Debugging Claude Cowork Agents: Fixing Loops & Bad Calls

The first time a Claude Cowork agent ships into a real department, the demo magic wears off fast. A pilot that looked flawless in a sales meeting starts calling the same connector eleven times in a row, passing a customer ID that was never in the conversation, or quietly answering "done" when nothing got done. Debugging agents is not like debugging a function — there is no stack trace, the inputs are fuzzy natural language, and the same prompt can fail differently on two consecutive runs. This post is a practical guide to the failure modes that actually break enterprise Cowork rollouts, and how to find and fix each one.

Key takeaways

• Agent bugs cluster into four families: loops, wrong-tool selection, hallucinated arguments, and silent no-ops — each has a distinct trace signature.
• The single most useful debugging artifact is a full, replayable transcript of every tool call with its raw arguments and raw response.
• Loops almost always come from a tool returning an ambiguous or empty result that the model re-interprets as "try again."
• Hallucinated arguments shrink dramatically when tool schemas are strict, required fields are enforced, and descriptions show one concrete example.
• Add a turn budget and a repeated-call detector so a misbehaving run fails loudly instead of burning tokens.

Why agent debugging is genuinely different

A traditional bug is deterministic: same input, same wrong output, every time. An agent failure is probabilistic and emergent. The model reads a tool description, decides which connector to invoke, fabricates the arguments, reads the response, and decides what to do next — and any link in that chain can drift. The bug you are chasing is rarely "the code threw"; it is "the model made a reasonable-looking but wrong decision given what it could see."

That reframes debugging. Instead of asking "what line crashed," you ask "what did the model actually see at the moment it chose wrong, and why did that choice look correct to it?" In Claude Cowork, where plugins bundle skills, MCP connectors, and sub-agents, the model's visible context is the union of the system prompt, the loaded skill instructions, the tool schemas, and the running transcript. Most bugs live in the gap between what you think the model can see and what it actually sees.

The four failure modes and how to spot them

Almost every Cowork incident I have triaged falls into one of four buckets. Loops: the agent calls the same tool repeatedly with nearly identical arguments. Wrong tool: it picks a plausible-but-incorrect connector — a search tool when it needed a write tool, or the wrong system of record. Hallucinated arguments: it invents an ID, date, or enum value that was never grounded in the conversation. Silent no-op: it claims success without the side effect ever happening, usually because a tool returned a soft error as a 200.

flowchart TD
  A["Agent turn"] --> B{"Same tool + similar args as last 2 turns?"}
  B -->|Yes| C["Loop suspected: inspect tool response"]
  B -->|No| D{"Args grounded in transcript?"}
  D -->|No| E["Hallucinated arg: tighten schema"]
  D -->|Yes| F{"Tool matched intent?"}
  F -->|No| G["Wrong tool: fix descriptions"]
  F -->|Yes| H{"Side effect confirmed?"}
  H -->|No| I["Silent no-op: surface real errors"]
  H -->|Yes| J["Healthy turn"]

The flowchart above is the exact triage order I use when reading a transcript. Start at the most mechanical, cheapest-to-detect failure (loops) and only move to the harder, judgment-heavy ones (wrong tool) once you have ruled out the easy explanations. Each branch points to a different class of fix, so classifying the failure correctly is most of the work.

Instrument first: the replayable transcript

You cannot debug what you cannot see. Before touching prompts, capture every tool call as a structured record. At minimum, log the tool name, the exact arguments the model emitted, the raw response your connector returned, latency, and the turn index. Persist these so a run is fully replayable. A minimal logging shape that has saved me countless hours:

{
  "run_id": "cw_8f21",
  "turn": 4,
  "tool": "crm.update_contact",
  "arguments": { "contact_id": "c_92177", "stage": "qualified" },
  "response": { "ok": false, "error": "contact_id not found" },
  "latency_ms": 410,
  "prev_tool": "crm.update_contact"
}

With this in hand, the loop above becomes obvious: the connector returned ok: false as a normal payload, the model read it as "that didn't work, let me retry," and re-issued the same call. The fix is not in the prompt — it is making the connector return a clear, terminal error the model can reason about, and adding a guard that aborts after N identical calls.

