diff --git a/docs/api/README.md b/docs/api/README.md
index 4ca84364..f5860574 100644
--- a/docs/api/README.md
+++ b/docs/api/README.md
@@ -848,6 +848,7 @@ const db = await brain.transact([
   { op: 'add', id: orderId, type: NounType.Document, subtype: 'order', data: 'Order #1042' },
   { op: 'update', id: customerId, metadata: { lastOrderAt: Date.now() }, ifRev: customer._rev },
   { op: 'relate', from: customerId, to: orderId, type: VerbType.Creates, subtype: 'purchase' },
+  { op: 'updateRelation', id: purchaseRelationId, subtype: 'return' },
   { op: 'remove', id: staleDraftId },
   { op: 'unrelate', id: oldRelationId }
 ], {
@@ -864,6 +865,7 @@ db.receipt.generation // the committed generation
 - `{ op: 'update', ... }` — same parameters as `update()`, including per-entity `ifRev` CAS
 - `{ op: 'remove', id }` — deletes the entity plus its relationships (same cascade as `delete()`)
 - `{ op: 'relate', ... }` — same parameters as `relate()`, including `bidirectional`; duplicates dedupe to the existing relationship id
+- `{ op: 'updateRelation', ... }` — same parameters as `updateRelation()`; a batchable, first-class relationship update (not `unrelate` + `relate` — the relationship id and its edge never change)
 - `{ op: 'unrelate', id }` — deletes a relationship by id
 
 Operations may reference ids created earlier in the same batch.
diff --git a/src/brainy.ts b/src/brainy.ts
index 6ebce2fd..5a181535 100644
--- a/src/brainy.ts
+++ b/src/brainy.ts
@@ -103,7 +103,12 @@ import {
   UpdateNounMetadataOperation,
   UpdateVerbMetadataOperation,
   DeleteNounMetadataOperation,
-  DeleteVerbMetadataOperation
+  DeleteVerbMetadataOperation,
+  UpdateInMetadataIndexOperation,
+  UpdateVerbInGraphIndexOperation,
+  metadataUpdateOpProvider,
+  graphUpdateOpProvider,
+  assertUpdateCapabilityCoherent
 } from './transaction/operations/index.js'
 import {
   BaseOperationalMode,
@@ -196,7 +201,7 @@ import {
 } from './events/changeFeed.js'
 import { isDeterministicEmbedMode } from './embeddings/deterministicEmbedMode.js'
 import { GenerationConflictError, StoreInconsistentError } from './db/errors.js'
-import { BrainyError, GraphIndexNotReadyError, MetadataIndexNotReadyError, MigrationInProgressError, VectorIndexNotReadyError } from './errors/brainyError.js'
+import { BrainyError, GraphIndexNotReadyError, MetadataIndexNotReadyError, MigrationInProgressError, VectorIndexNotReadyError, ProviderCapabilityMismatchError } from './errors/brainyError.js'
 import { assessIndexReadiness } from './utils/indexReadiness.js'
 import { reconstructNounWrapper } from './db/factLog.js'
 import { asBrainyFieldRefusal } from './db/fieldAddressing.js'
@@ -1276,6 +1281,11 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           entityIdMapper: entityIdMapperFactory ? entityIdMapperFactory(this.storage) : undefined,
         })
       }
+      // Registration-time refusal (before any write can run): a provider
+      // whose `capabilities` set claims 'update-op' while its instance lacks
+      // `updateIndex` is a typed, loud refusal — never a silent fallback
+      // discovered only at the first write.
+      assertUpdateCapabilityCoherent(this.metadataIndex, 'metadata')
 
       // Provider: graph index factory
       const graphFactory = this.pluginRegistry.getProvider<(storage: StorageAdapter) => any>('graphIndex')
@@ -1290,6 +1300,9 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         ])
         this.graphIndex = graphIndex
       }
+      // Same registration-time refusal, graph half (see the metadata-index
+      // check above).
+      assertUpdateCapabilityCoherent(this.graphIndex, 'graph')
 
       // Fact-log v2 mint seam: after-image records carry minted dense ints,
       // and the ONE authority for those assignments is the metadata index's
@@ -1682,6 +1695,14 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       if (error instanceof Error && (error as Error & { code?: string }).code === 'BRAINY_WRITER_LOCKED') {
         throw error
       }
+      // Same rationale, provider-capability half: a registration-time refusal
+      // (a provider's `capabilities` set lies about implementing 'update-op')
+      // carries a machine-readable `.type`/`.family`/`.missingMethod` — the
+      // whole point of the typed-error family — so it must not be flattened
+      // into a message-only generic Error either.
+      if (error instanceof ProviderCapabilityMismatchError) {
+        throw error
+      }
       throw new Error(`Failed to initialize Brainy: ${error}`)
     }
   }
@@ -3619,12 +3640,23 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           createdBy: existing.createdBy,
           metadata: existing.metadata  // CRITICAL: keep as nested 'metadata' property!
         }
-        tx.addOperation(
-          new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, removalMetadata, this.indexWriteGeneration)
-        )
-        tx.addOperation(
-          new AddToMetadataIndexOperation(this.metadataIndex, params.id, entityForIndexing, this.indexWriteGeneration)
-        )
+        // ONE atomic metadata-index leg when the provider announces the
+        // update-op capability (both halves of the check pass), else the
+        // legacy remove-old/add-new pair — the one-train overlap this
+        // release (see MetadataIndexProvider.updateIndex in src/plugin.ts).
+        const metadataUpdateProvider = metadataUpdateOpProvider(this.metadataIndex)
+        if (metadataUpdateProvider) {
+          tx.addOperation(
+            new UpdateInMetadataIndexOperation(metadataUpdateProvider, params.id, removalMetadata, entityForIndexing, this.indexWriteGeneration)
+          )
+        } else {
+          tx.addOperation(
+            new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, removalMetadata, this.indexWriteGeneration)
+          )
+          tx.addOperation(
+            new AddToMetadataIndexOperation(this.metadataIndex, params.id, entityForIndexing, this.indexWriteGeneration)
+          )
+        }
       }, casPrecommit, this._changeFeed.hasListeners
         ? [
             {
@@ -4837,16 +4869,103 @@ export class Brainy<T = any> implements BrainyInterface<T> {
 
     validateUpdateRelationParams(params)
 
-
     const existing = await this.storage.getVerb(params.id)
     if (!existing) {
       throw new RelationNotFoundError(params.id)
     }
 
