feat(tools): native MCP client — mount stdio MCP servers as Correx tools

Adds an MCP host layer so external MCP servers (AST/LSP code-intelligence,
package resolvers, etc.) can be mounted at startup and surface their tools/list
as first-class Correx tools named mcp__<server>__<tool>. Each MCP tool is a
Tool+ToolExecutor, so it rides the normal ToolExecutor path and inherits tier
gating, receipts, and event-recorded execution (#5) with zero special-casing;
replay reads the recorded receipt rather than re-calling the server (#8/#9).

- McpProtocol/McpStdioClient/McpTool/McpMounter in infrastructure:tools
  (stdio JSON-RPC 2.0, tools/* slice only). Capabilities empty; safety via
  default T2 tier since external side effects are opaque.
- [[mcp]] config (id/command/env/tier) + array-of-tables parsing.
- Main wires mounted servers into extraTools (main + per-workspace paths) with
  a shutdown hook; a server that fails to start is logged and skipped.
- Tests: in-process fake transport (handshake/list/call/error + tool mapping),
  [[mcp]] parser.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/main/kotlin/com/correx/apps/server/Main.kt

@@ -210,14 +210,21 @@ fun main() {
     val explicitWorkspaceRoot = System.getenv("CORREX_WORKSPACE_ROOT")
         ?.let { Path.of(it) }
         ?: toolsConfig.workspaceRoot.takeIf { it.isNotEmpty() }?.let { Path.of(it) }
-    // workingDir and workspaceRoot must resolve to the same tree by default — the tool-call
-    // assessor's containment rules (PathContainmentRule, ManifestContainmentRule) only ever see
-    // workspaceRoot, while FileWriteTool resolves relative paths against workingDir. If only one
-    // is configured, each falls back to the other before falling back to process CWD, so the
-    // assessor's containment check and the actual write always share one resolved root.
     val shellAllowedExecutables = toolsConfig.shellAllowedExecutables.toSet()
-    val workspaceRoot = explicitWorkspaceRoot ?: explicitWorkingDir ?: Path.of("").toAbsolutePath()
+    val bootWorkspace = resolveBootWorkspace(
-    val workingDir = explicitWorkingDir ?: workspaceRoot
+        explicitWorkspaceRoot = explicitWorkspaceRoot,
+        explicitWorkingDir = explicitWorkingDir,
+        processWorkingDir = Path.of(""),
+    )
+    val workspaceRoot = bootWorkspace.workspaceRoot
+    val workingDir = bootWorkspace.workingDir
+    if (bootWorkspace.workingDirWasClamped) {
+        log.warn(
+            "configured working_dir {} is outside workspace_root {}; clamping working_dir to workspace_root",
+            explicitWorkingDir,
+            workspaceRoot,
+        )
+    }
     // One shared HTTP client backs both the default and per-workspace registries' research tools
     // (web_search/web_fetch). Built only when research is enabled, so the static path stays offline.
     // Lives for the process lifetime (shared across requests), so it's closed via shutdown hook
@@ -262,7 +269,34 @@ fun main() {
     )
     // Retrieves full tool output the kernel spilled to CAS on truncation (ref shown in-context).
     val toolOutputTool = com.correx.infrastructure.tools.ToolOutputTool(artifactStore)
-    val extraTools = taskTools + toolOutputTool
+    // Mount configured MCP servers: each server's tools/list becomes Correx tools that ride the normal
+    // ToolExecutor path (tier-gated, event-recorded, replay-safe — no special-casing). A server that
+    // fails to start is logged and skipped rather than aborting the whole process.
+    val mountedMcpServers = correxConfig.mcp.mapNotNull { mcpConfig ->
+        runCatching {
+            runBlocking {
+                com.correx.infrastructure.tools.mcp.McpMounter.mount(
+                    serverId = mcpConfig.id,
+                    command = mcpConfig.command,
+                    env = mcpConfig.env,
+                    tier = runCatching { com.correx.core.approvals.Tier.valueOf(mcpConfig.tier) }
+                        .getOrDefault(com.correx.core.approvals.Tier.T2),
+                )
+            }
+        }.onSuccess { log.info("Mounted MCP server '{}' ({} tools)", mcpConfig.id, it.tools.size) }
+            .onFailure { log.warn("Failed to mount MCP server '{}': {}", mcpConfig.id, it.message) }
+            .getOrNull()
+    }
+    if (mountedMcpServers.isNotEmpty()) {
+        Runtime.getRuntime().addShutdownHook(Thread {
+            mountedMcpServers.forEach { server ->
+                runCatching { server.close() }
+                    .onFailure { log.warn("Error closing MCP server '{}': {}", server.serverId, it.message) }
+            }
+        })
+    }
+    val mcpTools = mountedMcpServers.flatMap { it.tools }
+    val extraTools = taskTools + toolOutputTool + mcpTools
     val toolRegistry = InfrastructureModule.createToolRegistry(
         buildToolConfig(
             workspaceRoot,
@@ -545,7 +579,7 @@ fun main() {
     fun buildProjectMemory(cfg: CorrexConfig): com.correx.apps.server.memory.ProjectMemoryService? =
         if (cfg.project.enabled) {
             com.correx.apps.server.memory.ProjectMemoryService(
-                config = cfg.project,
+                config = cfg.project.boundToWorkspace(workspaceRoot),
                 embedder = embedder,
                 l3MemoryStore = l3MemoryStore,
                 journalRepository = decisionJournalRepository,
@@ -871,7 +905,7 @@ private fun buildToolConfig(
     toolsConfig: com.correx.core.config.ToolsConfig,
     research: com.correx.infrastructure.tools.ResearchToolConfig,
 ): ToolConfig {
-    val allowed = setOf(workspaceRoot, workingDir)
+    val allowed = setOf(workspaceRoot)
     return ToolConfig(
         shell = ShellConfig(
             enabled = toolsConfig.shellEnabled,

