diff --git a/src/resources/extensions/gsd/tools/plan-milestone.ts b/src/resources/extensions/gsd/tools/plan-milestone.ts
index 95bc2ede8..44749a5ce 100644
--- a/src/resources/extensions/gsd/tools/plan-milestone.ts
+++ b/src/resources/extensions/gsd/tools/plan-milestone.ts
@@ -189,27 +189,34 @@ export async function handlePlanMilestone(
     return { error: `validation failed: ${(err as Error).message}` };
   }
 
-  // ── State machine preconditions ─────────────────────────────────────────
-  const existingMilestone = getMilestone(params.milestoneId);
-  if (existingMilestone && (existingMilestone.status === "complete" || existingMilestone.status === "done")) {
-    return { error: `cannot re-plan milestone ${params.milestoneId}: it is already complete` };
-  }
-
-  // Validate depends_on: all dependencies must exist and be complete
-  if (params.dependsOn && params.dependsOn.length > 0) {
-    for (const depId of params.dependsOn) {
-      const dep = getMilestone(depId);
-      if (!dep) {
-        return { error: `depends_on references unknown milestone: ${depId}` };
-      }
-      if (dep.status !== "complete" && dep.status !== "done") {
-        return { error: `depends_on milestone ${depId} is not yet complete (status: ${dep.status})` };
-      }
-    }
-  }
+  // ── Guards + DB writes inside a single transaction (prevents TOCTOU) ───
+  // Guards must be inside the transaction so the state they check cannot
+  // change between the read and the write (#2723).
+  let guardError: string | null = null;
 
   try {
     transaction(() => {
+      const existingMilestone = getMilestone(params.milestoneId);
+      if (existingMilestone && (existingMilestone.status === "complete" || existingMilestone.status === "done")) {
+        guardError = `cannot re-plan milestone ${params.milestoneId}: it is already complete`;
+        return;
+      }
+
+      // Validate depends_on: all dependencies must exist and be complete
+      if (params.dependsOn && params.dependsOn.length > 0) {
+        for (const depId of params.dependsOn) {
+          const dep = getMilestone(depId);
+          if (!dep) {
+            guardError = `depends_on references unknown milestone: ${depId}`;
+            return;
+          }
+          if (dep.status !== "complete" && dep.status !== "done") {
+            guardError = `depends_on milestone ${depId} is not yet complete (status: ${dep.status})`;
+            return;
+          }
+        }
+      }
+
       insertMilestone({
         id: params.milestoneId,
         title: params.title,
@@ -254,6 +261,10 @@ export async function handlePlanMilestone(
     return { error: `db write failed: ${(err as Error).message}` };
   }
 
+  if (guardError) {
+    return { error: guardError };
+  }
+
   let roadmapPath: string;
   try {
     const renderResult = await renderRoadmapFromDb(basePath, params.milestoneId);
diff --git a/src/resources/extensions/gsd/tools/plan-slice.ts b/src/resources/extensions/gsd/tools/plan-slice.ts
index 8229aee52..bb5ab8f3f 100644
--- a/src/resources/extensions/gsd/tools/plan-slice.ts
+++ b/src/resources/extensions/gsd/tools/plan-slice.ts
@@ -146,24 +146,33 @@ export async function handlePlanSlice(
     return { error: `validation failed: ${(err as Error).message}` };
   }
 
-  const parentMilestone = getMilestone(params.milestoneId);
-  if (!parentMilestone) {
-    return { error: `milestone not found: ${params.milestoneId}` };
-  }
-  if (parentMilestone.status === "complete" || parentMilestone.status === "done") {
-    return { error: `cannot plan slice in a closed milestone: ${params.milestoneId} (status: ${parentMilestone.status})` };
-  }
-
-  const parentSlice = getSlice(params.milestoneId, params.sliceId);
-  if (!parentSlice) {
-    return { error: `missing parent slice: ${params.milestoneId}/${params.sliceId}` };
-  }
-  if (parentSlice.status === "complete" || parentSlice.status === "done") {
-    return { error: `cannot re-plan slice ${params.sliceId}: it is already complete — use gsd_slice_reopen first` };
-  }
+  // ── Guards + DB writes inside a single transaction (prevents TOCTOU) ───
+  // Guards must be inside the transaction so the state they check cannot
+  // change between the read and the write (#2723).
+  let guardError: string | null = null;
 
