diff --git a/CHANGELOG.md b/CHANGELOG.md
index 74881cf3..23a80751 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `kimi-k2.7-code`, recognizes `kimi`/`kimi-k2` aliases for that model, keeps
   explicit `kimi-k2.6` selectable, and adds the OpenRouter
   `moonshotai/kimi-k2.7-code` registry row.
+- **Ephemeral generated project context (#3058).** Opening CodeWhale in a
+  directory with no instruction files now keeps the bounded generated project
+  overview in memory instead of creating `.codewhale/instructions.md`.
 - **Cursor-style activity metadata rows (#3146).** Dense successful tool-run
   summaries now render as a single muted `Explored ...` / `Updated metadata`
   row, include short command-family labels for successful generic verifier
diff --git a/crates/tui/CHANGELOG.md b/crates/tui/CHANGELOG.md
index 7e7d678f..1b47fea7 100644
--- a/crates/tui/CHANGELOG.md
+++ b/crates/tui/CHANGELOG.md
@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `kimi-k2.7-code`, recognizes `kimi`/`kimi-k2` aliases for that model, keeps
   explicit `kimi-k2.6` selectable, and adds the OpenRouter
   `moonshotai/kimi-k2.7-code` registry row.
+- **Ephemeral generated project context (#3058).** Opening CodeWhale in a
+  directory with no instruction files now keeps the bounded generated project
+  overview in memory instead of creating `.codewhale/instructions.md`.
 - **Cursor-style activity metadata rows (#3146).** Dense successful tool-run
   summaries now render as a single muted `Explored ...` / `Updated metadata`
   row, include short command-family labels for successful generic verifier
diff --git a/crates/tui/src/project_context.rs b/crates/tui/src/project_context.rs
index 54c8300f..055dca82 100644
--- a/crates/tui/src/project_context.rs
+++ b/crates/tui/src/project_context.rs
@@ -740,22 +740,14 @@ fn load_project_context_with_parents_and_home(
         }
     }
 
-    // Auto-generate .codewhale/instructions.md when no context file exists anywhere.
-    // This avoids the per-turn filesystem scan fallback in prompts.rs that
-    // breaks KV prefix cache stability.
+    // Generate a bounded in-memory fallback when no context file exists
+    // anywhere. This keeps prompt shape stable without creating project-local
+    // `.codewhale/` files merely because CodeWhale was opened in a directory.
     if !ctx.has_instructions()
-        && let Some(generated) = auto_generate_context(workspace)
+        && let Some(generated) = generate_ephemeral_context(workspace)
     {
-        let mut warnings = std::mem::take(&mut ctx.warnings);
-        ctx = load_project_context(workspace);
-        warnings.extend(ctx.warnings.iter().cloned());
-        ctx.warnings = warnings;
-        if !ctx.has_instructions() {
-            // Loaded from the file we just wrote — use the generated content
-            // directly as a last resort (shouldn't normally happen).
-            ctx.instructions = Some(generated);
-            ctx.source_path = None;
-        }
+        ctx.instructions = Some(generated);
+        ctx.source_path = None;
     }
 
     // Load the CodeWhale-specific repo authority policy
@@ -920,44 +912,17 @@ fn load_global_agents_context(workspace: &Path, home_dir: Option<&Path>) -> Opti
     None
 }
 
-/// Generate a context file from project tree + summary and write it to
-/// `.codewhale/instructions.md` (or `.deepseek/instructions.md` as legacy
-/// fallback). Returns the generated content on success.
-fn auto_generate_context(workspace: &Path) -> Option<String> {
-    let codewhale_dir = workspace.join(".codewhale");
-    let instructions_path = codewhale_dir.join("instructions.md");
-    let legacy_instructions_path = workspace.join(".deepseek/instructions.md");
-
-    // Don't overwrite an existing file (check both locations)
-    if instructions_path.exists() || legacy_instructions_path.exists() {
-        return None;
-    }
-
+/// Generate ephemeral context from the project tree. Returns the generated
+/// content on success without writing workspace files.
+fn generate_ephemeral_context(workspace: &Path) -> Option<String> {
     let overview = generate_bounded_project_overview(workspace)?;
 
