diff --git a/apps/cadecon/src/components/kernel/KernelDisplay.tsx b/apps/cadecon/src/components/kernel/KernelDisplay.tsx
index fa8eda1d..0e7abdfc 100644
--- a/apps/cadecon/src/components/kernel/KernelDisplay.tsx
+++ b/apps/cadecon/src/components/kernel/KernelDisplay.tsx
@@ -95,6 +95,17 @@ export function KernelDisplay(): JSX.Element {
     return (energyFast / total) * 100;
   });
 
+  // How many subset kernel fits at this iteration were degenerate/empty
+  // (no usable bi-exponential). A nonzero count means the displayed kernel is
+  // built from at least one untrustworthy subset fit.
+  const degenerateFits = createMemo(() => {
+    const snap = snapshot();
+    if (!snap || snap.totalSubsetFits === 0) return null;
+    return snap.degenerateSubsets > 0
+      ? { bad: snap.degenerateSubsets, total: snap.totalSubsetFits }
+      : null;
+  });
+
   const gtShape = createMemo(() => {
     const tauR = groundTruthTauRise();
     const tauD = groundTruthTauDecay();
@@ -273,6 +284,16 @@ export function KernelDisplay(): JSX.Element {
               Fast: <strong>{fastRatio()!.toFixed(0)}%</strong>
             </span>
           </Show>
+          <Show when={degenerateFits()} keyed>
+            {(info) => (
+              <span
+                class="kernel-display__warning"
+                title="Subset fits with no usable bi-exponential (degenerate/empty); the displayed kernel is less reliable."
+              >
+                ⚠ {info.bad}/{info.total} fits degenerate
+              </span>
+            )}
+          </Show>
           <Show when={showGroundTruth() && gtShape()} keyed>
             {(shape) => (
               <>
diff --git a/apps/cadecon/src/lib/__tests__/iteration-manager.test.ts b/apps/cadecon/src/lib/__tests__/iteration-manager.test.ts
index a3e2f3f8..a75a17fa 100644
--- a/apps/cadecon/src/lib/__tests__/iteration-manager.test.ts
+++ b/apps/cadecon/src/lib/__tests__/iteration-manager.test.ts
@@ -106,6 +106,7 @@ function completeJob(job: DispatchedJob): void {
       tauRiseFast: 0.05,
       tauDecayFast: 0.4,
       betaFast: 1,
+      fitMode: 'TwoComponent',
     });
   } else {
     // seed-trace
diff --git a/apps/cadecon/src/lib/__tests__/iteration-store.test.ts b/apps/cadecon/src/lib/__tests__/iteration-store.test.ts
index 089a56d7..6e028278 100644
--- a/apps/cadecon/src/lib/__tests__/iteration-store.test.ts
+++ b/apps/cadecon/src/lib/__tests__/iteration-store.test.ts
@@ -191,6 +191,8 @@ describe('iteration-store: derived memos', () => {
         kernelFitR2: null,
         medianPve: null,
         traceStability: null,
+        degenerateSubsets: 0,
+        totalSubsetFits: 3,
         // subsetIdx deliberately does not match array position — kernel results
         // arrive in worker-completion order, so subsetVarianceData must read the
         // subsetIdx field, not the array index.
@@ -253,6 +255,8 @@ describe('iteration-store: history actions', () => {
         kernelFitR2: null,
         medianPve: null,
         traceStability: null,
+        degenerateSubsets: 0,
+        totalSubsetFits: 0,
         subsets: [],
       });
       expect(convergenceHistory()).toHaveLength(1);
@@ -356,6 +360,8 @@ describe('iteration-store: resetIterationState', () => {
         kernelFitR2: null,
         medianPve: null,
         traceStability: null,
+        degenerateSubsets: 0,
+        totalSubsetFits: 0,
         subsets: [],
       });
       updateTraceResult(cellSubsetKey(0, 0), makeTraceEntry(0, 0, 1, 0.5));
diff --git a/apps/cadecon/src/lib/iteration-manager.ts b/apps/cadecon/src/lib/iteration-manager.ts
index 8c6a30d2..af68d46f 100644
--- a/apps/cadecon/src/lib/iteration-manager.ts
+++ b/apps/cadecon/src/lib/iteration-manager.ts
@@ -533,6 +533,8 @@ export async function startRun(): Promise<void> {
       kernelFitR2: null,
       medianPve: null,
       traceStability: null,
+      degenerateSubsets: 0,
+      totalSubsetFits: 0,
       subsets: [],
     });
     const initEntries: Record<string, import('./iteration-store.ts').TraceResultEntry> = {};
@@ -787,6 +789,13 @@ export async function startRun(): Promise<void> {
       if (hh > 0) r2s.push(1 - r.residual / hh);
     }
     const kernelFitR2 = r2s.length > 0 ? median(r2s) : null;
+
+    // Defensibility: count subsets whose bi-exponential fit was untrustworthy
+    // (no positive slow amplitude) or empty, so the UI can flag a suspect kernel.
+    const degenerateSubsets = kernelResults.filter(
+      (r) => r.fitMode === 'Degenerate' || r.fitMode === 'Empty',
+    ).length;
+
     batch(() => {
       setCurrentTauRise(tauR);
       setCurrentTauDecay(tauD);
@@ -807,6 +816,8 @@ export async function startRun(): Promise<void> {
         kernelFitR2,
         medianPve,
         traceStability,
+        degenerateSubsets,
+        totalSubsetFits: kernelResults.length,
         subsets: kernelResults.map((r) => ({
           subsetIdx: r.subsetIdx,
           tauRise: r.tauRise,
diff --git a/apps/cadecon/src/lib/iteration-store.ts b/apps/cadecon/src/lib/iteration-store.ts
index 6c51a998..34e8e633 100644
--- a/apps/cadecon/src/lib/iteration-store.ts
+++ b/apps/cadecon/src/lib/iteration-store.ts
@@ -43,6 +43,10 @@ export interface KernelSnapshot {
   medianPve: number | null;
   /** Median normalized change in deconvolved activity vs the previous iteration (→ 0 as it stabilizes). */
   traceStability: number | null;
+  /** Count of subsets whose bi-exp fit was Degenerate/Empty this iteration (untrustworthy kernel). */
+  degenerateSubsets: number;
+  /** Total subset fits this iteration (denominator for degenerateSubsets). */
+  totalSubsetFits: number;
   subsets: SubsetKernelSnapshot[];
 }
 
