diff --git a/src/resources/extensions/gsd/guided-flow.ts b/src/resources/extensions/gsd/guided-flow.ts
index 7202088ca..8025addcb 100644
--- a/src/resources/extensions/gsd/guided-flow.ts
+++ b/src/resources/extensions/gsd/guided-flow.ts
@@ -636,9 +636,11 @@ async function showQueueAdd(
   const existingContext = await buildExistingMilestonesContext(basePath, milestoneIds, state);
 
   // ── Determine next milestone ID ─────────────────────────────────────
+  // Note: the LLM will use the gsd_generate_milestone_id tool to get IDs
+  // at creation time, but we still mention the next ID in the preamble
+  // for context about where the sequence is.
   const uniqueEnabled = !!loadEffectiveGSDPreferences()?.preferences?.unique_milestone_ids;
   const nextId = nextMilestoneId(milestoneIds, uniqueEnabled);
-  const nextIdPlus1 = nextMilestoneId([...milestoneIds, nextId], uniqueEnabled);
 
   // ── Build preamble ──────────────────────────────────────────────────
   const activePart = state.activeMilestone
@@ -659,8 +661,6 @@ async function showQueueAdd(
   const queueInlinedTemplates = inlineTemplate("context", "Context");
   const prompt = loadPrompt("queue", {
     preamble,
-    nextId,
-    nextIdPlus1,
     existingMilestonesContext: existingContext,
     inlinedTemplates: queueInlinedTemplates,
     commitInstruction: buildDocsCommitInstruction("docs: queue <milestone list>"),
diff --git a/src/resources/extensions/gsd/index.ts b/src/resources/extensions/gsd/index.ts
index f387f5f5f..fc6416a97 100644
--- a/src/resources/extensions/gsd/index.ts
+++ b/src/resources/extensions/gsd/index.ts
@@ -36,7 +36,7 @@ import { loadPrompt } from "./prompt-loader.js";
 import { deriveState } from "./state.js";
 import { isAutoActive, isAutoPaused, handleAgentEnd, pauseAuto, getAutoDashboardData, markToolStart, markToolEnd } from "./auto.js";
 import { saveActivityLog } from "./activity-log.js";
-import { checkAutoStartAfterDiscuss, getDiscussionMilestoneId } from "./guided-flow.js";
+import { checkAutoStartAfterDiscuss, getDiscussionMilestoneId, findMilestoneIds, nextMilestoneId } from "./guided-flow.js";
 import { GSDDashboardOverlay } from "./dashboard-overlay.js";
 import {
   loadEffectiveGSDPreferences,
@@ -467,6 +467,46 @@ export default function (pi: ExtensionAPI) {
     },
   });
 
+  // ── gsd_generate_milestone_id — canonical milestone ID generation ──────
+  // The LLM cannot generate random suffixes for unique_milestone_ids on its
+  // own. This tool calls back into the TS code that owns ID generation,
+  // ensuring the preference is always respected and IDs are always valid.
+  pi.registerTool({
+    name: "gsd_generate_milestone_id",
+    label: "Generate Milestone ID",
+    description:
+      "Generate the next milestone ID for a new GSD milestone. " +
+      "Scans existing milestones on disk and respects the unique_milestone_ids preference. " +
+      "Always use this tool when creating a new milestone — never invent milestone IDs manually.",
+    promptSnippet: "Generate a valid milestone ID (respects unique_milestone_ids preference)",
+    promptGuidelines: [
+      "ALWAYS call gsd_generate_milestone_id before creating a new milestone directory or writing milestone files.",
+      "Never invent or hardcode milestone IDs like M001, M002 — always use this tool.",
+      "Call it once per milestone you need to create. For multi-milestone projects, call it once for each milestone in sequence.",
+      "The tool returns the correct format based on project preferences (e.g. M001 or M001-r5jzab).",
+    ],
+    parameters: Type.Object({}),
+    async execute(_toolCallId, _params, _signal, _onUpdate, _ctx) {
+      try {
+        const basePath = process.cwd();
+        const existingIds = findMilestoneIds(basePath);
+        const uniqueEnabled = !!loadEffectiveGSDPreferences()?.preferences?.unique_milestone_ids;
+        const newId = nextMilestoneId(existingIds, uniqueEnabled);
+        return {
+          content: [{ type: "text" as const, text: newId }],
+          details: { operation: "generate_milestone_id", id: newId, existingCount: existingIds.length, uniqueEnabled },
+        };
+      } catch (err) {
+        const msg = err instanceof Error ? err.message : String(err);
+        return {
+          content: [{ type: "text" as const, text: `Error generating milestone ID: ${msg}` }],
+          isError: true,
+          details: { operation: "generate_milestone_id", error: msg },
+        };
+      }
+    },
+  });
+
   // ── session_start: render branded GSD header + load tool keys + remote status ──
   pi.on("session_start", async (_event, ctx) => {
     // Theme access throws in RPC mode (no TUI) — header is decorative, skip it
diff --git a/src/resources/extensions/gsd/prompts/discuss-headless.md b/src/resources/extensions/gsd/prompts/discuss-headless.md
index 4a5afb0a2..fb908c9da 100644
--- a/src/resources/extensions/gsd/prompts/discuss-headless.md
+++ b/src/resources/extensions/gsd/prompts/discuss-headless.md
@@ -55,7 +55,7 @@ Use these templates exactly:
 9. Say exactly: "Milestone {{milestoneId}} ready."
 
