diff --git a/src/lib/services/installers/cliproxy.ts b/src/lib/services/installers/cliproxy.ts
index 61c3b6699b..64c3c01294 100644
--- a/src/lib/services/installers/cliproxy.ts
+++ b/src/lib/services/installers/cliproxy.ts
@@ -105,11 +105,13 @@ export function resolveSpawnArgs(port: number): SpawnArgs {
 
   fs.mkdirSync(CONFIG_DIR, { recursive: true });
   const configPath = path.join(CONFIG_DIR, "config.yaml");
-  fs.writeFileSync(configPath, `port: ${port}\nhost: 127.0.0.1\nlog_level: warn\n`, "utf8");
+  if (!fs.existsSync(configPath)) {
+    fs.writeFileSync(configPath, `port: ${port}\nhost: 127.0.0.1\nlog_level: warn\n`, "utf8");
+  }
 
   return {
     command: symlinkPath,
-    args: ["-c", configPath],
+    args: ["--config", configPath],
     env: { ...process.env },
     cwd: CONFIG_DIR,
   };
diff --git a/tests/unit/services/installers/cliproxy-resolve-spawn-args-6877.test.ts b/tests/unit/services/installers/cliproxy-resolve-spawn-args-6877.test.ts
new file mode 100644
index 0000000000..db14e2d854
--- /dev/null
+++ b/tests/unit/services/installers/cliproxy-resolve-spawn-args-6877.test.ts
@@ -0,0 +1,104 @@
+/**
+ * Regression test for #6877 — cliproxy resolveSpawnArgs() must:
+ *   (a) invoke the real CLIProxyAPI binary with the long-form `--config` flag
+ *       (the short `-c` flag is not recognized by the upstream binary and
+ *       silently falls back to its own default config, ignoring ours), and
+ *   (b) never clobber an existing config.yaml — only write the default
+ *       template the first time the file does not exist yet.
+ *
+ * Unlike the pre-existing tests/unit/services/installers/cliproxy.test.ts
+ * (which only re-asserts string literals and never calls the real function),
+ * this test imports and calls the real resolveSpawnArgs() against a real
+ * temp-directory filesystem.
+ */
+
+import { describe, it, beforeEach, after } from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
+
+function createTempDataDir(): string {
+  return fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-cliproxy-6877-"));
+}
+
+// DATA_DIR is captured into a module-level const the *first* time
+// src/lib/db/core.ts (transitively imported by cliproxy.ts) is loaded, and
+// dynamic import() results are cached per resolved URL for the lifetime of
+// the process — so the env var must be pinned to one fixed temp dir BEFORE
+// the module is imported for the first time. Each test then resets the
+// *contents* of that same directory rather than re-pointing DATA_DIR.
+const FIXED_DATA_DIR = createTempDataDir();
+process.env.DATA_DIR = FIXED_DATA_DIR;
+
+after(() => {
+  if (ORIGINAL_DATA_DIR === undefined) {
+    delete process.env.DATA_DIR;
+  } else {
+    process.env.DATA_DIR = ORIGINAL_DATA_DIR;
+  }
+  fs.rmSync(FIXED_DATA_DIR, { recursive: true, force: true });
+});
+
+describe("resolveSpawnArgs (#6877 — real filesystem)", () => {
+  const dataDir = FIXED_DATA_DIR;
+
+  beforeEach(() => {
+    fs.rmSync(dataDir, { recursive: true, force: true });
+    fs.mkdirSync(dataDir, { recursive: true });
+  });
+
+  it("uses the --config long flag and never the -c short flag", async () => {
+    const { resolveSpawnArgs } = await import(
+      "../../../../src/lib/services/installers/cliproxy.ts"
+    );
+
+    const port = 8317;
+    const result = resolveSpawnArgs(port);
+
+    const configPath = path.join(dataDir, "services", "cliproxy", "config.yaml");
+
+    assert.deepEqual(result.args, ["--config", configPath]);
+    assert.ok(!result.args.includes("-c"), "args must never contain the short -c flag");
+  });
+
+  it("writes the default config.yaml template when none exists yet", async () => {
+    const { resolveSpawnArgs } = await import(
+      "../../../../src/lib/services/installers/cliproxy.ts"
+    );
+
+    const port = 9123;
+    const result = resolveSpawnArgs(port);
+
+    const configPath = path.join(dataDir, "services", "cliproxy", "config.yaml");
+    assert.equal(result.args[1], configPath);
+    assert.ok(fs.existsSync(configPath), "config.yaml must be created on first run");
+
+    const content = fs.readFileSync(configPath, "utf8");
+    assert.equal(content, `port: ${port}\nhost: 127.0.0.1\nlog_level: warn\n`);
+  });
+
+  it("preserves a pre-existing config.yaml byte-for-byte instead of overwriting it", async () => {
+    const { resolveSpawnArgs } = await import(
+      "../../../../src/lib/services/installers/cliproxy.ts"
+    );
+
+    const configDir = path.join(dataDir, "services", "cliproxy");
+    fs.mkdirSync(configDir, { recursive: true });
+    const configPath = path.join(configDir, "config.yaml");
+    const customContent =
+      "port: 8317\nhost: 0.0.0.0\nlog_level: debug\n# hand-edited by the operator\napi-keys:\n  - custom-key\n";
+    fs.writeFileSync(configPath, customContent, "utf8");
+
+    resolveSpawnArgs(8317);
+
+    const contentAfter = fs.readFileSync(configPath, "utf8");
+    assert.equal(
+      contentAfter,
+      customContent,
+      "resolveSpawnArgs must not clobber an existing, operator-customized config.yaml"
+    );
+  });
+});