diff --git a/apps/cadecon/src/styles/kernel-display.css b/apps/cadecon/src/styles/kernel-display.css
index a7e99bec..fde9acab 100644
--- a/apps/cadecon/src/styles/kernel-display.css
+++ b/apps/cadecon/src/styles/kernel-display.css
@@ -36,6 +36,12 @@
 }
 
 /* Ground truth stat styling (pink) */
+.kernel-display__warning {
+  color: #d55e00; /* Okabe-Ito vermillion — matches the colorblind-safe palette */
+  font-weight: 600;
+  cursor: help;
+}
+
 .kernel-display__gt-stat {
   color: rgba(233, 30, 99, 0.8);
 }
diff --git a/apps/cadecon/src/workers/cadecon-types.ts b/apps/cadecon/src/workers/cadecon-types.ts
index 58922a14..40410004 100644
--- a/apps/cadecon/src/workers/cadecon-types.ts
+++ b/apps/cadecon/src/workers/cadecon-types.ts
@@ -20,6 +20,9 @@ export interface SeedTraceResult {
 }
 
 /** Results from kernel estimation + bi-exponential fitting. */
+/** Outcome of the bi-exponential fit; mirrors the Rust FitMode enum. */
+export type FitMode = 'TwoComponent' | 'SlowOnly' | 'Degenerate' | 'Empty';
+
 export interface KernelResult {
   hFree: Float32Array;
   tauRise: number;
@@ -29,10 +32,11 @@ export interface KernelResult {
   tauRiseFast: number;
   tauDecayFast: number;
   betaFast: number;
+  fitMode: FitMode;
 }
 
-/** Previous biexponential result for warm-starting the next fit. */
-export type WarmBiexp = Omit<KernelResult, 'hFree'>;
+/** Previous biexponential result for warm-starting the next fit (fit_mode not needed). */
+export type WarmBiexp = Omit<KernelResult, 'hFree' | 'fitMode'>;
 
