How do you stop an agent loop that never ends or costs too much?

Put hard limits around the loop in code: a maximum number of turns, a spending budget and a wall-clock timeout. Then find out why it loops. The usual causes are tool errors the model cannot understand, a goal with no clear end, and a tool that returns too much. Limits stop the damage; better tools and prompts stop the cause.

1. Hard limits (never optional)

• Turns. The Claude Agent SDK has max_turns (maxTurns in TypeScript), which counts tool-use turns. With no setting there is no limit. When it is hit, the run ends with the result subtype error_max_turns.
• Budget. max_budget_usd (maxBudgetUsd) stops the run once its cost passes a figure, with the subtype error_max_budget_usd. The check runs between turns, so a run can go over by about one response. Set it a little below what you can afford.
• Time. A timeout on the whole run and on each tool call, so one hung command cannot block everything.
• Tokens per response. max_tokens caps each reply.

Anthropic's "Building effective agents" lists a maximum number of iterations among the stopping conditions an agent should have, because errors compound over long runs.

2. Detect loops

• The same tool with the same input 3 times in a row: stop, or return a result that says "you already tried this; it failed because …".
• No progress: no new file changed, no new fact found for N turns.
• Log every turn with its tool, input size, output size and cost, so a stuck run is visible on a dashboard rather than on the invoice.

3. Fix the causes

• Unclear errors. "Error 500" makes the model retry blindly. "Table sales.orders does not exist; tables in sales are: order_lines, customers" lets it correct itself.
• No finish line. Say in the task what done looks like ("stop when all tests pass, or after 2 failed fixes, and report what you tried").
• Huge outputs. A tool returning 50,000 lines fills the context, and the model loses track. Truncate and paginate.
• Too much freedom. If the path is known, use a fixed workflow for that part.

4. Fail usefully

When a limit stops the run, return what was done and why it stopped, and keep the session so a person can resume it. Both "hit max turns after fixing 3 of 5 tests" and a session id are more useful than a blank failure.

A good closing line

"Limits are for safety, and transcripts are for fixing. I read the runs that hit a limit and change the tool or the prompt so the next one doesn't."