interactive-story/plan/14-phase-story-tree.md

Phase 14 — Story tree (branching adventures)

Goal: replace the linear action list with a tree. A retry becomes a sibling rather than a rewrite; continuing from one makes it a branch. Players can go back to any turn, take a different path, and keep both — switching between them freely.

Depends on 13-memory-embedding-cost.md shipping first: memory retrieval is the one read a tree cannot window, so it is the cost floor of a turn under this design. Fix the floor before building on it.

Why (beyond the feature)

Seven bug classes stop existing, and every one traces to the same root — the story is a mutable list:

Bug	Why it goes away
Deleting a middle action skips a later one forever (note_action_removed)	Cursors become node ids, not positions in a shifting list
prune_dangling_memories orphaning actions behind the cursor	Nothing is ever removed
Editing a summarised action leaves its memory stale forever — currently unfixed	Editing makes a new node; the old memory stays correct for the old path
The one-turn memory holdback (settled_story_actions)	Nothing is mutated, so nothing goes stale — the concept is unnecessary
The legacy-cursor no-rewind trap found while fixing that	Same
"Anything reading adventure.actions during generation must exclude the retried action" — leaked into 4 call sites	The replaced turn is not on your path; it cannot leak
variant_count / mirrored text drifting from variants	The denormalisation disappears entirely

It also resolves the 1NF violation: variants as a JSON repeating group becomes rows.

Design decisions (settled 2026-08-16)

• Full branching, with UI. Not the "tree schema, no branch picker" middle option — branching ships as a feature people use.
• branch_id + depth on every node, not parent pointers alone. Parent pointers alone mean walking N links to read a story, which throws away the round-two windowing work. depth replaces index as the ordering key.
• A branches table with parent_branch_id and fork_depth. The fork point is stored at fork time, never inferred. Nothing is copied on a fork — a branch borrows its ancestors' turns.
• Lineage cached on the branch row. lineage = [(D,∞), (C,45), (B,30), (A,10)], computed once at fork (parent's lineage + one entry). Reads never walk to reconstruct it. Each ancestor is capped at the fork_depth of the branch beneath it.
• Read the lineage lazily, windowed. Query the newest few lineage entries, measure, fetch more only if the context budget is not covered — the exact shape of history.window_covering(). Clause count is bounded by the context window, not by fork count, so a 200-fork story reads as cheaply as a 2-fork one.
• Promote to a branch only on continue. Attempts at the tip stay as sibling leaves; one becomes a branch the moment a turn is played past it. Keeps the lineage chain to "divergences I built a story on", not "every retry ever" — the difference between a handful of entries and fifty.
• Memories and the summary attach to the node that produced them, found by walking up. Shared ancestors are shared automatically, so a fork costs nothing and nothing needs recreating. A memory covering depths 37–42 hangs off that branch's node 42 and is invisible to any path not through it. Generalise the rule: anything derived attaches to the node that produced it — the summary included, so stop storing it per turn.
• Full lineage for memories, windowed lineage for the story. Memory retrieval is long-range recall and cannot be windowed, but memories are sparse (~1 per 6 actions), so a long OR-clause returning ~33 small rows is fine. Two queries, one lineage.
• Never auto-prune. Nothing is deleted without an explicit user action. This makes a branch-management UI a hard dependency, not a nice-to-have — storage grows without limit otherwise.
• Story cards stay adventure-wide. A card invented on branch B shows on branch A. Already true for undo today (script card mutations are not reverted), so this is a documented limit, not a regression. Explicitly rejected event-sourcing card changes onto nodes.
• State carries over almost free. state_before / world_state_before already snapshot the script scoreboard and RPG world state per action, and apply_variant() already restores them on a switch — a branch switch is the same move. Wrinkle: those are before pictures; a node wants the after. And they are NULL on pre-column rows.

Schema sketch

branches(id, adventure_id, parent_branch_id, fork_depth, lineage JSON, created_at)
actions(id, adventure_id, branch_id, depth, type, text, reasoning,
        world_delta, state_after, world_state_after, context_snapshot, created_at)
memories(..., branch_id, depth)          -- attached to the node that produced it
adventures(..., head_branch_id, head_depth)

Reading branch C, tip at depth 7, lineage [(C,7), (B,5), (A,3)]:

SELECT * FROM actions
WHERE (branch_id='C' AND depth <= 7)
   OR (branch_id='B' AND depth <= 5)
   OR (branch_id='A' AND depth <= 3)
ORDER BY depth DESC LIMIT 32

→ A0 A1 A2 A3 B4 B5 C6 C7. Depth is a position along a path, not a global turn number — A4 and B4 are alternatives, not duplicates.

