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+# Story 4.7: Convergence Criteria & Validation
+
+Status: done
+
+<!-- Note: Validation is optional. Run validate-create-story for quality check before dev-story. -->
+
+## Story
+
+As a simulation user,
+I want strict, multi-dimensional convergence criteria with per-circuit granularity and a sparse/block-diagonal Jacobian for multi-circuit systems,
+so that large systems remain tractable and convergence is only declared when ALL circuits physically satisfy pressure, mass, and energy balances simultaneously.
+
+## Acceptance Criteria
+
+1. **Δ Pressure convergence ≤ 1 Pa** (AC: #1)
+   - Given a system approaching solution
+   - When checking convergence with `ConvergenceCriteria::check()`
+   - Then the check passes only if `max |ΔP_i| < 1.0 Pa` across all pressure state variables
+   - And a configurable `pressure_tolerance_pa: f64` field (default 1.0 Pa) controls the threshold
+
+2. **Mass balance error < 1e-9 kg/s per circuit** (AC: #2)
+   - Given a cycle residuals vector
+   - When `ConvergenceCriteria::check()` is called
+   - Then mass balance error per circuit is extracted: `Σ |ṁ_in - ṁ_out| < 1e-9 kg/s`
+   - And `mass_balance_tolerance_kgs: f64` field (default 1e-9) controls the threshold
+
+3. **Energy balance error < 1e-6 kW per circuit** (AC: #3)
+   - Given a cycle residuals vector
+   - When `ConvergenceCriteria::check()` is called
+   - Then energy balance error per circuit satisfies `|Σ Q̇ + Ẇ - Σ (ṁ·h)| < 1e-6 kW (1e-3 W)`
+   - And `energy_balance_tolerance_w: f64` field (default 1e-3) controls the threshold
+
+4. **Per-circuit convergence tracking** (AC: #4)
+   - Given a multi-circuit system (N circuits)
+   - When checking convergence
+   - Then `ConvergenceReport` contains a `per_circuit: Vec<CircuitConvergence>` with one entry per active circuit
+   - And each `CircuitConvergence` contains: `circuit_id: u8`, `pressure_ok: bool`, `mass_ok: bool`, `energy_ok: bool`, `converged: bool`
+   - And `circuit.converged = pressure_ok && mass_ok && energy_ok`
+
+5. **Global convergence = ALL circuits converged** (AC: #5)
+   - Given a multi-circuit system with N circuits
+   - When `ConvergenceReport::is_globally_converged()` is called
+   - Then it returns `true` if and only if ALL entries in `per_circuit` have `converged == true`
+   - And a single-circuit system behaves identically (degenerate case of N=1)
+
+6. **Block-diagonal Jacobian for uncoupled multi-circuit** (AC: #6)
+   - Given a system with N circuits that have NO thermal couplings between them
+   - When `JacobianMatrix` is assembled via `system.assemble_jacobian()`
+   - Then the resulting matrix has block-diagonal structure (entries outside circuit blocks are zero)
+   - And `JacobianMatrix::block_structure(system)` returns `Vec<(row_start, row_end, col_start, col_end)>` describing each circuit block
+   - And `JacobianMatrix::is_block_diagonal(system, tolerance: f64)` returns `true` for uncoupled systems
+
+7. **ConvergenceCriteria integrates with solvers** (AC: #7)
+   - Given a `NewtonConfig` or `PicardConfig`
+   - When the user sets `solver.with_convergence_criteria(criteria)`
+   - Then the solver uses `ConvergenceCriteria::check()` instead of the raw L2-norm tolerance check
+   - And the old `tolerance: f64` field remains for backward-compat (ignored when `convergence_criteria` is `Some`)
+   - And `FallbackSolver` delegates `with_convergence_criteria()` to both sub-solvers
+
+8. **`ConvergenceReport` in `ConvergedState`** (AC: #8)
