parallel-dispatch

name	parallel-dispatch
description	Three-level parallelism (phase group → phase-task → intra-role fan-out), parallelize-by-default stance, disjoint-touches pre-flight, merge/rollback for horizontal fan-out, failure semantics, and dynamic concurrency caps for orchestrators consuming a cto plan.

Parallelize by Default

Every dispatchable unit runs in parallel unless a documented blocker applies. Default log field: parallelism_decision: parallel. When serializing, log parallelism_decision: serial+blocker:<reason> per dispatched unit.

The only recognized blockers (exhaustive list):

ID	Blocker
a	Same-file write conflict (detected by disjoint-touches pre-flight or post-edit verification).
b	Package-manifest contention or git-tracked file race in the same folder (serialize per folder when siblings both write it).
c	Sequential depends_on edge in the cto plan DAG (downstream cannot start until upstream completes).
d	Explicit user / plan serial: true flag on the affected unit or group.

R1 — plan graph is read-only for the consuming orchestrator. Any DAG mutation requires hand-back to cto with a re_plan_brief. Re-dispatch within an existing phase is allowed; mutating the DAG is not.

Level-1 Phase Group Fan-out

Level 1 — fan-out across sibling phases in one parallel group. Phases that share the same depends_on set form a group; dispatch every phase in that group as parallel Task calls in a single assistant turn when each is parallelizable and touches are disjoint per pre-flight.

Per-group execution (topological order):

1. Fan out every phase in the current group as parallel Task calls in one turn (Level-1 parallelism). A single-phase group is inherently serial.
2. Within each dispatched phase, the assignee may further parallelize at Level 2 per disjoint-touches rules.
3. Brief each phase/agent with steps, touches / rollback_scope, context, acceptance criteria — minimal briefs (no full plan dump).
4. Wait for all phases in the group; collect structured outputs; verify per-phase acceptance and that disjoint writes held at runtime.
5. Run sequential follow-ups inside the group only when the plan explicitly orders them inside that group.

Example — phase-group fan-out (Level 1):

CTO plan dependency graph:
G1: P1 → foundation
G2: P2a, P2b, P2c → all depend on [P1]; touches disjoint
G3: P3 → depends on [P2a, P2b, P2c]

Your execution:
→ Dispatch P1 (single phase, serial).
→ Await user checkpoint for G1.
→ Dispatch P2a, P2b, P2c as three parallel Task calls in one turn.
→ Await all three to complete; verify disjoint writes actually held.
→ Present group summary; await user checkpoint for G2.
→ Dispatch P3 (single phase, serial).
→ Await user checkpoint for G3 → plan complete.

After group execution: summarize per-phase work, parallelism vs lone dispatch, verification, failures, rollbacks if any — then checkpoint for explicit user approval before the next group (same semantics as orchestration rules).

Level-2 Phase-Task Fan-out within Group

Level 2 — task fan-out within a single phase. Split steps into agent-scoped tasks (dev-frontend, dev-backend, qa-unit, etc.) and parallelize those whose parallelizable flag is true and whose file touches are disjoint; respect task-level depends_on ordering inside the phase.

Example — task fan-out (Level 2, within one phase):

Phase P2a tasks:

- dev-frontend: implement login UI (parallelizable: true, touches ui/**)
- dev-backend: implement auth API (parallelizable: true, touches api/**)
- qa-unit: write unit tests (depends on dev output — runs after)

Execution:
→ Dispatch dev-frontend + dev-backend in parallel.
→ Wait for both.
→ Dispatch qa-unit (dependent).
→ Verify phase.

Do not parallelize at Level 2 when: phases are not parallelizable_with each other per plan; siblings' touches intersect at runtime; tasks have write dependencies on the same files; steps are explicitly sequential; or tasks need shared mutable state.

Level-3 Intra-Role Horizontal Fan-out

Given phase P, role R, and touches[] = [t1..tk]:

1. Partition into N disjoint groups with longest-common-prefix segmentation (greedy scan of sorted paths; grow each group while the common path prefix is stable; minimum group size 2 unless k < 2N, in which case degrade to k single-task instances — no fan-out).
2. Fan-out only if N ≥ 2 disjoint groups and N ≤ 8. If below threshold, use a single instance. Hard cap: max 8 instances per role per phase.
3. For each group g_i (1 ≤ i ≤ N): spawn an agent instance with instance_id = "<role>#<i>", pass disjoint slice touches_i, collect diff_i and diff_hash_i.
4. Merge: union all diff_i into merged_diff. After edits, verify ∀ i,j (i≠j): touches_i ∩ touches_j = ∅ and files_modified_i ∩ files_modified_j = ∅ (instances may touch files outside their slice — post-edit check is mandatory).
5. Post-edit overlap: rollback all N instances using each instance’s originating_diff, log [tech-lead] fanout_post_edit_overlap_rollback phase={p} instances={n}, then fall back to single-instance sequential for the phase slice.
6. Compute merged_diff_hash for downstream loops (e.g. merged-diff review).

