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/config.md

@@ -243,6 +243,25 @@ To disable reasoning summaries, set `model_reasoning_summary` to `"none"` in you
 model_reasoning_summary = "none"  # disable reasoning summaries
 ```
 
+## model_verbosity
+
+Controls output length/detail on GPT‑5 family models when using the Responses API. Supported values:
+
+- `"low"`
+- `"medium"` (default when omitted)
+- `"high"`
+
+When set, Codex includes a `text` object in the request payload with the configured verbosity, for example: `"text": { "verbosity": "low" }`.
+
+Example:
+
+```toml
+model = "gpt-5"
+model_verbosity = "low"
+```
+
+Note: This applies only to providers using the Responses API. Chat Completions providers are unaffected.
+
 ## model_supports_reasoning_summaries
 
 By default, `reasoning` is only set on requests to OpenAI models that are known to support them. To force `reasoning` to set on requests to the current model, you can force this behavior by setting the following in `config.toml`:

codex-rs/core/src/client.rs

@@ -28,6 +28,7 @@ use crate::client_common::ResponseEvent;
 use crate::client_common::ResponseStream;
 use crate::client_common::ResponsesApiRequest;
 use crate::client_common::create_reasoning_param_for_request;
+use crate::client_common::create_text_param_for_request;
 use crate::config::Config;
 use crate::error::CodexErr;
 use crate::error::Result;
@@ -164,6 +165,19 @@ impl ModelClient {
 
         let input_with_instructions = prompt.get_formatted_input();
 
+        // Only include `text.verbosity` for GPT-5 family models
+        let text = if self.config.model_family.family == "gpt-5" {
+            create_text_param_for_request(self.config.model_verbosity)
+        } else {
+            if self.config.model_verbosity.is_some() {
+                warn!(
+                    "model_verbosity is set but ignored for non-gpt-5 model family: {}",
+                    self.config.model_family.family
+                );
+            }
+            None
+        };
+
         let payload = ResponsesApiRequest {
             model: &self.config.model,
             instructions: &full_instructions,
@@ -176,6 +190,7 @@ impl ModelClient {
             stream: true,
             include,
             prompt_cache_key: Some(self.session_id.to_string()),
+            text,
         };
 
         let mut attempt = 0;

codex-rs/core/src/client_common.rs

@@ -1,3 +1,4 @@
+use crate::config_types::Verbosity as VerbosityConfig;
 use crate::error::Result;
 use crate::model_family::ModelFamily;
 use crate::models::ContentItem;
@@ -99,6 +100,32 @@ pub(crate) struct Reasoning {
     pub(crate) summary: ReasoningSummaryConfig,
 }
 
+/// Controls under the `text` field in the Responses API for GPT-5.
+#[derive(Debug, Serialize, Default, Clone, Copy)]
+pub(crate) struct TextControls {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(crate) verbosity: Option<OpenAiVerbosity>,
+}
+
+#[derive(Debug, Serialize, Default, Clone, Copy)]
+#[serde(rename_all = "lowercase")]
+pub(crate) enum OpenAiVerbosity {
+    Low,
+    #[default]
+    Medium,
+    High,
+}
+
+impl From<VerbosityConfig> for OpenAiVerbosity {
+    fn from(v: VerbosityConfig) -> Self {
+        match v {
+            VerbosityConfig::Low => OpenAiVerbosity::Low,
+            VerbosityConfig::Medium => OpenAiVerbosity::Medium,
+            VerbosityConfig::High => OpenAiVerbosity::High,
+        }
+    }
+}
+
 /// Request object that is serialized as JSON and POST'ed when using the
 /// Responses API.
