diff --git a/src/resources/extensions/gsd/guided-flow-queue.ts b/src/resources/extensions/gsd/guided-flow-queue.ts
index 5b0b21e94..1a5e10aa3 100644
--- a/src/resources/extensions/gsd/guided-flow-queue.ts
+++ b/src/resources/extensions/gsd/guided-flow-queue.ts
@@ -244,12 +244,22 @@ export async function buildExistingMilestonesContext(
     }
   }
 
-  // For each milestone, include context and status
+  // For each milestone, include context and status.
+  // Completed milestones get a compact summary line only — loading their full
+  // CONTEXT.md + SUMMARY.md files is expensive and triggers 429 rate limits on
+  // projects with many completed milestones (#2379).
   for (const mid of milestoneIds) {
     const registryEntry = state.registry.find(m => m.id === mid);
     const status = registryEntry?.status ?? "unknown";
     const title = registryEntry?.title ?? mid;
 
+    // Completed milestones: emit a one-liner — the LLM only needs to know
+    // they exist for dedup/dependency purposes, not their full content.
+    if (status === "complete") {
+      sections.push(`### ${mid}: ${title}\n**Status:** complete`);
+      continue;
+    }
+
     const parts: string[] = [];
     parts.push(`### ${mid}: ${title}\n**Status:** ${status}`);
 
@@ -271,17 +281,6 @@ export async function buildExistingMilestonesContext(
       }
     }
 