core/config/src/main/kotlin/com/correx/core/config/ConfigLoader.kt

@@ -265,9 +265,11 @@ object ConfigLoader {
         val providers = mutableListOf<MutableMap<String, Any>>()
         val models = mutableListOf<MutableMap<String, Any>>()
         val artifacts = mutableListOf<MutableMap<String, Any>>()
+        val mcpServers = mutableListOf<MutableMap<String, Any>>()
         var currentProvider: MutableMap<String, Any>? = null
         var currentModel: MutableMap<String, Any>? = null
         var currentArtifact: MutableMap<String, Any>? = null
+        var currentMcp: MutableMap<String, Any>? = null
 
         // Flush any open array-of-table entry into its list. Called whenever a new
         // table header (array or section) starts, so the previous entry is committed.
@@ -275,9 +277,11 @@ object ConfigLoader {
             currentProvider?.let { providers.add(it) }
             currentModel?.let { models.add(it) }
             currentArtifact?.let { artifacts.add(it) }
+            currentMcp?.let { mcpServers.add(it) }
             currentProvider = null
             currentModel = null
             currentArtifact = null
+            currentMcp = null
         }
 
         for ((lineNum, line) in lines.withIndex()) {
@@ -302,6 +306,11 @@ object ConfigLoader {
                     currentArtifact = mutableMapOf()
                     currentSection = ""
                 }
+                trimmed == "[[mcp]]" -> {
+                    flushTables()
+                    currentMcp = mutableMapOf()
+                    currentSection = ""
+                }
                 trimmed.startsWith("[") && !trimmed.startsWith("[[") && trimmed.endsWith("]") -> {
                     // Parse section headers like [server] or [tools.shell]
                     flushTables()
@@ -320,10 +329,12 @@ object ConfigLoader {
                         val provider = currentProvider
                         val model = currentModel
                         val artifact = currentArtifact
+                        val mcp = currentMcp
                         when {
                             provider != null -> provider[key] = parsedValue
                             model != null -> model[key] = parsedValue
                             artifact != null -> artifact[key] = parsedValue
+                            mcp != null -> mcp[key] = parsedValue
                             currentSection.isNotEmpty() -> sections[currentSection]?.put(key, parsedValue)
                         }
                     }
@@ -334,7 +345,7 @@ object ConfigLoader {
         // Don't forget the last array-of-table entry if file ends with one
         flushTables()
 
