diff --git a/src/config/schema/fallback-models.test.ts b/src/config/schema/fallback-models.test.ts
new file mode 100644
index 000000000..966348288
--- /dev/null
+++ b/src/config/schema/fallback-models.test.ts
@@ -0,0 +1,100 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+
+import { OhMyOpenCodeConfigSchema } from "../schema"
+import type { FallbackModelObject } from "./fallback-models"
+import { FallbackModelsSchema } from "./fallback-models"
+
+describe("FallbackModelsSchema", () => {
+  test("accepts string array fallback_models", () => {
+    // given
+    const fallbackModels = ["openai/gpt-5.4", "anthropic/claude-sonnet-4-6"]
+
+    // when
+    const result = FallbackModelsSchema.safeParse(fallbackModels)
+
+    // then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data).toEqual(fallbackModels)
+    }
+  })
+
+  test("accepts object array fallback_models", () => {
+    // given
+    const fallbackModels: FallbackModelObject[] = [
+      {
+        model: "openai/gpt-5.4",
+        variant: "high",
+        reasoningEffort: "high",
+        temperature: 0.3,
+      },
+    ]
+
+    // when
+    const result = FallbackModelsSchema.safeParse(fallbackModels)
+
+    // then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data).toEqual(fallbackModels)
+    }
+  })
+})
+
+describe("OhMyOpenCodeConfigSchema fallback_models", () => {
+  test("accepts object array fallback_models under agents", () => {
+    // given
+    const fallbackModels: FallbackModelObject[] = [
+      {
+        model: "openai/gpt-5.4",
+        variant: "low",
+        reasoningEffort: "medium",
+      },
+    ]
+    const config = {
+      agents: {
+        explore: {
+          fallback_models: fallbackModels,
+        },
+      },
+    }
+
+    // when
+    const result = OhMyOpenCodeConfigSchema.safeParse(config)
+
+    // then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.agents?.explore?.fallback_models).toEqual(config.agents.explore.fallback_models)
+    }
+  })
+
+  test("accepts object array fallback_models under categories", () => {
+    // given
+    const fallbackModels: FallbackModelObject[] = [
+      {
+        model: "openai/gpt-5.4",
+        maxTokens: 4096,
+        thinking: { type: "disabled" },
+      },
+    ]
+    const config = {
+      categories: {
+        deep: {
+          fallback_models: fallbackModels,
+        },
+      },
+    }
+
+    // when
+    const result = OhMyOpenCodeConfigSchema.safeParse(config)
+
+    // then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.categories?.deep?.fallback_models).toEqual(config.categories.deep.fallback_models)
+    }
+  })
+})
diff --git a/src/plugin-handlers/prometheus-agent-config-builder.test.ts b/src/plugin-handlers/prometheus-agent-config-builder.test.ts
new file mode 100644
index 000000000..e6440834a
--- /dev/null
+++ b/src/plugin-handlers/prometheus-agent-config-builder.test.ts
@@ -0,0 +1,199 @@
+import { describe, expect, test, spyOn, afterEach, beforeEach } from "bun:test";
+import { buildPrometheusAgentConfig } from "./prometheus-agent-config-builder";
+import * as shared from "../shared";
+import * as categoryResolver from "./category-config-resolver";
+import type { CategoryConfig } from "../config/schema";
+
+describe("buildPrometheusAgentConfig", () => {
+  let fetchAvailableModelsSpy: ReturnType<typeof spyOn>;
+  let readConnectedProvidersCacheSpy: ReturnType<typeof spyOn>;
+  let resolveCategoryConfigSpy: ReturnType<typeof spyOn>;
+  let logSpy: ReturnType<typeof spyOn>;
+
+  beforeEach(() => {
+    fetchAvailableModelsSpy = spyOn(shared, "fetchAvailableModels").mockResolvedValue(new Set());
+    readConnectedProvidersCacheSpy = spyOn(shared, "readConnectedProvidersCache").mockReturnValue(null);
+    resolveCategoryConfigSpy = spyOn(categoryResolver, "resolveCategoryConfig").mockImplementation(
+      (category) => ({ model: `${category}/default-model` } as CategoryConfig)
+    );
+    logSpy = spyOn(shared, "log").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    fetchAvailableModelsSpy.mockRestore();
+    readConnectedProvidersCacheSpy.mockRestore();
+    resolveCategoryConfigSpy.mockRestore();
+    logSpy.mockRestore();
+  });
+
+  describe("#given no explicit Prometheus model configured", () => {
