diff --git a/src/agent/index.ts b/src/agent/index.ts
index e38ce91c..9def7776 100644
--- a/src/agent/index.ts
+++ b/src/agent/index.ts
@@ -65,6 +65,8 @@ import {
   getProviderModelOptions,
   getProviderRegionEnvKey,
   getProviderSecretKeyEnvKey,
+  getProvidersForKnownModelId,
+  isModelIdForOtherProvider,
   DEFAULT_VERTEX_LOCATION,
   GOOGLE_CLOUD_PROJECT_ENV_KEY,
   isValidModelId,
@@ -590,9 +592,40 @@ export function resolveModelId(
     );
   }
 
+  warnOnProviderModelMismatch(options, provider, modelId);
+
   return modelId;
 }
 
+// Non-fatal: if the configured model is a known model of a different provider
+// (e.g. an Anthropic model left in OPENWIKI_MODEL_ID while the provider is now
+// OpenAI), surface an actionable warning instead of letting the request fail
+// later with an opaque provider-side 400/404. The run still proceeds, since a
+// custom endpoint or gateway may legitimately serve the model.
+function warnOnProviderModelMismatch(
+  options: OpenWikiRunOptions,
+  provider: OpenWikiProvider,
+  modelId: string,
+): void {
+  if (!isModelIdForOtherProvider(modelId, provider)) {
+    return;
+  }
+
+  const otherProviders = getProvidersForKnownModelId(modelId, provider)
+    .map((otherProvider) => getProviderLabel(otherProvider))
+    .join(", ");
+  const message =
+    `Warning: model "${modelId}" is not a known ${getProviderLabel(provider)} model ` +
+    `(it belongs to ${otherProviders}). The request may fail. ` +
+    `Set ${OPENWIKI_MODEL_ID_ENV_KEY} to a ${getProviderLabel(provider)} model, or switch providers.`;
+
+  emitDebug(options, `model.mismatch provider=${provider} model=${modelId}`);
+  options.onEvent?.({ type: "text", text: message });
+  // Also emit to stderr so the warning survives on failure, where the TUI
+  // re-renders the log away and --print discards buffered streamed text.
+  process.stderr.write(`${message}\n`);
+}
+
 export function createModel(
   provider: OpenWikiProvider,
   modelId: string,
diff --git a/src/constants.ts b/src/constants.ts
index ca74925f..b4372959 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -557,6 +557,54 @@ export function getDefaultModelId(provider: OpenWikiProvider): string {
   return getProviderModelOptions(provider)[0]?.id ?? DEFAULT_MODEL_ID;
 }
 
+// Returns the list of built-in providers whose known model options include the
+// given model ID by exact match, excluding the provider passed in. Used to warn
+// when a saved model plainly belongs to a different provider (e.g. an Anthropic
+// model left over while the provider is now OpenAI). Exact matching avoids false
+// positives from namespaced overlaps such as OpenRouter's "anthropic/claude-...".
+// Returns an empty array for custom/unknown model IDs, so gateway and
+// OpenAI-compatible model names are never flagged.
+export function getProvidersForKnownModelId(
+  modelId: string,
+  excludeProvider: OpenWikiProvider,
+): OpenWikiProvider[] {
+  const normalized = normalizeModelId(modelId);
+  const providers: OpenWikiProvider[] = [];
+
+  for (const provider of Object.keys(PROVIDER_CONFIGS) as OpenWikiProvider[]) {
+    if (provider === excludeProvider) {
+      continue;
+    }
+    if (
+      getProviderModelOptions(provider).some(
+        (option) => option.id === normalized,
+      )
+    ) {
+      providers.push(provider);
+    }
+  }
+
+  return providers;
+}
+
+// True when the model ID is a known model of some other provider and is NOT a
+// known model of the configured provider — a clear provider/model mismatch.
+export function isModelIdForOtherProvider(
+  modelId: string,
+  provider: OpenWikiProvider,
+): boolean {
+  const normalized = normalizeModelId(modelId);
+  const isKnownForProvider = getProviderModelOptions(provider).some(
+    (option) => option.id === normalized,
+  );
+
+  if (isKnownForProvider) {
+    return false;
+  }
+
+  return getProvidersForKnownModelId(normalized, provider).length > 0;
+}
+
 export function normalizeProvider(
   value: string | null | undefined,
 ): OpenWikiProvider | null {
diff --git a/test/constants.test.ts b/test/constants.test.ts
index 40db95d5..9d585942 100644
--- a/test/constants.test.ts
+++ b/test/constants.test.ts
@@ -11,6 +11,8 @@ import {
   getProviderModelOptions,
   getProviderRegionEnvKey,
   getProviderSecretKeyEnvKey,
+  getProvidersForKnownModelId,
+  isModelIdForOtherProvider,
   isValidBaseUrl,
   isValidProviderBaseUrl,
   isValidModelId,
@@ -416,3 +418,71 @@ describe("getDefaultModelId", () => {
     },
   );
 });
+
+describe("getProvidersForKnownModelId", () => {
+  test("finds the provider(s) whose known models include the id", () => {
+    // claude-opus-4-8 is a known model of both anthropic and the
+    // gemini-enterprise gateway, which also serves Claude models.
+    expect(getProvidersForKnownModelId("claude-opus-4-8", "openai")).toEqual([
+      "anthropic",
+      "gemini-enterprise",
+    ]);
+  });
+
+  test("excludes the provider passed in", () => {
+    // Excluding anthropic still leaves gemini-enterprise, which also lists it.
+    expect(getProvidersForKnownModelId("claude-opus-4-8", "anthropic")).toEqual(
+      ["gemini-enterprise"],
+    );
+  });
+
+  test("uses exact matching, so namespaced overlaps do not false-match", () => {
+    // Anthropic's bare "claude-opus-4-8" must not match OpenRouter's
+    // namespaced "anthropic/claude-opus-4-8" or vice versa.
+    expect(
+      getProvidersForKnownModelId("anthropic/claude-opus-4-8", "openai"),
+    ).toEqual(["openrouter"]);
+    expect(
+      getProvidersForKnownModelId("anthropic/claude-opus-4-8", "openrouter"),
+    ).toEqual([]);
+  });
+
+  test("returns empty for custom / unknown model ids", () => {
+    expect(
+      getProvidersForKnownModelId("my-gateway-model", "openai-compatible"),
+    ).toEqual([]);
+  });
+});
+
+describe("isModelIdForOtherProvider", () => {
+  test("flags a model that clearly belongs to a different provider", () => {
+    expect(isModelIdForOtherProvider("claude-opus-4-8", "openai")).toBe(true);
+  });
+
+  test("does not flag a model that is valid for the configured provider", () => {
+    expect(isModelIdForOtherProvider("claude-opus-4-8", "anthropic")).toBe(
+      false,
+    );
+  });
+
+  test("does not flag shared OpenAI models across openai / openai-chatgpt", () => {
+    const [firstOpenAiModel] = getProviderModelOptions("openai");
+    if (firstOpenAiModel) {
+      expect(
+        isModelIdForOtherProvider(firstOpenAiModel.id, "openai-chatgpt"),
+      ).toBe(false);
+    }
+  });
+
+  test("does not flag custom / unknown model ids", () => {
+    expect(
+      isModelIdForOtherProvider("my-gateway-model", "openai-compatible"),
+    ).toBe(false);
+  });
+
+  test("trims whitespace before comparing", () => {
+    expect(isModelIdForOtherProvider("  claude-opus-4-8  ", "openai")).toBe(
+      true,
+    );
+  });
+});