diff --git a/bun.lock b/bun.lock
index 3ef2700..08472a0 100644
--- a/bun.lock
+++ b/bun.lock
@@ -13,6 +13,7 @@
         "nostr-tools": "^2.12.0",
         "qrcode": "^1.5.4",
         "rxjs": "^7.8.1",
+        "yaml": "^2.9.0",
         "zustand": "^5.0.5",
       },
       "devDependencies": {
@@ -325,6 +326,8 @@
 
     "y18n": ["y18n@4.0.3", "", {}, "sha512-JKhqTOwSrqNA1NY5lSztJ1GrBiUodLMmIZuLiDaMRJ+itFd+ABVE8XBjOvIWL+rSqNDC74LCSFmlb/U4UZ4hJQ=="],
 
+    "yaml": ["yaml@2.9.0", "", { "bin": { "yaml": "bin.mjs" } }, "sha512-2AvhNX3mb8zd6Zy7INTtSpl1F15HW6Wnqj0srWlkKLcpYl/gMIMJiyuGq2KeI2YFxUPjdlB+3Lc10seMLtL4cA=="],
+
     "yargs": ["yargs@15.4.1", "", { "dependencies": { "cliui": "^6.0.0", "decamelize": "^1.2.0", "find-up": "^4.1.0", "get-caller-file": "^2.0.1", "require-directory": "^2.1.1", "require-main-filename": "^2.0.0", "set-blocking": "^2.0.0", "string-width": "^4.2.0", "which-module": "^2.0.0", "y18n": "^4.0.0", "yargs-parser": "^18.1.2" } }, "sha512-aePbxDmcYW++PaqBsJ+HYUFwCdv4LVvdnhBy78E57PIor8/OVvhMrADFFEDh8DHDFRv/O9i3lPhsENjO7QX0+A=="],
 
     "yargs-parser": ["yargs-parser@18.1.3", "", { "dependencies": { "camelcase": "^5.0.0", "decamelize": "^1.2.0" } }, "sha512-o50j0JeToy/4K6OZcaQmW6lyXXKhq7csREXcDwk2omFPJEwUNOVtJKvmDr9EI1fAJZUyZcRF7kxGBWmRXudrCQ=="],
diff --git a/package.json b/package.json
index 02c6114..a45bccc 100644
--- a/package.json
+++ b/package.json
@@ -32,6 +32,7 @@
     "nostr-tools": "^2.12.0",
     "qrcode": "^1.5.4",
     "rxjs": "^7.8.1",
+    "yaml": "^2.9.0",
     "zustand": "^5.0.5"
   }
 }
diff --git a/src/integrations/hermes.ts b/src/integrations/hermes.ts
index 6fb7bd9..f5f3a28 100644
--- a/src/integrations/hermes.ts
+++ b/src/integrations/hermes.ts
@@ -1,10 +1,65 @@
 import { existsSync, mkdirSync } from "fs";
 import { readFile, writeFile } from "fs/promises";
 import { dirname } from "path";
+import { parseDocument } from "yaml";
 import type { RoutstrdConfig } from "../utils/config";
 import type { IntegrationConfig, RoutstrModel } from "./registry";
 import { callDaemon, getDaemonBaseUrl } from "../utils/daemon-client";
 
+interface HermesRoutstrConfig {
+  baseUrl: string;
+  apiKey: string;
+  defaultModel: string;
+}
+
+interface HermesCustomProvider {
+  name?: string;
+  [key: string]: unknown;
+}
+
+export function mergeHermesConfig(
+  content: string,
+  routstr: HermesRoutstrConfig,
+): string {
+  const document = parseDocument(content || "{}", { prettyErrors: true });
+  if (document.errors.length > 0) {
+    throw document.errors[0];
+  }
+
+  const urlDisplay = routstr.baseUrl
+    .replace(/\/v1$/, "")
+    .replace(/^https?:\/\//, "");
+  const provider = {
+    name: `Routstr (${urlDisplay})`,
+    base_url: routstr.baseUrl,
+    api_key: routstr.apiKey,
+    model: routstr.defaultModel,
+  };
+
+  const isNewConfig = content.trim() === "";
+  if (isNewConfig) {
+    document.set("model", {
+      default: routstr.defaultModel,
+      provider: "custom",
+      base_url: routstr.baseUrl,
+      api_key: routstr.apiKey,
+    });
+  }
+
+  const existingConfig = document.toJS() as {
+    custom_providers?: HermesCustomProvider[];
+  };
+  const existingProviders = existingConfig.custom_providers;
+  const providers = Array.isArray(existingProviders) ? existingProviders : [];
+  if (providers.some((item) => item.name?.startsWith("Routstr ("))) {
+    return content;
+  }
+  providers.push(provider);
+  document.set("custom_providers", providers);
+
+  return document.toString();
+}
+
 export async function installHermesIntegration(
   config: RoutstrdConfig,
   apiKey: string,
@@ -26,10 +81,10 @@ export async function installHermesIntegration(
 
     if (models.length >= 3) {
       defaultModel = models[2]!.id;
-      console.log(`Set default model to 3rd available model: ${defaultModel}`);
+      console.log(`Using 3rd available model for new Hermes configurations: ${defaultModel}`);
     } else if (models.length > 0) {
       defaultModel = models[0]!.id;
-      console.log(`Only ${models.length} models available, using ${defaultModel} as default.`);
+      console.log(`Only ${models.length} models available, using ${defaultModel} for new Hermes configurations.`);
     } else {
       console.log("No models available from routstr daemon, using fallback default.");
     }
@@ -47,36 +102,24 @@ export async function installHermesIntegration(
     console.error(`Error reading ${configPath}, creating new one.`);
   }
 