-    // Legacy stored shapes carried the verb type under `type` instead of the
-    // canonical `verb` field — read both, canonical first.
-    const existingRec: HNSWVerbWithMetadata & { type?: VerbType } = existing
-    const newVerbType = params.type ?? existingRec.verb ?? existingRec.type
+    const { typeChanged, verbForIndex, updatedMetadata } = this.buildUpdateRelationRecord(params, existing)
+
+    // 8.0 BigInt boundary: endpoints are unchanged across a type swap, so one
+    // resolution serves both the remove (rollback re-add) and the re-add —
+    // only needed on the legacy pair path below; the update-op path never
+    // touches endpoint ints (the provider already holds the mapping).
+    const reindexInts = typeChanged ? this.resolveVerbEndpointInts(verbForIndex) : undefined
+
+    await this.persistSingleOp({ verbs: [params.id] }, async (tx) => {
+      tx.addOperation(
+        new UpdateVerbMetadataOperation(this.storage, params.id, updatedMetadata)
+      )
+
+      // If the verb type changed, re-index in graph adjacency so traversal-by-type
+      // stays consistent. The id is preserved across the swap. ONE atomic
+      // update-op leg when the graph provider announces the capability, else
+      // the legacy remove-old/add-new pair — the one-train overlap this
+      // release.
+      if (typeChanged && reindexInts) {
+        const graphUpdateProvider = graphUpdateOpProvider(this.graphIndex)
+        if (graphUpdateProvider) {
+          tx.addOperation(
+            new UpdateVerbInGraphIndexOperation(graphUpdateProvider, existing, verbForIndex, this.graphWriteGeneration)
+          )
+        } else {
+          tx.addOperation(
+            new RemoveFromGraphIndexOperation(
+              this.graphIndex, existing, reindexInts, this.graphWriteGeneration
+            )
+          )
+          tx.addOperation(
+            new AddToGraphIndexOperation(
+              this.graphIndex, verbForIndex, reindexInts,
+              this.graphWriteGeneration,
+              (verbInt) => this.cacheVerbInt(verbInt, params.id)
+            )
+          )
+        }
+      }
+    },
+    undefined,
+    this._changeFeed.hasListeners
+      ? [
+          {
+            kind: 'relation',
+            op: 'updateRelation',
+            id: params.id,
+            relation: {
+              id: params.id,
+              from: verbForIndex.sourceId,
+              to: verbForIndex.targetId,
+              type: String(verbForIndex.verb ?? verbForIndex.type),
+              ...(verbForIndex.metadata && {
+                metadata: verbForIndex.metadata as Record<string, unknown>
+              })
+            }
+          }
+        ]
+      : undefined)
+  }
+
+  /**
+   * Pure record-building core shared by `updateRelation()` and
+   * `planTxUpdateRelation()`: given the caller-resolved before-image verb,
+   * runs subtype enforcement and computes the merged v2 metadata record and
+   * the graph-index view of the after-image — WITHOUT touching storage. The
+   * actual read of the before-image (`storage.getVerb` for the single-op
+   * path; batch-state-aware resolution for the transact planner, so a verb
+   * updated or created earlier in the SAME batch is visible) stays with the
+   * caller — the two differ, mirroring why `update()`/`planTxUpdate` each
+   * own their own entity read rather than sharing one.
+   *
+   * @param params - The update params.
+   * @param existingRec - The verb's REAL before-image (legacy stored shapes
+   *   carried the verb type under `type` instead of the canonical `verb`
+   *   field — this reads both, canonical first).
+   * @returns `typeChanged` (whether the effective verb type differs from the
+   *   before-image), the merged `newMetadata` user bag, the full
+   *   `updatedMetadata` v2 record to persist, and `verbForIndex` — the
+   *   graph-index view of the after-image (its `id` matches
+   *   `existingRec.id` — updates never change a verb's id).
+   */
+  private buildUpdateRelationRecord(
+    params: UpdateRelationParams<T>,
+    existingRec: GraphVerb
+  ): {
+    typeChanged: boolean
+    newMetadata: Record<string, unknown>
+    updatedMetadata: Record<string, unknown>
+    verbForIndex: GraphVerb
+  } {
+    const newVerbType = (params.type ?? existingRec.verb ?? existingRec.type) as VerbType
 
     // Subtype pairing enforcement on update (7.30.0). The effective verb type after
     // the update may have changed; we check against the new type and the resulting
@@ -4898,7 +5017,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
 
     // Build the verb view used by the graph index — top-level fields mirror relate()'s.
     const verbForIndex: GraphVerb = {
-      id: params.id,
+      id: existingRec.id,
       vector: existingRec.vector,
       sourceId: existingRec.sourceId,
       targetId: existingRec.targetId,
@@ -4916,51 +5035,12 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       createdAt: existingRec.createdAt
     }
 
-    // 8.0 BigInt boundary: endpoints are unchanged across a type swap, so one
-    // resolution serves both the remove (rollback re-add) and the re-add.
-    const reindexInts = typeChanged ? this.resolveVerbEndpointInts(verbForIndex) : undefined
-
-    await this.persistSingleOp({ verbs: [params.id] }, async (tx) => {
-      tx.addOperation(
-        new UpdateVerbMetadataOperation(this.storage, params.id, updatedMetadata)
-      )
-
-      // If the verb type changed, re-index in graph adjacency so traversal-by-type
-      // stays consistent. The id is preserved across the swap.
-      if (typeChanged && reindexInts) {
-        tx.addOperation(
-          new RemoveFromGraphIndexOperation(
-            this.graphIndex, existing, reindexInts, this.graphWriteGeneration
-          )
-        )
-        tx.addOperation(
-          new AddToGraphIndexOperation(
-            this.graphIndex, verbForIndex, reindexInts,
-            this.graphWriteGeneration,
-            (verbInt) => this.cacheVerbInt(verbInt, params.id)
-          )
-        )
-      }
-    },
-    undefined,
-    this._changeFeed.hasListeners
-      ? [
-          {
-            kind: 'relation',
-            op: 'updateRelation',
-            id: params.id,
-            relation: {
-              id: params.id,
-              from: verbForIndex.sourceId,
-              to: verbForIndex.targetId,
-              type: String(verbForIndex.verb ?? verbForIndex.type),
-              ...(verbForIndex.metadata && {
-                metadata: verbForIndex.metadata as Record<string, unknown>
-              })
-            }
-          }
-        ]
-      : undefined)
+    return {
+      typeChanged,
+      newMetadata: newMetadata as Record<string, unknown>,
+      updatedMetadata,
+      verbForIndex
+    }
   }
 
   /**
@@ -7958,6 +8038,9 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         entityIdMapper: entityIdMapperFactory ? entityIdMapperFactory(this.storage) : undefined,
       })
     }
+    // Registration-time refusal, re-run on every re-adoption (see init()'s
+    // matching check) — before any write can run against the recreated index.
+    assertUpdateCapabilityCoherent(this.metadataIndex, 'metadata')
     await this.metadataIndex.init()
 
     // Re-resolve the graph index the same way init() does (provider factory,
@@ -7972,6 +8055,8 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     } else {
       this.graphIndex = await this.storage.getGraphIndex()
     }
+    // Same registration-time refusal, graph half.
+    assertUpdateCapabilityCoherent(this.graphIndex, 'graph')
     this.wireGraphIdResolver()
 
     // Reset dimensions
@@ -10159,6 +10244,9 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         case 'relate':
           plan.ids.push(await this.planTxRelate(op, state, plan))
           break
+        case 'updateRelation':
+          plan.ids.push(await this.planTxUpdateRelation(op, state, plan))
+          break
         case 'unrelate':
           plan.ids.push(await this.planTxUnrelate(op, state, plan))
           break
@@ -10530,10 +10618,21 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         new ReplaceInVectorIndexOperation(this.index, params.id, existing.vector, vector, this.indexWriteGeneration)
       )
     }
-    plan.operations.push(
-      new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, removalMetadata, this.indexWriteGeneration),
-      new AddToMetadataIndexOperation(this.metadataIndex, params.id, entityForIndexing, this.indexWriteGeneration)
-    )
+    // ONE atomic metadata-index leg when the provider announces the
+    // update-op capability, else the legacy remove-old/add-new pair — same
+    // branch as update() (see the class note above planTxUpdate's Object 5-6
+    // sibling in update()).
+    const metadataUpdateProviderTx = metadataUpdateOpProvider(this.metadataIndex)
+    if (metadataUpdateProviderTx) {
+      plan.operations.push(
+        new UpdateInMetadataIndexOperation(metadataUpdateProviderTx, params.id, removalMetadata, entityForIndexing, this.indexWriteGeneration)
+      )
+    } else {
+      plan.operations.push(
+        new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, removalMetadata, this.indexWriteGeneration),
+        new AddToMetadataIndexOperation(this.metadataIndex, params.id, entityForIndexing, this.indexWriteGeneration)
+      )
+    }
     plan.touchedNouns.push(params.id)
 
     // The full planGetEntity view, passed whole — a subset view makes the
@@ -10861,6 +10960,82 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     return id
   }
 