-    // For completed milestones, include the summary if it exists
-    if (status === "complete") {
-      const summaryFile = resolveMilestoneFile(basePath, mid, "SUMMARY");
-      if (summaryFile) {
-        const content = await loadFile(summaryFile);
-        if (content) {
-          parts.push(`\n**Summary:**\n${content.trim()}`);
-        }
-      }
-    }
-
     // For active/pending/parked milestones, include the roadmap if it exists
     // (shows what's planned but not yet built)
     if (status === "active" || status === "pending" || status === "parked") {
diff --git a/src/resources/extensions/gsd/tests/queue-completed-milestone-perf.test.ts b/src/resources/extensions/gsd/tests/queue-completed-milestone-perf.test.ts
new file mode 100644
index 000000000..75c1e871a
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/queue-completed-milestone-perf.test.ts
@@ -0,0 +1,155 @@
+/**
+ * Regression test for #2379: /gsd queue fails with 429 rate limit on projects
+ * with many completed milestones.
+ *
+ * The bug: buildExistingMilestonesContext iterates over ALL milestones
+ * (including completed ones) and calls loadFile for CONTEXT, SUMMARY,
+ * CONTEXT-DRAFT, and ROADMAP files on each — causing excessive I/O that
+ * triggers rate limits on large projects.
+ *
+ * The fix: completed milestones should emit a short summary line without
+ * loading their heavy artifact files (CONTEXT.md, SUMMARY.md, etc.).
+ */
+
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+
+import { buildExistingMilestonesContext } from "../guided-flow-queue.ts";
+import type { GSDState, MilestoneRegistryEntry } from "../types.ts";
+import { createTestContext } from "./test-helpers.ts";
+
+const { assertTrue, assertEq, report } = createTestContext();
+
+// ─── Fixture: project with many completed milestones ─────────────────────
+
+const tmpBase = mkdtempSync(join(tmpdir(), "gsd-queue-perf-"));
+const gsd = join(tmpBase, ".gsd");
+mkdirSync(join(gsd, "milestones"), { recursive: true });
+
+const COMPLETED_COUNT = 25;
+const ACTIVE_COUNT = 1;
+const PENDING_COUNT = 2;
+
+const allMilestoneIds: string[] = [];
+const registry: MilestoneRegistryEntry[] = [];
+
+// Create 25 completed milestones with CONTEXT.md and SUMMARY.md files
+for (let i = 1; i <= COMPLETED_COUNT; i++) {
+  const mid = `M${String(i).padStart(3, "0")}`;
+  allMilestoneIds.push(mid);
+  registry.push({ id: mid, title: `Completed milestone ${i}`, status: "complete" });
+  mkdirSync(join(gsd, "milestones", mid), { recursive: true });
+  writeFileSync(
+    join(gsd, "milestones", mid, `${mid}-CONTEXT.md`),
+    `# ${mid}: Completed milestone ${i}\n\nThis is a large context document for ${mid}.\n${"Lorem ipsum dolor sit amet. ".repeat(50)}\n`,
+  );
+  writeFileSync(
+    join(gsd, "milestones", mid, `${mid}-SUMMARY.md`),
+    `# ${mid} Summary\n\nDelivered feature ${i} successfully.\n`,
+  );
+}
+
+// Create 1 active milestone
+{
+  const mid = `M${String(COMPLETED_COUNT + 1).padStart(3, "0")}`;
+  allMilestoneIds.push(mid);
+  registry.push({ id: mid, title: "Active milestone", status: "active" });
+  mkdirSync(join(gsd, "milestones", mid), { recursive: true });
+  writeFileSync(
+    join(gsd, "milestones", mid, `${mid}-CONTEXT.md`),
+    `# ${mid}: Active milestone\n\nCurrently in progress.\n`,
+  );
+  writeFileSync(
+    join(gsd, "milestones", mid, `${mid}-ROADMAP.md`),
+    `# ${mid} Roadmap\n\nSlices planned.\n`,
+  );
+}
+
+// Create 2 pending milestones
+for (let i = 0; i < PENDING_COUNT; i++) {
+  const mid = `M${String(COMPLETED_COUNT + ACTIVE_COUNT + 1 + i).padStart(3, "0")}`;
+  allMilestoneIds.push(mid);
+  registry.push({ id: mid, title: `Pending milestone ${i + 1}`, status: "pending" });
+  mkdirSync(join(gsd, "milestones", mid), { recursive: true });
+  writeFileSync(
+    join(gsd, "milestones", mid, `${mid}-CONTEXT.md`),
+    `# ${mid}: Pending milestone ${i + 1}\n\nQueued work.\n`,
+  );
+}
+
+const state: GSDState = {
+  activeMilestone: { id: `M${String(COMPLETED_COUNT + 1).padStart(3, "0")}`, title: "Active milestone" },
+  activeSlice: null,
+  activeTask: null,
+  phase: "executing",
+  recentDecisions: [],
+  blockers: [],
+  nextAction: "",
+  registry,
+};
+
+// ─── Test: completed milestones should NOT have their files loaded ────────
+
+console.log("\n=== Queue completed milestone performance (#2379) ===");
+
+const context = await buildExistingMilestonesContext(tmpBase, allMilestoneIds, state);
+
+// Active and pending milestones SHOULD have full context loaded
+const activeMid = `M${String(COMPLETED_COUNT + 1).padStart(3, "0")}`;
+assertTrue(
+  context.includes("Currently in progress"),
+  "Active milestone context content should be loaded",
+);
+assertTrue(
+  context.includes("Slices planned"),
+  "Active milestone roadmap should be loaded",
+);
+
+for (let i = 0; i < PENDING_COUNT; i++) {
+  const mid = `M${String(COMPLETED_COUNT + ACTIVE_COUNT + 1 + i).padStart(3, "0")}`;
+  assertTrue(
+    context.includes(`Pending milestone ${i + 1}`),
+    `Pending milestone ${mid} context should be loaded`,
+  );
+}
+
+// Completed milestones should NOT have their CONTEXT.md body or SUMMARY.md
+// content loaded — only a status line
+for (let i = 1; i <= COMPLETED_COUNT; i++) {
+  const mid = `M${String(i).padStart(3, "0")}`;
+
+  // Should still mention the milestone ID and status
+  assertTrue(
+    context.includes(mid),
+    `Completed milestone ${mid} should still be referenced`,
+  );
+
+  // Should NOT contain the heavy context body text
+  assertTrue(
+    !context.includes(`This is a large context document for ${mid}`),
+    `Completed milestone ${mid} should NOT have its full CONTEXT.md body loaded`,
+  );
+
+  // Should NOT contain the summary body
+  assertTrue(
+    !context.includes(`Delivered feature ${i} successfully`),
+    `Completed milestone ${mid} should NOT have its SUMMARY.md body loaded`,
+  );
+}
+
+// ─── Test: the overall context should be reasonable in size ──────────────
+
+// With 25 completed milestones NOT loading files, the context should be
+// significantly smaller than if all files were loaded
+const contextLines = context.split("\n").length;
+assertTrue(
+  contextLines < 200,
+  `Context should be concise (got ${contextLines} lines); completed milestones should not inflate it`,
+);
+
+// ─── Cleanup ──────────────────────────────────────────────────────────────
+
+rmSync(tmpBase, { recursive: true, force: true });
+
+report();