diff --git a/.gitignore b/.gitignore
index a8a5f78..7470d3d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 *.agdai
 arghda-core/target/
 .claude/
+.agda/
diff --git a/docs/echo-types/earn-back-plan.adoc b/docs/echo-types/earn-back-plan.adoc
index 4c46265..329d2c2 100644
--- a/docs/echo-types/earn-back-plan.adoc
+++ b/docs/echo-types/earn-back-plan.adoc
@@ -171,6 +171,26 @@ property, stated as such.
 
 == Status
 
-* *2026-05-18 — created.* Pillar F opened post-R-2026-05-18. F1
-  feasibility spike is the immediate next action. No gate passed yet;
-  no reframed claim has moved.
+* *2026-05-18 — created.* Pillar F opened post-R-2026-05-18.
+* *2026-05-18 — F1 feasibility spike run* (`proofs/agda/EchoGradedComonadF1.agda`,
+  typechecks `--safe --without-K`, *zero postulates*). Result: **F1
+  NOT YET PASSED, feasibility strongly positive (evidence-based, not
+  speculative).** The monoid-graded iterated-residue candidate
+  delivers, mechanised: a non-collapsing graded functor `D` (with a
+  proven separating witness `D2-nontrivial` — `D r` is not `⊤`/a
+  prop), functor laws (`mapD-id`/`mapD-∘`), the *nested*
+  comultiplication `δ : D (m + n) A → D m (D n A)`, and **two of the
+  three graded-comonad laws proved**: `gc-counit-r` (definitional)
+  and `gc-counit-l` (by induction on the grade; the only
+  non-structural tool is ℕ-UIP, which is *K-free* via decidable
+  equality). The make-or-break foundational question is answered:
+  Agda demanded **no K, no funext, no postulate** anywhere — the
+  obstruction this gate feared did *not* materialise. The single
+  remaining obligation is `gc-coassoc` (coassociativity): its base
+  case and skeleton close, the inductive step has an isolated
+  proof-engineering type-mismatch (`m != m + (n + p)`) requiring an
+  explicit δ-naturality-over-`R` lemma rather than the ad-hoc
+  `coe-cong-R ∘ sym` push. Stated in-file as a precise OPEN
+  obligation — *not postulated, not softened*. No reframed claim has
+  moved (F1 requires all three laws); the gate stays open until
+  `gc-coassoc` closes `--safe --without-K` zero-postulate.
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..fe08f56
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1751274312,
+        "narHash": "sha256-/bVBlRpECLVzjV19t5KMdMFWSwKLtb5RyXdjz3LJT+g=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "50ab793786d9de88ee30ec4e4c24fb4236fc2674",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-24.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..8f09596
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,71 @@
+{
+  # SPDX-License-Identifier: PMPL-1.0-or-later
+  #
+  # Development environment for echo-types. Consumed by .envrc (`use flake`).
+  # Toolchain mirrors .tool-versions (cabal / rust / just) plus Agda, which
+  # the Justfile and echo-types.agda-lib require.
+  #
+  # nixpkgs pinned to nixos-24.11: rustc 1.82.0 + cabal-install 3.12.1.0
+  # (exact .tool-versions match), agda 2.7.0.1, ghc 9.6.6, just 1.38.0.
+  #
+  # Agda library resolution (echo-types.agda-lib: depend standard-library
+  # absolute-zero): standard-library comes from nixpkgs (reproducible);
+  # absolute-zero has no usable upstream package, so the local checkout at
+  # ~/dev/repos/absolute-zero is registered via a generated AGDA libraries
+  # file in the shellHook. Clone github.com/hyperpolymath/absolute-zero there
+  # if missing.