   try {
     transaction(() => {
+      const parentMilestone = getMilestone(params.milestoneId);
+      if (!parentMilestone) {
+        guardError = `milestone not found: ${params.milestoneId}`;
+        return;
+      }
+      if (parentMilestone.status === "complete" || parentMilestone.status === "done") {
+        guardError = `cannot plan slice in a closed milestone: ${params.milestoneId} (status: ${parentMilestone.status})`;
+        return;
+      }
+
+      const parentSlice = getSlice(params.milestoneId, params.sliceId);
+      if (!parentSlice) {
+        guardError = `missing parent slice: ${params.milestoneId}/${params.sliceId}`;
+        return;
+      }
+      if (parentSlice.status === "complete" || parentSlice.status === "done") {
+        guardError = `cannot re-plan slice ${params.sliceId}: it is already complete — use gsd_slice_reopen first`;
+        return;
+      }
+
       upsertSlicePlanning(params.milestoneId, params.sliceId, {
         goal: params.goal,
         successCriteria: params.successCriteria,
@@ -211,6 +220,10 @@ export async function handlePlanSlice(
     return { error: `db write failed: ${(err as Error).message}` };
   }
 
+  if (guardError) {
+    return { error: guardError };
+  }
+
   try {
     const renderResult = await renderPlanFromDb(basePath, params.milestoneId, params.sliceId);
     invalidateStateCache();
diff --git a/src/resources/extensions/gsd/tools/plan-task.ts b/src/resources/extensions/gsd/tools/plan-task.ts
index c640ee22d..3fb9495ca 100644
--- a/src/resources/extensions/gsd/tools/plan-task.ts
+++ b/src/resources/extensions/gsd/tools/plan-task.ts
@@ -77,21 +77,29 @@ export async function handlePlanTask(
     return { error: `validation failed: ${(err as Error).message}` };
   }
 
-  const parentSlice = getSlice(params.milestoneId, params.sliceId);
-  if (!parentSlice) {
-    return { error: `missing parent slice: ${params.milestoneId}/${params.sliceId}` };
-  }
-  if (parentSlice.status === "complete" || parentSlice.status === "done") {
-    return { error: `cannot plan task in a closed slice: ${params.sliceId} (status: ${parentSlice.status})` };
-  }
-
-  const existingTask = getTask(params.milestoneId, params.sliceId, params.taskId);
-  if (existingTask && (existingTask.status === "complete" || existingTask.status === "done")) {
-    return { error: `cannot re-plan task ${params.taskId}: it is already complete — use gsd_task_reopen first` };
-  }
+  // ── Guards + DB writes inside a single transaction (prevents TOCTOU) ───
+  // Guards must be inside the transaction so the state they check cannot
+  // change between the read and the write (#2723).
+  let guardError: string | null = null;
 
   try {
     transaction(() => {
+      const parentSlice = getSlice(params.milestoneId, params.sliceId);
+      if (!parentSlice) {
+        guardError = `missing parent slice: ${params.milestoneId}/${params.sliceId}`;
+        return;
+      }
+      if (parentSlice.status === "complete" || parentSlice.status === "done") {
+        guardError = `cannot plan task in a closed slice: ${params.sliceId} (status: ${parentSlice.status})`;
+        return;
+      }
+
+      const existingTask = getTask(params.milestoneId, params.sliceId, params.taskId);
+      if (existingTask && (existingTask.status === "complete" || existingTask.status === "done")) {
+        guardError = `cannot re-plan task ${params.taskId}: it is already complete — use gsd_task_reopen first`;
+        return;
+      }
+
       if (!existingTask) {
         insertTask({
           id: params.taskId,
@@ -117,6 +125,10 @@ export async function handlePlanTask(
     return { error: `db write failed: ${(err as Error).message}` };
   }
 
+  if (guardError) {
+    return { error: guardError };
+  }
+
   try {
     const renderResult = await renderTaskPlanFromDb(basePath, params.milestoneId, params.sliceId, params.taskId);
     invalidateStateCache();
diff --git a/src/resources/extensions/gsd/tools/reassess-roadmap.ts b/src/resources/extensions/gsd/tools/reassess-roadmap.ts
index db916bea9..3aa832120 100644
--- a/src/resources/extensions/gsd/tools/reassess-roadmap.ts
+++ b/src/resources/extensions/gsd/tools/reassess-roadmap.ts
@@ -104,47 +104,6 @@ export async function handleReassessRoadmap(
     return { error: `validation failed: ${(err as Error).message}` };
   }
 
