diff --git a/src/mir/builder/calls/lowering.rs b/src/mir/builder/calls/lowering.rs
index 2cc2f541..707f25c4 100644
--- a/src/mir/builder/calls/lowering.rs
+++ b/src/mir/builder/calls/lowering.rs
@@ -176,9 +176,28 @@ impl MirBuilder {
     fn lower_function_body(&mut self, body: Vec<ASTNode>) -> Result<(), String> {
         let trace = crate::mir::builder::control_flow::joinir::trace::trace();
 
-        if crate::config::env::joinir_dev_enabled() {
+        // Phase 112: StepTree capability guard (strict-only)
+        let strict = crate::config::env::joinir_dev::strict_enabled();
+        let dev = crate::config::env::joinir_dev_enabled();
+
+        if strict || dev {
             let tree = crate::mir::control_tree::StepTreeBuilderBox::build_from_block(&body);
-            trace.dev("control_tree/step_tree", &tree.to_compact_string());
+
+            if dev {
+                trace.dev("control_tree/step_tree", &tree.to_compact_string());
+            }
+
+            // Phase 112: Guard check (strict mode only)
+            let func_name = self
+                .scope_ctx
+                .current_function
+                .as_ref()
+                .map(|f| f.signature.name.clone())
+                .unwrap_or_else(|| "<unknown>".to_string());
+
+            crate::mir::builder::control_flow::joinir::control_tree_capability_guard::check(
+                &tree, &func_name, strict, dev,
+            )?;
         }
 
