diff --git a/packages/gittensory-miner/lib/harness-submission-trigger.d.ts b/packages/gittensory-miner/lib/harness-submission-trigger.d.ts
index 1a223030a..c18dc01ad 100644
--- a/packages/gittensory-miner/lib/harness-submission-trigger.d.ts
+++ b/packages/gittensory-miner/lib/harness-submission-trigger.d.ts
@@ -44,3 +44,26 @@ export type HarnessSubmissionResult = {
 export function countConsecutiveGateBlocks(eventLedger: HarnessSubmissionEventLedger, sinceMs: number): number;
 
 export function evaluateAndRecordHarnessSubmissionTrigger(candidate: HarnessSubmissionCandidateInput, deps: HarnessSubmissionDeps): HarnessSubmissionResult;
+
+/** The exact input shape buildOpenPrSpec (root src/mcp/local-write-tools.ts) expects. */
+export type OpenPrInput = {
+  repoFullName: string;
+  base: string;
+  head: string;
+  title: string;
+  body: string;
+  draft: boolean;
+};
+
+export type PrepareOpenPrSubmissionCandidate = HarnessSubmissionCandidateInput & {
+  base: string;
+  title: string;
+  body?: string;
+  draft?: boolean;
+};
+
+export type PrepareOpenPrSubmissionResult =
+  | { ready: true; decision: HarnessSubmissionDecision; event: HarnessSubmissionResult["event"]; openPrInput: OpenPrInput }
+  | { ready: false; decision: HarnessSubmissionDecision; event: HarnessSubmissionResult["event"] };
+
+export function prepareOpenPrSubmission(candidate: PrepareOpenPrSubmissionCandidate, deps: HarnessSubmissionDeps): PrepareOpenPrSubmissionResult;
diff --git a/packages/gittensory-miner/lib/harness-submission-trigger.js b/packages/gittensory-miner/lib/harness-submission-trigger.js
index 9eb580aa6..c6039052a 100644
--- a/packages/gittensory-miner/lib/harness-submission-trigger.js
+++ b/packages/gittensory-miner/lib/harness-submission-trigger.js
@@ -7,11 +7,16 @@ import { evaluateHarnessSubmissionTrigger } from "@jsonbored/gittensory-engine";
 // event -- regardless of outcome, so a paused-pending-human-review session leaves a full trail of why.
 //
 // NOT WIRED INTO ANY AUTOMATIC SCHEDULE: per this issue's own "manual owner sign-off on the wiring before this
-// ships to any default-on profile" deliverable. A real call site (root-side server/CLI integration) invokes
-// this function with a real `HandoffPacket`; on `allow: true` it may then build the `open_pr` local-write spec
-// itself -- this module does not, and cannot, do that (the spec builder lives in the private root `src/` tree,
-// unreachable from this package -- same cross-package-boundary reason self-review-adapter.ts's slop injection
-// exists).
+// ships to any default-on profile" deliverable. `prepareOpenPrSubmission` below is the call site up to the
+// cross-package boundary: on `allow: true` it shapes the exact input `buildOpenPrSpec` (root
+// `src/mcp/local-write-tools.ts`) needs -- but does not, and cannot, call that function itself, since the spec
+// builder lives in the private root `src/` tree, unreachable from this package (same cross-package-boundary
+// reason self-review-adapter.ts's slop injection exists). A real root-side/MCP call site (e.g. the existing
+// `gittensory_open_pr` tool, src/mcp/server.ts) takes `openPrInput` from a `ready: true` result and passes it
+// to `buildOpenPrSpec` (or the equivalent tool call) to actually produce the runnable local-write spec. The
+// CLI/driver entrypoint that instantiates a real `CodingAgentDriver` and calls `runIterateLoop` end to end with
+// live credentials does not exist yet in this package -- that is separate, larger scope from this decision-to-
+// payload bridge.
 //
 // SESSION-SCOPED, NOT PER-REPO: the circuit breaker's own "pauses the run entirely" wording means the tally is
 // counted across EVERY repo's decisions this session, not scoped to one repo -- distinct from #2338's loop-
@@ -82,3 +87,52 @@ export function evaluateAndRecordHarnessSubmissionTrigger(candidate, deps) {
 
   return { decision, event };
 }
+
+/**
+ * Bridge one completed handoff through the submission gate to a submission-READY payload -- the exact input
+ * shape `buildOpenPrSpec` expects (repoFullName/base/head/title/body/draft). On `allow: true` returns
+ * `{ ready: true, decision, event, openPrInput }`; otherwise `{ ready: false, decision, event }` -- the block
+ * reasons are on `decision.reasons` and already on the ledger via the wrapped call either way. Does NOT call
+ * `buildOpenPrSpec` itself (see this module's own doc comment for why it cannot) -- a real root-side/MCP call
+ * site takes `openPrInput` from a `ready: true` result and passes it to that function or the equivalent
+ * `gittensory_open_pr` MCP tool.
+ *
+ * Fails closed (throws) on a malformed candidate, mirroring evaluateAndRecordHarnessSubmissionTrigger's own
+ * validation -- a missing PR title/base is a caller bug that must never silently degrade into a garbage spec.