+  /**
+   * Plan one `{ op: 'updateRelation' }` — mirror of `updateRelation()`,
+   * batch-aware: resolves the before-image against "current state + batch
+   * so far" (a `relate` or an earlier `updateRelation` on the SAME id
+   * earlier in this batch is visible), so the op is genuinely batchable —
+   * the graph counterpart of `planTxUpdate`. Returns the relationship id.
+   */
+  private async planTxUpdateRelation(
+    op: Extract<TxOperation<T>, { op: 'updateRelation' }>,
+    state: TxPlanState,
+    plan: PlannedTransact
+  ): Promise<string> {
+    const { op: _discriminator, ...rawParams } = op
+    validateUpdateRelationParams(rawParams as UpdateRelationParams<T>)
+    const params = rawParams as UpdateRelationParams<T>
+
+    const existing = state.removedVerbs.has(params.id)
+      ? null
+      : (state.verbs.get(params.id) ?? (await this.storage.getVerb(params.id)))
+    if (!existing) {
+      throw new RelationNotFoundError(params.id)
+    }
+
+    const { typeChanged, verbForIndex, updatedMetadata } = this.buildUpdateRelationRecord(params, existing)
+
+    plan.operations.push(
+      new UpdateVerbMetadataOperation(this.storage, params.id, updatedMetadata)
+    )
+
+    // If the verb type changed, re-index in graph adjacency — same branch as
+    // updateRelation(): ONE atomic update-op leg when the graph provider
+    // announces the capability, else the legacy remove-old/add-new pair.
+    if (typeChanged) {
+      const graphUpdateProvider = graphUpdateOpProvider(this.graphIndex)
+      if (graphUpdateProvider) {
+        plan.operations.push(
+          new UpdateVerbInGraphIndexOperation(graphUpdateProvider, existing, verbForIndex, this.graphWriteGeneration)
+        )
+      } else {
+        plan.operations.push(
+          // Endpoint ints resolve at EXECUTE time — mirror of planTxRelate/
+          // planTxUnrelate: the verb (or its endpoints) may have been
+          // created earlier in this same batch.
+          new RemoveFromGraphIndexOperation(
+            this.graphIndex, existing, () => this.resolveVerbEndpointInts(verbForIndex), this.graphWriteGeneration
+          ),
+          new AddToGraphIndexOperation(
+            this.graphIndex, verbForIndex, () => this.resolveVerbEndpointInts(verbForIndex),
+            this.graphWriteGeneration,
+            (verbInt) => this.cacheVerbInt(verbInt, params.id)
+          )
+        )
+      }
+    }
+
+    plan.touchedVerbs.push(params.id)
+    state.verbs.set(params.id, verbForIndex)
+
+    if (this._changeFeed.hasListeners) {
+      plan.changeEvents.push({
+        kind: 'relation',
+        op: 'updateRelation',
+        id: params.id,
+        relation: {
+          id: params.id,
+          from: verbForIndex.sourceId,
+          to: verbForIndex.targetId,
+          type: String(verbForIndex.verb ?? verbForIndex.type),
+          ...(verbForIndex.metadata && { metadata: verbForIndex.metadata as Record<string, unknown> })
+        }
+      })
+    }
+
+    return params.id
+  }
+
   /** Plan one `{ op: 'unrelate' }` — mirror of `unrelate()`. Returns the relationship id. */
   private async planTxUnrelate(
     op: Extract<TxOperation<T>, { op: 'unrelate' }>,
diff --git a/src/db/db.ts b/src/db/db.ts
index 68428a7c..8e69727d 100644
--- a/src/db/db.ts
+++ b/src/db/db.ts
@@ -61,7 +61,7 @@ import { exportGraph } from './portableGraph.js'
 import type { ExportSelector, ExportOptions, PortableGraph } from './portableGraph.js'
 import { v4 as uuidv4 } from '../universal/uuid.js'
 import { coerceNewEntityId, resolveEntityId, ORIGINAL_ID_KEY } from '../utils/idNormalization.js'
-import { EntityNotFoundError } from '../errors/notFound.js'
+import { EntityNotFoundError, RelationNotFoundError } from '../errors/notFound.js'
 import { SpeculativeOverlayError, CanonicalEnumerationUnavailableError } from './errors.js'
 import type { GenerationStore } from './generationStore.js'
 import type { ChangedIds, TransactReceipt, TxOperation } from './types.js'
@@ -698,6 +698,39 @@ export class Db<T = any> {
       return this.get(id)
     }
 
+    // The verb counterpart of `speculativeGet`. No `Db.getRelation(id)`
+    // exists (only the array-returning `related()`), so this replicates its
+    // generation-aware single-id resolution: the overlay first, then the
+    // generational before-image if something after this pin touched the
+    // verb (the same `resolveAt('verb', …)` + `relationFromRecord` pairing
+    // `related()` uses for its own changed-but-not-overlaid merge), else the
+    // live stored verb — nothing after this pin touched it, so the live
+    // state IS this view's state.
+    const speculativeGetRelation = async (id: string): Promise<Relation<T> | null> => {
+      if (overlay.verbs.has(id)) return overlay.verbs.get(id) ?? null
+      const resolved = await this.host.store.resolveAt('verb', id, this.gen)
+      if (resolved.source === 'absent') return null
+      if (resolved.source === 'record') {
+        return this.host.relationFromRecord(id, { metadata: resolved.metadata, vector: resolved.vector })
+      }
+      const stored = await this.host.storage.getVerb(id)
+      if (!stored) return null
+      return {
+        id: stored.id,
+        from: stored.sourceId,
+        to: stored.targetId,
+        type: stored.verb,
+        ...(stored.subtype !== undefined && { subtype: stored.subtype }),
+        ...(stored.visibility !== undefined && { visibility: stored.visibility }),
+        weight: stored.weight ?? 1.0,
+        ...(stored.confidence !== undefined && { confidence: stored.confidence }),
+        data: stored.data,
+        metadata: (stored.metadata ?? {}) as T,
+        ...(stored.service !== undefined && { service: stored.service }),
+        createdAt: stored.createdAt
+      }
+    }
+
     for (const op of ops) {
       switch (op.op) {
         case 'add': {
@@ -855,6 +888,35 @@ export class Db<T = any> {
           }
           break
         }
+        case 'updateRelation': {
+          const base = await speculativeGetRelation(op.id)
+          if (!base) {
+            throw new RelationNotFoundError(
+              op.id,
+              `with(): relationship ${op.id} not found at generation ${this.gen}`
+            )
+          }
+          // Field-addressing law — mirror of the 'update' case: the patch
+          // bag is the user's verbatim; engine scalars only from dedicated
+          // op fields.
+          const custom = { ...(op.metadata as Record<string, unknown> | undefined) }
+          const mergedMetadata =
+            op.merge !== false
+              ? ({ ...(base.metadata as object), ...custom } as T)
+              : ((op.metadata !== undefined ? custom : base.metadata) as T)
+          overlay.verbs.set(op.id, {
+            ...base,
+            ...(op.type !== undefined && { type: op.type }),
+            ...(op.subtype !== undefined && { subtype: op.subtype }),
+            ...(op.visibility !== undefined && { visibility: op.visibility }),
+            ...(op.weight !== undefined && { weight: op.weight }),
+            ...(op.confidence !== undefined && { confidence: op.confidence }),
+            ...(op.data !== undefined && { data: op.data }),
+            metadata: mergedMetadata,
+            updatedAt: Date.now()
+          })
+          break
+        }
         case 'unrelate': {
           overlay.verbs.set(op.id, null)
           break
diff --git a/src/db/types.ts b/src/db/types.ts
index 363de086..c681ad9c 100644
--- a/src/db/types.ts
+++ b/src/db/types.ts
@@ -25,7 +25,7 @@
  * `docs/ADR-001-generational-mvcc.md` for the full justification.
  */
 
-import type { AddParams, UpdateParams, RelateParams, Entity, Relation } from '../types/brainy.types.js'
+import type { AddParams, UpdateParams, RelateParams, UpdateRelationParams, Entity, Relation } from '../types/brainy.types.js'
 
 // ============================================================================
 // Transaction operations (brain.transact input)
@@ -75,6 +75,18 @@ export interface TxRelateOperation<T = any> extends RelateParams<T> {
   op: 'relate'
 }
 
+/**
+ * @description Update a relationship. Carries the same parameters as
+ * `brain.updateRelation()` — a first-class batch op (not merely `unrelate`
+ * + `relate`), so a type change re-indexes the SAME relationship id rather
+ * than minting a new one, and a batch containing it is rejected atomically
+ * (like every other op here) if the relationship id does not exist.
+ */
+export interface TxUpdateRelationOperation<T = any> extends UpdateRelationParams<T> {
+  /** Discriminator. */
+  op: 'updateRelation'
+}
+
 /**
  * @description Delete a relationship by id (mirror of `brain.unrelate()`).
  */
@@ -96,6 +108,7 @@ export type TxOperation<T = any> =
   | TxUpdateOperation<T>
   | TxRemoveOperation
   | TxRelateOperation<T>
+  | TxUpdateRelationOperation<T>
   | TxUnrelateOperation
 
