| name | prp-spike |
| description | Prove or disprove an idea by building the smallest throwaway artifact that could falsify it, in an isolated worktree, ending in a PROVEN / DISPROVEN / CONDITIONAL verdict backed by evidence - never a PR. Use when the user wants to "spike this", "build a proof of concept", "POC this", "prototype it to find out", asks "is this possible", "is this feasible", "does this fit our stack", or "would it be worth changing X to allow Y" and wants it settled by building rather than by analysis, wants to compare approaches on evidence before picking one, or invokes /prp-spike. |
| argument-hint | <idea or question> [--here] |
PRP Spike — Prove or Disprove
Settle an open question by building the smallest thing that could prove it wrong. The deliverable is a verdict backed by evidence, not shippable code.
Input: $ARGUMENTS (if absent, take the question from the conversation).
When to use
- Feasibility — can this be built at all, here, with what we have?
- Fit — does this sit naturally in our stack, or fight it the whole way?
- Cost of admission — what would have to change (a primitive, an abstraction, a product constraint) to make it possible, and is that trade worth it?
- Choice — which of several approaches survives contact with the real constraints?
Not for work whose feasibility is already settled — that is prp-plan then prp-implement. Not for something already broken — that is prp-debug.
The contract
- A spike answers one falsifiable question — or several sub-claims that share a single falsifier. No falsifiable question, no spike.
- Kill criteria are written before the build. Deciding what counts as failure after seeing results turns a spike into a rationalization.
- DISPROVEN is a success. It bought a decision with evidence and closed a path that would otherwise have cost weeks.
- Spike code is throwaway by construction and never opens a PR.
Phase 1 — Frame
Convert the request into a claim that can fail, and fix the kill criteria before touching code.
Reconnaissance before framing is allowed, and usually required. A hypothesis that names a version, a field, a threshold, or a mechanism cannot be written cold — read the issue, the source, and the environment until the claim can be stated precisely. The line is the falsifier: never let findings produced by the thing built to test the claim reshape the claim. Recon sharpens the question; results must only answer it.
Produce four things:
- Hypothesis — one sentence, falsifiable, specific enough that two people would agree on whether it held.
- Slug — 2–4 lowercase hyphenated words from the hypothesis. It names the branch, the worktree, and the report file; derive it once here and reuse it everywhere.
- Kill criteria — the specific observations that would end this as DISPROVEN, fixed now rather than after results exist.
- Boundaries between verdicts — what separates PROVEN from CONDITIONAL, and CONDITIONAL from DISPROVEN. The kill criteria say what failure looks like; this says which kind of failure it is. Deciding that boundary after seeing results is how CONDITIONAL gets rounded to whichever verdict is more convenient.
Split on the falsifier, not the claim count. Sub-claims one artifact can test together are one spike — build it once, list them in the frame, and report a verdict per sub-claim, because the mix is the output. Sub-claims each needing their own setup are separate spikes: take the riskiest first here, and list the rest in the report's Recommendation as follow-up spikes. For the framing craft (vague-to-falsifiable rewrites, constraint questions, splitting, sizing), read references/framing.md.
If the spike compares approaches, read references/evidence.md → Fair comparison now. The winning metric must be fixed and recorded here, before either variant exists — a metric chosen afterwards will be the one the favourite happens to win.
State the frame back to the user. When running interactively, confirm an ambiguous hypothesis before building — a spike that answers the wrong question is total waste, and framing is the cheapest thing to correct. When running unattended, record the interpretation chosen and the alternatives rejected, and proceed.
Record the frame before building
Pre-commitment only counts if it outlives the conversation. Write the frame to disk now — a transcript does not survive compaction, and an agent that has seen its results can reconstruct criteria that flatter them.
