feat(fase1): spike B - magentic unbounded + concurrent state isolation [skip-docs]

docs/fase1-spikes/findings-b.md

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+# Spike B findings — Magentic unbounded footgun (G1/B4) + fan-out state isolation (G2/B7)
+
+**Assumptions:** (G1/B4) Magentic with `limits=None` does NOT self-terminate, so an
+explicit limit is mandatory; (G2/B7) reusing a shared fan-out builder/workflow instance
+corrupts state, while a fresh-instance helper prevents it.
+
+This spike drives the **real** GA builders with the fake client (de-risked by the Step 2
+builder smoke), so it runs entirely in the quality gate — **no live LLM**.
+
+## (a) Magentic unbounded termination — CONFIRMED
+
+- A `StandardMagenticManager` whose progress ledger always reports *not satisfied /
+  progress being made* never finalizes.
+- With **`max_round_count=None`** the workflow streams orchestration events indefinitely;
+  only the shared harness round/iteration guard (`Budget` + `TokenMeter`) stops it.
+  `unbounded_magentic_self_terminates()` returns **False** → it does **not** self-terminate.
+- With an explicit **`max_round_count=2`** the same manager stops cleanly
+  ("Magentic Orchestrator: Max round count reached") and yields an output.
+- **Implication (B4):** a stop-criterion/round cap is mandatory at startup for any
+  Magentic use — never rely on natural termination. (Magentic stays experimental and is
+  exercised here ONLY to confirm the footgun, never as the debate default — that remains
+  Group Chat maker-checker, A2/G8.)
+
+### Minor MAF observation (recorded)
+
+Cutting an *instrumented* Magentic stream short — the only way to observe an unbounded run
+— makes OpenTelemetry log a benign `Failed to detach context` on generator close, and the
+in-flight fake response surfaces a `RuntimeWarning: coroutine was never awaited`. Both are
+cosmetic (no functional effect); the spike silences the OTel context logger and filters the
+warning. Worth knowing if Fase 2 observes streamed orchestrations.
+
+## (b) Fan-out state isolation — CONFIRMED
+
+- **Shared instance:** one `ConcurrentBuilder` workflow reused across the three reference
+  projects → the shared fake clients accumulate calls (max call count == 3) → **state
+  bleed**.
+- **Fresh instance:** `fresh_workflow()` builds a new workflow + fresh clients per run →
+  each run's clients see exactly one call (`[1, 1, 1]`) → **zero bleed** across all 3
+  projects (B7).
+- **Implication:** Fase 2 fan-out must build a fresh workflow/executor instance per
+  project run (a factory like `fresh_workflow()`), never reuse a shared instance.
+
+## Token use
+
+**0 — no live LLM.** All behavior is driven by the deterministic `FakeChatClient`; its
+"tokens" are word-counts of canned replies and are not meaningful cost numbers here.
+
+## Implication for Fase 2
+
+Both footguns behave exactly as the §15 register predicted. The framework MUST: (1) require
+explicit round/stop caps for any Magentic-style loop (fail-fast at startup, D6/B4); and
+(2) use a fresh-instance-per-run factory for fan-out to avoid state corruption (B7).