diff --git a/src/config/io.ts b/src/config/io.ts
index 73fa35296b7d..581c6f43747b 100644
--- a/src/config/io.ts
+++ b/src/config/io.ts
@@ -510,7 +510,7 @@ function resolveConfigStatMetadata(
 function resolveConfigWriteSuspiciousReasons(params: {
   existsBefore: boolean;
   unreadableBefore: boolean;
-  previousBytes: number | null;
+  sizeBaselineBytes: number | null;
   nextBytes: number | null;
   hasMetaBefore: boolean;
   gatewayModeBefore: string | null;
@@ -524,12 +524,12 @@ function resolveConfigWriteSuspiciousReasons(params: {
     reasons.push("unreadable-config-before-write");
   }
   if (
-    typeof params.previousBytes === "number" &&
+    typeof params.sizeBaselineBytes === "number" &&
     typeof params.nextBytes === "number" &&
-    params.previousBytes >= 512 &&
-    params.nextBytes < Math.floor(params.previousBytes * 0.5)
+    params.sizeBaselineBytes >= 512 &&
+    params.nextBytes < Math.floor(params.sizeBaselineBytes * 0.5)
   ) {
-    reasons.push(`size-drop:${params.previousBytes}->${params.nextBytes}`);
+    reasons.push(`size-drop:${params.sizeBaselineBytes}->${params.nextBytes}`);
   }
   if (!params.hasMetaBefore) {
     reasons.push("missing-meta-before-write");
@@ -991,6 +991,29 @@ function stampConfigVersion(cfg: OpenClawConfig, version?: string): OpenClawConf
   return stampConfigWriteMetadata(cfg, new Date().toISOString(), version);
 }
 
+function resolveConfigSizeBaselineBytes(params: {
+  raw: string | null;
+  json5: { parse: (value: string) => unknown };
+  lastTouchedVersionOverride?: string;
+}): number | null {
+  if (params.raw === null) {
+    return null;
+  }
+  const rawBytes = Buffer.byteLength(params.raw, "utf-8");
+  const parsed = parseConfigJson5(params.raw, params.json5);
+  if (!parsed.ok || !isRecord(parsed.parsed)) {
+    return rawBytes;
+  }
+  const canonical = JSON.stringify(
+    stampConfigVersion(parsed.parsed as OpenClawConfig, params.lastTouchedVersionOverride),
+    null,
+    2,
+  )
+    .trimEnd()
+    .concat("\n");
+  return Buffer.byteLength(canonical, "utf-8");
+}
+
 function warnIfConfigFromFuture(cfg: OpenClawConfig, logger: Pick<typeof console, "warn">): void {
   const touched = cfg.meta?.lastTouchedVersion;
   if (!touched) {
@@ -2499,6 +2522,13 @@ export function createConfigIO(
     const changedPathCount = changedPaths?.size;
     const previousBytes =
       typeof snapshot.raw === "string" ? Buffer.byteLength(snapshot.raw, "utf-8") : null;
+    // Formatting is not data. Keep malformed/non-object files on the raw-byte
+    // baseline, but compare parseable authored config in its canonical form.