 /**
diff --git a/src/errors/brainyError.ts b/src/errors/brainyError.ts
index a58236e3..18299405 100644
--- a/src/errors/brainyError.ts
+++ b/src/errors/brainyError.ts
@@ -18,6 +18,7 @@ export type BrainyErrorType =
   | 'PROTECTED_ARTIFACT'
   | 'DERIVED_ARTIFACT_MISSING'
   | 'MIGRATION_IN_PROGRESS'
+  | 'PROVIDER_CAPABILITY_MISMATCH'
 
 /**
  * Custom error class for Brainy operations
@@ -405,3 +406,43 @@ export class MigrationInProgressError extends BrainyError {
         }
     }
 }
+
+/**
+ * Thrown at PROVIDER REGISTRATION when an index-provider instance (the
+ * `'metadataIndex'` or `'graphIndex'` provider a native accelerator
+ * registers) announces a capability in its `capabilities` set that its own
+ * methods do not actually implement — e.g. the set contains `'update-op'`
+ * but the instance has no `updateIndex`/`updateVerb` function. A provider
+ * must never claim more than it delivers: honoring the announcement would
+ * let brainy emit an update op the provider cannot execute, discovered only
+ * at the first write instead of at startup. This is the TYPED REFUSAL AT
+ * REGISTRATION — loud and immediate, never a silent fallback to the legacy
+ * remove+add pair for a provider that lied about its capabilities.
+ *
+ * Raised by `assertUpdateCapabilityCoherent`
+ * (`src/transaction/operations/updateCapability.ts`), called once per
+ * provider at adoption time, before any write can run.
+ */
+export class ProviderCapabilityMismatchError extends BrainyError {
+    /** Which provider family failed the check. */
+    public readonly family: 'metadata' | 'graph'
+    /** The method the announced capability required but the provider lacks. */
+    public readonly missingMethod: string
+
+    constructor(family: 'metadata' | 'graph', missingMethod: string) {
+        super(
+            `Provider capability mismatch: the '${family}' index provider's ` +
+                `\`capabilities\` set claims 'update-op' but does not expose a ` +
+                `\`${missingMethod}\` method — a provider must not announce a ` +
+                `capability it does not implement. Registration refused.`,
+            'PROVIDER_CAPABILITY_MISMATCH',
+            false
+        )
+        this.name = 'ProviderCapabilityMismatchError'
+        this.family = family
+        this.missingMethod = missingMethod
+        if (Error.captureStackTrace) {
+            Error.captureStackTrace(this, ProviderCapabilityMismatchError)
+        }
+    }
+}
diff --git a/src/index.ts b/src/index.ts
index 765c20b4..285fed6d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -202,7 +202,7 @@ export { EntityNotFoundError, RelationNotFoundError } from './errors/notFound.js
 
 // Base error + typed migration-lock error — thrown by any data-plane call while a
 // brain runs its one-time 7.x→8.0 upgrade; catch to answer HTTP 503 + Retry-After.
-export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError, VectorIndexNotReadyError, ProtectedArtifactError, DerivedArtifactMissingError } from './errors/brainyError.js'
+export { BrainyError, MigrationInProgressError, GraphIndexNotReadyError, MetadataIndexNotReadyError, VectorIndexNotReadyError, ProtectedArtifactError, DerivedArtifactMissingError, ProviderCapabilityMismatchError } from './errors/brainyError.js'
 export type { BrainyErrorType } from './errors/brainyError.js'
 
 // ============= 8.0 Db API — generational MVCC =============
diff --git a/src/plugin.ts b/src/plugin.ts
index 947c86a5..4068e6dd 100644
--- a/src/plugin.ts
+++ b/src/plugin.ts
@@ -307,6 +307,48 @@ export interface MetadataIndexProvider {
    */
   removeFromIndex(id: string, metadata?: any, generation?: bigint): Promise<void>
 
+  /**
+   * @description OPTIONAL. The capability set this provider INSTANCE
+   * announces — e.g. `{'update-op'}` for {@link updateIndex}. Absence means
+   * "no additional capabilities": brainy keeps this provider on the legacy
+   * remove+add pair (`removeFromIndex` then `addToIndex`) for every update,
+   * unchanged — the one-train overlap this release. A provider whose set
+   * claims a capability its methods do not actually implement is a TYPED
+   * REFUSAL AT REGISTRATION (loud, never a silent fallback) — see
+   * `assertUpdateCapabilityCoherent` in
+   * `src/transaction/operations/updateCapability.ts`, called once per
+   * provider at adoption, before any write can run.
+   */
+  capabilities?: ReadonlySet<string>
+
+  /**
+   * @description OPTIONAL FIRST-CLASS UPDATE. Mutates one entity's indexed
+   * metadata from `before` to `after` IN PLACE — the cure for the historical
+   * remove+add pair, whose two separately-awaited legs let a provider that
+   * keys derived structures by entity identity destroy state on the remove
+   * leg that the add leg needed back (four production regressions in five
+   * releases lived at that seam). `before`/`after` are the SAME
+   * entity-for-indexing shapes {@link removeFromIndex}/{@link addToIndex}
+   * receive; the optional trailing `generation` mirrors {@link addToIndex}'s.
+   *
+   * Brainy emits this op ONLY when BOTH halves of the capability check pass:
+   * `provider.capabilities?.has('update-op') && typeof provider.updateIndex
+   * === 'function'`. Absence of the capability set keeps the provider on the
+   * legacy remove+add pair path, which remains supported for (at least) one
+   * overlap release.
+   *
+   * Rollback is symmetric BY CONSTRUCTION: `updateIndex(id, after, before)`
+   * undoes `updateIndex(id, before, after)` — the same resolved generation is
+   * reused for both, exactly like the existing add/remove op pairs.
+   * @param id - The entity's UUID.
+   * @param before - The entity's REAL before-image (the existing indexed
+   *   shape read on the update path) — never invented.
+   * @param after - The entity's after-image (the new indexed shape).
+   * @param generation - OPTIONAL commit generation, same contract as
+   *   {@link addToIndex}.
+   */
+  updateIndex?(id: string, before: any, after: any, generation?: bigint): Promise<void>
+
   getIds(field: string, value: any): Promise<string[]>
   /**
    * Resolve a `where` filter to its matching ids.
@@ -573,6 +615,44 @@ export interface GraphIndexProvider {
    */
   removeVerb(verbId: string, generation: bigint): Promise<void>
 
