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Pass MCP secrets via Claude Code env expansion instead of disk #2035

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40 changes: 26 additions & 14 deletions source/Calamari.AiAgent.Tests/ClaudeCodeBehaviour/McpWriterFixture.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -44,23 +44,30 @@ public void SetupMcpConfig_WritesValidJson_WithServers()
});
vars.Set(SpecialVariables.Action.Claude.McpServers, mcpJson);

var configPath = new McpWriter(vars).SetupMcpConfig(workingDir);
var result = new McpWriter(vars).SetupMcpConfig(workingDir);

File.Exists(configPath).Should().BeTrue();
File.Exists(result.Path).Should().BeTrue();

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
doc.RootElement.TryGetProperty("mcpServers", out var mcpServers).Should().BeTrue();
mcpServers.TryGetProperty("github", out var github).Should().BeTrue();
github.GetProperty("command").GetString().Should().Be("npx");

// The secret env value is referenced as a ${VAR} placeholder on disk, with the real
// value surfaced for injection into the claude process env.
var tokenPlaceholder = github.GetProperty("env").GetProperty("TOKEN").GetString();
tokenPlaceholder.Should().Be("${MCP_GITHUB_TOKEN}");
result.SecretEnvVars["MCP_GITHUB_TOKEN"].Should().Be("abc123");
json.Should().NotContain("abc123");
}

[Test]
public void SetupMcpConfig_WritesEmptyServers_WhenNoneProvided()
{
var configPath = new McpWriter(new CalamariVariables()).SetupMcpConfig(workingDir);
var result = new McpWriter(new CalamariVariables()).SetupMcpConfig(workingDir);

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
doc.RootElement.TryGetProperty("mcpServers", out var mcpServers).Should().BeTrue();
mcpServers.EnumerateObject().Should().BeEmpty();
Expand Down Expand Up @@ -98,17 +105,22 @@ public void SetupMcpConfig_AddsOctopusMcpServer_WhenTokenAndUrlProvided()
vars.Set(SpecialVariables.Action.Claude.OctopusToken, "API-TESTKEY");
vars.Set(SpecialVariables.Web.ServerUri, "https://octopus.example.com");

var configPath = new McpWriter(vars).SetupMcpConfig(workingDir);
var result = new McpWriter(vars).SetupMcpConfig(workingDir);

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
var mcpServers = doc.RootElement.GetProperty("mcpServers");
mcpServers.TryGetProperty("octopus", out var octopus).Should().BeTrue();
octopus.GetProperty("command").GetString().Should().Be("npx");

var env = octopus.GetProperty("env");
env.GetProperty("OCTOPUS_SERVER_URL").GetString().Should().Be("https://octopus.example.com");
env.GetProperty("OCTOPUS_API_KEY").GetString().Should().Be("API-TESTKEY");

// The Octopus token is referenced as a ${VAR} placeholder on disk, with the real value
// surfaced for injection into the claude process env.
env.GetProperty("OCTOPUS_API_KEY").GetString().Should().Be("${OCTOPUS_API_KEY}");
result.SecretEnvVars["OCTOPUS_API_KEY"].Should().Be("API-TESTKEY");
json.Should().NotContain("API-TESTKEY");
}

[Test]
Expand All @@ -117,9 +129,9 @@ public void SetupMcpConfig_SkipsOctopusMcpServer_WhenTokenMissing()
var vars = new CalamariVariables();
vars.Set(SpecialVariables.Web.ServerUri, "https://octopus.example.com");

var configPath = new McpWriter(vars).SetupMcpConfig(workingDir);
var result = new McpWriter(vars).SetupMcpConfig(workingDir);

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
doc.RootElement.GetProperty("mcpServers").TryGetProperty("octopus", out _).Should().BeFalse();
}
Expand Down Expand Up @@ -169,9 +181,9 @@ public void SetupMcpConfig_InjectsPathEnvVar_WhenNotProvidedByUser()
});
vars.Set(SpecialVariables.Action.Claude.McpServers, mcpJson);

var configPath = new McpWriter(vars).SetupMcpConfig(workingDir);
var result = new McpWriter(vars).SetupMcpConfig(workingDir);

