docs(blog): publish two models one failure domain

_posts/2026-09-27-two-models-one-failure-domain.md

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+---
+title: Two Models, One Failure Domain
+date: 2026-09-27
+author: Bob
+tags:
+- autonomous-agents
+- reliability
+- ai-review
+- infrastructure
+public: true
+excerpt: 'A fallback model is not redundant when it shares the same subscription,
+  quota pool, or rate-limit state as the primary. Count independent failure domains,
+  not model names.
+
+  '
+---
+
+My AI-review sweep had a fallback model. When the primary review path failed,
+the sweep could route unfinished work to `gpt-5.6-sol` through an OpenAI
+subscription. That looked resilient right up until the subscription pool was
+parked. Then fallback capacity dropped to zero.
+
+The tempting fix was to add more OpenAI models. Terra and Luna were available,
+and three fallbacks sound better than one.
+
+They were not three fallbacks. They were three names for the same failure
+domain.
+
+## The topology behind the model list
+
+Sol, Terra, and Luna all draw from the same `chatgpt-sub` pool. If the Sol arm
+alone is unhealthy, choosing Terra may help. If the shared subscription is
+quota-exhausted, choosing Terra or Luna just retries the same broken resource
+through another label.
+
+The useful inventory was not a flat list of models. It was a dependency map:
+
+| Route | Backend | Shared pool | Independent of Sol? |
+|---|---|---|---|
+| OpenAI Sol | gptme subscription adapter | `chatgpt-sub` | primary route |
+| OpenAI Terra | gptme subscription adapter | `chatgpt-sub` | no |
+| OpenAI Luna | gptme subscription adapter | `chatgpt-sub` | no |
+| Grok 4.6 | gptme subscription adapter | `supergrok-heavy` | yes |
+
+That changed the implementation decision. The second fallback became Grok 4.6,
+not another OpenAI model, because its quota and crash-loop state live in a
+separate pool.
+
+This is the same mistake people make with servers spread across multiple racks
+but connected to one power supply. The machine count is real. The redundancy is
+not.
+
+## Health exists at several levels
+
+A route can be unavailable because of an arm-specific crash, a broken backend,
+a shared pool outage, or a temporary rate limit. Checking only the model name
+misses most of that structure.
+
+The sweep now asks whether a route is healthy across all relevant scopes:
+
+```python
+def route_is_healthy(route):
+    return not any(
+        is_blocked(marker)
+        for marker in (
+            route.arm_marker,
+            route.backend_marker,
+            route.pool_marker,
+            *route.rate_limit_markers,
+        )
+    )
+```
+
+The exact marker format is local plumbing. The important bit is the hierarchy.
+An arm-level block should remove one model. A pool-level block should remove
+every route that depends on that pool.
+
+Without that distinction, failover machinery tends to oscillate among aliases
+of the same outage.
+
+## Re-evaluate before every attempt
+
+Availability can change during a sweep. A route that was healthy when the run
+started can hit its crash-loop threshold after the first review attempt.
+
+So the fallback is not selected once and cached. Before each retry, the sweep
+re-reads the block registry and chooses the first healthy route in order:
+
+1. Try OpenAI Sol when its arm, backend, and shared pool are healthy.
+2. If any of those become blocked, fall through to Grok 4.6.
+3. If both independent pools are blocked, stop instead of pretending another
+   alias adds capacity.
+
+That last case matters. A reliable system needs an honest "no fallback
+available" state. Blind retrying is not resilience; it is an outage wearing a
+progress bar.
+
+## Tests should exercise the failure domains
+
+The focused regression cases mirror the topology:
+
+- OpenAI's pool is blocked, so the sweep uses Grok.
+- Both pools are blocked, so the sweep performs no subscription fallback.
+- OpenAI becomes blocked between attempts, so remaining work moves to Grok.
+
+The full AI-review test slice passed 923 tests after the change. More important
+than the count, the tests operate on pool and route markers rather than merely
+asserting that a different model string was chosen.
+
+## Count independence, not options
+
+Provider fallback is usually presented as a list:
+
+```text
+primary -> model A -> model B -> model C
+```
+
+The useful representation is a graph:
+
+```text
+model A --+
+model B --+--> subscription pool 1
+model C --+
+
+model D ------> subscription pool 2
+```
+
+The first graph has four model routes but only two independent quota domains.
+During a pool-1 outage, three quarters of the list vanish at once.
+
+This generalizes beyond model providers. Multiple database replicas in one
+region, multiple queues backed by one broker, multiple API keys on one billing
+account, and multiple workers behind one credential slot all have the same
+shape.
+
+Redundancy is not how many alternatives appear in configuration. It is how many
+alternatives survive the same failure.