diff --git a/packages/gittensory-engine/src/index.ts b/packages/gittensory-engine/src/index.ts
index cb7ff228d2..af7bd6f173 100644
--- a/packages/gittensory-engine/src/index.ts
+++ b/packages/gittensory-engine/src/index.ts
@@ -155,6 +155,15 @@ export {
   type AttemptLogEventType,
   type NormalizedAttemptLogEvent,
 } from "./miner/attempt-log.js";
+export {
+  ACCEPTANCE_CRITERIA_FILENAME,
+  ACCEPTANCE_CRITERIA_VERSION,
+  buildAcceptanceCriteria,
+  serializeAcceptanceCriteria,
+  shouldWriteAcceptanceCriteria,
+  type AcceptanceCriteria,
+  type AcceptanceCriteriaInput,
+} from "./miner/acceptance-criteria.js";
 export {
   codingAgentModeExecutes,
   isGlobalMinerCodingAgentPause,
diff --git a/packages/gittensory-engine/src/miner/acceptance-criteria.ts b/packages/gittensory-engine/src/miner/acceptance-criteria.ts
new file mode 100644
index 0000000000..d60e763799
--- /dev/null
+++ b/packages/gittensory-engine/src/miner/acceptance-criteria.ts
@@ -0,0 +1,105 @@
+import { sanitizePromptPacketField, type PromptPacket } from "../prompt-packet.js";
+import type { FeasibilityGateResult, FeasibilityVerdict } from "../feasibility.js";
+
+// Acceptance-criteria composer (#4271). Before a coding-agent driver (#4262's interface; #4266/#4267's
+// implementations) starts editing, the miner pins down — immutably, so the agent cannot quietly redefine its own
+// success bar mid-attempt — what "done" means for this attempt. This module is the pure composition step: it folds
+// the two already-shipped Phase 2 primitives — the sanitized `PromptPacket` (prompt-packet.ts, the analyze→coding-
+// agent "boundary membrane") and the `FeasibilityGateResult` go/raise/avoid verdict (feasibility.ts) — into one
+// document. Producing the document is this module's job; actually writing it into the attempt's worktree is the
+// worktree primitive's (#4269), and handing it to the driver is the driver interface's (#4262) — neither is here,
+// so this stays pure and side-effect-free like the rest of gittensory-engine.
+//
+// DECISIONS this file makes (per the issue's open questions):
+// - Serialization format: JSON, not markdown. The acceptance criteria are an immutable, checksum-verifiable success
+//   bar consumed by tooling (a self-review step must be able to prove the target did not move across iterations), so
+//   a deterministic, canonically-ordered JSON document beats prose. `serializeAcceptanceCriteria` emits stable-key-
+//   order JSON with a trailing newline so a checksum recorded alongside stays byte-stable.
+// - Filename: a single fixed name, `ACCEPTANCE_CRITERIA_FILENAME`.
+// - Immutability: the built document is deep-frozen (`Object.freeze`, arrays copied+frozen) so it cannot be mutated
+//   in-memory for the lifetime of the attempt; the byte-stable serialization is what a caller checksums on disk.
+// - Written only on `go`: a `raise`/`avoid` verdict means the attempt should not start, so no criteria file is
+//   written. The builder still returns a document (with `writable: false`) so a caller can log *why* it was skipped;
+//   `shouldWriteAcceptanceCriteria` is the gate for the write itself.
+//
+// Redaction is delegated to `sanitizePromptPacketField` rather than re-implemented: this document is exactly as
+// exposed to a prompt-injectable coding-agent session as the prompt packet, so it gets the same scrub (idempotent
+// on already-sanitized packet text).
+
+/** Fixed on-disk filename for the per-attempt acceptance-criteria document written into the attempt worktree. */
+export const ACCEPTANCE_CRITERIA_FILENAME = "acceptance-criteria.json";
+
+/** Schema version of the serialized document; bump on any field-shape change. */
+export const ACCEPTANCE_CRITERIA_VERSION = 1;
+
+/** Inputs to the composer: the sanitized prompt packet and the feasibility verdict for this attempt. */
+export type AcceptanceCriteriaInput = {
+  promptPacket: PromptPacket;
+  feasibility: FeasibilityGateResult;
+};
+
+/** The composed, immutable per-attempt success bar. All fields are read-only; the document is frozen once built. */
+export type AcceptanceCriteria = {
+  readonly version: number;
+  readonly verdict: FeasibilityVerdict;
+  /** Whether this attempt is authorized to start (and therefore the file should be written): `verdict === "go"`. */
+  readonly writable: boolean;
+  readonly taskBrief: string;
+  readonly constraints: string;
+  readonly feasibilityNotes: string;
+  readonly retrievalContext: string;
+  readonly feasibilitySummary: string;
+  readonly avoidReasons: readonly string[];
+  readonly raiseReasons: readonly string[];
+};
+
+/**
+ * Only a `go` feasibility verdict authorizes the attempt to start, so only `go` gets an acceptance-criteria file
+ * written to the worktree. `raise`/`avoid` should be handled upstream (the attempt does not begin) — this predicate
+ * is the single source of truth for that gate.
