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. It survived SP3 as memorybank.settled_after, which is the - 1 in "how much story is past the mark"; that subtraction is the whole of it.

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.

--rich: a correctness fixture beside the scale one

The measuring fixture is sized from production and leaves every column it does not weigh at its default. Checked against a freshly built one, that is exactly the set of columns a tree has to migrate: state_before/world_state_before NULL on all 600 rows, no scenario and so no RPG layer, no adventure scripts, both cursors 0, and 100 retry histories whose two attempts carry byte-identical text with variant_index always 0 — so "which attempt is live?", the one question SP4's migration answers, had no observable answer.

tools.stress_session --rich fills in precisely those and nothing else:

cd backend
.venv/Scripts/python.exe -m tools.stress_session --rich --actions 30 --memories 12

An RPG scenario built from the real seed schema with a world state played forward (hp 100 → 71, flags flipping partway); per-action state_before/world_state_before snapshots that are monotonic, so a bad rollback reads as a wrong number rather than as nothing; a gold script on the adventure; story cards; non-zero memory and summary cursors with a real story_summary; pinned and forgotten memories; retry attempts with distinct texts, counts of 2 and 3, and a live attempt that is often not the last one written; and a second adventure, so "does this leak across adventures?" is answerable — a branch clause that forgot its adventure would still look correct on a database holding exactly one.

It is a correctness fixture, so prefer it small: what it is for is variety per row, not rows. Its byte figures are not comparable to a plain run and it does not replace the scale fixture — the plain one still holds the egress ceilings, and was re-measured unchanged (1.8 kB, actions 1.7 kB) after --rich was added.

The invariant text == variants[variant_index]["text"] holds on every retried row, and is asserted when the fixture is built. SP4's migration reads exactly that to decide which sibling becomes the head.

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.

Done, 2026-08-17 (branch sp1-tree-schema). 297 tests green, the 283 from SP0 plus 14 in test_tree_migration.py. The baseline contract passes unmodified, which was the pass condition. Five things the plan did not anticipate, all of them found by building it:

• The writes could not wait for SP2. The file table above lists only models.py and migrations.py, but a migration never visits a row written after it runs — so shipping the columns without a writer would leave every turn played between the two deploys with no branch, and from SP2 on a row with no branch is a row no read can see. app/tree.py is that writer: root_branch / head_branch (get-or-create), place_action, place_memory, refresh_head. One module for the same reason SP2 gets one — a node written without a branch fails by disappearing, not by raising. Wired into create/turn/import/undo/delete/memory, plus seed_demo.py and tools.stress_session (a fixture built by create_all is stamped LATEST, so no migration ever runs against it).
• adventures.head_branch_id cannot be a foreign key. branches.adventure_id already points the other way, and the pair is then a cycle create_all refuses to order; the fix for that is use_alter, which SQLite has no ALTER for. It is a plain integer, documented as a cache, and head_branch recovers onto the root if it ever names a branch that is gone.
• lineage is NOT NULL, because branches comes from create_all. The backfill cannot insert a row and fill the lineage afterwards via a NULL marker, so it inserts '[]' and guards step two on json_array_length(lineage) = 0 — not = '[]', because Postgres json has no equality operator.
• SQLite cannot drop a column a foreign key names. So a current-schema database cannot be rewound past branch_id at all, which broke the two existing tests that simulate an old database by rewinding only the stamp. Fixed properly: migrations._column_already_there makes every ADD COLUMN idempotent (the IF NOT EXISTS the module docstring asks for and SQLite has no syntax for), and tests/schema_rewind.py holds the inverse of the migrations that can be undone. The SP1 fixture therefore builds a genuine schema 45 by dropping the three tables and recreating them from frozen pre-tree DDL, so the real ALTERs run — including the one that adds a foreign key.
• Deleting a branch takes its nodes with it, and deleting an adventure takes its branch — both verified, both at the database level via ON DELETE CASCADE on the two branch_id columns. SP7's delete-a-branch needs no code of its own for the nodes.

Measured: branches costs 0.1 kB of a 733.5 kB turn (0 %), and the page load (62.6 kB) and index (1.8 kB) shapes are byte-identical to the figures in STATUS.md. One cost that is not free: the new table and its index add ~47 ms to every create_all/drop_all cycle on SQLite, and the suite does one per test — 20 s → 38 s. Test-only (DDL fsync), so no model change; if it ever matters, the fix is the test harness, not the schema.

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.

Done, 2026-08-17 (branch sp2-branch-clause). 317 tests green, the 297 from SP1 plus 20 in test_branch_clause.py. The baseline contract passes unmodified, which was the pass condition. Six things worth not rediscovering:

• The contract forced the write side, not the read side. The SP0 baseline writes its actions straight to the database and must pass unmodified — so "every writer calls place_action" could not be the invariant, because the baseline is a writer and does not. Neither did any of the eleven other test fixtures. The alternative was a read tolerant of a NULL branch, which is the quiet-wrong-story failure this subphase exists to make impossible. So the session enforces it instead: tree.place_new_nodes runs from Session.before_flush and places anything unplaced, registered in models.py so that importing the models arms it. The call sites keep their explicit calls — a node placed at the call site is placed before the code around it reads the row back.
• A branch could no longer be created with a flush. root_branch did db.add(); db.flush() to get the id its lineage names, and a nested flush inside before_flush raises. It inserts through Core and reads the row back — same transaction, three statements, once per adventure ever.
• The identity map holds weak references, and it cost 25 % of the suite. Resolving the head branch per node re-read the branches row for every node in the flush, because nothing held a strong reference between two calls: 201 SELECTs to write 200 actions, and 36 s → 45 s on the same 297 tests measured back to back. The head is now resolved once per adventure per flush — 2 SELECTs, and the same 297 tests then time within noise of SP1 (44.2 s each; this machine's load drifts by ~20 % between runs, so trust the statement count, not the stopwatch). Pinned by a test that counts reads of branches, because the stopwatch is all the symptom there ever was.
• The index screen is the one read scoped by head branch rather than by lineage. _latest_narration picks one row per adventure for a hundred adventures at once, and a lineage clause each would put hundreds of OR-terms on that query. The two answers differ only for a branch with no nodes of its own, which cannot exist — a branch is created by playing a turn onto it.
• Two reads are deliberately left un-pathed, both documented where they live. max_action_index allocates the legacy index, which must stay adventure-wide or two branches issue the same number; and export is a flat v1 bundle whose reader has no idea branches exist, which is why SP6 replaces the format rather than widening the query. A third is a known divergence, not a decision: the index screen's action_count counts the tree, and will overstate a branched story until SP5.
• Adventure.actions was left ordered by index on purpose. Ordering the collection by depth would not make it a story — it is every branch's actions, and a path is a selection out of it. What the relationship is still for is ownership and the delete-orphan cascade.

Measured: a story forked 20 times reads its newest 32-action window for 3,178 B against the 2,961 B an unforked story of the same length costs (1.07×), naming one branch of its 22 lineage entries. The pre-tree 600-action --keep fixture was migrated and then driven over HTTP end to end: index 1,840 B, page load 64,149 B — the same shapes as before the phase — and scrolling to the start took 9 pages and saw all 600 actions exactly once.

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.

Done, 2026-08-18 (branch sp3-node-cursors). 330 tests green, the 318 the branch started from plus 12 — 11 in test_branch_clause's new sibling test_memory_nodes.py and one on migration 56. The baseline contract passes unmodified, which was the pass condition. app/context/cursors.py is the new module; migrations 53–56 add memory_cursor_branch_id/_depth and summary_cursor_branch_id/_depth and translate the old counts into them.

Seven things worth not rediscovering:

• An anchor is a coordinate, not a pointer, and that is what deleted the machinery. Half of this subphase was expected to be rewriting the cursor bookkeeping in depth terms. None of it needed rewriting: count_after(41) is well defined with node 41 deleted, and deleting node 12 does not change what "past node 41" means. So note_action_removed, _rewind_cursors_to_index, position_of_index and the post-turn clamp did not become depth-shaped versions of themselves — they became nothing. If a mark still needs correcting when the story changes, it is still a position.
• The clamp had its own trap and it also goes. run_post_turn clamped both cursors to the story length every pass, deliberately against the full count, because clamping to the settled count rewound a caught-up adventure a step and re-covered an action. An anchor past the tip is not a broken value: settled_after reports nothing to do, and the story growing back past it resumes exactly where it left off.
• prune_dangling_memories became a lookup, and got stricter by accident. A memory hangs off the node its block ends on, so "what did this node produce?" is (branch_id, depth) — memorybank.forget_node. The scan it replaces could only ever notice damage after the fact (a covered range past max(index)), and could not notice at all when the node was deleted from the middle of a story that still had later actions. Withdrawing the memory is half the job: the ground it covered is still behind the mark, so the mark goes back to source_start - 1 — a depth, whether or not a row still sits there.
• A memory with no node had to be spelled out in the clause. A hand-written memory summarises nothing, so it carries a branch and a NULL depth. Every ancestor entry in a lineage clause is capped depth <= fork, and NULL fails that — so a typed memory would have become invisible at the first fork after it was written, with nothing to see but a prompt that stopped mentioning it. Path.clause(unanchored=True) is that case, and actions never pass it: an action with no depth is a pre-tree row no read should see.
• Retrieval reads the whole lineage, and it is free. Measured on two stories of 84 actions and 14 memories each, one flat and one forked twenty times: 1,807 B against 1,823 B. The clause carries 22 branch terms instead of one, and the clause is not what crosses the wire. The egress shapes are otherwise byte-identical to SP1's — index 1.8 kB, page load 62.7 kB, turn 733.8 kB.
• Three reads stay adventure-wide, deliberately. Embedding and eviction are facts about the row and about the bank, not about the path — skipping a sibling's memories would only mean embedding them at the moment somebody switched to them, and evicting the memories of a story nobody is reading is the right thing to evict first. The Memories drawer is management rather than retrieval, and hiding a branch's memories there would make them unfindable in a phase whose rule is that nothing is removed automatically.
• The v1 bundle still speaks positions, in exactly two places. Export counts the anchor back into a position; import translates the other way, but only after the actions exist, because that is the one moment the two coordinate systems can be lined up. cursors.position_of and cursors.anchor_at_position are the whole of what still knows about positions, and SP6's v2 format retires them.

Migrations 53–56 rewrite adventures, not actions — a few hundred rows against a few hundred thousand — so this deploy needs no VACUUM FULL of its own. The one SP1 owes is still owed.

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;. Also adventures.memory_cursor and summary_cursor — unread since SP3, kept only so a rolled-back build resumes from a real number. They are on adventures, so dropping them costs no vacuum.

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.