+   - Given a converged system
+   - When `ConvergedState` is returned
+   - Then it optionally contains `convergence_report: Option<ConvergenceReport>`
+   - And `ConvergedState::convergence_report()` returns `None` when criteria is not set (backward-compat)
+
+## Tasks / Subtasks
+
+- [x] Create `crates/solver/src/criteria.rs` module (AC: #1–#6)
+  - [x] Define `ConvergenceCriteria` struct with fields:
+    - [x] `pressure_tolerance_pa: f64` (default 1.0)
+    - [x] `mass_balance_tolerance_kgs: f64` (default 1e-9)
+    - [x] `energy_balance_tolerance_w: f64` (default 1e-3)
+  - [x] Implement `ConvergenceCriteria::default()` using the values above
+  - [x] Define `CircuitConvergence` struct: `circuit_id: u8`, `pressure_ok: bool`, `mass_ok: bool`, `energy_ok: bool`, `converged: bool`
+  - [x] Define `ConvergenceReport` struct: `per_circuit: Vec<CircuitConvergence>`, `globally_converged: bool`
+  - [x] Implement `ConvergenceReport::is_globally_converged() -> bool`
+  - [x] Implement `ConvergenceCriteria::check(state: &[f64], residuals: &[f64], system: &System) -> ConvergenceReport`:
+    - [x] Extract per-circuit pressure deltas from state vector using `system.circuit_edges(circuit_id)` and `system.edge_state_indices(edge)`
+    - [x] For each circuit: compute `max |ΔP| < pressure_tolerance_pa` (use state increments from previous Newton step if available, else use residuals as proxy)
+    - [x] For each circuit: compute mass balance residual sum `< mass_balance_tolerance_kgs`
+    - [x] For each circuit: compute energy balance residual sum `< energy_balance_tolerance_w`
+    - [x] Return `ConvergenceReport` with per-circuit and global status
+  - [x] Expose `criteria.rs` in `lib.rs` (AC: #7)
+
+- [x] Add `block_structure` support to `JacobianMatrix` (AC: #6)
+  - [x] Implement `JacobianMatrix::block_structure(system: &System) -> Vec<(usize, usize, usize, usize)>` (row_start, row_end, col_start, col_end per circuit)
+  - [x] Implement `JacobianMatrix::is_block_diagonal(system: &System, tolerance: f64) -> bool` — verify off-block entries are < tolerance
+  - [x] These are pure analysis helpers; they do NOT change how the Jacobian is built (nalgebra `DMatrix<f64>` is retained)
+
+- [x] Integrate `ConvergenceCriteria` into solvers (AC: #7, #8)
+  - [x] Add `convergence_criteria: Option<ConvergenceCriteria>` field to `NewtonConfig`
+  - [x] Add `with_convergence_criteria(mut self, criteria: ConvergenceCriteria) -> Self` builder to `NewtonConfig`
+  - [x] In `NewtonConfig::solve()`: if `convergence_criteria.is_some()`, call `criteria.check()` for convergence test instead of raw `current_norm < self.tolerance`
+  - [x] Same pattern for `PicardConfig`
+  - [x] Add `with_convergence_criteria()` to `FallbackSolver` (delegates to both)
+  - [x] Add `convergence_report: Option<ConvergenceReport>` to `ConvergedState`
+  - [x] Update `ConvergedState::new()` or add `ConvergedState::with_report()` constructor
+
+- [x] Expose everything in `lib.rs`
+  - [x] `pub mod criteria;`
+  - [x] Re-export: `ConvergenceCriteria`, `ConvergenceReport`, `CircuitConvergence`
+
+- [x] Unit tests in `criteria.rs` (AC: #1–#5)
+  - [x] `test_default_thresholds`: assert `ConvergenceCriteria::default()` has correct values (1.0, 1e-9, 1e-3)
+  - [x] `test_single_circuit_converged`: 2-edge single-circuit system with zero residuals → `globally_converged = true`
+  - [x] `test_single_circuit_not_converged_pressure`: residuals indicating ΔP = 2 Pa → `pressure_ok = false`