 **For multi-milestone**, write in this order:
-1. Create all milestone directories: `mkdir -p .gsd/milestones/{M###}/slices` for each
+1. For each milestone, call `gsd_generate_milestone_id` to get its ID — never invent milestone IDs manually. Then `mkdir -p .gsd/milestones/<ID>/slices` for each.
 2. Write `.gsd/PROJECT.md` — full vision across ALL milestones (using Project template)
 3. Write `.gsd/REQUIREMENTS.md` — full capability contract (using Requirements template)
 4. Seed `.gsd/DECISIONS.md` (using Decisions template)
@@ -82,5 +82,5 @@ Use these templates exactly:
 - **Investigate before writing** — always scout the codebase first
 - **Use depends_on frontmatter** for multi-milestone sequences (the state machine reads this field to determine execution order)
 - **Anti-reduction rule** — if the spec describes a big vision, plan the big vision. Do not ask "what's the minimum viable version?" or reduce scope. Phase complex/risky work into later milestones — do not cut it.
-- **Naming convention** — directories use bare IDs (`M001/`, `S01/`), files use ID-SUFFIX format (`M001-CONTEXT.md`, `M001-ROADMAP.md`)
+- **Naming convention** — always use `gsd_generate_milestone_id` to get milestone IDs. Directories use bare IDs (e.g. `M001/` or `M001-r5jzab/`), files use ID-SUFFIX format (e.g. `M001-CONTEXT.md` or `M001-r5jzab-CONTEXT.md`). Never invent milestone IDs manually.
 - **End with "Milestone {{milestoneId}} ready."** — this triggers auto-start detection
diff --git a/src/resources/extensions/gsd/prompts/discuss.md b/src/resources/extensions/gsd/prompts/discuss.md
index de1f5a56f..3687d587c 100644
--- a/src/resources/extensions/gsd/prompts/discuss.md
+++ b/src/resources/extensions/gsd/prompts/discuss.md
@@ -211,7 +211,7 @@ Once the user confirms the milestone split:
 
 #### Phase 1: Shared artifacts
 
-1. `mkdir -p .gsd/milestones/{{milestoneId}}/slices` for each milestone
+1. For each milestone, call `gsd_generate_milestone_id` to get its ID — never invent milestone IDs manually. Then `mkdir -p .gsd/milestones/<ID>/slices`.
 2. Write `.gsd/PROJECT.md` — use the **Project** output template below.
 3. Write `.gsd/REQUIREMENTS.md` — use the **Requirements** output template below. Capture Active, Deferred, Out of Scope, and any already Validated requirements. Later milestones may have provisional ownership where slice plans do not exist yet.
 4. Seed `.gsd/DECISIONS.md` — use the **Decisions** output template below.
diff --git a/src/resources/extensions/gsd/prompts/queue.md b/src/resources/extensions/gsd/prompts/queue.md
index 55406fba7..69bdf5316 100644
--- a/src/resources/extensions/gsd/prompts/queue.md
+++ b/src/resources/extensions/gsd/prompts/queue.md
@@ -79,9 +79,9 @@ Determine where the new milestones should go in the overall sequence. Consider d
 
 ## Output Phase
 
-Once the user is satisfied, in a single pass for **each** new milestone (starting from {{nextId}}):
+Once the user is satisfied, in a single pass for **each** new milestone:
 
-1. `mkdir -p .gsd/milestones/<ID>/slices`
+1. Call `gsd_generate_milestone_id` to get the milestone ID — never invent milestone IDs manually. Then `mkdir -p .gsd/milestones/<ID>/slices`.
 2. Write `.gsd/milestones/<ID>/<ID>-CONTEXT.md` — use the **Context** output template below. Capture intent, scope, risks, constraints, integration points, and relevant requirements. Mark the status as "Queued — pending auto-mode execution." **If this milestone depends on other milestones, add YAML frontmatter with `depends_on`:**
    ```yaml
    ---