Reasoning Failure Taxonomy

Six reasoning failure modes account for the vast majority of chain-of-thought errors in production. Each has distinct diagnostic signatures and prompt-level fixes. Naming the failure mode is the prerequisite for fixing it.

1. 1. Wrong Premise The model begins with an incorrect assumption about the problem. Signature: early steps are logically valid but the initial framing is factually wrong. Fix: inject the correct premise explicitly in the prompt ("The current onboarding process takes 3 weeks" rather than letting the model assume). Or add step-back prompting to force premise verification.
2. 2. Missed Constraint The model acknowledges a constraint early in the trace, then violates it later. Signature: the constraint appears in step 1-2 but disappears from the reasoning by step 4+. Fix: add a constraint tracking instruction: "At each step, verify that your reasoning remains within the constraints: [list constraints]."
3. 3. Invalid Inference The model draws a conclusion that does not logically follow from the stated premises. Signature: premises are correct, but the "therefore" statement does not follow. Fix: add intermediate steps. Invalid inferences are most common when the model jumps from premise to conclusion without intermediate steps. More steps = more checkpoints.
4. 4. Hallucinated Fact The model introduces a false factual claim to fill a gap in its knowledge. Signature: a specific claim appears in the reasoning without corresponding support in the provided context, and the claim is wrong. Fix: add grounding constraints ("Use only the information provided. If required information is absent, state it is absent."). Or add explicit uncertainty requirements.
5. 5. Reasoning Loop The model revisits the same consideration without making progress. Signature: steps repeat variations of the same point without advancing toward a conclusion. Fix: add progress requirements to the CoT instruction: "Each step must advance toward the conclusion. Do not repeat a consideration that has already been addressed."
6. 6. Premature Conclusion The model concludes before completing the required reasoning steps. Signature: the conclusion appears too early; subsequent steps are justifications rather than reasoning. Fix: require a minimum number of steps, or require that specific criteria be addressed before the conclusion is allowed: "Do not state a conclusion until you have addressed [criteria list]."