diff --git a/php/Mcp/Dependencies/DependencyType.php b/php/Mcp/Dependencies/DependencyType.php
new file mode 100644
index 00000000..243a2734
--- /dev/null
+++ b/php/Mcp/Dependencies/DependencyType.php
@@ -0,0 +1,59 @@
+<?php
+
+// SPDX-License-Identifier: EUPL-1.2
+
+declare(strict_types=1);
+
+namespace Core\Mcp\Dependencies;
+
+/**
+ * Types of tool dependencies.
+ *
+ * Defines how a prerequisite must be satisfied before a tool can execute.
+ */
+enum DependencyType: string
+{
+    /**
+     * Another tool must have been called in the current session.
+     * Example: task_update requires plan_create to have been called.
+     */
+    case TOOL_CALLED = 'tool_called';
+
+    /**
+     * A specific state key must exist in the session context.
+     * Example: session_log requires session_id to be set.
+     */
+    case SESSION_STATE = 'session_state';
+
+    /**
+     * A specific context value must be present.
+     * Example: workspace_id must exist for workspace-scoped tools.
+     */
+    case CONTEXT_EXISTS = 'context_exists';
+
+    /**
+     * A database entity must exist (checked by ID or slug).
+     * Example: task_update requires the plan_slug to reference an existing plan.
+     */
+    case ENTITY_EXISTS = 'entity_exists';
+
+    /**
+     * A custom condition evaluated at runtime.
+     * Example: Complex business rules that don't fit other types.
+     */
+    case CUSTOM = 'custom';
+
+    /**
+     * Get a human-readable label for this dependency type.
+     */
+    public function label(): string
+    {
+        return match ($this) {
+            self::TOOL_CALLED => 'Tool must be called first',
+            self::SESSION_STATE => 'Session state required',
+            self::CONTEXT_EXISTS => 'Context value required',
+            self::ENTITY_EXISTS => 'Entity must exist',
+            self::CUSTOM => 'Custom condition',
+        };
+    }
+}
diff --git a/php/Mcp/Dependencies/HasDependencies.php b/php/Mcp/Dependencies/HasDependencies.php
new file mode 100644
index 00000000..2f5d1e65
--- /dev/null
+++ b/php/Mcp/Dependencies/HasDependencies.php
@@ -0,0 +1,23 @@
+<?php
+
+// SPDX-License-Identifier: EUPL-1.2
+
+declare(strict_types=1);
+
+namespace Core\Mcp\Dependencies;
+
+/**
+ * Interface for tools that declare dependencies.
+ *
+ * Tools implementing this interface can specify prerequisites
+ * that must be satisfied before execution.
+ */
+interface HasDependencies
+{
+    /**
+     * Get the dependencies for this tool.
+     *
+     * @return array<ToolDependency>
+     */
+    public function dependencies(): array;
+}
diff --git a/php/Mcp/Dependencies/ToolDependency.php b/php/Mcp/Dependencies/ToolDependency.php
new file mode 100644
index 00000000..a9503241
--- /dev/null
+++ b/php/Mcp/Dependencies/ToolDependency.php
@@ -0,0 +1,136 @@
+<?php
+
+// SPDX-License-Identifier: EUPL-1.2
+
+declare(strict_types=1);
+
+namespace Core\Mcp\Dependencies;
+
+/**
+ * Represents a single tool dependency.
+ *
+ * Defines what must be satisfied before a tool can execute.
+ */
+class ToolDependency
+{
+    /**
+     * Create a new tool dependency.
+     *
+     * @param  DependencyType  $type  The type of dependency
+     * @param  string  $key  The identifier (tool name, state key, context key, etc.)
+     * @param  string|null  $description  Human-readable description for error messages
+     * @param  bool  $optional  If true, this is a soft dependency (warning, not error)
+     * @param  array  $metadata  Additional metadata for custom validation
+     */
+    public function __construct(
+        public readonly DependencyType $type,
+        public readonly string $key,
+        public readonly ?string $description = null,
+        public readonly bool $optional = false,
+        public readonly array $metadata = [],
+    ) {}
+
+    /**
+     * Create a tool_called dependency.
