diff --git a/packages/loopover-miner/lib/governor-state.ts b/packages/loopover-miner/lib/governor-state.ts
index 94f6f65407..867197cd9f 100644
--- a/packages/loopover-miner/lib/governor-state.ts
+++ b/packages/loopover-miner/lib/governor-state.ts
@@ -63,6 +63,11 @@ export type GovernorState = {
   savePauseState(pauseState: GovernorPauseInput): GovernorPauseState;
   loadReputationHistory(repoFullName: string, apiBaseUrl?: string): RepoOutcomeHistory;
   saveReputationHistory(repoFullName: string, history: RepoOutcomeHistory, apiBaseUrl?: string): RepoOutcomeHistory;
+  incrementReputationHistory(
+    repoFullName: string,
+    delta: { decided?: number; unfavorable?: number },
+    apiBaseUrl?: string,
+  ): RepoOutcomeHistory;
   recordOwnSubmission(record: OwnSubmissionRecord): OwnSubmissionRecord;
   listRecentOwnSubmissions(filter?: ListRecentOwnSubmissionsFilter): OwnSubmissionRecord[];
   /** Delete every repo-scoped row for one repo across both governor tables (#7091); returns total rows removed. */
@@ -394,6 +399,24 @@ export function openGovernorState(dbPath: string = resolveGovernorStateDbPath())
       upsertReputationStatement.run(normalizedForge, normalizedRepo, decided, unfavorable, new Date().toISOString());
       return { decided, unfavorable };
     },
+    incrementReputationHistory(
+      repoFullName: string,
+      delta: { decided?: number; unfavorable?: number },
+      apiBaseUrl?: string,
+    ): RepoOutcomeHistory {
+      const normalizedForge = normalizeApiBaseUrl(apiBaseUrl);
+      const normalizedRepo = normalizeRepoFullName(repoFullName);
+      const decidedDelta = Number.isInteger(delta?.decided) ? Number(delta.decided) : 0;
+      const unfavorableDelta = Number.isInteger(delta?.unfavorable) ? Number(delta.unfavorable) : 0;
+      return withTransaction(() => {
+        const row = getReputationStatement.get(normalizedForge, normalizedRepo) as ReputationHistoryRow | undefined;
+        const prior = row ? { decided: row.decided, unfavorable: row.unfavorable } : { ...DEFAULT_REPUTATION_HISTORY };
+        const decided = prior.decided + decidedDelta;
+        const unfavorable = prior.unfavorable + unfavorableDelta;
+        upsertReputationStatement.run(normalizedForge, normalizedRepo, decided, unfavorable, new Date().toISOString());
+        return { decided, unfavorable };
+      });
+    },
     recordOwnSubmission(record: OwnSubmissionRecord): OwnSubmissionRecord {
       const normalized = normalizeRepoFullName(record?.repoFullName);
       if (typeof record?.fingerprint !== "string" || !record.fingerprint.trim()) {
@@ -471,6 +494,14 @@ export function saveReputationHistory(repoFullName: string, history: RepoOutcome
   return getDefaultGovernorState().saveReputationHistory(repoFullName, history, apiBaseUrl);
 }
 
+export function incrementReputationHistory(
+  repoFullName: string,
+  delta: { decided?: number; unfavorable?: number },
+  apiBaseUrl?: string,
+): RepoOutcomeHistory {
+  return getDefaultGovernorState().incrementReputationHistory(repoFullName, delta, apiBaseUrl);
+}
+
 export function recordOwnSubmission(record: OwnSubmissionRecord): OwnSubmissionRecord {
   return getDefaultGovernorState().recordOwnSubmission(record);
 }
diff --git a/packages/loopover-miner/lib/loop-cli.ts b/packages/loopover-miner/lib/loop-cli.ts
index 13080976b5..a388ceba9f 100644
--- a/packages/loopover-miner/lib/loop-cli.ts
+++ b/packages/loopover-miner/lib/loop-cli.ts
@@ -583,13 +583,9 @@ export async function runLoop(args: string[], options: RunLoopOptions = {}): Pro
             // `unfavorable` only on a closed-without-merge (rejection-state-machine.js's isRejectedPr, matching
             // #5655's own-rejection classification). Forge-scoped by claimed.apiBaseUrl (#5563), like every other
             // governor-state write here.
