diff --git a/packages/loopover-engine/src/index.ts b/packages/loopover-engine/src/index.ts
index 1e7ebe0d83..6c86cd3948 100644
--- a/packages/loopover-engine/src/index.ts
+++ b/packages/loopover-engine/src/index.ts
@@ -593,6 +593,16 @@ export {
   type ResultChangedFile,
   type ResultsPayload,
 } from "./results-payload.js";
+export {
+  buildProgressSnapshot,
+  progressChanged,
+  MAX_PROGRESS_ACTIVITY,
+  type LoopPhase,
+  type LoopProgressActivity,
+  type LoopProgressState,
+  type LoopRunStatus,
+  type ProgressSnapshot,
+} from "./loop-progress.js";
 export {
   buildMetadataRankInput,
   computeMetadataDupRisk,
diff --git a/packages/loopover-engine/src/loop-progress.ts b/packages/loopover-engine/src/loop-progress.ts
new file mode 100644
index 0000000000..6efe71816b
--- /dev/null
+++ b/packages/loopover-engine/src/loop-progress.ts
@@ -0,0 +1,67 @@
+// Loop progress model (pure) — the near-real-time progress a customer watches while their rented loop runs
+// (#4800, part of the Rent-a-Loop path #4778). This owns the DETERMINISTIC brain of the stream: it builds a
+// progress snapshot from already-computed loop state, and decides when the snapshot has meaningfully changed
+// so a customer-facing surface (#4807) can push ON CHANGE rather than poll on a fixed interval. No IO, no
+// transport — a plain in/out transform, mirroring the intake bridge (#4798) and results composer (#4801).
+
+// Cap the streamed activity tail so a long run never floods the surface; the loop's full log lives elsewhere.
+export const MAX_PROGRESS_ACTIVITY = 10;
+
+export type LoopPhase = "queued" | "claiming" | "coding" | "reviewing" | "submitting" | "done";
+export type LoopRunStatus = "running" | "converged" | "abandoned" | "error";
+
+export type LoopProgressActivity = {
+  step: string;
+  detail?: string | undefined;
+  at?: string | undefined;
+};
+
+/** The already-computed state of one running loop, the input to a progress snapshot. */
+export type LoopProgressState = {
+  iteration: number;
+  maxIterations?: number | null | undefined;
+  phase: LoopPhase;
+  status: LoopRunStatus;
+  recentActivity?: LoopProgressActivity[] | undefined;
+};
+
+export type ProgressSnapshot = {
+  phase: LoopPhase;
+  status: LoopRunStatus;
+  iteration: number;
+  maxIterations: number | null;
+  /** Progress through the iteration budget (0-100), or null when the budget is unknown. */
+  percentComplete: number | null;
+  /** The most recent activity, newest last, capped at {@link MAX_PROGRESS_ACTIVITY}. */
+  recentActivity: LoopProgressActivity[];
+  done: boolean;
+};
+
+/** Build a customer-facing progress snapshot from already-computed loop state (#4800). Pure. */
+export function buildProgressSnapshot(state: LoopProgressState): ProgressSnapshot {
+  const maxIterations = state.maxIterations ?? null;
+  const percentComplete =
+    maxIterations !== null && maxIterations > 0 ? Math.min(100, Math.round((state.iteration / maxIterations) * 100)) : null;
+  return {
+    phase: state.phase,
+    status: state.status,
+    iteration: state.iteration,
+    maxIterations,
+    percentComplete,
+    recentActivity: (state.recentActivity ?? []).slice(-MAX_PROGRESS_ACTIVITY),
+    done: state.status !== "running",
+  };
+}
+
+/** True when `next` differs from `prev` in a way worth pushing to the customer — so the surface streams
+ *  ON CHANGE instead of polling on a fixed interval (#4800's acceptance). A null `prev` (the first snapshot)
+ *  always pushes. Compares the displayed axes: phase, status, iteration, and the activity tail's length. */
+export function progressChanged(prev: ProgressSnapshot | null, next: ProgressSnapshot): boolean {
+  if (prev === null) return true;
+  return (
+    prev.phase !== next.phase ||
+    prev.status !== next.status ||
+    prev.iteration !== next.iteration ||
+    prev.recentActivity.length !== next.recentActivity.length
+  );
+}
diff --git a/src/loop-progress.ts b/src/loop-progress.ts
new file mode 100644
index 0000000000..5fa782f5fa
--- /dev/null
+++ b/src/loop-progress.ts
@@ -0,0 +1,5 @@
+// Loop progress model (#4800) — thin re-export shim. The canonical implementation lives in
+// `@loopover/engine` (packages/loopover-engine/src/loop-progress.ts), imported via the relative source
+// path (matching src/results-payload.ts / src/idea-intake.ts) so the published loopover-mcp / loopover-miner
+// CLIs share one model, and so this never depends on the engine's built dist/ during typecheck/test.
