diff --git a/src/runtime/dispatch.zig b/src/runtime/dispatch.zig
index 84dbe3d..553676d 100644
--- a/src/runtime/dispatch.zig
+++ b/src/runtime/dispatch.zig
@@ -13,14 +13,18 @@ const types = @import("types.zig");
 const Backend = types.Backend;
 const ResolvedAgent = types.ResolvedAgent;
 
+const usage = @import("usage.zig");
+
 /// Dispatch an agent run to the appropriate backend.
 /// Writes the agent's text output to `out`.
+/// Records usage for the dispatched backend.
 pub fn dispatch(
     alloc: std.mem.Allocator,
     resolved: ResolvedAgent,
     prompt: []const u8,
     out: *std.ArrayList(u8),
 ) void {
+    usage.global().record(resolved.backend);
     switch (resolved.backend) {
         .claude => spawnClaude(alloc, resolved, prompt, out),
         .codex  => spawnCodex(alloc, resolved, prompt, out),
diff --git a/src/runtime/usage.zig b/src/runtime/usage.zig
new file mode 100644
index 0000000..7205c20
--- /dev/null
+++ b/src/runtime/usage.zig
@@ -0,0 +1,213 @@
+// runtime/usage.zig — Usage tracking (#268)
+//
+// Tracks API calls per backend within a rolling 5-hour window.
+// Used by resolve() to overflow to a secondary provider when the primary
+// is nearing its quota.
+//
+// Thread-safe: all mutations guarded by mutex.
+
+const std = @import("std");
+const types = @import("types.zig");
+const Backend = types.Backend;
+
+const WINDOW_NS: i128 = 5 * 60 * 60 * std.time.ns_per_s;
+
+pub const UsageTracker = struct {
+    claude_calls: u32 = 0,
+    codex_calls: u32 = 0,
+    window_start_ns: i128,
+    mu: std.Thread.Mutex = .{},
+
+    pub fn init() UsageTracker {
+        return .{
+            .window_start_ns = std.time.nanoTimestamp(),
+        };
+    }
+
+    pub fn initWithStart(start_ns: i128) UsageTracker {
+        return .{
+            .window_start_ns = start_ns,
+        };
+    }
+
+    /// Record a call to the given backend. Resets window if expired.
+    pub fn record(self: *UsageTracker, backend: Backend) void {
+        self.mu.lock();
+        defer self.mu.unlock();
+
+        self.maybeResetWindow();
+        switch (backend) {
+            .claude => self.claude_calls += 1,
+            .codex => self.codex_calls += 1,
+        }
+    }
+
+    /// Check if the given backend is near its quota limit.
+    /// `warn_at_percent` is a fraction (0.0 to 1.0), e.g. 0.8 for 80%.
+    /// `quota` is the maximum calls allowed in a 5h window.
+    pub fn nearLimit(self: *UsageTracker, backend: Backend, quota: u32, warn_at_percent: f64) bool {
+        self.mu.lock();
+        defer self.mu.unlock();
+
+        self.maybeResetWindow();
+        const calls = switch (backend) {
+            .claude => self.claude_calls,
+            .codex => self.codex_calls,
+        };
+        if (quota == 0) return false;
+        const threshold: u32 = @intFromFloat(@as(f64, @floatFromInt(quota)) * warn_at_percent);
+        return calls >= threshold;
+    }
+
+    /// Get current call count for a backend.
+    pub fn callCount(self: *UsageTracker, backend: Backend) u32 {
+        self.mu.lock();
+        defer self.mu.unlock();
+
+        self.maybeResetWindow();
+        return switch (backend) {
+            .claude => self.claude_calls,
+            .codex => self.codex_calls,
+        };
+    }
+
+    /// Get total calls across all backends in current window.
+    pub fn totalCalls(self: *UsageTracker) u32 {
+        self.mu.lock();
+        defer self.mu.unlock();
+
+        self.maybeResetWindow();
+        return self.claude_calls + self.codex_calls;
+    }
+
+    /// Time remaining in the current window, in seconds.