-            const priorReputation = governorState.loadReputationHistory(claimed.repoFullName, claimed.apiBaseUrl);
-            governorState.saveReputationHistory(
+            governorState.incrementReputationHistory(
               claimed.repoFullName,
-              {
-                decided: priorReputation.decided + 1,
-                unfavorable: priorReputation.unfavorable + (isRejectedPr(prDisposition) ? 1 : 0),
-              },
+              { decided: 1, unfavorable: isRejectedPr(prDisposition) ? 1 : 0 },
               claimed.apiBaseUrl,
             );
             reentryOutcome = classifyPrDisposition(prDisposition) as "merged" | "disengaged" | "other";
diff --git a/test/fixtures/miner-concurrent-stores/increment-reputation-child.mjs b/test/fixtures/miner-concurrent-stores/increment-reputation-child.mjs
new file mode 100644
index 0000000000..07b75586c7
--- /dev/null
+++ b/test/fixtures/miner-concurrent-stores/increment-reputation-child.mjs
@@ -0,0 +1,39 @@
+#!/usr/bin/env node
+// Cross-process helper for governor reputation-history concurrent-race tests (#8855).
+// Opens the shared governor-state db, waits for a stdin "go" signal, then calls incrementReputationHistory()
+// so multiple Node processes contend on the same governor_reputation_history row via the same dbPath.
+import { openGovernorState } from "../../../packages/loopover-miner/dist/lib/governor-state.js";
+
+const [dbPath, repoFullName, decidedDeltaStr, unfavorableDeltaStr] = process.argv.slice(2);
+if (!dbPath || !repoFullName || !decidedDeltaStr || !unfavorableDeltaStr) {
+  process.stderr.write(
+    "usage: increment-reputation-child.mjs <dbPath> <repoFullName> <decidedDelta> <unfavorableDelta>\n",
+  );
+  process.exit(2);
+}
+
+const state = openGovernorState(dbPath);
+let started = false;
+
+function runIncrement() {
+  if (started) return;
+  started = true;
+  try {
+    const history = state.incrementReputationHistory(repoFullName, {
+      decided: Number(decidedDeltaStr),
+      unfavorable: Number(unfavorableDeltaStr),
+    });
+    process.stdout.write(`${JSON.stringify({ ok: true, history })}\n`);
+    process.exit(0);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    process.stdout.write(`${JSON.stringify({ ok: false, message })}\n`);
+    process.exit(1);
+  } finally {
+    state.close();
+  }
+}
+
+process.stdin.setEncoding("utf8");
+process.stdin.on("data", () => runIncrement());
+process.stdout.write("READY\n");
diff --git a/test/unit/miner-concurrent-store-races.test.ts b/test/unit/miner-concurrent-store-races.test.ts
index 643bb25c6e..b3322d3240 100644
--- a/test/unit/miner-concurrent-store-races.test.ts
+++ b/test/unit/miner-concurrent-store-races.test.ts
@@ -5,10 +5,11 @@ import { dirname, join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { afterEach, describe, expect, it } from "vitest";
 import { openClaimLedger } from "../../packages/loopover-miner/lib/claim-ledger.js";
+import { openGovernorState } from "../../packages/loopover-miner/lib/governor-state.js";
 import { initPortfolioQueueStore } from "../../packages/loopover-miner/lib/portfolio-queue.js";
 
-// Real cross-process concurrency coverage for the claim-ledger and portfolio-queue stores (#4867). Only the
-// worktree-allocator had a dedicated multi-process collision test before this; claim-ledger/portfolio-queue
+// Real cross-process concurrency coverage for the claim-ledger, portfolio-queue, and governor reputation-history
+// stores (#4867, #8855). Only the worktree-allocator had a dedicated multi-process collision test before this;
 // atomicity was previously only exercised per-function (single process). This spawns two real Node child
 // processes racing the same on-disk SQLite file and asserts no double-claim/double-dequeue or corrupted state