+  - [x] `test_multi_circuit_global_needs_all`: 2-circuit system, circuit 0 converged, circuit 1 not → `is_globally_converged() = false`
+  - [x] `test_multi_circuit_all_converged`: 2-circuit system, both converged → `is_globally_converged() = true`
+  - [x] `test_custom_thresholds`: custom `pressure_tolerance_pa = 0.1` → tighter convergence check
+
+- [x] Unit tests in `jacobian.rs` (AC: #6)
+  - [x] `test_block_structure_single_circuit`: single circuit → one block covering full matrix
+  - [x] `test_block_structure_two_circuits`: 2 uncoupled circuits → 2 blocks, no overlap
+  - [x] `test_is_block_diagonal_uncoupled`: build uncoupled 2-circuit system, assert `is_block_diagonal(system, 1e-10) = true`
+  - [x] `test_is_block_diagonal_coupled`: manually add off-block entries, assert `is_block_diagonal(...) = false`
+
+- [x] Integration test in `crates/solver/tests/convergence_criteria.rs`
+  - [x] `test_newton_with_criteria_single_circuit`: Newton solver + `ConvergenceCriteria::default()` on 2-edge system, assert `is_globally_converged = true` in report
+  - [x] `test_newton_with_criteria_backward_compat`: Newton solver without `convergence_criteria` set, assert `convergence_report = None` in `ConvergedState`
+
+## Dev Notes
+
+### Epic Context
+
+**Epic 4: Intelligent Solver Engine** — Solve any system with < 1s guarantee, Newton-Raphson ↔ Sequential Substitution fallback.
+
+**Story Dependencies (all DONE):**
+- **Story 4.1** — `Solver` trait, `SolverError`, `ConvergedState`, `ConvergenceStatus` defined
+- **Story 4.2** — Full Newton with line search, `NewtonConfig`
+- **Story 4.3** — Picard with relaxation, `PicardConfig`
+- **Story 4.4** — `FallbackSolver` with Newton↔Picard switching
+- **Story 4.5** — `TimeoutConfig`, best-state tracking, ZOH (adds `previous_state`, `timeout_config` to configs)
+- **Story 4.6** — `SmartInitializer`, `initial_state` field added to `NewtonConfig`/`PicardConfig`/`FallbackSolver`
+- **Story 4.8 (NEXT)** — Jacobian Freezing Optimization
+
+**FRs covered:**
+- **FR20** — Convergence criterion: max |ΔP| < 1 Pa (1e-5 bar)
+- **FR21** — Global multi-circuit convergence: ALL circuits must converge
+
+### Architecture Context
+
+**Technical Stack:**
+- `entropyk_core::{Pressure, Temperature, Enthalpy, MassFlow}` — NewType wrappers (MUST use in public API, no bare f64)
+- `nalgebra::DMatrix<f64>` — backing type of `JacobianMatrix` (already in `jacobian.rs`)
+- `tracing` — structured logging (already in solver crate)
+- `thiserror` — error handling (already in solver crate)
+- `approx` — `assert_relative_eq!` for float tests (already in dev-dependencies)
+
+**Existing Infrastructure to Leverage:**
+
+```rust
+// crates/solver/src/system.rs — EXISTING
+pub struct System {
+    node_to_circuit: HashMap<NodeIndex, CircuitId>,  // node → circuit
+    thermal_couplings: Vec<ThermalCoupling>,  // inter-circuit heat transfer
+    // ...
+}
+
+impl System {
+    pub fn circuit_count(&self) -> usize { ... }  // distinct circuit IDs
+    pub fn circuit_edges(&self, circuit_id: CircuitId) -> impl Iterator<Item=EdgeIndex> + '_ { ... }
+    pub fn edge_circuit(&self, edge: EdgeIndex) -> CircuitId { ... }
+    pub fn edge_state_indices(&self, edge_id: EdgeIndex) -> (usize, usize)  // (p_idx, h_idx)
+    pub fn circuit_nodes(&self, circuit_id: CircuitId) -> impl Iterator<Item=NodeIndex> + '_ { ... }
+    pub fn state_vector_len(&self) -> usize { ... }  // 2 * edge_count
+    pub fn thermal_coupling_count(&self) -> usize { ... }  // number of couplings
+}
+
+// State vector layout: [P_edge0, h_edge0, P_edge1, h_edge1, ...]
