diff --git a/packages/server/json-preview.js b/packages/server/json-preview.js
new file mode 100644
index 0000000..c730632
--- /dev/null
+++ b/packages/server/json-preview.js
@@ -0,0 +1,43 @@
+const BUFFER_PREVIEW_BYTES = 16;
+
+/**
+ * @param {unknown} value
+ */
+export function isBinary(value) {
+  return (
+    Buffer.isBuffer(value) ||
+    ArrayBuffer.isView(value) ||
+    value instanceof ArrayBuffer
+  );
+}
+
+/**
+ * Compact stand-in for JSON (Buffer.toJSON dumps every byte as a number).
+ * @param {Buffer | ArrayBufferView | ArrayBuffer} value
+ */
+export function summarizeBinary(value) {
+  const buf = Buffer.isBuffer(value)
+    ? value
+    : value instanceof ArrayBuffer
+      ? Buffer.from(value)
+      : Buffer.from(value.buffer, value.byteOffset, value.byteLength);
+  const take = Math.min(buf.length, BUFFER_PREVIEW_BYTES);
+  return {
+    type: "Buffer",
+    length: buf.length,
+    preview: buf.subarray(0, take).toString("hex"),
+    truncated: buf.length > take,
+  };
+}
+
+/**
+ * JSON.stringify replacer. Must be a real function so `this` is the holder:
+ * Buffer#toJSON already ran on `value`, but `this[key]` is still the Buffer.
+ * @param {string} key
+ * @param {unknown} value
+ */
+export function jsonPreviewReplacer(key, value) {
+  const raw = this[key];
+  if (isBinary(raw)) return summarizeBinary(raw);
+  return value;
+}
diff --git a/packages/server/src/api/dry-run-logger.js b/packages/server/src/api/dry-run-logger.js
index 7f347c0..660322c 100644
--- a/packages/server/src/api/dry-run-logger.js
+++ b/packages/server/src/api/dry-run-logger.js
@@ -1,4 +1,5 @@
 import pino from "pino";
+import { isBinary, summarizeBinary } from "../../json-preview.js";
 import { redactString } from "../../secret-value.js";
 
 const LEVEL_TO_NUM = {
@@ -64,7 +65,9 @@ export function safeSerialize(value) {
   try {
     return JSON.parse(
       redactString(
-        JSON.stringify(value, (_key, v) => {
+        JSON.stringify(value, function (key, v) {
+          const raw = this[key];
+          if (isBinary(raw)) return summarizeBinary(raw);
           if (typeof v === "bigint") return v.toString();
           if (typeof v === "object" && v !== null) {
             if (seen.has(v)) return "[Circular]";
diff --git a/packages/server/src/api/workflows.js b/packages/server/src/api/workflows.js
index f017f58..116d79f 100644
--- a/packages/server/src/api/workflows.js
+++ b/packages/server/src/api/workflows.js
@@ -418,7 +418,7 @@ export default function workflowsPluginFactory(registry) {
       return {
         runId: result.runId,
         status: result.status,
-        result: result.result,
+        result: store.toDisplayValue(result.result),
       };
     });
 
diff --git a/packages/server/store.js b/packages/server/store.js
index d8510dd..6132725 100644
--- a/packages/server/store.js
+++ b/packages/server/store.js
@@ -1,5 +1,6 @@
 import { randomUUID } from "node:crypto";
 import { db } from "./db.js";
+import { jsonPreviewReplacer } from "./json-preview.js";
 import { redactString } from "./secret-value.js";
 
 const MAX_JSON_BYTES = 64 * 1024;
@@ -12,7 +13,7 @@ export function serialize(value) {
   if (value === undefined || value === null) return null;
   let json;
   try {
-    json = JSON.stringify(value);
+    json = JSON.stringify(value, jsonPreviewReplacer);
   } catch {
     json = JSON.stringify({ truncated: true, reason: "unserializable" });
   }
@@ -24,6 +25,20 @@ export function serialize(value) {
   });
 }
 
+/**
+ * Parsed JSON-safe copy for API / UI (buffers summarized, size-capped).
+ * @param {unknown} value
+ */
+export function toDisplayValue(value) {
+  const json = serialize(value);
+  if (json == null) return null;
+  try {
+    return JSON.parse(json);
+  } catch {
+    return null;
+  }
+}
+
 /**
  * @param {string | null} value
  * @returns {unknown}
diff --git a/packages/server/test/json-preview-smoke.js b/packages/server/test/json-preview-smoke.js
new file mode 100644
index 0000000..bb87cda
--- /dev/null
+++ b/packages/server/test/json-preview-smoke.js
@@ -0,0 +1,56 @@
+import { jsonPreviewReplacer, summarizeBinary } from "../json-preview.js";
+import { serialize, toDisplayValue } from "../store.js";
+import { safeSerialize } from "../src/api/dry-run-logger.js";
+
+const png = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 1, 2, 3]);
+const summary = summarizeBinary(png);
+if (summary.length !== png.length || summary.preview !== "89504e470d0a1a0a010203" || summary.truncated !== false) {
+  throw new Error(`summarizeBinary: ${JSON.stringify(summary)}`);
+}
+
+const long = Buffer.alloc(32, 0xff);
+const longSummary = summarizeBinary(long);
+if (longSummary.length !== 32 || longSummary.preview.length !== 32 || longSummary.truncated !== true) {
+  throw new Error(`long summarizeBinary: ${JSON.stringify(longSummary)}`);
+}
+
+const dumped = JSON.stringify({ file: png });
+if (!dumped.includes('"data":[')) {
+  throw new Error("expected default Buffer JSON to include data array");
+}
+
+const previewed = JSON.stringify({ file: png }, jsonPreviewReplacer);
+if (previewed.includes('"data":[')) {
+  throw new Error(`replacer still dumped bytes: ${previewed}`);
+}
+if (!previewed.includes('"preview":"89504e470d0a1a0a010203"')) {
+  throw new Error(`replacer missing hex preview: ${previewed}`);
+}
+
+const stored = serialize({ output: { file: png, filename: "test.png" } });
+if (stored.includes('"data":[')) {
+  throw new Error(`serialize dumped bytes: ${stored.slice(0, 200)}`);
+}
+
+const display = toDisplayValue({
+  output: { file: png },
+  context: { file: png },
+});
+if (display.output.file.length !== png.length || display.context.file.truncated !== false) {
+  throw new Error(`toDisplayValue: ${JSON.stringify(display)}`);
+}
+if (Array.isArray(display.output.file.data)) {
+  throw new Error("toDisplayValue should not keep Buffer.data");
+}
+
+const dry = safeSerialize({ file: png, n: 1n });
+if (dry.n !== "1" || Array.isArray(dry.file.data)) {
+  throw new Error(`safeSerialize: ${JSON.stringify(dry)}`);
+}
+
+const typed = safeSerialize({ file: new Uint8Array(png) });
+if (typed.file.length !== png.length || typed.file.type !== "Buffer") {
+  throw new Error(`Uint8Array: ${JSON.stringify(typed)}`);
+}
+
+console.log("json-preview-smoke: ok");