         trace.emit_if(
diff --git a/src/mir/builder/control_flow/joinir/control_tree_capability_guard.rs b/src/mir/builder/control_flow/joinir/control_tree_capability_guard.rs
new file mode 100644
index 00000000..4156509e
--- /dev/null
+++ b/src/mir/builder/control_flow/joinir/control_tree_capability_guard.rs
@@ -0,0 +1,200 @@
+//! Phase 112: StepTree Capability Guard (strict-only)
+//!
+//! ## Purpose
+//! Detects unsupported capabilities in StepTree required_caps during strict mode.
+//! Provides actionable 1-line hints for developers to fix their code.
+//!
+//! ## Design
+//! - **Allowlist**: If, NestedIf, Loop, Return, Break, Continue
+//! - **Deny (strict)**: NestedLoop, TryCatch, Throw, Lambda, While, ForRange, Match, Arrow
+//! - **Default behavior unchanged**: strict=false always returns Ok(())
+//!
+//! ## Integration
+//! - Called from `lower_function_body()` in `calls/lowering.rs`
+//! - Uses existing `HAKO_JOINIR_STRICT` / `NYASH_JOINIR_STRICT` env vars
+//! - Error format: `[joinir/control_tree/cap_missing/<Cap>] <msg>  Hint: <hint>`
+
+use crate::mir::control_tree::{StepCapability, StepTree};
+use crate::mir::join_ir::lowering::error_tags;
+use std::collections::BTreeSet;
+
+/// Check StepTree capabilities against allowlist (Phase 112)
+///
+/// # Arguments
+/// - `tree`: StepTree to check
+/// - `func_name`: Function name for error messages
+/// - `strict`: If false, always returns Ok (default behavior unchanged)
+/// - `dev`: If true, trace missing caps (debug info)
+///
+/// # Returns
+/// - Ok(()) if all required_caps are allowed (or strict=false)
+/// - Err with freeze_with_hint format if unsupported cap found in strict mode
+pub fn check(tree: &StepTree, func_name: &str, strict: bool, dev: bool) -> Result<(), String> {
+    if !strict {
+        return Ok(()); // Default behavior: always pass
+    }
+
+    // Allowlist (supported capabilities)
+    let allowed: BTreeSet<StepCapability> = [
+        StepCapability::If,
+        StepCapability::NestedIf,
+        StepCapability::Loop,
+        StepCapability::Return,
+        StepCapability::Break,
+        StepCapability::Continue,
+    ]
+    .into_iter()
+    .collect();
+
+    // Check for unsupported caps
+    for cap in &tree.contract.required_caps {
+        if !allowed.contains(cap) {
+            let cap_name = format!("{:?}", cap);
+            let tag = format!("control_tree/cap_missing/{}", cap_name);
+            let msg = format!(
+                "{} detected in '{}' (step_tree_sig={})",
+                cap_name,
+                func_name,
+                tree.signature.to_hex()
+            );
+            let hint = get_hint_for_cap(cap);
+
+            if dev {
+                // Trace for debugging
+                eprintln!(
+                    "[joinir/control_tree] missing cap: {} in {}",
+                    cap_name, func_name
+                );
+            }
+
+            return Err(error_tags::freeze_with_hint(&tag, &msg, &hint));
+        }
+    }
+
+    Ok(())
+}
+
+fn get_hint_for_cap(cap: &StepCapability) -> String {
+    match cap {
+        StepCapability::NestedLoop => {
+            "refactor to avoid nested loops (not supported yet) or run without HAKO_JOINIR_STRICT=1".to_string()
+        }
+        StepCapability::TryCatch => {
+            "try/catch not supported in JoinIR yet, use HAKO_JOINIR_STRICT=0".to_string()
+        }
+        StepCapability::Throw => {
+            "throw not supported in JoinIR yet, use HAKO_JOINIR_STRICT=0".to_string()
+        }
+        StepCapability::Lambda => {
+            "lambda not supported in JoinIR yet, extract to named function".to_string()
+        }
+        StepCapability::While => {
+            "use 'loop(cond)' instead of 'while(cond)' syntax".to_string()
+        }
+        StepCapability::ForRange => {
+            "use 'loop(i < n)' instead of 'for i in range' syntax".to_string()
+        }
+        StepCapability::Match => {
+            "match expressions not supported in JoinIR yet, use if-else chain".to_string()
+        }
+        StepCapability::Arrow => {
+            "arrow functions not supported in JoinIR yet, use regular functions".to_string()
+        }
+        _ => format!("{:?} not supported in JoinIR yet", cap),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ast::{ASTNode, BinaryOperator, LiteralValue, Span};
+    use crate::mir::control_tree::StepTreeBuilderBox;
+
+    fn var(name: &str) -> ASTNode {
+        ASTNode::Variable {
+            name: name.to_string(),
+            span: Span::unknown(),
+        }
+    }
+
+    fn int_lit(v: i64) -> ASTNode {
+        ASTNode::Literal {
+            value: LiteralValue::Integer(v),
+            span: Span::unknown(),
+        }
+    }
+
+    fn bin_lt(lhs: ASTNode, rhs: ASTNode) -> ASTNode {
+        ASTNode::BinaryOp {
+            operator: BinaryOperator::Less,
+            left: Box::new(lhs),
+            right: Box::new(rhs),
+            span: Span::unknown(),
+        }
+    }
+
+    #[test]
+    fn test_nested_loop_strict_rejects() {
+        // AST: loop(i < 3) { loop(j < 2) { ... } }
+        let nested_loop_ast = vec![ASTNode::Loop {
+            condition: Box::new(bin_lt(var("i"), int_lit(3))),
+            body: vec![ASTNode::Loop {
+                condition: Box::new(bin_lt(var("j"), int_lit(2))),
+                body: vec![],
+                span: Span::unknown(),
+            }],
+            span: Span::unknown(),
+        }];
+
+        let tree = StepTreeBuilderBox::build_from_block(&nested_loop_ast);
+
+        // strict=true should reject NestedLoop
+        let result = check(&tree, "test_func", true, false);
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+        assert!(err.contains("[joinir/control_tree/cap_missing/NestedLoop]"));
+        assert!(err.contains("Hint:"));
+    }
+
+    #[test]
+    fn test_if_only_strict_passes() {
+        // AST: if x == 1 { ... } else { ... }
+        let if_only_ast = vec![ASTNode::If {
+            condition: Box::new(ASTNode::BinaryOp {
+                operator: BinaryOperator::Equal,
+                left: Box::new(var("x")),
+                right: Box::new(int_lit(1)),
+                span: Span::unknown(),
+            }),
+            then_body: vec![],
+            else_body: Some(vec![]),
+            span: Span::unknown(),
+        }];
+
+        let tree = StepTreeBuilderBox::build_from_block(&if_only_ast);
+
+        // strict=true should pass (If is allowed)
+        let result = check(&tree, "test_func", true, false);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_strict_false_always_passes() {
+        // Even with NestedLoop, strict=false should pass
+        let nested_loop_ast = vec![ASTNode::Loop {
+            condition: Box::new(bin_lt(var("i"), int_lit(3))),
+            body: vec![ASTNode::Loop {
+                condition: Box::new(bin_lt(var("j"), int_lit(2))),
+                body: vec![],
+                span: Span::unknown(),
+            }],
+            span: Span::unknown(),
+        }];
+
+        let tree = StepTreeBuilderBox::build_from_block(&nested_loop_ast);
+
+        // strict=false should always pass
+        let result = check(&tree, "test_func", false, false);
+        assert!(result.is_ok());
+    }
+}
diff --git a/src/mir/builder/control_flow/joinir/mod.rs b/src/mir/builder/control_flow/joinir/mod.rs
index 050a9614..6bb1c80f 100644
--- a/src/mir/builder/control_flow/joinir/mod.rs
+++ b/src/mir/builder/control_flow/joinir/mod.rs
@@ -7,7 +7,9 @@
 //! - Loop processing context (loop_context.rs) ✅ Phase 140-P5
 //! - MIR block merging (merge/) ✅ Phase 4
 //! - Unified tracing (trace.rs) ✅ Phase 195
+//! - Control tree capability guard (control_tree_capability_guard.rs) ✅ Phase 112
 
+pub(in crate::mir::builder) mod control_tree_capability_guard;
 pub(in crate::mir::builder) mod loop_context;
 pub(in crate::mir::builder) mod merge;
 pub(in crate::mir::builder) mod parity_checker;
diff --git a/src/mir/control_tree/mod.rs b/src/mir/control_tree/mod.rs
index fa5eed40..85be1f87 100644
--- a/src/mir/control_tree/mod.rs
+++ b/src/mir/control_tree/mod.rs
@@ -7,6 +7,7 @@
 mod step_tree;
 
 pub use step_tree::{
-    AstSummary, StepNode, StepStmtKind, StepTree, StepTreeBuilderBox, StepTreeFeatures,
+    AstSummary, ExitKind, StepCapability, StepNode, StepStmtKind, StepTree, StepTreeBuilderBox,
+    StepTreeContract, StepTreeFeatures, StepTreeSignature,
 };