+ */
+export function shouldWriteAcceptanceCriteria(verdict: FeasibilityVerdict): boolean {
+  return verdict === "go";
+}
+
+/**
+ * Pure builder: compose a {@link PromptPacket} and a {@link FeasibilityGateResult} into one immutable
+ * acceptance-criteria document. Text fields are re-sanitized with {@link sanitizePromptPacketField} (idempotent),
+ * and the returned document is deep-frozen so it cannot be mutated for the lifetime of the attempt.
+ */
+export function buildAcceptanceCriteria(input: AcceptanceCriteriaInput): AcceptanceCriteria {
+  const { promptPacket, feasibility } = input;
+  return Object.freeze({
+    version: ACCEPTANCE_CRITERIA_VERSION,
+    verdict: feasibility.verdict,
+    writable: shouldWriteAcceptanceCriteria(feasibility.verdict),
+    taskBrief: sanitizePromptPacketField(promptPacket.taskBrief),
+    constraints: sanitizePromptPacketField(promptPacket.constraints),
+    feasibilityNotes: sanitizePromptPacketField(promptPacket.feasibilityNotes),
+    retrievalContext: sanitizePromptPacketField(promptPacket.retrievalContext),
+    feasibilitySummary: feasibility.summary,
+    avoidReasons: Object.freeze([...feasibility.avoidReasons]),
+    raiseReasons: Object.freeze([...feasibility.raiseReasons]),
+  });
+}
+
+/**
+ * Deterministic canonical JSON serialization of a built document: fixed key order (independent of the input
+ * object's own key order) plus a trailing newline, so a checksum recorded alongside the file stays byte-stable
+ * across processes. `JSON.parse` round-trips it back to the same field values.
+ */
+export function serializeAcceptanceCriteria(doc: AcceptanceCriteria): string {
+  const ordered = {
+    version: doc.version,
+    verdict: doc.verdict,
+    writable: doc.writable,
+    taskBrief: doc.taskBrief,
+    constraints: doc.constraints,
+    feasibilityNotes: doc.feasibilityNotes,
+    retrievalContext: doc.retrievalContext,
+    feasibilitySummary: doc.feasibilitySummary,
+    avoidReasons: [...doc.avoidReasons],
+    raiseReasons: [...doc.raiseReasons],
+  };
+  return `${JSON.stringify(ordered, null, 2)}\n`;
+}
diff --git a/test/unit/miner-acceptance-criteria.test.ts b/test/unit/miner-acceptance-criteria.test.ts
new file mode 100644
index 0000000000..c9e7c539b0
--- /dev/null
+++ b/test/unit/miner-acceptance-criteria.test.ts
@@ -0,0 +1,119 @@
+import { describe, expect, it } from "vitest";
+import {
+  ACCEPTANCE_CRITERIA_FILENAME,
+  ACCEPTANCE_CRITERIA_VERSION,
+  buildAcceptanceCriteria,
+  serializeAcceptanceCriteria,
+  shouldWriteAcceptanceCriteria,
+  buildFeasibilityVerdict,
+  type AcceptanceCriteria,
+  type PromptPacket,
+} from "../../packages/gittensory-engine/src/index";
+
+const CLEAN_PACKET: PromptPacket = {
+  taskBrief: "Fix the off-by-one in the pagination cursor.",
+  feasibilityNotes: "Unclaimed, no dup cluster, issue ready.",
+  retrievalContext: "cursor.ts handles the page window.",
+  constraints: "No public API changes; add a unit test.",
+};
+
+const GO = buildFeasibilityVerdict({ claimStatus: "unclaimed", duplicateClusterRisk: "none", issueStatus: "ready" });
+const RAISE = buildFeasibilityVerdict({ claimStatus: "claimed", duplicateClusterRisk: "none", issueStatus: "ready" });
+const AVOID = buildFeasibilityVerdict({ claimStatus: "solved", duplicateClusterRisk: "none", issueStatus: "ready" });
+
+describe("acceptance-criteria composer (#4271)", () => {
+  it("re-exports the composer API from the engine barrel", () => {
+    expect(typeof buildAcceptanceCriteria).toBe("function");
+    expect(typeof serializeAcceptanceCriteria).toBe("function");
+    expect(typeof shouldWriteAcceptanceCriteria).toBe("function");
+    expect(ACCEPTANCE_CRITERIA_FILENAME).toBe("acceptance-criteria.json");
+    expect(ACCEPTANCE_CRITERIA_VERSION).toBe(1);
+  });
+
+  it("composes a go-verdict document from both inputs and marks it writable", () => {
+    const doc = buildAcceptanceCriteria({ promptPacket: CLEAN_PACKET, feasibility: GO });