 #[derive(Debug, Serialize)]
@@ -119,6 +146,8 @@ pub(crate) struct ResponsesApiRequest<'a> {
     pub(crate) include: Vec<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub(crate) prompt_cache_key: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(crate) text: Option<TextControls>,
 }
 
 pub(crate) fn create_reasoning_param_for_request(
@@ -133,6 +162,14 @@ pub(crate) fn create_reasoning_param_for_request(
     }
 }
 
+pub(crate) fn create_text_param_for_request(
+    verbosity: Option<VerbosityConfig>,
+) -> Option<TextControls> {
+    verbosity.map(|v| TextControls {
+        verbosity: Some(v.into()),
+    })
+}
+
 pub(crate) struct ResponseStream {
     pub(crate) rx_event: mpsc::Receiver<Result<ResponseEvent>>,
 }
@@ -161,4 +198,57 @@ mod tests {
         let full = prompt.get_full_instructions(&model_family);
         assert_eq!(full, expected);
     }
+
+    #[test]
+    fn serializes_text_verbosity_when_set() {
+        let input: Vec<ResponseItem> = vec![];
+        let tools: Vec<serde_json::Value> = vec![];
+        let req = ResponsesApiRequest {
+            model: "gpt-5",
+            instructions: "i",
+            input: &input,
+            tools: &tools,
+            tool_choice: "auto",
+            parallel_tool_calls: false,
+            reasoning: None,
+            store: true,
+            stream: true,
+            include: vec![],
+            prompt_cache_key: None,
+            text: Some(TextControls {
+                verbosity: Some(OpenAiVerbosity::Low),
+            }),
+        };
+
+        let v = serde_json::to_value(&req).expect("json");
+        assert_eq!(
+            v.get("text")
+                .and_then(|t| t.get("verbosity"))
+                .and_then(|s| s.as_str()),
+            Some("low")
+        );
+    }
+
+    #[test]
+    fn omits_text_when_not_set() {
+        let input: Vec<ResponseItem> = vec![];
+        let tools: Vec<serde_json::Value> = vec![];
+        let req = ResponsesApiRequest {
+            model: "gpt-5",
+            instructions: "i",
+            input: &input,
+            tools: &tools,
+            tool_choice: "auto",
+            parallel_tool_calls: false,
+            reasoning: None,
+            store: true,
+            stream: true,
+            include: vec![],
+            prompt_cache_key: None,
+            text: None,
+        };
+
+        let v = serde_json::to_value(&req).expect("json");
+        assert!(v.get("text").is_none());
+    }
 }

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,

codex-rs/core/src/config_profile.rs

@@ -1,6 +1,7 @@
 use serde::Deserialize;
 use std::path::PathBuf;
 
+use crate::config_types::Verbosity;
 use crate::protocol::AskForApproval;
 use codex_protocol::config_types::ReasoningEffort;
 use codex_protocol::config_types::ReasoningSummary;
@@ -17,6 +18,7 @@ pub struct ConfigProfile {
     pub disable_response_storage: Option<bool>,
     pub model_reasoning_effort: Option<ReasoningEffort>,
     pub model_reasoning_summary: Option<ReasoningSummary>,
+    pub model_verbosity: Option<Verbosity>,
     pub chatgpt_base_url: Option<String>,
     pub experimental_instructions_file: Option<PathBuf>,
 }

codex-rs/core/src/config_types.rs

@@ -8,6 +8,8 @@ use std::path::PathBuf;
 use wildmatch::WildMatchPattern;
 
 use serde::Deserialize;
+use serde::Serialize;
+use strum_macros::Display;
 
 #[derive(Deserialize, Debug, Clone, PartialEq)]
 pub struct McpServerConfig {
@@ -183,3 +185,43 @@ impl From<ShellEnvironmentPolicyToml> for ShellEnvironmentPolicy {
         }
     }
 }
+
+/// See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#get-started-with-reasoning
+#[derive(Debug, Serialize, Deserialize, Default, Clone, Copy, PartialEq, Eq, Display)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum ReasoningEffort {
+    Low,
+    #[default]
+    Medium,
+    High,
+    /// Option to disable reasoning.
+    None,
+}
+
+/// A summary of the reasoning performed by the model. This can be useful for
+/// debugging and understanding the model's reasoning process.
+/// See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#reasoning-summaries
+#[derive(Debug, Serialize, Deserialize, Default, Clone, Copy, PartialEq, Eq, Display)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum ReasoningSummary {
+    #[default]
+    Auto,
+    Concise,
+    Detailed,
+    /// Option to disable reasoning summaries.
+    None,
+}
+
+/// Controls output length/detail on GPT-5 models via the Responses API.
+/// Serialized with lowercase values to match the OpenAI API.
+#[derive(Debug, Serialize, Deserialize, Default, Clone, Copy, PartialEq, Eq, Display)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum Verbosity {
+    Low,
+    #[default]
+    Medium,
+    High,
+}