diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index d190486ab..e323c15f5 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -231,7 +231,35 @@ The CI parity workflow (`.github/workflows/ci-scope-parity.yml`) runs both paths
 
 #### Semantic-model source of truth
 
-`ParsedFile` (`gitnexus-shared/src/scope-resolution/parsed-file.ts`) is the single semantic model both paths consume. Scope-resolution passes MUST NOT build a parallel parse representation. If a per-language hook needs AST-level facts that `ParsedFile` doesn't expose, it should reuse the orchestrator's `treeCache` (`RunScopeResolutionInput.treeCache`) rather than re-invoking `parser.parse(...)` on its own — the C# `populateNamespaceSiblings` hook is the reference implementation of this pattern.
+Two independent invariants.
+
+**ParsedFile = the AST-level truth.** `ParsedFile` (`gitnexus-shared/src/scope-resolution/parsed-file.ts`) is the single per-file artifact both resolution paths consume. Scope-resolution passes MUST NOT build a parallel parse representation. If a per-language hook needs AST-level facts that `ParsedFile` doesn't expose, it should reuse the orchestrator's `treeCache` (`RunScopeResolutionInput.treeCache`) rather than re-invoking `parser.parse(...)` on its own — the C# `populateNamespaceSiblings` hook is the reference implementation of this pattern.
+
+**SemanticModel = the symbol-level truth.** `SemanticModel` (`gitnexus/src/core/ingestion/model/semantic-model.ts`) is the authoritative store for every symbol-indexed lookup (by `nodeId`, `simpleName`, `qualifiedName`, or `filePath`). Both paths read from here:
+
+- Legacy Call-Resolution DAG → `call-processor` Tier 1/2/3 via `model.symbols.lookupExactAll`, `model.methods.lookupMethodByName`, `model.types.lookupClassByName`, `lookupMethodByOwnerWithMRO`.
+- Scope-resolution pipeline → `findOwnedMember`, `pickOverload`, `findExportedDefByName` all consult `model.methods` / `model.fields` / `model.symbols`.
+
+The scope-resolution pipeline additionally carries `WorkspaceResolutionIndex` for `Scope`-valued lookups (`classScopeByDefId`, `moduleScopeByFile`) that `SemanticModel` structurally cannot hold. No symbol-indexed duplicates exist outside `SemanticModel`.
+
+**Write / read phase contract.** The model is mutable during three ordered phases and read-only afterward:
+
+```
+ Phase 1: legacy parse     ──► symbolTable.add fans into types/methods/fields
+ Phase 2: scope-resolution ──► reconcileOwnership() registers corrected ownerIds
+ Phase 3: finalize         ──► model.attachScopeIndexes(bundle) — one-shot freeze
+ ─────────────────────────── phase boundary ───────────────────────────
+ Read phase: all resolution passes + MCP + HTTP + embeddings see
+             SemanticModel (read-only handle); writes are type-errors.
+```
+
+`runScopeResolution` narrows `MutableSemanticModel` → `SemanticModel` at the phase boundary so downstream passes physically cannot mutate the model even accidentally.
+
+**Transitional: reconciliation pass.** `reconcileOwnership` (`scope-resolution/pipeline/reconcile-ownership.ts`) is a shim for languages whose legacy extractor doesn't resolve `enclosingClassId` at parse time (Python class-body methods are the canonical case). It walks `parsed.localDefs[i].ownerId` after `populateOwners` and registers any missed methods/fields into the model. Idempotent — safe to re-run, safe alongside languages whose legacy extractor already carries `ownerId` (C#).
+
+The architectural end state is for every language's parse-time extractor to emit the correct `ownerId` directly, making reconciliation a no-op (tracked as a follow-up refactor). The dev-mode validator `validateOwnershipParity` surfaces any drift via `onWarn` under `NODE_ENV !== 'production' && VALIDATE_SEMANTIC_MODEL !== '0'`.
+
+References: `semantic-model.ts` file-head (full write/read contract); `contract/scope-resolver.ts` Contract Invariant I9 (scope-resolution-side rule).
 