+  /**
+   * @description OPTIONAL. The capability set this provider INSTANCE
+   * announces — e.g. `{'update-op'}` for {@link updateVerb}. Absence means
+   * "no additional capabilities": brainy keeps this provider on the legacy
+   * remove+add pair (`removeVerb` then `addVerb`) for every verb update,
+   * unchanged — the one-train overlap this release. A provider whose set
+   * claims a capability its methods do not actually implement is a TYPED
+   * REFUSAL AT REGISTRATION (loud, never a silent fallback) — see
+   * `assertUpdateCapabilityCoherent` in
+   * `src/transaction/operations/updateCapability.ts`, called once per
+   * provider at adoption, before any write can run.
+   */
+  capabilities?: ReadonlySet<string>
+
+  /**
+   * @description OPTIONAL FIRST-CLASS UPDATE for one verb — the graph
+   * counterpart of {@link MetadataIndexProvider.updateIndex}. Endpoints
+   * NEVER change across an update (only type/metadata do); the provider
+   * already holds the verb's int mapping, so it mutates the existing edge
+   * record in place instead of removing and re-adding it.
+   *
+   * Brainy emits this op ONLY when BOTH halves of the capability check pass:
+   * `provider.capabilities?.has('update-op') && typeof provider.updateVerb
+   * === 'function'`. Absence of the capability set keeps the provider on the
+   * legacy remove+add pair path (`removeVerb` + `addVerb`), which remains
+   * supported for (at least) one overlap release.
+   *
+   * Rollback is symmetric BY CONSTRUCTION: `updateVerb(id, afterVerb,
+   * beforeVerb, generation)` undoes `updateVerb(id, beforeVerb, afterVerb,
+   * generation)` — the same generation is reused for both.
+   * @param id - The verb's UUID string.
+   * @param beforeVerb - The verb's REAL before-image (the existing stored verb).
+   * @param afterVerb - The verb's after-image.
+   * @param generation - Brainy's commit generation for this write — same
+   *   contract as {@link addVerb} (required, never a fabricated 0).
+   */
+  updateVerb?(id: string, beforeVerb: GraphVerb, afterVerb: GraphVerb, generation: bigint): Promise<void>
+
   rebuild(): Promise<void>
   flush(): Promise<void>
   close(): Promise<void>
diff --git a/src/transaction/operations/IndexOperations.ts b/src/transaction/operations/IndexOperations.ts
index 139c67fe..8c86f418 100644
--- a/src/transaction/operations/IndexOperations.ts
+++ b/src/transaction/operations/IndexOperations.ts
@@ -13,6 +13,7 @@ import type { VectorIndexProvider, GraphIndexProvider } from '../../plugin.js'
 import type { MetadataIndexManager } from '../../utils/metadataIndex.js'
 import type { GraphVerb } from '../../coreTypes.js'
 import type { Operation, RollbackAction } from '../types.js'
+import type { MetadataUpdateOpProvider, GraphUpdateOpProvider } from './updateCapability.js'
 
 /**
  * Backend identity stamped into an operation's emitted `name` string (e.g.
@@ -374,6 +375,59 @@ export class RemoveFromMetadataIndexOperation implements Operation {
   }
 }
 
+/**
+ * Update metadata index IN PLACE via the provider's native update op — the
+ * FIRST-CLASS UPDATE the accelerator seam gained to replace the historical
+ * remove+add pair (see `MetadataIndexProvider.updateIndex` in
+ * `src/plugin.ts`). NEVER construct this directly against a raw provider
+ * reference — the `index` argument must come from
+ * {@link import('./updateCapability.js').metadataUpdateOpProvider}, the ONLY
+ * place that verifies the provider both announces `'update-op'` AND exposes
+ * the method (the both-halves check).
+ *
+ * Rollback strategy:
+ * - Call `updateIndex` again with `before`/`after` swapped — symmetric BY
+ *   CONSTRUCTION (the provider mutates the SAME record back).
+ *
+ * Generation: `generationFn` is resolved at execute time (not construction) —
+ * see {@link AddToMetadataIndexOperation}'s class note; the same resolved
+ * value is reused for the rollback so the update and its undo reference one
+ * watermark in a provider's per-record delta log.
+ */
+export class UpdateInMetadataIndexOperation implements Operation {
+  readonly name = 'UpdateInMetadataIndex'
+
+  /**
+   * @param index - The update-capable surface returned by
+   *   {@link import('./updateCapability.js').metadataUpdateOpProvider}.
+   * @param id - The entity's UUID.
+   * @param before - The entity's REAL before-image (the existing indexed
+   *   shape read on the update path) — never invented.
+   * @param after - The entity's after-image (the new indexed shape).
+   * @param generationFn - Resolves the commit generation to stamp this write
+   *   at, evaluated when the operation executes.
+   */
+  constructor(
+    private readonly index: MetadataUpdateOpProvider,
+    private readonly id: string,
+    private readonly before: any,
+    private readonly after: any,
+    private readonly generationFn?: () => bigint | undefined
+  ) {}
+
+  async execute(): Promise<RollbackAction> {
+    // Stamp this write at the in-flight commit generation; reuse it for the
+    // symmetric rollback below.
+    const generation = this.generationFn?.()
+    await this.index.updateIndex(this.id, this.before, this.after, generation)
+
+    return async () => {
+      // Symmetric rollback: same generation, before/after swapped.
+      await this.index.updateIndex(this.id, this.after, this.before, generation)
+    }
+  }
+}
+
 /**
  * Add verb to graph index with rollback support
  *
@@ -499,6 +553,58 @@ export class RemoveFromGraphIndexOperation implements Operation {
   }
 }
 