-  // ── Verify milestone exists and is active ────────────────────────
-  const milestone = getMilestone(params.milestoneId);
-  if (!milestone) {
-    return { error: `milestone not found: ${params.milestoneId}` };
-  }
-  if (milestone.status === "complete" || milestone.status === "done") {
-    return { error: `cannot reassess a closed milestone: ${params.milestoneId} (status: ${milestone.status})` };
-  }
-
-  // ── Verify completedSliceId is actually complete ──────────────────
-  const completedSlice = getSlice(params.milestoneId, params.completedSliceId);
-  if (!completedSlice) {
-    return { error: `completedSliceId not found: ${params.milestoneId}/${params.completedSliceId}` };
-  }
-  if (completedSlice.status !== "complete" && completedSlice.status !== "done") {
-    return { error: `completedSliceId ${params.completedSliceId} is not complete (status: ${completedSlice.status}) — reassess can only be called after a slice finishes` };
-  }
-
-  // ── Structural enforcement ────────────────────────────────────────
-  const existingSlices = getMilestoneSlices(params.milestoneId);
-  const completedSliceIds = new Set<string>();
-  for (const slice of existingSlices) {
-    if (slice.status === "complete" || slice.status === "done") {
-      completedSliceIds.add(slice.id);
-    }
-  }
-
-  // Reject modifications to completed slices
-  for (const modifiedSlice of params.sliceChanges.modified) {
-    if (completedSliceIds.has(modifiedSlice.sliceId)) {
-      return { error: `cannot modify completed slice ${modifiedSlice.sliceId}` };
-    }
-  }
-
-  // Reject removal of completed slices
-  for (const removedId of params.sliceChanges.removed) {
-    if (completedSliceIds.has(removedId)) {
-      return { error: `cannot remove completed slice ${removedId}` };
-    }
-  }
-
   // ── Compute assessment artifact path ──────────────────────────────
   // Assessment lives in the completed slice's directory
   const assessmentRelPath = join(
@@ -153,9 +112,58 @@ export async function handleReassessRoadmap(
     `${params.completedSliceId}-ASSESSMENT.md`,
   );
 
-  // ── Transaction: DB mutations ─────────────────────────────────────
+  // ── Guards + DB writes inside a single transaction (prevents TOCTOU) ───
+  // Guards must be inside the transaction so the state they check cannot
+  // change between the read and the write (#2723).
+  let guardError: string | null = null;
+
   try {
     transaction(() => {
+      // Verify milestone exists and is active
+      const milestone = getMilestone(params.milestoneId);
+      if (!milestone) {
+        guardError = `milestone not found: ${params.milestoneId}`;
+        return;
+      }
+      if (milestone.status === "complete" || milestone.status === "done") {
+        guardError = `cannot reassess a closed milestone: ${params.milestoneId} (status: ${milestone.status})`;
+        return;
+      }
+
+      // Verify completedSliceId is actually complete
+      const completedSlice = getSlice(params.milestoneId, params.completedSliceId);
+      if (!completedSlice) {
+        guardError = `completedSliceId not found: ${params.milestoneId}/${params.completedSliceId}`;
+        return;
+      }
+      if (completedSlice.status !== "complete" && completedSlice.status !== "done") {
+        guardError = `completedSliceId ${params.completedSliceId} is not complete (status: ${completedSlice.status}) — reassess can only be called after a slice finishes`;
+        return;
+      }
+
+      // Structural enforcement — reject modifications/removal of completed slices
+      const existingSlices = getMilestoneSlices(params.milestoneId);
+      const completedSliceIds = new Set<string>();
+      for (const slice of existingSlices) {
+        if (slice.status === "complete" || slice.status === "done") {
+          completedSliceIds.add(slice.id);
+        }
+      }
+
+      for (const modifiedSlice of params.sliceChanges.modified) {
+        if (completedSliceIds.has(modifiedSlice.sliceId)) {
+          guardError = `cannot modify completed slice ${modifiedSlice.sliceId}`;
+          return;
+        }
+      }
+
+      for (const removedId of params.sliceChanges.removed) {
+        if (completedSliceIds.has(removedId)) {
+          guardError = `cannot remove completed slice ${removedId}`;
+          return;
+        }
+      }
+
       // Record assessment
       insertAssessment({
         path: assessmentRelPath,
@@ -198,6 +206,10 @@ export async function handleReassessRoadmap(
     return { error: `db write failed: ${(err as Error).message}` };
   }
 
+  if (guardError) {
+    return { error: guardError };
+  }
+
   // ── Render artifacts ──────────────────────────────────────────────
   try {
     const roadmapResult = await renderRoadmapFromDb(basePath, params.milestoneId);
diff --git a/src/resources/extensions/gsd/tools/replan-slice.ts b/src/resources/extensions/gsd/tools/replan-slice.ts
index f96474825..c6b060597 100644
--- a/src/resources/extensions/gsd/tools/replan-slice.ts
+++ b/src/resources/extensions/gsd/tools/replan-slice.ts
@@ -90,52 +90,61 @@ export async function handleReplanSlice(
     return { error: `validation failed: ${(err as Error).message}` };
   }
 
