diff --git a/tests/julia/runtests.jl b/tests/julia/runtests.jl
index 4ec598d..df11e24 100644
--- a/tests/julia/runtests.jl
+++ b/tests/julia/runtests.jl
@@ -8,4 +8,6 @@ using .Concore
     include("test_config.jl")
     include("test_sync.jl")
     include("test_protocol.jl")
+    include("test_wire_compat.jl")
+    include("test_interop.jl")
 end
diff --git a/tests/julia/test_interop.jl b/tests/julia/test_interop.jl
new file mode 100644
index 0000000..662e8a8
--- /dev/null
+++ b/tests/julia/test_interop.jl
@@ -0,0 +1,210 @@
+# File-backend round trips against Python and C++ bindings.
+@testset "Cross-language interop" begin
+
+    repo_root = normpath(joinpath(@__DIR__, "..", ".."))
+    repo_include = replace(repo_root, "\\" => "/")
+    python_path(path) = replace(path, "\\" => "/")
+
+    function reset_interop_state!()
+        Concore.simtime = 0.0
+        Concore.delay = 0.0
+        Concore.s = ""
+        Concore.olds = ""
+        Concore.retrycount = 0
+    end
+
+    function run_cmd(cmd; dir=repo_root)
+        try
+            return 0, strip(read(Cmd(cmd; dir=dir), String)), ""
+        catch err
+            return 1, "", sprint(showerror, err)
+        end
+    end
+
+    @testset "Python writes, Julia reads" begin
+        python = Sys.which("python")
+        python === nothing && (python = Sys.which("python3"))
+
+        if python === nothing
+            @test_skip "python executable not found"
+        else
+            mktempdir() do dir
+                mkpath(joinpath(dir, "out1"))
+
+                script = """
+import sys
+sys.path.insert(0, "$(python_path(repo_root))")
+import concore
+concore.delay = 0
+concore.simtime = 5.0
+concore.outpath = "$(python_path(joinpath(dir, "out")))"
+concore.write(1, "signal", [42.0, 3.0])
+"""
+
+                code, _out, err = run_cmd(`$python -c $script`)
+                @test code == 0
+                err != "" && @info err
+
+                old_inpath = Concore.inpath
+                try
+                    reset_interop_state!()
+                    Concore.inpath = joinpath(dir, "out")
+                    result = Concore.concore_read(1, "signal", "[0.0, 0.0, 0.0]")
+
+                    @test result == [42.0, 3.0]
+                    @test Concore.simtime == 5.0
+                    @test read(joinpath(dir, "out1", "signal"), String) == "[5.0, 42.0, 3.0]"
+                finally
+                    Concore.inpath = old_inpath
+                    reset_interop_state!()
+                end
+            end
+        end
+    end
+
+    @testset "Julia writes, Python reads" begin
+        python = Sys.which("python")
+        python === nothing && (python = Sys.which("python3"))
+
+        if python === nothing
+            @test_skip "python executable not found"
+        else
+            mktempdir() do dir
+                old_outpath = Concore.outpath
+                try
+                    reset_interop_state!()
+                    Concore.outpath = joinpath(dir, "out")
+                    Concore.simtime = 7.0
+                    Concore.concore_write(1, "signal", [11.0, 12.5])
+
+                    script = """
+import sys
+sys.path.insert(0, "$(python_path(repo_root))")
+import concore
+concore.delay = 0
+concore.simtime = 0.0
+concore.inpath = "$(python_path(joinpath(dir, "out")))"
+value = concore.read(1, "signal", "[0.0, 0.0, 0.0]")
+assert value == [11.0, 12.5], value
+assert concore.simtime == 7.0, concore.simtime
+"""
+
+                    code, _out, err = run_cmd(`$python -c $script`)
+                    @test code == 0
+                    err != "" && @info err
+                finally
+                    Concore.outpath = old_outpath
+                    reset_interop_state!()
+                end
+            end
+        end
+    end
+
+    function cxx_compiler()
+        for name in ("g++", "clang++")
+            compiler = Sys.which(name)
+            compiler !== nothing && return compiler
+        end
+        return nothing
+    end
+
+    @testset "C++ writes, Julia reads" begin
+        compiler = cxx_compiler()
+
+        if compiler === nothing
+            @test_skip "C++ compiler not found"
+        else
+            mktempdir() do dir
+                source = joinpath(dir, "write_signal.cpp")
+                exe = joinpath(dir, Sys.iswindows() ? "write_signal.exe" : "write_signal")
+                write(source, """
+#include <vector>
+#include "$repo_include/concore.hpp"
+
+int main() {
+    Concore c;
+    c.delay = 0;
+    c.simtime = 9.0;
+    c.write_FM(1, "signal", std::vector<double>{13.0, 14.0});
+    return 0;
+}
+""")
+                mkpath(joinpath(dir, "out1"))
+
+                code, _out, err = run_cmd(`$compiler -std=c++11 -I$repo_root -o $exe $source`; dir=dir)
+                @test code == 0
+                err != "" && @info err
+
+                code, _out, err = run_cmd(`$exe`; dir=dir)
+                @test code == 0
+                err != "" && @info err