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
var env = doc.RootElement
.GetProperty("mcpServers")
Expand All @@ -195,9 +207,9 @@ public void SetupMcpConfig_PreservesUserProvidedPathEnvVar()
});
vars.Set(SpecialVariables.Action.Claude.McpServers, mcpJson);

var configPath = new McpWriter(vars).SetupMcpConfig(workingDir);
var result = new McpWriter(vars).SetupMcpConfig(workingDir);

var json = File.ReadAllText(configPath);
var json = File.ReadAllText(result.Path);
var doc = JsonDocument.Parse(json);
var env = doc.RootElement
.GetProperty("mcpServers")
Expand Down
19 changes: 13 additions & 6 deletions source/Calamari.AiAgent/ClaudeCodeBehaviour/InvokeClaudeCodeBehaviour.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -104,16 +104,23 @@ public async Task Execute(RunningDeployment context)
}

argsBuilder.WithSystemPromptFile(new SystemPromptWriter().WriteSystemPromptFile(workingDir));
argsBuilder.WithMcpConfigPath(mcpConfig);
argsBuilder.WithMcpConfigPath(mcpConfig.Path);

// MCP secrets are referenced as ${VAR} in mcp-config.json and supplied to the claude
// process env here, so they aren't written to disk in plaintext. Claude expands them
// into each stdio server's env block at launch.
var alwaysSet = new Dictionary<string, string>
{
["ANTHROPIC_API_KEY"] = apiToken,
["CLAUDE_CODE_SUBPROCESS_ENV_SCRUB"] = "1", // Strips Anthropic/cloud credentials from Bash, hook, and MCP subprocess environments
};
foreach (var kvp in mcpConfig.SecretEnvVars)
alwaysSet[kvp.Key] = kvp.Value;

var environment = ClaudeCodeEnvironment.Build(
ClaudeCodeEnvironment.GetCurrentEnvironmentVariables(),
PassThroughEnvironmentVariables(variables),
new Dictionary<string, string>
{
["ANTHROPIC_API_KEY"] = apiToken,
["CLAUDE_CODE_SUBPROCESS_ENV_SCRUB"] = "1", // Strips Anthropic/cloud credentials from Bash, hook, and MCP subprocess environments
});
alwaysSet);

var response = await new ClaudeCodeCliRunner(log).RunAsync(
argsBuilder,
Expand Down
68 changes: 52 additions & 16 deletions source/Calamari.AiAgent/ClaudeCodeBehaviour/McpWriter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,14 +13,18 @@ public class McpWriter(IVariables variables)
{
static readonly string ConfigName = "mcp-config.json";

public string SetupMcpConfig(string workingDir)
public McpConfigResult SetupMcpConfig(string workingDir)
{
var mcpServers = BuildMcpServers();
// Secret env values are referenced from mcp-config.json as ${VAR} placeholders and
// passed to the claude process env instead of being written to disk in plaintext.
// Claude expands ${VAR} in stdio server env values from its own process env at launch.
var secretEnvVars = new Dictionary<string, string>();
var mcpServers = BuildMcpServers(secretEnvVars);
var config = new { mcpServers };
var json = JsonSerializer.Serialize(config, new JsonSerializerOptions { WriteIndented = true });
var path = Path.Combine(workingDir, ConfigName);
File.WriteAllText(path, json);
return path;
return new McpConfigResult(path, secretEnvVars);
}

public IEnumerable<string> GetAllowedTools()
Expand All @@ -31,16 +35,16 @@ public IEnumerable<string> GetAllowedTools()
return mcpServers.Select(serverName => $"mcp__{serverName.Name}__*");
}