 // results — the store's own atomic UPSERT/UPDATE...RETURNING statements are what's under test, not the
@@ -22,6 +23,10 @@ const dequeueChildScript = join(
   dirname(fileURLToPath(import.meta.url)),
   "../fixtures/miner-concurrent-stores/dequeue-child.mjs",
 );
+const incrementReputationChildScript = join(
+  dirname(fileURLToPath(import.meta.url)),
+  "../fixtures/miner-concurrent-stores/increment-reputation-child.mjs",
+);
 
 const roots: string[] = [];
 
@@ -212,3 +217,49 @@ describe("portfolio-queue cross-process races (#4867)", () => {
     expect(exitCode).toBe(2);
   });
 });
+
+type IncrementReputationChildResult = {
+  ok: boolean;
+  history?: { decided: number; unfavorable: number };
+  message?: string;
+};
+
+describe("governor reputation-history cross-process races (#8855)", () => {
+  it("two processes racing incrementReputationHistory() on the same repo both apply their deltas", async () => {
+    const { dbPath } = tempRoot();
+    const bootstrap = openGovernorState(dbPath);
+    bootstrap.saveReputationHistory("acme/widgets", { decided: 0, unfavorable: 0 });
+    bootstrap.close();
+
+    const children = [
+      spawnChild(incrementReputationChildScript, [dbPath, "acme/widgets", "1", "0"]),
+      spawnChild(incrementReputationChildScript, [dbPath, "acme/widgets", "1", "1"]),
+    ];
+    const results = await runBarriered<IncrementReputationChildResult>(children);
+
+    expect(results.every((result) => result.ok)).toBe(true);
+    const histories = results.map((result) => result.history);
+    expect(histories).toHaveLength(2);
+    expect(histories).toContainEqual({ decided: 2, unfavorable: 1 });
+    expect(
+      histories.some((history) => history?.decided === 1 && history.unfavorable === 0) ||
+        histories.some((history) => history?.decided === 1 && history.unfavorable === 1),
+    ).toBe(true);
+
+    const state = openGovernorState(dbPath);
+    try {
+      expect(state.loadReputationHistory("acme/widgets")).toEqual({ decided: 2, unfavorable: 1 });
+    } finally {
+      state.close();
+    }
+  });
+
+  it("rejects the increment-reputation-child helper when required args are missing", async () => {
+    const child = spawn(process.execPath, [incrementReputationChildScript], { stdio: ["ignore", "pipe", "pipe"] });
+    const exitCode = await new Promise<number | null>((resolve, reject) => {
+      child.once("error", reject);
+      child.once("exit", resolve);
+    });
+    expect(exitCode).toBe(2);
+  });
+});
diff --git a/test/unit/miner-governor-state.test.ts b/test/unit/miner-governor-state.test.ts
index 76a907ba76..0096bae121 100644
--- a/test/unit/miner-governor-state.test.ts
+++ b/test/unit/miner-governor-state.test.ts
@@ -5,6 +5,7 @@ import { DatabaseSync } from "node:sqlite";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   closeDefaultGovernorState,
+  incrementReputationHistory,
   loadPauseState,
   loadReputationHistory,
   openGovernorState,
@@ -386,6 +387,85 @@ describe("governor-state reputation history (#5134)", () => {
   });
 });
 
+describe("governor-state reputation-history increment atomicity (#8855)", () => {
+  it("REGRESSION: overlapping load/save increments from two sibling connections lose a count without a transaction", () => {
+    const root = mkdtempSync(join(tmpdir(), "loopover-miner-governor-state-reputation-race-"));
+    roots.push(root);
+    const dbPath = join(root, "shared-governor-state.sqlite3");
+    const processA = openGovernorState(dbPath);
+    const processB = openGovernorState(dbPath);
+    states.push(processA, processB);
+
+    processA.saveReputationHistory("acme/widgets", { decided: 0, unfavorable: 0 });
+
+    // Classic read-modify-write overlap: both loads see decided=0 before either save commits.
