diff --git a/frontend/src/components/editor/ai/__tests__/completion-utils.test.ts b/frontend/src/components/editor/ai/__tests__/completion-utils.test.ts
index 449c283ce27..e5c4c8ac7d9 100644
--- a/frontend/src/components/editor/ai/__tests__/completion-utils.test.ts
+++ b/frontend/src/components/editor/ai/__tests__/completion-utils.test.ts
@@ -1,6 +1,15 @@
 /* Copyright 2026 Marimo. All rights reserved. */
-import { beforeEach, describe, expect, it, type Mock, vi } from "vitest";
+import {
+  afterEach,
+  beforeEach,
+  describe,
+  expect,
+  it,
+  type Mock,
+  vi,
+} from "vitest";
 import { variableName } from "@/__tests__/branded";
+import * as aiContext from "@/core/ai/context/context";
 import { getCodes } from "@/core/codemirror/copilot/getCodes";
 import { dataSourceConnectionsAtom } from "@/core/datasets/data-source-connections";
 import { DUCKDB_ENGINE } from "@/core/datasets/engines";
@@ -8,10 +17,11 @@ import { datasetsAtom } from "@/core/datasets/state";
 import type { DatasetsState } from "@/core/datasets/types";
 import { store } from "@/core/state/jotai";
 import { variablesAtom } from "@/core/variables/state";
-import type { UIMessage } from "ai";
+import type { FileUIPart, UIMessage } from "ai";
 import {
   codeToCells,
   getAICompletionBody,
+  getAICompletionBodyWithAttachments,
   isContextAttachment,
   MARIMO_CONTEXT_PART_TYPE,
   resolveChatContext,
@@ -440,6 +450,42 @@ describe("isContextAttachment", () => {
   });
 });
 
+describe("context attachment stamping", () => {
+  const rawAttachment: FileUIPart = {
+    type: "file",
+    mediaType: "image/png",
+    url: "data:image/png;base64,abc",
+  };
+
+  beforeEach(() => {
+    vi.spyOn(aiContext, "getAIContextRegistry").mockReturnValue({
+      parseAllContextIds: () => ["data://t1"],
+      formatContextForAI: () => '<data name="t1" />',
+      getAttachmentsForContext: async () => [rawAttachment],
+    } as unknown as ReturnType<typeof aiContext.getAIContextRegistry>);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("stamps chat attachments as context-derived", async () => {
+    const { attachments } = await resolveChatContext("see @data://t1");
+    expect(attachments).toHaveLength(1);
+    expect(isContextAttachment(attachments[0])).toBe(true);
+    // The original attachment is left untouched (we return a stamped copy).
+    expect(rawAttachment.providerMetadata).toBeUndefined();
+  });
+
+  it("stamps completion attachments the same way as chat", async () => {
+    const { attachments } = await getAICompletionBodyWithAttachments({
+      input: "see @data://t1",
+    });
+    expect(attachments).toHaveLength(1);
+    expect(isContextAttachment(attachments[0])).toBe(true);
+  });
+});
+
 describe("codeToCells", () => {
   it("should return empty array for empty string", () => {
     const code = "";
diff --git a/frontend/src/components/editor/ai/completion-utils.ts b/frontend/src/components/editor/ai/completion-utils.ts
index ef408595d42..d41cb095892 100644
--- a/frontend/src/components/editor/ai/completion-utils.ts
+++ b/frontend/src/components/editor/ai/completion-utils.ts
@@ -9,6 +9,7 @@ import {
 import type { ReactCodeMirrorRef } from "@uiw/react-codemirror";
 import type { DataUIPart, FileUIPart, UIMessage } from "ai";
 import { getAIContextRegistry } from "@/core/ai/context/context";
+import type { ContextLocatorId } from "@/core/ai/context/registry";
 import { getCodes } from "@/core/codemirror/copilot/getCodes";
 import type { LanguageAdapterType } from "@/core/codemirror/language/types";
 import type { AiCompletionRequest } from "@/core/network/types";
@@ -89,20 +90,51 @@ export function isContextAttachment(part: UIMessage["parts"][number]): boolean {
 }
 