Dictionary<string, McpServerConfig> BuildMcpServers()
Dictionary<string, McpServerConfig> BuildMcpServers(Dictionary<string, string> secretEnvVars)
{
var path = Environment.GetEnvironmentVariable("PATH") ?? "";
var servers = AddCustomMcpServer(path);
AddOctopusMcp(servers, path);

var servers = AddCustomMcpServer(path, secretEnvVars);
AddOctopusMcp(servers, path, secretEnvVars);
return servers;
}

void AddOctopusMcp(Dictionary<string, McpServerConfig> servers, string path)
void AddOctopusMcp(Dictionary<string, McpServerConfig> servers, string path, Dictionary<string, string> secretEnvVars)
{
var octopusToken = variables.Get(SpecialVariables.Action.Claude.OctopusToken);
if (string.IsNullOrWhiteSpace(octopusToken))
Expand All @@ -56,14 +60,15 @@ void AddOctopusMcp(Dictionary<string, McpServerConfig> servers, string path)
else
{
Log.Verbose("Octopus Server URL: " + octopusServerUrl);
secretEnvVars["OCTOPUS_API_KEY"] = octopusToken;
servers["octopus"] = new McpServerConfig
{
Command = "npx",
Args = new[] { "-y", "@octopusdeploy/mcp-server" },
Env = new Dictionary<string, string>
{
["OCTOPUS_SERVER_URL"] = octopusServerUrl,
["OCTOPUS_API_KEY"] = octopusToken,
["OCTOPUS_API_KEY"] = "${OCTOPUS_API_KEY}",
["PATH"] = path,
},
};
Expand All @@ -90,7 +95,7 @@ List<McpServerEntry> GetCustomMcpServers()
}
}

Dictionary<string, McpServerConfig> AddCustomMcpServer(string path)
Dictionary<string, McpServerConfig> AddCustomMcpServer(string path, Dictionary<string, string> secretEnvVars)
{
var entries = GetCustomMcpServers();

Expand All @@ -104,9 +109,25 @@ Dictionary<string, McpServerConfig> AddCustomMcpServer(string path)
if (string.IsNullOrWhiteSpace(entry.Command))
throw new CommandException($"MCP server '{entry.Name}' must have a command.");

var env = entry.Env != null
? new Dictionary<string, string>(entry.Env)
: new Dictionary<string, string>();
var env = new Dictionary<string, string>();
if (entry.Env != null)
{
foreach (var kvp in entry.Env)
{
// PATH is not a secret and is needed verbatim to locate the server command.
if (kvp.Key == "PATH")
{
env[kvp.Key] = kvp.Value;
continue;
}

// Custom server env values may be secrets, so reference them as ${VAR}
// placeholders and pass the real values via the claude process env.
var placeholder = ReserveEnvVarName(secretEnvVars, entry.Name, kvp.Key);
secretEnvVars[placeholder] = kvp.Value;
env[kvp.Key] = $"${{{placeholder}}}";
}
}

if (!env.ContainsKey("PATH"))
env["PATH"] = path;
Expand All @@ -125,7 +146,22 @@ Dictionary<string, McpServerConfig> AddCustomMcpServer(string path)
return mcpServerConfigs;
}

// Builds a deterministic, collision-safe env var name for a custom server's env entry.
static string ReserveEnvVarName(Dictionary<string, string> secretEnvVars, string serverName, string key)
{
var raw = $"MCP_{serverName}_{key}".ToUpperInvariant();
var chars = raw.Select(c => char.IsLetterOrDigit(c) ? c : '_').ToArray();
var name = new string(chars);
if (name.Length == 0 || char.IsDigit(name[0]))
name = "_" + name;

var candidate = name;
var suffix = 1;
while (secretEnvVars.ContainsKey(candidate))
candidate = $"{name}_{suffix++}";

return candidate;
}
}



}
public record McpConfigResult(string Path, IReadOnlyDictionary<string, string> SecretEnvVars);