_gd="$(git rev-parse --path-format=absolute --git-common-dir 2>/dev/null)"
case "$_gd" in */.git) _root="${_gd%/.git}" ;; "") _root="$PWD" ;; *) _root="$_gd" ;; esac
_root="$(cd "$_root" && pwd -P)"
_name="$(basename "$_root" | tr '[:upper:]' '[:lower:]' | tr -cs 'a-z0-9' '-' | sed 's/^-*//;s/-*$//')"
PRP_DIR="${PRP_HOME:-$HOME/.prp}/${_name:-project}-$(printf %s "$_root" | git hash-object --stdin | cut -c1-8)"
mkdir -p "$PRP_DIR"; [ -f "$PRP_DIR/project.json" ] || printf '{"path": "%s", "name": "%s"}\n' "$_root" "${_name:-project}" > "$PRP_DIR/project.json"
mkdir -p "$PRP_DIR/spikes"
The resolver keys off the main repo, so the file lands in the shared store and stays readable from the main checkout while the spike branch is untouched.
Read templates/spike-report.md now (mandatory) and create $PRP_DIR/spikes/spike-<slug>.md with its header, ## The question section, and **Verdict**: (pending). Phase 6 fills in the rest; the hypothesis, kill criteria, and verdict boundaries recorded here are never edited after this point. The pending marker keeps an abandoned spike visibly unfinished instead of reading as a report with a missing verdict.
GATE: the file exists and contains the hypothesis, kill criteria, and verdict boundaries. Do not build without it.
Phase 2 — Isolate
Spike code is disposable and often invasive. Keep it away from the working checkout.
If already in an isolated worktree — spawned there by an orchestrator — stay put and do not nest a second one. EnterWorktree is unavailable to a pinned agent; where a branch name is wanted, plain git switch -c spike/<slug> inside the current worktree is enough.
Otherwise use the prp-worktree skill to create a worktree named spike/<slug> and work there. A spike branch is never merged.
--here skips isolation entirely — use it only when a fresh checkout cannot run the project (gitignored build prerequisites, an expensive bootstrap, a running local stack). Record in the report that it was used and why, and leave the checkout as it was found.
Phase 3 — Research
Read only enough to choose a credible approach — one a competent engineer would defend. A spike that fails because of a naive approach has proven nothing about the idea.
- Prefer primary sources: official docs, the dependency's actual source, specs, the codebase itself.
- Use
prp-core:web-researcher for anything outside the training cutoff, and prp-core:codebase-analyst to learn how the relevant subsystem really works before assuming what it allows.
- Stop when the approach is defensible. Research is not the deliverable here; the build is.
Phase 4 — Build the falsifier
Build the smallest artifact that could prove the hypothesis wrong, then stop.
The shape follows the question, not a catalogue — an interactive demo, a comparison of N implementations against a stated metric, a probe against a real API, a load harness, a type-level sketch, a patched dependency proving a primitive change works. Ask: what would I have to see to stop believing this? Build that.
Hold to:
- Aim at the risk. Build the part that might fail. Scaffolding, auth, styling, and persistence are not the question — stub them.
- No production habits. No tests, no error handling beyond what keeps it running, no abstractions. Every hour spent making spike code good is an hour not spent learning.
- Real inputs. Real data shapes, real API, real volumes. A spike on synthetic happy-path data proves nothing about production.
- Surface the state. Print or render what the artifact is doing, so the evidence is observable rather than asserted.
- Track what got faked. Every stub is a hole in the proof; the report has to name them.
Phase 5 — Stress
A spike that only ran the happy path has not been tested — it has been demoed. Attack the claim where it is weakest.
Work the kill criteria from Phase 1 deliberately: push the volume, break the assumption the approach rests on, feed the edge case, pull the dependency. Most spikes earn their keep here.
For evidence standards — what counts as proof, how to make a comparison fair, and the traps that make spikes lie — read references/evidence.md.
Phase 6 — Verdict
Re-read the kill criteria and verdict boundaries recorded in Phase 1 before choosing — not after. They were written by someone who had not yet seen these results. That is the only reason they are worth anything.