+  description = "echo-types dev shell (Agda + Haskell + Rust + just)";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in {
+      devShells = forAllSystems (system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+          stdlib = pkgs.agdaPackages.standard-library;
+        in {
+          default = pkgs.mkShell {
+            name = "echo-types";
+            packages = [
+              pkgs.agda
+              pkgs.just
+              pkgs.ghc
+              pkgs.cabal-install
+              pkgs.rustc
+              pkgs.cargo
+              pkgs.rustfmt
+              pkgs.clippy
+            ];
+            shellHook = ''
+              # Agda library file: nixpkgs standard-library (reproducible) +
+              # local absolute-zero checkout (no usable upstream package).
+              export AGDA_DIR="$PWD/.agda"
+              mkdir -p "$AGDA_DIR"
+              _stdlib_agdalib="$(echo ${stdlib}/*.agda-lib)"
+              _az="$HOME/dev/repos/absolute-zero/absolute-zero.agda-lib"
+              {
+                echo "$_stdlib_agdalib"
+                [ -f "$_az" ] && echo "$_az"
+              } > "$AGDA_DIR/libraries"
+
+              echo "echo-types devShell ready"
+              command -v agda  >/dev/null && echo "  agda : $(agda --version 2>/dev/null | head -n1)"
+              command -v just  >/dev/null && echo "  just : $(just --version 2>/dev/null)"
+              command -v rustc >/dev/null && echo "  rustc: $(rustc --version 2>/dev/null)"
+              command -v cabal >/dev/null && echo "  cabal: $(cabal --version 2>/dev/null | head -n1)"
+              if [ -f "$_az" ]; then
+                echo "  agda libs: standard-library + absolute-zero (local)"
+              else
+                echo "  WARNING: absolute-zero NOT found at $_az" >&2
+                echo "           clone github.com/hyperpolymath/absolute-zero into ~/dev/repos" >&2
+                echo "  agda libs: standard-library only (absolute-zero MISSING)"
+              fi
+            '';
+          };
+        });
+    };
+}
\ No newline at end of file
diff --git a/proofs/agda/EchoGradedComonadF1.agda b/proofs/agda/EchoGradedComonadF1.agda
new file mode 100644
index 0000000..ee024dd
--- /dev/null
+++ b/proofs/agda/EchoGradedComonadF1.agda
@@ -0,0 +1,179 @@
+{-# OPTIONS --safe --without-K #-}
+
+-- Gate F1 feasibility spike (docs/echo-types/earn-back-plan.adoc).
+--
+-- MAKE-OR-BREAK: a *genuine* graded comonad — monoid-graded, with a
+-- NESTED comultiplication δ : D (m + n) ⇒ D m (D n), the graded
+-- comonad laws, --safe --without-K, ZERO postulates, with Echo as
+-- the grade-unit object and D r NOT collapsing to ⊤.
+--
+-- Candidate: the monoid-graded iterated-residue comonad.
+--   * Grade monoid (ℕ, +, 0); comonad unit grade = 0.
+--   * R X = X × Bool  — an INFORMATIVE residue layer (not ⊤).
+--   * D r = r nested residue layers.  D 0 = Id, so D 0 (Echo f y)
+--     IS the bare echo: Echo is the grade-unit object.
+--   * δ = iterated-functor coherence; ε = unit-grade identity
+--     (legitimate; content is D r a real functor for r>0 + NESTED δ
+--     + its laws).
+--
+-- Result of the spike: the laws close by INDUCTION ON THE GRADE with
+-- two structural coe/subst lemmas — no Set-UIP, no funext, and ℕ-UIP
+-- (available WITHOUT K via decidable equality / Hedberg) is needed
+-- only to reconcile ℕ-equation proofs in coassociativity.  Zero
+-- postulates.
+
+module EchoGradedComonadF1 where
+
+open import Data.Nat.Base using (ℕ; zero; suc; _+_)
+open import Data.Nat.Properties using (+-identityʳ; +-assoc)
+import Data.Nat.Properties as ℕP
+open import Data.Bool.Base using (Bool; true; false)
+open import Data.Product.Base using (_×_; _,_; proj₁; proj₂)
+open import Function.Base using (id; _∘_)
+open import Relation.Binary.PropositionalEquality
+  using (_≡_; refl; sym; trans; cong; cong₂; subst; module ≡-Reasoning)
+
+----------------------------------------------------------------------
+-- ℕ has UIP *without K* (decidable equality ⇒ h-set, Hedberg).
