overview → Windows npm Shim Resolution.
Process exits with 1 on first prompt	CLI not authenticated in this user account	Run the CLI's auth command in the same shell environment Burp inherits (claude /login, gemini auth login, etc.), then retry.
Output is blank or truncated	OpenCode idle timeout fires before first token	The plugin terminates an OpenCode subprocess after 30 seconds of idle stdout. Retry after warming the model, or pick a different backend for cold-start workloads.
Very long prompts produce no output	Claude/Copilot CLI fallback path threshold (32000 chars)	The CLI is being fed a smaller prompt by design. Trim context (lower the Manual context body chars caps) or switch to an HTTP backend that streams.
AI: Offline after a successful run	Supervisor decided the process exited unexpectedly	Auto-Restart is on by default — wait one health cycle. If it never recovers, check Burp's Output tab for the supervisor's diagnostic.

Switching Backend Without Losing Work

The active chat session stores its conversation history per-session, not per-backend. Changing Preferred Backend affects only new sessions and the scanner pipelines. To move a stuck session to a different backend:

1. Note the title of the stuck session.

2. Right-click the session → Export as Markdown.

3. Switch Preferred Backend.

4. Open a fresh chat session and paste the relevant context back.

For scanners, switching the backend takes effect on the next analysis cycle. Anything mid-flight finishes (or fails) on the original backend.

Cross-references

• Backends Overview for capability matrix, retry schedule, output token limits, and CLI detection rules.

• Per-backend pages — each has its own troubleshooting block for provider-specific quirks:

  • Burp AI (built-in)

  • Ollama, LM Studio

  • NVIDIA NIM, Perplexity, Generic OpenAI-compatible

  • Gemini CLI, Claude CLI, Codex CLI, Copilot CLI, OpenCode CLI

• Troubleshooting (general)

• AI Request Logger