diff --git a/packages/core/src/__tests__/task-detail-prompt-resilience.test.ts b/packages/core/src/__tests__/task-detail-prompt-resilience.test.ts
index 2d685a89ca..750d65ffb3 100644
--- a/packages/core/src/__tests__/task-detail-prompt-resilience.test.ts
+++ b/packages/core/src/__tests__/task-detail-prompt-resilience.test.ts
@@ -83,6 +83,11 @@ describe("getTask PROMPT.md read resilience (task-write-API 500 regression)", ()
       await expect(store.moveTask(task.id, "in-progress")).resolves.toBeTruthy();
       await expect(store.moveTask(task.id, "todo")).resolves.toBeTruthy();
 
+      // Directly updating a step whose definition lives only in the unreadable
+      // PROMPT.md genuinely cannot succeed, but the error must name the real
+      // cause (PROMPT.md) rather than a misleading "task has 0 steps".
+      await expect(store.updateStep(task.id, 0, "in-progress")).rejects.toThrow(/PROMPT\.md/);
+
       await expect(store.archiveTask(task.id)).resolves.toBeTruthy();
       const archived = await store.getTask(task.id);
       expect(archived.column).toBe("archived");
diff --git a/packages/core/src/store.ts b/packages/core/src/store.ts
index a8a9530f72..82c0c95862 100644
--- a/packages/core/src/store.ts
+++ b/packages/core/src/store.ts
@@ -9965,11 +9965,17 @@ ${TASK_UPSERT_SQL_ASSIGNMENTS}
       // FNXC:TaskDetailPromptResilience 2026-07-10-15:00: step auto-init is
       // best-effort — an unreadable PROMPT.md must not fail updateStep (on the
       // reported reset path); proceed with the persisted (empty) steps.
+      let promptStepsUnavailable: string | undefined;
       if (task.steps.length === 0 && !graphSource) {
         try {
           task.steps = await this.parseStepsFromPrompt(id);
         } catch (err) {
-          storeLog.warn(`[task-detail] failed to auto-init steps from PROMPT.md for ${id}: ${getErrorMessage(err)}`);
+          // Remember WHY steps couldn't be resolved so the range check below
+          // attributes the failure to the unreadable PROMPT.md rather than a
+          // misleading "0 steps". A step defined only in an unreadable PROMPT.md
+          // genuinely cannot be updated — but the error should say so.
+          promptStepsUnavailable = getErrorMessage(err);
+          storeLog.warn(`[task-detail] failed to auto-init steps from PROMPT.md for ${id}: ${promptStepsUnavailable}`);
         }
       }
 
@@ -9979,6 +9985,14 @@ ${TASK_UPSERT_SQL_ASSIGNMENTS}
       }
 
       if (stepIndex < 0 || stepIndex >= task.steps.length) {
+        // FNXC:TaskDetailPromptResilience 2026-07-10-16:30: when the range failure
+        // is caused by an unreadable PROMPT.md (not a genuinely stepless task),
+        // surface the real cause instead of a confusing "task has 0 steps".
+        if (promptStepsUnavailable !== undefined && task.steps.length === 0) {
+          throw new Error(
+            `Cannot update step ${stepIndex} for ${id}: its steps are defined in PROMPT.md, which could not be read (${promptStepsUnavailable}).`,
+          );
+        }
         throw new Error(
           `Step ${stepIndex} out of range (task has ${task.steps.length} steps)`,
         );