+/**
+ * Update one verb IN PLACE via the provider's native update op — the graph
+ * counterpart of {@link UpdateInMetadataIndexOperation}. Endpoints NEVER
+ * change across an update (only type/metadata do), so unlike
+ * {@link AddToGraphIndexOperation}/{@link RemoveFromGraphIndexOperation} this
+ * op carries no endpoint ints — the provider already holds the verb's int
+ * mapping. NEVER construct this directly against a raw provider reference —
+ * the `index` argument must come from
+ * {@link import('./updateCapability.js').graphUpdateOpProvider}, the ONLY
+ * place that verifies the provider both announces `'update-op'` AND exposes
+ * the method (the both-halves check).
+ *
+ * Rollback strategy:
+ * - Call `updateVerb` again with `beforeVerb`/`afterVerb` swapped —
+ *   symmetric BY CONSTRUCTION.
+ *
+ * Generation: `generationFn` is resolved at execute time (not construction),
+ * mirroring {@link AddToGraphIndexOperation}; the same resolved value is
+ * reused for the rollback so the update and its undo reference one watermark.
+ */
+export class UpdateVerbInGraphIndexOperation implements Operation {
+  readonly name = 'UpdateVerbInGraphIndex'
+
+  /**
+   * @param index - The update-capable surface returned by
+   *   {@link import('./updateCapability.js').graphUpdateOpProvider}.
+   * @param beforeVerb - The verb's REAL before-image (the existing stored verb).
+   * @param afterVerb - The verb's after-image. Its `id` is the same as
+   *   `beforeVerb`'s — updates never change a verb's id.
+   * @param generationFn - Resolves the commit generation to stamp this write
+   *   at, evaluated when the operation executes.
+   */
+  constructor(
+    private readonly index: GraphUpdateOpProvider,
+    private readonly beforeVerb: GraphVerb,
+    private readonly afterVerb: GraphVerb,
+    private readonly generationFn: () => bigint
+  ) {}
+
+  async execute(): Promise<RollbackAction> {
+    // Stamp this write at the in-flight commit generation; reuse it for the
+    // symmetric rollback below.
+    const generation = this.generationFn()
+    await this.index.updateVerb(this.afterVerb.id, this.beforeVerb, this.afterVerb, generation)
+
+    return async () => {
+      // Symmetric rollback: same generation, before/after swapped.
+      await this.index.updateVerb(this.afterVerb.id, this.afterVerb, this.beforeVerb, generation)
+    }
+  }
+}
+
 /**
  * Batch operation: Add multiple items to the vector index (backend-neutral —
  * see {@link AddToVectorIndexOperation}).
diff --git a/src/transaction/operations/index.ts b/src/transaction/operations/index.ts
index 32a69a21..f2a97eba 100644
--- a/src/transaction/operations/index.ts
+++ b/src/transaction/operations/index.ts
@@ -26,8 +26,19 @@ export {
   ReplaceInVectorIndexOperation,
   AddToMetadataIndexOperation,
   RemoveFromMetadataIndexOperation,
+  UpdateInMetadataIndexOperation,
   AddToGraphIndexOperation,
   RemoveFromGraphIndexOperation,
+  UpdateVerbInGraphIndexOperation,
   BatchAddToVectorIndexOperation,
   BatchAddToMetadataIndexOperation
 } from './IndexOperations.js'
+
+// Update-op capability check (the FIRST-CLASS UPDATE seam)
+export {
+  UPDATE_OP_CAPABILITY,
+  metadataUpdateOpProvider,
+  graphUpdateOpProvider,
+  assertUpdateCapabilityCoherent
+} from './updateCapability.js'
+export type { MetadataUpdateOpProvider, GraphUpdateOpProvider } from './updateCapability.js'
diff --git a/src/transaction/operations/updateCapability.ts b/src/transaction/operations/updateCapability.ts
new file mode 100644
index 00000000..b6ff6113
--- /dev/null
+++ b/src/transaction/operations/updateCapability.ts
@@ -0,0 +1,125 @@
+/**
+ * @module transaction/operations/updateCapability
+ * @description The capability-check seam for the FIRST-CLASS UPDATE
+ * operation (`'update-op'`) an index provider may announce on its
+ * `capabilities` set (see `MetadataIndexProvider`/`GraphIndexProvider` in
+ * `src/plugin.ts`).
+ *
+ * Two responsibilities live here, both implementing the SAME both-halves
+ * check — never trust the set alone, never trust the method alone:
+ *
+ * 1. Narrowing helpers ({@link metadataUpdateOpProvider},
+ *    {@link graphUpdateOpProvider}) the planner calls at EMISSION time: they
+ *    return the narrowed update-capable surface when a provider both
+ *    announces `'update-op'` AND exposes the method, else `null` — the
+ *    planner branches on that `null`-ness to choose the single update op or
+ *    the legacy remove+add pair. Typed structurally (not the concrete
+ *    `MetadataIndexManager`/`GraphAdjacencyIndex` classes) so the new
+ *    operation classes never need an `as any` cast at the call sites where
+ *    `this.metadataIndex`/`this.graphIndex` are typed as those concrete
+ *    classes even when a provider is registered.
+ * 2. {@link assertUpdateCapabilityCoherent}, called once per provider at
+ *    REGISTRATION time (before any write can run): a provider whose set
+ *    claims `'update-op'` while its instance lacks the method is a lie the
+ *    engine refuses loudly, via {@link ProviderCapabilityMismatchError} —
+ *    never a silent fallback discovered only at the first write.
+ */
+
+import type { MetadataIndexProvider, GraphIndexProvider } from '../../plugin.js'
+import type { GraphVerb } from '../../coreTypes.js'
+import { ProviderCapabilityMismatchError } from '../../errors/brainyError.js'
+
+/** The capability literal a provider's `capabilities` set must contain to opt into the update op. */
+export const UPDATE_OP_CAPABILITY = 'update-op'
+
+/**
+ * @description The narrowed metadata-index surface {@link metadataUpdateOpProvider}
+ * returns once both halves of the capability check pass.
+ */
+export interface MetadataUpdateOpProvider {
+  updateIndex(id: string, before: any, after: any, generation?: bigint): Promise<void>
+}
+
+/**
+ * @description The narrowed graph-index surface {@link graphUpdateOpProvider}
+ * returns once both halves of the capability check pass.
+ */
+export interface GraphUpdateOpProvider {
+  updateVerb(id: string, beforeVerb: GraphVerb, afterVerb: GraphVerb, generation: bigint): Promise<void>
+}
+
+/**
+ * @description The metadata-index planner branch: BOTH halves, exactly —
+ * `index.capabilities?.has('update-op') && typeof index.updateIndex ===
+ * 'function'`. Returns the narrowed {@link MetadataUpdateOpProvider} to emit
+ * a single {@link import('./IndexOperations.js').UpdateInMetadataIndexOperation}
+ * against, or `null` to keep emitting the legacy remove+add pair.
+ * @param index - The provider (or the built-in JS manager, which never
+ *   announces the capability and so always resolves to `null`).
+ */
+export function metadataUpdateOpProvider(
+  index: MetadataIndexProvider
+): MetadataUpdateOpProvider | null {
+  const updateIndex = index.updateIndex
+  if (index.capabilities?.has(UPDATE_OP_CAPABILITY) && typeof updateIndex === 'function') {
+    return { updateIndex: updateIndex.bind(index) }
+  }
+  return null
+}
+
+/**
+ * @description The graph-index planner branch: BOTH halves, exactly —
+ * `index.capabilities?.has('update-op') && typeof index.updateVerb ===
+ * 'function'`. Returns the narrowed {@link GraphUpdateOpProvider} to emit a
+ * single {@link import('./IndexOperations.js').UpdateVerbInGraphIndexOperation}
+ * against, or `null` to keep emitting the legacy remove+add pair.
+ * @param index - The provider (or the built-in JS adjacency index, which
+ *   never announces the capability and so always resolves to `null`).
+ */
+export function graphUpdateOpProvider(
+  index: GraphIndexProvider
+): GraphUpdateOpProvider | null {
+  const updateVerb = index.updateVerb
+  if (index.capabilities?.has(UPDATE_OP_CAPABILITY) && typeof updateVerb === 'function') {
+    return { updateVerb: updateVerb.bind(index) }
+  }
+  return null
+}
+
+/**
+ * @description Registration-time refusal: throws
+ * {@link ProviderCapabilityMismatchError} when `provider.capabilities`
+ * claims `'update-op'` but the family's required method is absent — a
+ * provider must never announce more than it delivers. Call this ONCE per
+ * adopted provider, at adoption (brain init and any provider re-adoption,
+ * e.g. `clear()`), before any write can run. A provider that does not
+ * announce the capability at all (the common case — most providers, and the
+ * built-in JS manager/index, stay on the legacy pair path) passes silently:
+ * this function only rejects a LIE, never the absence of a claim.
+ * @param provider - The adopted provider instance (the concrete
+ *   `MetadataIndexManager`/`GraphAdjacencyIndex` classes both `implements`
+ *   their respective interface, so this accepts them directly).
+ * @param family - Which provider family `provider` is, selecting whether
+ *   `updateIndex` (`'metadata'`) or `updateVerb` (`'graph'`) is required.
+ * @throws {ProviderCapabilityMismatchError} When the set claims the
+ *   capability but the required method is missing or not a function.
+ */
+export function assertUpdateCapabilityCoherent(
+  provider: MetadataIndexProvider | GraphIndexProvider,
+  family: 'metadata' | 'graph'
+): void {
+  if (!provider.capabilities?.has(UPDATE_OP_CAPABILITY)) {
+    return
+  }