+// Pressure entries: state[2*i], Enthalpy entries: state[2*i + 1]
+
+// crates/solver/src/solver.rs — EXISTING (post Story 4.6)
+pub struct NewtonConfig {
+    pub max_iterations: usize,        // default 100
+    pub tolerance: f64,               // default 1e-6 (keep for backward-compat)
+    pub line_search: bool,
+    pub timeout: Option<Duration>,
+    pub use_numerical_jacobian: bool,
+    pub line_search_armijo_c: f64,
+    pub line_search_max_backtracks: usize,
+    pub divergence_threshold: f64,
+    pub timeout_config: TimeoutConfig,
+    pub previous_state: Option<Vec<f64>>,
+    pub initial_state: Option<Vec<f64>>,
+    // ADD: pub convergence_criteria: Option<ConvergenceCriteria>,
+}
+
+pub struct PicardConfig {
+    pub max_iterations: usize,
+    pub tolerance: f64,               // keep for backward-compat
+    pub relaxation_factor: f64,
+    pub timeout: Option<Duration>,
+    pub divergence_threshold: f64,
+    pub divergence_patience: usize,
+    pub timeout_config: TimeoutConfig,
+    pub previous_state: Option<Vec<f64>>,
+    pub initial_state: Option<Vec<f64>>,
+    // ADD: pub convergence_criteria: Option<ConvergenceCriteria>,
+}
+
+pub struct ConvergedState {
+    pub state: Vec<f64>,
+    pub iterations: usize,
+    pub final_residual: f64,
+    pub status: ConvergenceStatus,
+    // ADD: pub convergence_report: Option<ConvergenceReport>,
+}
+
+// Currently in NewtonConfig::solve():
+// if current_norm < self.tolerance → converged
+// CHANGE TO: if let Some(ref c) = self.convergence_criteria { c.check(...).is_globally_converged() }
+//            else { current_norm < self.tolerance }
+```
+
+**crates/solver/src/jacobian.rs — EXISTING:**
+
+```rust
+pub struct JacobianMatrix(DMatrix<f64>);
+
+impl JacobianMatrix {
+    pub fn from_builder(entries: &[(usize, usize, f64)], n_rows: usize, n_cols: usize) -> Self { ... }
+    pub fn solve(&self, residuals: &[f64]) -> Option<Vec<f64>> { ... }
+    pub fn numerical<F>(...) -> Self { ... }
+    pub fn as_matrix(&self) -> &DMatrix<f64> { ... }
+    pub fn nrows(&self) -> usize { ... }
+    pub fn ncols(&self) -> usize { ... }
+    pub fn get(&self, row: usize, col: usize) -> Option<f64> { ... }
+    pub fn set(&mut self, row: usize, col: usize, value: f64) { ... }
+    pub fn norm(&self) -> f64 { ... }
+    pub fn condition_number(&self) -> Option<f64> { ... }
+    // ADD:
+    // pub fn block_structure(&self, system: &System) -> Vec<(usize, usize, usize, usize)>
+    // pub fn is_block_diagonal(&self, system: &System, tolerance: f64) -> bool
+}
+```
+
+**`ConvergenceCriteria::check()` Implementation Guide:**
+
+The key challenge is mapping the residual vector back to per-circuit quantities. The residual vector is assembled in component order (same order as `traverse_for_jacobian()`). For now, the simplest approach that meets the AC:
+
+1. **Pressure check**: For each circuit, iterate `system.circuit_edges(circuit_id)`. For each edge, extract `(p_idx, _) = system.edge_state_indices(edge)`. Compute `|state[p_idx] - prev_state[p_idx]|` if previous state available, or use the residuals at those indices as a proxy (residuals ≈ pressure errors at convergence).
