fix: revert model ID rename (backward-compat) + resolve review comments

Addresses two review findings:

1. cubic P1 — renaming the internal model ID from `altimate-default` to
   `altimate-llm-gateway` broke backward compatibility. Opencode
   persists selected model IDs to `model.json` (favorites, recents,
   variants) and users can pin `model: altimate-backend/altimate-default`
   in their opencode.json. After the rename those references would
   silently go stale. Keep the ID as `altimate-default`; the polish the
   user actually wanted was the display name, which stays as
   "Altimate LLM Gateway". Added a comment explaining why the ID is
   preserved. Rephrased the auto-selection tip in
   `docs/docs/configure/providers.md` to refer to the display name
   rather than the internal ID.

2. Coderabbit — the custom-URL example in `docs/docs/getting-started.md`
   used `api-url::instance-name::api-key` which is ambiguous; the parser
   requires `http(s)://`. Replaced with a concrete
   `https://api.example.com::...` example and added a sentence stating
   the scheme is required.

Also reverted provider.ts, acp/agent.ts, provider.test.ts, and
release-v0.5.20-adversarial.test.ts to use the original `altimate-default`
ID.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: suryaiyer95

docs/docs/configure/providers.md

@@ -39,7 +39,7 @@ Managed LLM access with dynamic routing across Sonnet 4.6, Opus 4.6, GPT-5.4, GP
 For pricing, security, and data handling details, see the [Altimate LLM Gateway guide](https://datamates-docs.myaltimate.com/user-guide/components/llm-gateway/).
 
 !!! tip "Automatic model selection"
-    When Altimate credentials are configured and no model is explicitly chosen, `altimate-backend/altimate-llm-gateway` is selected automatically. You can override this by setting `model` in your config or by restricting the `provider` section to specific providers only.
+    When Altimate credentials are configured and no model is explicitly chosen, the Altimate LLM Gateway is selected automatically. You can override this by setting `model` in your config or by restricting the `provider` section to specific providers only.
 
 ## Anthropic
 

docs/docs/getting-started.md

@@ -94,10 +94,10 @@ For example: `acme::your-api-key` — this uses the default API URL `https://api
 - **Instance Name** — the subdomain from your Altimate dashboard URL (e.g. `acme` from `https://acme.app.myaltimate.com`)
 - **API Key** — go to **Settings > API Keys** in your Altimate dashboard and click **Copy**
 
-If your instance uses a different API URL (e.g. a self-hosted or `getaltimate.com` deployment), prepend it:
+If your instance uses a different API URL (e.g. a self-hosted or `getaltimate.com` deployment), prepend the full URL — it must include the `http://` or `https://` scheme, hostname-only values will fail validation:
 
 ```text
-api-url::instance-name::api-key
+https://api.example.com::instance-name::api-key
 ```
 
 For example: `https://api.getaltimate.com::acme::your-api-key`

packages/opencode/src/acp/agent.ts

@@ -1581,12 +1581,12 @@ export namespace ACP {
     const altimateProvider = providers.find((p) => p.id === "altimate-backend")
     if (
       altimateProvider &&
-      altimateProvider.models["altimate-llm-gateway"] &&
+      altimateProvider.models["altimate-default"] &&
       (!providerFilter || Object.keys(providerFilter).includes("altimate-backend"))
     ) {
       return {
         providerID: ProviderID.make("altimate-backend"),
-        modelID: ModelID.make("altimate-llm-gateway"),
+        modelID: ModelID.make("altimate-default"),
       }
     }
     // altimate_change end

packages/opencode/src/provider/provider.ts

@@ -1022,12 +1022,16 @@ export namespace Provider {
     // altimate_change start — register altimate-backend as an OpenAI-compatible provider
     if (!database["altimate-backend"]) {
       const backendModels: Record<string, Model> = {
-        "altimate-llm-gateway": {
-          id: ModelID.make("altimate-llm-gateway"),
+        // ID "altimate-default" is kept for backward compatibility — existing
+        // users have it persisted in their model.json favorites/recents and in
+        // opencode.json `model:` entries. Display name ("Altimate LLM Gateway")
+        // is what the TUI actually shows, so branding stays correct.
+        "altimate-default": {
+          id: ModelID.make("altimate-default"),
           providerID: ProviderID.make("altimate-backend"),
           name: "Altimate LLM Gateway",
           family: "openai",
-          api: { id: "altimate-llm-gateway", url: "", npm: "@ai-sdk/openai-compatible" },
+          api: { id: "altimate-default", url: "", npm: "@ai-sdk/openai-compatible" },
           status: "active",
           headers: {},
           options: {},
@@ -1642,15 +1646,15 @@ export namespace Provider {
     const altimateProvider = providers[altimateProviderID]
     if (
       altimateProvider &&
-      altimateProvider.models[ModelID.make("altimate-llm-gateway")] &&
+      altimateProvider.models[ModelID.make("altimate-default")] &&
       (!cfg.provider || Object.keys(cfg.provider).includes(String(altimateProviderID)))
     ) {
       // altimate_change start — log when altimate-backend auto-selected
-      log.info("defaulting to altimate-backend/altimate-llm-gateway (no model configured)")
+      log.info("defaulting to altimate-backend/altimate-default (no model configured)")
       // altimate_change end
       return {
         providerID: altimateProviderID,
-        modelID: ModelID.make("altimate-llm-gateway"),
+        modelID: ModelID.make("altimate-default"),
       }
     }
     // altimate_change end

packages/opencode/test/provider/provider.test.ts

@@ -2362,7 +2362,7 @@ test("defaultModel returns altimate-backend when altimate credentials exist and
       fn: async () => {
         const model = await Provider.defaultModel()
         expect(String(model.providerID)).toBe("altimate-backend")
-        expect(String(model.modelID)).toBe("altimate-llm-gateway")
+        expect(String(model.modelID)).toBe("altimate-default")
       },
     })
   } finally {
@@ -2408,7 +2408,7 @@ test("defaultModel prefers altimate-backend over other providers when altimate i
         // But defaultModel should prefer altimate-backend
         const model = await Provider.defaultModel()
         expect(String(model.providerID)).toBe("altimate-backend")
-        expect(String(model.modelID)).toBe("altimate-llm-gateway")
+        expect(String(model.modelID)).toBe("altimate-default")
       },
     })
   } finally {

packages/opencode/test/skill/release-v0.5.20-adversarial.test.ts

@@ -256,9 +256,9 @@ describe("v0.5.20 release: listTracesPaginated adversarial", () => {
 
 describe("v0.5.20 release: Provider.parseModel adversarial", () => {
   test("model string with multiple slashes preserves all parts", () => {
-    const result = Provider.parseModel("altimate-backend/altimate-llm-gateway")
+    const result = Provider.parseModel("altimate-backend/altimate-default")
     expect(String(result.providerID)).toBe("altimate-backend")
-    expect(String(result.modelID)).toBe("altimate-llm-gateway")
+    expect(String(result.modelID)).toBe("altimate-default")
   })
 
   test("model string with nested slashes preserves full model ID", () => {