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Freedom251 › Blog › Claude Code

Moving Decision Logic Out of an LLM: What Can Actually Break

By Freedom251 Team · · 9 min read

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The specific first-person experiment behind this headline cannot be verified: no primary account establishes which application, decision boundary, baseline, tests, or regressions were involved. The closest documented evidence points to a practical risk that is easy to miss: moving a decision from model-generated text into deterministic code can improve repeatability while introducing new failures at the handoff, trigger, schema, and fallback layers. Related Claude Code reports measured whether instructions were invoked at all, rather than assuming a correct-looking answer proved that they were.

What is known—and what is not

No verifiable primary account for the exact headline is available. That means the article cannot honestly claim that a particular routing rule, classifier, workflow, latency figure, cost change, or error rate came from the titled experiment. Its original and revised architectures, test set, and observed breakages remain unspecified.

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That uncertainty matters because “moving decision logic out of an LLM” can describe several very different changes. A team might replace a model’s choice of tool with an if/else branch, move policy checks into a service, require structured output before executing an action, or keep the model’s recommendation while making a deterministic controller responsible for approval. Each boundary creates different failure modes.

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What can be established is narrower but useful. A NewSumm roundup dated October 6, 2026 summarized independent reports about Claude Code skills, instruction files, trigger reliability, and testing. Those reports show why a before-and-after account must measure invocation and failure visibility, not just the quality of final prose. They do not prove what happened in the experiment named by this headline.

What “decision logic” means in an LLM system

An LLM can perform several jobs that are often casually called “decision-making.” Separating them makes the migration boundary explicit.

Function Typical model role Deterministic alternative New risk when moved
Interpretation Infer intent from natural language Usually remains with the model, or uses a constrained classifier Ambiguous inputs can be forced into the wrong category
Eligibility Decide whether an action appears allowed Policy engine, schema validation, or explicit rule set Rules may omit an edge case the model previously handled
Routing Select a tool, specialist, or workflow Code-based router using validated fields Missing, stale, or malformed fields can send work to the wrong path
Sequencing Choose the next step in a multi-step task State machine or workflow controller Unexpected states can halt execution instead of being recovered conversationally
Presentation Explain the result to the user Template or response formatter Messages become precise but less adaptable to unusual context

The safest boundary is usually not “model versus code” in the abstract. It is a contract: the model supplies a bounded, typed proposal; deterministic code validates it, applies non-negotiable policy, records the outcome, and decides whether execution may continue. If the model still chooses among unconstrained actions, the critical decision has not really moved.

Why teams move decisions into deterministic code

  • Repeatability: identical validated inputs can produce the same route or authorization result.
  • Auditability: a rule, version, and input can be logged instead of inferred from a long prompt and hidden model state.
  • Safety: limits such as allowed tools, spending ceilings, or data boundaries can be enforced without relying on instruction following.
  • Testability: a rule can be exercised against a fixed case table and boundary values.
  • Operational control: deterministic branches can be rolled back, feature-flagged, and monitored independently of model changes.

These benefits do not make the system automatically more correct. Code executes exactly what was specified, including an incomplete specification. A model may have handled an unanticipated phrasing through broad contextual reasoning; a strict router may instead reject it, choose a default, or silently discard information. The migration trades one class of uncertainty for another.

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Evidence from adjacent Claude Code evaluations

Reference files were read only when they were linked

One small Claude Code skill experiment tested whether the model would open a linked reference.md file. In that setup, the file was read in 16 of 16 runs when SKILL.md linked to it, and in 0 of 4 runs when no link was present. The experiment author, as summarized in NewSumm’s October 6, 2026 roundup, cautioned that this was one skill, one task, and a very small run count. It does not establish a general trigger probability for larger or longer sessions.

The implication for a decision-logic migration is architectural: a component can exist and still be functionally absent if the model is not reliably directed to invoke it. A deterministic policy service does not solve that problem if the model can bypass the call, omit the required field, or produce text that downstream code never validates.

A trigger-description change produced a separate regression

A different report deliberately changed a skill’s trigger description. Its evaluation-suite score fell from 1.00 to 0.36, while a trigger-only tripwire fell from 1.00 to 0.00. The author used repeated runs to distinguish a reliably broken trigger from ordinary model variability. These figures describe that author’s sabotage test in a distinct Claude Code setup; they are not measurements from the headline’s experiment.

This distinction is important when a team claims that “the new logic broke.” A failing end-to-end score may reflect a trigger, schema, tool-call, or execution problem rather than a bad rule. A dedicated invocation check helps locate the layer that changed.

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A public-skill scan is a warning, not a population estimate

NewSumm also reported a SkillCrossroads scan of 215 public Claude Code skills. The scan classified 69% as unlikely or borderline to trigger reliably under its rubric. That is a sample- and rubric-specific result from public repositories, not a reliability rate for all skills, agents, or LLM applications. It nevertheless supports a practical rule: treat activation as a measurable behavior, not an assumption.

What can break after the boundary moves

1. The input contract becomes narrower than the conversation

Natural language contains qualifiers, exceptions, and implied context. A rule engine may receive only a short label such as refund and lose facts such as purchase age, jurisdiction, prior attempts, or user authorization. Define the complete input schema before deleting model context. Reject missing fields explicitly, and preserve the original request for audit and recovery.

2. The model can bypass the new controller

If the model is still allowed to call tools directly, deterministic logic is advisory. Put the controller on the execution path: tool credentials, network permissions, or service endpoints should be available only after validation. Log attempted bypasses as failures, not merely as unusual text.

