diff --git a/lib/artdag/conformance.sh b/lib/artdag/conformance.sh
index 0551abb3..ff32eaf2 100755
--- a/lib/artdag/conformance.sh
+++ b/lib/artdag/conformance.sh
@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute optimize fed)
+SUITES=(dag analyze plan execute optimize fed cost)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -47,6 +47,7 @@ run_suite() {
 (load "lib/artdag/execute.sx")
 (load "lib/artdag/optimize.sx")
 (load "lib/artdag/federation.sx")
+(load "lib/artdag/cost.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")
diff --git a/lib/artdag/cost.sx b/lib/artdag/cost.sx
new file mode 100644
index 00000000..6cbf9547
--- /dev/null
+++ b/lib/artdag/cost.sx
@@ -0,0 +1,66 @@
+; lib/artdag/cost.sx — cost model for the scheduler: per-node weights, critical
+; path (min makespan with unlimited parallelism), plan makespan under batching/cap,
+; total serial work, and the resulting speedup. Costs come from an injected
+; cost-fn (op params) -> number so media-op costs stay opaque. Depends on dag.sx.
+
+(define artdag/const-cost (fn (op params) 1))
+
+(define
+  artdag/op-cost
+  (fn
+    (table)
+    (fn (op params) (if (has-key? table op) (get table op) 1))))
+
+(define
+  artdag/-node-cost
+  (fn
+    (dag cost-fn id)
+    (let
+      ((n (artdag/dag-get dag id)))
+      (cost-fn (artdag/node-op n) (artdag/node-params n)))))
+
+(define
+  artdag/-max
+  (fn (xs) (reduce (fn (mx x) (if (> x mx) x mx)) 0 xs)))
+
+; longest weighted path through the dag = makespan with unlimited workers.
+(define
+  artdag/critical-path
+  (fn
+    (dag cost-fn)
+    (let
+      ((ft (reduce (fn (m id) (let ((maxdep (artdag/-max (map (fn (d) (get m d)) (artdag/node-inputs (artdag/dag-get dag id)))))) (assoc m id (+ (artdag/-node-cost dag cost-fn id) maxdep)))) {} (artdag/dag-order dag))))
+      (artdag/-max (map (fn (id) (get ft id)) (keys ft))))))
+
+; estimated wall-clock for a plan: each batch runs in parallel (costs its
+; slowest node), batches run in sequence.
+(define
+  artdag/makespan
+  (fn
+    (dag plan cost-fn)
+    (reduce
+      (fn
+        (total batch)
+        (+
+          total
+          (artdag/-max
+            (map (fn (id) (artdag/-node-cost dag cost-fn id)) batch))))
+      0
+      plan)))
+
+; total serial work = sum of all node costs.
+(define
+  artdag/total-work
+  (fn
+    (dag cost-fn)
+    (reduce
+      (fn (s id) (+ s (artdag/-node-cost dag cost-fn id)))
+      0
+      (keys (artdag/dag-nodes dag)))))
+
+; speedup of a plan vs running everything serially.
+(define
+  artdag/speedup
+  (fn
+    (dag plan cost-fn)
+    (/ (artdag/total-work dag cost-fn) (artdag/makespan dag plan cost-fn))))
diff --git a/lib/artdag/scoreboard.json b/lib/artdag/scoreboard.json
index 1c4bedd3..e707a6d3 100644
--- a/lib/artdag/scoreboard.json
+++ b/lib/artdag/scoreboard.json
@@ -5,9 +5,10 @@
     "plan": {"pass": 18, "fail": 0},
     "execute": {"pass": 15, "fail": 0},
     "optimize": {"pass": 18, "fail": 0},
-    "fed": {"pass": 15, "fail": 0}
+    "fed": {"pass": 15, "fail": 0},
+    "cost": {"pass": 13, "fail": 0}
   },
-  "total_pass": 102,
+  "total_pass": 115,
   "total_fail": 0,
-  "total": 102
+  "total": 115
 }
diff --git a/lib/artdag/scoreboard.md b/lib/artdag/scoreboard.md
index e5d97fb9..6440f9d5 100644
--- a/lib/artdag/scoreboard.md
+++ b/lib/artdag/scoreboard.md
@@ -10,4 +10,5 @@ _Generated by `lib/artdag/conformance.sh`_
 | execute | 15 | 0 | 15 |
 | optimize | 18 | 0 | 18 |
 | fed | 15 | 0 | 15 |
-| **Total** | **102** | **0** | **102** |
+| cost | 13 | 0 | 13 |
+| **Total** | **115** | **0** | **115** |
diff --git a/lib/artdag/tests/cost.sx b/lib/artdag/tests/cost.sx
new file mode 100644
index 00000000..fbc71da5
--- /dev/null
+++ b/lib/artdag/tests/cost.sx
@@ -0,0 +1,117 @@
+; cost model: critical path, makespan under cap, total work, speedup.
+
+(define
+  cost-CHAIN
+  (artdag/build
+    (list
+      (list "a" "in" (list) {})
+      (list "b" "f" (list "a") {})
+      (list "c" "f" (list "b") {})
+      (list "d" "f" (list "c") {}))))
+
+(define
+  cost-DIA
+  (artdag/build
+    (list
+      (list "a" "in" (list) {})
+      (list "b" "f" (list "a") {})
+      (list "c" "g" (list "a") {})
+      (list "d" "add" (list "b" "c") {} true))))
+
+(define cost-W (artdag/op-cost {:f 2 :add 5}))
+
+; ---- unit cost ----
+
+(artdag-test
+  "critical path: chain is its length"
+  (artdag/critical-path cost-CHAIN artdag/const-cost)
+  4)
+
+(artdag-test
+  "critical path: diamond longest path"
+  (artdag/critical-path cost-DIA artdag/const-cost)
+  3)
+
+(artdag-test
+  "total work: unit cost equals node count"
+  (artdag/total-work cost-DIA artdag/const-cost)
+  4)
+
+(artdag-test
+  "single node critical path is its cost"
+  (artdag/critical-path
+    (artdag/build (list (list "a" "in" (list) {})))
+    artdag/const-cost)
+  1)
+
+; ---- makespan vs cap ----
+
+(artdag-test
+  "full plan makespan equals critical path"
+  (artdag/makespan
+    cost-DIA
+    (artdag/plan cost-DIA 0)
+    artdag/const-cost)
+  (artdag/critical-path cost-DIA artdag/const-cost))
+
+(artdag-test
+  "serial plan makespan equals total work"
+  (artdag/makespan
+    cost-DIA
+    (artdag/plan cost-DIA 1)
+    artdag/const-cost)
+  (artdag/total-work cost-DIA artdag/const-cost))
+
+(artdag-test
+  "capped makespan is never below the critical path"
+  (>=
+    (artdag/makespan
+      cost-DIA
+      (artdag/plan cost-DIA 1)
+      artdag/const-cost)
+    (artdag/critical-path cost-DIA artdag/const-cost))
+  true)
+
+; ---- weighted costs ----
+
+(artdag-test
+  "weighted critical path follows heavy ops"
+  (artdag/critical-path cost-DIA cost-W)
+  8)
+
+(artdag-test
+  "weighted total work sums all node costs"
+  (artdag/total-work cost-DIA cost-W)
+  9)
+
+(artdag-test
+  "op-cost defaults unknown ops to 1"
+  (artdag/total-work
+    (artdag/build (list (list "a" "in" (list) {})))
+    cost-W)
+  1)
+
+(artdag-test
+  "weighted full-plan makespan equals critical path"
+  (artdag/makespan cost-DIA (artdag/plan cost-DIA 0) cost-W)
+  (artdag/critical-path cost-DIA cost-W))
+
+; ---- speedup ----
+
+(artdag-test
+  "serial plan has no speedup"
+  (artdag/speedup
+    cost-DIA
+    (artdag/plan cost-DIA 1)
+    artdag/const-cost)
+  1)
+
+(artdag-test
+  "parallel plan beats serial"
+  (>
+    (artdag/speedup
+      cost-DIA
+      (artdag/plan cost-DIA 0)
+      artdag/const-cost)
+    1)
+  true)
diff --git a/plans/artdag-on-sx.md b/plans/artdag-on-sx.md
index 95e168b0..af9ee3a7 100644
--- a/plans/artdag-on-sx.md
+++ b/plans/artdag-on-sx.md
@@ -30,7 +30,9 @@ edges.
 