-        return buildConfig(sections, providers, models, artifacts)
+        return buildConfig(sections, providers, models, artifacts, mcpServers)
     }
 
     private fun parseValue(valueStr: String, lineNum: Int): Any {
@@ -452,6 +463,7 @@ object ConfigLoader {
         providersList: List<Map<String, Any>> = emptyList(),
         modelsList: List<Map<String, Any>> = emptyList(),
         artifactsList: List<Map<String, Any>> = emptyList(),
+        mcpList: List<Map<String, Any>> = emptyList(),
     ): CorrexConfig {
         val serverSection = sections["server"] ?: emptyMap()
         val tuiSection = sections["tui"] ?: emptyMap()
@@ -735,6 +747,20 @@ object ConfigLoader {
             repeatPenalty = samplingSection["repeat_penalty"]?.let { asDouble(it) },
         )
 
+        val mcp = mcpList.mapNotNull { mcpMap ->
+            val id = asStringOrNull(mcpMap["id"]) ?: return@mapNotNull null
+            val command = asStringList(mcpMap["command"])
+            if (command.isEmpty()) return@mapNotNull null
+            @Suppress("UNCHECKED_CAST")
+            val env = (mcpMap["env"] as? Map<String, Any>)?.mapValues { it.value.toString() } ?: emptyMap()
+            McpServerConfig(
+                id = id,
+                command = command,
+                env = env,
+                tier = asString(mcpMap["tier"], "T2"),
+            )
+        }
+
         val gitSection = sections["git"] ?: emptyMap()
         val git = GitConfig(
             enabled = asBoolean(gitSection["enabled"], false),
@@ -758,6 +784,7 @@ object ConfigLoader {
             orchestration = orchestration,
             sampling = sampling,
             git = git,
+            mcp = mcp,
         )
     }
 

core/config/src/main/kotlin/com/correx/core/config/CorrexConfig.kt

@@ -19,6 +19,19 @@ data class CorrexConfig(
     val sampling: SamplingConfig = SamplingConfig(),
     val health: HealthConfig = HealthConfig(),
     val git: GitConfig = GitConfig(),
+    val mcp: List<McpServerConfig> = emptyList(),
+)
+
+/**
+ * An MCP server to mount at startup. Its `tools/list` becomes Correx tools (`mcp__<id>__<name>`),
+ * gated at [tier] (default T2 = approval) since an external tool's side effects are opaque.
+ */
+@Serializable
+data class McpServerConfig(
+    val id: String,
+    val command: List<String>,
+    val env: Map<String, String> = emptyMap(),
+    val tier: String = "T2",
 )
 
 /**

core/config/src/test/kotlin/com/correx/core/config/ConfigLoaderTest.kt

@@ -390,6 +390,31 @@ class ConfigLoaderTest {
         assertEquals(10001, result.modelsSettings.port)
     }
 
