diff --git a/packages/prompts-core/src/index.ts b/packages/prompts-core/src/index.ts
index f12a83c8b..d5b68ec06 100644
--- a/packages/prompts-core/src/index.ts
+++ b/packages/prompts-core/src/index.ts
@@ -6,3 +6,5 @@ export type {
   RuntimeInjection,
   VariantTable,
 } from "./types"
+export { resolveVariant } from "./variant-resolver"
+export type { ResolveVariantInput } from "./variant-resolver"
diff --git a/packages/prompts-core/src/variant-resolver.test.ts b/packages/prompts-core/src/variant-resolver.test.ts
new file mode 100644
index 000000000..303e3d028
--- /dev/null
+++ b/packages/prompts-core/src/variant-resolver.test.ts
@@ -0,0 +1,50 @@
+import { describe, expect, test } from "bun:test"
+import type { PromptSource, VariantTable } from "./types"
+import { resolveVariant } from "./variant-resolver"
+
+const promptSource = (baseDir: string): PromptSource => ({ baseDir })
+
+const variants = {
+  planner: promptSource("/prompts/planner"),
+  gpt: promptSource("/prompts/gpt"),
+  gemini: promptSource("/prompts/gemini"),
+  kimi: promptSource("/prompts/kimi"),
+  glm: promptSource("/prompts/glm"),
+  default: promptSource("/prompts/default"),
+} satisfies VariantTable
+
+describe("resolveVariant", () => {
+  test("#given Claude Opus 4.7 model #then resolves default variant", () => {
+    expect(resolveVariant({ modelID: "claude-opus-4-7", variants })).toBe("default")
+  })
+
+  test("#given GPT model #then resolves gpt variant", () => {
+    expect(resolveVariant({ modelID: "gpt-5-5", variants })).toBe("gpt")
+  })
+
+  test("#given Gemini model #then resolves gemini variant", () => {
+    expect(resolveVariant({ modelID: "gemini-3-1-pro", variants })).toBe("gemini")
+  })
+
+  test("#given Kimi K2 model #then resolves kimi variant", () => {
+    expect(resolveVariant({ modelID: "kimi-k2-6", variants })).toBe("kimi")
+  })
+
+  test("#given GLM model #then resolves glm variant", () => {
+    expect(resolveVariant({ modelID: "glm-5-1", variants })).toBe("glm")
+  })
+
+  test("#given Prometheus agent #then planner overrides model variant", () => {
+    expect(resolveVariant({ agentName: "prometheus", modelID: "gpt-5-5", variants })).toBe(
+      "planner"
+    )
+  })
+
+  test("#given unknown model #then falls back to default variant", () => {
+    expect(resolveVariant({ modelID: "claude-haiku-4-5", variants })).toBe("default")
+  })
+
+  test("#given empty variants table #then throws TypeError", () => {
+    expect(() => resolveVariant({ modelID: "gpt-5-5", variants: {} })).toThrow(TypeError)
+  })
+})
diff --git a/packages/prompts-core/src/variant-resolver.ts b/packages/prompts-core/src/variant-resolver.ts
new file mode 100644
index 000000000..f2f5e1d81
--- /dev/null
+++ b/packages/prompts-core/src/variant-resolver.ts
@@ -0,0 +1,59 @@
+import {
+  isClaudeOpus47Model,
+  isGeminiModel,
+  isGlmModel,
+  isGptModel,
+  isKimiK2Model,
+  isMiniMaxModel,
+} from "@oh-my-opencode/model-core"
+import type { VariantTable } from "./types"
+
+export type ResolveVariantInput = {
+  readonly modelID?: string
+  readonly agentName?: string
+  readonly variants: VariantTable
+}
+
+const PLANNER_AGENT_NAMES = new Set(["prometheus"])
+
+const MODEL_MATCHERS: Record<string, (modelID: string) => boolean> = {
+  gpt: isGptModel,
+  gemini: isGeminiModel,
+  kimi: isKimiK2Model,
+  glm: isGlmModel,
+  "opus-4-7": isClaudeOpus47Model,
+  minimax: isMiniMaxModel,
+}
+
+export function resolveVariant(input: ResolveVariantInput): string {
+  const variantNames = Object.keys(input.variants)
+  if (variantNames.length === 0) {
+    throw new TypeError("resolveVariant requires at least one prompt variant")
+  }
+
+  if (isPlannerAgent(input.agentName) && variantNames.includes("planner")) {
+    return "planner"
+  }
+
+  if (input.modelID !== undefined) {
+    for (const variantName of variantNames) {
+      if (matchesModelVariant(variantName, input.modelID)) return variantName
+    }
+  }
+
+  if (variantNames.includes("default")) return "default"
+
+  const firstVariant = variantNames[0]
+  if (firstVariant !== undefined) return firstVariant
+
+  throw new TypeError("resolveVariant requires at least one prompt variant")
+}
+
+function isPlannerAgent(agentName: string | undefined): boolean {
+  return agentName !== undefined && PLANNER_AGENT_NAMES.has(agentName.toLowerCase())
+}
+
+function matchesModelVariant(variantName: string, modelID: string): boolean {
+  const matcher = MODEL_MATCHERS[variantName]
+  return matcher?.(modelID) ?? false
+}