diff --git a/src/mir/control_tree/normalized_shadow/available_inputs_collector.rs b/src/mir/control_tree/normalized_shadow/available_inputs_collector.rs
new file mode 100644
index 00000000..c1328a77
--- /dev/null
+++ b/src/mir/control_tree/normalized_shadow/available_inputs_collector.rs
@@ -0,0 +1,142 @@
+//! Phase 126: available_inputs collector (SSOT)
+//!
+//! ## Responsibility
+//!
+//! - Collect available_inputs from function params + CapturedEnv
+//! - Returns BTreeMap<String, ValueId> with deterministic order
+//! - No AST inference (SSOT sources only)
+//!
+//! ## Design
+//!
+//! - Input sources (priority order):
+//!   1. Function params (from ScopeContext + VariableContext)
+//!   2. CapturedEnv (pinned/captured variables from outer scope)
+//! - Forbidden: AST-based capture inference (Phase 100 CapturedEnv is SSOT)
+
+// Phase 126: Import contexts from MirBuilder (pub(in crate::mir) visibility)
+// Since we're in crate::mir::control_tree::normalized_shadow, we can access these
+use crate::mir::builder::MirBuilder;
+use crate::mir::loop_pattern_detection::function_scope_capture::CapturedEnv;
+use crate::mir::ValueId;
+use std::collections::BTreeMap;
+
+/// Phase 126: Box-First available_inputs collector
+pub struct AvailableInputsCollectorBox;
+
+impl AvailableInputsCollectorBox {
+    /// Collect available_inputs from SSOT sources (via MirBuilder)
+    ///
+    /// ## Contract
+    ///
+    /// - Sources (priority order):
+    ///   1. Function params: builder.scope_ctx.function_param_names + builder.variable_ctx.variable_map
+    ///   2. CapturedEnv: captured_env.vars (pinned/captured from outer scope)
+    /// - Output: BTreeMap<String, ValueId> (deterministic order)
+    /// - No AST inference: only use pre-computed SSOT sources
+    ///
+    /// ## Implementation
+    ///
+    /// - Collect function params first (higher priority)
+    /// - Collect CapturedEnv vars (lower priority, don't override params)
+    /// - Use BTreeMap for deterministic iteration
+    pub fn collect(
+        builder: &MirBuilder,
+        captured_env: Option<&CapturedEnv>,
+    ) -> BTreeMap<String, ValueId> {
+        let mut available_inputs = BTreeMap::new();
+
+        // 1. Function params (SSOT: scope_ctx + variable_ctx)
+        for param_name in &builder.scope_ctx.function_param_names {
+            if let Some(value_id) = builder.variable_ctx.lookup(param_name) {
+                available_inputs.insert(param_name.clone(), value_id);
+            }
+        }
+
+        // 2. CapturedEnv (SSOT: pinned/captured vars)
+        if let Some(env) = captured_env {
+            for var in &env.vars {
+                // Don't override function params (params have higher priority)
+                if !available_inputs.contains_key(&var.name) {
+                    available_inputs.insert(var.name.clone(), var.host_id);
+                }
+            }
+        }
+
+        available_inputs
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::mir::loop_pattern_detection::function_scope_capture::CapturedVar;
+
+    #[test]
+    fn test_collect_empty() {
+        let builder = MirBuilder::new();
+        let result = AvailableInputsCollectorBox::collect(&builder, None);
+        assert_eq!(result.len(), 0);
+    }
+
+    #[test]
+    fn test_collect_function_params() {
+        let mut builder = MirBuilder::new();
+
+        // Simulate function param: x -> ValueId(1)
+        builder.scope_ctx.function_param_names.insert("x".to_string());
+        builder.variable_ctx.insert("x".to_string(), ValueId(1));
+
+        let result = AvailableInputsCollectorBox::collect(&builder, None);
+        assert_eq!(result.len(), 1);
+        assert_eq!(result.get("x"), Some(&ValueId(1)));
+    }
+
+    #[test]
+    fn test_collect_captured_env() {
+        let builder = MirBuilder::new();
+
+        let mut captured = CapturedEnv::new();
+        captured.insert("outer_x".to_string(), ValueId(42));
+
+        let result = AvailableInputsCollectorBox::collect(&builder, Some(&captured));
+        assert_eq!(result.len(), 1);
+        assert_eq!(result.get("outer_x"), Some(&ValueId(42)));
+    }
+
+    #[test]
+    fn test_collect_params_override_captured() {
+        let mut builder = MirBuilder::new();
+
+        // Function param: x -> ValueId(1)
+        builder.scope_ctx.function_param_names.insert("x".to_string());
+        builder.variable_ctx.insert("x".to_string(), ValueId(1));
+
+        // Captured: x -> ValueId(42) (should be ignored)
+        let mut captured = CapturedEnv::new();
+        captured.insert("x".to_string(), ValueId(42));
+
+        let result = AvailableInputsCollectorBox::collect(&builder, Some(&captured));
+        assert_eq!(result.len(), 1);
+        // Function param (ValueId(1)) should win over captured (ValueId(42))
+        assert_eq!(result.get("x"), Some(&ValueId(1)));
+    }
+
+    #[test]
+    fn test_collect_deterministic_order() {
+        let mut builder = MirBuilder::new();
+
+        // Add params in non-alphabetical order
+        builder.scope_ctx.function_param_names.insert("z".to_string());
+        builder.scope_ctx.function_param_names.insert("a".to_string());
+        builder.scope_ctx.function_param_names.insert("m".to_string());
+        builder.variable_ctx.insert("z".to_string(), ValueId(3));
+        builder.variable_ctx.insert("a".to_string(), ValueId(1));
+        builder.variable_ctx.insert("m".to_string(), ValueId(2));
+
+        let result = AvailableInputsCollectorBox::collect(&builder, None);
+        let keys: Vec<_> = result.keys().collect();
+
+        // BTreeMap ensures alphabetical order
+        assert_eq!(keys, vec![&"a".to_string(), &"m".to_string(), &"z".to_string()]);
+    }
+}
diff --git a/src/mir/control_tree/normalized_shadow/mod.rs b/src/mir/control_tree/normalized_shadow/mod.rs
index d31320d5..585333a7 100644
--- a/src/mir/control_tree/normalized_shadow/mod.rs
+++ b/src/mir/control_tree/normalized_shadow/mod.rs
@@ -32,6 +32,7 @@
 pub mod builder;
 pub mod contracts;
 pub mod parity;
+pub mod available_inputs_collector; // Phase 126: available_inputs SSOT
 
 pub use builder::StepTreeNormalizedShadowLowererBox;
 pub use contracts::{CapabilityCheckResult, UnsupportedCapability};