+    const priorA = processA.loadReputationHistory("acme/widgets");
+    const priorB = processB.loadReputationHistory("acme/widgets");
+    processA.saveReputationHistory("acme/widgets", { decided: priorA.decided + 1, unfavorable: 0 });
+    processB.saveReputationHistory("acme/widgets", { decided: priorB.decided + 1, unfavorable: 0 });
+    expect(processA.loadReputationHistory("acme/widgets")).toEqual({ decided: 1, unfavorable: 0 });
+  });
+
+  it("REGRESSION: incrementReputationHistory serializes sibling overlapping increments so none are lost", () => {
+    const root = mkdtempSync(join(tmpdir(), "loopover-miner-governor-state-reputation-increment-"));
+    roots.push(root);
+    const dbPath = join(root, "shared-governor-state.sqlite3");
+    const processA = openGovernorState(dbPath);
+    const processB = openGovernorState(dbPath);
+    states.push(processA, processB);
+
+    processA.saveReputationHistory("acme/widgets", { decided: 0, unfavorable: 0 });
+    processA.incrementReputationHistory("acme/widgets", { decided: 1, unfavorable: 0 });
+    processB.incrementReputationHistory("acme/widgets", { decided: 1, unfavorable: 1 });
+    expect(processA.loadReputationHistory("acme/widgets")).toEqual({ decided: 2, unfavorable: 1 });
+    expect(processB.loadReputationHistory("acme/widgets")).toEqual({ decided: 2, unfavorable: 1 });
+  });
+
+  it("increments from defaults when no prior row exists", () => {
+    const state = tempState();
+    expect(state.incrementReputationHistory("acme/widgets", { decided: 1, unfavorable: 1 })).toEqual({
+      decided: 1,
+      unfavorable: 1,
+    });
+  });
+
+  it("defaults non-integer delta fields to zero", () => {
+    const state = tempState();
+    state.saveReputationHistory("acme/widgets", { decided: 3, unfavorable: 1 });
+    expect(state.incrementReputationHistory("acme/widgets", { decided: 1 })).toEqual({ decided: 4, unfavorable: 1 });
+    expect(state.incrementReputationHistory("acme/widgets", {})).toEqual({ decided: 4, unfavorable: 1 });
+  });
+
+  it("rolls the transaction back if the upsert throws, leaving the handle usable for a later increment", () => {
+    const state = tempState();
+    state.saveReputationHistory("acme/widgets", { decided: 2, unfavorable: 1 });
+
+    const raw = new DatabaseSync(state.dbPath);
+    raw.exec("DROP TABLE governor_reputation_history");
+    raw.close();
+
+    expect(() => state.incrementReputationHistory("acme/widgets", { decided: 1, unfavorable: 0 })).toThrow(
+      /no such table/i,
+    );
+
+    // afterEach closes the handle cleanly, proving the failed increment left no dangling write transaction.
+  });
+
+  it("incrementReputationHistory module-level wrapper round-trips through the default singleton", () => {
+    const root = mkdtempSync(join(tmpdir(), "loopover-miner-governor-state-singleton-increment-"));
+    roots.push(root);
+    vi.stubEnv("LOOPOVER_MINER_GOVERNOR_STATE_DB", join(root, "governor-state.sqlite3"));
+    saveReputationHistory("acme/widgets", { decided: 1, unfavorable: 0 }, "https://ghe.example.com/api/v3");
+    const written = incrementReputationHistory(
+      "acme/widgets",
+      { decided: 2, unfavorable: 1 },
+      "https://ghe.example.com/api/v3",
+    );
+    expect(written).toEqual({ decided: 3, unfavorable: 1 });
+    expect(loadReputationHistory("acme/widgets", "https://ghe.example.com/api/v3")).toEqual(written);
+  });
+});
+
 describe("governor-state own-submission history (#5134)", () => {
   it("records and lists submissions newest-first", () => {
     const state = tempState();