+
+2. **Mass/Energy balance**: The convergence check needs mass and energy residuals per circuit. The simplest AC-compliant implementation:
+   - When `ConvergenceCriteria` is used, the solver passes the full residual vector
+   - The check partitions residuals by circuit using the equation ordering from `compute_residuals()` (same as `traverse_for_jacobian()`)
+   - For mass: residuals with unit Pa (pressure continuity) at convergence are near 0; use residual norm per circuit < threshold
+   - **NOTE**: A fully correct mass/energy extraction requires component-level metadata that doesn't exist yet. For Story 4-7, use a pragmatic approach: treat the residual norm per circuit as the mass/energy proxy. Full mass/energy balance validation is Epic 7 (Stories 7-1, 7-2). Document this clearly in criteria.rs.
+
+3. **Block structure**: For `block_structure(system)`, iterate each circuit ID 0..circuit_count. For each circuit, find the range of state column indices (from `edge_state_indices`) and the range of equation rows (from `traverse_for_jacobian()` filtered by circuit). Return `(row_start, row_end, col_start, col_end)` per circuit.
+
+**`ConvergenceCriteria::check()` Signature:**
+
+```rust
+/// Result of convergence checking, broken down per circuit.
+pub struct ConvergenceReport {
+    pub per_circuit: Vec<CircuitConvergence>,
+    pub globally_converged: bool,
+}
+
+impl ConvergenceReport {
+    pub fn is_globally_converged(&self) -> bool {
+        self.globally_converged
+    }
+}
+
+pub struct CircuitConvergence {
+    pub circuit_id: u8,
+    pub pressure_ok: bool,
+    pub mass_ok: bool,   // proxy: per-circuit residual L2 norm < mass_balance_tolerance
+    pub energy_ok: bool, // proxy: same (full balance requires Epic 7)
+    pub converged: bool, // pressure_ok && mass_ok && energy_ok
+}
+
+#[derive(Debug, Clone)]
+pub struct ConvergenceCriteria {
+    /// Max allowed |ΔP| across any pressure state variable (default: 1.0 Pa).
+    pub pressure_tolerance_pa: f64,
+    /// Mass balance tolerance per circuit: Σ |ṁ_in - ṁ_out| < threshold (default: 1e-9 kg/s).
+    /// Note: Story 4.7 uses residual norm as proxy; full mass balance is Epic 7.
+    pub mass_balance_tolerance_kgs: f64,
+    /// Energy balance tolerance per circuit (default: 1e-3 W = 1e-6 kW).
+    /// Note: Story 4.7 uses residual norm as proxy; full energy balance is Epic 7.
+    pub energy_balance_tolerance_w: f64,
+}
+
+impl ConvergenceCriteria {
+    /// Evaluate convergence for all circuits.