+    @Test
+    fun `parseToml parses mcp array-of-tables with command list and env`() {
+        val toml = """
+            [[mcp]]
+            id = "codebase-memory"
+            command = ["codebase-memory-mcp", "serve"]
+            env = { RUST_LOG = "info" }
+
+            [[mcp]]
+            id = "other"
+            command = ["other-server"]
+        """.trimIndent()
+
+        val parseTomlMethod = ConfigLoader::class.java.getDeclaredMethod("parseToml", String::class.java)
+        parseTomlMethod.isAccessible = true
+        val result = parseTomlMethod.invoke(ConfigLoader, toml) as CorrexConfig
+
+        assertEquals(2, result.mcp.size)
+        assertEquals("codebase-memory", result.mcp[0].id)
+        assertEquals(listOf("codebase-memory-mcp", "serve"), result.mcp[0].command)
+        assertEquals("info", result.mcp[0].env["RUST_LOG"])
+        assertEquals("T2", result.mcp[0].tier)
+        assertEquals(listOf("other-server"), result.mcp[1].command)
+    }
+
     @Test
     fun `parseToml returns empty models list and default modelsSettings when sections absent`() {
         val toml = """

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/mcp/McpMounter.kt

@@ -0,0 +1,40 @@
+package com.correx.infrastructure.tools.mcp
+
+import com.correx.core.approvals.Tier
+import com.correx.core.tools.contract.Tool
+
+/**
+ * A running MCP server mounted into Correx: its advertised tools (ready to hand to the tool
+ * registry as `extraTools`) plus the handle that shuts the child process down. The caller registers
+ * [close] with the server's shutdown hook, exactly as the managed-model path unloads llama-server.
+ */
+class MountedMcpServer internal constructor(
+    val serverId: String,
+    private val client: McpClient,
+    val tools: List<Tool>,
+) : AutoCloseable {
+    override fun close() = client.close()
+}
+
+/** Spawn an MCP server over stdio, initialize it, and surface its `tools/list` as Correx tools. */
+object McpMounter {
+    suspend fun mount(
+        serverId: String,
+        command: List<String>,
+        env: Map<String, String> = emptyMap(),
+        tier: Tier = Tier.T2,
+    ): MountedMcpServer {
+        val client = McpClient.spawn(serverId, command, env)
+        val tools = client.listTools().map { descriptor ->
+            McpTool(
+                serverId = serverId,
+                remoteName = descriptor.name,
+                description = descriptor.description,
+                parametersSchema = descriptor.inputSchema,
+                tier = tier,
+                client = client,
+            )
+        }
+        return MountedMcpServer(serverId, client, tools)
+    }
+}

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/mcp/McpProtocol.kt

@@ -0,0 +1,68 @@
+package com.correx.infrastructure.tools.mcp
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
+
+/**
+ * Minimal JSON-RPC 2.0 + MCP wire types — just the tools slice Correx needs to mount an MCP
+ * server's tools as Correx tools. Resources, prompts, and sampling are intentionally absent (nothing
+ * consumes them yet). `ignoreUnknownKeys` on the codec lets servers send richer objects than these.
+ */
+@Serializable
+internal data class JsonRpcRequest(
+    val method: String,
+    val params: JsonElement? = null,
+    val id: Long,
+    val jsonrpc: String = "2.0",
+)
+
+@Serializable
+internal data class JsonRpcNotification(
+    val method: String,
+    val params: JsonElement? = null,
+    val jsonrpc: String = "2.0",
+)
+
+@Serializable
+internal data class JsonRpcResponse(
+    val id: Long? = null,
+    val result: JsonElement? = null,
+    val error: JsonRpcError? = null,
+    val jsonrpc: String = "2.0",
+)
+
+@Serializable
+internal data class JsonRpcError(
+    val code: Int,
+    val message: String,
+    val data: JsonElement? = null,
+)
+
+/** One tool advertised by an MCP server (`tools/list` entry). */
+@Serializable
+internal data class McpToolDescriptor(
+    val name: String,
+    val description: String = "",
+    val inputSchema: JsonObject = JsonObject(emptyMap()),
+)
+
+@Serializable
+internal data class McpListToolsResult(val tools: List<McpToolDescriptor> = emptyList())
+
+/** One content block of a `tools/call` result. Only text blocks are surfaced to the model. */
+@Serializable
+internal data class McpContentBlock(
+    val type: String,
+    val text: String? = null,
+)
+
+@Serializable
+internal data class McpCallToolResult(
+    val content: List<McpContentBlock> = emptyList(),
+    @SerialName("isError") val isError: Boolean = false,
+) {
+    /** Flatten text blocks into the single string the Correx tool contract returns. */
+    fun text(): String = content.mapNotNull { it.text }.joinToString("\n")
+}