+export * from "../packages/loopover-engine/src/loop-progress";
diff --git a/src/mcp/server.ts b/src/mcp/server.ts
index 6b0c69514b..92faccbb07 100644
--- a/src/mcp/server.ts
+++ b/src/mcp/server.ts
@@ -157,6 +157,7 @@ import { buildIssueSlopAssessment } from "../signals/issue-slop";
 import { buildSlopAssessment } from "../signals/slop";
 import { validateIdeaSubmission, buildTaskGraph, buildClaimPlan } from "../idea-intake";
 import { buildResultsPayload } from "../results-payload";
+import { buildProgressSnapshot } from "../loop-progress";
 import { buildStructuralImprovementAssessment } from "../signals/improvement";
 import { buildBoundaryTestGenerationFinding, buildBoundaryTestGenerationSpec } from "../signals/boundary-test-generation";
 import { buildRepoDataQuality } from "../signals/data-quality";
@@ -976,6 +977,28 @@ const buildResultsPayloadOutputSchema = {
   totals: z.unknown().optional(),
 };
 
+// Loop progress-snapshot input (#4800): a running loop's already-computed state.
+const buildProgressSnapshotShape = {
+  iteration: z.number().int(),
+  maxIterations: z.number().int().nullable().optional(),
+  phase: z.enum(["queued", "claiming", "coding", "reviewing", "submitting", "done"]),
+  status: z.enum(["running", "converged", "abandoned", "error"]),
+  recentActivity: z
+    .array(z.object({ step: z.string(), detail: z.string().optional(), at: z.string().optional() }))
+    .max(1000)
+    .optional(),
+};
+
+const buildProgressSnapshotOutputSchema = {
+  phase: z.string().optional(),
+  status: z.string().optional(),
+  iteration: z.number().optional(),
+  maxIterations: z.number().nullable().optional(),
+  percentComplete: z.number().nullable().optional(),
+  recentActivity: z.unknown().optional(),
+  done: z.boolean().optional(),
+};
+
 // Deterministic structural-improvement counterpart to checkSlopRiskShape (#4746, sub-issue I of epic #4737):
 // the positive-axis mirror of checkSlopRisk, same pure local-metadata contract. changedFiles/tests/testFiles
 // are reused verbatim (same shape as checkSlopRiskShape) so the two signals never disagree about what counts
@@ -1759,6 +1782,17 @@ export class LoopoverMcp {
       async (input) => this.toolResult(await this.buildLoopResults(input)),
     );
 
+    server.registerTool(
+      "loopover_build_progress_snapshot",
+      {
+        description:
+          "Build a near-real-time progress snapshot for a running rented loop (#4800): phase, status, iteration/percent-complete, and a bounded recent-activity tail, from already-computed loop state. Deterministic and source-free; a customer surface pushes it on change (via the engine's progressChanged) rather than polling on a fixed interval.",
+        inputSchema: buildProgressSnapshotShape,
+        outputSchema: buildProgressSnapshotOutputSchema,
+      },
+      async (input) => this.toolResult(await this.buildLoopProgress(input)),
+    );
+
     server.registerTool(
       "loopover_check_slop_risk",
       {
@@ -3072,6 +3106,15 @@ export class LoopoverMcp {
     };
   }
 
+  private async buildLoopProgress(input: z.infer<z.ZodObject<typeof buildProgressSnapshotShape>>): Promise<ToolPayload> {
+    await this.enforceToolRateLimit("loopover_build_progress_snapshot");
+    const snapshot = buildProgressSnapshot(input);
+    return {
+      summary: `Loop progress: ${snapshot.phase} (${snapshot.status}), iteration ${snapshot.iteration}.`,
+      data: snapshot as unknown as Record<string, unknown>,
+    };
+  }
+
   private async checkSlopRisk(input: z.infer<z.ZodObject<typeof checkSlopRiskShape>>): Promise<ToolPayload> {
     await this.enforceToolRateLimit("loopover_check_slop_risk");
     const assessment = buildSlopAssessment(input);
diff --git a/test/unit/loop-progress.test.ts b/test/unit/loop-progress.test.ts
new file mode 100644
index 0000000000..4b89cf0535
--- /dev/null
+++ b/test/unit/loop-progress.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildProgressSnapshot,
+  progressChanged,
+  MAX_PROGRESS_ACTIVITY,
+  type LoopProgressState,
+} from "../../packages/loopover-engine/src/loop-progress";
+
+function running(overrides: Partial<LoopProgressState> = {}): LoopProgressState {
+  return { iteration: 2, maxIterations: 5, phase: "coding", status: "running", ...overrides };
+}
+
+describe("buildProgressSnapshot (#4800)", () => {
+  it("builds a snapshot with percent-complete from the iteration budget", () => {
+    const s = buildProgressSnapshot(running({ recentActivity: [{ step: "claimed" }, { step: "coding" }] }));
+    expect(s).toMatchObject({ phase: "coding", status: "running", iteration: 2, maxIterations: 5, percentComplete: 40, done: false });
+    expect(s.recentActivity).toHaveLength(2);