+    const sizeBaselineBytes = resolveConfigSizeBaselineBytes({
+      raw: snapshot.raw,
+      json5: deps.json5,
+      lastTouchedVersionOverride: options.lastTouchedVersionOverride,
+    });
     const nextBytes = Buffer.byteLength(json, "utf-8");
     const previousStat = snapshot.exists
       ? await deps.fs.promises.stat(configPath).catch(() => null)
@@ -2510,7 +2540,7 @@ export function createConfigIO(
     const suspiciousReasons = resolveConfigWriteSuspiciousReasons({
       existsBefore: snapshot.exists,
       unreadableBefore: snapshot.readError != null,
-      previousBytes,
+      sizeBaselineBytes,
       nextBytes,
       hasMetaBefore,
       gatewayModeBefore,
diff --git a/src/config/io.write-config.test.ts b/src/config/io.write-config.test.ts
index 1ae0831c1adf..85db12f013d2 100644
--- a/src/config/io.write-config.test.ts
+++ b/src/config/io.write-config.test.ts
@@ -1178,6 +1178,114 @@ describe("config io write", () => {
     });
   });
 
+  it("ignores verbose BOM formatting but still rejects a destructive size drop", async () => {
+    await withSuiteHome(async (home) => {
+      const configPath = path.join(home, ".openclaw", "openclaw.json");
+      await fs.mkdir(path.dirname(configPath), { recursive: true });
+      const original = {
+        meta: { lastTouchedVersion: "2026.4.23" },
+        gateway: { mode: "local" },
+        channels: {
+          telegram: {
+            enabled: true,
+            allowFrom: Array.from({ length: 80 }, (_, index) => `telegram:${index}`),
+          },
+        },
+      } satisfies ConfigFileSnapshot["config"];
+      const powerShellRaw = `\uFEFF${JSON.stringify(original, null, 12)}\n`;
+      await fs.writeFile(configPath, powerShellRaw, "utf-8");
+      const io = createConfigIO({
+        env: { VITEST: "true" } as NodeJS.ProcessEnv,
+        homedir: () => home,
+        logger: silentLogger,
+      });
+
+      const snapshot = await io.readConfigFileSnapshot();
+      expect(snapshot.valid).toBe(true);
+      await expectConfigWriteRejected(
+        io.writeConfigFile(
+          { meta: original.meta, gateway: { mode: "local" } },
+          { baseSnapshot: snapshot },
+        ),
+      );
+      await expect(fs.readFile(configPath, "utf-8")).resolves.toBe(powerShellRaw);
+
+      await io.writeConfigFile(
+        {
+          ...original,
+          gateway: { mode: "local", port: 18789 },
+        },
+        { baseSnapshot: snapshot },
+      );
+      const canonicalRaw = await fs.readFile(configPath, "utf-8");
+      expect(Buffer.byteLength(powerShellRaw, "utf-8")).toBeGreaterThan(
+        Buffer.byteLength(canonicalRaw, "utf-8") * 2,
+      );
+    });
+  });
+
+  it("canonicalizes parseable schema-invalid config for the size baseline", async () => {
+    await withSuiteHome(async (home) => {
+      const configPath = path.join(home, ".openclaw", "openclaw.json");
+      await fs.mkdir(path.dirname(configPath), { recursive: true });
+      const channels = {
+        telegram: {
+          enabled: true,
+          allowFrom: Array.from({ length: 60 }, (_, index) => `telegram:${index}`),
+        },
+      };
+      const invalid = {
+        gateway: { mode: "local" },
+        channels,
+        agents: { list: "not-an-array" },
+      };
+      const invalidRaw = `\uFEFF${JSON.stringify(invalid, null, 12)}\n`;
+      await fs.writeFile(configPath, invalidRaw, "utf-8");
+      const io = createConfigIO({
+        env: { VITEST: "true" } as NodeJS.ProcessEnv,
+        homedir: () => home,
+        logger: silentLogger,
+      });
+      const snapshot = {
+        path: configPath,
+        exists: true,
+        raw: invalidRaw,
+        parsed: invalid,
+        sourceConfig: {},
+        resolved: {},
+        valid: false,
+        runtimeConfig: {},
+        config: {},
+        issues: [],
+        warnings: [],
+        legacyIssues: [],
+      } satisfies ConfigFileSnapshot;
+      await expect(
+        io.writeConfigFile(
+          { gateway: { mode: "local", port: 18789 }, channels },
+          { baseSnapshot: snapshot, skipPluginValidation: true },
+        ),
+      ).resolves.toBeDefined();
+    });
+  });
+
+  it("keeps the raw-byte size baseline for malformed config", async () => {
+    await withSuiteHome(async (home) => {
+      const configPath = path.join(home, ".openclaw", "openclaw.json");
+      await fs.mkdir(path.dirname(configPath), { recursive: true });
+      const malformedRaw = `not-json\n${"x".repeat(2048)}\n`;
+      await fs.writeFile(configPath, malformedRaw, "utf-8");
+      const io = createConfigIO({
+        env: { VITEST: "true" } as NodeJS.ProcessEnv,
+        homedir: () => home,
+        logger: silentLogger,
+      });
+
+      await expectConfigWriteRejected(io.writeConfigFile({ gateway: { mode: "local" } }));
+      await expect(fs.readFile(configPath, "utf-8")).resolves.toBe(malformedRaw);
+    });
+  });
+
   it("allows intentional size-drop writes without disabling gateway-mode protection", async () => {
     await withSuiteHome(async (home) => {
       const configPath = path.join(home, ".openclaw", "openclaw.json");