fix(agents): honor user atlas model when resolution returns undefined (#4255)

Atlas was the only agent that bailed early (returning undefined) when
applyModelResolution returned undefined, even if the user had explicitly
configured agents.atlas.model. The general-agents.ts path already handled
this case by falling back to the user's override; sisyphus has its own
explicit-config check. Atlas had neither, so under edge cases (cold
provider cache, no system default, empty availableModels) Atlas was
silently dropped and OpenCode used its built-in default — surfaced as
the hardcoded claude-sonnet-4-6 in the reported bug.

This aligns atlas-agent.ts with the same defensive pattern used in
general-agents.ts: when resolution fails but the user has an explicit
model override, honor it directly instead of dropping the agent.

Adds a regression test covering the scenario where availableModels is
empty AND systemDefaultModel is undefined AND no provider cache exists.

src/agents/builtin-agents/atlas-agent.ts

@@ -38,7 +38,7 @@ export function maybeCreateAtlasConfig(input: {
   const orchestratorOverride = agentOverrides["atlas"]
   const atlasRequirement = AGENT_MODEL_REQUIREMENTS["atlas"]
 
-  const atlasResolution = applyModelResolution({
+  let atlasResolution = applyModelResolution({
     uiSelectedModel: orchestratorOverride?.model !== undefined ? undefined : uiSelectedModel,
     userModel: orchestratorOverride?.model,
     requirement: atlasRequirement,
@@ -46,6 +46,12 @@ export function maybeCreateAtlasConfig(input: {
     systemDefaultModel,
   })
 
+  if (!atlasResolution && orchestratorOverride?.model) {
+    // User explicitly configured a model but resolution failed (e.g., cold cache, no system default).
+    // Honor the user's choice directly instead of dropping Atlas entirely.
+    atlasResolution = { model: orchestratorOverride.model, provenance: "override" as const }
+  }
+
   if (!atlasResolution) return undefined
   const { model: atlasModel, variant: atlasResolvedVariant } = atlasResolution
 

src/agents/utils.test.ts

@@ -170,6 +170,30 @@ describe("createBuiltinAgents with model overrides", () => {
     }
   })
 
+  test("atlas honors user config model when resolution fails (no available models, no system default)", async () => {
+    // #given - regression for #4255: user sets agents.atlas.model but availableModels is empty
+    // and systemDefaultModel is undefined, so applyModelResolution returns undefined.
+    // Previous behavior: atlas was silently dropped, OpenCode used its built-in default.
+    // Expected behavior: honor the user's explicit model override.
+    const cacheSpy = spyOn(connectedProvidersCache, "readConnectedProvidersCache").mockReturnValue(null)
+    const fetchSpy = spyOn(shared, "fetchAvailableModels").mockResolvedValue(new Set())
+    const overrides = {
+      atlas: { model: "minimax-cn-coding-plan/MiniMax-M2.5-highspeed" },
+    }
+
+    try {
+      // #when - no systemDefaultModel, no availableModels, no cache
+      const agents = await createBuiltinAgents([], overrides, undefined, undefined)
+
+      // #then
+      expect(agents.atlas).toBeDefined()
+      expect(agents.atlas.model).toBe("minimax-cn-coding-plan/MiniMax-M2.5-highspeed")
+    } finally {
+      cacheSpy.mockRestore()
+      fetchSpy.mockRestore()
+    }
+  })
+
   test("Sisyphus is created on first run when no availableModels or cache exist", async () => {
     // #given
     const systemDefaultModel = "anthropic/claude-opus-4-7"