fix(FN-8456): keep prompt writer in triage toolset

.changeset/fix-triage-prompt-writer-tools.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Let task planning persist complete specifications before Plan Review starts.
+category: fix
+dev: Triage sessions now use the coding tool surface so `fn_task_prompt_write` remains available.

packages/engine/src/__tests__/pi-create-fn-agent.test.ts

@@ -2210,6 +2210,27 @@ describe("createFnAgent", () => {
     expect(createSessionArgs.customTools.map((tool) => tool.name)).toContain("fn_list_agents");
   });
 
+  it("keeps fn_task_prompt_write in coding session tools", async () => {
+    const promptWriter = {
+      name: "fn_task_prompt_write",
+      label: "Write PROMPT.md",
+      description: "Persist the task specification",
+      parameters: {},
+      execute: vi.fn(),
+    };
+
+    const { createFnAgent } = await import("../pi.js");
+    await createFnAgent({
+      cwd: "/tmp",
+      systemPrompt: "test",
+      tools: "coding",
+      customTools: [promptWriter as any],
+    });
+
+    const createSessionArgs = createAgentSessionMock.mock.calls[0]?.[0] as { customTools: Array<{ name: string }> };
+    expect(createSessionArgs.customTools.map((tool) => tool.name)).toContain("fn_task_prompt_write");
+  });
+
   it("does not allow extra builtin tools in readonly sessions by default", async () => {
     const { createFnAgent } = await import("../pi.js");
 

packages/engine/src/__tests__/triage-planning-prompt-single-source.test.ts

@@ -158,7 +158,7 @@ describe("triage planning prompt single source", () => {
     await new TriageProcessor(store, "/tmp/root").specifyTask(task);
 
     expect(customTools.map((tool) => tool.name)).toContain("fn_task_prompt_write");
-    expect(sessionTools).toBe("readonly");
+    expect(sessionTools).toBe("coding");
   });
 
   it("requires triage plans to use the durable prompt writer instead of generic filesystem writes", () => {

packages/engine/src/triage.ts

@@ -1612,8 +1612,11 @@ export class TriageProcessor {
           : { provider: undefined, modelId: undefined };
 
         /*
-        FNXC:TriagePromptPersistence 2026-07-21-16:30:
-        `tools: "readonly"` intentionally coexists with the custom prompt writer above: readonly governs general tools, while fn_task_prompt_write performs the one authorized synchronized task-artifact mutation.
+        FNXC:TriagePromptPersistence 2026-07-21-17:50:
+        Planning must use the coding tool surface. The shared readonly policy filters
+        mutation tools, including fn_task_prompt_write, before the model sees them;
+        advertising that writer in the prompt while running readonly stranded triage
+        on the original PROMPT.md stub and sent the stub into Plan Review.
         */
         const { session } = await createResolvedAgentSession({
           sessionPurpose: "triage",
@@ -1622,7 +1625,7 @@ export class TriageProcessor {
           cwd: this.rootDir,
           systemPrompt: triageSystemPromptFinal,
           systemPromptLayers: triageLayers,
-          tools: "readonly",
+          tools: "coding",
           customTools,
           onText: agentLogger.onText,
           onThinking: agentLogger.onThinking,