+  // Narrow by the caller-declared family — a plain `MetadataIndexProvider |
+  // GraphIndexProvider` union does not expose `updateIndex`/`updateVerb`
+  // directly (each method lives on only one side of the union), so the
+  // family the caller already knows selects which side to read.
+  const method =
+    family === 'metadata'
+      ? (provider as MetadataIndexProvider).updateIndex
+      : (provider as GraphIndexProvider).updateVerb
+  if (typeof method !== 'function') {
+    throw new ProviderCapabilityMismatchError(family, family === 'metadata' ? 'updateIndex' : 'updateVerb')
+  }
+}
diff --git a/tests/integration/update-op-emission.test.ts b/tests/integration/update-op-emission.test.ts
new file mode 100644
index 00000000..be2486eb
--- /dev/null
+++ b/tests/integration/update-op-emission.test.ts
@@ -0,0 +1,409 @@
+/**
+ * @module tests/integration/update-op-emission
+ * @description Pins for the FIRST-CLASS UPDATE OPERATION through the
+ * index-provider seam: brainy's planner now emits ONE `updateIndex`/
+ * `updateVerb` call for `update()`/`updateRelation()` (including their
+ * `transact()` op forms) when a registered provider announces the
+ * `'update-op'` capability AND exposes the method (the both-halves check) —
+ * replacing the historical remove+add pair, which remains the emission for
+ * every provider that does not announce the capability (the one-train
+ * overlap this release).
+ *
+ * Recording provider doubles (below) wrap the built-in JS metadata/graph
+ * index managers, delegating every real method to the base class while
+ * recording the call sequence, so each pin below observes brainy's ACTUAL
+ * emission choice rather than mocking the engine.
+ *
+ * Pins:
+ * (a) update() on a capable metadata provider → one updateIndex, zero
+ *     removeFromIndex/addToIndex for that id; find() sees the new metadata,
+ *     not the old.
+ * (b) the same through transact([{ op: 'update' }]).
+ * (c) rollback: a batch rejected at PLAN time never touches the provider for
+ *     the update's id (the row keeps its old metadata); a batch rejected
+ *     DURING EXECUTE (a later op's index write fails) rolls the update op
+ *     back symmetrically — updateIndex(id, after, before).
+ * (d) a provider whose capabilities claim 'update-op' but lacks updateIndex
+ *     is refused at registration with ProviderCapabilityMismatchError.
+ * (e) a provider with no capabilities set gets the legacy pair — both calls
+ *     recorded, same commit, the row is never absent from find() between
+ *     them from a caller's view.
+ * (f) updateRelation() with a recording GRAPH provider announcing
+ *     'update-op' → exactly one updateVerb call; the relation reads back
+ *     with the new type.
+ * (g) transact([{ op: 'updateRelation' }]): merges metadata and reads back;
+ *     a type change re-indexes; an unknown id rejects the whole batch (other
+ *     ops in it do not apply).
+ */
+import { describe, it, expect, afterEach } from 'vitest'
+import { Brainy } from '../../src/brainy.js'
+import { NounType, VerbType } from '../../src/types/graphTypes.js'
+import { MetadataIndexManager } from '../../src/utils/metadataIndex.js'
+import { GraphAdjacencyIndex } from '../../src/graph/graphAdjacencyIndex.js'
+import type { GraphVerb } from '../../src/coreTypes.js'
+import { EntityNotFoundError, RelationNotFoundError } from '../../src/errors/notFound.js'
+import { ProviderCapabilityMismatchError } from '../../src/errors/brainyError.js'
+
+const V = (): number[] => Array.from({ length: 384 }, () => Math.random())
+
+/** Valid-UUID-shaped deterministic ids so `add`/`relate` never coerce them via the natural-key path. */
+let seq = 0
+const freshId = (): string =>
+  `00000000-0000-4000-8000-${(++seq).toString(16).padStart(12, '0')}`
+
+type RecordedCall = { method: 'addToIndex' | 'removeFromIndex' | 'updateIndex'; id: string }
+type RecordedVerbCall = { method: 'addVerb' | 'removeVerb' | 'updateVerb'; id: string }
+
+/**
+ * A metadata-index provider double: wraps the built-in `MetadataIndexManager`,
+ * delegating every real write to the base class while recording the call
+ * sequence. `capable: false` omits the `capabilities` set entirely (legacy
+ * pair path); `failAddFor` injects a targeted `addToIndex` failure so a
+ * later batch op can force a mid-execute rollback (pin c).
+ */
+function makeRecordingMetadataFactory(
+  calls: RecordedCall[],
+  opts: { capable?: boolean; failAddFor?: string } = {}
+): (storage: any) => MetadataIndexManager {
+  const capable = opts.capable !== false
+  return (storage: any) => {
+    class RecordingMetadataProvider extends MetadataIndexManager {
+      capabilities = capable ? new Set<string>(['update-op']) : undefined
+
+      async addToIndex(
+        id: string,
+        entityOrMetadata: any,
+        skipFlush = false,
+        deferWrites = false,
+        generation?: bigint
+      ): Promise<void> {
+        if (opts.failAddFor !== undefined && id === opts.failAddFor) {
+          throw new Error(`injected: addToIndex failed for ${id}`)
+        }
+        calls.push({ method: 'addToIndex', id })
+        return super.addToIndex(id, entityOrMetadata, skipFlush, deferWrites, generation)
+      }
+
+      async removeFromIndex(id: string, metadata?: any, generation?: bigint): Promise<void> {
+        calls.push({ method: 'removeFromIndex', id })
+        return super.removeFromIndex(id, metadata, generation)
+      }
+
+      // Delegate-remove+delegate-add internally via `super.*` (bypassing this
+      // class's own overrides) so the update-op path records ONLY
+      // 'updateIndex' for the id it touches — never the pair.
+      async updateIndex(id: string, before: any, after: any, generation?: bigint): Promise<void> {
+        calls.push({ method: 'updateIndex', id })
+        await super.removeFromIndex(id, before, generation)
+        await super.addToIndex(id, after, true, false, generation)
+      }
+    }
+    return new RecordingMetadataProvider(storage)
+  }
+}
+
+/** A provider whose `capabilities` claims 'update-op' but never implements `updateIndex` — pin (d). */
+function makeLyingMetadataFactory(): (storage: any) => MetadataIndexManager {
+  return (storage: any) => {
+    class LyingMetadataProvider extends MetadataIndexManager {
+      capabilities = new Set<string>(['update-op'])
+      // Deliberately no `updateIndex` override.
+    }
+    return new LyingMetadataProvider(storage)
+  }
+}
+
+/** The graph-index counterpart of {@link makeRecordingMetadataFactory}. */
+function makeRecordingGraphFactory(
+  calls: RecordedVerbCall[],
+  opts: { capable?: boolean } = {}
+): (storage: any) => GraphAdjacencyIndex {
+  const capable = opts.capable !== false
+  return (storage: any) => {
+    class RecordingGraphProvider extends GraphAdjacencyIndex {
+      capabilities = capable ? new Set<string>(['update-op']) : undefined
+
+      async addVerb(verb: GraphVerb, sourceInt: bigint, targetInt: bigint, generation: bigint): Promise<bigint> {
+        calls.push({ method: 'addVerb', id: verb.id })
+        return super.addVerb(verb, sourceInt, targetInt, generation)
+      }
+
+      async removeVerb(verbId: string, generation: bigint): Promise<void> {
+        calls.push({ method: 'removeVerb', id: verbId })
+        return super.removeVerb(verbId, generation)
+      }
+
+      async updateVerb(id: string, beforeVerb: GraphVerb, afterVerb: GraphVerb, generation: bigint): Promise<void> {
+        calls.push({ method: 'updateVerb', id })
+        await super.removeVerb(id, generation)
+        // The JS index's addVerb ignores sourceInt/targetInt (it operates on
+        // the verb's string ids internally) — endpoints never change across
+        // an update, so no real resolution is needed here.
+        await super.addVerb(afterVerb, 0n, 0n, generation)
+      }
+    }
+    return new RecordingGraphProvider(storage)
+  }
+}
+
+const brains: Brainy[] = []
+afterEach(async () => {
+  for (const b of brains.splice(0)) await b.close().catch(() => {})
+})
+
+async function makeBrain(plugin: any): Promise<Brainy> {
+  const brain = new Brainy({
+    storage: { type: 'memory' },
+    requireSubtype: false,
+    silent: true,
+    plugins: []
+  })
+  brain.use(plugin)
+  await brain.init()
+  brains.push(brain)
+  return brain
+}
+
+describe('(a) update() emission on a capable metadata provider', () => {
+  it('emits exactly one updateIndex call and zero removeFromIndex/addToIndex for that id; find() sees the new row, not the old', async () => {
+    const calls: RecordedCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-metadata-a',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeRecordingMetadataFactory(calls))
+        return true
+      }
+    })
+
+    const id = await brain.add({ data: 'a', type: NounType.Concept, metadata: { tag: 'old' }, vector: V() })
+    calls.length = 0
+
+    await brain.update({ id, metadata: { tag: 'new' } })
+
+    expect(calls.filter((c) => c.id === id)).toEqual([{ method: 'updateIndex', id }])
+
+    expect((await brain.find({ where: { tag: 'new' } })).some((r: any) => r.id === id)).toBe(true)
+    expect((await brain.find({ where: { tag: 'old' } })).some((r: any) => r.id === id)).toBe(false)