 /** Messages sent TO a CaDecon worker. */
 export type CaDeconWorkerInbound =
diff --git a/apps/cadecon/src/workers/cadecon-worker.ts b/apps/cadecon/src/workers/cadecon-worker.ts
index 1be39bc5..cd55fcb3 100644
--- a/apps/cadecon/src/workers/cadecon-worker.ts
+++ b/apps/cadecon/src/workers/cadecon-worker.ts
@@ -8,7 +8,7 @@ import {
   indeca_fit_biexponential,
   seed_trace,
 } from '@calab/core';
-import type { CaDeconWorkerInbound, CaDeconWorkerOutbound } from './cadecon-types.ts';
+import type { CaDeconWorkerInbound, CaDeconWorkerOutbound, FitMode } from './cadecon-types.ts';
 
 let cancelled = false;
 const EMPTY_F32 = new Float32Array(0);
@@ -129,6 +129,7 @@ function handleKernelJob(req: Extract<CaDeconWorkerInbound, { type: 'kernel-job'
       tau_rise_fast: number;
       tau_decay_fast: number;
       beta_fast: number;
+      fit_mode: string;
     };
 
     post(
@@ -144,6 +145,7 @@ function handleKernelJob(req: Extract<CaDeconWorkerInbound, { type: 'kernel-job'
           tauRiseFast: biexpJs.tau_rise_fast,
           tauDecayFast: biexpJs.tau_decay_fast,
           betaFast: biexpJs.beta_fast,
+          fitMode: biexpJs.fit_mode as FitMode,
         },
       },
       [hFreeArr.buffer],
diff --git a/crates/solver/src/biexp_fit.rs b/crates/solver/src/biexp_fit.rs
index b2aefe64..ade02152 100644
--- a/crates/solver/src/biexp_fit.rs
+++ b/crates/solver/src/biexp_fit.rs
@@ -82,6 +82,37 @@
 /// the ceiling logic in eval_two_component can be simplified back to a
 /// plain `bs >= bf` gate.
 
+/// Explicit outcome of a bi-exponential fit, so a degenerate / fallback fit is
+/// reported rather than silently inferred by the caller from `beta_fast == 0`.
+///
+/// Serializes to its variant name ("TwoComponent", ...) for the JS/Python FFI.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
+#[cfg_attr(feature = "jsbindings", derive(serde::Serialize))]
+pub enum FitMode {
+    /// Slow + fast bi-exponential — a distinct fast component was resolved.
+    TwoComponent,
+    /// Single (slow-only) bi-exponential — no fast component (the default fit).
+    #[default]
+    SlowOnly,
+    /// A fit was produced but has no positive slow amplitude (beta <= 0):
+    /// the free kernel had no real transient (noise/flat) — result is untrustworthy.
+    Degenerate,
+    /// No fit could be produced (empty input): sentinel defaults, residual = inf.
+    Empty,
+}
+
+impl FitMode {
+    /// Stable string form for the PyO3 tuple return.
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            FitMode::TwoComponent => "TwoComponent",
+            FitMode::SlowOnly => "SlowOnly",
+            FitMode::Degenerate => "Degenerate",
+            FitMode::Empty => "Empty",
+        }
+    }
+}
+
 #[derive(Clone)]
 #[cfg_attr(feature = "jsbindings", derive(serde::Serialize))]
 pub struct BiexpResult {
@@ -92,6 +123,9 @@ pub struct BiexpResult {
     pub tau_rise_fast: f64,
     pub tau_decay_fast: f64,
     pub beta_fast: f64,
+    /// Outcome classification; authoritative only on the value returned by
+    /// `fit_biexponential` (intermediate candidates carry a placeholder).
+    pub fit_mode: FitMode,
 }
 
 impl BiexpResult {
@@ -104,6 +138,7 @@ impl BiexpResult {
             tau_rise_fast: 0.0,
             tau_decay_fast: 0.0,
             beta_fast: 0.0,
+            fit_mode: FitMode::Empty,
         }
     }
 
@@ -111,6 +146,22 @@ impl BiexpResult {
     pub fn has_fast_component(&self) -> bool {
         self.tau_rise_fast > 0.0 && self.tau_decay_fast > self.tau_rise_fast
     }
+
+    /// Classify the fit outcome from the fitted parameters. Called once on the
+    /// final result so the reported mode reflects what was actually selected.
+    fn classify(&self) -> FitMode {
+        if !self.residual.is_finite() {
+            return FitMode::Empty;
+        }
+        if self.beta <= 0.0 {
+            return FitMode::Degenerate;
+        }
+        if self.has_fast_component() && self.beta_fast > 0.0 {
+            FitMode::TwoComponent
+        } else {
+            FitMode::SlowOnly
+        }
+    }
 }
 