Reach one top-level verdict for the original hypothesis. When sub-claims have mixed results, keep each sub-claim's own verdict and use the top-level verdict to answer whether the original hypothesis held:
| Verdict | Meaning |
|---|
| PROVEN | Holds within current constraints. Evidence attached. |
| DISPROVEN | Does not hold. Name the wall it hit and why it is not the approach's fault. |
| CONDITIONAL | Holds only if a named constraint changes. Name the constraint, the cost of changing it, and what else that change would unlock. |
CONDITIONAL is the verdict most spikes should reach and most reports dodge. "Impossible" is usually shorthand for "impossible without changing something we were treating as fixed" — a primitive, a schema, a dependency, a product rule. Surfacing that trade is the point: it converts a dead end into a priced decision. Never collapse it into DISPROVEN. Never let it drift into PROVEN by quietly assuming the change is free.
Label each verdict proved or inferred, and record the seat it was proved from — which process, layer or surface, under which mode. Then re-run the inferred ones before finalizing: the setup already exists by this point, and an inference is where a spike is most confidently wrong. references/evidence.md carries the craft, including what to do when a kill criterion turns out to have named the wrong observation points.
Complete $PRP_DIR/spikes/spike-<slug>.md — read templates/spike-report.md again (mandatory) and fill every remaining section, replacing (pending) with the verdict. Keep every heading except ## Conditional constraints, which exists only when the top-level hypothesis or at least one sub-claim is CONDITIONAL.
One report, at that path. The Phase 1 file is the report — finish it in place. Do not write a second copy under research/, reports/, or anywhere else: a stub pointing at a fuller document elsewhere splits the record, and nothing keeps the two in agreement.
Phase 7 — Dispose
The evidence's permanent home is the store. Copy whatever the verdict rests on — harness scripts, fixtures, captured output — into $PRP_DIR/spikes/<slug>/. It survives a discarded worktree, is shared across the project's worktrees, and needs no git operation an isolated agent may be unable to perform.
But $PRP_DIR is local-only, so a store path is unfollowable by anyone else. Read references/handoff.md for the routes that make evidence followable off this machine — a secret gist when the verdict travels somewhere others read, a branch only when the spike code is substantial enough to re-run — and for the --here patch capture.
- Never open a PR. Every other terminal skill in this pack ends in one; this one ends in a verdict.
- Do not fold spike code into production. A validated approach gets rewritten under normal standards, by
prp-plan and prp-implement.
- Verify the Evidence pointer before calling the report done. A named branch must exist and carry a commit; otherwise point at the store directory. Evidence that cannot be followed is a claim, not a result.
- Leave the worktree in place if the user may want to poke at it; otherwise tear it down with the prp-worktree skill.
Phase 8 — Route the verdict
A spike that ends in the operator's terminal changes nothing. Propose where the verdict should land — a comment on the issue that commissioned it, a new item when the spike started from free text and nothing fits, or nothing at all when the question is closed and no work follows.
Propose; do not act. Creating or commenting on a tracker item is outward-facing, and this skill's terminal act is a verdict — never a merge, a PR, or an unrequested ticket. references/handoff.md has the routing table and what each verdict should ask for; a CONDITIONAL in particular must be framed as a decision, not filed as a task.
Report to the user: the hypothesis, the verdict, the two or three pieces of evidence that decided it, where the evidence lives, the report path, and the proposed destination. Lead with the verdict.
Gotchas
- The build phase eats spikes. Producing something impressive rather than something decisive is the default failure. Return to the hypothesis whenever the next step is unclear.
- A working artifact is not a PROVEN hypothesis. It proves only what it exercised.
- Spike effort does not estimate real effort. Something built in an hour without tests, error handling, or edge cases is not an hour of work. Say this wherever the report could be read as a plan.
- Do not let a spike become the implementation. When the verdict is PROVEN and the code looks decent, the pull to keep going is strong. Stop and hand off.
Resources
references/framing.md — turning a vague idea into a falsifiable hypothesis with kill criteria; constraint and comparison questions; splitting and sizingreferences/evidence.md — evidence standards, fair comparisons, proving a negative, the traps that make spikes lie (mandatory read in Phase 1 for a comparison spike, before either variant is built; otherwise read in Phase 5)references/handoff.md — making evidence followable off this machine (store / gist / branch), and routing the verdict to where it changes something. Read in Phase 7–8templates/spike-report.md — the report to fill (mandatory read in Phase 1 to record the frame, and again in Phase 6 to complete it)
