Add configurable path_suffix for OpenAI-compat endpoints

[RefuseOdd]

Summary

Adds an optional path_suffix field to provider config that lets users override the API path for OpenAI-compatible endpoints. Some third-party endpoints reject /v1/chat/completions and only accept /chat/completions. This change allows users to set path_suffix = "/chat/completions" in their provider config to bypass the default /v1 versioning.

Changes

• ProviderConfigToml (config crate): added path_suffix: Option<String>
• ProviderConfig (tui crate): added path_suffix: Option<String>
• merge_provider_config: propagates the new field
• merge_project_provider_config: propagates the new field
• api_url: delegates to new api_url_with_suffix
• api_url_with_suffix: when suffix is set, uses base URL directly with the suffix path (skips /v1)
• DeepSeekClient::new: reads path_suffix from config
• DeepSeekClient methods: all URL-building calls use api_url_with_suffix
• config.example.toml: documents the new option
• Tests for the new URL building behavior

Closes #2089

Greptile Summary

Adds an optional path_suffix field to provider config so users can override the API path for OpenAI-compatible endpoints that reject the default /v1/chat/completions route (e.g. providers that only accept /chat/completions).

• api_url_with_suffix correctly uses unversioned_base_url to strip the version segment before appending the suffix, and the suffix guard is scoped to path == \"chat/completions\" only — models, health probes, and beta/ FIM paths are all unaffected.
• path_suffix is wired through DeepSeekClient, both chat call sites in chat.rs, and all three config merge paths; tests cover the main URL-building cases including the no-suffix default, version stripping, and path isolation.

Confidence Score: 4/5

Safe to merge with one design decision worth revisiting before shipping.

The core URL-building logic is correct and well-tested. The one open question is whether path_suffix should be settable from untrusted project-level config: the merge_project_provider_config helper is explicitly documented to exclude endpoint fields, yet path_suffix — which redirects chat API calls to an arbitrary path on the provider — was added to it. This doesn't cross a host boundary, so exploitation is limited, but it contradicts the function's stated security policy and may deserve a second look before the feature is released.

crates/config/src/lib.rs — specifically whether path_suffix belongs in merge_project_provider_config given that function's documented exclusion of endpoint overrides.

Important Files Changed

Filename	Overview
crates/tui/src/client.rs	Adds api_url_with_suffix that correctly strips the version segment via unversioned_base_url and only applies the suffix for the chat/completions path; path_suffix field added to DeepSeekClient and read from provider config on construction.
crates/tui/src/client/chat.rs	Both streaming and non-streaming chat call sites updated from api_url to api_url_with_suffix with self.path_suffix.as_deref(); straightforward mechanical change.
crates/config/src/lib.rs	path_suffix added to ProviderConfigToml and propagated in merge_project_provider_config, but that helper is explicitly documented to exclude endpoint-related fields from untrusted project config.
crates/tui/src/config.rs	path_suffix field added to ProviderConfig and merge_provider_config; all changes are consistent with the existing field pattern.
config.example.toml	Adds a commented-out path_suffix example under the custom provider block with a clear description.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["api_url_with_suffix(base_url, path, suffix)"] --> B{path starts with 'beta/'?}
    B -- yes --> C["format: unversioned_base_url(base_url) / path"]
    B -- no --> D{path == 'chat/completions' AND suffix set?}
    D -- yes --> E["format: unversioned_base_url(base_url) / suffix.trim_start_matches('/')"]
    D -- no --> F["versioned = versioned_base_url(base_url)"]
    F --> G{versioned ends with 'beta'?}
    G -- yes --> H["versioned = unversioned_base_url / v1"]
    G -- no --> I["use versioned as-is"]
    H --> J["format: versioned / path"]
    I --> J

    subgraph Call Sites
        K["chat.rs streaming"] -->|path_suffix| A
        L["chat.rs non-streaming"] -->|path_suffix| A
        M["client.rs translate"] -->|path_suffix| A
        N["client.rs FIM beta/completions"] -->|None| A
        O["client.rs list_models"] -->|api_url wraps with None| A
        P["client.rs health probe"] -->|api_url wraps with None| A
    end

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_{Reviews (2): Last reviewed commit: "Limit path suffix to chat completions" | Re-trigger Greptile}

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[Hmbown]

Thanks for tackling path_suffix; the user need is real, especially for OpenAI-compatible endpoints that accept /chat/completions but not /v1/chat/completions.

I am not harvesting this revision into v0.8.50 yet because the current plumbing applies the suffix too broadly. The fix should be narrow:

1. In the suffix branch, build from unversioned_base_url(base_url) so base_url = "https://api.example.com/v1" plus path_suffix = "/chat/completions" yields https://api.example.com/chat/completions.
2. Apply path_suffix only for chat-completions request construction. Keep model listing, health checks, and probes on the normal models path.
3. Pass None for beta/FIM paths so it is explicit that suffix overrides do not affect /beta/completions.
4. Add tests for all three paths: chat suffix stripping /v1, model listing ignoring suffix, and beta path ignoring suffix.

Once those pass, this is the kind of small provider-compatibility feature that should be harvestable.

[Hmbown]

Hey @RefuseOdd — this is a solid feature! Several third-party OpenAI-compatible endpoints do need a custom path, and a config field is the right place for it.

Two small things before this is harvest-ready:

1. `api_url_with_suffix` should strip the existing version segment from `base_url` before appending the suffix — right now a config with `base_url = ".../v1"` and `path_suffix = "/chat/completions"` still produces `.../v1/chat/completions`.
2. `path_suffix` should only apply to chat-completions endpoints, not model-listing (health checks and `/models` should keep using the default path).

Fix those two and this is good to go! Happy to review the update. 🐋

[RefuseOdd]

@Hmbown pushed the requested fixes in 1c5c1212:

• api_url_with_suffix now appends path_suffix from unversioned_base_url(base_url), so base_url = ".../v1" with path_suffix = "/chat/completions" resolves to .../chat/completions.
• path_suffix is now only applied to chat-completions URL construction. /models, health/recovery probes, and /beta/completions continue using the normal URL paths.

I also added focused tests for chat suffix stripping, model listing ignoring suffix, and beta paths ignoring suffix. cargo fmt --check and cargo test -p codewhale-tui api_url_with_suffix pass.