feat(backends): LocalFileBinding implements verbs (PEFT/aLoRA path) + from_catalog() (Epic #929 Phase 2) #1141

Parent epic: #929
Design proposal: PR #1080 §8.1, §9.2, §9.3
Phase: 2 (Wave 4 — start after 2.1 merges)
Depends on: #1134, #1135, #1140
Blocks: #1144, #385, #1465

Problem

LocalFileBinding is a stub from issue 0.1, raising NotImplementedError on each verb. Reality A (today's IntrinsicAdapter — local PEFT/aLoRA file) needs the four verbs working before the binding is usable.

Agreed design

Four verbs

• prepare() — resolves configured HF revision (from catalogue pinned SHA; defaults to the pinned SHA if unspecified), downloads PEFT weights via the existing HF path, registers with AdapterMixin.add_adapter, then loads with AdapterMixin.load_peft_adapter.
• activate() — switches adapter on (PEFT layer enabled) via the backend's PEFT activation primitive.
• deactivate() — switches off. Auto-called by adapter_scope even on exception.
• release() — removes PEFT adapter via AdapterMixin.unload_peft_adapter. Second call is a no-op.

Session-scoped: prepare once per session. Call-scoped: activate/deactivate.

Corrected 2026-08-11. This line previously read "prepare once per session (or
explicit release()+prepare() cycle)". That contradicts the merged WeightsBinding
ABC from #1134, which states: "release() — terminal; releases all resources. The
binding is not reusable after release()." The implementation follows the ABC, so the
cycle this issue promised is not available. Which of the two is right is an open
question, tracked in #1528 — that issue also covers the related defect that
release() leaves the adapter registered in the backend's _added_adapters, so its
qualified_name stays claimed for the backend's lifetime.

from_catalog() classmethod

LocalFileBinding.from_catalog(name: str) -> LocalFileBinding — looks up the catalogue entry by name (post-0.2, with pinned revision), returns a fully configured binding. This is the user-facing standard path:

Adapter(name="answerability", weights=LocalFileBinding.from_catalog("answerability"))

OTel spans — moved to #1466 (2026-07-29)

Superseded. Do not implement from this section. The span work moved to #1466;
see "Criteria moved out" below for why. The names originally written here
(intrinsic.call, intrinsic.prepare, intrinsic.activate, intrinsic.deactivate,
intrinsic.parse) were never the convention this repo uses and must not be
copied. The merged naming, established by #1140 / PR #1422 and documented in
docs/dev/adapter_observability.md, is adapter_function for the parent and
adapter_function.<phase> for the children, with mellea.adapter_function.*
span attributes. Likewise the content-capture gate is MELLEA_TRACES_CONTENT
(plural TRACES), not MELLEA_TRACE_CONTENT — the latter never existed; see
docs/dev/adapter_observability.md:133, which already records this same mistake
against #1140's criteria.

Metric hooks (ship with this PR)

ADAPTER_FUNCTION_PHASE_COMPLETE and ADAPTER_FUNCTION_INVOCATION_COMPLETE are fired
from the lifecycle, and AdapterFunctionMetricsPlugin (already merged, at
mellea/telemetry/metrics_plugins.py) turns them into metrics. Firing the hooks is
correct and stays in scope here; only span production moved to #1466.

Docs updates

• docs/dev/adapter_observability.md (from 2.1) — cover the LocalFile lifecycle and
  which metric hooks it fires. Not span attributes: those moved to feat(telemetry): add complete adapter-function lifecycle observability #1466 with the
  spans themselves.
• docs/docs/advanced/intrinsics.md — add new Adapter(weights=LocalFileBinding.from_catalog(...)) construction pattern; old IntrinsicAdapter(...) shown as deprecated with migration note
• docs/examples/intrinsics/ — at least one example shows new construction; existing examples gain model_options=

Both user-facing docs items deferred, 2026-08-11. Rationale, recorded here rather
than left as a silently unmet criterion: this PR makes LocalFileBinding and
adapter_scope real, but nothing in production calls them — _generate_from_intrinsic
still generates outside the scope, and resolve_adapter still returns adapters whose
io_contract raises. Documenting Adapter(weights=LocalFileBinding.from_catalog(...))
as a construction pattern now would be documenting a path a user cannot yet use, and
deprecating IntrinsicAdapter(...) in favour of it would be actively wrong. Both belong
with the cutover that makes the path real: #1465 for the generation path, #1144 for the
shim removal and tutorial rewrite.

Out of scope

EmbeddedBinding (2.3), ServerMediatedBinding (3.1), long-running session refresh policy (deferred — PR #1080 §17 Q5), full rewrite of docs/dev/intrinsics_and_adapters.md (4.1).

Also out of scope, added 2026-07-29: routing _generate_from_intrinsic through adapter_scope (#1465) and emitting the adapter-function spans from a tracing plugin (#1466). See "Criteria moved out" below.

