diff --git a/src/v3/compiler/src/bootstrap_generated.rs b/src/v3/compiler/src/bootstrap_generated.rs
index b1d262c53b2..18f53ce4017 100644
--- a/src/v3/compiler/src/bootstrap_generated.rs
+++ b/src/v3/compiler/src/bootstrap_generated.rs
@@ -2490,5 +2490,6 @@ pub(crate) fn bootstrapped_fixture_dag() -> Dag {
         callable_strategy_variants: CallableStrategyVariants::default(),
         clusters: vec![],
         optional_match_disjs: HashMap::new(),
+        user_declaration_append_begin: 0,
     }
 }
diff --git a/src/v3/compiler/src/bootstrap_generated_without_parse_surface.rs b/src/v3/compiler/src/bootstrap_generated_without_parse_surface.rs
index d1048d1a666..ff7230b6c92 100644
--- a/src/v3/compiler/src/bootstrap_generated_without_parse_surface.rs
+++ b/src/v3/compiler/src/bootstrap_generated_without_parse_surface.rs
@@ -2407,5 +2407,6 @@ pub(crate) fn bootstrapped_fixture_without_parse_surface_dag() -> Dag {
         callable_strategy_variants: CallableStrategyVariants::default(),
         clusters: vec![],
         optional_match_disjs: HashMap::new(),
+        user_declaration_append_begin: 0,
     }
 }
diff --git a/src/v3/compiler/src/bootstrap_std_generated.rs b/src/v3/compiler/src/bootstrap_std_generated.rs
index 9e6c869ffa5..f470b27a09c 100644
--- a/src/v3/compiler/src/bootstrap_std_generated.rs
+++ b/src/v3/compiler/src/bootstrap_std_generated.rs
@@ -12413,5 +12413,6 @@ pub(crate) fn bootstrapped_std_fixture_dag() -> Dag {
         callable_strategy_variants: CallableStrategyVariants::default(),
         clusters: vec![],
         optional_match_disjs: HashMap::new(),
+        user_declaration_append_begin: 0,
     }
 }
diff --git a/src/v3/compiler/src/dag.rs b/src/v3/compiler/src/dag.rs
index bef81ed4788..02286b4f8d9 100644
--- a/src/v3/compiler/src/dag.rs
+++ b/src/v3/compiler/src/dag.rs
@@ -2358,6 +2358,10 @@ pub struct Dag {
     /// inference needs stable `Some` / `None` variant identities without
     /// promoting optionals into named top-level declarations.
     optional_match_disjs: HashMap<DeclarationId, DeclarationId>,
+    /// First [`DeclarationId::raw`] value minted **after** the embedded bootstrap fixture
+    /// rows loaded at `Dag` construction (`lower` / `lower_into` append here; callers cannot
+    /// claim fixture provenance for these rows by stamping an authority-looking `span.file`).
+    user_declaration_append_begin: u32,
 }
 
 static BOOTSTRAPPED_DAG: LazyLock<Dag> = LazyLock::new(|| {
@@ -2366,6 +2370,7 @@ static BOOTSTRAPPED_DAG: LazyLock<Dag> = LazyLock::new(|| {
     assert_bootstrap_fixture_paths_match_regen_keys(&dag);
     dag.populate_primitive_cache();
     crate::int_literal_ranges::validate_rust_pilot_integer_primitives(&mut dag);
+    dag.user_declaration_append_begin = dag.declarations.len() as u32;
     dag
 });
 
@@ -2376,6 +2381,7 @@ static BOOTSTRAPPED_STD_FIXTURE_DAG: LazyLock<Dag> = LazyLock::new(|| {
     let mut dag = bootstrap_std_generated::bootstrapped_std_fixture_dag();
     dag.mark_bootstrap_secret_nominal_opacity();
     dag.populate_primitive_cache();
+    dag.user_declaration_append_begin = dag.declarations.len() as u32;
     dag
 });
 
@@ -2386,6 +2392,7 @@ static BOOTSTRAPPED_DAG_WITHOUT_PARSE_SURFACE_FIXTURE: LazyLock<Dag> = LazyLock:
     assert_bootstrap_fixture_paths_match_regen_keys(&dag);
     dag.populate_primitive_cache();
     crate::int_literal_ranges::validate_rust_pilot_integer_primitives(&mut dag);
+    dag.user_declaration_append_begin = dag.declarations.len() as u32;
     dag
 });
 
@@ -2417,6 +2424,7 @@ impl Dag {
             callable_strategy_variants: CallableStrategyVariants::default(),
             clusters: Vec::new(),
             optional_match_disjs: HashMap::new(),
+            user_declaration_append_begin: 0,
         }
     }
 
