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+# Frontier Membership Without Partitioning
+
+## Status
+
+Prospective design note only. Do not implement yet.
+
+The current single-frontier posture is acceptable for now. This note captures a
+clean future cut once overlapping frontier work becomes real enough to justify
+the model change.
+
+## Thesis
+
+Frontiers should be scopes, not partitions.
+
+`hypothesis` and `experiment` are the real scientific graph vertices.
+`frontier` is a control object with a brief and a bounded grounding projection.
+It should not own vertices exclusively.
+
+The current model still treats frontier as an ownership partition:
+
+- every hypothesis has one `frontier_id`
+- every experiment has one `frontier_id`
+- experiments inherit that frontier from their owning hypothesis
+- influence edges may not cross frontier boundaries
+
+That is stricter than the real ontology. In practice:
+
+- one hypothesis may matter to multiple frontiers
+- one experiment may be relevant to multiple frontiers
+- later hypotheses may be directly informed by experiments from another frontier
+
+The current shape is tolerable while there is effectively one live frontier. It
+is not the right long-term model.
+
+## Desired Ontology
+
+### Graph vertices
+
+The true graph vertices remain:
+
+- `hypothesis`
+- `experiment`
+
+### Control objects
+
+`frontier` is not a graph vertex. It is a scope object that owns:
+
+- label
+- objective
+- status
+- brief
+
+### Sidecars
+
+`artifact` remains an attachable reference-only sidecar.
+
+## Relations
+
+Two relations remain fundamental.
+
+### 1. Ownership
+
+Every experiment has exactly one owning hypothesis.
+
+This is the canonical tree spine. It should remain mandatory and singular.
+
+### 2. Influence
+
+Hypotheses and experiments may both influence later hypotheses or experiments.
+
+This is the sparse DAG laid over the ownership spine. It should no longer be
+artificially constrained by frontier boundaries.
+
+### 3. Membership
+
+Frontier membership becomes a separate many-to-many relation.
+
+A hypothesis or experiment may belong to zero, one, or many frontiers.
+
+This relation is about scope and visibility, not causality.
+
+## Recommended Data Model
+
+### Remove vertex-owned frontier identity
+
+Delete the idea that hypotheses or experiments intrinsically belong to exactly
+one frontier.
+
+Concretely, the long-term cut should remove:
+
+- `HypothesisRecord.frontier_id`
+- `ExperimentRecord.frontier_id`
+- `CrossFrontierInfluence`
+
+### Add explicit membership tables
+
+Use explicit membership relations instead:
+
+- `frontier_hypotheses(frontier_id, hypothesis_id, added_at)`
+- `frontier_experiments(frontier_id, experiment_id, added_at)`
+
+The split tables are preferable to one polymorphic membership table because the
+types and invariants are simpler, and queries stay more direct.
+
+### Preserve one hard invariant
+
+An experiment may only be attached to a frontier if its owning hypothesis is
+also attached to that frontier.
+
+This prevents the frontier scope from containing orphan experiments whose
+canonical spine is missing from the same view.
+
+That still allows:
+
+- one hypothesis in multiple frontiers
+- one experiment in multiple frontiers
+- one experiment in a subset of its hypothesis frontiers
+
+but disallows:
+
+- experiment in frontier `B` while owning hypothesis is absent from `B`
+
+## Query Semantics
+
+### `frontier.open`
+
+`frontier.open` should derive its surface from membership, not ownership.
+
+Its bounded output should still be:
+
+- frontier brief
+- active tags
+- live metric keys
+- active hypotheses with deduped current state
+- open experiments
+
+But all of those should be computed from frontier members.
+
+### Active hypotheses
+
+Active hypotheses should be derived from a bounded combination of:
+
+- roadmap membership in the frontier brief
+- hypotheses with open experiments in the frontier
+- hypotheses with latest non-rejected closed experiments still relevant to the
+  frontier
+
+The exact rule can stay implementation-local as long as the result is bounded
+and legible.
+
+### Live metrics
+
+The right default is not “all metrics touched by frontier members.”
+
+The live metric set should be derived from:
+
+- all open experiments in the frontier
+- the immediate comparison context for those open experiments
+
+A good default comparison context is:
+
+- the union of metric keys on all open experiments
+- plus the metric keys on immediate experiment ancestors of those open
+  experiments
+
+This keeps the hot path focused on the active A/B comparison set rather than
+every historical metric ever observed in the scope.
+
+## Surface Changes
+
+When this cut happens, the public model should grow explicit membership
+operations rather than smuggling scope through create-time ownership.
+
+Likely surfaces:
+
+- `frontier.member.add`
+- `frontier.member.remove`
+- `frontier.member.list`
+
+Or, if we prefer type-specific verbs:
+
+- `frontier.hypothesis.add`
+- `frontier.hypothesis.remove`
+- `frontier.experiment.add`
+- `frontier.experiment.remove`
+
+I prefer the type-specific form because it is clearer for agents and avoids a
+generic weakly-typed membership tool.
+
+Read/list filters should then interpret `frontier=` as membership selection.
+
+## Brief and Roadmap Semantics
+
+The frontier brief remains a singleton owned by the frontier.
+
+Its roadmap should reference frontier-member hypotheses only.
+
+That is a healthy constraint:
+
+- the brief is a scoped grounding object
+- roadmap entries should not point outside the scope they summarize
+
+If a hypothesis becomes relevant to a frontier roadmap, attach it to that
+frontier first.
+
+## Migration Shape
+
+This should be a red cut when it happens.
+
+No backward-compatibility layer is needed if the project is still early enough
+that re-seeding is acceptable.
+
+The migration is straightforward:
+
+1. Create frontier membership tables.
+2. For every hypothesis, insert one membership row from its current
+   `frontier_id`.
+3. For every experiment, insert one membership row from its current
+   `frontier_id`.
+4. Drop `frontier_id` from hypotheses and experiments.
+5. Delete `CrossFrontierInfluence`.
+6. Rewrite frontier-scoped queries to use membership joins.
+
+Because the current world is effectively single-frontier, this is mostly a
+normalization cut rather than a semantic salvage operation.
+
+## Why This Is Better
+
+This model matches the clarified ontology:
+
+- the scientific truth lives in hypotheses and experiments
+- the frontier is a bounded lens over that truth
+- scope should not distort causality
+
+It also makes later comparative work cleaner:
+
+- one hypothesis can be reused across multiple frontier narratives
+- one experiment can inform more than one frontier without duplication
+- influence edges can remain honest even when they cross scope boundaries
+
+That is a better fit for a system whose real purpose is an austere experimental
+record spine rather than a partitioned project tracker.