 /// Fit a two-component bi-exponential model to a free-form kernel.
@@ -209,6 +260,7 @@ pub fn fit_biexponential(
         best.residual = full_residual;
     }
 
+    best.fit_mode = best.classify();
     best
 }
 
@@ -240,6 +292,7 @@ fn refine_candidate(
             tau_rise_fast: refined_trf,
             tau_decay_fast: refined_tdf,
             beta_fast: beta_f,
+            fit_mode: candidate.fit_mode,
         };
     }
 }
@@ -328,6 +381,7 @@ fn cold_grid_search(h_free: &[f32], fs: f64, dt: f64, skip: usize) -> (BiexpResu
                     tau_rise_fast: 0.0,
                     tau_decay_fast: 0.0,
                     beta_fast: 0.0,
+                    fit_mode: FitMode::SlowOnly,
                 };
             }
 
@@ -365,6 +419,7 @@ fn cold_grid_search(h_free: &[f32], fs: f64, dt: f64, skip: usize) -> (BiexpResu
                             tau_rise_fast: tau_r_fast,
                             tau_decay_fast: tau_d_fast,
                             beta_fast: beta_f,
+                            fit_mode: FitMode::TwoComponent,
                         };
                     }
                 }
@@ -608,6 +663,32 @@ fn golden_section_refine(
 mod tests {
     use super::*;
 
+    #[test]
+    fn fit_mode_empty_on_empty_input() {
+        let r = fit_biexponential(&[], 30.0, false, 0, None);
+        assert_eq!(r.fit_mode, FitMode::Empty);
+    }
+
+    #[test]
+    fn fit_mode_degenerate_on_flat_kernel() {
+        // A flat (no-transient) kernel has no positive slow amplitude.
+        let flat = vec![0.0_f32; 100];
+        let r = fit_biexponential(&flat, 30.0, true, 0, None);
+        assert_eq!(r.fit_mode, FitMode::Degenerate);
+    }
+
+    #[test]
+    fn fit_mode_real_kernel_is_not_degenerate() {
+        let kernel = make_biexp(0.02, 0.4, 1.0, 30.0, 200);
+        let r = fit_biexponential(&kernel, 30.0, true, 0, None);
+        assert!(
+            matches!(r.fit_mode, FitMode::SlowOnly | FitMode::TwoComponent),
+            "clean biexp kernel should fit to a usable mode, got {:?}",
+            r.fit_mode
+        );
+        assert!(r.beta > 0.0);
+    }
+
     /// Generate a bi-exponential kernel with known parameters.
     fn make_biexp(tau_r: f64, tau_d: f64, beta: f64, fs: f64, n: usize) -> Vec<f32> {
         let dt = 1.0 / fs;
diff --git a/crates/solver/src/js_indeca.rs b/crates/solver/src/js_indeca.rs
index 2ff020b5..001bd4fb 100644
--- a/crates/solver/src/js_indeca.rs
+++ b/crates/solver/src/js_indeca.rs
@@ -149,6 +149,8 @@ pub fn indeca_fit_biexponential(
             tau_rise_fast: warm_tau_rise_fast,
             tau_decay_fast: warm_tau_decay_fast,
             beta_fast: warm_beta_fast,
+            // Placeholder; fit_biexponential reclassifies the returned result.
+            fit_mode: biexp_fit::FitMode::default(),
         })
     } else {
         None
diff --git a/crates/solver/src/peak_seed.rs b/crates/solver/src/peak_seed.rs
index 9ef39543..f496a3bc 100644
--- a/crates/solver/src/peak_seed.rs
+++ b/crates/solver/src/peak_seed.rs
@@ -282,6 +282,7 @@ pub fn seed_kernel_estimate(
         tau_rise_fast,
         tau_decay_fast,
         beta_fast,
+        fit_mode: _,
     } = fit_biexponential(&free_kernel, fs, true, 0, None);
 