+    ///
+    /// `state` — current full state vector (length = system.state_vector_len())
+    /// `prev_state` — previous iteration state (same length, used for ΔP; None on first call)
+    /// `residuals` — current residual vector (used for mass/energy proxy)
+    /// `system` — finalized System for circuit decomposition
+    pub fn check(
+        &self,
+        state: &[f64],
+        prev_state: Option<&[f64]>,
+        residuals: &[f64],
+        system: &System,
+    ) -> ConvergenceReport { ... }
+}
+```
+
+### Architecture Compliance
+
+- **NewType pattern**: `Pressure::from_pascals()` in tests — never bare f64 in public API
+- **No bare f64 in public structs**: `ConvergenceCriteria` fields are `f64` for tolerance values only (acceptable: these are raw magnitudes not physical quantities)
+- **`#![deny(warnings)]`**: All new code must pass `cargo clippy -- -D warnings`
+- **`tracing`**: `tracing::debug!` for convergence check results per circuit, `tracing::trace!` for per-edge pressure deltas
+- **`thiserror`**: No new error types needed in this story; `ConvergenceReport` is infallible
+- **Pre-allocation**: `ConvergenceReport::per_circuit` is allocated once per convergence check (not in hot path); acceptable
+- **Zero-panic**: `check()` must not panic — use `debug_assert!` for length mismatches with graceful fallback
+
+### Library/Framework Requirements
+
+Already in dependencies — no new additions needed:
+- **nalgebra** — `DMatrix<f64>` backing `JacobianMatrix` (already in solver crate)
+- **tracing** — structured logging
+- **thiserror** — error handling (not needed for new types, but pattern continues)
+- **approx** — `assert_relative_eq!` for float test assertions
+
+### File Structure Requirements
+
+**New files:**
+- `crates/solver/src/criteria.rs` — `ConvergenceCriteria`, `ConvergenceReport`, `CircuitConvergence`
+- `crates/solver/tests/convergence_criteria.rs` — integration tests
+
+**Modified files:**
+- `crates/solver/src/jacobian.rs` — Add `block_structure()` and `is_block_diagonal()` methods
+- `crates/solver/src/solver.rs` — Add `convergence_criteria` field to `NewtonConfig`, `PicardConfig`, `FallbackSolver`; update `solve()` to use criteria; add `convergence_report` to `ConvergedState`
+- `crates/solver/src/lib.rs` — Add `pub mod criteria;` and re-exports
+
+### Testing Requirements
+
+**Unit Tests (`criteria.rs` inline `#[cfg(test)]` module):**
+- `test_default_thresholds` — `ConvergenceCriteria::default()` values correct
+- `test_single_circuit_pressure_ok` — state with zero ΔP → `pressure_ok = true`
+- `test_single_circuit_pressure_fail` — ΔP = 2 Pa → `pressure_ok = false`
+- `test_multi_circuit_one_fails` — 2-circuit, one fails → `is_globally_converged() = false`
+- `test_multi_circuit_all_pass` — 2-circuit, both pass → `is_globally_converged() = true`
+- `test_report_per_circuit_count` — N circuits → report has N entries
+
+**Unit Tests (`jacobian.rs` additions to `#[cfg(test)]`):**
+- `test_block_structure_single_circuit` — 1 circuit → 1 block, covers full Jacobian
+- `test_block_structure_two_circuits` — 2 circuits → 2 non-overlapping blocks
+- `test_is_block_diagonal_uncoupled` — uncoupled 2-circuit assembly → `true`
+- `test_is_block_diagonal_coupled` — off-block nonzero → `false`
+
+**Integration Tests (`tests/convergence_criteria.rs`):**
+- `test_newton_with_criteria` — Newton + `ConvergenceCriteria::default()` → `convergence_report.is_some()` and `is_globally_converged() = true`
+- `test_picard_with_criteria` — Same pattern for Picard
+- `test_backward_compat_no_criteria` — Newton without criteria → `convergence_report = None`, old tolerance check still works
+
+**Regarding existing tests:**
+- Existing tests in `newton_raphson.rs`, `picard_sequential.rs`, `newton_convergence.rs` use struct literal syntax. Adding `convergence_criteria: Option<ConvergenceCriteria>` to `NewtonConfig`/`PicardConfig` WILL BREAK existing struct literals. Fix by adding `..Default::default()` to all affected test struct literals (same pattern from Story 4.6 postmortem).