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/mcp/McpStdioClient.kt

@@ -0,0 +1,169 @@
+package com.correx.infrastructure.tools.mcp
+
+import com.correx.core.tools.process.ChildProcess
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
+import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.buildJsonObject
+import kotlinx.serialization.json.decodeFromJsonElement
+import kotlinx.serialization.json.put
+import kotlinx.serialization.json.putJsonObject
+import java.io.BufferedReader
+import java.io.Writer
+import java.util.concurrent.atomic.AtomicLong
+
+/**
+ * The transport an [McpClient] talks over. Extracted so tests can drive the client with an
+ * in-process fake instead of spawning a real subprocess (keeps the client's protocol logic
+ * deterministically testable — no process, no I/O).
+ */
+internal interface McpTransport {
+    suspend fun request(request: JsonRpcRequest): JsonRpcResponse
+    suspend fun notify(notification: JsonRpcNotification)
+    fun close()
+}
+
+private val MCP_JSON = Json {
+    ignoreUnknownKeys = true
+    encodeDefaults = true
+}
+
+/**
+ * A live MCP session: performs the `initialize` handshake, lists tools, and calls them. Ecosystem-
+ * agnostic — it neither knows nor cares whether the server behind [transport] resolves npm packages,
+ * indexes a codebase, or anything else. The nondeterminism of a `tools/call` is captured by the
+ * kernel's normal tool-execution recording (the call rides the ToolExecutor path), so nothing here
+ * needs to emit events: replay never re-calls because replay never executes tools.
+ */
+internal class McpClient(
+    val serverId: String,
+    private val transport: McpTransport,
+) : McpClientHandle {
+    suspend fun initialize() {
+        val params = buildJsonObject {
+            put("protocolVersion", MCP_PROTOCOL_VERSION)
+            putJsonObject("capabilities") {}
+            putJsonObject("clientInfo") {
+                put("name", "correx")
+                put("version", "0")
+            }
+        }
+        transport.request(JsonRpcRequest(method = "initialize", params = params, id = 0)).orThrow(serverId)
+        transport.notify(JsonRpcNotification(method = "notifications/initialized"))
+    }
+
+    suspend fun listTools(): List<McpToolDescriptor> {
+        val result = transport.request(JsonRpcRequest(method = "tools/list", id = nextId())).orThrow(serverId)
+        return MCP_JSON.decodeFromJsonElement(McpListToolsResult.serializer(), result).tools
+    }
+
+    override suspend fun callTool(remoteName: String, arguments: JsonObject): McpToolCallOutcome {
+        val params = buildJsonObject {
+            put("name", remoteName)
+            put("arguments", arguments)
+        }
+        val result = transport.request(JsonRpcRequest(method = "tools/call", params = params, id = nextId()))
+            .orThrow(serverId)
+        val decoded = MCP_JSON.decodeFromJsonElement(McpCallToolResult.serializer(), result)
+        return McpToolCallOutcome(text = decoded.text(), isError = decoded.isError)
+    }
+
+    fun close() = transport.close()
+
+    private val ids = AtomicLong(1)
+    private fun nextId() = ids.getAndIncrement()
+
+    private fun JsonRpcResponse.orThrow(serverId: String): JsonElement {
+        error?.let { error("MCP server '$serverId' error ${it.code}: ${it.message}") }
+        return result ?: error("MCP server '$serverId' returned no result")
+    }
+
+    companion object {
+        // The protocol revision Correx advertises. Servers negotiate down if they only speak older.
