diff --git a/packages/zero-client/src/client/crud.ts b/packages/zero-client/src/client/crud.ts
index 224e64a29d..f9fed81082 100644
--- a/packages/zero-client/src/client/crud.ts
+++ b/packages/zero-client/src/client/crud.ts
@@ -13,10 +13,12 @@ import type {TableSchema} from '../../../zero-schema/src/table-schema.ts';
 import type {Schema} from '../../../zero-types/src/schema.ts';
 import type {
   CRUDExecutor,
+  CRUDExecutorOptions,
   DeleteID,
   InsertValue,
   TableMutator,
   UpdateValue,
+  UpsertOptions,
   UpsertValue,
 } from '../../../zql/src/mutate/crud.ts';
 import * as crudImpl from './crud-impl.ts';
@@ -107,12 +109,13 @@ function makeEntityCRUDMutate<S extends TableSchema>(
       };
       return zeroCRUD({ops: [op]});
     },
-    upsert: (value: UpsertValue<S>) => {
+    upsert: (value: UpsertValue<S>, options?: UpsertOptions<S>) => {
       const op: UpsertOp = {
         op: 'upsert',
         tableName,
         primaryKey,
         value,
+        conflictColumns: options?.onConflict,
       };
       return zeroCRUD({ops: [op]});
     },
@@ -158,12 +161,13 @@ export function makeBatchCRUDMutate<S extends TableSchema>(
       ops.push(op);
       return promiseVoid;
     },
-    upsert: (value: UpsertValue<S>) => {
+    upsert: (value: UpsertValue<S>, options?: UpsertOptions<S>) => {
       const op: UpsertOp = {
         op: 'upsert',
         tableName,
         primaryKey,
         value,
+        conflictColumns: options?.onConflict,
       };
       ops.push(op);
       return promiseVoid;
@@ -207,12 +211,12 @@ export function makeCRUDExecutor(
   schema: Schema,
   ivmBranch: IVMSourceBranch | undefined,
 ): CRUDExecutor {
-  return (tableName, kind, value) => {
+  return (tableName, kind, value, options?: CRUDExecutorOptions) => {
     const {primaryKey} = schema.tables[tableName];
     return crudImpl[kind](
       tx,
       // oxlint-disable-next-line @typescript-eslint/no-explicit-any
-      {op: kind, tableName, primaryKey, value} as any,
+      {op: kind, tableName, primaryKey, value, conflictColumns: options?.onConflict} as any,
       schema,
       ivmBranch,
     );
@@ -230,7 +234,11 @@ export function makeCRUDMutator(schema: Schema): CRUDMutator {
   ): Promise<void> => {
     const executor = makeCRUDExecutor(tx, schema, undefined);
     for (const op of crudArg.ops) {
-      await executor(op.tableName, op.op, op.value);
+      const options: CRUDExecutorOptions | undefined =
+        op.op === 'upsert' && op.conflictColumns
+          ? {onConflict: op.conflictColumns}
+          : undefined;
+      await executor(op.tableName, op.op, op.value, options);
     }
   };
 }
diff --git a/packages/zero-client/src/client/mutate-request.test.ts b/packages/zero-client/src/client/mutate-request.test.ts
index 6e23e89ced..3b88d1aa4d 100644
--- a/packages/zero-client/src/client/mutate-request.test.ts
+++ b/packages/zero-client/src/client/mutate-request.test.ts
@@ -6,6 +6,7 @@ import type {
   InsertValue,
   SchemaCRUD,
   UpdateValue,
+  UpsertOptions,
   UpsertValue,
 } from '../../../zql/src/mutate/crud.ts';
 import type {Transaction} from '../../../zql/src/mutate/custom.ts';
@@ -127,7 +128,7 @@ describe('zero.mutate(mr) with MutateRequest', () => {
       (value: {id: string; readonly name?: string | undefined}) => Promise<void>
     >();
     expectTypeOf(z.mutate.user.upsert).toEqualTypeOf<
-      (value: {id: string; readonly name: string}) => Promise<void>
+      (value: {id: string; readonly name: string}, options?: UpsertOptions<typeof schema.tables.user>) => Promise<void>
     >();
     expectTypeOf(z.mutate.user.delete).toEqualTypeOf<
       (id: {id: string}) => Promise<void>
@@ -420,7 +421,7 @@ describe('CRUD patterns on client', () => {
         (value: UpdateValue<UserTableSchema>) => Promise<void>
       >();
       expectTypeOf<UserMutators['upsert']>().toEqualTypeOf<
-        (value: UpsertValue<UserTableSchema>) => Promise<void>
+        (value: UpsertValue<UserTableSchema>, options?: UpsertOptions<UserTableSchema>) => Promise<void>
       >();
       expectTypeOf<UserMutators['delete']>().toEqualTypeOf<
         (id: DeleteID<UserTableSchema>) => Promise<void>
diff --git a/packages/zero-protocol/src/push.ts b/packages/zero-protocol/src/push.ts
index 1a8419f37e..d21a0c0182 100644
--- a/packages/zero-protocol/src/push.ts
+++ b/packages/zero-protocol/src/push.ts
@@ -52,12 +52,17 @@ const insertOpSchema = v.object({
 /**
  * Upsert semantics. Inserts if entity with id does not already exist,
  * otherwise updates existing entity with id.
+ *
+ * By default, conflict detection uses the primary key. If `conflictColumns`
+ * is provided, those columns are used instead (for unique constraints).
  */
 const upsertOpSchema = v.object({
   op: v.literal('upsert'),
   tableName: v.string(),
   primaryKey: primaryKeySchema,
   value: rowSchema,
+  // Optional: columns to use for ON CONFLICT instead of primary key
+  conflictColumns: v.array(v.string()).optional(),
 });
 