+    describe("#when currentModel is NOT in Prometheus fallback chain", () => {
+      test("falls through to fallback chain instead of using currentModel as override", async () => {
+        // given - currentModel is a model NOT in Prometheus fallback chain
+        // Prometheus chain: claude-opus-4-6, gpt-5.4, glm-5, gemini-3.1-pro
+        const currentModel = "some-provider/gpt-5.3-codex";
+
+        // when
+        await buildPrometheusAgentConfig({
+          configAgentPlan: undefined,
+          pluginPrometheusOverride: undefined,
+          userCategories: undefined,
+          currentModel,
+        });
+
+        // then - should NOT have resolved via override (currentModel)
+        // The model should fall through to fallback chain
+        const lastLogCall = logSpy.mock.calls[logSpy.mock.calls.length - 1];
+        const lastLogMessage = lastLogCall?.[0] as string;
+        expect(lastLogMessage).not.toContain("UI selection");
+        expect(lastLogMessage).not.toContain("config override");
+      });
+    });
+
+    describe("#when currentModel IS in Prometheus fallback chain", () => {
+      test("preserves currentModel as uiSelectedModel for claude-opus-4-6", async () => {
+        // given - currentModel matches a Prometheus fallback chain entry
+        const currentModel = "anthropic/claude-opus-4-6";
+
+        // when - should not throw and should produce a valid config
+        const result = await buildPrometheusAgentConfig({
+          configAgentPlan: undefined,
+          pluginPrometheusOverride: undefined,
+          userCategories: undefined,
+          currentModel,
+        });
+
+        // then - config should be produced (currentModel accepted as valid)
+        expect(result).toBeDefined();
+      });
+
+      test("accepts gpt-5.4 from fallback chain", async () => {
+        const result = await buildPrometheusAgentConfig({
+          configAgentPlan: undefined,
+          pluginPrometheusOverride: undefined,
+          userCategories: undefined,
+          currentModel: "openai/gpt-5.4",
+        });
+        expect(result).toBeDefined();
+      });
+
+      test("accepts glm-5 from fallback chain", async () => {
+        const result = await buildPrometheusAgentConfig({
+          configAgentPlan: undefined,
+          pluginPrometheusOverride: undefined,
+          userCategories: undefined,
+          currentModel: "opencode-go/glm-5",
+        });
+        expect(result).toBeDefined();
+      });
+
+      test("accepts gemini-3.1-pro from fallback chain", async () => {
+        const result = await buildPrometheusAgentConfig({
+          configAgentPlan: undefined,
+          pluginPrometheusOverride: undefined,
+          userCategories: undefined,
+          currentModel: "google/gemini-3.1-pro",
+        });
+        expect(result).toBeDefined();
+      });
+    });
+  });
+
+  describe("#given explicit Prometheus model configured via plugin override", () => {
+    test("explicit config wins over currentModel and fallback chain", async () => {
+      // given
+      const currentModel = "anthropic/claude-opus-4-6";
+      const explicitModel = "custom-provider/custom-model";
+
+      // when
+      await buildPrometheusAgentConfig({
+        configAgentPlan: undefined,
+        pluginPrometheusOverride: { model: explicitModel },
+        userCategories: undefined,
+        currentModel,
+      });
+
+      // then - should resolve via config override, not UI selection
+      const configOverrideLog = logSpy.mock.calls.find(
+        (call) => (call[0] as string).includes("config override")
+      );
+      expect(configOverrideLog).toBeDefined();
+      expect(configOverrideLog?.[1]).toEqual({ model: explicitModel });
+    });
+  });
+
+  describe("#given category with model configured", () => {
+    test("category model wins when no explicit override", async () => {
+      // given
+      const currentModel = "anthropic/claude-opus-4-6";
+      const categoryModel = "category-provider/category-model";
+
+      resolveCategoryConfigSpy.mockReturnValue({
+        model: categoryModel,
+      } as CategoryConfig);
+
+      // when
+      await buildPrometheusAgentConfig({
+        configAgentPlan: undefined,
+        pluginPrometheusOverride: { category: "test-category" },
