Question 1

How to fix 'Failed to open acpRegistry provider session' error when using Cline with T3 Code (ACP)?

Accepted Answer

Root cause is a bug in Cline 3.0.73's ACP mode: `cline --acp` returns two session config options tagged `category: "model"` — `provider` (auth selector) first, `model` second. Per the ACP spec, clients break same-category ties by array order, so spec-compliant clients like T3 Code treat `provider` as the model selector and send the chosen model id to it via `session/set_config_option`. Cline rejects this with: `Invalid value "openrouter/free" for session config option "provider": expected one of cline, cline-pass, openai-codex`, and the session fails to open.

Verified workaround: insert a stdio proxy between the client and `cline --acp` that rewrites the `provider` option's category from `"model"` to `"provider"` in the `session/new` JSON-RPC response. With this rewrite, sessions open, models respond, and tools run normally.

Proper fix (upstream): Cline must keep `category: "model"` only on the `model` option and assign `provider` a custom category such as `_provider` (underscore-prefixed names are reserved for custom use in the ACP spec) or `model_config`. Track Cline issue #15158 and upgrade the `cline` npm package once a patched release is published.

Question 2

Why does the Cline desktop app sidebar sort mode reset to time-sorted on every launch?

Accepted Answer

Known bug in Cline Desktop 0.0.37: the Sessions ↔ Projects sort toggle is never persisted. In `apps/examples/desktop-app/webview/components/agent-sidebar.tsx`, the state is initialized with `useState<SidebarSortMode>("time")` and the toggle only flips local component state—there is no localStorage read/write for the sort mode, so the component always remounts into the `"time"` default on launch. There is no in-app setting to fix this. Workarounds: (1) manually re-click the sort toggle after each launch; (2) if building from source, patch the component with a lazy `useState` initializer that reads a key such as `cline.code.sidebar-sort.v1` from localStorage (falling back to `"time"`) and write the new value back inside the toggle's `onClick` handler—the same persistence pattern already used by the sibling preferences `cline.code.model-selection.v1` and `cline.code.workspace-selection.v2`.

Question 3

How to fix "Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead" error in Cline with OpenAI Compatible provider?

Accepted Answer

**Cause:** Cline's OpenAI Compatible provider only converts `max_tokens` to `max_completion_tokens` for model IDs matching its built-in reasoning-model patterns (`isOpenAIReasoningEraModelId`: o1/o3/o4 and the GPT-5 family). A custom model ID like `gpt-6-sol` does not match these patterns, so Cline sends the legacy `max_tokens` parameter, which GPT-6-era endpoints reject.

**Workaround 1 – Use a model ID alias that matches a recognized pattern.** Since the endpoint is fronted by your own Azure APIM gateway (which routes by model ID), publish an alias whose name matches Cline's reasoning patterns, e.g. `gpt-5-sol` or any ID containing `gpt-5`/`o3`, route it to the actual GPT-6 deployment in APIM, and enter that alias as the model ID in Cline. Cline will then send `max_completion_tokens`.

**Workaround 2 – Rewrite the parameter at the gateway.** Add an APIM inbound policy that transforms the request JSON body, renaming `max_tokens` to `max_completion_tokens` (e.g., via a `set-body` policy with a Liquid template) before forwarding to Azure OpenAI.

**Permanent fix:** Upgrade Cline once a release extends the reasoning-model ID patterns to cover the GPT-6 family (tracked in issue #14478). Until then, the alias or gateway rewrite is required when a maximum output token limit is configured.

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