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+# RFC 013: Add `F9_NOT_IMPLEMENTED` to `FailureTaxonomy`
+
+**Gap IDs:** `provisional-gap-004` (provisional — replace before opening upstream)
+
+## Problem
+
+[`failure-taxonomy.schema.json`](../schemas/events/failure-taxonomy.schema.json)
+defines nine classes, F0 through F8:
+
+```
+F0_NONE | F1_MISSING_ARTIFACT | F2_ENVIRONMENT | F3_DATA_ACCESS |
+F4_POLICY | F5_UNSPECIFIED_CONFIG | F6_HPC_QUEUE | F7_RUNTIME |
+F8_DOMAIN_ASSUMPTION
+```
+
+None of the nine cleanly captures the "the capability is declared but the
+backend is not yet implemented" case that every implementation hits while
+building toward full conformance. Operationally, this happens in three
+distinct shapes:
+
+1. **Stub endpoint returning HTTP 501.** ARIA's optional operations
+   (`pauseRun`, `resumeRun`, `listRunCheckpoints`, sandbox/supervision ops)
+   commonly ship as `501 + StructuredError{code: "NOT_IMPLEMENTED"}` until the
+   underlying durable-execution or HITL work lands. Today implementations
+   reach for `F5_UNSPECIFIED_CONFIG` as the closest existing class, but the
+   semantics are wrong — there is no config to set; the feature is simply not
+   built.
+2. **Feature disabled by deployment flag.** A capability is implemented but
+   turned off (e.g., LLM features without provider credentials, HITL routes
+   when `ENABLE_HITL=false`). Today implementations either reuse
+   `F5_UNSPECIFIED_CONFIG` (close, but conflates two ops concerns) or
+   `F4_POLICY` (wrong — it isn't a policy decision, it's a deployment
+   posture).
+3. **Capability declared but smoke test failed.**
+   [`capability.schema.json`](../schemas/common/capability.schema.json) already
+   has a `status: smoke_test_failed` value, but the corresponding Run that
+   *would* exercise the capability has no precise `failureClass` to record.
+
+Three different operational states, all currently sharing one approximate
+class. The ambiguity makes the `failure_taxonomy_consistent` conformance row
+misleading: a deployment with 30 unimplemented stubs and a deployment with 30
+misconfigurations both report `F5_UNSPECIFIED_CONFIG` and look identical to an
+auditor.
+
+## Proposal
+
+Add one new class:
+
+```diff
+ "enum": [
+   "F0_NONE",
+   "F1_MISSING_ARTIFACT",
+   "F2_ENVIRONMENT",
+   "F3_DATA_ACCESS",
+   "F4_POLICY",
+   "F5_UNSPECIFIED_CONFIG",
+   "F6_HPC_QUEUE",
+   "F7_RUNTIME",
+-  "F8_DOMAIN_ASSUMPTION"
++  "F8_DOMAIN_ASSUMPTION",
++  "F9_NOT_IMPLEMENTED"
+ ]
+```
+
+### Semantics
+
+- **`F9_NOT_IMPLEMENTED`** — the operation is declared in the
+  `capability.schema.json` registry but the backend implementation is absent
+  or disabled at the deployment. Distinct from `F2_ENVIRONMENT` (the
+  environment is broken), `F4_POLICY` (a policy denied it), and
+  `F5_UNSPECIFIED_CONFIG` (configuration is missing for an *implemented*
+  capability).
+- Paired with `HTTP 501 + StructuredError{code: "NOT_IMPLEMENTED",
+  recoverable: false}` on the wire.
+- A Run carrying `failureClass: F9_NOT_IMPLEMENTED` MUST also have
+  `status: failed`.
+
+Backward-compatible (no Run ever previously emitted F9; existing F5 emitters
+keep working).
+
+### Alternative considered: split F9 vs. F10
+
+A single F9 conflates "never implemented" with "implemented but disabled." The
+argument for splitting is auditability — a deployment can know how many of its
+501s are missing code vs. deliberately gated. The argument against is taxonomy
+proliferation: every additional class adds reporting overhead, and the
+deployment knows the difference internally via its own feature flags. This RFC
+proposes the single-class addition and leaves a possible
+`F10_FEATURE_DISABLED` split to a follow-up RFC if demand emerges.
+
+### Alternative considered: document `F5_UNSPECIFIED_CONFIG` overload
+
+Lighter-touch: update the schema description for F5 to explicitly cover "not
+implemented" alongside "misconfigured." Rejected because the words "unspecified
+config" don't fit "no code exists" — the overload is what creates the auditor
+confusion in the first place.
+
+### Relationship to RFC 015
+
+[RFC 015](015-conformance-stub-semantics.md) asks the higher-order governance
+question: do 501 stubs count toward conformance at all? This RFC scopes purely
+to the failure taxonomy and is useful regardless of how RFC 015 resolves —
+even if stubs *do* count, the Runs they generate still need a non-misleading
+`failureClass`.
+
+## Deferred (out of scope for this RFC)
+
+- **`F10_FEATURE_DISABLED` split** (see Alternative above).
+- **`StructuredError` ⇄ `failureClass` mapping table.** Codifying which
+  `StructuredError.code` values map to which `failureClass` would be useful
+  but is a documentation/governance task, not a schema change.
+- **Per-operation declaration of expected `failureClass` set.** A capability
+  could in principle declare which failure classes its Runs can produce.
+  Useful for static analysis; out of scope here.
+
+## Platform reference
+
+[ARIAPlatform `platform-v0-implementation-profile.json`](https://github.com/brettin/ARIAPlatform_v0/blob/main/reference/platform-v0-implementation-profile.json)
+— `FailureClass`, `StructuredError`. ARIAPlatform v0 stubs surface as 501 +
+`code: "NOT_IMPLEMENTED"` today; this RFC adds the matching failure class so
+the wire response and the Run record agree on the same vocabulary.