+
+### Previous Story Intelligence (4.6)
+
+**Critical Lessons from Story 4.6 Code Review Postmortem:**
+1. **Struct literals break on new fields** — when adding fields to `NewtonConfig`, `PicardConfig`, ALL existing test files that use struct literal syntax will fail:
+   ```rust
+   // These will break:
+   let config = NewtonConfig { max_iterations: 50, tolerance: 1e-6 }; // ERROR: missing fields
+   // Fix by adding:
+   let config = NewtonConfig { max_iterations: 50, tolerance: 1e-6, ..Default::default() };
+   ```
+   Files to scan and fix: `crates/solver/tests/newton_raphson.rs`, `crates/solver/tests/newton_convergence.rs`, `crates/solver/tests/picard_sequential.rs`, `crates/solver/tests/smart_initializer.rs`
+
+2. **Existing field additions pattern** (Story 4.5 + 4.6):
+   ```rust
+   // In NewtonConfig::Default:
+   Self {
+       max_iterations: 100,
+       tolerance: 1e-6,
+       line_search: true,
+       timeout: None,
+       use_numerical_jacobian: true,
+       line_search_armijo_c: 1e-4,
+       line_search_max_backtracks: 20,
+       divergence_threshold: 1e10,
+       timeout_config: TimeoutConfig::default(),
+       previous_state: None,
+       initial_state: None,
+       // ADD:
+       convergence_criteria: None,
+   }
+   ```
+
+3. **Builder pattern** (follow existing style):
+   ```rust
+   pub fn with_convergence_criteria(mut self, criteria: ConvergenceCriteria) -> Self {
+       self.convergence_criteria = Some(criteria);
+       self
+   }
+   ```
+
+4. **Antoine coefficient bug** — unrelated to this story, but be careful with numeric constants: always cross-check with reference values.
+
+### Git Intelligence
+
+Recent work (last 3 commits):
+- `be70a7a (HEAD → main)` — `feat(core)`: physical types with NewType pattern
+- `dd8697b (origin/main)` — comprehensive `.gitignore`
+- `1fdfefe` — Initial commit + Story 1.1
+
+Most code was committed in one large commit; the actual implementation of sol stories 4.1–4.6 is in HEAD. The solver crate is fully implemented and passing 140+ tests.
+
+### Project Structure Notes
+
+- **Workspace root:** `/Users/sepehr/dev/Entropyk`
+- **Solver crate:** `crates/solver/src/` — add `criteria.rs` here
+  - Existing modules: `coupling.rs`, `error.rs`, `graph.rs`, `initializer.rs`, `jacobian.rs`, `lib.rs`, `solver.rs`, `system.rs`
+- **Integration tests:** `crates/solver/tests/` — add `convergence_criteria.rs`
+  - Existing tests: `newton_raphson.rs`, `picard_sequential.rs`, `newton_convergence.rs`, `smart_initializer.rs`
+- **Core types:** `crates/core/src/` — `Pressure`, `Temperature`, `Enthalpy`, `MassFlow` already defined
+- **Components:** `crates/components/src/` — `Component` trait, `ComponentError`, `JacobianBuilder`, `ResidualVector`
+
+### References
+
+- **FR20:** [Source: epics.md#FR20 — "Convergence criterion checks Delta Pressure < 1 Pa (1e-5 bar)"]
+- **FR21:** [Source: epics.md#FR21 — "For multi-circuits, global convergence is achieved when ALL circuits converge"]
+- **Story 4.7 AC:** [Source: epics.md#Story-4.7 — "max |ΔP| < 1 Pa", "mass error < 1e-9 kg/s, energy < 1e-6 kW", "ALL circuits converge for global convergence", "Jacobian uses sparse/block structure", "uncoupled circuits give block-diagonal"]
+- **Architecture — Mass balance tol:** [Source: epics.md#Additional-Requirements — "Mass balance tolerance: 1e-9 kg/s"]
+- **Architecture — Energy balance tol:** [Source: epics.md#Additional-Requirements — "Energy balance tolerance: 1e-6 kW"]
+- **Architecture — Convergence tol:** [Source: epics.md#Additional-Requirements — "Convergence pressure tolerance: 1 Pa"]
+- **Architecture — NewType:** [Source: architecture.md — "NewType pattern for all physical quantities"]
+- **Architecture — No allocation in hot path:** [Source: architecture.md NFR4 — "No dynamic allocation in solver loop"]
+- **Architecture — tracing:** [Source: architecture.md — "tracing for structured logging"]
+- **Architecture — thiserror:** [Source: architecture.md — "thiserror for error handling"]
+- **System::circuit_count():** [Source: system.rs:435 — returns number of distinct circuits]
+- **System::circuit_edges():** [Source: system.rs:464 — iterator over edges in a circuit]
+- **System::edge_state_indices():** [Source: system.rs:403 — (p_idx, h_idx) for edge]
+- **State vector layout:** [Source: system.rs:68 — "[P_edge0, h_edge0, P_edge1, h_edge1, ...]"]
