feat(gpt5): add model_verbosity for GPT‑5 via Responses API (#2108)

**Summary**
- Adds `model_verbosity` config (values: low, medium, high).
- Sends `text.verbosity` only for GPT‑5 family models via the Responses
API.
- Updates docs and adds serialization tests.

**Motivation**
- GPT‑5 introduces a verbosity control to steer output length/detail
without pro
mpt surgery.
- Exposing it as a config knob keeps prompts stable and makes behavior
explicit
and repeatable.

**Changes**
- Config:
  - Added `Verbosity` enum (low|medium|high).
- Added optional `model_verbosity` to `ConfigToml`, `Config`, and
`ConfigProfi
le`.
- Request wiring:
  - Extended `ResponsesApiRequest` with optional `text` object.
- Populates `text.verbosity` only when model family is `gpt-5`; omitted
otherw
ise.
- Tests:
- Verifies `text.verbosity` serializes when set and is omitted when not
set.
- Docs:
  - Added “GPT‑5 Verbosity” section in `codex-rs/README.md`.
  - Added `model_verbosity` section to `codex-rs/config.md`.

**Usage**
- In `~/.codex/config.toml`:
  - `model = "gpt-5"`
  - `model_verbosity = "low"` (or `"medium"` default, `"high"`)
- CLI override example:
  - `codex -c model="gpt-5" -c model_verbosity="high"`

**API Impact**
- Requests to GPT‑5 via Responses API include: `text: { verbosity:
"low|medium|h
igh" }` when configured.
- For legacy models or Chat Completions providers, `text` is omitted.

**Backward Compatibility**
- Default behavior unchanged when `model_verbosity` is not set (server
default “
medium”).

**Testing**
- Added unit tests for serialization/omission of `text.verbosity`.
- Ran `cargo fmt` and `cargo test --all-features` (all green).

**Docs**
- `README.md`: new “GPT‑5 Verbosity” note under Config with example.
- `config.md`: new `model_verbosity` section.

**Out of Scope**
- No changes to temperature/top_p or other GPT‑5 parameters.
- No changes to Chat Completions wiring.

**Risks / Notes**
- If OpenAI changes the wire shape for verbosity, we may need to update
`Respons
esApiRequest`.
- Behavior gated to `gpt-5` model family to avoid unexpected effects
elsewhere.

**Checklist**
- [x] Code gated to GPT‑5 family only
- [x] Docs updated (`README.md`, `config.md`)
- [x] Tests added and passing
- [x] Formatting applied

Release note: Add `model_verbosity` config to control GPT‑5 output verbosity via the Responses API (low|medium|high).

codex-rs/core/src/config.rs

@@ -6,6 +6,7 @@ use crate::config_types::ShellEnvironmentPolicy;
 use crate::config_types::ShellEnvironmentPolicyToml;
 use crate::config_types::Tui;
 use crate::config_types::UriBasedFileOpener;
+use crate::config_types::Verbosity;
 use crate::model_family::ModelFamily;
 use crate::model_family::find_family_for_model;
 use crate::model_provider_info::ModelProviderInfo;
@@ -150,6 +151,9 @@ pub struct Config {
     /// request using the Responses API.
     pub model_reasoning_summary: ReasoningSummary,
 
+    /// Optional verbosity control for GPT-5 models (Responses API `text.verbosity`).
+    pub model_verbosity: Option<Verbosity>,
+
     /// Base URL for requests to ChatGPT (as opposed to the OpenAI API).
     pub chatgpt_base_url: String,
 
@@ -441,6 +445,8 @@ pub struct ConfigToml {
 
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
+    /// Optional verbosity control for GPT-5 models (Responses API `text.verbosity`).
+    pub model_verbosity: Option<Verbosity>,
 
     /// Override to force-enable reasoning summaries for the configured model.
     pub model_supports_reasoning_summaries: Option<bool>,
@@ -718,7 +724,7 @@ impl Config {
                 .model_reasoning_summary
                 .or(cfg.model_reasoning_summary)
                 .unwrap_or_default(),
-
+            model_verbosity: config_profile.model_verbosity.or(cfg.model_verbosity),
             chatgpt_base_url: config_profile
                 .chatgpt_base_url
                 .or(cfg.chatgpt_base_url)
@@ -1087,6 +1093,7 @@ disable_response_storage = true
                 show_raw_agent_reasoning: false,
                 model_reasoning_effort: ReasoningEffort::High,
                 model_reasoning_summary: ReasoningSummary::Detailed,
+                model_verbosity: None,
                 chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
                 experimental_resume: None,
                 base_instructions: None,
@@ -1140,6 +1147,7 @@ disable_response_storage = true
             show_raw_agent_reasoning: false,
             model_reasoning_effort: ReasoningEffort::default(),
             model_reasoning_summary: ReasoningSummary::default(),
+            model_verbosity: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             experimental_resume: None,
             base_instructions: None,
@@ -1208,6 +1216,7 @@ disable_response_storage = true
             show_raw_agent_reasoning: false,
             model_reasoning_effort: ReasoningEffort::default(),
             model_reasoning_summary: ReasoningSummary::default(),
+            model_verbosity: None,
             chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
             experimental_resume: None,
             base_instructions: None,