diff --git a/apps/mcp-server/src/tools/complete-task.ts b/apps/mcp-server/src/tools/complete-task.ts
new file mode 100644
index 0000000..9233ef1
--- /dev/null
+++ b/apps/mcp-server/src/tools/complete-task.ts
@@ -0,0 +1,140 @@
+import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import { z } from "zod";
+import { db } from "../db.js";
+import { eq } from "../drizzle.js";
+import { agentRuns, auditLog, markdownBacklogItems } from "../schema.js";
+import { toolCatch, toolErr, toolOk } from "./tool-result.js";
+
+const outcomeSchema = z.enum(["succeeded", "failed", "cancelled", "stalled"]);
+
+const finalStatusSchema = z.enum([
+  "done",
+  "blocked",
+  "ready",
+  "in_progress",
+  "cancelled",
+]);
+
+const tokenCount = z.number().int().nonnegative();
+
+const completeTaskInputSchema = z.object({
+  runId: z.string().uuid(),
+  outcome: outcomeSchema,
+  tokensInput: tokenCount.optional(),
+  tokensOutput: tokenCount.optional(),
+  tokensTotal: tokenCount.optional(),
+  notes: z.string().max(2000).optional(),
+  error: z.string().max(2000).optional(),
+  finalStatus: finalStatusSchema.optional(),
+});
+
+type CompleteTaskInput = z.infer<typeof completeTaskInputSchema>;
+
+function resolveTokensTotal(input: CompleteTaskInput): number | null {
+  if (typeof input.tokensTotal === "number") return input.tokensTotal;
+  if (
+    typeof input.tokensInput === "number" &&
+    typeof input.tokensOutput === "number"
+  ) {
+    return input.tokensInput + input.tokensOutput;
+  }
+  return null;
+}
+
+function resolveFinalStatus(
+  input: CompleteTaskInput,
+): z.infer<typeof finalStatusSchema> | null {
+  if (input.finalStatus) return input.finalStatus;
+  if (input.outcome === "succeeded") return "done";
+  if (input.outcome === "failed") return "blocked";
+  return null;
+}
+
+export function registerCompleteTaskTool(mcp: McpServer): void {
+  mcp.registerTool(
+    "complete_task",
+    {
+      description:
+        "Close an agent_runs row at session end with an outcome and token totals. Optionally flips the backlog item to a terminal status. Errors if the run is already closed.",
+      inputSchema: completeTaskInputSchema,
+    },
+    async (args) => {
+      try {
+        const input = completeTaskInputSchema.parse(args);
+
+        const [run] = await db
+          .select()
+          .from(agentRuns)
+          .where(eq(agentRuns.id, input.runId))
+          .limit(1);
+
+        if (!run) return toolErr("Run not found");
+        if (run.finishedAt !== null) {
+          return toolErr(
+            `Run already finished (outcome: ${run.outcome ?? "unknown"})`,
+          );
+        }
+
+        const [backlogItem] = await db
+          .select()
+          .from(markdownBacklogItems)
+          .where(eq(markdownBacklogItems.id, run.backlogItemId))
+          .limit(1);
+
+        if (!backlogItem || backlogItem.workspaceId !== run.workspaceId) {
+          return toolErr("Backlog item missing");
+        }
+
+        const tokensTotal = resolveTokensTotal(input);
+        const finalStatus = resolveFinalStatus(input);
+        const finishedAt = new Date();
+
+        await db.transaction(async (tx) => {
+          await tx
+            .update(agentRuns)
+            .set({
+              finishedAt,
+              outcome: input.outcome,
+              tokensInput: input.tokensInput ?? null,
+              tokensOutput: input.tokensOutput ?? null,
+              tokensTotal,
+              notes: input.notes ?? null,
+              error: input.error ?? null,
+            })
+            .where(eq(agentRuns.id, input.runId));
+
+          if (finalStatus) {
+            await tx
+              .update(markdownBacklogItems)
+              .set({ status: finalStatus, updatedAt: new Date() })
+              .where(eq(markdownBacklogItems.id, run.backlogItemId));
+          }
+
+          await tx.insert(auditLog).values({
+            workspaceId: run.workspaceId,
+            actorUserId: run.actorUserId ?? null,
+            action: "task.completed",
+            targetType: "agent_run",
+            targetId: input.runId,
+            metadata: {
+              outcome: input.outcome,
+              finalStatus,
+              backlogItemId: run.backlogItemId,
+              tokensTotal,
+            },
+          });
+        });
+
+        return toolOk({
+          runId: input.runId,
+          finishedAt: finishedAt.toISOString(),
+          finalStatus: finalStatus ?? null,
+          tokensTotal,
+          outcome: input.outcome,
+        });
+      } catch (e) {
+        return toolCatch(e);
+      }
+    },
+  );
+}