anthropic: Allow configured models to opt into fast mode (#58225)

Adds support for enabling Anthropic fast mode on configured models.

Previously, Anthropic models configured through
`language_models.anthropic.available_models` were always marked as not
supporting fast mode, so `speed: "fast"` would be stripped before
sending requests even when the configured model supported Anthropic fast
mode.

This adds an optional `supports_fast_mode` field to configured Anthropic
models. When enabled, the model is marked as supporting fast mode and
the required Anthropic beta header is added automatically. Built-in
fast-mode model detection remains the fallback when the setting is
omitted.

Testing:
- `cargo fmt --package anthropic --package language_models --package
settings_content`
- `cargo test -p language_models available_model --lib`
- `cargo test -p anthropic from_listed_enables_fast_mode --lib`
- `cargo check -p language_models`
- Manual: verified in a local build that a configured Anthropic model
can send fast mode requests correctly.

Release Notes:

- agent: Allow specifying if fast mode is supported for custom anthropic
models

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

Author: Moeblack

crates/anthropic/src/anthropic.rs

@@ -19,7 +19,14 @@ pub mod batches;
 pub mod completion;
 
 pub const ANTHROPIC_API_URL: &str = "https://api.anthropic.com";
-const FAST_MODE_BETA_HEADER: &str = "fast-mode-2026-02-01";
+pub const FAST_MODE_BETA_HEADER: &str = "fast-mode-2026-02-01";
+
+pub fn supports_fast_mode(model_id: &str) -> bool {
+    matches!(
+        model_id,
+        "claude-opus-4-6" | "claude-opus-4-7" | "claude-opus-4-8"
+    )
+}
 
 pub const FABLE_MODEL_ID_PREFIX: &str = "claude-fable-5";
 pub const FABLE_FALLBACK_MODEL_ID: &str = "claude-opus-4-8";
@@ -156,10 +163,7 @@ impl Model {
             AnthropicModelMode::Default
         };
 
-        let supports_speed = matches!(
-            entry.id.as_str(),
-            "claude-opus-4-6" | "claude-opus-4-7" | "claude-opus-4-8"
-        );
+        let supports_speed = supports_fast_mode(&entry.id);
 
         // <https://platform.claude.com/docs/en/build-with-claude/compaction#supported-models>
         let supports_compaction = matches!(

crates/language_models/src/provider/anthropic.rs

@@ -333,6 +333,17 @@ fn available_model_to_anthropic_model(available: &AvailableModel) -> anthropic::
         AnthropicModelMode::Thinking { .. } | AnthropicModelMode::AdaptiveThinking
     );
     let supports_adaptive_thinking = matches!(mode, AnthropicModelMode::AdaptiveThinking);
+    let supports_speed = available
+        .supports_fast_mode
+        .unwrap_or_else(|| anthropic::supports_fast_mode(&available.name));
+    let mut extra_beta_headers = available.extra_beta_headers.clone();
+    if supports_speed
+        && !extra_beta_headers
+            .iter()
+            .any(|header| header.trim() == anthropic::FAST_MODE_BETA_HEADER)
+    {
+        extra_beta_headers.push(anthropic::FAST_MODE_BETA_HEADER.to_string());
+    }
 
     anthropic::Model {
         display_name: available
@@ -347,7 +358,7 @@ fn available_model_to_anthropic_model(available: &AvailableModel) -> anthropic::
         supports_thinking,
         supports_adaptive_thinking,
         supports_images: true,
-        supports_speed: false,
+        supports_speed,
         supports_compaction: false,
         supported_effort_levels: if supports_adaptive_thinking {
             vec![
@@ -361,7 +372,7 @@ fn available_model_to_anthropic_model(available: &AvailableModel) -> anthropic::
             vec![]
         },
         tool_override: available.tool_override.clone(),
-        extra_beta_headers: available.extra_beta_headers.clone(),
+        extra_beta_headers,
     }
 }
 

crates/settings_content/src/language_model.rs

@@ -106,6 +106,8 @@ pub struct AnthropicAvailableModel {
     pub default_temperature: Option<f32>,
     #[serde(default)]
     pub extra_beta_headers: Vec<String>,
+    /// Whether Anthropic's fast mode is available for this model.
+    pub supports_fast_mode: Option<bool>,
     /// The model's mode (e.g. thinking)
     pub mode: Option<ModelMode>,
 }