Work

1. branches table, branch_id/depth on actions, head_* on adventures.
2. Lineage computation + a single module that owns the branch clause — same role context/history.py plays today. Every query must go through it; one forgotten clause shows the wrong story, quietly.
3. history.py rewritten against lineage windowing. window_covering keeps its shape.
4. Memories/summary attached to nodes; delete the cursor-position machinery (position_of_index, settled_*, note_action_removed, _rewind_cursors_to_index).
5. Sibling storage for un-promoted tip attempts + the promotion step.
6. Node state moves from before to after snapshots.
7. Migration: every existing adventure becomes branch A; index → depth; variants entries become sibling nodes; variant_index becomes the head pointer.
8. Frontend: branch picker replacing VariantPager, plus branch management (rename, delete, switch) — required, given no auto-pruning.

Open

• retry_of.index reuse stops the world-state cooldown clock advancing on a re-run of the same turn. Whatever replaces it must preserve that. (Carried into SP5 below.)

Everything else that stood open here was decided on 2026-08-17 — see the next section.

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Implementation plan (decided 2026-08-17)

Four decisions, taken before any code was written:

Question	Decision
Phased or single migration?	Structural-first, no runtime flag. SP1–SP4 migrate to the tree representation with behaviour identical to today. Branching features layer on after.
How destructive is the migration?	Keep the legacy columns for one release. index, variants, variant_index, variant_count stay, unread, until the tree is proven live (SP8 drops them).
Branch-management UI scope	Full tree visualisation — a spatial view of forks, not just a picker.
Export bundle	ai-dnd-adventure-v2, with a v1 legacy reader so existing bundles keep importing.

Why no feature flag. A linear story is a tree with one branch, so the intermediate states are not half-migrated — they are the same product with a superset schema underneath. That makes "current adventures are unaffected" a literal, testable pass condition for every subphase up to SP4, which a flag would have replaced with two live code paths through the context builder, the memory bank, undo and retry at once.

The regression contract

tests/test_story_tree_baseline.py (built in SP0) drives the whole product over HTTP — create, play, retry, switch, undo, page with before_id, memories, summary, export — and asserts only on API responses, never internals.

It must pass unmodified through SP1, SP2 and SP3. That is the contract those subphases are verified against. SP4 is the first subphase permitted to change it, and even there only where the change is deliberate and named below.

Traps found while reading (these are the ones that bite quietly)

• history._from_memory() slices adventure.actions. The "never load twice" shortcut returns whatever is already in the session — which under a tree is every branch's actions, not the path. It would silently assemble context from siblings. This is the single highest-risk line in the change; SP2 must make the shortcut branch-aware or delete it.
• scripting/pipeline.py::_history() and _info() read adventure.actions directly, and hand it to user scripts as the documented history API. Same trap, user-visible.
• Adventure.actions is order_by="Action.index". Ordering it by depth is not enough — the collection is still every branch. Anything iterating it needs the path.
• limits.check_row_cap("actions") counts every action in the adventure. With no auto-pruning, a branched adventure hits MAX_ACTIONS_PER_ADVENTURE while its story is far shorter. The cap has to count the tree but be explained as the tree, or move.
• The holdback cannot die in SP3. settled_story_actions exists because retry mutates a row in place. Retry stops mutating in SP4, so the holdback is only safe to delete there — deleting it in SP3 reopens the exact bug it was written for.

Subphases

Each ships independently, on its own branch, green before the next starts.

SP0 — Baseline and regression net (no product change)

• tests/test_story_tree_baseline.py — the contract above. 24 tests covering opening a windowed adventure, the turn engine (do/say/story/continue), script effects, retry and variant switching, undo, paging up to the start, edit/delete, memories, export/import round-trip, and world state.

Done, 2026-08-17. 259 existing tests green (17.8s), then 283 green with the baseline added, against unmodified main. That is the number every subphase below is measured against.

A pre-migration (schema 45) fixture is needed too, built by a script rather than committed as a binary — but its only consumer is the SP1 migration test, so it lands there.

SP1 — Schema and migration (no behaviour change)

File	Change
app/models.py	New Branch model. Action.branch_id, Action.depth. Adventure.head_branch_id, head_depth. Memory.branch_id, Memory.depth. Legacy columns untouched.
app/migrations.py	Migrations 46+: create branches; add columns; index (branch_id, depth); backfill. Dialect map wherever BLOB/BYTEA-style spellings diverge.

Backfill: one branch per adventure, lineage = [(A, ∞)]; actions.branch_id = A, depth = index; adventures.head_* from max(index); memories take branch A and a depth derived from source_end.

Verify: baseline test unmodified. New tests/test_tree_migration.py — every action carries a branch and depth == old index, no row lost, head pointers correct, memories mapped. Bootstrap run twice is a no-op. tests/test_egress.py ceilings unmoved (two integers a row). Post-deploy VACUUM FULL actions; is mandatory — this rewrites every row, which is the 144 MB lesson at the top of STATUS.md.