-  // Remove existing model block
-  content = content.replace(/^model:\n(?:  .*\n)*/gm, "");
-  // Remove existing custom_providers block
-  content = content.replace(/^custom_providers:\n(?:- .*\n(?:  .*\n)*)*/gm, "");
-  // Clean up extra blank lines
-  content = content.replace(/\n{3,}/g, "\n\n").trim();
-
-  const urlDisplay = baseUrl.replace(/^https?:\/\//, "");
-
-  const modelBlock = `model:
-  default: ${defaultModel}
-  provider: custom
-  base_url: ${baseUrlV1}
-  api_key: ${apiKey}`;
-
-  const providerBlock = `custom_providers:
-- name: Routstr (${urlDisplay})
-  base_url: ${baseUrlV1}
-  api_key: ${apiKey}
-  model: ${defaultModel}`;
-
-  const parts: string[] = [modelBlock];
-  if (content) {
-    parts.push(content);
+  let newContent: string;
+  try {
+    newContent = mergeHermesConfig(content, {
+      baseUrl: baseUrlV1,
+      apiKey,
+      defaultModel,
+    });
+  } catch (error) {
+    console.error(`Failed to parse ${configPath} as YAML; leaving it unchanged:`, error);
+    return;
   }
-  parts.push(providerBlock);
-
-  const newContent = parts.join("\n\n") + "\n";
 
   try {
+    if (newContent === content) {
+      console.log(`${configPath} already contains current routstr settings.`);
+      return;
+    }
+
     mkdirSync(dirname(configPath), { recursive: true });
     await writeFile(configPath, newContent);
     console.log(`Successfully updated ${configPath} with routstr settings.`);
diff --git a/tests/integrations/hermes.test.ts b/tests/integrations/hermes.test.ts
new file mode 100644
index 0000000..12ac581
--- /dev/null
+++ b/tests/integrations/hermes.test.ts
@@ -0,0 +1,97 @@
+import { describe, expect, it } from "bun:test";
+import { parse } from "yaml";
+import { mergeHermesConfig } from "../../src/integrations/hermes";
+
+const ROUTSTR = {
+  baseUrl: "http://localhost:8008/v1",
+  apiKey: "routstr-key",
+  defaultModel: "deepseek-v4-flash",
+};
+
+describe("mergeHermesConfig", () => {
+  it("preserves the user's model and unrelated custom providers", () => {
+    const existing = `model:
+  default: user-model
+  provider: custom
+  base_url: https://user.example/v1
+  api_key: user-key
+
+custom_providers:
+  - name: User Provider
+    base_url: https://user.example/v1
+    api_key: user-key
+    model: user-model
+
+other_setting: true
+`;
+
+    const merged = parse(mergeHermesConfig(existing, ROUTSTR));
+
+    expect(merged.model).toEqual({
+      default: "user-model",
+      provider: "custom",
+      base_url: "https://user.example/v1",
+      api_key: "user-key",
+    });
+    expect(merged.other_setting).toBe(true);
+    expect(merged.custom_providers).toContainEqual({
+      name: "User Provider",
+      base_url: "https://user.example/v1",
+      api_key: "user-key",
+      model: "user-model",
+    });
+    expect(merged.custom_providers).toContainEqual({
+      name: "Routstr (localhost:8008)",
+      base_url: ROUTSTR.baseUrl,
+      api_key: ROUTSTR.apiKey,
+      model: ROUTSTR.defaultModel,
+    });
+  });
+
+  it("does not add a model configuration to an existing Hermes file", () => {
+    const merged = parse(mergeHermesConfig("other_setting: true\n", ROUTSTR));
+
+    expect(merged.model).toBeUndefined();
+    expect(merged.other_setting).toBe(true);
+  });
+
+  it("creates a Routstr default when creating a new Hermes file", () => {
+    const merged = parse(mergeHermesConfig("", ROUTSTR));
+
+    expect(merged.model).toEqual({
+      default: ROUTSTR.defaultModel,
+      provider: "custom",
+      base_url: ROUTSTR.baseUrl,
+      api_key: ROUTSTR.apiKey,
+    });
+  });
+
+  it("keeps an existing Routstr provider unchanged without duplicating it", () => {
+    const existing = `custom_providers:
+  - name: Routstr (old-host:8008)
+    base_url: http://old-host:8008/v1
+    api_key: old-key
+    model: old-model
+`;
+
+    const once = mergeHermesConfig(existing, ROUTSTR);
+    const twice = mergeHermesConfig(once, ROUTSTR);
+    const merged = parse(twice);
+    const routstrProviders = merged.custom_providers.filter(
+      (provider: { name?: string }) => provider.name?.startsWith("Routstr ("),
+    );
+
+    expect(routstrProviders).toHaveLength(1);
+    expect(routstrProviders[0]).toEqual({
+      name: "Routstr (old-host:8008)",
+      base_url: "http://old-host:8008/v1",
+      api_key: "old-key",
+      model: "old-model",
+    });
+    expect(twice).toBe(once);
+  });
+
+  it("rejects malformed YAML instead of replacing it", () => {
+    expect(() => mergeHermesConfig("model: [unterminated", ROUTSTR)).toThrow();
+  });
+});