-    let content = format!(
-        "# Project Context (Auto-generated)\n\n\
-         > This file was automatically generated by CodeWhale.\n\
-         > You can edit or delete it at any time.\n\n\
+    Some(format!(
+        "# Project Context (Auto-generated, ephemeral)\n\n\
+         > This context was generated in memory by CodeWhale.\n\
+         > No .codewhale/instructions.md file was written.\n\n\
          {overview}"
-    );
-
-    // Create .codewhale/ directory
-    if let Err(e) = std::fs::create_dir_all(&codewhale_dir) {
-        tracing::warn!("Failed to create .codewhale/ directory: {e}");
-        return None;
-    }
-
-    match std::fs::write(&instructions_path, &content) {
-        Ok(()) => {
-            tracing::info!("Auto-generated {}", instructions_path.display());
-            Some(content)
-        }
-        Err(e) => {
-            tracing::warn!("Failed to write {}: {e}", instructions_path.display());
-            None
-        }
-    }
+    ))
 }
 
 /// Load a context file with size checking
@@ -1488,7 +1453,7 @@ mod tests {
     }
 
     #[test]
-    fn auto_generated_context_is_bounded_for_many_file_workspace() {
+    fn generated_context_is_bounded_and_ephemeral_for_many_file_workspace() {
         let workspace = tempdir().expect("workspace tempdir");
         let home = tempdir().expect("home tempdir");
         let noisy = workspace.path().join("aaa-many-files");
@@ -1513,9 +1478,17 @@ mod tests {
         assert!(ctx.has_instructions());
 
         let generated_path = workspace.path().join(".codewhale").join("instructions.md");
-        assert_eq!(ctx.source_path.as_deref(), Some(generated_path.as_path()));
-        let generated = fs::read_to_string(&generated_path).expect("read generated");
-        assert!(generated.contains("Project Context (Auto-generated)"));
+        assert_eq!(ctx.source_path, None);
+        assert!(
+            !generated_path.exists(),
+            "generated project context should stay ephemeral"
+        );
+        assert!(
+            !workspace.path().join(".codewhale").exists(),
+            "loading context should not create a .codewhale directory"
+        );
+        let generated = ctx.instructions.as_ref().expect("generated instructions");
+        assert!(generated.contains("Project Context (Auto-generated, ephemeral)"));
         assert!(generated.contains("Bounded Project Overview"));
         assert!(!generated.contains("<project_context_pack>"));
         assert!(
@@ -1613,7 +1586,7 @@ mod tests {
     }
 
     #[test]
-    fn cached_context_regenerates_after_auto_generated_context_is_deleted() {
+    fn cached_generated_context_stays_ephemeral() {
         crate::project_context_cache::clear();
         let workspace = tempdir().expect("workspace tempdir");
         let home = tempdir().expect("home tempdir");
@@ -1622,17 +1595,17 @@ mod tests {
             load_project_context_with_parents_cached_and_home(workspace.path(), Some(home.path()));
         assert!(first.has_instructions());
         let generated_path = workspace.path().join(".codewhale").join("instructions.md");
-        assert!(generated_path.is_file(), "expected generated instructions");
-
-        fs::remove_file(&generated_path).expect("remove generated instructions");
-        assert!(!generated_path.exists());
+        assert!(
+            !generated_path.exists(),
+            "first load should not write generated instructions"
+        );
 
         let second =
             load_project_context_with_parents_cached_and_home(workspace.path(), Some(home.path()));
         assert!(second.has_instructions());
         assert!(
-            generated_path.is_file(),
-            "cache hit under the missing-file signature would skip regeneration"
+            !generated_path.exists(),
+            "cached generated context should remain in memory-only state"
         );
     }
 
@@ -1937,7 +1910,7 @@ mod tests {
             ctx.instructions
                 .as_ref()
                 .unwrap()
-                .contains("Project Context (Auto-generated)")
+                .contains("Project Context (Auto-generated, ephemeral)")
         );
     }
 }
diff --git a/crates/tui/src/prompts.rs b/crates/tui/src/prompts.rs
index 564eb34a..a1a69f2b 100644
--- a/crates/tui/src/prompts.rs
+++ b/crates/tui/src/prompts.rs
@@ -1088,9 +1088,9 @@ pub fn system_prompt_for_mode_with_context_skills_session_and_approval(
     };
 
     // 1–2. Mode prompt + project context.
-    // `load_project_context_with_parents` auto-generates .codewhale/instructions.md
-    // (or .deepseek/instructions.md as fallback) when no context file exists,
-    // so the fallback should always be available.
+    // `load_project_context_with_parents` generates an in-memory bounded
+    // overview when no context file exists, so the fallback should usually be
+    // available without writing project-local files.
     let mut full_prompt = if let Some(project_block) = project_context.as_system_block() {
         format!("{mode_prompt}\n\n{project_block}")
     } else {