+ * The one field evaluateAndRecordHarnessSubmissionTrigger does NOT itself require -- handoffPacket.branchRef,
+ * optional there because iterate-loop.ts deliberately does not manage worktrees/branches -- IS required here,
+ * but only once the decision is known to be `allow: true`: a PR cannot be opened without a source branch, but a
+ * blocked candidate needs no branch at all, and must not throw for a reason unrelated to why it was blocked.
+ *
+ * @param {{ killSwitchScope: "global"|"repo"|"none", repoFullName: string, handoffPacket: { branchRef?: string, [key: string]: unknown }, slopThreshold: "clean"|"low"|"elevated"|"high", mode: "observe"|"enforce", maxConsecutiveGateBlocks?: number, base: string, title: string, body?: string, draft?: boolean }} candidate
+ * @param {{ eventLedger: object, sessionStartMs?: number }} deps
+ */
+export function prepareOpenPrSubmission(candidate, deps) {
+  if (!candidate || typeof candidate !== "object") throw new Error("invalid_harness_submission_candidate");
+  const base = typeof candidate.base === "string" ? candidate.base.trim() : "";
+  if (!base) throw new Error("invalid_pr_base");
+  const title = typeof candidate.title === "string" ? candidate.title.trim() : "";
+  if (!title) throw new Error("invalid_pr_title");
+
+  const { decision, event } = evaluateAndRecordHarnessSubmissionTrigger(candidate, deps);
+  if (!decision.allow) return { ready: false, decision, event };
+
+  // Only reached once evaluateAndRecordHarnessSubmissionTrigger has already validated handoffPacket is a
+  // well-formed object -- safe to read .branchRef directly.
+  const head = typeof candidate.handoffPacket.branchRef === "string" ? candidate.handoffPacket.branchRef.trim() : "";
+  if (!head) throw new Error("invalid_pr_head_branch");
+
+  return {
+    ready: true,
+    decision,
+    event,
+    openPrInput: {
+      repoFullName: candidate.repoFullName.trim(),
+      base,
+      head,
+      title,
+      body: typeof candidate.body === "string" ? candidate.body : "",
+      draft: candidate.draft === true,
+    },
+  };
+}
diff --git a/test/unit/miner-harness-submission-trigger.test.ts b/test/unit/miner-harness-submission-trigger.test.ts
index 77df563f2..78adabc3a 100644
--- a/test/unit/miner-harness-submission-trigger.test.ts
+++ b/test/unit/miner-harness-submission-trigger.test.ts
@@ -7,7 +7,12 @@ vi.mock("@jsonbored/gittensory-engine", async () => {
   return import("../../packages/gittensory-engine/src/index");
 });
 
-import { evaluateAndRecordHarnessSubmissionTrigger, countConsecutiveGateBlocks, HARNESS_SUBMISSION_TRIGGER_DECISION_EVENT } from "../../packages/gittensory-miner/lib/harness-submission-trigger.js";
+import {
+  evaluateAndRecordHarnessSubmissionTrigger,
+  countConsecutiveGateBlocks,
+  prepareOpenPrSubmission,
+  HARNESS_SUBMISSION_TRIGGER_DECISION_EVENT,
+} from "../../packages/gittensory-miner/lib/harness-submission-trigger.js";
 import { initEventLedger } from "../../packages/gittensory-miner/lib/event-ledger.js";
 
 const roots: string[] = [];
@@ -285,3 +290,148 @@ describe("evaluateAndRecordHarnessSubmissionTrigger (#2337)", () => {
     ).toThrow("invalid_event_ledger");
   });
 });
+
+describe("prepareOpenPrSubmission (#2337 open-pr call site)", () => {
+  it("shapes a ready:true openPrInput exactly matching buildOpenPrSpec's expected fields when the gate allows", () => {
+    const eventLedger = tempEventLedger();
+
+    const result = prepareOpenPrSubmission(
+      {
+        killSwitchScope: "none",
+        repoFullName: "acme/widgets",
+        handoffPacket: handoffPacket({ branchRef: "attempt/1" }),
+        slopThreshold: "low",
+        mode: "enforce",
+        base: "main",
+        title: "fix: add retry logic",
+        body: "Closes #1.",
+        draft: true,
+      },
+      { eventLedger },
+    );
+
+    expect(result.ready).toBe(true);
+    if (!result.ready) throw new Error("expected ready:true");
+    expect(result.decision.allow).toBe(true);
+    expect(result.openPrInput).toEqual({
+      repoFullName: "acme/widgets",
+      base: "main",
+      head: "attempt/1",
+      title: "fix: add retry logic",
+      body: "Closes #1.",
+      draft: true,
+    });
+  });
+
+  it("returns ready:false (no openPrInput) when the gate blocks, without requiring a branch to open a PR from at all", () => {
+    const eventLedger = tempEventLedger();
+
+    // Kill-switch active AND no branchRef on the handoff -- proves the head-branch check never runs on the
+    // blocked path (a candidate that will never open a PR must not throw for an unrelated missing-field reason).
