diff --git a/app/store/chat.ts b/app/store/chat.ts
index 87c1a8beba0..d264b149c23 100644
--- a/app/store/chat.ts
+++ b/app/store/chat.ts
@@ -166,10 +166,12 @@ function fillTemplateWith(input: string, modelConfig: ModelConfig) {
 
   var serviceProvider = "OpenAI";
   if (modelInfo) {
-    // TODO: auto detect the providerName from the modelConfig.model
-
     // Directly use the providerName from the modelInfo
     serviceProvider = modelInfo.provider.providerName;
+  } else if (modelConfig.providerName) {
+    // For custom models not in DEFAULT_MODELS, use the configured providerName
+    // so that {{ServiceProvider}} in templates reflects the actual provider
+    serviceProvider = modelConfig.providerName;
   }
 
   const vars = {
diff --git a/app/store/config.ts b/app/store/config.ts
index 45e21b02697..0dbb448e188 100644
--- a/app/store/config.ts
+++ b/app/store/config.ts
@@ -17,7 +17,8 @@ import {
 import { createPersistStore } from "../utils/store";
 import type { Voice } from "rt-client";
 
-export type ModelType = (typeof DEFAULT_MODELS)[number]["name"];
+// Include `string & {}` to allow custom model names while preserving autocomplete for built-in models
+export type ModelType = (typeof DEFAULT_MODELS)[number]["name"] | (string & {});
 export type TTSModelType = (typeof DEFAULT_TTS_MODELS)[number];
 export type TTSVoiceType = (typeof DEFAULT_TTS_VOICES)[number];
 export type TTSEngineType = (typeof DEFAULT_TTS_ENGINES)[number];
diff --git a/app/utils/model.ts b/app/utils/model.ts
index f460babcd25..a0ecb63bfc0 100644
--- a/app/utils/model.ts
+++ b/app/utils/model.ts
@@ -254,5 +254,18 @@ export function isModelNotavailableInServer(
     const fullName = `${modelName}@${providerName.toLowerCase()}`;
     if (modelTable?.[fullName]?.available === true) return false;
   }
+
+  // For custom models added without an explicit provider (e.g. just "chatglm3-6b"),
+  // the table entry key is "<modelName>@<modelName>". Check if any available entry
+  // in the custom model table matches this model name, regardless of provider.
+  // This ensures user-defined custom models are never incorrectly blocked.
+  const hasAvailableCustomEntry = Object.values(modelTable).some(
+    (entry) =>
+      entry.name === modelName &&
+      entry.available === true &&
+      entry.provider?.providerType === "custom",
+  );
+  if (hasAvailableCustomEntry) return false;
+
   return true;
 }
diff --git a/test/custom-model.test.ts b/test/custom-model.test.ts
new file mode 100644
index 00000000000..a5eddeeabe9
--- /dev/null
+++ b/test/custom-model.test.ts
@@ -0,0 +1,104 @@
+/**
+ * Tests for custom model support (issue #4221).
+ *
+ * Verifies that models added via the customModels setting (e.g. "chatglm3-6b")
+ * are handled correctly throughout the codebase:
+ *  - collectModelTable builds the right entries
+ *  - isModelNotavailableInServer correctly allows custom models
+ */
+
+import { collectModelTable, isModelNotavailableInServer } from "../app/utils/model";
+import { DEFAULT_MODELS } from "../app/constant";
+
+describe("collectModelTable with custom models", () => {
+  test("adds a custom model with provider derived from model name when no @ given", () => {
+    const table = collectModelTable(DEFAULT_MODELS, "chatglm3-6b");
+    const entry = table["chatglm3-6b@chatglm3-6b"];
+    expect(entry).toBeDefined();
+    expect(entry.name).toBe("chatglm3-6b");
+    expect(entry.available).toBe(true);
+    expect(entry.provider?.providerType).toBe("custom");
+  });
+
+  test("adds a custom model with explicit provider (model@OpenAI format)", () => {
+    const table = collectModelTable(DEFAULT_MODELS, "chatglm3-6b@OpenAI");
+    const entry = table["chatglm3-6b@openai"];
+    expect(entry).toBeDefined();
+    expect(entry.name).toBe("chatglm3-6b");
+    expect(entry.available).toBe(true);
+    expect(entry.provider?.providerType).toBe("custom");
+  });
+
+  test("custom model is marked unavailable when prefixed with -", () => {
+    const table = collectModelTable(DEFAULT_MODELS, "-chatglm3-6b");
+    // Custom model created with available=false
+    const entry = table["chatglm3-6b@chatglm3-6b"];
+    expect(entry).toBeDefined();
+    expect(entry.available).toBe(false);
+  });
+
+  test("custom model with display name alias (model=Alias format)", () => {
+    const table = collectModelTable(DEFAULT_MODELS, "chatglm3-6b=GLM-3");
+    const entry = table["chatglm3-6b@chatglm3-6b"];
+    expect(entry).toBeDefined();
+    expect(entry.displayName).toBe("GLM-3");
+    expect(entry.available).toBe(true);
+  });
+});
+
+describe("isModelNotavailableInServer with custom models", () => {
+  afterEach(() => {
+    delete process.env.DISABLE_GPT4;
+  });
+
+  test("allows a custom model when providerNames includes the model name itself (existing behaviour)", () => {
+    // This is the pattern used in api/common.ts: third provider is the model name
+    const result = isModelNotavailableInServer(
+      "chatglm3-6b",
+      "chatglm3-6b",
+      ["OpenAI", "Azure", "chatglm3-6b"],
+    );
+    expect(result).toBe(false);
+  });
+
+  test("allows a custom model even when providerNames does NOT include the model name (new fallback)", () => {
+    // Custom model added to server customModels without explicit provider.
+    // The request comes in with only standard provider names — the new fallback
+    // should still allow it because the model's providerType is "custom".
+    const result = isModelNotavailableInServer(
+      "chatglm3-6b",
+      "chatglm3-6b",
+      ["OpenAI", "Azure"],
+    );
+    expect(result).toBe(false);
+  });
+
+  test("blocks a model that is not in customModels and not in DEFAULT_MODELS", () => {
+    // No custom model configured — completely unknown model should be blocked
+    // only if customModels is non-empty (the check is skipped when customModels is empty)
+    const result = isModelNotavailableInServer(
+      "-all,gpt-4o-mini", // customModels is set, but chatglm3-6b is not listed
+      "chatglm3-6b",
+      ["OpenAI", "Azure"],
+    );
+    expect(result).toBe(true);
+  });
+
+  test("allows a built-in model when no custom model filter is applied", () => {
+    const result = isModelNotavailableInServer(
+      "",
+      "gpt-4o",
+      "OpenAI",
+    );
+    expect(result).toBe(false);
+  });
+
+  test("custom model added via -all,+model format is allowed", () => {
+    const result = isModelNotavailableInServer(
+      "-all,chatglm3-6b",
+      "chatglm3-6b",
+      ["OpenAI", "Azure", "chatglm3-6b"],
+    );
+    expect(result).toBe(false);
+  });
+});