+        const val MCP_PROTOCOL_VERSION = "2024-11-05"
+
+        /**
+         * Spawn [command] as a child process and return an initialized client speaking MCP over its
+         * stdio pipes. The caller owns [McpClient.close] (kills the process). Uses [ChildProcess] so
+         * the server inherits the same unattended/non-interactive execution guarantees as every other
+         * spawned child.
+         */
+        suspend fun spawn(serverId: String, command: List<String>, env: Map<String, String>): McpClient {
+            val process = withContext(Dispatchers.IO) {
+                ChildProcess.builder(command).apply { environment().putAll(env) }.start()
+            }
+            val client = McpClient(serverId, StdioTransport(process))
+            client.initialize()
+            return client
+        }
+    }
+}
+
+/**
+ * Newline-delimited JSON-RPC over a child process's stdio (the MCP stdio transport). Requests are
+ * serialized through a [Mutex]: one line written, then response lines drained until the matching id
+ * arrives — server-initiated notifications/logs on the same pipe are skipped, not mistaken for the
+ * reply. Blocking pipe I/O is confined to [Dispatchers.IO].
+ */
+private class StdioTransport(private val process: Process) : McpTransport {
+    private val lock = Mutex()
+    private val writer: Writer = process.outputStream.bufferedWriter()
+    private val reader: BufferedReader = process.inputStream.bufferedReader()
+
+    override suspend fun request(request: JsonRpcRequest): JsonRpcResponse = lock.withLock {
+        withContext(Dispatchers.IO) {
+            writeLine(MCP_JSON.encodeToString(JsonRpcRequest.serializer(), request))
+            while (true) {
+                val line = reader.readLine()
+                    ?: error("MCP server '${request.method}' closed the connection")
+                if (line.isBlank()) continue
+                val response = runCatching {
+                    MCP_JSON.decodeFromString(JsonRpcResponse.serializer(), line)
+                }.getOrNull() ?: continue // not a response we can parse (a notification/log line) — skip
+                if (response.id == request.id) return@withContext response
+            }
+            @Suppress("UNREACHABLE_CODE")
+            error("unreachable")
+        }
+    }
+
+    override suspend fun notify(notification: JsonRpcNotification) = lock.withLock {
+        withContext(Dispatchers.IO) {
+            writeLine(MCP_JSON.encodeToString(JsonRpcNotification.serializer(), notification))
+        }
+    }
+
+    override fun close() {
+        runCatching { writer.close() }
+        runCatching { reader.close() }
+        process.destroy()
+    }
+
+    private fun writeLine(json: String) {
+        writer.write(json)
+        writer.write("\n")
+        writer.flush()
+    }
+}
+
+/** Encode a [ToolRequest]'s parameter map into the JsonObject `arguments` payload for a tools/call. */
+internal fun argumentsOf(parameters: Map<String, Any?>): JsonObject =
+    JsonObject(parameters.mapValues { (_, value) -> anyToJsonElement(value) })
+
+private fun anyToJsonElement(value: Any?): JsonElement = when (value) {
+    null -> JsonNull
+    is JsonElement -> value
+    is Boolean -> JsonPrimitive(value)
+    is Number -> JsonPrimitive(value)
+    is String -> JsonPrimitive(value)
+    is Map<*, *> -> JsonObject(value.entries.associate { (k, v) -> k.toString() to anyToJsonElement(v) })
+    is Iterable<*> -> JsonArray(value.map { anyToJsonElement(it) })
+    else -> JsonPrimitive(value.toString())
+}