+        userCategories: { "test-category": { model: categoryModel } },
+        currentModel,
+      });
+
+      // then - should resolve via category default
+      const categoryDefaultLog = logSpy.mock.calls.find(
+        (call) => (call[0] as string).includes("category default")
+      );
+      expect(categoryDefaultLog).toBeDefined();
+    });
+
+    test("explicit model override wins over category model", async () => {
+      // given
+      const categoryModel = "category-provider/category-model";
+      const explicitModel = "explicit-provider/explicit-model";
+
+      resolveCategoryConfigSpy.mockReturnValue({
+        model: categoryModel,
+      } as CategoryConfig);
+
+      // when
+      await buildPrometheusAgentConfig({
+        configAgentPlan: undefined,
+        pluginPrometheusOverride: {
+          category: "test-category",
+          model: explicitModel,
+        },
+        userCategories: { "test-category": { model: categoryModel } },
+        currentModel: undefined,
+      });
+
+      // then - should resolve via config override, not category default
+      const configOverrideLog = logSpy.mock.calls.find(
+        (call) => (call[0] as string).includes("config override")
+      );
+      expect(configOverrideLog).toBeDefined();
+      expect(configOverrideLog?.[1]).toEqual({ model: explicitModel });
+    });
+  });
+
+  describe("#given no currentModel and no explicit config", () => {
+    test("falls through to fallback chain", async () => {
+      // given - no currentModel, no explicit config
+      readConnectedProvidersCacheSpy.mockReturnValue(["anthropic"]);
+
+      // when
+      await buildPrometheusAgentConfig({
+        configAgentPlan: undefined,
+        pluginPrometheusOverride: undefined,
+        userCategories: undefined,
+        currentModel: undefined,
+      });
+
+      // then - should resolve via fallback chain
+      const fallbackChainLog = logSpy.mock.calls.find(
+        (call) => (call[0] as string).includes("fallback chain")
+      );
+      expect(fallbackChainLog).toBeDefined();
+    });
+  });
+});
diff --git a/src/plugin-handlers/prometheus-agent-config-builder.ts b/src/plugin-handlers/prometheus-agent-config-builder.ts
index 63824f95b..620e9d721 100644
--- a/src/plugin-handlers/prometheus-agent-config-builder.ts
+++ b/src/plugin-handlers/prometheus-agent-config-builder.ts
@@ -2,6 +2,7 @@ import type { CategoryConfig } from "../config/schema";
 import { PROMETHEUS_PERMISSION, getPrometheusPrompt } from "../agents/prometheus";
 import { resolvePromptAppend } from "../agents/builtin-agents/resolve-file-uri";
 import { AGENT_MODEL_REQUIREMENTS } from "../shared/model-requirements";
+import type { FallbackEntry } from "../shared/model-requirements";
 import {
   fetchAvailableModels,
   readConnectedProvidersCache,
@@ -22,6 +23,20 @@ type PrometheusOverride = Record<string, unknown> & {
   prompt_append?: string;
 };
 
+function isModelInFallbackChain(
+  model: string | undefined,
+  fallbackChain: FallbackEntry[] | undefined,
+): boolean {
+  if (!model || !fallbackChain || fallbackChain.length === 0) {
+    return false;
+  }
+
+  const modelParts = model.split("/");
+  const modelName = modelParts.length >= 2 ? modelParts.slice(1).join("/") : model;
+
+  return fallbackChain.some((entry) => entry.model === modelName);
+}
+
 export async function buildPrometheusAgentConfig(params: {
   configAgentPlan: Record<string, unknown> | undefined;
   pluginPrometheusOverride: PrometheusOverride | undefined;
@@ -42,9 +57,18 @@ export async function buildPrometheusAgentConfig(params: {
   const configuredPrometheusModel =
     params.pluginPrometheusOverride?.model ?? categoryConfig?.model;
 
+  const shouldUseCurrentModel = isModelInFallbackChain(
+    params.currentModel,
+    requirement?.fallbackChain,
+  );
+
   const modelResolution = resolveModelPipeline({
     intent: {
-      uiSelectedModel: configuredPrometheusModel ? undefined : params.currentModel,
+      uiSelectedModel: configuredPrometheusModel
+        ? undefined
+        : shouldUseCurrentModel
+          ? params.currentModel
+          : undefined,
       userModel: params.pluginPrometheusOverride?.model,
       categoryDefaultModel: categoryConfig?.model,
     },