diff --git a/crates/tui/src/tools/fim.rs b/crates/tui/src/tools/fim.rs
index 0510a2c3b..f59dc53ba 100644
--- a/crates/tui/src/tools/fim.rs
+++ b/crates/tui/src/tools/fim.rs
@@ -94,8 +94,10 @@ impl ToolSpec for FimEditTool {
     }
 
     fn capabilities(&self) -> Vec<ToolCapability> {
+        // Writes the target file atomically (see execute step 7), so ReadOnly
+        // would be false; repo_law::WRITE_TOOLS also relies on this tool being
+        // recognized as write-capable.
         vec![
-            ToolCapability::ReadOnly,
             ToolCapability::WritesFiles,
             ToolCapability::RequiresApproval,
         ]
@@ -180,3 +182,24 @@ impl ToolSpec for FimEditTool {
         ToolResult::json(&result).map_err(|e| ToolError::execution_failed(e.to_string()))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fim_edit_declares_write_capability_without_read_only() {
+        let tool = FimEditTool::new(None, "fim-model".to_string());
+        let capabilities = tool.capabilities();
+
+        assert!(
+            capabilities.contains(&ToolCapability::WritesFiles),
+            "fim_edit writes the target file atomically"
+        );
+        assert!(
+            !capabilities.contains(&ToolCapability::ReadOnly),
+            "a file-writing tool must not claim ReadOnly"
+        );
+        assert!(!tool.is_read_only());
+    }
+}
diff --git a/crates/tui/src/tools/plan.rs b/crates/tui/src/tools/plan.rs
index b1f1ca12f..68e1ce3c1 100644
--- a/crates/tui/src/tools/plan.rs
+++ b/crates/tui/src/tools/plan.rs
@@ -570,7 +570,8 @@ impl ToolSpec for UpdatePlanTool {
     }
 
     fn capabilities(&self) -> Vec<ToolCapability> {
-        vec![ToolCapability::WritesFiles]
+        // Mutates only in-memory session plan state; no filesystem writes.
+        vec![]
     }
 
     fn approval_requirement(&self) -> ApprovalRequirement {
@@ -664,9 +665,24 @@ fn string_vec_field(input: &serde_json::Value, field: &str) -> Vec<String> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::tools::spec::{ToolContext, ToolSpec};
+    use crate::tools::spec::{ToolCapability, ToolContext, ToolSpec};
     use serde_json::json;
 
+    #[test]
+    fn update_plan_declares_no_filesystem_write_capability() {
+        let tool = UpdatePlanTool::new(new_shared_plan_state());
+        let capabilities = tool.capabilities();
+
+        assert!(
+            !capabilities.contains(&ToolCapability::WritesFiles),
+            "update_plan mutates only in-memory session plan state"
+        );
+        assert!(
+            !tool.is_read_only(),
+            "update_plan still mutates session state"
+        );
+    }
+
     #[test]
     fn update_plan_description_keeps_work_update_as_primary_progress() {
         let tool = UpdatePlanTool::new(new_shared_plan_state());