diff --git a/poc-agent/src/tools/bash.rs b/poc-agent/src/tools/bash.rs
index cf5bcac..fdf6d0e 100644
--- a/poc-agent/src/tools/bash.rs
+++ b/poc-agent/src/tools/bash.rs
@@ -7,6 +7,7 @@
 // display running commands and the user can kill them (Ctrl+K).
 
 use anyhow::{Context, Result};
+use serde::Deserialize;
 use serde_json::json;
 use std::process::Stdio;
 use std::sync::Arc;
@@ -16,6 +17,15 @@ use tokio::sync::Mutex;
 
 use crate::types::ToolDef;
 
+#[derive(Deserialize)]
+struct Args {
+    command: String,
+    #[serde(default = "default_timeout")]
+    timeout_secs: u64,
+}
+
+fn default_timeout() -> u64 { 120 }
+
 /// Info about a running child process, visible to the TUI.
 #[derive(Debug, Clone)]
 pub struct ProcessInfo {
@@ -94,8 +104,10 @@ pub fn definition() -> ToolDef {
 }
 
 pub async fn run_bash(args: &serde_json::Value, tracker: &ProcessTracker) -> Result<String> {
-    let command = args["command"].as_str().context("command is required")?;
-    let timeout_secs = args["timeout_secs"].as_u64().unwrap_or(120);
+    let a: Args = serde_json::from_value(args.clone())
+        .context("invalid bash arguments")?;
+    let command = &a.command;
+    let timeout_secs = a.timeout_secs;
 
     let mut child = tokio::process::Command::new("bash")
         .arg("-c")
diff --git a/poc-agent/src/tools/grep.rs b/poc-agent/src/tools/grep.rs
index 64e0bde..f49f5da 100644
--- a/poc-agent/src/tools/grep.rs
+++ b/poc-agent/src/tools/grep.rs
@@ -4,11 +4,25 @@
 // isn't installed. Both produce compatible output.
 
 use anyhow::{Context, Result};
+use serde::Deserialize;
 use serde_json::json;
 use std::process::Command;
 
 use crate::types::ToolDef;
 
+#[derive(Deserialize)]
+struct Args {
+    pattern: String,
+    #[serde(default = "default_path")]
+    path: String,
+    glob: Option<String>,
+    #[serde(default)]
+    show_content: bool,
+    context_lines: Option<u64>,
+}
+
+fn default_path() -> String { ".".into() }
+
 pub fn definition() -> ToolDef {
     ToolDef::new(
         "grep",
@@ -51,16 +65,13 @@ fn has_rg() -> bool {
 }
 
 pub fn grep(args: &serde_json::Value) -> Result<String> {
-    let pattern = get_str(args, "pattern")?;
-    let path = args["path"].as_str().unwrap_or(".");
-    let file_glob = args["glob"].as_str();
-    let show_content = args["show_content"].as_bool().unwrap_or(false);
-    let context = args["context_lines"].as_u64();
+    let a: Args = serde_json::from_value(args.clone())
+        .context("invalid grep arguments")?;
 
     let output = if has_rg() {
-        run_search("rg", pattern, path, file_glob, show_content, context, true)?
+        run_search("rg", &a.pattern, &a.path, a.glob.as_deref(), a.show_content, a.context_lines, true)?
     } else {
-        run_search("grep", pattern, path, file_glob, show_content, context, false)?
+        run_search("grep", &a.pattern, &a.path, a.glob.as_deref(), a.show_content, a.context_lines, false)?
     };
 
     if output.is_empty() {
@@ -116,10 +127,3 @@ fn run_search(
     let output = cmd.output().with_context(|| format!("Failed to run {}", tool))?;
     Ok(String::from_utf8_lossy(&output.stdout).to_string())
 }
-
-/// Helper: get required string argument.
-fn get_str<'a>(args: &'a serde_json::Value, name: &'a str) -> Result<&'a str> {
-    args.get(name)
-        .and_then(|v| v.as_str())
-        .context(format!("{} is required", name))
-}
diff --git a/poc-agent/src/tools/vision.rs b/poc-agent/src/tools/vision.rs
index 01173ed..f9ed968 100644
--- a/poc-agent/src/tools/vision.rs
+++ b/poc-agent/src/tools/vision.rs
@@ -6,10 +6,21 @@
 
 use anyhow::{Context, Result};
 use base64::Engine;
+use serde::Deserialize;
 
 use super::ToolOutput;
 use crate::types::ToolDef;
 
+#[derive(Deserialize)]
+struct Args {
+    file_path: Option<String>,
+    pane_id: Option<String>,
+    #[serde(default = "default_lines")]
+    lines: usize,
+}
+
+fn default_lines() -> usize { 50 }
+
 pub fn definition() -> ToolDef {
     ToolDef::new(
         "view_image",
@@ -39,13 +50,15 @@ pub fn definition() -> ToolDef {
 
 /// View an image file or capture a tmux pane.
 pub fn view_image(args: &serde_json::Value) -> Result<ToolOutput> {
-    if let Some(pane_id) = args.get("pane_id").and_then(|v| v.as_str()) {
-        return capture_tmux_pane(pane_id, args);
+    let a: Args = serde_json::from_value(args.clone())
+        .context("invalid view_image arguments")?;
+
+    if let Some(ref pane_id) = a.pane_id {
+        return capture_tmux_pane(pane_id, a.lines);
     }
 
-    let file_path = args
-        .get("file_path")
-        .and_then(|v| v.as_str())
+    let file_path = a.file_path
+        .as_deref()
         .context("view_image requires either file_path or pane_id")?;
 
     let path = std::path::Path::new(file_path);
@@ -83,13 +96,8 @@ pub fn view_image(args: &serde_json::Value) -> Result<ToolOutput> {
     })
 }
 
-/// Capture a tmux pane to a PNG screenshot using tmux's capture-pane.
-/// Falls back to text capture if image capture isn't available.
-fn capture_tmux_pane(pane_id: &str, args: &serde_json::Value) -> Result<ToolOutput> {
-    let lines = args
-        .get("lines")
-        .and_then(|v| v.as_u64())
-        .unwrap_or(50) as usize;
+/// Capture a tmux pane's text content.
+fn capture_tmux_pane(pane_id: &str, lines: usize) -> Result<ToolOutput> {
 
     // Use tmux capture-pane to get text content, then render to image
     // via a simple approach: capture text and return it (the model can