diff --git a/packages/loopover-engine/src/miner/worktree-pool.ts b/packages/loopover-engine/src/miner/worktree-pool.ts
index 5520b6e44f..2302131c93 100644
--- a/packages/loopover-engine/src/miner/worktree-pool.ts
+++ b/packages/loopover-engine/src/miner/worktree-pool.ts
@@ -41,9 +41,18 @@ export function isWorktreeAllocated(state: WorktreePoolState, attemptId: string)
   return state.allocations.some((allocation) => allocation.attemptId === attemptId);
 }
 
+/** Normalize a possibly-untrusted concurrency cap before it gates allocation: a non-finite (`NaN`/`Infinity`),
+ *  negative, or fractional `maxConcurrency` can never disable the cap or yield a `NaN` slot count — it floors to
+ *  a non-negative integer, so an invalid value behaves as the documented non-positive cap (allocates nothing).
+ *  Mirrors the `finiteNonNegativeInt` discipline already applied in governor/rate-limit.ts, governor/budget-cap.ts,
+ *  and tenant-quota.ts (#5828). */
+function finiteNonNegativeInt(value: number): number {
+  return Number.isFinite(value) ? Math.max(0, Math.floor(value)) : 0;
+}
+
 /** Slots still available before the concurrency cap (never negative). Pure. */
 export function availableWorktreeSlots(state: WorktreePoolState, config: WorktreePoolConfig): number {
-  return Math.max(0, config.maxConcurrency - state.allocations.length);
+  return Math.max(0, finiteNonNegativeInt(config.maxConcurrency) - state.allocations.length);
 }
 
 /**
@@ -60,7 +69,7 @@ export function acquireWorktree(
   if (isWorktreeAllocated(state, input.attemptId)) {
     return { ok: false, reason: "already_allocated", state };
   }
-  if (state.allocations.length >= config.maxConcurrency) {
+  if (state.allocations.length >= finiteNonNegativeInt(config.maxConcurrency)) {
     return { ok: false, reason: "at_capacity", state };
   }
   const allocation: WorktreeAllocation = {
diff --git a/test/unit/miner-worktree-pool.test.ts b/test/unit/miner-worktree-pool.test.ts
index d67829d18b..b3c3abb051 100644
--- a/test/unit/miner-worktree-pool.test.ts
+++ b/test/unit/miner-worktree-pool.test.ts
@@ -78,4 +78,38 @@ describe("worktree pool allocator (#4297)", () => {
     const s = acquired(acquired(EMPTY_WORKTREE_POOL, "a"), "b");
     expect(availableWorktreeSlots(s, { maxConcurrency: 1 })).toBe(0);
   });
+
+  it("treats a NaN maxConcurrency as a zero cap instead of disabling the limit", () => {
+    const cfg = { maxConcurrency: NaN };
+    expect(availableWorktreeSlots(EMPTY_WORKTREE_POOL, cfg)).toBe(0);
+    const r = acquireWorktree(EMPTY_WORKTREE_POOL, cfg, { attemptId: "a", repoPath: REPO });
+    expect(r.ok).toBe(false);
+    if (r.ok) return;
+    expect(r.reason).toBe("at_capacity");
+  });
+
+  it("treats a negative maxConcurrency as a zero cap", () => {
+    const cfg = { maxConcurrency: -1 };
+    expect(availableWorktreeSlots(EMPTY_WORKTREE_POOL, cfg)).toBe(0);
+    const r = acquireWorktree(EMPTY_WORKTREE_POOL, cfg, { attemptId: "a", repoPath: REPO });
+    expect(r.ok).toBe(false);
+    if (r.ok) return;
+    expect(r.reason).toBe("at_capacity");
+  });
+
+  it("floors a fractional maxConcurrency to the nearest integer", () => {
+    const cfg = { maxConcurrency: 2.5 };
+    expect(availableWorktreeSlots(EMPTY_WORKTREE_POOL, cfg)).toBe(2);
+    const one = acquireWorktree(EMPTY_WORKTREE_POOL, cfg, { attemptId: "a", repoPath: REPO });
+    expect(one.ok).toBe(true);
+    if (!one.ok) return;
+    const two = acquireWorktree(one.state, cfg, { attemptId: "b", repoPath: REPO });
+    expect(two.ok).toBe(true);
+    if (!two.ok) return;
+    expect(availableWorktreeSlots(two.state, cfg)).toBe(0);
+    const three = acquireWorktree(two.state, cfg, { attemptId: "c", repoPath: REPO });
+    expect(three.ok).toBe(false);
+    if (three.ok) return;
+    expect(three.reason).toBe("at_capacity");
+  });
 });