+    pub fn windowRemainingSeconds(self: *UsageTracker) u32 {
+        self.mu.lock();
+        defer self.mu.unlock();
+
+        const now = std.time.nanoTimestamp();
+        const elapsed = now - self.window_start_ns;
+        if (elapsed >= WINDOW_NS) return 0;
+        const remaining_ns = WINDOW_NS - elapsed;
+        return @intCast(@divTrunc(remaining_ns, std.time.ns_per_s));
+    }
+
+    fn maybeResetWindow(self: *UsageTracker) void {
+        const now = std.time.nanoTimestamp();
+        if (now - self.window_start_ns >= WINDOW_NS) {
+            self.claude_calls = 0;
+            self.codex_calls = 0;
+            self.window_start_ns = now;
+        }
+    }
+};
+
+// Global singleton — initialized lazily on first use.
+var g_tracker: ?UsageTracker = null;
+var g_init_mu: std.Thread.Mutex = .{};
+
+pub fn global() *UsageTracker {
+    g_init_mu.lock();
+    defer g_init_mu.unlock();
+    if (g_tracker == null) {
+        g_tracker = UsageTracker.init();
+    }
+    return &g_tracker.?;
+}
+
+/// Reset global tracker (for testing).
+pub fn resetGlobal() void {
+    g_init_mu.lock();
+    defer g_init_mu.unlock();
+    g_tracker = null;
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+test "usage: record increments correct backend" {
+    var t = UsageTracker.init();
+    t.record(.claude);
+    t.record(.claude);
+    t.record(.codex);
+
+    try std.testing.expectEqual(@as(u32, 2), t.callCount(.claude));
+    try std.testing.expectEqual(@as(u32, 1), t.callCount(.codex));
+    try std.testing.expectEqual(@as(u32, 3), t.totalCalls());
+}
+
+test "usage: nearLimit triggers at threshold" {
+    var t = UsageTracker.init();
+    // quota=10, warn at 80% = 8 calls
+    for (0..8) |_| t.record(.claude);
+
+    try std.testing.expect(t.nearLimit(.claude, 10, 0.8));
+    try std.testing.expect(!t.nearLimit(.codex, 10, 0.8));
+}
+
+test "usage: nearLimit below threshold" {
+    var t = UsageTracker.init();
+    for (0..7) |_| t.record(.claude);
+
+    try std.testing.expect(!t.nearLimit(.claude, 10, 0.8));
+}
+
+test "usage: nearLimit with zero quota" {
+    var t = UsageTracker.init();
+    t.record(.claude);
+    try std.testing.expect(!t.nearLimit(.claude, 0, 0.8));
+}
+
+test "usage: window reset clears counts" {
+    // Start window 6 hours ago (past the 5h window)
+    const past_ns = std.time.nanoTimestamp() - (6 * 60 * 60 * std.time.ns_per_s);
+    var t = UsageTracker.initWithStart(past_ns);
+    t.claude_calls = 100;
+    t.codex_calls = 50;
+
+    // Accessing counts triggers window reset
+    try std.testing.expectEqual(@as(u32, 0), t.callCount(.claude));
+    try std.testing.expectEqual(@as(u32, 0), t.callCount(.codex));
+}
+
+test "usage: windowRemainingSeconds within window" {
+    var t = UsageTracker.init();
+    const remaining = t.windowRemainingSeconds();
+    // Should be close to 5 hours (18000 seconds)
+    try std.testing.expect(remaining > 17900);
+    try std.testing.expect(remaining <= 18000);
+}
+
+test "usage: windowRemainingSeconds expired window" {
+    const past_ns = std.time.nanoTimestamp() - (6 * 60 * 60 * std.time.ns_per_s);
+    var t = UsageTracker.initWithStart(past_ns);
+    try std.testing.expectEqual(@as(u32, 0), t.windowRemainingSeconds());
+}
+
+test "usage: global singleton" {
+    resetGlobal();
+    const a = global();
+    const b = global();
+    try std.testing.expect(a == b);
+    a.record(.claude);
+    try std.testing.expectEqual(@as(u32, 1), b.callCount(.claude));
+    resetGlobal();
+}
+
+test "usage: concurrent records are safe" {
+    var t = UsageTracker.init();
+    const N = 100;
+
+    const thread_fn = struct {
+        fn run(tracker: *UsageTracker) void {
+            for (0..N) |_| tracker.record(.claude);
+        }
+    }.run;
+
+    var threads: [4]std.Thread = undefined;
+    for (&threads) |*th| {
+        th.* = std.Thread.spawn(.{}, thread_fn, .{&t}) catch unreachable;
+    }
+    for (&threads) |*th| th.join();
+
+    try std.testing.expectEqual(@as(u32, 4 * N), t.callCount(.claude));
+}