+     */
+    public static function toolCalled(string $toolName, ?string $description = null): self
+    {
+        return new self(
+            type: DependencyType::TOOL_CALLED,
+            key: $toolName,
+            description: $description ?? "Tool '{$toolName}' must be called first",
+        );
+    }
+
+    /**
+     * Create a session_state dependency.
+     */
+    public static function sessionState(string $stateKey, ?string $description = null): self
+    {
+        return new self(
+            type: DependencyType::SESSION_STATE,
+            key: $stateKey,
+            description: $description ?? "Session state '{$stateKey}' is required",
+        );
+    }
+
+    /**
+     * Create a context_exists dependency.
+     */
+    public static function contextExists(string $contextKey, ?string $description = null): self
+    {
+        return new self(
+            type: DependencyType::CONTEXT_EXISTS,
+            key: $contextKey,
+            description: $description ?? "Context '{$contextKey}' is required",
+        );
+    }
+
+    /**
+     * Create an entity_exists dependency.
+     */
+    public static function entityExists(string $entityType, ?string $description = null, array $metadata = []): self
+    {
+        return new self(
+            type: DependencyType::ENTITY_EXISTS,
+            key: $entityType,
+            description: $description ?? "Entity '{$entityType}' must exist",
+            metadata: $metadata,
+        );
+    }
+
+    /**
+     * Create a custom dependency with callback metadata.
+     */
+    public static function custom(string $name, ?string $description = null, array $metadata = []): self
+    {
+        return new self(
+            type: DependencyType::CUSTOM,
+            key: $name,
+            description: $description,
+            metadata: $metadata,
+        );
+    }
+
+    /**
+     * Mark this dependency as optional (soft dependency).
+     */
+    public function asOptional(): self
+    {
+        return new self(
+            type: $this->type,
+            key: $this->key,
+            description: $this->description,
+            optional: true,
+            metadata: $this->metadata,
+        );
+    }
+
+    /**
+     * Convert to array representation.
+     */
+    public function toArray(): array
+    {
+        return [
+            'type' => $this->type->value,
+            'key' => $this->key,
+            'description' => $this->description,
+            'optional' => $this->optional,
+            'metadata' => $this->metadata,
+        ];
+    }
+
+    /**
+     * Create from array representation.
+     */
+    public static function fromArray(array $data): self
+    {
+        return new self(
+            type: DependencyType::from($data['type']),
+            key: $data['key'],
+            description: $data['description'] ?? null,
+            optional: $data['optional'] ?? false,
+            metadata: $data['metadata'] ?? [],
+        );
+    }
+}
diff --git a/php/Mcp/Exceptions/MissingDependencyException.php b/php/Mcp/Exceptions/MissingDependencyException.php
new file mode 100644
index 00000000..04ae0115
--- /dev/null
+++ b/php/Mcp/Exceptions/MissingDependencyException.php
@@ -0,0 +1,62 @@
+<?php
+
+// SPDX-License-Identifier: EUPL-1.2
+
+declare(strict_types=1);
+
+namespace Core\Mcp\Exceptions;
+
+use RuntimeException;
+
+/**
+ * Thrown when a tool's declared dependencies are not satisfied.
+ *
+ * Shaped for this repo's ToolDependencyService, not copied from dappcore/mcp's
+ * version of the same class. That one takes
+ * (string $toolName, array $missingDependencies, array $suggestedOrder) and
+ * builds its own message, while this service raises it with a single,
+ * already-composed message:
+ *
+ *     $exceptionClass = 'Core\Mcp\Exceptions\MissingDependencyException';
+ *     if (class_exists($exceptionClass)) {
+ *         return new $exceptionClass($message);
+ *     }
+ *
+ * Rolling the upstream signature in verbatim would have turned a class-not-found
+ * into an ArgumentCountError the first time a dependency went unmet — a
+ * different fatal, not a fix. The extra detail stays available as optional
+ * constructor arguments, so callers that have it can pass it.
+ */
+class MissingDependencyException extends RuntimeException
+{
+    /**
+     * @param  array<int, array{tool: string, type: string, key: string, message: string}>  $missingDependencies
+     *                                                                                                            The rows ToolDependencyService::missing() returns — arrays, not
+     *                                                                                                            ToolDependency objects, because that is what this service produces.
+     * @param  array<int, string>  $suggestedOrder  Tools worth calling first.