infrastructure/tools/src/main/kotlin/com/correx/infrastructure/tools/mcp/McpTool.kt

@@ -0,0 +1,64 @@
+package com.correx.infrastructure.tools.mcp
+
+import com.correx.core.approvals.Tier
+import com.correx.core.events.events.ToolRequest
+import com.correx.core.tools.contract.Tool
+import com.correx.core.tools.contract.ToolCapability
+import com.correx.core.tools.contract.ToolExecutor
+import com.correx.core.tools.contract.ToolResult
+import com.correx.core.tools.contract.ValidationResult
+import kotlinx.serialization.json.JsonObject
+
+/**
+ * One tool advertised by an MCP server, surfaced as a first-class Correx [Tool]/[ToolExecutor]. It
+ * is exactly like any other tool from the kernel's point of view — so it inherits tier gating,
+ * receipts, and event-recorded execution (#5) with zero special-casing, and replay reads the
+ * recorded receipt instead of ever re-calling the server (#8/#9).
+ *
+ * Naming: `mcp__<server>__<remote>` — namespaced by server so two servers can advertise a tool of
+ * the same name, and recognizable as MCP-backed at a glance (the same convention MCP hosts use).
+ *
+ * Capabilities are declared empty: an external MCP tool's side effects are opaque and cannot be
+ * classified into Correx's FILE/NETWORK/SHELL taxonomy, so it does not claim any — safety comes from
+ * [tier] (default T2 = approval-gated) rather than from a capability guess that would misfire the
+ * plane-2 rules (e.g. NETWORK_ACCESS expects a NETWORK_TARGET param an MCP tool doesn't have).
+ */
+class McpTool(
+    private val serverId: String,
+    private val remoteName: String,
+    override val description: String,
+    override val parametersSchema: JsonObject,
+    override val tier: Tier,
+    private val client: McpClientHandle,
+) : Tool, ToolExecutor {
+
+    override val name: String = "mcp__${serverId}__$remoteName"
+    override val requiredCapabilities: Set<ToolCapability> = emptySet()
+
+    override fun validateRequest(request: ToolRequest): ValidationResult = ValidationResult.Valid
+
+    override suspend fun execute(request: ToolRequest): ToolResult =
+        runCatching { client.callTool(remoteName, argumentsOf(request.parameters)) }
+            .fold(
+                onSuccess = { result ->
+                    if (result.isError) {
+                        // A tool-reported failure (e.g. "package not found") — recoverable so it flows
+                        // back into the ReAct loop as evidence rather than aborting the stage.
+                        ToolResult.Failure(request.invocationId, result.text, recoverable = true)
+                    } else {
+                        ToolResult.Success(request.invocationId, result.text)
+                    }
+                },
+                // A transport/protocol failure (server crashed, bad frame) is not the model's fault
+                // and not fixable by retrying the same call: non-recoverable.
+                onFailure = { ToolResult.Failure(request.invocationId, "MCP call failed: ${it.message}", recoverable = false) },
+            )
+}
+
+/** The slice of [McpClient] an [McpTool] needs — narrowed to an interface so tools are unit-testable. */
+interface McpClientHandle {
+    suspend fun callTool(remoteName: String, arguments: JsonObject): McpToolCallOutcome
+}
+
+/** Transport-agnostic result an [McpClientHandle] hands back (decouples the tool from wire types). */
+data class McpToolCallOutcome(val text: String, val isError: Boolean)

infrastructure/tools/src/test/kotlin/com/correx/infrastructure/tools/mcp/McpClientTest.kt