 /**
diff --git a/packages/zero-server/src/custom.test.ts b/packages/zero-server/src/custom.test.ts
index b84811d340..2bef4f03b8 100644
--- a/packages/zero-server/src/custom.test.ts
+++ b/packages/zero-server/src/custom.test.ts
@@ -12,6 +12,7 @@ import type {
   DeleteID,
   InsertValue,
   UpdateValue,
+  UpsertOptions,
   UpsertValue,
 } from '../../zql/src/mutate/crud.ts';
 import type {DBTransaction} from '../../zql/src/mutate/custom.ts';
@@ -163,7 +164,7 @@ describe('server CRUD patterns', () => {
         (value: UpdateValue<BasicTable>) => Promise<void>
       >();
       expectTypeOf<BasicCRUD['upsert']>().toEqualTypeOf<
-        (value: UpsertValue<BasicTable>) => Promise<void>
+        (value: UpsertValue<BasicTable>, options?: UpsertOptions<BasicTable>) => Promise<void>
       >();
       expectTypeOf<BasicCRUD['delete']>().toEqualTypeOf<
         (id: DeleteID<BasicTable>) => Promise<void>
diff --git a/packages/zero-server/src/custom.ts b/packages/zero-server/src/custom.ts
index 91e2e18600..31d71b00c1 100644
--- a/packages/zero-server/src/custom.ts
+++ b/packages/zero-server/src/custom.ts
@@ -15,12 +15,14 @@ import type {
 } from '../../zero-types/src/server-schema.ts';
 import {
   type CRUDExecutor,
+  type CRUDExecutorOptions,
   type CRUDKind,
   makeCRUDMutate,
   makeTransactionMutate,
   type SchemaCRUD,
   type TableCRUD,
   type TransactionMutate,
+  type UpsertOptions,
 } from '../../zql/src/mutate/crud.ts';
 import type {
   DBTransaction,
@@ -222,8 +224,11 @@ export class CRUDMutatorFactory<S extends Schema> {
       mapValues(tableCRUD, method => method.bind(txHolder)),
     ) as unknown as SchemaCRUD<S>;
 
-    return (table: string, kind: CRUDKind, args: unknown) => {
+    return (table: string, kind: CRUDKind, args: unknown, options?: CRUDExecutorOptions) => {
       const tableCRUD = boundCRUDs[table as keyof S['tables']];
+      if (kind === 'upsert' && options?.onConflict) {
+        return tableCRUD.upsert(args as never, {onConflict: options.onConflict});
+      }
       // oxlint-disable-next-line @typescript-eslint/no-explicit-any
       return (tableCRUD as any)[kind](args);
     };
@@ -334,8 +339,11 @@ function makeServerCRUDExecutor<S extends Schema>(
   ) as unknown as SchemaCRUD<S>;
 
   // Return executor that dispatches to bound methods
-  return (table: string, kind: CRUDKind, args: unknown) => {
+  return (table: string, kind: CRUDKind, args: unknown, options?: CRUDExecutorOptions) => {
     const tableCRUD = boundCRUDs[table as keyof S['tables']];
+    if (kind === 'upsert' && options?.onConflict) {
+      return tableCRUD.upsert(args as never, {onConflict: options.onConflict});
+    }
     // oxlint-disable-next-line @typescript-eslint/no-explicit-any
     return (tableCRUD as any)[kind](args);
   };
@@ -366,12 +374,13 @@ function makeServerTableCRUD(schema: TableSchema): TableCRUD<TableSchema> {
       const tx = this[dbTxSymbol];
       await tx.query(stmt.text, stmt.values);
     },
-    async upsert(this: WithHiddenTxAndSchema, value) {
+    async upsert(this: WithHiddenTxAndSchema, value, options?: UpsertOptions<TableSchema>) {
       value = removeUndefined(value);
       const serverTableSchema = this[serverSchemaSymbol][serverName(schema)];
       const targetedColumns = origAndServerNamesFor(Object.keys(value), schema);
-      const primaryKeyColumns = origAndServerNamesFor(
-        schema.primaryKey,
+      // use custom conflict columns if provided, otherwise fall back to primary key
+      const conflictColumns = origAndServerNamesFor(
+        options?.onConflict ?? schema.primaryKey,
         schema,
       );
       const stmt = formatPgInternalConvert(
@@ -384,7 +393,7 @@ function makeServerTableCRUD(schema: TableSchema): TableCRUD<TableSchema> {
           ),
           ', ',
         )}) ON CONFLICT (${sql.join(
-          primaryKeyColumns.map(([, serverName]) => sql.ident(serverName)),
+          conflictColumns.map(([, serverName]) => sql.ident(serverName)),
           ', ',
         )}) DO UPDATE SET ${sql.join(
           Object.entries(value).map(
diff --git a/packages/zql/src/mutate/crud.ts b/packages/zql/src/mutate/crud.ts
index 4ff4ada8d9..630154e3c1 100644
--- a/packages/zql/src/mutate/crud.ts
+++ b/packages/zql/src/mutate/crud.ts
@@ -10,6 +10,17 @@ export type SchemaCRUD<S extends Schema> = {
 
