diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 48c19f243..6a07e86d9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        crystal-version: [1.19.0, 1.20.0, 1.21.0]
+        crystal-version: [1.21.0]
     steps:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1
@@ -103,7 +103,7 @@ jobs:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1
         with:
-          crystal: 1.20.0
+          crystal: 1.21.0
       - name: Install dependencies
         run: shards install
       - name: Run Ameba linter
@@ -118,7 +118,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        crystal-version: [1.19.0, 1.20.0, 1.21.0]
+        crystal-version: [1.21.0]
     steps:
       - uses: actions/checkout@v7
       - uses: crystal-lang/install-crystal@v1
diff --git a/AGENTS.md b/AGENTS.md
index 1fb9e4a58..fe27ad0b6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -17,7 +17,7 @@ Crystal-based attack surface detector that identifies endpoints by static analys
 
 ```bash
 # Docker build (for CI or consistent environments)
-docker run --rm -v $(pwd):/app -w /app crystallang/crystal:1.19.0-alpine sh -c "apk add --no-cache yaml-dev zstd-dev && shards install && shards build"
+docker run --rm -v $(pwd):/app -w /app crystallang/crystal:1.21.0-alpine sh -c "apk add --no-cache yaml-dev zstd-dev && shards install && shards build"
 
 # Local install (Ubuntu/Debian)
 curl -fsSL https://crystal-lang.org/install.sh | sudo bash
@@ -161,6 +161,6 @@ Key design notes:
 4. Verify basic functionality: `./bin/noir -b spec/functional_test/fixtures/crystal`
 
 ## Environment
-- Crystal ~> 1.19 (CI: 1.19.0)
-- Docker image: `crystallang/crystal:1.19.0-alpine`
+- Crystal ~> 1.19 (CI: 1.21.0)
+- Docker image: `crystallang/crystal:1.21.0-alpine`
 - Dependencies: `libyaml-dev`, `libzstd-dev`, `zlib1g-dev`, `pkg-config`