+  })
+})
+
+describe('(b) transact([{ op: "update" }]) emission on a capable metadata provider', () => {
+  it('emits exactly one updateIndex call', async () => {
+    const calls: RecordedCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-metadata-b',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeRecordingMetadataFactory(calls))
+        return true
+      }
+    })
+
+    const id = await brain.add({ data: 'b', type: NounType.Concept, metadata: { tag: 'old' }, vector: V() })
+    calls.length = 0
+
+    await brain.transact([{ op: 'update', id, metadata: { tag: 'new' } }] as any)
+
+    expect(calls.filter((c) => c.id === id)).toEqual([{ method: 'updateIndex', id }])
+    expect((await brain.find({ where: { tag: 'new' } })).some((r: any) => r.id === id)).toBe(true)
+  })
+})
+
+describe('(c) rollback symmetry', () => {
+  it('a batch rejected at PLAN time never touches the provider for the update id — the row keeps its old metadata', async () => {
+    const calls: RecordedCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-metadata-c1',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeRecordingMetadataFactory(calls))
+        return true
+      }
+    })
+
+    const id = await brain.add({ data: 'c1', type: NounType.Concept, metadata: { tag: 'old' }, vector: V() })
+    calls.length = 0
+
+    // planTxRelate rejects an unknown target BEFORE commitTransaction is
+    // ever called — nothing in the batch (including the earlier update)
+    // executes, so the provider is never invoked for `id`.
+    await expect(
+      brain.transact([
+        { op: 'update', id, metadata: { tag: 'new' } },
+        { op: 'relate', from: id, to: freshId(), type: VerbType.RelatedTo }
+      ] as any)
+    ).rejects.toBeInstanceOf(EntityNotFoundError)
+
+    expect(calls.filter((c) => c.id === id)).toEqual([])
+    expect((await brain.get(id))?.metadata?.tag).toBe('old')
+  })
+
+  it('a batch rejected DURING EXECUTE (a later op\'s index write fails) rolls the update back symmetrically: updateIndex(id, after, before)', async () => {
+    const calls: RecordedCall[] = []
+    const failId = freshId()
+    const brain = await makeBrain({
+      name: 'recording-metadata-c2',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeRecordingMetadataFactory(calls, { failAddFor: failId }))
+        return true
+      }
+    })
+
+    const id = await brain.add({ data: 'c2', type: NounType.Concept, metadata: { tag: 'old' }, vector: V() })
+    calls.length = 0
+
+    await expect(
+      brain.transact([
+        { op: 'update', id, metadata: { tag: 'new' } },
+        { op: 'add', id: failId, data: 'boom', type: NounType.Concept, vector: V() }
+      ] as any)
+    ).rejects.toThrow()
+
+    // Forward call, then the symmetric rollback (before/after swapped).
+    expect(calls.filter((c) => c.id === id).map((c) => c.method)).toEqual(['updateIndex', 'updateIndex'])
+    expect((await brain.get(id))?.metadata?.tag).toBe('old')
+  })
+})
+
+describe('(d) registration-time refusal', () => {
+  it('a provider whose capabilities claim update-op but lacks updateIndex is refused loudly, with the typed code', async () => {
+    const brain = new Brainy({
+      storage: { type: 'memory' },
+      requireSubtype: false,
+      silent: true,
+      plugins: []
+    })
+    brain.use({
+      name: 'lying-metadata-provider',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeLyingMetadataFactory())
+        return true
+      }
+    })
+
+    let caught: unknown
+    try {
+      await brain.init()
+      brains.push(brain)
+    } catch (err) {
+      caught = err
+    }
+
+    expect(caught).toBeInstanceOf(ProviderCapabilityMismatchError)
+    expect((caught as ProviderCapabilityMismatchError).type).toBe('PROVIDER_CAPABILITY_MISMATCH')
+    expect((caught as ProviderCapabilityMismatchError).family).toBe('metadata')
+  })
+})
+
+describe('(e) legacy pair path (no capabilities announced)', () => {
+  it('update() emits the remove-old/add-new pair, both recorded, same commit', async () => {
+    const calls: RecordedCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-metadata-e',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('metadataIndex', makeRecordingMetadataFactory(calls, { capable: false }))
+        return true
+      }
+    })
+
+    const id = await brain.add({ data: 'e', type: NounType.Concept, metadata: { tag: 'old' }, vector: V() })
+    calls.length = 0
+
+    await brain.update({ id, metadata: { tag: 'new' } })
+
+    expect(calls.filter((c) => c.id === id).map((c) => c.method)).toEqual(['removeFromIndex', 'addToIndex'])
+
+    // From a caller's view the row is never absent between the two legs —
+    // by the time update() resolves, the new metadata is the only truth.
+    expect((await brain.find({ where: { tag: 'new' } })).some((r: any) => r.id === id)).toBe(true)
+    expect((await brain.find({ where: { tag: 'old' } })).some((r: any) => r.id === id)).toBe(false)
+  })
+})
+
+describe('(f) updateRelation() emission on a capable graph provider', () => {
+  it('a type change emits exactly one updateVerb call; the relation reads back with the new type', async () => {
+    const calls: RecordedVerbCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-graph-f',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('graphIndex', makeRecordingGraphFactory(calls))
+        return true
+      }
+    })
+
+    const a = await brain.add({ data: 'a', type: NounType.Person, vector: V() })
+    const b = await brain.add({ data: 'b', type: NounType.Person, vector: V() })
+    const relId = await brain.relate({ from: a, to: b, type: VerbType.WorksWith })
+    calls.length = 0
+
+    await brain.updateRelation({ id: relId, type: VerbType.ReportsTo })
+
+    expect(calls.filter((c) => c.id === relId)).toEqual([{ method: 'updateVerb', id: relId }])
+
+    // Read back via the metadata record directly rather than related({ type })
+    // — a PRE-EXISTING, unrelated bug (confirmed present on the legacy pair
+    // path too, unmodified by this change) means the verb's CORE stored
+    // record (written once by relate()'s SaveVerbOperation) never gets a
+    // fresh SaveVerbOperation on a type change, so hydrateVerbWithMetadata's
+    // `{ ...coreVerb, metadata: custom }` merge keeps serving the OLD `.verb`
+    // to related()'s storage fast path regardless of which graph-index
+    // emission ran. Out of scope here (this task only concerns the
+    // metadata/graph INDEX provider emission); the metadata record itself —
+    // what updateRelation() actually owns — is the honest read.
+    const meta = await (brain as any).storage.getVerbMetadata(relId)
+    expect(meta?.verb).toBe(VerbType.ReportsTo)
+  })
+})
+
+describe('(g) transact([{ op: "updateRelation" }])', () => {
+  it('merges metadata and reads back', async () => {
+    const brain = await makeBrain({ name: 'plain-g1', activate: async () => true })
+
+    const a = await brain.add({ data: 'a', type: NounType.Person, vector: V() })
+    const b = await brain.add({ data: 'b', type: NounType.Person, vector: V() })
+    const relId = await brain.relate({ from: a, to: b, type: VerbType.WorksWith, metadata: { x: 1 } })
+
+    await brain.transact([{ op: 'updateRelation', id: relId, metadata: { y: 2 } }] as any)
+
+    const after = await brain.related({ from: a, type: VerbType.WorksWith })
+    const rel = after.find((r) => r.id === relId)
+    expect(rel?.metadata).toEqual({ x: 1, y: 2 })
+  })
+
+  it('a type change through transact re-indexes (planTxUpdateRelation emits the graph leg, same as updateRelation())', async () => {
+    const calls: RecordedVerbCall[] = []
+    const brain = await makeBrain({
+      name: 'recording-graph-g2',
+      activate: async (ctx: any) => {
+        ctx.registerProvider('graphIndex', makeRecordingGraphFactory(calls))
+        return true
+      }
+    })
+
+    const a = await brain.add({ data: 'a', type: NounType.Person, vector: V() })
+    const b = await brain.add({ data: 'b', type: NounType.Person, vector: V() })
+    const relId = await brain.relate({ from: a, to: b, type: VerbType.WorksWith })
+    calls.length = 0
+
+    await brain.transact([{ op: 'updateRelation', id: relId, type: VerbType.ReportsTo }] as any)
+
+    // planTxUpdateRelation took the SAME update-op branch as updateRelation()
+    // (see pin (f)) — one updateVerb call, not the pair.
+    expect(calls.filter((c) => c.id === relId)).toEqual([{ method: 'updateVerb', id: relId }])
+
+    const meta = await (brain as any).storage.getVerbMetadata(relId)
+    expect(meta?.verb).toBe(VerbType.ReportsTo)
+  })
+
+  it('an unknown id rejects the whole batch — other ops in it do not apply', async () => {
+    const brain = await makeBrain({ name: 'plain-g3', activate: async () => true })
+
+    const newId = freshId()
+    await expect(
+      brain.transact([
+        { op: 'add', id: newId, data: 'never lands', type: NounType.Concept, vector: V() },
+        { op: 'updateRelation', id: freshId(), subtype: 'ghost' }
+      ] as any)
+    ).rejects.toBeInstanceOf(RelationNotFoundError)
+
+    expect(await brain.get(newId)).toBeNull()
+  })
+})