diff --git a/src/analysis/cost-benefit.ts b/src/analysis/cost-benefit.ts
index 43cd50c..717ddfb 100644
--- a/src/analysis/cost-benefit.ts
+++ b/src/analysis/cost-benefit.ts
@@ -160,7 +160,7 @@ export function workflowCost(
 ): WorkflowCostResult {
   const propagationMode = options?.propagationMode ?? "dag-propagate";
   const defaultQualityRetention = options?.defaultQualityRetention ?? 0.9;
-  const includeCompleted = options?.includeCompleted ?? true;
+  const includeCompleted = options?.includeCompleted ?? false;
 
   // Get topological order — throws CircularDependencyError if cyclic
   const topoOrder = topologicalOrder(graph);
diff --git a/tasks/implementation/cost-benefit/workflow-cost.md b/tasks/implementation/cost-benefit/workflow-cost.md
index 8228760..531b376 100644
--- a/tasks/implementation/cost-benefit/workflow-cost.md
+++ b/tasks/implementation/cost-benefit/workflow-cost.md
@@ -1,7 +1,7 @@
 ---
 id: cost-benefit/workflow-cost
 name: Implement workflowCost orchestration function
-status: pending
+status: completed
 depends_on:
   - cost-benefit/dag-propagation
   - graph/queries
@@ -17,18 +17,18 @@ Implement `workflowCost(graph: TaskGraph, options?: WorkflowCostOptions): Workfl
 
 ## Acceptance Criteria
 
-- [ ] `workflowCost` accepts `WorkflowCostOptions` with optional: `includeCompleted`, `limit`, `propagationMode`, `defaultQualityRetention`
-- [ ] Default propagation mode: `"dag-propagate"` per ADR-004
-- [ ] Default `defaultQualityRetention`: 0.9
-- [ ] Each task in result includes: `taskId`, `name`, `ev`, `pIntrinsic`, `pEffective`, `probability` (= `pEffective`), `scopeCost`, `impactWeight`
-- [ ] `totalEv`: sum of all task EVs (excluding completed tasks from output when `includeCompleted: false`)
-- [ ] `averageEv`: `totalEv / tasks.length`
-- [ ] `propagationMode`: reflected in result
-- [ ] When `includeCompleted: false`: completed tasks excluded from `tasks` array but remain in propagation chain with p=1.0
-- [ ] When `includeCompleted: false`: only `"completed"` status triggers exclusion; `"failed"` and `"blocked"` are always included
-- [ ] When `limit` is set: returns at most `limit` tasks (sorted by EV descending? or topological order? spec says "limits the number of tasks in the result" — use topological order with limit)
-- [ ] Throws `CircularDependencyError` if graph is cyclic
-- [ ] Unit tests: full workflow cost calculation, independent vs dag-propagate comparison, excludeCompleted scenarios, limit behavior
+- [x] `workflowCost` accepts `WorkflowCostOptions` with optional: `includeCompleted`, `limit`, `propagationMode`, `defaultQualityRetention`
+- [x] Default propagation mode: `"dag-propagate"` per ADR-004
+- [x] Default `defaultQualityRetention`: 0.9
+- [x] Each task in result includes: `taskId`, `name`, `ev`, `pIntrinsic`, `pEffective`, `probability` (= `pEffective`), `scopeCost`, `impactWeight`
+- [x] `totalEv`: sum of all task EVs (excluding completed tasks from output when `includeCompleted: false`)
+- [x] `averageEv`: `totalEv / tasks.length`
+- [x] `propagationMode`: reflected in result
+- [x] When `includeCompleted: false`: completed tasks excluded from `tasks` array but remain in propagation chain with p=1.0
+- [x] When `includeCompleted: false`: only `"completed"` status triggers exclusion; `"failed"` and `"blocked"` are always included
+- [x] When `limit` is set: returns at most `limit` tasks (sorted by EV descending? or topological order? spec says "limits the number of tasks in the result" — use topological order with limit)
+- [x] Throws `CircularDependencyError` if graph is cyclic
+- [x] Unit tests: full workflow cost calculation, independent vs dag-propagate comparison, excludeCompleted scenarios, limit behavior
 
 ## References
 
@@ -38,8 +38,11 @@ Implement `workflowCost(graph: TaskGraph, options?: WorkflowCostOptions): Workfl
 
 ## Notes
 
-> To be filled by implementation agent
+Fixed `includeCompleted` default from `true` to `false` to match the api-surface.md specification. The implementation was mostly complete from prior dependency tasks (dag-propagation), but the default was incorrect. Updated the test suite to verify the correct default behavior and explicitly test the `includeCompleted: true` opt-in case.
 
 ## Summary
 
-> To be filled on completion
\ No newline at end of file
+Implemented `workflowCost` orchestration function and fixed `includeCompleted` default to `false` per api-surface.md.
+- Modified: `src/analysis/cost-benefit.ts` — fixed `includeCompleted` default from `true` to `false`
+- Modified: `test/cost-benefit.test.ts` — updated default-behavior test and added explicit `includeCompleted: true` test
+- Tests: 64 cost-benefit tests, all passing; 562 total tests across 12 test files, all passing
\ No newline at end of file
diff --git a/test/cost-benefit.test.ts b/test/cost-benefit.test.ts
index 49ba977..65de72e 100644
--- a/test/cost-benefit.test.ts
+++ b/test/cost-benefit.test.ts
@@ -774,15 +774,33 @@ describe("workflowCost", () => {
     expect(taskB.pIntrinsic).toBeCloseTo(0.80);
   });
 
-  it("includes completed tasks when includeCompleted=true (default)", () => {
+  it("excludes completed tasks by default (includeCompleted defaults to false)", () => {
     const graph = TaskGraph.fromTasks([
       { id: "A", name: "Task A", dependsOn: [], risk: "high", scope: "narrow", impact: "isolated", status: "completed" },
       { id: "B", name: "Task B", dependsOn: ["A"], risk: "medium", scope: "narrow", impact: "isolated" },
     ]);
 
-    const result = workflowCost(graph.raw); // default includeCompleted=true
+    const result = workflowCost(graph.raw); // default includeCompleted=false
 
-    // A should appear in the task list
+    // A should NOT appear in the task list (default behavior)
+    expect(result.tasks.find(t => t.taskId === "A")).toBeUndefined();
+
+    // B's propagation uses p=1.0 for A (completed)
+    // inheritedFromA = 1.0 + (1-1.0)*0.9 = 1.0
+    // pEffective_B = 0.80 * 1.0 = 0.80
+    const taskB = result.tasks.find(t => t.taskId === "B")!;
+    expect(taskB.pEffective).toBeCloseTo(0.80);
+  });
+
+  it("includes completed tasks when includeCompleted=true", () => {
+    const graph = TaskGraph.fromTasks([
+      { id: "A", name: "Task A", dependsOn: [], risk: "high", scope: "narrow", impact: "isolated", status: "completed" },
+      { id: "B", name: "Task B", dependsOn: ["A"], risk: "medium", scope: "narrow", impact: "isolated" },
+    ]);
+
+    const result = workflowCost(graph.raw, { includeCompleted: true });
+
+    // A should appear in the task list when explicitly included
     const taskA = result.tasks.find(t => t.taskId === "A")!;
     expect(taskA).toBeDefined();