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 selected backend every five seconds. The backend-specific result is rendered as a colored pill plus tooltip:

Pill	Internal state	What you should do
AI: OK	Healthy	The configured check passed. For CLI backends this checks executable resolution, not authentication; for Anthropic it checks only that a key is present.
AI: Degraded	Degraded	Read the tooltip. Current HTTP checks use this for authentication failures, and NVIDIA NIM/Perplexity also use it for HTTP 429.
AI: Offline	Unavailable	Read the tooltip. The endpoint/configuration check failed, the CLI executable is unresolved, or Burp AI is disabled.

Status transitions are not logger events. Use the AI Request Logger ERROR / RETRY filters for failures and retries produced by actual model requests.

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

Each launched HTTP connection has a circuit breaker that opens after 5 recorded transient failures. While open, the next model request on that connection fails fast with <backend> backend is temporarily unavailable (circuit open) and the AI Request Logger records the error instead of sending it upstream.

• The breaker stays open for 30 seconds, then allows one half-open model request. Success closes it; failure reopens it.
• Transport exceptions count per failed attempt. HTTP 429 and 5xx responses count once; other 4xx responses do not open the breaker.
• The breaker belongs to the launched connection, not the top-bar health checker. Start a new chat session for a fresh connection, or wait for the half-open attempt; changing the preferred backend does not rewrite an existing session's breaker.

Per-Backend Error Signatures

HTTP backends (Ollama, LM Studio, NVIDIA NIM, Perplexity, Anthropic, 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).
Degraded with HTTP 401/403 in the tooltip	The live health endpoint was reachable but rejected authentication	Re-paste the key and verify it belongs to the configured endpoint.
Degraded with HTTP 429 while NVIDIA NIM or Perplexity is selected	The five-second live completion used for health is being rate-limited	Review quota and provider logs, or select another backend to stop that health traffic. HTTP 429 is not retried inside the same model call.
<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 UI health cycle (about 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.
Burp AI is present in Preferred Backend but remains unavailable	Burp Community, disabled Use AI, or unavailable Montoya AI surface	Use Burp 2026.2+ Professional with Use AI for extensions enabled, or select an independent backend. Registered backends remain listed even when their health check reports unavailable.

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.
AI: OK, but the first prompt exits with an authentication error	CLI health checks executable resolution, not provider authentication	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 full combined prompt was written to an owner-only temp file where POSIX permissions are supported, and the CLI received an instruction containing that path. Check that the CLI can read the local temp path, or trim the manual-context caps.
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, Anthropic, Generic OpenAI-compatible
  • Gemini CLI, Claude CLI, Codex CLI, Copilot CLI, OpenCode CLI
• Troubleshooting (general)
• AI Request Logger