Disjoint-Touches Pre-flight

Dispatch rules (Level 1 and Level 2):

1. Identify independent units — different files/modules, no shared state; at Level 1, each unit’s parallelizable_with must include the sibling IDs you dispatch with.
2. Check parallelizable — only background-parallelize agents/tasks marked parallelizable: true where the plan allows it.
3. Invoke in parallel — parallel Task calls in one turn, or run_in_background: true for all but one as required by the host.
4. Verify disjoint writes pre-dispatch — for every pair of siblings, touches sets must not intersect; if they do, stop and escalate to cto (plan parallel-safety broken).
5. Collect and confirm — wait for all units in the group; confirm acceptance from agent reports or verification passes — not by editing or running implementation yourself.

Build the execution DAG from ## Phase Dependency Graph: group phases by depends_on; cross-check each phase’s parallelizable_with matches its group’s sibling IDs; escalate if malformed.

R8 — cross-folder: Phases in a parallel group MUST share a single folder_root. Cross-folder phases in one parallel group are a plan defect; halt and escalate to cto. Within one folder, serialize any phase that touches package manifests or git-tracked files if a sibling also writes that same folder.

Failure Semantics within a Group

Collect-then-report: For an approved parallel group, run all in-flight tasks to completion (or each task’s timeout) before tech-lead reports outcomes to the user. Do not surface partial failures mid-group.

Per-instance failure (Level-3 fan-out): Re-dispatch only the failed instance’s slice alone — one retry; if that fails, escalate. Keep the other N−1 instances’ diffs in a staging area on disk until the retried instance returns, then proceed with merge.

Legacy sibling-phase behavior (after group completes): If one sibling phase fails and others succeed: roll back only the failed phase per its rollback_scope; report partial success; await user guidance before re-dispatching the failed phase — do not roll back successful siblings. If multiple siblings fail: roll back each independently; escalate with a combined summary. If failure shows touches were not actually disjoint: halt, roll back all siblings to be safe, escalate to cto.

Concurrency Caps

• Per-group concurrency = count of parallelizable phases in the current cto plan group (dynamic). The legacy fixed cap of 4 parallel phases per group is removed — do not apply a hardcoded 4.
• Per-phase intra-role fan-out: maximum 8 instances per role per phase (see Level-3).
• Global safety net: 12 concurrent in-flight Task calls per orchestrator session (across all groups and fan-outs).
• Rate-limit backoff: On 3 consecutive HTTP 429 (or equivalent rate-limit signals), halve effective concurrency for 60 seconds, then reassess.

Platform caps + delegation floor (YAML)

Canonical file (stowed): ~/.config/opencode/configurations/orchestration-policies/subagent-caps.yml
Repo source: ../../configurations/orchestration-policies/subagent-caps.yml

Key	Cursor (policy default)	Gemini (policy default)
platforms.*.max_concurrent_subagents	10	5
platforms.*.max_depth	3	2
platforms.*.max_total_per_session	50	30
floor.min_subagents_per_coordinator	6 (unless coordinator_overrides)	same file
floor.target_subagents	8	same
floor.half_floor_recursive	3 min direct Tasks when using recursive fan-out	same
defaults.fan_out_minimum	2 — if ≥2 independent shards exist, parallelize	same

coordinator_overrides: remotion-builder and n8n-builder → min_subagents_direct: 4, target_subagents: 6 (single map entry each).

Floor vs platform cap: When **max_concurrent_subagents < floor.min_subagents_per_coordinator** for the active platform row, **do not** drop the floor silently — run **batched waves**: wave-1 = up to **max_concurrent_subagents** parallel **Tasks**, await all envelopes + merges, wave-2+ until **effective** dispatched count (including recursive tree per **mandatory-delegation.md**) meets floor or a valid **below_floor_justification** is emitted. Log each wave in **dispatch-audit.md** with parallelism: partial` when caps force sequencing across waves.

Serial fallback: When choosing serial over parallel and defaults.serial_requires_justification: true, write user-visible reason and append swarm_override_reason / serial justification per agent-orchestration.md audit row.