diff --git a/src/common/errors.ts b/src/common/errors.ts
index 084d45ca7..1ef987de4 100644
--- a/src/common/errors.ts
+++ b/src/common/errors.ts
@@ -2,6 +2,7 @@ export enum ErrorCodes {
     NotConnectedToMongoDB = 1_000_000,
     MisconfiguredConnectionString = 1_000_001,
     ForbiddenCollscan = 1_000_002,
+    ForbiddenWriteOperation = 1_000_003,
 }
 
 export class MongoDBError<ErrorCode extends ErrorCodes = ErrorCodes> extends Error {
diff --git a/src/tools/mongodb/read/aggregate.ts b/src/tools/mongodb/read/aggregate.ts
index 8492a61ce..c61603459 100644
--- a/src/tools/mongodb/read/aggregate.ts
+++ b/src/tools/mongodb/read/aggregate.ts
@@ -5,6 +5,7 @@ import type { ToolArgs, OperationType } from "../../tool.js";
 import { formatUntrustedData } from "../../tool.js";
 import { checkIndexUsage } from "../../../helpers/indexCheck.js";
 import { EJSON } from "bson";
+import { ErrorCodes, MongoDBError } from "../../../common/errors.js";
 
 export const AggregateArgs = {
     pipeline: z.array(z.object({}).passthrough()).describe("An array of aggregation stages to execute"),
@@ -26,6 +27,8 @@ export class AggregateTool extends MongoDBToolBase {
     }: ToolArgs<typeof this.argsShape>): Promise<CallToolResult> {
         const provider = await this.ensureConnected();
 
+        this.assertOnlyUsesPermittedStages(pipeline);
+
         // Check if aggregate operation uses an index if enabled
         if (this.config.indexCheck) {
             await checkIndexUsage(provider, database, collection, "aggregate", async () => {
@@ -44,4 +47,19 @@ export class AggregateTool extends MongoDBToolBase {
             ),
         };
     }
+
+    private assertOnlyUsesPermittedStages(pipeline: Record<string, unknown>[]): void {
+        if (!this.config.readOnly) {
+            return;
+        }
+
+        for (const stage of pipeline) {
+            if (stage.$out || stage.$merge) {
+                throw new MongoDBError(
+                    ErrorCodes.ForbiddenWriteOperation,
+                    "In readOnly mode you can not run pipelines with $out or $merge stages."
+                );
+            }
+        }
+    }
 }
diff --git a/tests/integration/tools/mongodb/read/aggregate.test.ts b/tests/integration/tools/mongodb/read/aggregate.test.ts
index fbe72ae80..643c5ef37 100644
--- a/tests/integration/tools/mongodb/read/aggregate.test.ts
+++ b/tests/integration/tools/mongodb/read/aggregate.test.ts
@@ -95,6 +95,40 @@ describeWithMongoDB("aggregate tool", (integration) => {
         );
     });
 
+    it("can not run $out stages in readOnly mode", async () => {
+        await integration.connectMcpClient();
+        integration.mcpServer().userConfig.readOnly = true;
+        const response = await integration.mcpClient().callTool({
+            name: "aggregate",
+            arguments: {
+                database: integration.randomDbName(),
+                collection: "people",
+                pipeline: [{ $out: "outpeople" }],
+            },
+        });
+        const content = getResponseContent(response);
+        expect(content).toEqual(
+            "Error running aggregate: In readOnly mode you can not run pipelines with $out or $merge stages."
+        );
+    });
+
+    it("can not run $merge stages in readOnly mode", async () => {
+        await integration.connectMcpClient();
+        integration.mcpServer().userConfig.readOnly = true;
+        const response = await integration.mcpClient().callTool({
+            name: "aggregate",
+            arguments: {
+                database: integration.randomDbName(),
+                collection: "people",
+                pipeline: [{ $merge: "outpeople" }],
+            },
+        });
+        const content = getResponseContent(response);
+        expect(content).toEqual(
+            "Error running aggregate: In readOnly mode you can not run pipelines with $out or $merge stages."
+        );
+    });
+
     validateAutoConnectBehavior(integration, "aggregate", () => {
         return {
             args: {