diff --git a/apps/api/src/tools/task-board/automations.test.ts b/apps/api/src/tools/task-board/automations.test.ts
new file mode 100644
index 0000000000..241013c783
--- /dev/null
+++ b/apps/api/src/tools/task-board/automations.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from "bun:test";
+import { TASK_BOARD_AUTOMATION_UPSERT } from "./automations";
+
+/**
+ * Regression: `prompt` had no length cap — an unbounded `text` column,
+ * writable directly by any org member's tool call. Same gap as the task
+ * description cap (#6574's sibling), just on the automation's instruction.
+ */
+describe("TASK_BOARD_AUTOMATION_UPSERT prompt length", () => {
+  it("rejects a prompt over the cap", () => {
+    const result = TASK_BOARD_AUTOMATION_UPSERT.inputSchema.safeParse({
+      columnKey: "todo",
+      prompt: "x".repeat(50_001),
+    });
+    expect(result.success).toBe(false);
+  });
+
+  it("accepts a prompt at the cap", () => {
+    const result = TASK_BOARD_AUTOMATION_UPSERT.inputSchema.safeParse({
+      columnKey: "todo",
+      prompt: "x".repeat(50_000),
+    });
+    expect(result.success).toBe(true);
+  });
+
+  it("accepts a null prompt (the default instruction)", () => {
+    const result = TASK_BOARD_AUTOMATION_UPSERT.inputSchema.safeParse({
+      columnKey: "todo",
+      prompt: null,
+    });
+    expect(result.success).toBe(true);
+  });
+});
diff --git a/apps/api/src/tools/task-board/automations.ts b/apps/api/src/tools/task-board/automations.ts
index d3560d1de0..326c3f1c54 100644
--- a/apps/api/src/tools/task-board/automations.ts
+++ b/apps/api/src/tools/task-board/automations.ts
@@ -2,6 +2,7 @@ import { z } from "zod";
 import { defineTool } from "@/core/define-tool";
 import { requireAuth } from "@/core/studio-context";
 import { boardHandler } from "./board-handler";
+import { MAX_AUTOMATION_PROMPT_LENGTH } from "./schema";
 
 const AutomationSchema = z.object({
   columnKey: z.string(),
@@ -46,6 +47,7 @@ export const TASK_BOARD_AUTOMATION_UPSERT = defineTool({
       .describe("A column of this board — see TASK_BOARD_ITEM_LIST."),
     prompt: z
       .string()
+      .max(MAX_AUTOMATION_PROMPT_LENGTH)
       .nullable()
       .optional()
       .describe("What to do with a card landing here; null for the default."),
diff --git a/apps/api/src/tools/task-board/schema.ts b/apps/api/src/tools/task-board/schema.ts
index ceb8721067..07d11e91f7 100644
--- a/apps/api/src/tools/task-board/schema.ts
+++ b/apps/api/src/tools/task-board/schema.ts
@@ -16,6 +16,10 @@ export const MAX_TASK_TITLE_LENGTH = 500;
  *  so nothing legitimate approaches this; same reasoning as the caps above. */
 export const MAX_TASK_REPO_LENGTH = 200;
 
+/** A column automation's prompt is an instruction, not the message body —
+ *  same reasoning as MAX_TASK_DESCRIPTION_LENGTH. */
+export const MAX_AUTOMATION_PROMPT_LENGTH = 50_000;
+
 export const TaskBoardItemStatusSchema = z.enum([
   "triage",
   "todo",