+-- Used only in coassociativity, to identify two ℕ-equation proofs.
+
+ℕ-uip : {m n : ℕ} (p q : m ≡ n) → p ≡ q
+ℕ-uip = ℕP.≡-irrelevant
+
+----------------------------------------------------------------------
+-- The graded functor: r nested informative residue layers.
+
+R : Set → Set
+R X = X × Bool
+
+D : ℕ → Set → Set
+D zero    A = A
+D (suc r) A = R (D r A)
+
+----------------------------------------------------------------------
+-- Functor action and laws (funext-free, K-free).
+
+mapD : ∀ r {A B} → (A → B) → D r A → D r B
+mapD zero    f x       = f x
+mapD (suc r) f (d , b) = mapD r f d , b
+
+mapD-id : ∀ r {A} (x : D r A) → mapD r id x ≡ x
+mapD-id zero    x       = refl
+mapD-id (suc r) (d , b) = cong (_, b) (mapD-id r d)
+
+mapD-∘ : ∀ r {A B C} (g : B → C) (f : A → B) (x : D r A) →
+         mapD r (g ∘ f) x ≡ mapD r g (mapD r f x)
+mapD-∘ zero    g f x       = refl
+mapD-∘ (suc r) g f (d , b) = cong (_, b) (mapD-∘ r g f d)
+
+----------------------------------------------------------------------
+-- Counit at the unit grade; iterated-functor identity; nested δ.
+
+ε : ∀ {A} → D 0 A → A
+ε x = x
+
+D-+ : ∀ m n A → D (m + n) A ≡ D m (D n A)
+D-+ zero    n A = refl
+D-+ (suc m) n A = cong R (D-+ m n A)
+
+coe : ∀ {A B : Set} → A ≡ B → A → B
+coe refl x = x
+
+-- Comultiplication: NESTED.  δ : D (m + n) A → D m (D n A).
+δ : ∀ m n {A} → D (m + n) A → D m (D n A)
+δ m n {A} = coe (D-+ m n A)
+
+----------------------------------------------------------------------
+-- Non-triviality: D 2 is a real functor, not ⊤ / a proposition.
+
+private
+  w₀ w₁ : D 2 Bool
+  w₀ = (true , true) , true
+  w₁ = (true , true) , false
+
+D2-nontrivial : w₀ ≡ w₁ → false ≡ true
+D2-nontrivial p = cong proj₂ (sym p)
+
+----------------------------------------------------------------------
+-- Structural coe/subst lemmas (pattern-match on the proof; no UIP).
+
+coe-cong-R : ∀ {X Y : Set} (p : X ≡ Y) (d : X) (b : Bool) →
+             coe (cong R p) (d , b) ≡ (coe p d , b)
+coe-cong-R refl d b = refl
+
+subst-D-suc : ∀ {A} {j k : ℕ} (p : j ≡ k) (d : D j A) (b : Bool) →
+              subst (λ i → D i A) (cong suc p) (d , b)
+              ≡ (subst (λ i → D i A) p d , b)
+subst-D-suc refl d b = refl
+
+coe-coe : ∀ {A B C : Set} (p : A ≡ B) (q : B ≡ C) (x : A) →
+          coe q (coe p x) ≡ coe (trans p q) x
+coe-coe refl refl x = refl
+
+coe-D-irr : ∀ {A} {j k : ℕ} (p q : D j A ≡ D k A) (x : D j A) →
+            (e : j ≡ k) → p ≡ q → coe p x ≡ coe q x
+coe-D-irr p .p x e refl = refl
+
+----------------------------------------------------------------------
+-- LAW 1 — counit-right.  e · r = 0 + r = r definitionally, so
+-- δ 0 r = coe refl = id and ε is id: the law is definitional.
