diff --git a/server/src/http/mcp/mod.rs b/server/src/http/mcp/mod.rs
index 76601995..8debde48 100644
--- a/server/src/http/mcp/mod.rs
+++ b/server/src/http/mcp/mod.rs
@@ -513,6 +513,28 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                         "required": ["story_id", "criterion_index"]
                     }
                 },
+                {
+                    "name": "edit_criterion",
+                    "description": "Update the text of an existing acceptance criterion in place, preserving its checked/unchecked state. Uses a 0-based index counting all criteria (both checked and unchecked).",
+                    "inputSchema": {
+                        "type": "object",
+                        "properties": {
+                            "story_id": {
+                                "type": "string",
+                                "description": "Story identifier (filename stem, e.g. '28_my_story')"
+                            },
+                            "criterion_index": {
+                                "type": "integer",
+                                "description": "0-based index of the criterion to edit (counts all criteria)"
+                            },
+                            "new_text": {
+                                "type": "string",
+                                "description": "New text for the criterion (without the '- [ ] ' or '- [x] ' prefix)"
+                            }
+                        },
+                        "required": ["story_id", "criterion_index", "new_text"]
+                    }
+                },
                 {
                     "name": "add_criterion",
                     "description": "Add an acceptance criterion to an existing story file. Appends '- [ ] {criterion}' after the last existing criterion in the '## Acceptance Criteria' section. Auto-commits via the filesystem watcher.",
@@ -1242,6 +1264,7 @@ async fn handle_tools_call(id: Option<Value>, params: &Value, ctx: &AppContext)
         "accept_story" => story_tools::tool_accept_story(&args, ctx),
         // Story mutation tools (auto-commit to master)
         "check_criterion" => story_tools::tool_check_criterion(&args, ctx),
+        "edit_criterion" => story_tools::tool_edit_criterion(&args, ctx),
         "add_criterion" => story_tools::tool_add_criterion(&args, ctx),
         "update_story" => story_tools::tool_update_story(&args, ctx),
         // Spike lifecycle tools
@@ -1426,7 +1449,7 @@ mod tests {
         assert!(names.contains(&"loc_file"));
         assert!(names.contains(&"dump_crdt"));
         assert!(names.contains(&"get_version"));
-        assert_eq!(tools.len(), 63);
+        assert_eq!(tools.len(), 64);
     }
 
     #[test]
diff --git a/server/src/http/mcp/story_tools.rs b/server/src/http/mcp/story_tools.rs
index df9a98d4..9a1a90ec 100644
--- a/server/src/http/mcp/story_tools.rs
+++ b/server/src/http/mcp/story_tools.rs
@@ -5,8 +5,9 @@ use crate::agents::{
 use crate::http::context::AppContext;
 use crate::http::workflow::{
     add_criterion_to_file, check_criterion_in_file, create_bug_file, create_refactor_file,
-    create_spike_file, create_story_file, list_bug_files, list_refactor_files, load_pipeline_state,
-    load_upcoming_stories, update_story_in_file, validate_story_dirs,
+    create_spike_file, create_story_file, edit_criterion_in_file, list_bug_files,
+    list_refactor_files, load_pipeline_state, load_upcoming_stories, update_story_in_file,
+    validate_story_dirs,
 };
 use crate::io::story_metadata::{
     check_archived_deps, check_archived_deps_from_list, parse_front_matter, parse_unchecked_todos,
@@ -331,6 +332,28 @@ pub(super) fn tool_check_criterion(args: &Value, ctx: &AppContext) -> Result<Str
     ))
 }
 
+pub(super) fn tool_edit_criterion(args: &Value, ctx: &AppContext) -> Result<String, String> {
+    let story_id = args
+        .get("story_id")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: story_id")?;
+    let criterion_index = args
+        .get("criterion_index")
+        .and_then(|v| v.as_u64())
+        .ok_or("Missing required argument: criterion_index")? as usize;
+    let new_text = args
+        .get("new_text")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: new_text")?;
+
+    let root = ctx.state.get_project_root()?;
+    edit_criterion_in_file(&root, story_id, criterion_index, new_text)?;
+
+    Ok(format!(
+        "Criterion {criterion_index} updated for story '{story_id}'."
+    ))
+}
+
 pub(super) fn tool_add_criterion(args: &Value, ctx: &AppContext) -> Result<String, String> {
     let story_id = args
         .get("story_id")
diff --git a/server/src/http/workflow/mod.rs b/server/src/http/workflow/mod.rs
index 82901172..c3d05293 100644
--- a/server/src/http/workflow/mod.rs
+++ b/server/src/http/workflow/mod.rs
@@ -7,7 +7,8 @@ pub use bug_ops::{
     create_bug_file, create_refactor_file, create_spike_file, list_bug_files, list_refactor_files,
 };
 pub use story_ops::{
-    add_criterion_to_file, check_criterion_in_file, create_story_file, update_story_in_file,
+    add_criterion_to_file, check_criterion_in_file, create_story_file, edit_criterion_in_file,
+    update_story_in_file,
 };
 pub use test_results::{
     read_test_results_from_story_file, write_coverage_baseline_to_story_file,
diff --git a/server/src/http/workflow/story_ops.rs b/server/src/http/workflow/story_ops.rs
index 4ccd184e..8f15178f 100644
--- a/server/src/http/workflow/story_ops.rs
+++ b/server/src/http/workflow/story_ops.rs
@@ -126,6 +126,64 @@ pub fn check_criterion_in_file(
     Ok(())
 }
 
+/// Edit the text of an existing acceptance criterion without changing its checked state.
+///
+/// Finds the criterion at `criterion_index` (0-based, counting all criteria regardless
+/// of checked state) and replaces its text with `new_text`.
+pub fn edit_criterion_in_file(
+    project_root: &Path,
+    story_id: &str,
+    criterion_index: usize,
+    new_text: &str,
+) -> Result<(), String> {
+    let contents = read_story_content(project_root, story_id)?;
+
+    let mut count: usize = 0;
+    let mut found = false;
+    let new_lines: Vec<String> = contents
+        .lines()
+        .map(|line| {
+            let trimmed = line.trim();
+            let prefix = if trimmed.starts_with("- [ ] ") {
+                Some("- [ ] ")
+            } else if trimmed.starts_with("- [x] ") {
+                Some("- [x] ")
+            } else {
+                None
+            };
+            if let Some(p) = prefix {
+                if count == criterion_index {
+                    count += 1;
+                    found = true;
+                    let indent_len = line.len() - trimmed.len();
+                    let indent = &line[..indent_len];
+                    return format!("{indent}{p}{new_text}");
+                }
+                count += 1;
+            }
+            line.to_string()
+        })
+        .collect();
+
+    if !found {
+        return Err(format!(
+            "Criterion index {criterion_index} out of range. Story '{story_id}' has \
+             {count} criteria (indices 0..{}).",
+            count.saturating_sub(1)
+        ));
+    }
+
+    let mut new_str = new_lines.join("\n");
+    if contents.ends_with('\n') {
+        new_str.push('\n');
+    }
+
+    let stage = story_stage(story_id).unwrap_or_else(|| "2_current".to_string());
+    write_story_content(project_root, story_id, &stage, &new_str);
+
+    Ok(())
+}
+
 /// Add a new acceptance criterion to a story.
 ///
 /// Appends `- [ ] {criterion}` after the last existing criterion line in the