+
+                old_inpath = Concore.inpath
+                try
+                    reset_interop_state!()
+                    Concore.inpath = joinpath(dir, "out")
+                    result = Concore.concore_read(1, "signal", "[0.0, 0.0, 0.0]")
+
+                    @test result == [13.0, 14.0]
+                    @test Concore.simtime == 9.0
+                finally
+                    Concore.inpath = old_inpath
+                    reset_interop_state!()
+                end
+            end
+        end
+    end
+
+    @testset "Julia writes, C++ reads" begin
+        compiler = cxx_compiler()
+
+        if compiler === nothing
+            @test_skip "C++ compiler not found"
+        else
+            mktempdir() do dir
+                old_outpath = Concore.outpath
+                try
+                    reset_interop_state!()
+                    Concore.outpath = joinpath(dir, "out")
+                    Concore.simtime = 10.0
+                    Concore.concore_write(1, "signal", [21.0, 22.0])
+                    mkpath(joinpath(dir, "in1"))
+                    cp(joinpath(dir, "out1", "signal"), joinpath(dir, "in1", "signal"))
+                finally
+                    Concore.outpath = old_outpath
+                end
+
+                source = joinpath(dir, "read_signal.cpp")
+                exe = joinpath(dir, Sys.iswindows() ? "read_signal.exe" : "read_signal")
+                write(source, """
+#include <cmath>
+#include <vector>
+#include "$repo_include/concore.hpp"
+
+int main() {
+    Concore c;
+    c.delay = 0;
+    c.simtime = 0.0;
+    std::vector<double> value = c.read_FM(1, "signal", "[0.0, 0.0, 0.0]");
+    if (value.size() != 2) return 1;
+    if (std::fabs(value[0] - 21.0) > 1e-9) return 2;
+    if (std::fabs(value[1] - 22.0) > 1e-9) return 3;
+    if (std::fabs(c.simtime - 10.0) > 1e-9) return 4;
+    return 0;
+}
+""")
+
+                code, _out, err = run_cmd(`$compiler -std=c++11 -I$repo_root -o $exe $source`; dir=dir)
+                @test code == 0
+                err != "" && @info err
+
+                code, _out, err = run_cmd(`$exe`; dir=dir)
+                @test code == 0
+                err != "" && @info err
+
+                reset_interop_state!()
+            end
+        end
+    end
+
+end
diff --git a/tests/julia/test_wire_compat.jl b/tests/julia/test_wire_compat.jl
new file mode 100644
index 0000000..340ed23
--- /dev/null
+++ b/tests/julia/test_wire_compat.jl
@@ -0,0 +1,70 @@
+# Exact wire strings that must stay readable across Concore runtimes.
+@testset "Wire compatibility" begin
+
+    function reset_wire_state!()
+        Concore.simtime = 0.0
+        Concore.delay = 0.0
+        Concore.s = ""
+        Concore.olds = ""
+        Concore.retrycount = 0
+    end
+
+    @testset "Julia writes Python-style wire strings" begin
+        cases = [
+            (0.0, [0.0], "[0.0, 0.0]"),
+            (0.0, [1.0, 2.0, 3.0], "[0.0, 1.0, 2.0, 3.0]"),
+            (5.0, [42.0, 3.0], "[5.0, 42.0, 3.0]"),
+            (2.0, [3.14, -1.5, 0.0], "[2.0, 3.14, -1.5, 0.0]"),
+        ]
+
+        mktempdir() do dir
+            old_outpath = Concore.outpath
+            Concore.outpath = joinpath(dir, "out")
+
+            try
+                for (simtime, values, expected) in cases
+                    reset_wire_state!()
+                    Concore.simtime = simtime
+                    Concore.concore_write(1, "wire", values)
+                    content = read(joinpath(dir, "out1", "wire"), String)
+
+                    @test content == expected
+                    @test !occursin('\n', content)
+                    @test !endswith(content, ",]")
+                    @test !endswith(content, ", ]")
+                end
+            finally
+                Concore.outpath = old_outpath
+                reset_wire_state!()
+            end
+        end
+    end
+
+    @testset "Julia parses Python and C++ wire strings" begin
+        @test Concore.safe_parse_list("[5.0, 42.0, 3.0]") == [5.0, 42.0, 3.0]
+        @test Concore.safe_parse_list("[5,42,3]") == [5.0, 42.0, 3.0]
+        @test Concore.safe_parse_list("[np.float64(5.0), np.float64(42.0)]") == [5.0, 42.0]
+        @test Concore.safe_parse_list("np.array([1.0, 2.0, 3.0])") == [1.0, 2.0, 3.0]
+    end
+
+    @testset "delta affects written simtime only" begin
+        mktempdir() do dir
+            old_outpath = Concore.outpath
+            Concore.outpath = joinpath(dir, "out")
+
+            try
+                reset_wire_state!()
+                Concore.simtime = 5.0
+                Concore.concore_write(1, "ym", [3.01]; delta=1)
+
+                content = read(joinpath(dir, "out1", "ym"), String)
+                @test content == "[6.0, 3.01]"
+                @test Concore.simtime == 5.0
+            finally
+                Concore.outpath = old_outpath
+                reset_wire_state!()
+            end
+        end
+    end
+
+end