diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index f935ab4857..eb45850dc7 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -1577,6 +1577,16 @@ export namespace ACP {
 
     if (specified && !providers.length) return specified
 
+    // altimate_change start — default to altimate-backend when configured and no model chosen yet
+    const altimateProvider = providers.find((p) => p.id === "altimate-backend")
+    if (altimateProvider && altimateProvider.models["altimate-default"]) {
+      return {
+        providerID: ProviderID.make("altimate-backend"),
+        modelID: ModelID.make("altimate-default"),
+      }
+    }
+    // altimate_change end
+
     const opencodeProvider = providers.find((p) => p.id === "opencode")
     if (opencodeProvider) {
       if (opencodeProvider.models["big-pickle"]) {
diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index 9e81a4ff38..6221bec5ff 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -1637,6 +1637,21 @@ export namespace Provider {
       return { providerID: entry.providerID, modelID: entry.modelID }
     }
 
+    // altimate_change start — default to altimate-backend when configured and no model chosen yet
+    const altimateProviderID = ProviderID.make("altimate-backend")
+    const altimateProvider = providers[altimateProviderID]
+    if (
+      altimateProvider &&
+      altimateProvider.models[ModelID.make("altimate-default")] &&
+      (!cfg.provider || Object.keys(cfg.provider).includes(String(altimateProviderID)))
+    ) {
+      return {
+        providerID: altimateProviderID,
+        modelID: ModelID.make("altimate-default"),
+      }
+    }
+    // altimate_change end
+
     const provider = Object.values(providers).find((p) => !cfg.provider || Object.keys(cfg.provider).includes(p.id))
     if (!provider) throw new Error("no providers found")
     const [model] = sort(Object.values(provider.models))
diff --git a/packages/opencode/test/provider/provider.test.ts b/packages/opencode/test/provider/provider.test.ts
index 05a6064051..42dd6c437e 100644
--- a/packages/opencode/test/provider/provider.test.ts
+++ b/packages/opencode/test/provider/provider.test.ts
@@ -1,5 +1,6 @@
 import { test, expect } from "bun:test"
 import path from "path"
+import fs from "fs/promises"
 
 import { tmpdir } from "../fixture/fixture"
 import { Instance } from "../../src/project/instance"
@@ -2330,4 +2331,158 @@ test("github-copilot is excluded when CODESPACES=true and only GITHUB_TOKEN is s
     },
   })
 })
+
+// altimate_change start — tests for altimate-backend default model preference
+test("defaultModel returns altimate-backend when altimate credentials exist and no model configured", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://altimate.ai/config.json",
+        }),
+      )
+    },
+  })
+  const originalHome = process.env.OPENCODE_TEST_HOME
+  process.env.OPENCODE_TEST_HOME = tmp.path
+  const altimateDir = path.join(tmp.path, ".altimate")
+  await fs.mkdir(altimateDir, { recursive: true })
+  await Bun.write(
+    path.join(altimateDir, "altimate.json"),
+    JSON.stringify({
+      altimateUrl: "https://test.altimate.ai",
+      altimateInstanceName: "test-instance",
+      altimateApiKey: "test-api-key",
+    }),
+  )
+  try {
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const model = await Provider.defaultModel()
+        expect(String(model.providerID)).toBe("altimate-backend")
+        expect(String(model.modelID)).toBe("altimate-default")
+      },
+    })
+  } finally {
+    if (originalHome === undefined) delete process.env.OPENCODE_TEST_HOME
+    else process.env.OPENCODE_TEST_HOME = originalHome
+  }
+})
+
+test("defaultModel prefers altimate-backend over other providers when altimate is configured", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://altimate.ai/config.json",
+        }),
+      )
+    },
+  })
+  const originalHome = process.env.OPENCODE_TEST_HOME
+  process.env.OPENCODE_TEST_HOME = tmp.path
+  const altimateDir = path.join(tmp.path, ".altimate")
+  await fs.mkdir(altimateDir, { recursive: true })
+  await Bun.write(
+    path.join(altimateDir, "altimate.json"),
+    JSON.stringify({
+      altimateUrl: "https://test.altimate.ai",
+      altimateInstanceName: "test-instance",
+      altimateApiKey: "test-api-key",
+    }),
+  )
+  try {
+    await Instance.provide({
+      directory: tmp.path,
+      init: async () => {
+        Env.set("ANTHROPIC_API_KEY", "test-api-key")
+      },
+      fn: async () => {
+        const providers = await Provider.list()
+        // Both providers should be available
+        expect(providers["anthropic"]).toBeDefined()
+        expect(providers["altimate-backend"]).toBeDefined()
+        // But defaultModel should prefer altimate-backend
+        const model = await Provider.defaultModel()
+        expect(String(model.providerID)).toBe("altimate-backend")
+        expect(String(model.modelID)).toBe("altimate-default")
+      },
+    })
+  } finally {
+    if (originalHome === undefined) delete process.env.OPENCODE_TEST_HOME
+    else process.env.OPENCODE_TEST_HOME = originalHome
+  }
+})
+
+test("defaultModel respects explicit config model over altimate-backend", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://altimate.ai/config.json",
+          model: "anthropic/claude-sonnet-4-20250514",
+        }),
+      )
+    },
+  })
+  const originalHome = process.env.OPENCODE_TEST_HOME
+  process.env.OPENCODE_TEST_HOME = tmp.path
+  const altimateDir = path.join(tmp.path, ".altimate")
+  await fs.mkdir(altimateDir, { recursive: true })
+  await Bun.write(
+    path.join(altimateDir, "altimate.json"),
+    JSON.stringify({
+      altimateUrl: "https://test.altimate.ai",
+      altimateInstanceName: "test-instance",
+      altimateApiKey: "test-api-key",
+    }),
+  )
+  try {
+    await Instance.provide({
+      directory: tmp.path,
+      init: async () => {
+        Env.set("ANTHROPIC_API_KEY", "test-api-key")
+      },
+      fn: async () => {
+        const model = await Provider.defaultModel()
+        expect(String(model.providerID)).toBe("anthropic")
+        expect(String(model.modelID)).toBe("claude-sonnet-4-20250514")
+      },
+    })
+  } finally {
+    if (originalHome === undefined) delete process.env.OPENCODE_TEST_HOME
+    else process.env.OPENCODE_TEST_HOME = originalHome
+  }
+})
+
+test("defaultModel falls through to other providers when altimate is not configured", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://altimate.ai/config.json",
+        }),
+      )
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    init: async () => {
+      Env.set("ANTHROPIC_API_KEY", "test-api-key")
+    },
+    fn: async () => {
+      const providers = await Provider.list()
+      // altimate-backend should NOT be available (no credentials file)
+      expect(providers["altimate-backend"]).toBeUndefined()
+      const model = await Provider.defaultModel()
+      // Should fall through to anthropic
+      expect(String(model.providerID)).toBe("anthropic")
+    },
+  })
+})
 // altimate_change end