Acceptance criteria

• All four verbs implemented for LocalFileBinding
• prepare() uses pinned revision from catalogue; revision="main" opts into tracking-latest
• activate/deactivate toggle PEFT layer correctly
• release() unloads the weights and is idempotent on a second call
• release() cleanly unregisters — not met, deferred to refactor(backends): release() leaves the adapter registered; is a binding reusable after release? (Epic #929) #1528. release()
  calls unload_peft_adapter, which removes the entry from the backend's loaded
  set but not from _added_adapters, the registered set. There is no inverse verb
  for add_adapter to call. PR feat(backends): LocalFileBinding implements verbs (PEFT/aLoRA path) + from_catalog() (Epic #929 Phase 2) #1454 corrects release()'s docstring to state what
  it does and does not release, and makes the resulting "Backend refused to register"
  error name both possible causes; whether a released name should become re-claimable
  at all is the open question on refactor(backends): release() leaves the adapter registered; is a binding reusable after release? (Epic #929) #1528.
• LocalFileBinding.from_catalog("answerability") returns correct binding with pinned revision
• AdapterFunctionMetricsPlugin: mellea.adapter_function.invocations counter increments; mellea.adapter_function.parse_failures on AdapterSchemaMismatchError; mellea.adapter_function.phase_duration records prepare + activate durations
  (the parse_failures half needed a fix: adapter_scope declared
  outcome: Literal["success", "error"] and so could never emit "schema_error",
  the only value that increments that counter. It now classifies
  AdapterSchemaMismatchError separately.)
• docs/docs/advanced/intrinsics.md updated: new construction pattern present, deprecated old pattern noted — deferred to refactor(backends): route intrinsic generation through adapter_scope (Epic #929 Phase 2) #1465 / refactor(intrinsics): remove deprecation shims; rewrite intrinsics_and_adapters.md; write 3 tutorials (Epic #929 Phase 4) #1144, see "Docs updates"
• docs/examples/intrinsics/ updated; all examples pass uv run pytest docs/examples/intrinsics/ — deferred to refactor(backends): route intrinsic generation through adapter_scope (Epic #929 Phase 2) #1465 / refactor(intrinsics): remove deprecation shims; rewrite intrinsics_and_adapters.md; write 3 tutorials (Epic #929 Phase 4) #1144, see "Docs updates"
• docs/dev/adapter_observability.md updated for the LocalFile lifecycle and metric hooks (span-attribute sections moved to feat(telemetry): add complete adapter-function lifecycle observability #1466)
• ruff format, ruff check, mypy clean

Criteria moved out (2026-07-29)

PR #1454 removes the inline span implementation, because spans must be produced
by a plugin rather than opened inside mellea/backends/ — see the
PR discussion
for the reasoning. These were acceptance criteria here and are now tracked where
the work will actually happen, rather than left unticked on this issue:

Criterion text below is restated in the merged naming convention
(adapter_function / MELLEA_TRACES_CONTENT), not in the intrinsic.* /
MELLEA_TRACE_CONTENT spellings this issue was originally written with — neither of
those ever existed in the codebase.

The metrics criterion is unaffected and still applies — firing the hooks is
correct, and prepare() is reachable through from_catalog(). The last of the
four moved because generation does not yet run inside adapter_scope; that
cutover is #1465, and it is not "4.1" (#1144, which is shim removal plus a
docs rewrite).

Test plan

Unit tests (test/backends/test_adapters/test_local_file_binding.py) with mocked HF download + mocked backend:

• test_prepare_uses_pinned_revision
• test_prepare_allows_main_override
• test_release_is_idempotent
• test_from_catalog_returns_binding_with_correct_revision
• test_activate_deactivate_call_correct_mixin_verbs
• Span tests via synthetic OTel exporter: test_call_span_emitted, test_prepare_span_has_revision_attribute, test_content_events_absent_by_default, test_content_events_present_with_gate_set — moved to feat(telemetry): add complete adapter-function lifecycle observability #1466 with the span criteria
• test_metrics_invocation_counter_increments
• test_metrics_parse_failures_increments — inject synthetic schema-mismatch

Integration tests (test/backends/test_adapters/test_local_file_integration.py, @pytest.mark.integration, @pytest.mark.hf, @pytest.mark.slow):

• LocalHFBackend × LocalFileBinding × {lora, alora} × {check_answerability, requirement_check} — full lifecycle
• Regression: pre-fix: move test_huggingface.py to granite4.1; and small rag intrinsic … #1008 shape {"requirement_likelihood": 0.9} raises AdapterSchemaMismatchError

Qualitative (optional, @pytest.mark.qualitative): test_check_answerability_quality

Breaking changes

None for end users. Replaces internal stubs; behaviour matches existing IntrinsicAdapter runtime path post-1.A.

References

• PR docs: design proposal for adapter lifecycle (#929) — review channel, do not merge #1080 §8.1, §9.2, §9.3
• OTel omissions/gaps: chat content events, gen_ai.conversation.id, prompt templates, error status #1035 / PR feat(telemetry): close five OTel GenAI semantic convention emission gaps (#1035) #1036 — MELLEA_TRACES_CONTENT content-capture gate convention
• refactor(backends): AdapterMixin verb rename/narrow + resolve_model_options + IntrinsicMetricsPlugin (Epic #929 Phase 2) #1140 / PR refactor(backends)!: narrow AdapterMixin verbs, centralize option resolution, add AdapterFunctionMetricsPlugin skeleton #1422 — adapter_function span and metric naming convention; AdapterFunctionMetricsPlugin
• docs/dev/adapter_observability.md — the merged naming of record