Debugging with GitNexus

When to Use

• "Why is this function failing?"
• "Trace where this error comes from"
• "Who calls this method?"
• "This endpoint returns 500"
• Investigating bugs, errors, or unexpected behavior

Bind the repository first

A root cause traced in the wrong repository is a wrong root cause.

Call list_repos {} before the first tool call. With one indexed repository, use the examples below as written. With more than one, pass repo on every call: an omitted repo normally errors, but under an MCP policy with a configured default it resolves to that default silently. If you cannot tell which repository is meant, stop and ask. This matters most for cypher, whose statement carries no in-band hint of which database it ran against.

list_repos is paginated, so page with offset: pagination.nextOffset until hasMore is false before concluding a repository is absent.

A stale index describes the code from before your bug, so refresh before trusting a trace, and state the repository and index freshness with the diagnosis.

Workflow

0. list_repos {}                                          → Bind repo
1. query({search_query: "<error or symptom>"})            → Find related execution flows
2. context({name: "<suspect>"})                    → See callers/callees/processes
3. READ gitnexus://repo/{name}/process/{name}                → Trace execution flow
4. cypher({statement: "MATCH path..."})                 → Custom traces if needed

If "Index is stale" → run node .gitnexus/run.cjs analyze in terminal.

Checklist

- [ ] list_repos {} — bind repo; explicit repo when >1 indexed, ask if ambiguous
- [ ] Understand the symptom (error message, unexpected behavior)
- [ ] query for error text or related code
- [ ] Identify the suspect function from returned processes
- [ ] context to see callers and callees
- [ ] Trace execution flow via process resource if applicable
- [ ] cypher for custom call chain traces if needed
- [ ] Read source files to confirm root cause
- [ ] State the repository and index freshness with the diagnosis

Debugging Patterns

Symptom	GitNexus Approach
Error message	query for error text → context on throw sites
Wrong return value	context on the function → trace callees for data flow
Intermittent failure	context → look for external calls, async deps
Performance issue	context → find symbols with many callers (hot paths)
Recent regression	detect_changes to see what your changes affect — pass worktree for a linked worktree
"How does A reach B?"	trace between the two symbols — shortest call chain in one call

Tools

query — find code related to error:

query({search_query: "payment validation error", repo: "my-app"})
→ Processes: CheckoutFlow, ErrorHandling
→ Symbols: validatePayment, handlePaymentError, PaymentException

context — full context for a suspect:

context({name: "validatePayment", repo: "my-app"})
→ Incoming calls: processCheckout, webhookHandler
→ Outgoing calls: verifyCard, fetchRates (external API!)
→ Processes: CheckoutFlow (step 3/7)

cypher — custom call chain traces. Pass repo alongside the statement; the Cypher text itself names no repository, so the result is unattributable without it:

MATCH path = (a)-[:CodeRelation {type: 'CALLS'}*1..2]->(b:Function {name: "validatePayment"})
RETURN [n IN nodes(path) | n.name] AS chain

trace — shortest call chain between two symbols ("how does A reach B?"), one call instead of chaining context hops:

trace({ from: "processCheckout", to: "fetchRates", repo: "my-app" })
→ status: ok, hopCount: 3
→ hops: processCheckout → validatePayment → verifyCard → fetchRates
→ edges: CALLS (1.0), CALLS (0.95), CALLS (1.0)

When no path exists, trace reports the furthest reachable node — exactly where the chain breaks (dynamic dispatch, reflection, or an external boundary).

Example: "Payment endpoint returns 500 intermittently"

0. list_repos {}
   → total: 2 (my-app, billing-api) — bind my-app explicitly on every call

1. query({search_query: "payment error handling", repo: "my-app"})
   → Processes: CheckoutFlow, ErrorHandling
   → Symbols: validatePayment, handlePaymentError

2. context({name: "validatePayment", repo: "my-app"})
   → Outgoing calls: verifyCard, fetchRates (external API!)

3. READ gitnexus://repo/my-app/process/CheckoutFlow
   → Step 3: validatePayment → calls fetchRates (external)

4. Root cause: fetchRates calls external API without proper timeout
   Repository: my-app  Index: current

With a single indexed repository, step 0 returns total: 1 and the repo argument drops out of every call above.