 ## Status (rolling)
 
-`bash lib/artdag/conformance.sh` → **102/102** (6 suites: dag, analyze, plan, execute, optimize, fed)
+`bash lib/artdag/conformance.sh` → **115/115** (7 suites: dag, analyze, plan, execute, optimize, fed, cost)
+
+Base roadmap (Phases 1–6) COMPLETE. Now extending.
 
 ## Ground rules
 
@@ -136,6 +138,15 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Progress log
 
+- **Ext: cost-based scheduling** (cost suite 13/13, total 115/115).
+  `lib/artdag/cost.sx`: an injected `cost-fn (op params)` keeps media-op costs opaque
+  (`const-cost`, `op-cost table`). `critical-path` = longest weighted path (finish-time
+  fold over topo order) = min makespan with unlimited workers. `makespan dag plan
+  cost-fn` sums each batch's slowest node — full plan (cap 0) makespan == critical
+  path, serial (cap 1) == `total-work`. `speedup` = total-work / makespan. Verified
+  weighted paths follow heavy ops and capped makespan never dips below the critical
+  path.
+
 - **Phase 6 — Federation (shared content-addressed cache)** (fed suite 15/15, total
   102/102). `lib/artdag/federation.sx`: an instance = `{:cache <persist kv> :prov
   {cid->origin-peer}}`. `fed-export` dumps the whole cache as `{:cid :result :peer}`