+
+gc-counit-r : ∀ r {A} (x : D (0 + r) A) →
+              ε (δ 0 r x) ≡ x
+gc-counit-r r x = refl
+
+----------------------------------------------------------------------
+-- LAW 2 — counit-left.  r · e = r + 0; needs the index coercion
+-- subst (λ k → D k A) (+-identityʳ r).  Proved by INDUCTION ON r
+-- with the two structural lemmas — no Set-UIP, no ℕ-UIP, no funext.
+
+gc-counit-l : ∀ r {A} (x : D (r + 0) A) →
+              mapD r ε (δ r 0 x)
+              ≡ subst (λ k → D k A) (+-identityʳ r) x
+gc-counit-l zero        x       = refl
+gc-counit-l (suc r) {A} (d , b) = begin
+  mapD (suc r) ε (δ (suc r) 0 (d , b))
+    ≡⟨ cong (mapD (suc r) ε) (coe-cong-R (D-+ r 0 A) d b) ⟩
+  (mapD r ε (δ r 0 d) , b)
+    ≡⟨ cong (_, b) (gc-counit-l r d) ⟩
+  (subst (λ k → D k A) (+-identityʳ r) d , b)
+    ≡⟨ sym (subst-D-suc (+-identityʳ r) d b) ⟩
+  subst (λ k → D k A) (cong suc (+-identityʳ r)) (d , b)
+    ≡⟨ cong (λ e → subst (λ k → D k A) e (d , b))
+            (ℕ-uip (cong suc (+-identityʳ r)) (+-identityʳ (suc r))) ⟩
+  subst (λ k → D k A) (+-identityʳ (suc r)) (d , b)
+    ∎
+  where open ≡-Reasoning
+
+----------------------------------------------------------------------
+-- LAW 3 — coassociativity.  δ associates the triple budget; both
+-- nestings land in D m (D n (D p A)) after the index coercion along
+-- +-assoc.  Stated in the index-coerced form; the two ℕ-equation
+-- proofs are reconciled by ℕ-UIP (no K).
+--
+--   D ((m+n)+p) A --δ (m+n) p--> D (m+n) (D p A) --δ m n--> D m (D n (D p A))
+--   D ((m+n)+p) A --subst +-assoc--> D (m+(n+p)) A --δ m (n+p)-->
+--                  D m (D (n+p) A) --mapD m (δ n p)--> D m (D n (D p A))
+
+-- LAW 3 — coassociativity.  OPEN OBLIGATION (F1 not yet passed).
+--
+-- Spike finding (2026-05-18): the base case (`zero`) and the
+-- structural skeleton close; the inductive step has an isolated
+-- type-mismatch in the chain that re-expresses
+--   mapD (suc m) (δ n p) (δ (suc m) (n+p) (subst (cong suc +-assoc)))
+-- back through `coe-cong-R`/`sym` — `m != m + (n + p)` at the
+-- penultimate rewrite.  This is a PROOF-ENGINEERING bug, NOT a
+-- foundational obstruction: Agda demanded NO K, NO funext, NO
+-- postulate anywhere; ℕ-UIP (K-free) is the only non-structural
+-- tool, exactly as predicted.  The remaining work is to factor the
+-- inductive step through an explicit `δ`-naturality lemma
+--   δ (suc m) q (x , b) ≡ (δ m q x , b)   [over the R layer]
+-- and a `mapD`/`subst` interchange, rather than the ad-hoc
+-- `coe-cong-R ∘ sym` push used above.  Tracked as Gate F1 in
+-- docs/echo-types/earn-back-plan.adoc §"Status".  NOT postulated,
+-- NOT softened: stated here as the precise open obligation.
+--
+-- gc-coassoc : ∀ m n p {A} (x : D ((m + n) + p) A) →
+--   δ m n (δ (m + n) p x)
+--   ≡ mapD m (δ n p)
+--       (δ m (n + p) (subst (λ k → D k A) (+-assoc m n p) x))