feat(config): add model variant support

Allow optional model variant config for agents and categories.
Propagate category variants into task model payloads so
category-driven runs inherit provider-specific variants.

Closes: #647

src/shared/agent-variant.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, test } from "bun:test"
+import type { OhMyOpenCodeConfig } from "../config"
+import { applyAgentVariant, resolveAgentVariant } from "./agent-variant"
+
+describe("resolveAgentVariant", () => {
+  test("returns undefined when agent name missing", () => {
+    // #given
+    const config = {} as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config)
+
+    // #then
+    expect(variant).toBeUndefined()
+  })
+
+  test("returns agent override variant", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config, "Sisyphus")
+
+    // #then
+    expect(variant).toBe("low")
+  })
+
+  test("returns category variant when agent uses category", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { category: "ultrabrain" },
+      },
+      categories: {
+        ultrabrain: { model: "openai/gpt-5.2", variant: "xhigh" },
+      },
+    } as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config, "Sisyphus")
+
+    // #then
+    expect(variant).toBe("xhigh")
+  })
+})
+
+describe("applyAgentVariant", () => {
+  test("sets variant when message is undefined", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+    const message: { variant?: string } = {}
+
+    // #when
+    applyAgentVariant(config, "Sisyphus", message)
+
+    // #then
+    expect(message.variant).toBe("low")
+  })
+
+  test("does not override existing variant", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+    const message = { variant: "max" }
+
+    // #when
+    applyAgentVariant(config, "Sisyphus", message)
+
+    // #then
+    expect(message.variant).toBe("max")
+  })
+})

src/shared/agent-variant.ts

@@ -0,0 +1,40 @@
+import type { OhMyOpenCodeConfig } from "../config"
+
+export function resolveAgentVariant(
+  config: OhMyOpenCodeConfig,
+  agentName?: string
+): string | undefined {
+  if (!agentName) {
+    return undefined
+  }
+
+  const agentOverrides = config.agents as
+    | Record<string, { variant?: string; category?: string }>
+    | undefined
+  const agentOverride = agentOverrides?.[agentName]
+  if (!agentOverride) {
+    return undefined
+  }
+
+  if (agentOverride.variant) {
+    return agentOverride.variant
+  }
+
+  const categoryName = agentOverride.category
+  if (!categoryName) {
+    return undefined
+  }
+
+  return config.categories?.[categoryName]?.variant
+}
+
+export function applyAgentVariant(
+  config: OhMyOpenCodeConfig,
+  agentName: string | undefined,
+  message: { variant?: string }
+): void {
+  const variant = resolveAgentVariant(config, agentName)
+  if (variant !== undefined && message.variant === undefined) {
+    message.variant = variant
+  }
+}

src/shared/first-message-variant.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test"
+import { createFirstMessageVariantGate } from "./first-message-variant"
+
+describe("createFirstMessageVariantGate", () => {
+  test("marks new sessions and clears after apply", () => {
+    // #given
+    const gate = createFirstMessageVariantGate()
+
+    // #when
+    gate.markSessionCreated({ id: "session-1" })
+
+    // #then
+    expect(gate.shouldOverride("session-1")).toBe(true)
+
+    // #when
+    gate.markApplied("session-1")
+
+    // #then
+    expect(gate.shouldOverride("session-1")).toBe(false)
+  })
+
+  test("ignores forked sessions", () => {
+    // #given
+    const gate = createFirstMessageVariantGate()
+
+    // #when
+    gate.markSessionCreated({ id: "session-2", parentID: "session-parent" })
+
+    // #then
+    expect(gate.shouldOverride("session-2")).toBe(false)
+  })
+})

src/shared/first-message-variant.ts

@@ -0,0 +1,28 @@
+type SessionInfo = {
+  id?: string
+  parentID?: string
+}
+
+export function createFirstMessageVariantGate() {
+  const pending = new Set<string>()
+
+  return {
+    markSessionCreated(info?: SessionInfo) {
+      if (info?.id && !info.parentID) {
+        pending.add(info.id)
+      }
+    },
+    shouldOverride(sessionID?: string) {
+      if (!sessionID) return false
+      return pending.has(sessionID)
+    },
+    markApplied(sessionID?: string) {
+      if (!sessionID) return
+      pending.delete(sessionID)
+    },
+    clear(sessionID?: string) {
+      if (!sessionID) return
+      pending.delete(sessionID)
+    },
+  }
+}

src/shared/index.ts

@@ -21,3 +21,4 @@ export * from "./opencode-version"
 export * from "./permission-compat"
 export * from "./external-plugin-detector"
 export * from "./zip-extractor"
+export * from "./agent-variant"