 export type TransactionMutate<S extends Schema> = SchemaCRUD<S>;
 
+/**
+ * Options for upsert operations.
+ */
+export type UpsertOptions<S extends TableSchema> = {
+  /**
+   * Columns to use for conflict detection instead of the primary key.
+   * Use this when upserting based on a unique constraint other than the primary key.
+   */
+  onConflict?: (keyof S['columns'] & string)[];
+};
+
 export type TableCRUD<S extends TableSchema> = {
   /**
    * Writes a row if a row with the same primary key doesn't already exist.
@@ -21,11 +32,15 @@ export type TableCRUD<S extends TableSchema> = {
 
   /**
    * Writes a row unconditionally, overwriting any existing row with the same
-   * primary key. Non-primary-key fields that are 'optional' can be omitted or
-   * set to `undefined`. Such fields will be assigned the value `null`
-   * optimistically and then the default value as defined by the server.
+   * primary key (or custom conflict columns if specified). Non-primary-key
+   * fields that are 'optional' can be omitted or set to `undefined`. Such
+   * fields will be assigned the value `null` optimistically and then the
+   * default value as defined by the server.
+   *
+   * @param value - The row data to upsert
+   * @param options - Optional settings including custom conflict columns
    */
-  upsert: (value: UpsertValue<S>) => Promise<void>;
+  upsert: (value: UpsertValue<S>, options?: UpsertOptions<S>) => Promise<void>;
 
   /**
    * Updates a row with the same primary key. If no such row exists, this
@@ -90,11 +105,15 @@ export type TableMutator<TS extends TableSchema> = {
 
   /**
    * Writes a row unconditionally, overwriting any existing row with the same
-   * primary key. Non-primary-key fields that are 'optional' can be omitted or
-   * set to `undefined`. Such fields will be assigned the value `null`
-   * optimistically and then the default value as defined by the server.
+   * primary key (or custom conflict columns if specified). Non-primary-key
+   * fields that are 'optional' can be omitted or set to `undefined`. Such
+   * fields will be assigned the value `null` optimistically and then the
+   * default value as defined by the server.
+   *
+   * @param value - The row data to upsert
+   * @param options - Optional settings including custom conflict columns
    */
-  upsert: (value: UpsertValue<TS>) => Promise<void>;
+  upsert: (value: UpsertValue<TS>, options?: UpsertOptions<TS>) => Promise<void>;
 
   /**
    * Updates a row with the same primary key. If no such row exists, this
@@ -110,6 +129,14 @@ export type TableMutator<TS extends TableSchema> = {
   delete: (id: DeleteID<TS>) => Promise<void>;
 };
 
+/**
+ * Options passed to the executor for CRUD operations.
+ */
+export type CRUDExecutorOptions = {
+  /** For upsert: columns to use for ON CONFLICT instead of primary key */
+  onConflict?: string[];
+};
+
 /**
  * A function that executes a CRUD operation.
  * Client and server provide different implementations.
@@ -118,6 +145,7 @@ export type CRUDExecutor = (
   table: string,
   kind: CRUDKind,
   args: unknown,
+  options?: CRUDExecutorOptions,
 ) => Promise<void>;
 
 /**
@@ -180,12 +208,13 @@ function makeTableCRUD(
   tableName: string,
   executor: CRUDExecutor,
 ): TableCRUD<TableSchema> {
-  return Object.fromEntries(
-    CRUD_KINDS.map(kind => [
-      kind,
-      (value: unknown) => executor(tableName, kind, value),
-    ]),
-  ) as TableCRUD<TableSchema>;
+  return {
+    insert: (value: unknown) => executor(tableName, 'insert', value),
+    upsert: (value: unknown, options?: UpsertOptions<TableSchema>) =>
+      executor(tableName, 'upsert', value, options?.onConflict ? {onConflict: options.onConflict} : undefined),
+    update: (value: unknown) => executor(tableName, 'update', value),
+    delete: (value: unknown) => executor(tableName, 'delete', value),
+  };
 }
 
 export type CRUDMutator<