SP2 — The branch clause (reads move to lineage; still one branch)

One module owns the clause; every action read goes through it. A forgotten clause shows the wrong story, quietly, which is why it is one module and not a convention.

File	Change
app/context/lineage.py (new)	Lineage computation and the branch clause. The only place that knows how a path is selected.
app/context/history.py	_filters takes the clause; order by depth; window_covering keeps its shape. Fix _from_memory.
app/routers/adventures.py	action_window anchors on the anchor's depth; last_action, next_index→next_depth, _latest_narration.
app/scripting/pipeline.py	_history()/_info() read the path, not the collection.

Verify: baseline test unmodified. test_history_window.py, test_action_paging.py green. New test builds a two-branch fixture directly in the DB and asserts the design doc's own example reads back as A0 A1 A2 A3 B4 B5 C6 C7, and that a sibling's nodes are invisible. Egress: a 20-fork fixture costs within a small factor of a 1-fork one — clause count is bounded by the context window, not by fork count.

SP3 — Memories and summary attach to nodes

Cursors stop being positions in a shifting list. memory_cursor/summary_cursor become node-anchored (branch_id, depth).

Deleted: position_of_index, note_action_removed, _rewind_cursors_to_index, prune_dangling_memories, and their call sites in undo_turn and delete_action. Kept until SP4: settled_story_actions and the holdback (see traps).

Verify: baseline test unmodified. test_memory_settling.py and test_memory_retrieval.py updated, plus a new branch-isolation test — a memory created on branch B is invisible from branch A, and shared ancestors are visible from both. Memory retrieval reads the full lineage (it cannot be windowed) but stays sparse: assert the byte cost on a deep fork.

SP4 — Variants become sibling nodes

Retry stops mutating a row. It writes a sibling leaf at the same depth.

Deleted: set_variants, variant_of, apply_variant, VARIANT_SNAPSHOT_KEYS, and the holdback. Node state moves from before to after snapshots (state_after, world_state_after) — a sibling needs its own outcome, so this belongs here rather than in its own subphase. Pre-column rows are NULL and must stay tolerated.

A migration converts existing variants JSON into sibling rows — reading the legacy column that decision 2 kept, which is the whole reason it was kept.

/variants and /variant keep their URLs and response shapes here, re-implemented over sibling rows, so the frontend keeps working until SP7 replaces it.

Verify: the first subphase allowed to move the baseline test, and only for variant_count/variant_index semantics — the retry outcomes must not move. test_retry_variants.py rewritten against siblings but asserting the same observable results, including that the gold script is not double-applied. test_state_revert.py against after-snapshots. Migration test: attempt count preserved, the active attempt becomes the head.

SP5 — Fork on continue

Playing past a non-head sibling promotes it: a branches row with parent_branch_id, fork_depth, and a lineage computed once from the parent's. Nothing is copied. adventures.head_* moves. The cooldown clock must not advance on a re-run of the same turn — the one item still open above.

Verify: a fork creates exactly one branch row and copies no actions; lineage is correct and capped at each fork_depth; both branches read independently; switching restores the right script/world state; the cooldown test from test_worldstate.py still holds across a retry.

SP6 — Export/import v2

ai-dnd-adventure-v2 carries branches and nodes; import accepts v1 and v2, mapping a v1 bundle's linear actions plus variants onto one branch with siblings. limits.check_bundle_lists learns about branches.

Verify: v2 round-trip of a branched adventure is lossless; a v1 bundle still imports; a bundle claiming more branches than rows is rejected rather than half-applied.

SP7 — Frontend: full tree visualisation

VariantPager is removed. A spatial tree view replaces it, plus switch, rename and delete-with-confirm. api.js gains the branch endpoints.

Verify: this is where the standing open gap gets closed — drive the 600-action --keep fixture in a browser by hand, on the same scroll path that has never been driven and already hid one bug. A vitest + jsdom harness covers the prepend arithmetic; jsdom has no layout, so scroll position still needs eyes.

SP8 — Drop the legacy columns

Only once the tree is proven live. Migration drops index, variants, variant_index, variant_count, followed by VACUUM FULL actions;.

Verify: full suite; egress ceilings; a measured before/after size, aggregates only.

Standing constraints

• No production data is read at any point in this phase. Migrations, the e2e baseline and every egress measurement run on local SQLite and the synthetic --keep fixture. If a real Postgres is ever needed for a write path, it is a throwaway database whose name contains stress/scratch, and it is asked for first.
• Any migration that rewrites actions is followed by VACUUM FULL actions; on the direct endpoint, not -pooler. SP1, SP4 and SP8 each rewrite every row.