diff --git a/.ci/force_json_020.jl b/.ci/force_json_020.jl
new file mode 100644
index 00000000..4fd33a4c
--- /dev/null
+++ b/.ci/force_json_020.jl
@@ -0,0 +1,32 @@
+# Rewrites the JSON entry in the [compat] section of Project.toml to "0.20" so
+# that every subsequent resolve can only pick JSON 0.20.x — the last JSON
+# version without non-stdlib dependencies, which is what the VS Code extension
+# uses on every Julia version.
+#
+# This must run on Julia 1.0 (no TOML stdlib), so the file is edited line by
+# line, tracking the current section: [deps] also has a `JSON = "<uuid>"` line
+# that must be left alone.
+
+function force_json_020()
+    lines = readlines("Project.toml")
+    section = ""
+    replaced = false
+    for i in eachindex(lines)
+        stripped = strip(lines[i])
+        if startswith(stripped, "[")
+            section = stripped
+        elseif section == "[compat]" && occursin(r"^JSON\s*=", stripped)
+            lines[i] = "JSON = \"0.20\""
+            replaced = true
+        end
+    end
+    replaced || error("No JSON entry found in the [compat] section of Project.toml")
+    open("Project.toml", "w") do io
+        for line in lines
+            println(io, line)
+        end
+    end
+    println("Restricted Project.toml compat to JSON = \"0.20\"")
+end
+
+force_json_020()
diff --git a/.ci/verify_json_020.jl b/.ci/verify_json_020.jl
new file mode 100644
index 00000000..67eaef78
--- /dev/null
+++ b/.ci/verify_json_020.jl
@@ -0,0 +1,31 @@
+# Fails loudly unless the resolved Manifest.toml has JSON at version 0.20.x,
+# so a silent fallback to a newer JSON cannot go unnoticed after
+# force_json_020.jl tightened the compat bound.
+#
+# Must run on Julia 1.0 (no TOML stdlib): scans the manifest line by line for
+# the JSON stanza, handling both manifest formats — `[[JSON]]` (old) and
+# `[[deps.JSON]]` (Julia >= 1.6.2).
+
+function verify_json_020()
+    in_json_stanza = false
+    for line in readlines("Manifest.toml")
+        stripped = strip(line)
+        if startswith(stripped, "[")
+            in_json_stanza = stripped == "[[JSON]]" || stripped == "[[deps.JSON]]"
+        elseif in_json_stanza
+            m = match(r"^version\s*=\s*\"(.*)\"", stripped)
+            if m !== nothing
+                version = m.captures[1]
+                if startswith(version, "0.20.")
+                    println("JSON resolved to version ", version)
+                    return
+                else
+                    error("JSON resolved to version $version, expected 0.20.x")
+                end
+            end
+        end
+    end
+    error("No JSON version found in Manifest.toml")
+end
+
+verify_json_020()
diff --git a/.github/workflows/juliaci-oldjson.yml b/.github/workflows/juliaci-oldjson.yml
new file mode 100644
index 00000000..01e06d41
--- /dev/null
+++ b/.github/workflows/juliaci-oldjson.yml
@@ -0,0 +1,83 @@
+name: Julia CI (JSON 0.20)
+
+# Tests JSONRPC against JSON 0.20 on every supported Julia version. The VS Code
+# extension uses JSON 0.20 on all Julia versions because it is the last JSON
+# version without non-stdlib dependencies, so this combination has to keep
+# working even though a plain resolve would pick JSON 0.21 (Julia <= 1.8) or
+# JSON 1.x (Julia >= 1.9). This is deliberately a standalone workflow built
+# from the individual julia-actions rather than a second invocation of the
+# testitem-workflow template, which would drag along duplicate lint, docs and
+# TagBot jobs.
+
+on:
+  push: {branches: [main,master]}
+  pull_request: {types: [opened,synchronize,reopened,ready_for_review,converted_to_draft]}
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
+  cancel-in-progress: true
+
+jobs:
+  compute-test-matrix:
+    runs-on: ubuntu-latest
+    outputs:
+      test-matrix: ${{ steps.matrix.outputs.test-matrix }}
+    steps:
+      - uses: actions/checkout@v7
+      - uses: julia-actions/julia-compute-test-matrix@v2
+        id: matrix
+        with:
+          include-release-versions: true
+          include-lts-versions: true
+          include-rc-versions: true
+          include-all-compatible-minor-versions: true
+
+  test:
+    needs: compute-test-matrix
+    strategy:
+      fail-fast: false
+      matrix:
+        include: ${{ fromJson(needs.compute-test-matrix.outputs.test-matrix) }}
+    runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ matrix.allow-failure }}
+    steps:
+      - uses: actions/checkout@v7
+      - uses: julia-actions/install-juliaup@v3
+        with:
+          channel: ${{ matrix.juliaup-channel }}
+      - uses: julia-actions/cache@v3
+      # Same bootstrap as the testitem-workflow template: `julia-buildpkg` only
+      # installs a registry from Julia 1.5 on, and Pkg before that does not
+      # clone one on demand either.
+      - name: Install the General registry on Julia < 1.5
+        shell: bash
+        run: |
+          julia -e '
+            VERSION < v"1.5" || exit()
+            general = joinpath(DEPOT_PATH[1], "registries", "General")
+            isfile(joinpath(general, "Registry.toml")) && exit()
+            if ispath(general)
+              println("Registry at $general is unusable, replacing it")
+              # git marks its object files read-only, which blocks the delete on Windows
+              for (root, dirs, files) in walkdir(general), f in files
+                try chmod(joinpath(root, f), 0o777) catch end
+              end
+              rm(general, recursive=true)
+            end
+            mkpath(dirname(general))
+            run(`git clone --quiet --depth=1 https://github.com/JuliaRegistries/General.git $general`)
+          '
+      # Rewriting the compat entry before buildpkg means the ordinary
+      # instantiate resolves JSON 0.20.x, and any later re-resolution is
+      # equally constrained.
+      - name: Restrict JSON compat to 0.20
+        shell: bash
+        run: julia .ci/force_json_020.jl
+      - uses: julia-actions/julia-buildpkg@v1
+      - name: Verify JSON 0.20 was resolved
+        shell: bash
+        run: julia .ci/verify_json_020.jl
+      - uses: julia-actions/julia-run-testitems@v2
+        with:
+          juliaup-channel: ${{ matrix.juliaup-channel }}
+          profile-name: ${{ format('Julia {0}:{1} (JSON 0.20)', matrix.juliaup-channel, matrix.os) }}
diff --git a/.github/workflows/juliaci.yml b/.github/workflows/juliaci.yml
index 79056ef5..888a57b3 100644
--- a/.github/workflows/juliaci.yml
+++ b/.github/workflows/juliaci.yml
@@ -17,21 +17,3 @@ jobs:
     permissions: write-all
     secrets:
       codecov_token: ${{ secrets.CODECOV_TOKEN }}
-
-  json-compat:
-    name: JSON ${{ matrix.json-version }}
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        json-version: ["0.21", "1.7"]
-    steps:
-      - uses: actions/checkout@v4
-      - uses: julia-actions/setup-julia@v2
-        with:
-          version: "1"
-      - uses: julia-actions/cache@v2
-      - name: Install and verify JSON version
-        run: |
-          julia --project=. -e 'using Pkg; Pkg.add(PackageSpec(name="JSON", version="${{ matrix.json-version }}")); using JSON; @assert startswith(string(pkgversion(JSON)), "${{ matrix.json-version }}."); Pkg.status()'
-      - uses: julia-actions/julia-runtest@v1