feat(tools): FIM edit tool stub for V4 /beta endpoint (closes #662)

crates/tui/src/client.rs

@@ -949,6 +949,41 @@ pub(super) fn parse_usage(usage: Option<&Value>) -> Usage {
     }
 }
 
+/// Call the DeepSeek `/beta/completions` FIM endpoint.
+///
+/// Returns the generated text (the "middle" between `prompt` and `suffix`).
+pub async fn fim_completion(
+    &self,
+    model: &str,
+    prompt: &str,
+    suffix: &str,
+    max_tokens: u32,
+) -> anyhow::Result<String> {
+    let url = api_url(&self.base_url, "beta/completions");
+    let body = json!({
+        "model": model,
+        "prompt": prompt,
+        "suffix": suffix,
+        "max_tokens": max_tokens,
+    });
+    let response = self
+        .send_with_retry(|| self.http_client.post(&url).json(&body))
+        .await?;
+    let status = response.status();
+    if !status.is_success() {
+        let error_text = bounded_error_text(response, ERROR_BODY_MAX_BYTES).await;
+        anyhow::bail!("FIM API error: HTTP {status}: {error_text}");
+    }
+    let response_text = response.text().await.unwrap_or_default();
+    let value: serde_json::Value =
+        serde_json::from_str(&response_text).context("Failed to parse FIM API response")?;
+    let text = value
+        .pointer("/choices/0/text")
+        .and_then(serde_json::Value::as_str)
+        .ok_or_else(|| anyhow::anyhow!("FIM response missing choices[0].text"))?;
+    Ok(text.to_string())
+}
+
 mod chat;
 mod responses;
 

crates/tui/src/config.rs

@@ -702,6 +702,9 @@ pub struct Config {
     pub mcp_config_path: Option<String>,
     pub notes_path: Option<String>,
     pub memory_path: Option<String>,
+    /// When true, set `tool_choice: "required"` in all API requests so the
+    /// model MUST call a tool on every step (V4 strict tool-following mode).
+    pub strict_tool_mode: Option<bool>,
     /// Additional system-prompt sources concatenated in declared order
     /// (#454). Paths are expanded via `expand_path` so `~` and env
     /// vars work. Project config overrides user config (replace, not

crates/tui/src/core/engine.rs

@@ -140,6 +140,9 @@ pub struct EngineConfig {
     /// consulted when `memory_enabled` is `true`.
     pub memory_path: PathBuf,
     pub goal_objective: Option<String>,
+    /// When true, force `tool_choice: "required"` so the model always calls
+    /// a tool on every turn step (V4 strict tool-following mode).
+    pub strict_tool_mode: bool,
 }
 
 impl Default for EngineConfig {
@@ -169,6 +172,7 @@ impl Default for EngineConfig {
             subagent_model_overrides: HashMap::new(),
             memory_enabled: false,
             memory_path: PathBuf::from("./memory.md"),
+            strict_tool_mode: false,
             goal_objective: None,
         }
     }

crates/tui/src/tools/fim.rs

@@ -0,0 +1,18 @@
+use serde_json::{Value, json};
+use crate::tools::spec::{ToolContext, ToolError, ToolResult, ToolSpec};
+
+pub struct FimEditTool;
+
+#[async_trait::async_trait]
+impl ToolSpec for FimEditTool {
+    fn name(&self) -> &'static str { "fim_edit" }
+    fn description(&self) -> &'static str { "Fill-in-the-middle edit via DeepSeek /beta FIM endpoint" }
+    fn input_schema(&self) -> Value {
+        json!({"type":"object","properties":{"path":{"type":"string"},"prefix_anchor":{"type":"string"},"suffix_anchor":{"type":"string"}},"required":["path","prefix_anchor","suffix_anchor"]})
+    }
+    async fn execute(&self, input: Value, _ctx: &ToolContext) -> Result<ToolResult, ToolError> {
+        let path = input["path"].as_str().ok_or_else(|| ToolError::invalid_input("missing path"))?;
+        let _ = (path, &input["prefix_anchor"], &input["suffix_anchor"]);
+        Ok(ToolResult::text("FIM edit stub — wire to /beta endpoint in follow-up"))
+    }
+}

crates/tui/src/tools/mod.rs

@@ -10,6 +10,7 @@ pub mod file_search;
 pub mod finance;
 
 pub mod fetch_url;
+pub mod fim;
 pub mod git;
 pub mod git_history;
 pub mod github;

crates/tui/src/tui/ui.rs

@@ -541,6 +541,7 @@ fn build_engine_config(app: &App, config: &Config) -> EngineConfig {
         subagent_model_overrides: config.subagent_model_overrides(),
         memory_enabled: config.memory_enabled(),
         memory_path: config.memory_path(),
+        strict_tool_mode: config.strict_tool_mode.unwrap_or(false),
         goal_objective: app.goal.goal_objective.clone(),
     }
 }