+    expect(doc.version).toBe(ACCEPTANCE_CRITERIA_VERSION);
+    expect(doc.verdict).toBe("go");
+    expect(doc.writable).toBe(true);
+    expect(doc.taskBrief).toBe(CLEAN_PACKET.taskBrief);
+    expect(doc.constraints).toBe(CLEAN_PACKET.constraints);
+    expect(doc.feasibilityNotes).toBe(CLEAN_PACKET.feasibilityNotes);
+    expect(doc.retrievalContext).toBe(CLEAN_PACKET.retrievalContext);
+    expect(doc.feasibilitySummary).toBe(GO.summary);
+    expect(doc.avoidReasons).toEqual([]);
+    expect(doc.raiseReasons).toEqual([]);
+  });
+
+  it("delegates redaction to the prompt-packet scrubber (unsafe terms + local paths)", () => {
+    const dirty: PromptPacket = {
+      taskBrief: "Stop leaking the wallet hotkey in the payout log.",
+      feasibilityNotes: "Notes live at /home/miner/notes.txt on the box.",
+      retrievalContext: "Windows path C:\\Users\\miner\\ctx.md is fine to mention.",
+      constraints: "Do not change the reward weighting.",
+    };
+    const doc = buildAcceptanceCriteria({ promptPacket: dirty, feasibility: GO });
+    expect(doc.taskBrief).toBe("Stop leaking the [redacted] [redacted] in the [redacted] log.");
+    expect(doc.feasibilityNotes).toBe("Notes live at <local-path> on the box.");
+    expect(doc.retrievalContext).toBe("Windows path <local-path> is fine to mention.");
+    expect(doc.constraints).toBe("Do not change the [redacted] weighting.");
+  });
+
+  it("marks a raise verdict non-writable and carries its raiseReasons", () => {
+    const doc = buildAcceptanceCriteria({ promptPacket: CLEAN_PACKET, feasibility: RAISE });
+    expect(doc.verdict).toBe("raise");
+    expect(doc.writable).toBe(false);
+    expect(doc.raiseReasons).toEqual(["claim_status_claimed"]);
+    expect(doc.avoidReasons).toEqual([]);
+  });
+
+  it("marks an avoid verdict non-writable and carries its avoidReasons", () => {
+    const doc = buildAcceptanceCriteria({ promptPacket: CLEAN_PACKET, feasibility: AVOID });
+    expect(doc.verdict).toBe("avoid");
+    expect(doc.writable).toBe(false);
+    expect(doc.avoidReasons).toEqual(["claim_status_solved"]);
+    expect(doc.raiseReasons).toEqual([]);
+  });
+
+  it("gates the write on a go verdict only", () => {
+    expect(shouldWriteAcceptanceCriteria("go")).toBe(true);
+    expect(shouldWriteAcceptanceCriteria("raise")).toBe(false);
+    expect(shouldWriteAcceptanceCriteria("avoid")).toBe(false);
+  });
+
+  it("deep-freezes the built document so the success bar cannot be mutated mid-attempt", () => {
+    const doc = buildAcceptanceCriteria({ promptPacket: CLEAN_PACKET, feasibility: AVOID });
+    expect(Object.isFrozen(doc)).toBe(true);
+    expect(Object.isFrozen(doc.avoidReasons)).toBe(true);
+    expect(() => {
+      (doc as { taskBrief: string }).taskBrief = "tampered";
+    }).toThrow();
+    expect(() => {
+      (doc.avoidReasons as string[]).push("injected");
+    }).toThrow();
+  });
+
+  it("serializes deterministically with a stable key order and trailing newline", () => {
+    const doc = buildAcceptanceCriteria({ promptPacket: CLEAN_PACKET, feasibility: GO });
+    const a = serializeAcceptanceCriteria(doc);
+    const b = serializeAcceptanceCriteria(doc);
+    expect(a).toBe(b);
+    expect(a.endsWith("\n")).toBe(true);
+    expect(a.startsWith('{\n  "version": 1,')).toBe(true);
+    expect(JSON.parse(a)).toMatchObject({ verdict: "go", writable: true, taskBrief: CLEAN_PACKET.taskBrief });
+
+    // Canonical order is independent of the input object's own key order.
+    const shuffled: AcceptanceCriteria = {
+      raiseReasons: [],
+      avoidReasons: [],
+      feasibilitySummary: GO.summary,
+      retrievalContext: CLEAN_PACKET.retrievalContext,
+      feasibilityNotes: CLEAN_PACKET.feasibilityNotes,
+      constraints: CLEAN_PACKET.constraints,
+      taskBrief: CLEAN_PACKET.taskBrief,
+      writable: true,
+      verdict: "go",
+      version: ACCEPTANCE_CRITERIA_VERSION,
+    };
+    expect(serializeAcceptanceCriteria(shuffled)).toBe(a);
+  });
+});