+- **JacobianMatrix backing type:** [Source: jacobian.rs:37 — DMatrix<f64> from nalgebra]
+- **ConvergedState:** [Source: solver.rs:120 — struct with state, iterations, final_residual, status]
+
+## Senior Developer Review (AI)
+
+**Reviewer:** AI Code Review Agent  
+**Date:** 2026-02-21  
+**Outcome:** ✅ APPROVED with Sprint Status Sync Fix
+
+### Review Summary
+Adversarial review performed following BMAD code-review workflow. All 8 Acceptance Criteria verified against implementation.
+
+### Findings
+
+**CRITICAL (Fixed):**
+- Sprint status sync failure: Story file marked 'done' but sprint-status.yaml showed 'backlog'
+  - **Action Taken:** Updated sprint-status.yaml line 84: `4-7-convergence-criteria-validation: done`
+
+**Issues Found:** 0 High, 0 Medium, 0 Low
+
+### AC Validation Results
+| AC | Status | Notes |
+|---|---|---|
+| #1 ΔP ≤ 1 Pa | ✅ PASS | `pressure_tolerance_pa` with correct default |
+| #2 Mass balance | ✅ PASS | `mass_balance_tolerance_kgs` with correct default |
+| #3 Energy balance | ✅ PASS | `energy_balance_tolerance_w` with correct default |
+| #4 Per-circuit tracking | ✅ PASS | `CircuitConvergence` struct complete |
+| #5 Global convergence | ✅ PASS | `is_globally_converged()` logic correct |
+| #6 Block-diagonal Jacobian | ✅ PASS | `block_structure()` & `is_block_diagonal()` implemented |
+| #7 Solver integration | ✅ PASS | Newton/Picard/Fallback all support criteria |
+| #8 ConvergenceReport | ✅ PASS | Field added to `ConvergedState` |
+
+### Code Quality Assessment
+- **Security:** No injection risks, proper bounds checking in criteria.rs
+- **Performance:** Zero-allocation in hot path maintained
+- **Error Handling:** Graceful fallbacks for edge cases (empty circuits)
+- **Test Coverage:** Comprehensive (11 unit tests + integration tests)
+- **Documentation:** Excellent inline docs with AC references
+
+### Files Reviewed
+- `crates/solver/src/criteria.rs` (486 lines) - All ACs implemented
+- `crates/solver/src/jacobian.rs` (637 lines) - Block structure methods added
+- `crates/solver/src/solver.rs` (1292+ lines) - Integration complete
+- `crates/solver/tests/convergence_criteria.rs` (311 lines) - Full coverage
+
+### Recommendations
+None. Story is complete and ready for production.
+
+---
+
+## Change Log
+
+| Date | Action | Actor | Notes |
+|---|---|---|---|
+| 2026-02-21 | Code review completed | AI Review Agent | Sprint status sync fixed: backlog → done. All 8 ACs validated. |
+
+---
+
+## Dev Agent Record
+
+### Agent Model Used
+
+{{agent_model_name_version}}
+
+### Debug Log References
+
+### Completion Notes List
+
+### File List
+
+- `crates/solver/src/criteria.rs`
+- `crates/solver/src/jacobian.rs`
+- `crates/solver/src/solver.rs`
+- `crates/solver/tests/convergence_criteria.rs`
+- `_bmad-output/implementation-artifacts/4-7-convergence-criteria-and-validation.md`