 ---
 
diff --git a/gitnexus/src/core/ingestion/model/semantic-model.ts b/gitnexus/src/core/ingestion/model/semantic-model.ts
index d1c5f1446..b989ff5b3 100644
--- a/gitnexus/src/core/ingestion/model/semantic-model.ts
+++ b/gitnexus/src/core/ingestion/model/semantic-model.ts
@@ -44,6 +44,45 @@
  * direct `createSymbolTable()` caller (e.g. an isolated unit test) gets
  * the pure, registry-free behavior — no surprises, no hidden side
  * effects.
+ *
+ * ## Single-source-of-truth invariant
+ *
+ * `SemanticModel` is the authoritative symbol store for the whole
+ * ingestion pipeline. Both the legacy Call-Resolution DAG and the
+ * new scope-resolution pipeline read symbol-keyed lookups from here
+ * exclusively — no parallel owner-keyed, name-keyed, or file-keyed
+ * symbol indexes exist outside this module. The scope-resolution
+ * pipeline does carry a small `WorkspaceResolutionIndex` for
+ * `Scope`-valued maps (`classScopeByDefId`, `moduleScopeByFile`) that
+ * `SemanticModel` structurally cannot hold, but nothing else.
+ *
+ * ## Write / read phase contract
+ *
+ * Writes to the model happen in three clearly-ordered phases during a
+ * single ingestion run:
+ *
+ *   1. **Legacy parse phase** (`parsing-processor`) calls
+ *      `symbols.add(...)` per extracted symbol, which fans out via
+ *      the dispatch table into `types` / `methods` / `fields`.
+ *   2. **Scope-resolution reconciliation** (`reconcileOwnership` in
+ *      `scope-resolution/pipeline/reconcile-ownership.ts`) registers
+ *      any `parsed.localDefs[i]` with a scope-resolution-corrected
+ *      `ownerId` that the legacy pass missed (Python class-body
+ *      methods are the canonical case). Idempotent.
+ *   3. **Finalize-orchestrator** calls `attachScopeIndexes(...)` to
+ *      stamp the materialized `ScopeResolutionIndexes` bundle onto
+ *      `model.scopes`. One-shot; throws on a second call.
+ *
+ * After these three phases, the model is effectively frozen:
+ *   - `attachScopeIndexes` applied `Object.freeze` to its bundle.
+ *   - Downstream passes receive the narrowed `SemanticModel` reader
+ *     handle (not `MutableSemanticModel`), so `.register()` /
+ *     `.clear()` / `attachScopeIndexes()` are structurally absent.
+ *
+ * See `scope-resolution/contract/scope-resolver.ts` Contract
+ * Invariant I9 for the scope-resolution-side rule and
+ * `ARCHITECTURE.md` § "Semantic-model source of truth" for the
+ * overall architecture.
  */
 
 import type { NodeLabel } from 'gitnexus-shared';
diff --git a/gitnexus/src/core/ingestion/scope-resolution/contract/scope-resolver.ts b/gitnexus/src/core/ingestion/scope-resolution/contract/scope-resolver.ts
index d04afbc3d..c23e90c74 100644
--- a/gitnexus/src/core/ingestion/scope-resolution/contract/scope-resolver.ts
+++ b/gitnexus/src/core/ingestion/scope-resolution/contract/scope-resolver.ts
@@ -138,6 +138,44 @@
  *     `ScopeResolutionIndexes` is a read-guidance surface for
  *     consumers, NOT an immutability promise during the resolve phase.
  *
+ *   - **I9 — `SemanticModel` is the single authoritative symbol store.**
+ *     Every symbol-indexed lookup (key = `nodeId | simpleName |
+ *     qualifiedName | filePath`) resolves through
+ *     `SemanticModel.{symbols,types,methods,fields}`. Scope-resolution
+ *     passes MUST NOT maintain parallel owner-keyed or name-keyed
+ *     symbol indexes — `WorkspaceResolutionIndex` is reserved for
+ *     `Scope`-valued lookups that `SemanticModel` structurally cannot
+ *     carry.
+ *
+ *     The `runScopeResolution` orchestrator guarantees this invariant
+ *     in two steps:
+ *       1. The legacy `parse` phase populates `SemanticModel` via
+ *          `symbolTable.add(...)`. For languages whose extractor
+ *          resolves `enclosingClassId` at parse time, class-body defs
+ *          are correctly owner-keyed there.
+ *       2. The `reconcileOwnership` pass runs after
+ *          `provider.populateOwners(parsed)` and registers any def in
+ *          `parsed.localDefs[i]` with a corrected `ownerId` that the
+ *          legacy pass missed (primarily Python class-body methods).
+ *          Idempotent — duplicates are skipped by `nodeId`.
+ *
+ *     Contract for consumers: `model` is `MutableSemanticModel` only
+ *     during those two write phases. Downstream passes receive a
+ *     narrowed `SemanticModel` (read-only) handle. This is enforced by
+ *     `runScopeResolution`'s type-level narrowing at the phase
+ *     boundary.
+ *
+ *     The dev-mode runtime validator (`validateOwnershipParity`)
+ *     surfaces any drift between `parsed.localDefs` ownership and the
+ *     registries via `onWarn` when
+ *     `NODE_ENV !== 'production' && VALIDATE_SEMANTIC_MODEL !== '0'`.
+ *
+ *     This invariant is a **transitional shim**: the architectural
+ *     end state is for every language's parse-time extractor to emit
+ *     the correct `ownerId` directly, removing the need for
+ *     reconciliation. Tracked as a follow-up; see ARCHITECTURE.md §
+ *     "Semantic-model source of truth".
+ *
  * ## Semantic-model source of truth
  *
  * `ParsedFile` (from `gitnexus-shared/src/scope-resolution/parsed-file.ts`)