-  // ── Verify parent slice exists and is not closed ─────────────────
-  const parentSlice = getSlice(params.milestoneId, params.sliceId);
-  if (!parentSlice) {
-    return { error: `missing parent slice: ${params.milestoneId}/${params.sliceId}` };
-  }
-  if (parentSlice.status === "complete" || parentSlice.status === "done") {
-    return { error: `cannot replan a closed slice: ${params.sliceId} (status: ${parentSlice.status})` };
-  }
-
-  // ── Verify blocker task exists and is complete ────────────────────
-  const blockerTask = getTask(params.milestoneId, params.sliceId, params.blockerTaskId);
-  if (!blockerTask) {
-    return { error: `blockerTaskId not found: ${params.milestoneId}/${params.sliceId}/${params.blockerTaskId}` };
-  }
-  if (blockerTask.status !== "complete" && blockerTask.status !== "done") {
-    return { error: `blockerTaskId ${params.blockerTaskId} is not complete (status: ${blockerTask.status}) — the blocker task must be finished before a replan is triggered` };
-  }
-
-  // ── Structural enforcement ────────────────────────────────────────
-  const existingTasks = getSliceTasks(params.milestoneId, params.sliceId);
-  const completedTaskIds = new Set<string>();
-  for (const task of existingTasks) {
-    if (task.status === "complete" || task.status === "done") {
-      completedTaskIds.add(task.id);
-    }
-  }
-
-  // Reject updates to completed tasks
-  for (const updatedTask of params.updatedTasks) {
-    if (completedTaskIds.has(updatedTask.taskId)) {
-      return { error: `cannot modify completed task ${updatedTask.taskId}` };
-    }
-  }
-
-  // Reject removal of completed tasks
-  for (const removedId of params.removedTaskIds) {
-    if (completedTaskIds.has(removedId)) {
-      return { error: `cannot remove completed task ${removedId}` };
-    }
-  }
-
-  // ── Transaction: DB mutations ─────────────────────────────────────
-  const existingTaskIds = new Set(existingTasks.map((t) => t.id));
+  // ── Guards + DB writes inside a single transaction (prevents TOCTOU) ───
+  // Guards must be inside the transaction so the state they check cannot
+  // change between the read and the write (#2723).
+  let guardError: string | null = null;
+  let existingTaskIds: Set<string> = new Set();
 
   try {
     transaction(() => {
+      // Verify parent slice exists and is not closed
+      const parentSlice = getSlice(params.milestoneId, params.sliceId);
+      if (!parentSlice) {
+        guardError = `missing parent slice: ${params.milestoneId}/${params.sliceId}`;
+        return;
+      }
+      if (parentSlice.status === "complete" || parentSlice.status === "done") {
+        guardError = `cannot replan a closed slice: ${params.sliceId} (status: ${parentSlice.status})`;
+        return;
+      }
+
+      // Verify blocker task exists and is complete
+      const blockerTask = getTask(params.milestoneId, params.sliceId, params.blockerTaskId);
+      if (!blockerTask) {
+        guardError = `blockerTaskId not found: ${params.milestoneId}/${params.sliceId}/${params.blockerTaskId}`;
+        return;
+      }
+      if (blockerTask.status !== "complete" && blockerTask.status !== "done") {
+        guardError = `blockerTaskId ${params.blockerTaskId} is not complete (status: ${blockerTask.status}) — the blocker task must be finished before a replan is triggered`;
+        return;
+      }
+
+      // Structural enforcement — reject modifications/removal of completed tasks
+      const existingTasks = getSliceTasks(params.milestoneId, params.sliceId);
+      const completedTaskIds = new Set<string>();
+      for (const task of existingTasks) {
+        if (task.status === "complete" || task.status === "done") {
+          completedTaskIds.add(task.id);
+        }
+      }
+
+      for (const updatedTask of params.updatedTasks) {
+        if (completedTaskIds.has(updatedTask.taskId)) {
+          guardError = `cannot modify completed task ${updatedTask.taskId}`;
+          return;
+        }
+      }
+
+      for (const removedId of params.removedTaskIds) {
+        if (completedTaskIds.has(removedId)) {
+          guardError = `cannot remove completed task ${removedId}`;
+          return;
+        }
+      }
+
+      existingTaskIds = new Set(existingTasks.map((t) => t.id));
+
       // Record replan history
       insertReplanHistory({
         milestoneId: params.milestoneId,
@@ -189,6 +198,10 @@ export async function handleReplanSlice(
     return { error: `db write failed: ${(err as Error).message}` };
   }
 
+  if (guardError) {
+    return { error: guardError };
+  }
+
   // ── Render artifacts ──────────────────────────────────────────────
   try {
     const renderResult = await renderPlanFromDb(basePath, params.milestoneId, params.sliceId);