@@ -2470,6 +2478,16 @@ impl Dag {
         self.primitives.string
     }
 
+    /// Raw [`DeclarationId`] boundary between embedded bootstrap fixture rows and any
+    /// declarations minted afterward (`lower_into`, tests, etc.).
+    ///
+    /// Lifetime extraction and similar consumers must **not** classify post-boundary rows as
+    /// “fixture authority” based on [`Declaration::span`] alone — callers can supply an
+    /// authority-*looking* `span.file` string without being part of the checked-in corpus.
+    pub fn declaration_append_boundary(&self) -> u32 {
+        self.user_declaration_append_begin
+    }
+
     /// The std **scalar** `TypeShape` roots used to prune algebra / ring
     /// `TypeConnective::Arrow` from the first-class-`fn` *data* walk in
     /// `emit::rust_target` (`decl_includes_first_class_arrow_data` and related).
diff --git a/src/v3/compiler/src/regen_bootstrap_emit.rs b/src/v3/compiler/src/regen_bootstrap_emit.rs
index ec2454dfb30..794133657d3 100644
--- a/src/v3/compiler/src/regen_bootstrap_emit.rs
+++ b/src/v3/compiler/src/regen_bootstrap_emit.rs
@@ -96,6 +96,7 @@ fn emit_bootstrap_module(dag: &Dag, function_name: &str) -> String {
         &render_optional_match_disjs(dag),
         2,
     );
+    out.push_str("        user_declaration_append_begin: 0,\n");
     out.push_str("    }\n");
     out.push_str("}\n");
     out
diff --git a/src/v3/grounding_lifetime/src/extract.rs b/src/v3/grounding_lifetime/src/extract.rs
index ea2c2cc15ef..ac23224e45f 100644
--- a/src/v3/grounding_lifetime/src/extract.rs
+++ b/src/v3/grounding_lifetime/src/extract.rs
@@ -5,9 +5,11 @@
 //! so extraction returns an **empty** program (the structural fold is validated via
 //! [`LifetimeProgram`] fixtures).
 //!
-//! Any **`data` or `fn` declaration** rooted in a **non-authority** source file (user
-//! or test modules) is treated as load-bearing program surface we cannot project yet:
-//! fail-closed per C-8 instead of returning `Ok(empty)` and dropping facts.
+//! Any **`data` or `fn` declaration** minted **after** the embedded fixture snapshot
+//! ([`Dag::declaration_append_boundary`]) is treated as load-bearing program surface we
+//! cannot project yet — **even if** `span.file` looks like an authority path (callers
+//! control the compile-time file string). Fail-closed per C-8 instead of returning
+//! `Ok(empty)` and dropping facts.
 
 use v3_compiler::dag::{Dag, TypeConnective};
 
@@ -21,6 +23,9 @@ use std::collections::BTreeMap;
 
 /// Span roots that appear on [`Dag::new()`] bootstrap fixtures (regenerated snapshots).
 ///
+/// Used only as **defense in depth** for rows with `DeclarationId::raw` **below**
+/// [`Dag::declaration_append_boundary`]: those rows are expected to be corpus-only.
+///
 /// Keep aligned with `bootstrap_generated.rs` / `bootstrap_generated_without_parse_surface.rs`
 /// when new fixture corpora land; otherwise user/test modules may be misclassified.
 ///
@@ -42,11 +47,9 @@ fn is_bootstrap_fixture_authority_source_file(file: &str) -> bool {
     )
 }
 
-fn first_non_authority_lifetime_surface_declaration(dag: &Dag) -> Option<(String, String)> {
+fn first_unextracted_lifetime_surface_declaration(dag: &Dag) -> Option<(String, String)> {
+    let boundary = dag.declaration_append_boundary();
     for decl in dag.declarations() {
-        if is_bootstrap_fixture_authority_source_file(&decl.span.file) {
-            continue;
-        }
         let surface =
             decl.value_body.is_some() || matches!(&decl.connective, TypeConnective::Arrow { .. });
         if !surface {
@@ -56,7 +59,14 @@ fn first_non_authority_lifetime_surface_declaration(dag: &Dag) -> Option<(String
             .name
             .clone()
             .unwrap_or_else(|| format!("declaration#{}", decl.id.raw()));
-        return Some((name, decl.span.file.clone()));
+        let post_fixture = decl.id.raw() >= boundary;
+        if post_fixture {
+            return Some((name, decl.span.file.clone()));
+        }
+        if !is_bootstrap_fixture_authority_source_file(&decl.span.file) {
+            // Fixture-id range but not stamped like the embedded corpus — do not trust.
+            return Some((name, decl.span.file.clone()));
+        }
     }
     None
 }
@@ -65,7 +75,7 @@ fn first_non_authority_lifetime_surface_declaration(dag: &Dag) -> Option<(String
 ///
 /// Fail-closed on constructs the R2 analyzer does not model (once lowering surfaces them).
 pub fn extract_lifetime_program(dag: &Dag) -> Result<LifetimeProgram, EmissionDiagnostic> {
-    if let Some((name, file)) = first_non_authority_lifetime_surface_declaration(dag) {
+    if let Some((name, file)) = first_unextracted_lifetime_surface_declaration(dag) {
         return Err(EmissionDiagnostic::LifetimeProgramExtractionPending {
             detail: format!("{name} ({file})"),
         });
diff --git a/src/v3/grounding_lifetime/src/lib.rs b/src/v3/grounding_lifetime/src/lib.rs
index 3a3610bbd61..5c254c9945d 100644
--- a/src/v3/grounding_lifetime/src/lib.rs
+++ b/src/v3/grounding_lifetime/src/lib.rs
@@ -165,6 +165,21 @@ mod tests {
         }
     }
 
+    /// `span.file` is caller-controlled; authority-looking paths must not bypass C-8.
+    #[test]
+    fn extract_fail_closed_when_user_code_uses_authority_looking_span_path() {
+        let source = "module spoof\n\nfn f() -> Int = 0\n";
+        let file = "dsl/std/user_spoof_under_extractor_test.dag";
+        let dag = v3_compiler::compile_to_dag(source, file).expect("compile");
+        let err = extract_lifetime_program(&dag).expect_err("must not Ok(empty) on spoof path");
+        match err {
+            EmissionDiagnostic::LifetimeProgramExtractionPending { detail } => {
+                assert!(detail.contains('f'), "unexpected detail: {detail}");
+            }
+            other => panic!("unexpected diagnostic: {other:?}"),
+        }
+    }
+
     #[test]
     fn function_param_indeterminate_growability_fails_closed_even_when_borrowed() {
         let mut bindings = BTreeMap::new();