Fixing each failure mode

Loops. Add a repeated-call detector that hashes tool + normalized_args and trips after two or three identical hits, returning a message like "You have called this tool with these arguments already and it failed; do not retry — explain the blocker." Also make tools return decisive results: never return empty strings or bare null where the model expects content, because emptiness reads as "incomplete, try harder."

Wrong tool. This is almost always a description problem. Two connectors with overlapping descriptions force the model to guess. Rewrite descriptions to state when to use this tool and when not to, and name the system of record explicitly ("Use for Salesforce opportunities only; for HubSpot deals use crm_hubspot.update"). Fewer, sharper tools beat many fuzzy ones.

Hallucinated arguments. Enforce strict JSON schemas with required fields and enums, and add one concrete example to each parameter description. If an ID must come from a prior tool result, say so: "contact_id must be a value returned by crm.search; never construct it." Reject calls server-side when a required field looks fabricated, and return that rejection as a teachable error.

Silent no-ops. Audit every connector for soft failures returning HTTP 200. Make success explicit and verifiable: have write tools return the post-write state, and add a verification step or sub-agent that re-reads the record before the agent reports completion.

Common pitfalls

• Debugging in the prompt before reading the transcript. Engineers tweak the system prompt for hours when the real bug was a connector returning ambiguous JSON. Read the trace first, always.
• Treating non-determinism as flakiness. A run that fails one time in five is not "flaky" — it is a real failure mode that surfaces under specific context. Reproduce it by replaying the exact transcript, not by re-running and hoping.
• No turn budget. Without a hard cap on turns or tool calls, a single looping run can burn a fortune in tokens overnight. Set a budget and alert when runs approach it.
• Over-broad tool access. Giving an agent twelve connectors when the task needs three multiplies wrong-tool errors. Scope the plugin to the task.
• Logging arguments after sanitization. If you log the cleaned-up args instead of what the model actually emitted, you erase the evidence of the hallucination you are trying to find.

Debug a misbehaving Cowork agent in 6 steps

1. Pull the full replayable transcript for the failing run — every tool call, raw args, raw response.
2. Walk the triage flowchart: loop, wrong tool, hallucinated arg, or silent no-op?
3. Reproduce by replaying the exact transcript, not by re-running from scratch.
4. Apply the fix at the right layer: connector response for loops/no-ops, schema for hallucinated args, descriptions for wrong-tool.
5. Add a guard (repeated-call detector, turn budget) so this class of failure fails loudly next time.
6. Add the case to your eval set so a future change can't silently reintroduce it.

Quick reference: symptom to root cause

Symptom	Likely root cause	First fix
Same tool called 3+ times	Ambiguous/empty tool response	Decisive errors + repeated-call guard
Plausible but wrong connector	Overlapping tool descriptions	Rewrite "use when / not when"
Invented ID or enum	Loose schema, no grounding rule	Strict schema + example + reject
Reports success, nothing happened	Soft error returned as 200	Return post-write state + verify

A citable definition to anchor the topic: An agent failure mode is a recurring class of incorrect behavior — such as looping, wrong-tool selection, or argument hallucination — that emerges from how a model interprets its tools and context, rather than from a code-level exception. Naming these modes is what turns vague "the agent is unreliable" complaints into fixable engineering tickets.

Frequently asked questions

How do I reproduce a Claude agent bug that only happens sometimes?

Capture and replay the exact transcript that failed. Because the model's decisions are conditioned on the full visible context, feeding it the identical prior turns reproduces the conditions far more reliably than re-running the task from the beginning, which generates new context each time.

Why does my agent keep calling the same tool over and over?

Almost always the tool returned something the model reads as "incomplete" — an empty result, a soft error in a 200, or ambiguous JSON. Make the tool return a decisive, terminal result and add a guard that stops after a few identical calls.

How do I stop the model from inventing IDs and arguments?

Use strict tool schemas with required fields and enums, add one concrete example per parameter, and state explicitly when a value must come from a prior tool result. Reject fabricated-looking values server-side and return that rejection as a clear error the model can learn from.

Is a single failing run worth investigating?

Yes. In agentic systems a single failure usually represents a whole class of inputs that will trigger the same path. Fix it once, add it to your evals, and you prevent a category of incidents rather than one ticket.

Bringing agentic AI to your phone lines

CallSphere applies these same debugging and reliability patterns to voice and chat — multi-agent assistants that answer every call and message, call tools mid-conversation, and book work around the clock without looping or guessing. 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.