Option D separates durable case history from volatile operational data. The new session receives a compact, structured summary describing the billing dispute, the customer's objective, actions previously taken, and the unresolved status. It does not inherit outdated backend observations as though they were still authoritative.
Fresh tool calls then retrieve the current refund or order state before the agent responds.
Agent SDK sessions preserve conversation history, including earlier tool calls and tool results. Resuming the complete transcript therefore reintroduces stale system data into the active context, even though the external backend may have changed substantially during the four-hour gap. Conversation persistence must not be confused with persistence of external-system truth.
Option A performs unnecessary calls to every previously used tool, including tools unrelated to the returning customer's current question. Option B relies on prompt compliance while retaining contradictory historical evidence in context. Option C manually removes tool results from an existing transcript and may damage the logical relationship between prior tool_use and tool_result blocks while still retaining a long, unstructured conversation.
A structured summary should preserve stable identifiers, previous actions, customer commitments, and unresolved issues. Time-sensitive fields such as refund status, delivery state, account balance, or expected resolution should always be refreshed through authoritative tools.
Official references/topics: Session persistence, stale tool-result management, context compaction, fresh-data retrieval.

Option C is the only approach that guarantees the required terminal condition independently of the model's behavior. When the agent loop stops, the orchestration layer evaluates the recorded outcome. If neither a successful resolution nor a confirmed human escalation occurred, deterministic application code invokes escalate_to_human using the information accumulated before termination.
The max_turns limit is a host-controlled safety boundary. Once that boundary is reached, the model may no longer have an opportunity to follow a prompt instruction or make another tool call. Therefore, the required fallback cannot depend entirely on Claude deciding to escalate before its remaining turns are exhausted.
Option A escalates based on an arbitrary 80% threshold and may prematurely terminate cases that could have been resolved successfully within the remaining budget. Option B provides additional turn capacity but does not guarantee that the second agent will complete its work or escalate before its own limit is reached. Option D remains probabilistic and cannot operate after an unexpected termination has already occurred.
The orchestration fallback should be idempotent, ensuring that repeated completion checks cannot create duplicate escalation records. It should also include verified customer information, relevant order data, completed actions, failure reasons, and the unresolved request in the handoff payload.
Official references/topics: Agent-loop termination, deterministic fallback orchestration, terminal-state validation, human handoff guarantees.

Option A establishes a clear destination without forcing the subagent to follow a brittle route. The prompt should define the research question, boundaries, expected deliverable, acceptable source types, recency requirements, coverage expectations, and verification criteria. The subagent can then broaden, narrow, reformulate, or follow emerging leads according to observed search results.
Anthropic's research-agent guidance emphasizes giving each subagent an objective, output format, source guidance, and clear task boundaries. It also describes research as dynamic and path-dependent, requiring agents to adapt their strategy as new information emerges.
Option B removes too much structure and may cause duplicated work, incomplete coverage, or inconsistent output. Option C handles only one known failure condition and still restricts the agent to a predetermined procedure. Option D adds classification complexity and creates an artificial boundary between topic types that may change during investigation. Objective-based delegation retains accountability because results can be evaluated against explicit quality criteria, while preserving the agent's ability to discover alternative terminology, unexpected sources, and useful adjacent evidence. This is controlled autonomy rather than either procedural micromanagement or unrestricted exploration.