Debugging Claude Skills: loops, bad tool calls, fixes

The first time a Claude agent loaded one of your Skills and immediately got stuck calling the same tool nine times in a row, you probably did what everyone does: stared at the transcript, scrolled up, and tried to reverse-engineer what the model was thinking. Skills make agents far more capable, but they also introduce a new debugging surface. A Skill is a folder of instructions, scripts, and resources Claude loads when it judges the work to be relevant — and when something goes wrong, the bug could live in the Skill's prose, in a referenced script, in the tool definitions an MCP server exposes, or in the model's own reasoning. This post is a practical map of the failure modes you will actually hit, and how to isolate each one.

Before debugging anything, get your terms straight. An Agent Skill is a discoverable bundle of instructions and optional code that Claude reads into context only when the current task matches the Skill's described purpose. That "only when relevant" loading is a feature for token economy and a liability for debugging, because the same prompt can behave differently depending on whether a Skill fired. Your first diagnostic question is always: did the Skill even load?

Why agents loop, and how to break the cycle

Loops are the most common and most maddening failure. The classic shape is a tool call that returns an error or an empty result, followed by Claude retrying the identical call with identical arguments, over and over until it exhausts a step budget. The root cause is almost never "the model is dumb." It is that the tool's error message gave Claude nothing actionable to change. A tool that returns {"error": "failed"} teaches the model nothing; a tool that returns {"error": "no record for id 4471; valid ids are returned by list_accounts"} tells Claude exactly what to do differently next turn.

The second loop pattern is the "two-tool ping-pong": Claude calls tool A, which suggests it needs data from tool B, calls B, which points back to A, and the agent oscillates. This is usually a Skill-instruction problem. If your Skill says "always verify the customer before updating" but never describes a terminal state, the agent has no exit condition. Fix it by writing the success criterion into the Skill explicitly: "Once you have a confirmed customer id and the update returns status ok, stop and report the result."

The third pattern is reasoning-level looping where no tool is even called — Claude keeps restating the plan without acting. That signals an over-long or contradictory Skill. When a Skill's instructions conflict ("be thorough" vs. "minimize steps"), the model can stall trying to satisfy both. Cut the contradictions and give a clear priority order.

flowchart TD
  A["Tool call returns result"] --> B{"Error or empty?"}
  B -->|No| C["Make progress, next step"]
  B -->|Yes| D{"Does error name a fix?"}
  D -->|Yes| E["Claude adjusts args, retries once"]
  D -->|No| F["Claude retries blindly = LOOP"]
  F --> G["Add actionable error + retry cap"]
  E --> H{"Succeeded?"}
  H -->|No, 2nd fail| I["Stop, surface error to user"]
  H -->|Yes| C

Wrong tool, right intent: dispatch failures

When Claude understands the goal but reaches for the wrong tool, the problem is almost always ambiguous tool descriptions. If you expose both search_orders and search_invoices with one-line descriptions that both say "find records," the model has to guess. Tool descriptions are prompts. Spend real words on them: state what the tool is for, what it is not for, and one example of when to choose it over a sibling tool. A description like "Use search_invoices ONLY for billing documents; for shipped-product lookups use search_orders" eliminates a whole class of misroutes.

Skills compound this because a Skill can introduce its own preferred tool while an MCP server exposes a generic one. If your Skill says "use the reporting helper" but three tools could plausibly be "the reporting helper," Claude picks inconsistently across runs. Name the exact tool in the Skill. Determinism in your instructions buys you determinism in behavior.

To diagnose dispatch errors, log every tool call with its full arguments and the Skill (if any) that was active. Then look for the divergence point: the step where a correct plan turned into the wrong call. Nine times out of ten you will find two tools whose descriptions overlap, and the fix is a one-line edit to make them mutually exclusive.