+  });
+
+  it("leaves percent-complete null when the iteration budget is unknown", () => {
+    expect(buildProgressSnapshot(running({ maxIterations: undefined })).percentComplete).toBeNull();
+    expect(buildProgressSnapshot(running({ maxIterations: null })).maxIterations).toBeNull();
+    expect(buildProgressSnapshot(running({ maxIterations: 0 })).percentComplete).toBeNull(); // 0 is not > 0
+  });
+
+  it("caps percent-complete at 100 when iteration exceeds the budget", () => {
+    expect(buildProgressSnapshot(running({ iteration: 7, maxIterations: 5 })).percentComplete).toBe(100);
+  });
+
+  it("defaults recent activity to empty and caps the tail at MAX_PROGRESS_ACTIVITY", () => {
+    expect(buildProgressSnapshot(running()).recentActivity).toEqual([]); // omitted
+    const many = Array.from({ length: MAX_PROGRESS_ACTIVITY + 4 }, (_, i) => ({ step: `s${i}` }));
+    const s = buildProgressSnapshot(running({ recentActivity: many }));
+    expect(s.recentActivity).toHaveLength(MAX_PROGRESS_ACTIVITY);
+    expect(s.recentActivity.at(-1)?.step).toBe(`s${MAX_PROGRESS_ACTIVITY + 3}`); // newest kept
+  });
+
+  it("marks the loop done once its status is no longer running", () => {
+    expect(buildProgressSnapshot(running({ status: "converged" })).done).toBe(true);
+    expect(buildProgressSnapshot(running({ status: "running" })).done).toBe(false);
+  });
+});
+
+describe("progressChanged — push on change, not on a fixed interval (#4800)", () => {
+  const base = buildProgressSnapshot(running({ recentActivity: [{ step: "a" }] }));
+
+  it("always pushes the first snapshot (no prior)", () => {
+    expect(progressChanged(null, base)).toBe(true);
+  });
+
+  it("pushes when phase, status, iteration, or the activity tail changes", () => {
+    expect(progressChanged(base, buildProgressSnapshot(running({ phase: "reviewing", recentActivity: [{ step: "a" }] })))).toBe(true);
+    expect(progressChanged(base, buildProgressSnapshot(running({ status: "converged", recentActivity: [{ step: "a" }] })))).toBe(true);
+    expect(progressChanged(base, buildProgressSnapshot(running({ iteration: 3, recentActivity: [{ step: "a" }] })))).toBe(true);
+    expect(progressChanged(base, buildProgressSnapshot(running({ recentActivity: [{ step: "a" }, { step: "b" }] })))).toBe(true);
+  });
+
+  it("does not push when nothing displayed has changed", () => {
+    expect(progressChanged(base, buildProgressSnapshot(running({ recentActivity: [{ step: "a" }] })))).toBe(false);
+  });
+});
diff --git a/test/unit/mcp-loop-progress.test.ts b/test/unit/mcp-loop-progress.test.ts
new file mode 100644
index 0000000000..161681ce67
--- /dev/null
+++ b/test/unit/mcp-loop-progress.test.ts
@@ -0,0 +1,42 @@
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { InMemoryTransport } from "@modelcontextprotocol/sdk/inMemory.js";
+import { describe, expect, it } from "vitest";
+import { LoopoverMcp } from "../../src/mcp/server";
+import { createTestEnv } from "../helpers/d1";
+
+async function connect() {
+  const server = new LoopoverMcp(createTestEnv()).createServer();
+  const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+  await server.connect(serverTransport);
+  const client = new Client({ name: "gittensory-progress-test", version: "0.1.0" }, { capabilities: {} });
+  await client.connect(clientTransport);
+  return client;
+}
+
+describe("MCP loopover_build_progress_snapshot", () => {
+  it("builds a progress snapshot for a running loop", async () => {
+    const client = await connect();
+    const result = await client.callTool({
+      name: "loopover_build_progress_snapshot",
+      arguments: {
+        iteration: 2, maxIterations: 5, phase: "coding", status: "running",
+        recentActivity: [{ step: "claimed issue-1" }, { step: "editing src/upload.ts" }],
+      },
+    });
+    expect(result.isError).toBeFalsy();
+    const data = result.structuredContent as { phase: string; status: string; iteration: number; percentComplete: number; done: boolean; recentActivity: unknown[] };
+    expect(data).toMatchObject({ phase: "coding", status: "running", iteration: 2, percentComplete: 40, done: false });
+    expect(data.recentActivity).toHaveLength(2);
+  });
+
+  it("marks a finished loop done", async () => {
+    const client = await connect();
+    const result = await client.callTool({
+      name: "loopover_build_progress_snapshot",
+      arguments: { iteration: 3, phase: "done", status: "converged" },
+    });
+    const data = result.structuredContent as { done: boolean; percentComplete: number | null };
+    expect(data.done).toBe(true);
+    expect(data.percentComplete).toBeNull(); // no maxIterations given
+  });
+});