fix: propagate variant field in all promptAsync continuation paths (#3081)

All 5 continuation paths now send variant as top-level body field:
- boulder-continuation-injector.ts
- ralph-loop/continuation-prompt-injector.ts
- todo-continuation-enforcer/continuation-injection.ts
- unstable-agent-babysitter-hook.ts
- session-recovery/resume.ts

Plus type/helper updates in atlas, todo-continuation-enforcer,
unstable-agent-babysitter, and session-recovery.

TDD: 18 regression tests added, all pass. tsc clean.

src/hooks/atlas/boulder-continuation-injector.test.ts

@@ -121,4 +121,64 @@ describe("injectBoulderContinuation", () => {
     expect(result).toBe("skipped_agent_unavailable")
     expect(promptAsyncMock).not.toHaveBeenCalled()
   })
+
+  test("#given recent prompt context includes variant #when injecting boulder continuation #then promptAsync receives variant as a top-level field", async () => {
+    // given
+    registerAgentName("atlas")
+    const capturedRequests: Array<{
+      body?: {
+        model?: { providerID: string; modelID: string }
+        variant?: string
+      }
+    }> = []
+    const promptAsyncMock = mock(async (request: unknown) => {
+      capturedRequests.push(request as typeof capturedRequests[number])
+      return undefined
+    })
+    const recentModel = {
+      providerID: "anthropic",
+      modelID: "claude-sonnet-4-20250514",
+      variant: "max",
+    }
+    const messagesMock = mock(async () => ({
+      data: [{
+        id: "msg_1",
+        info: {
+          agent: "atlas",
+          model: recentModel,
+          time: { created: Date.now() },
+        },
+      }],
+    }))
+
+    const ctx = {
+      directory: "/tmp",
+      client: {
+        session: {
+          messages: messagesMock,
+          promptAsync: promptAsyncMock,
+        },
+      },
+    } as unknown as PluginInput
+
+    // when
+    const result = await injectBoulderContinuation({
+      ctx,
+      sessionID: "ses_test_variant",
+      planName: "test-plan",
+      remaining: 1,
+      total: 2,
+      agent: "atlas",
+      sessionState: { promptFailureCount: 0 },
+    })
+
+    // then
+    expect(result).toBe("injected")
+    expect(capturedRequests).toHaveLength(1)
+    expect(capturedRequests[0]?.body?.model).toEqual({
+      providerID: "anthropic",
+      modelID: "claude-sonnet-4-20250514",
+    })
+    expect(capturedRequests[0]?.body?.variant).toBe("max")
+  })
 })

src/hooks/atlas/boulder-continuation-injector.ts

@@ -71,12 +71,18 @@ export async function injectBoulderContinuation(input: {
     const promptContext = await resolveRecentPromptContextForSession(ctx, sessionID)
     const inheritedTools = resolveInheritedPromptTools(sessionID, promptContext.tools)
 
-		await ctx.client.session.promptAsync({
-			path: { id: sessionID },
-			body: {
-				agent: continuationAgent,
-				...(promptContext.model !== undefined ? { model: promptContext.model } : {}),
-				...(inheritedTools ? { tools: inheritedTools } : {}),
+    const launchModel = promptContext.model
+      ? { providerID: promptContext.model.providerID, modelID: promptContext.model.modelID }
+      : undefined
+    const launchVariant = promptContext.model?.variant
+
+    await ctx.client.session.promptAsync({
+      path: { id: sessionID },
+      body: {
+        agent: continuationAgent,
+        ...(launchModel ? { model: launchModel } : {}),
+        ...(launchVariant ? { variant: launchVariant } : {}),
+        ...(inheritedTools ? { tools: inheritedTools } : {}),
         parts: [createInternalAgentTextPart(prompt)],
       },
       query: { directory: ctx.directory },

src/hooks/atlas/recent-model-resolver.ts

@@ -40,7 +40,14 @@ export async function resolveRecentPromptContextForSession(
       const model = info?.model
       const tools = normalizePromptTools(info?.tools)
       if (model?.providerID && model?.modelID) {
-        return { model: { providerID: model.providerID, modelID: model.modelID }, tools }
+        return {
+          model: {
+            providerID: model.providerID,
+            modelID: model.modelID,
+            ...(model.variant ? { variant: model.variant } : {}),
+          },
+          tools,
+        }
       }
 
       if (info?.providerID && info?.modelID) {
@@ -63,7 +70,14 @@ export async function resolveRecentPromptContextForSession(
   if (!model?.providerID || !model?.modelID) {
     return { tools }
   }
-  return { model: { providerID: model.providerID, modelID: model.modelID }, tools }
+  return {
+    model: {
+      providerID: model.providerID,
+      modelID: model.modelID,
+      ...(model.variant ? { variant: model.variant } : {}),
+    },
+    tools,
+  }
 }
 
 export async function resolveRecentModelForSession(

src/hooks/atlas/types.ts

@@ -2,7 +2,7 @@ import type { AgentOverrides } from "../../config"
 import type { BackgroundManager } from "../../features/background-agent"
 import type { TopLevelTaskRef } from "../../features/boulder-state"
 
-export type ModelInfo = { providerID: string; modelID: string }
+export type ModelInfo = { providerID: string; modelID: string; variant?: string }
 
 export interface AtlasHookOptions {
   directory: string