Hallucinated arguments and how to catch them early

The most dangerous failure is a confidently malformed tool call: Claude invents an account_id that looks plausible but does not exist, or passes a date in the wrong format, or fills a required field with a guessed value rather than asking. This happens when the model lacks the real value and the path of least resistance is to fabricate one. The structural defense is to make required inputs come from prior tool results, not from the model's imagination. If a delete operation needs an id, the Skill should instruct Claude to obtain that id from a list/search call first, never to construct it.

Defense in depth means validating at the tool boundary. Your MCP server should reject obviously invalid arguments with a precise error rather than executing them. A handler that checks if not is_valid_uuid(id): return error("id must be a UUID from list_accounts") turns a silent data-corruption bug into a visible, self-correcting loop — Claude reads the error and fetches a real id. Treat the tool layer as a contract, and the model's hallucinations become recoverable instead of destructive.

For numeric and enum arguments, constrain the schema. JSON Schema enums and ranges in your tool definitions give Claude a tighter target and let the runtime reject out-of-band values before they reach your business logic. The narrower the input space you advertise, the less room there is to hallucinate.

Tracing the bug to its real layer

The skill that separates fast debuggers from slow ones is layer isolation. When a Skill-driven run misbehaves, ask in order: Did the Skill load? (Check whether its instructions appear in context.) Were the tool descriptions clear? (Read them as the model would.) Did the tool return something actionable? (Inspect the raw result.) Did the model reason correctly given all that? Only the last question is genuinely about the model; the first three are about your configuration, and they account for the large majority of real bugs.

Reproduce with a frozen transcript. Capture the exact messages, tool definitions, and Skill contents from a failing run and replay them. Because the failure is usually configuration-driven, a frozen replay reproduces reliably even though raw sampling has variance. Change one variable at a time — sharpen one tool description, add one error detail — and re-run. This disciplined bisection beats rewriting the whole Skill and hoping.

Guardrails that prevent whole bug classes

Some bugs you fix; better ones you make impossible. Cap retries per tool so no loop runs forever. Set a total step budget per task so a confused agent fails loudly instead of burning tokens silently. Add a "when unsure, ask" instruction so the model surfaces missing inputs rather than fabricating them. And keep Skills small and single-purpose: a Skill that does one thing well produces failure modes you can reason about, while a sprawling Skill that does ten things produces emergent misbehavior you cannot.

Finally, write a few regression cases for every bug you fix. Each one becomes a tiny eval: the failing prompt, the expected tool sequence, and an assertion. Re-run them whenever you touch a Skill or tool description. Debugging agentic systems is less about clever fixes in the moment and more about turning each fix into a permanent guardrail.

Frequently asked questions

How do I tell whether a Skill actually loaded?

Inspect the model's context for the Skill's instruction text, or add a unique marker phrase to the Skill and check whether the run references it. If the marker is absent, the Skill never fired — fix its description so it matches the task, before debugging anything downstream.

Why does Claude retry the same failing call?

Because the error gave it nothing to change. Return errors that name the cause and the corrective action, add a per-tool retry cap, and the blind-retry loop disappears in most cases.

How do I stop hallucinated arguments?

Source required values from prior tool results rather than the model's guesses, constrain inputs with JSON Schema enums and formats, and validate at the tool boundary so bad arguments produce a corrective error instead of executing.

Are loops a model problem or a config problem?

Overwhelmingly a config problem: vague tool descriptions, unactionable errors, or missing exit conditions in the Skill. Fix the configuration first; reach for model or prompt changes only after the layers below are clean.

Bringing agentic AI to your phone lines

CallSphere takes these same debugging disciplines — actionable errors, tight tool contracts, hard exit conditions — and applies them to voice and chat agents that handle every call and message, call tools mid-conversation, and book work around the clock. See it live at callsphere.ai.

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Source & attribution: This is an independent, original explainer inspired by Anthropic's coverage on the Claude blog. Claude, Claude Code, Claude Cowork, Claude Opus, and the Model Context Protocol are products and trademarks of Anthropic. CallSphere is not affiliated with or endorsed by Anthropic.