+    const result = prepareOpenPrSubmission(
+      {
+        killSwitchScope: "global",
+        repoFullName: "acme/widgets",
+        handoffPacket: handoffPacket(),
+        slopThreshold: "low",
+        mode: "enforce",
+        base: "main",
+        title: "fix: add retry logic",
+      },
+      { eventLedger },
+    );
+
+    expect(result.ready).toBe(false);
+    expect(result.decision).toEqual({ allow: false, reasons: ["global_kill_switch_active"], circuitBreakerTripped: false });
+    expect((result as { openPrInput?: unknown }).openPrInput).toBeUndefined();
+  });
+
+  it("throws invalid_pr_base on a missing/blank base, before any gate evaluation or ledger write", () => {
+    const eventLedger = tempEventLedger();
+    expect(() =>
+      prepareOpenPrSubmission(
+        { killSwitchScope: "none", repoFullName: "acme/widgets", handoffPacket: handoffPacket({ branchRef: "attempt/1" }), slopThreshold: "low", mode: "enforce", title: "t" } as never,
+        { eventLedger },
+      ),
+    ).toThrow("invalid_pr_base");
+    expect(() =>
+      prepareOpenPrSubmission(
+        { killSwitchScope: "none", repoFullName: "acme/widgets", handoffPacket: handoffPacket({ branchRef: "attempt/1" }), slopThreshold: "low", mode: "enforce", base: "   ", title: "t" },
+        { eventLedger },
+      ),
+    ).toThrow("invalid_pr_base");
+    expect(eventLedger.readEvents({})).toHaveLength(0); // failed before the wrapped gate call ever wrote an event
+  });
+
+  it("throws invalid_pr_title on a missing/blank title, before any gate evaluation or ledger write", () => {
+    const eventLedger = tempEventLedger();
+    expect(() =>
+      prepareOpenPrSubmission(
+        { killSwitchScope: "none", repoFullName: "acme/widgets", handoffPacket: handoffPacket({ branchRef: "attempt/1" }), slopThreshold: "low", mode: "enforce", base: "main" } as never,
+        { eventLedger },
+      ),
+    ).toThrow("invalid_pr_title");
+    expect(eventLedger.readEvents({})).toHaveLength(0);
+  });
+
+  it("throws invalid_pr_head_branch when the gate allows but the handoff packet has no branch to open a PR from -- the allow decision is still recorded to the ledger", () => {
+    const eventLedger = tempEventLedger();
+
+    expect(() =>
+      prepareOpenPrSubmission(
+        {
+          killSwitchScope: "none",
+          repoFullName: "acme/widgets",
+          handoffPacket: handoffPacket(), // no branchRef
+          slopThreshold: "low",
+          mode: "enforce",
+          base: "main",
+          title: "t",
+        },
+        { eventLedger },
+      ),
+    ).toThrow("invalid_pr_head_branch");
+
+    // The gate's own allow:true decision is still on the audit trail even though the spec-build step failed.
+    const events = eventLedger.readEvents({ repoFullName: "acme/widgets" });
+    expect(events).toHaveLength(1);
+    expect(events[0]?.payload).toMatchObject({ allow: true });
+  });
+
+  it("defaults draft to false and body to an empty string when omitted", () => {
+    const eventLedger = tempEventLedger();
+    const result = prepareOpenPrSubmission(
+      { killSwitchScope: "none", repoFullName: "acme/widgets", handoffPacket: handoffPacket({ branchRef: "attempt/1" }), slopThreshold: "low", mode: "enforce", base: "main", title: "t" },
+      { eventLedger },
+    );
+    expect(result.ready).toBe(true);
+    if (!result.ready) throw new Error("expected ready:true");
+    expect(result.openPrInput.draft).toBe(false);
+    expect(result.openPrInput.body).toBe("");
+  });
+
+  it("trims repoFullName/base/title/head", () => {
+    const eventLedger = tempEventLedger();
+    const result = prepareOpenPrSubmission(
+      {
+        killSwitchScope: "none",
+        repoFullName: " acme/widgets ",
+        handoffPacket: handoffPacket({ branchRef: "  attempt/1  " }),
+        slopThreshold: "low",
+        mode: "enforce",
+        base: "  main  ",
+        title: "  t  ",
+      },
+      { eventLedger },
+    );
+    expect(result.ready).toBe(true);
+    if (!result.ready) throw new Error("expected ready:true");
+    expect(result.openPrInput).toMatchObject({ repoFullName: "acme/widgets", base: "main", head: "attempt/1", title: "t" });
+  });
+
+  it("fails closed on a malformed candidate (null, or a non-object primitive) rather than silently allowing", () => {
+    const eventLedger = tempEventLedger();
+    expect(() => prepareOpenPrSubmission(null as never, { eventLedger })).toThrow("invalid_harness_submission_candidate");
+    expect(() => prepareOpenPrSubmission("nope" as never, { eventLedger })).toThrow("invalid_harness_submission_candidate");
+  });
+});