3. Rules drift from prompts, tools, and data

A policy may encode an old tool name, status value, account tier, or API response. Version the rule set alongside its schema and record which version made each decision. Add contract tests that fail when an upstream field changes type or meaning.

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4. Defaults hide errors

“Otherwise, continue” is convenient but dangerous. A missing or invalid decision should produce a visible state such as needs_review, insufficient_data, or blocked. A safe fallback can still return a helpful explanation, but it should not look like a successful authorization.

5. Edge cases become dead ends

State machines are excellent for known transitions and poor at unknown ones unless an escape path exists. Define recovery for contradictory inputs, partial tool results, timeouts, duplicate events, and user corrections. Keep a human or model-assisted clarification route outside the irreversible action path.

6. Determinism is mistaken for correctness

A reproducible wrong answer is still wrong. Keep a labeled evaluation set containing ordinary cases, boundary values, adversarial wording, missing fields, and previously observed incidents. Measure false approvals and false blocks separately; a single accuracy number can conceal a serious safety trade-off.

7. Performance claims are assumed rather than measured

Moving a branch into code may reduce model calls, but an added validation service, database lookup, or retry loop can offset that gain. The available evidence does not establish latency, token, infrastructure-cost, or throughput changes for the titled experiment. Measure those dimensions under the same workload if they matter to the decision.

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How to run a credible before-and-after evaluation

  1. Draw the boundary. Document every decision in the original path and mark whether it is made by the model, code, a human, or an external service. Name the exact point that will change.
  2. Freeze a case set. Include representative requests, ambiguous wording, malformed payloads, missing context, policy boundaries, tool failures, and regression cases. Store expected outcomes and the reason for each label.
  3. Define ownership and contracts. Specify the model’s permitted outputs, required fields, enumerated values, maximum sizes, and behavior for uncertainty. Specify which checks the controller must enforce regardless of model output.
  4. Instrument invocation. Record whether the model produced the required structure, whether the controller was called, whether validation passed, which branch ran, and whether an action executed. A polished final response is not proof that every intended component ran.
  5. Repeat stochastic paths. Run identical cases multiple times when a model trigger, classifier, or tool-selection step remains. Report the number of runs and the distribution of outcomes, not only the best result.
  6. Compare separate metrics. Track decision correctness, invalid or missing actions, unauthorized actions, fallback frequency, clarification rate, latency, token use, and operator-visible failures. Keep invocation reliability separate from end-task quality.
  7. Test the failure path deliberately. Remove a required field, rename a tool, return an empty result, delay a service, and present conflicting instructions. Verify that the system blocks or recovers in the intended way.
  8. Review changes with paired traces. For each regression, compare the original and revised traces: inputs, model output, parsed fields, rule version, tool calls, errors, and final user message. Classify the cause before changing a rule.
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How to interpret small-run results

Small experiments are useful for finding a failure, not for proving a universal reliability rate. Sixteen successful linked-reference reads show what happened in those 16 runs; they do not show how the behavior scales across tasks, models, prompt lengths, or sessions. Four unlinked-reference runs provide an equally narrow observation.

Repeated runs help with stochastic systems, but repetition does not repair a biased case set. If every test uses one phrasing and one happy path, a perfectly repeatable result can still miss the production failure. Report the model and configuration, sampling settings when relevant, case construction, run count, pass criteria, and all observed failure categories.

When a score changes, preserve both the aggregate and the denominator. A fall from a perfect score to a lower score is interpretable only when readers know how many cases were tested, whether cases were weighted, and whether the change affected invocation, parsing, policy, execution, or presentation.

A practical architecture for keeping decisions visible

A robust pattern is a staged pipeline:

  • Interpret: the model converts the request into a typed proposal and can mark uncertainty.
  • Validate: a parser rejects malformed or incomplete proposals before any side effect.
  • Authorize: deterministic policy checks identity, scope, limits, and current state.
  • Execute: only the controller can call the permitted tool or service.
  • Explain: the model or a template turns the recorded outcome into user-facing language without changing the decision.
  • Observe: logs retain the request identifier, proposal, rule version, branch, errors, retries, and final status.

This arrangement keeps language flexibility where it helps and places irreversible choices behind code that can be tested. It also makes a regression easier to localize: an absent controller call is different from a rejected schema, which is different from an incorrect policy branch.

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What a complete account of this experiment would need to show

  • The application, model and configuration, and the exact decision that moved.
  • The original and revised control-flow diagrams or an equivalent description.
  • The fixed task set, labels, run count, repeated-run method, and scoring formula.
  • Separate results for trigger or invocation, parsing, decision correctness, execution safety, and final response quality.
  • Concrete failure traces showing what broke, how it was detected, and whether recovery was automatic.
  • Any latency, token, infrastructure-cost, or throughput comparison, with identical workload conditions.
  • The rule, prompt, schema, and tool versions used during each measurement.

Without those details, a headline can suggest a compelling lesson while leaving the decisive facts unknown. The adjacent Claude Code evidence supports measuring activation and regression directly, but it cannot fill in the missing account.

The decision a reader can apply now

Move high-consequence choices behind deterministic validation when the inputs and policy can be specified clearly. Keep the model for interpretation, clarification, and explanation where flexibility is valuable. Then test the boundary itself: prove that the controller is invoked, that malformed proposals are visible, that defaults are safe, and that repeated runs do not hide a trigger failure. That is the part of the experiment that can be reproduced even though the original first-person measurements remain unavailable.

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Claude Codedeterministic systemsevaluationLarge Language ModelsLLM agentssoftware architecture

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