+     */
+    public function __construct(
+        string $message,
+        public readonly array $missingDependencies = [],
+        public readonly string $toolName = '',
+        public readonly array $suggestedOrder = [],
+    ) {
+        parent::__construct($message);
+    }
+
+    /**
+     * The dependency keys that were not satisfied.
+     *
+     * @return array<int, string>
+     *
+     * @example
+     * $exception->missingKeys(); // ['workspace_id']
+     */
+    public function missingKeys(): array
+    {
+        return array_values(array_filter(array_map(
+            static fn (array $dependency): string => (string) ($dependency['key'] ?? ''),
+            $this->missingDependencies,
+        )));
+    }
+}
diff --git a/php/Mcp/Services/ToolDependencyService.php b/php/Mcp/Services/ToolDependencyService.php
index bc2ea2bf..7dd36cee 100644
--- a/php/Mcp/Services/ToolDependencyService.php
+++ b/php/Mcp/Services/ToolDependencyService.php
@@ -7,6 +7,7 @@
 namespace Core\Mod\Agentic\Mcp\Services;
 
 use Carbon\CarbonImmutable;
+use Core\Mcp\Dependencies\ToolDependency;
 use Illuminate\Container\Container;
 use InvalidArgumentException;
 use RuntimeException;
@@ -272,6 +273,21 @@ private function normaliseDependency(mixed $dependency): array
             ];
         }
 
+        if ($dependency instanceof ToolDependency) {
+            // toArray(), not get_object_vars(): the object holds $type as a
+            // DependencyType enum and names its text $description. The array
+            // branch below casts type to string — which fatals on an enum — and
+            // looks for 'message', so a raw property dump both breaks and
+            // silently loses the description.
+            $fields = $dependency->toArray();
+            $fields['message'] = $fields['description'] ?? null;
+
+            return $this->normaliseDependency(array_filter(
+                $fields,
+                static fn (mixed $value): bool => $value !== null,
+            ));
+        }
+
         if (is_object($dependency)) {
             $vars = get_object_vars($dependency);
 
diff --git a/php/Mcp/Tools/Concerns/ValidatesDependencies.php b/php/Mcp/Tools/Concerns/ValidatesDependencies.php
new file mode 100644
index 00000000..eb462a41
--- /dev/null
+++ b/php/Mcp/Tools/Concerns/ValidatesDependencies.php
@@ -0,0 +1,158 @@
+<?php
+
+// SPDX-License-Identifier: EUPL-1.2
+
+declare(strict_types=1);
+
+namespace Core\Mcp\Tools\Concerns;
+
+use Core\Mcp\Dependencies\ToolDependency;
+use Core\Mcp\Exceptions\MissingDependencyException;
+use Core\Mod\Agentic\Mcp\Services\ToolDependencyService;
+
+/**
+ * Inline dependency validation for tools.
+ *
+ * AgentTool has used this trait since it was written, but the trait was never
+ * present in this repository — it lives in dappcore/mcp, which is not a
+ * dependency here. Every tool extending AgentTool was therefore fatal on load
+ * ("Trait ... not found"), so none of the forty registered tool classes could
+ * be constructed in any environment.
+ *
+ * Written against THIS repository's ToolDependencyService rather than copied
+ * from upstream, because the two services share a name and not an API:
+ *
+ *   upstream trait calls        this service provides
+ *   ------------------------    ------------------------------------------
+ *   validateDependencies(...)   validateDependencies(mixed ...$arguments)
+ *                               — variadic, so upstream's named arguments
+ *                                 (sessionId:, toolName:, args:) bind to
+ *                                 nothing at all
+ *   checkDependencies(...)      canExecute($toolId, $context, $args, $session)
+ *   getMissingDependencies(...) missing($toolId, $context, $args, $session)
+ *   recordToolCall(...)         recordToolCall($sessionId, $toolId, $payload)
+ *
+ * A verbatim roll would have swapped a missing trait for a missing method.
+ */
+trait ValidatesDependencies
+{
+    /**
+     * Dependencies this tool requires. Override to declare them.
+     *
+     * @return array<int, ToolDependency>
+     *
+     * @example
+     * return [ToolDependency::contextExists('workspace_id')];
+     */
+    public function dependencies(): array
+    {
+        return [];
+    }
+
+    /**
+     * Throw unless every declared dependency is satisfied.