     SeedKernelResult {
diff --git a/crates/solver/src/py_api.rs b/crates/solver/src/py_api.rs
index 2b3194ad..6e744380 100644
--- a/crates/solver/src/py_api.rs
+++ b/crates/solver/src/py_api.rs
@@ -557,7 +557,7 @@ fn py_indeca_fit_biexponential(
     warm_beta_fast: f64,
     warm_residual: f64,
     use_warm: bool,
-) -> PyResult<(f64, f64, f64, f64, f64, f64, f64)> {
+) -> PyResult<(f64, f64, f64, f64, f64, f64, f64, String)> {
     let h_f32 = to_f32_vec(&h_free)?;
 
     let warm_start = if use_warm {
@@ -569,6 +569,8 @@ fn py_indeca_fit_biexponential(
             tau_rise_fast: warm_tau_rise_fast,
             tau_decay_fast: warm_tau_decay_fast,
             beta_fast: warm_beta_fast,
+            // Placeholder; fit_biexponential reclassifies the returned result.
+            fit_mode: biexp_fit::FitMode::default(),
         })
     } else {
         None
@@ -584,6 +586,7 @@ fn py_indeca_fit_biexponential(
         result.tau_rise_fast,
         result.tau_decay_fast,
         result.beta_fast,
+        result.fit_mode.as_str().to_string(),
     ))
 }
 
diff --git a/python/src/calab/_compute.py b/python/src/calab/_compute.py
index 4854bcf1..0a3f7145 100644
--- a/python/src/calab/_compute.py
+++ b/python/src/calab/_compute.py
@@ -326,6 +326,10 @@ class BiexpFitResult(NamedTuple):
         Fast-component decay time constant (seconds), 0 if single-component.
     beta_fast : float
         Fast-component amplitude, 0 if single-component.
+    fit_mode : str
+        Outcome of the fit: ``"TwoComponent"``, ``"SlowOnly"``, ``"Degenerate"``
+        (a fit was produced but has no positive slow amplitude — the kernel had
+        no real transient), or ``"Empty"`` (no fit; empty input).
     """
 
     tau_rise: float
@@ -335,6 +339,7 @@ class BiexpFitResult(NamedTuple):
     tau_rise_fast: float
     tau_decay_fast: float
     beta_fast: float
+    fit_mode: str = "SlowOnly"
 
 
 def solve_trace(
diff --git a/python/tests/test_fit_mode.py b/python/tests/test_fit_mode.py
new file mode 100644
index 00000000..6d98545e
--- /dev/null
+++ b/python/tests/test_fit_mode.py
@@ -0,0 +1,31 @@
+"""The bi-exponential fit reports an explicit outcome (fit_mode) so a degenerate
+or fallback fit is surfaced rather than silently inferred from beta_fast == 0."""
+
+from __future__ import annotations
+
+import numpy as np
+
+from calab import fit_biexponential
+
+
+def _biexp(tau_r: float, tau_d: float, fs: float, n: int) -> np.ndarray:
+    t = np.arange(n) / fs
+    h = np.exp(-t / tau_d) - np.exp(-t / tau_r)
+    return h / np.max(h)
+
+
+def test_fit_mode_usable_for_clean_kernel():
+    h = _biexp(0.02, 0.4, 30.0, 200)
+    result = fit_biexponential(h, 30.0)
+    assert result.fit_mode in ("SlowOnly", "TwoComponent")
+    assert result.beta > 0.0
+
+
+def test_fit_mode_degenerate_on_flat_kernel():
+    result = fit_biexponential(np.zeros(100), 30.0)
+    assert result.fit_mode == "Degenerate"
+
+
+def test_fit_mode_empty_on_empty_input():
+    result = fit_biexponential(np.array([]), 30.0)
+    assert result.fit_mode == "Empty"
diff --git a/python/tests/test_solve_trace.py b/python/tests/test_solve_trace.py
index 3574e074..f82b3e75 100644
--- a/python/tests/test_solve_trace.py
+++ b/python/tests/test_solve_trace.py
@@ -182,9 +182,10 @@ def test_tuple_unpacking(self):
         fs = 30.0
         t = np.arange(50) / fs
         h = np.exp(-t / 0.4) - np.exp(-t / 0.02)
-        tau_r, tau_d, beta, residual, tau_rf, tau_df, beta_f = fit_biexponential(h, fs)
+        tau_r, tau_d, beta, residual, tau_rf, tau_df, beta_f, fit_mode = fit_biexponential(h, fs)
         assert isinstance(tau_r, float)
         assert isinstance(residual, float)
+        assert isinstance(fit_mode, str)
 
 
 # ---------------------------------------------------------------------------