FN-6630: return text for empty task-list columns

Ensure filtered fn_task_list calls always return host-safe text for empty target columns.

- Add explicit empty-state copy for column filters with no matching tasks.
- Keep formatter fallback output non-empty before returning tool content.
- Cover empty active-column filters and existing small listings in CLI extension tests.
- Add a patch changeset for the published CLI package.

Files changed:
 .changeset/fn-6630-task-list-empty-column.md |  5 ++++
 packages/cli/src/__tests__/extension.test.ts | 36 +++++++++++++++++++++++++++-
 packages/cli/src/extension.ts                | 15 ++++++++++--
 3 files changed, 53 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-6630

Fusion-Task-Lineage: e371bf11-5841-47d0-ac72-4cdf2f1716ba

.changeset/fn-6630-task-list-empty-column.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix `fn_task_list` column filters so empty target columns return explicit text instead of an empty content block.

packages/cli/src/__tests__/extension.test.ts

@@ -23,7 +23,7 @@ vi.mock("../commands/task.js", () => ({
 }));
 
 import kbExtension, { resolveTaskListFormatter } from "../extension.js";
-import { TaskStore, AgentStore, MANUAL_RETRY_RESET_COUNTER_KEYS, RESEARCH_RUN_STATUSES, MAX_TASK_LIST_TEXT_CHARS, formatTaskListText } from "@fusion/core";
+import { TaskStore, AgentStore, MANUAL_RETRY_RESET_COUNTER_KEYS, RESEARCH_RUN_STATUSES, MAX_TASK_LIST_TEXT_CHARS, formatTaskListText, COLUMN_LABELS } from "@fusion/core";
 import type { WorkflowIr } from "@fusion/core";
 import { isGhAvailable, isGhAuthenticated, runGhJsonAsync } from "@fusion/core/gh-cli";
 import { hasBuiltCoreDistBarrel } from "@fusion/test-utils";
@@ -2588,6 +2588,37 @@ describe("fn pi extension (runnable structured-output regression slice)", () =>
       }
     });
 
+    it("returns explicit text for empty active-column filters on a non-empty board", async () => {
+      const store = new TaskStore(tmpDir);
+      await store.init();
+      try {
+        await store.createTask({ description: "Finished task keeps the board non-empty", column: "done" });
+      } finally {
+        store.close();
+      }
+
+      const listTool = api.tools.get("fn_task_list")!;
+      for (const column of ["triage", "todo", "in-progress", "in-review"] as const) {
+        const result = await listTool.execute(
+          `empty-${column}`,
+          { column },
+          undefined,
+          undefined,
+          makeCtx(tmpDir),
+        );
+        const text = result.content[0].text;
+
+        expect(result.content).toHaveLength(1);
+        expect(result.content[0].type).toBe("text");
+        expect(result.content.some((block: any) => block.type === "image")).toBe(false);
+        expect(text).toBeTruthy();
+        expect(text.trim()).not.toBe("");
+        expect(text).toContain(COLUMN_LABELS[column]);
+        expect(text).toContain(column);
+        expect(result.details.count).toBe(1);
+      }
+    });
+
     it("keeps small column-filtered listings complete without the clamp marker", async () => {
       const store = new TaskStore(tmpDir);
       await store.init();
@@ -2611,10 +2642,13 @@ describe("fn pi extension (runnable structured-output regression slice)", () =>
       expect(result.content).toHaveLength(1);
       expect(result.content[0].type).toBe("text");
       expect(result.content.some((block: any) => block.type === "image")).toBe(false);
+      expect(text).toBeTruthy();
+      expect(text.trim()).not.toBe("");
       expect(text).toContain("Todo (2):");
       expect(text).toContain("FN-001");
       expect(text).toContain("FN-002");
       expect(text).toContain("[deps: FN-001]");
+      expect(text).not.toContain("No tasks in Todo (todo).");
       expect(text).not.toContain("truncated to fit; narrow with column/limit");
       expect(result.details.count).toBe(2);
     });

packages/cli/src/extension.ts

@@ -854,9 +854,10 @@ export default function kbExtension(pi: ExtensionAPI) {
       }
 
       const perColumn = params.limit ?? 10;
+      const requestedColumn = params.column as ColumnId | undefined;
       const lines: string[] = [];
       for (const col of COLUMNS) {
-        if (params.column && params.column !== col) continue;
+        if (requestedColumn && requestedColumn !== col) continue;
 
         const colTasks = tasks.filter((t) => t.column === col);
         if (colTasks.length === 0) continue;
@@ -873,6 +874,10 @@ export default function kbExtension(pi: ExtensionAPI) {
         lines.push("");
       }
 
+      const emptyStateText = requestedColumn
+        ? `No tasks in ${columnLabel(requestedColumn)} (${requestedColumn}).`
+        : "No matching tasks.";
+
       /*
       FNXC:TaskListOutput 2026-06-16-17:47:
       FN-6492 routes CLI fn_task_list through the shared clamp so large column-filtered board reads remain text-only instead of being converted to host attachments.
@@ -882,10 +887,16 @@ export default function kbExtension(pi: ExtensionAPI) {
 
       FNXC:TaskListOutput 2026-06-17-07:25:
       FN-6573 requires CLI fn_task_list to resolve formatTaskListText from the runtime @fusion/core namespace with a typeof guard and a self-contained bounded fallback. A stale @fusion/core dist missing the FN-6570 formatter export crashed ambient heartbeat agents as `(0 , _core.formatTaskListText) is not a function`; the tool must now return bounded text instead.
+
+      FNXC:TaskListOutput 2026-06-18-04:46:
+      FN-6630 refines FN-6492 by requiring filtered fn_task_list calls against empty target columns to return explicit empty-state text. Host runtimes can imageify empty content blocks as `(see attached image)`, so this call site must never emit empty or whitespace-only text.
       */
       const formatter = resolveTaskListFormatter(fusionCore);
+      const text = lines.length === 0
+        ? emptyStateText
+        : formatter(lines, { clamp: fusionCore.clampTaskListText }).trimEnd();
       return {
-        content: [{ type: "text", text: formatter(lines, { clamp: fusionCore.clampTaskListText }).trimEnd() }],
+        content: [{ type: "text", text: text.trim().length > 0 ? text : emptyStateText }],
         details: { count: tasks.length },
       };
     },