P3.3.3 — Gossip Adaptive Fanout — Audit

Step 1 of the 5-step chain (CLAUDE.md §6). Phase 3 θ Wave 4 — the connectivity-driven fanout helper plus the local exchange tracker that recomputes the connectivity score every N epochs. Companion P3.3.2 (Bloom dedup) and P3.7.1 (MCP tool surface) ship in parallel.

§1. Surface inventory at base SHA 9c7165d5

Path	Exists?	Role
src/domains/consensus/	Yes — θ domain root (P3.1.1 / P3.1.2 / P3.1.3 / P3.3.1 / P3.4.1 / P3.5.1 / P3.6.1 shipped)	θ domain root
src/domains/consensus/adaptive-fanout.ts	No — to create	computeFanout, clamp, FanoutTracker class
src/__tests__/domains/consensus/adaptive-fanout.test.ts	No — to create	Worked table + clamping + tracker lifecycle
src/domains/consensus/gossip-wire.ts	Yes (P3.3.1 — e63a8bcf)	Consumed peer/event shape reference — NOT imported (slice is pure data, no wire dependency)
src/domains/rules/builtins.ts	Yes (κ P1.3.2)	Reference for the clamp ternary pattern (line ~clamp const) — NOT imported (κ’s clamp works on BuiltinValue tuples; this slice uses a bare bigint helper)

§2. Spec inventory

2.1 Worked table — s08 §Adaptive fanout

docs/spec/s08-gossip.md §Adaptive fanout (lines 75–92):

fanout = max(3, min(10, 15 - connectivity_score))

Where connectivity_score is the number of live peers in the routing table, clamped to [0, 12].

connectivity_score	fanout
0 (isolated)	10
3	10
5	10
7	8
10	5
12	3 (minimum)

Minimum fanout = 3 (always gossip somewhere); maximum = 10 (bandwidth ceiling on isolated nodes). Recomputation period = 5 epochs, driven by successful IHAVE/IWANT exchanges during the window.

2.2 Acceptance criteria (source prompt §P3.3.3 lines 1314–1328)

• computeFanout(connectivity_score: bigint): bigint returns max(3n, min(10n, 15n - connectivity_score))
• Worked-table fixture passes for score ∈ {0, 3, 5, 7, 10, 12}
• Score clamped to [0n, 12n] — values outside coerced (negative → 0n; >12n → 12n)
• Recomputed every 5 epochs based on successful IHAVE/IWANT exchanges
• trackExchange(peer_id, success, epoch) records exchange outcome
• recomputeIfDue(current_epoch, last_recompute_epoch, period_epochs = 5n) returns updated score iff current - last >= period
• Score uses live-peer count: peers with ≥ 1 successful exchange in last period_epochs

2.3 Determinism guardrails

docs/spec/s08-gossip.md does not list a forbidden-token sweep, but the task prompt’s FORBIDDENS section requires:

• No socket I/O — tracker is pure data
• bigint throughout, no Number
• No Math.*, no Date.*, no Math.random, no floats
• No new npm deps
• Not editing main checkout

These match the existing P3 forbidden-token scanners (messages.test.ts Group 9, gossip-wire.test.ts Group 15, time-anchors.test.ts Group 8).

§3. Reuse map

Symbol / pattern	Source	Action
bigint ternary clamp pattern	src/domains/rules/builtins.ts clamp const	Re-implement narrowly — κ’s clamp works on BuiltinValue tuples plus throws on lo > hi; this slice is a flat bigint helper
Header banner conventions	src/domains/consensus/gossip-wire.ts lines 1–57	Replicate banner + forbidden-token self-scan note
Per-slice forbidden-token scanner	src/__tests__/domains/consensus/time-anchors.test.ts Group 8	Replicate, narrowed to adaptive-fanout.ts
Test fixture style (bufN(seed))	src/__tests__/domains/consensus/gossip-wire.test.ts lines 38–44	Not needed — this slice uses only string peer_id keys, no Buffers

§4. Open questions

None — task prompt §P3.3.3 + s08 §Adaptive fanout fully resolve all design points (clamp order, period default, live-peer semantics, retention).

§5. Out of scope

• Wiring FanoutTracker into a live gossip loop (deferred — P3.3.1 / P3.3.2 / P3.7.1 handle wire + dedup + MCP surface)
• Persisting the score across process restarts (in-memory only, by design — exchange records are ephemeral over period_epochs)
• Multi-arbiter convergence of fanout scores (no global view; each node computes its own)
• Connectivity-aware peer selection (which peers to gossip to — task prompt scopes this to the count, not the picker)