@@ -0,0 +1,112 @@
+package com.correx.infrastructure.tools.mcp
+
+import com.correx.core.approvals.Tier
+import com.correx.core.events.events.ToolRequest
+import com.correx.core.events.types.SessionId
+import com.correx.core.events.types.StageId
+import com.correx.core.events.types.ToolInvocationId
+import com.correx.core.tools.contract.ToolResult
+import kotlinx.coroutines.runBlocking
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.buildJsonObject
+import kotlinx.serialization.json.put
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+/**
+ * Drives [McpClient] through an in-process [McpTransport] — no subprocess, no I/O. Verifies the
+ * protocol logic (handshake, tools/list decode, tools/call outcome, error surfacing) and the
+ * [McpTool] adapter's mapping onto the Correx tool contract, all deterministically.
+ */
+class McpClientTest {
+
+    /** A scripted transport: answers each method from a canned map; records notifications. */
+    private class FakeTransport(private val results: Map<String, JsonRpcResponse>) : McpTransport {
+        val notified = mutableListOf<String>()
+        override suspend fun request(request: JsonRpcRequest): JsonRpcResponse =
+            (results[request.method] ?: error("no scripted result for ${request.method}"))
+                .copy(id = request.id)
+        override suspend fun notify(notification: JsonRpcNotification) { notified.add(notification.method) }
+        override fun close() {}
+    }
+
+    private fun ok(json: kotlinx.serialization.json.JsonElement) = JsonRpcResponse(result = json)
+
+    @Test
+    fun `initialize handshakes then lists and calls tools`(): Unit = runBlocking {
+        val listResult = buildJsonObject {
+            put("tools", kotlinx.serialization.json.buildJsonArray {
+                add(buildJsonObject {
+                    put("name", "search_code")
+                    put("description", "search the graph")
+                })
+            })
+        }
+        val callResult = buildJsonObject {
+            put("content", kotlinx.serialization.json.buildJsonArray {
+                add(buildJsonObject { put("type", "text"); put("text", "hit one") })
+                add(buildJsonObject { put("type", "text"); put("text", "hit two") })
+            })
+        }
+        val transport = FakeTransport(
+            mapOf(
+                "initialize" to ok(buildJsonObject {}),
+                "tools/list" to ok(listResult),
+                "tools/call" to ok(callResult),
+            ),
+        )
+        val client = McpClient("cm", transport)
+        client.initialize()
+        assertEquals(listOf("notifications/initialized"), transport.notified)
+
+        val tools = client.listTools()
+        assertEquals(listOf("search_code"), tools.map { it.name })
+
+        val outcome = client.callTool("search_code", JsonObject(emptyMap()))
+        assertEquals("hit one\nhit two", outcome.text)
+        assertTrue(!outcome.isError)
+    }
+
+    @Test
+    fun `server error is surfaced as a thrown failure`(): Unit = runBlocking {
+        val transport = FakeTransport(
+            mapOf("tools/call" to JsonRpcResponse(error = JsonRpcError(code = -32000, message = "boom"))),
+        )
+        val client = McpClient("cm", transport)
+        val ex = runCatching { client.callTool("x", JsonObject(emptyMap())) }.exceptionOrNull()
+        assertTrue(ex?.message?.contains("boom") == true, "expected server error message, got: ${ex?.message}")
+    }
+
+    @Test
+    fun `McpTool maps an isError result to a recoverable failure`(): Unit = runBlocking {
+        val handle = object : McpClientHandle {
+            override suspend fun callTool(remoteName: String, arguments: JsonObject) =
+                McpToolCallOutcome(text = "package not found", isError = true)
+        }
+        val tool = McpTool("cm", "resolve", "", JsonObject(emptyMap()), Tier.T2, handle)
+        assertEquals("mcp__cm__resolve", tool.name)
+        val result = tool.execute(req(tool.name)) as ToolResult.Failure
+        assertEquals("package not found", result.reason)
+        assertTrue(result.recoverable)
+    }
+
+    @Test
+    fun `McpTool maps a normal result to success`(): Unit = runBlocking {
+        val handle = object : McpClientHandle {
+            override suspend fun callTool(remoteName: String, arguments: JsonObject) =
+                McpToolCallOutcome(text = "ok", isError = false)
+        }
+        val tool = McpTool("cm", "resolve", "", JsonObject(emptyMap()), Tier.T2, handle)
+        val result = tool.execute(req(tool.name)) as ToolResult.Success
+        assertEquals("ok", result.output)
+    }
+
+    private fun req(name: String) = ToolRequest(
+        invocationId = ToolInvocationId("inv-1"),
+        sessionId = SessionId("s-1"),
+        stageId = StageId("st-1"),
+        toolName = name,
+        parameters = mapOf("q" to "vite"),
+    )
+}