diff --git a/mateclaw-server/src/main/java/vip/mate/channel/MediaPathGuard.java b/mateclaw-server/src/main/java/vip/mate/channel/MediaPathGuard.java
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+++ b/mateclaw-server/src/main/java/vip/mate/channel/MediaPathGuard.java
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+package vip.mate.channel;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Locale;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * RFC-03 Lane K3 — centralized validation for media files passing through
+ * channel adapters (incoming attachments + outgoing replies).
+ *
+ * <p>Each {@code *ChannelAdapter} (DingTalk / Feishu / Discord / Telegram /
+ * QQ / WeCom / Weixin / Slack) historically inlined its own ad-hoc set of
+ * checks — extension allowlist here, size cap there, no path-traversal
+ * guard at all. The result is 8 places to keep in sync when a new file
+ * type is supported (or when a new exploit needs a defensive patch).
+ * Same gap as QwenPaw #1220.
+ *
+ * <p>This guard centralizes four checks that every adapter needs:
+ * <ol>
+ *   <li><b>Path containment</b> — the file resolves under the configured
+ *       workspace root (after symlink resolution). Blocks {@code ../}
+ *       traversal even when the attacker controls just a filename string.</li>
+ *   <li><b>Existence + regular file</b> — not a directory, not a symlink
+ *       to a special device.</li>
+ *   <li><b>Extension allowlist</b> — case-insensitive, dot-prefix tolerant.</li>
+ *   <li><b>Size cap</b> — checked once via {@link Files#size(Path)} so a
+ *       gigabyte-plus file gets rejected before any adapter sends it.</li>
+ * </ol>
+ *
+ * <p>Adapter migration is staged: this utility lands and ships with tests,
+ * then individual adapters convert their inline checks one PR at a time
+ * (Phase 1 starts with DingTalk per the RFC). Keeping this class
+ * single-purpose so it doesn't depend on any adapter-specific service
+ * means the migration can proceed without ripple-edits.
+ */
+public final class MediaPathGuard {
+
+    private MediaPathGuard() {}
+
+    /**
+     * Lookup-friendly representation of a violation. The {@link #reason}
+     * codes are stable so audit logs / metrics can group failures without
+     * parsing the message text.
+     */
+    public enum Reason {
+        PATH_OUTSIDE_WORKSPACE,
+        FILE_MISSING,
+        NOT_A_REGULAR_FILE,
+        EXTENSION_NOT_ALLOWED,
+        FILE_TOO_LARGE,
+        IO_ERROR
+    }
+
+    /** Validation policy — immutable, share across adapter calls. */
+    public record Policy(
+            Path workspaceRoot,
+            Set<String> allowedExtensions,
+            long maxBytes
+    ) {
+        public Policy {
+            Objects.requireNonNull(workspaceRoot, "workspaceRoot must not be null");
+            Objects.requireNonNull(allowedExtensions, "allowedExtensions must not be null");
+            if (maxBytes <= 0) {
+                throw new IllegalArgumentException("maxBytes must be positive: " + maxBytes);
+            }
+            // Normalize allowed extensions: lowercase, strip leading dots.
+            allowedExtensions = Set.copyOf(allowedExtensions.stream()
+                    .filter(Objects::nonNull)
+                    .map(e -> e.toLowerCase(Locale.ROOT))
+                    .map(e -> e.startsWith(".") ? e.substring(1) : e)
+                    .toList());
+        }
+    }
+
+    /** Thrown when validation fails — caller decides whether to log + drop
+     *  silently or surface to the user. */
+    public static final class MediaValidationException extends RuntimeException {
+        private final Reason reason;
+
+        public MediaValidationException(Reason reason, String message) {
+            super(message);
+            this.reason = reason;
+        }
+
+        public MediaValidationException(Reason reason, String message, Throwable cause) {
+            super(message, cause);
+            this.reason = reason;
+        }
+
+        public Reason reason() {
+            return reason;
+        }
+    }
+
+    /**
+     * Validate {@code file} against {@code policy}. Returns the canonical
+     * (real, symlink-resolved) path on success — callers that need to act
+     * on the file (read bytes, upload to platform) should use the returned
+     * value to avoid TOCTOU between validation and use.
+     *
+     * @throws MediaValidationException with a {@link Reason} that maps 1:1
+     *         to the four documented checks; the message is human-readable
+     *         and includes the offending path / extension / size.
+     */
+    public static Path validate(Path file, Policy policy) {
+        Objects.requireNonNull(file, "file must not be null");
+        Objects.requireNonNull(policy, "policy must not be null");
+
+        Path canonicalRoot;
+        Path canonicalFile;
+        try {
+            canonicalRoot = policy.workspaceRoot().toRealPath();
+        } catch (IOException e) {
+            throw new MediaValidationException(Reason.IO_ERROR,
+                    "workspace root cannot be resolved: " + policy.workspaceRoot(), e);
+        }
+        try {
+            canonicalFile = file.toRealPath();
+        } catch (IOException e) {
+            // toRealPath fails when file does not exist OR when traversal
+            // hits a non-readable segment. Distinguish the common case
+            // (missing file) for clearer audit output.
+            if (!Files.exists(file)) {
+                throw new MediaValidationException(Reason.FILE_MISSING,
+                        "file does not exist: " + file, e);
+            }
+            throw new MediaValidationException(Reason.IO_ERROR,
+                    "file path cannot be resolved: " + file, e);
+        }
+
+        // Containment: canonicalFile must be inside canonicalRoot. startsWith on
+        // Path does element-wise comparison so "/ws-foo" doesn't mistakenly look
+        // like a prefix of "/ws-foobar".
+        if (!canonicalFile.startsWith(canonicalRoot)) {
+            throw new MediaValidationException(Reason.PATH_OUTSIDE_WORKSPACE,
+                    "file is outside the workspace: " + canonicalFile + " not under " + canonicalRoot);
+        }
+
+        if (!Files.isRegularFile(canonicalFile)) {
+            throw new MediaValidationException(Reason.NOT_A_REGULAR_FILE,
+                    "not a regular file (directory / device / fifo): " + canonicalFile);
+        }
+
+        String ext = extensionOf(canonicalFile);
+        if (!policy.allowedExtensions().contains(ext)) {
+            throw new MediaValidationException(Reason.EXTENSION_NOT_ALLOWED,
+                    "extension '" + ext + "' not in allowlist; got " + canonicalFile);
+        }
+
+        long size;
+        try {
+            size = Files.size(canonicalFile);
+        } catch (IOException e) {
+            throw new MediaValidationException(Reason.IO_ERROR,
+                    "cannot read file size: " + canonicalFile, e);
+        }
+        if (size > policy.maxBytes()) {
+            throw new MediaValidationException(Reason.FILE_TOO_LARGE,
+                    "file size " + size + " exceeds policy cap " + policy.maxBytes() + ": " + canonicalFile);
+        }
+
+        return canonicalFile;
+    }
+
+    /** Lowercase extension without leading dot; empty string when there is none. */
+    static String extensionOf(Path file) {
+        String name = file.getFileName().toString();
+        int dot = name.lastIndexOf('.');
+        if (dot < 0 || dot == name.length() - 1) return "";
+        return name.substring(dot + 1).toLowerCase(Locale.ROOT);
+    }
+}