Reproduce the symptom with the smallest faithful path. If it cannot be reproduced, state that limitation and keep the eventual fix unverified.
Gather the actual error, state, inputs, and timing. Use existing logs and observability first; instrument only when the request authorizes that edit.
Follow the causal chain by repeatedly asking why each observed condition exists. Stop at the layer whose behavior explains the full reproduction.
Reject symptom patches such as a nil guard that merely silences a crash or a long comment defending a workaround.
Search for the same causal pattern across the relevant codebase. Report out-of-scope instances, but modify only the authorized scope.
For failures that appear only after restart, inspect changing persistent state before unchanged code: configuration, caches, lockfiles, serialized state, and abandoned temporary artifacts. Clearing state can support a diagnosis; it is not itself a durable fix.

Report the reproduction, evidence, causal chain, root cause, pattern scan, proposed repair, and verification path. Implement only when the user's request includes fixing the problem.

Trace a constraint

A cost, an estimate, a limit, or "we always do it this way" is a claim with a source, not a fact to plan around. Follow its causal chain the same way.

Quote the constraint as stated and name its source: a person, a ticket, a vendor quote, a past plan, or a measurement. "The ticket says so" locates the source; it is not the evidence.
Break the constraint into the parts that produce it. Stop one or two levels below the claim, where the verdict stops changing.
Tag each part. Fixed: physics, math, or a measured limit, with the evidence cited. Priced: a contract, regulation, or policy that can change, with its change cost named. Inherited: a habit, an old estimate, or an analogy to a past project, with nothing behind it.
Compute the floor from the fixed parts alone, using the few parts that dominate it. Say what the gap between the floor and the claim is made of and whether it is removable.

Report the constraint, its source, each part with its tag and evidence, the floor, the gap, and the cheapest test that would settle it. Inside a plan, an inherited constraint becomes an open question, never a requirement.

Adapted from Lauren Tan's PStack principle-fix-root-causes at commit 60c641e4fad674784b30abcf9f8915dea39df38d under the MIT License. See third-party notices. The constraint section follows the method in Farnam Street's What is First Principles Thinking?.

qstack-fix-root-causes

Fix root causes

Diagnose a failure