 /**
- * Resolve @-context for messages. They represent referenced
- * datasets, variables, or other context from the user's prompt.
+ * Stamp a context-derived attachment with a provenance marker.
+ *
+ * Some @-mentions resolve to file attachments (e.g. a cell's image output),
+ * which get appended to the user message right alongside files the user
+ * uploaded by hand. Once they're in the message the two are indistinguishable,
+ * so we mark the context-derived ones. This matters on message edit: we
+ * re-resolve context from the edited text, and `isContextAttachment` lets us
+ * drop only the stale context attachments while preserving the user's own
+ * uploads
  */
-export async function resolveChatContext(
+function stampContextAttachment(attachment: FileUIPart): FileUIPart {
+  return {
+    ...attachment,
+    providerMetadata: {
+      ...attachment.providerMetadata,
+      // Merge within the `marimo` namespace so we don't clobber any other
+      // marimo metadata a provider may have already set.
+      marimo: {
+        ...attachment.providerMetadata?.marimo,
+        ...CONTEXT_ATTACHMENT_METADATA.marimo,
+      },
+    },
+  };
+}
+
+interface ResolvedContext {
+  plainText: string;
+  contextIds: ContextLocatorId[];
+  attachments: FileUIPart[];
+}
+
+/**
+ * Parse @-context for messages
+ */
+async function resolveContextAttachments(
   input: string,
-): Promise<ResolvedChatContext> {
+): Promise<ResolvedContext> {
   if (!input.includes(CONTEXT_TRIGGER)) {
-    return { contextPart: null, attachments: [] };
+    return { plainText: "", contextIds: [], attachments: [] };
   }
 
   const registry = getAIContextRegistry(store);
   const contextIds = registry.parseAllContextIds(input);
   if (contextIds.length === 0) {
-    return { contextPart: null, attachments: [] };
+    return { plainText: "", contextIds: [], attachments: [] };
   }
 
   const plainText = registry.formatContextForAI(contextIds);
@@ -110,20 +142,24 @@ export async function resolveChatContext(
   let attachments: FileUIPart[] = [];
   try {
     const resolved = await registry.getAttachmentsForContext(contextIds);
-    attachments = resolved.map((attachment) => ({
-      ...attachment,
-      providerMetadata: {
-        ...attachment.providerMetadata,
-        marimo: {
-          ...attachment.providerMetadata?.marimo,
-          ...CONTEXT_ATTACHMENT_METADATA.marimo,
-        },
-      },
-    }));
+    attachments = resolved.map(stampContextAttachment);
   } catch (error) {
     Logger.error("Error getting attachments:", error);
   }
 
+  return { plainText, contextIds, attachments };
+}
+
+/**
+ * Resolve @-context for messages. They represent referenced
+ * datasets, variables, or other context from the user's prompt.
+ */
+export async function resolveChatContext(
+  input: string,
+): Promise<ResolvedChatContext> {
+  const { plainText, contextIds, attachments } =
+    await resolveContextAttachments(input);
+
   let contextPart: MarimoContextUIPart | null = null;
   if (plainText.trim()) {
     contextPart = {
@@ -141,31 +177,13 @@ export async function resolveChatContext(
 export async function getAICompletionBodyWithAttachments({
   input,
 }: Opts): Promise<AICompletionBodyWithAttachments> {
-  let contextString = "";
-  let attachments: FileUIPart[] = [];
-
-  // Skip if no '@' in the input
-  if (input.includes("@")) {
-    const registry = getAIContextRegistry(store);
-    const contextIds = registry.parseAllContextIds(input);
-
-    // Get context string
-    contextString = registry.formatContextForAI(contextIds);
-
-    // Get attachments
-    try {
-      attachments = await registry.getAttachmentsForContext(contextIds);
-      Logger.debug("Included attachments", attachments.length);
-    } catch (error) {
-      Logger.error("Error getting attachments:", error);
-    }
-  }
+  const { plainText, attachments } = await resolveContextAttachments(input);
 
   return {
     body: {
       includeOtherCode: getCodes(""),
       context: {
-        plainText: contextString,
+        plainText,
         schema: [],
         variables: [],
       },