+     *
+     * Positional, not named: the service takes mixed ...$arguments and parses
+     * them by position and type.
+     *
+     * @throws MissingDependencyException
+     *
+     * @example
+     * $this->validateDependencies(['session_id' => 'abc'], $args);
+     */
+    protected function validateDependencies(array $context = [], array $args = []): void
+    {
+        app(ToolDependencyService::class)->validateDependencies(
+            $this->dependencySessionId($context),
+            $this->name(),
+            $context,
+            $args,
+        );
+    }
+
+    /**
+     * Whether every declared dependency is satisfied, without throwing.
+     *
+     * @example
+     * if (! $this->dependenciesMet($context, $args)) { ... }
+     */
+    protected function dependenciesMet(array $context = [], array $args = []): bool
+    {
+        return app(ToolDependencyService::class)->canExecute(
+            $this->name(),
+            $context,
+            $args,
+            $this->dependencySessionId($context),
+        );
+    }
+
+    /**
+     * The dependencies that are not satisfied.
+     *
+     * Returns the service's own row shape — arrays of
+     * {tool, type, key, message} — rather than ToolDependency objects, because
+     * that is what it produces.
+     *
+     * @return array<int, array{tool: string, type: string, key: string, message: string}>
+     *
+     * @example
+     * $missing = $this->getMissingDependencies($context, $args);
+     */
+    protected function getMissingDependencies(array $context = [], array $args = []): array
+    {
+        return app(ToolDependencyService::class)->missing(
+            $this->name(),
+            $context,
+            $args,
+            $this->dependencySessionId($context),
+        );
+    }
+
+    /**
+     * Record this call so later tools can depend on it having happened.
+     *
+     * @example
+     * $this->recordToolCall($context, $args);
+     */
+    protected function recordToolCall(array $context = [], array $args = []): void
+    {
+        app(ToolDependencyService::class)->recordToolCall(
+            $this->dependencySessionId($context),
+            $this->name(),
+            $args,
+        );
+    }
+
+    /**
+     * Shape an unmet-dependency failure as a tool error response.
+     *
+     * @example
+     * return $this->dependencyError($exception);
+     */
+    protected function dependencyError(MissingDependencyException $exception): array
+    {
+        return [
+            'error' => 'dependency_not_met',
+            'message' => $exception->getMessage(),
+            'missing' => array_map(
+                static fn (array $dependency): array => [
+                    'type' => $dependency['type'] ?? 'unknown',
+                    'key' => $dependency['key'] ?? '',
+                    'description' => $dependency['message'] ?? '',
+                ],
+                $exception->missingDependencies,
+            ),
+            'suggested_order' => $exception->suggestedOrder,
+        ];
+    }
+
+    /**
+     * The session a dependency check is scoped to.
+     */
+    private function dependencySessionId(array $context): string
+    {
+        $sessionId = $context['session_id'] ?? null;
+
+        return is_string($sessionId) && $sessionId !== '' ? $sessionId : 'anonymous';
+    }
+}
diff --git a/php/Services/AgentToolRegistry.php b/php/Services/AgentToolRegistry.php
index 1db01d03..0dbbcd3d 100644
--- a/php/Services/AgentToolRegistry.php
+++ b/php/Services/AgentToolRegistry.php
@@ -6,7 +6,8 @@
 
 use Core\Api\Models\ApiKey;
 use Core\Mcp\Dependencies\HasDependencies;
-use Core\Mcp\Services\ToolDependencyService;
+use Core\Mcp\Exceptions\MissingDependencyException;
+use Core\Mod\Agentic\Mcp\Services\ToolDependencyService;
 use Core\Mod\Agentic\Mcp\Tools\Agent\Contracts\AgentToolInterface;
 use Illuminate\Support\Collection;
 use Illuminate\Support\Facades\Cache;
@@ -181,7 +182,7 @@ public function apiKeyCanAccessTool(ApiKey $apiKey, string $toolName): bool
      *
      * @throws \InvalidArgumentException If tool not found
      * @throws \RuntimeException If permission denied
-     * @throws \Core\Mcp\Exceptions\MissingDependencyException If dependencies not met
+     * @throws MissingDependencyException If dependencies not met
      */
     public function execute(
         string $name,