diff --git a/sdk-core/api/sdk-core.api b/sdk-core/api/sdk-core.api
index de3a5d14..729e204b 100644
--- a/sdk-core/api/sdk-core.api
+++ b/sdk-core/api/sdk-core.api
@@ -201,12 +201,14 @@ public final class org/dexpace/sdk/core/http/auth/KeyCredential : org/dexpace/sd
 	public final fun getApiKey ()Ljava/lang/String;
 	public final fun getHeaderName ()Lorg/dexpace/sdk/core/http/common/HttpHeaderName;
 	public final fun getPrefix ()Ljava/lang/String;
+	public fun toString ()Ljava/lang/String;
 }
 
 public final class org/dexpace/sdk/core/http/auth/NamedKeyCredential : org/dexpace/sdk/core/http/auth/Credential {
 	public fun <init> (Ljava/lang/String;Ljava/lang/String;)V
 	public final fun getKey ()Ljava/lang/String;
 	public final fun getName ()Ljava/lang/String;
+	public fun toString ()Ljava/lang/String;
 }
 
 public final class org/dexpace/sdk/core/http/common/CommonMediaTypes {
diff --git a/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/KeyCredential.kt b/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/KeyCredential.kt
index 92513063..b5c9c1ba 100644
--- a/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/KeyCredential.kt
+++ b/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/KeyCredential.kt
@@ -34,4 +34,12 @@ public class KeyCredential
         init {
             require(apiKey.isNotBlank()) { "apiKey must not be blank" }
         }
+
+        /**
+         * Redacts the secret [apiKey]. Without this override any log line, exception message,
+         * or debugger that stringifies the credential would expose the key; this emits
+         * `apiKey=***` instead while keeping the non-secret [headerName] and [prefix] for
+         * diagnostics. Identity equality is unaffected.
+         */
+        override fun toString(): String = "KeyCredential(apiKey=***, headerName=$headerName, prefix=$prefix)"
     }
diff --git a/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/NamedKeyCredential.kt b/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/NamedKeyCredential.kt
index 2fb69f1b..e5e2be54 100644
--- a/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/NamedKeyCredential.kt
+++ b/sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/auth/NamedKeyCredential.kt
@@ -24,4 +24,12 @@ public class NamedKeyCredential(public val name: String, public val key: String)
         require(name.isNotBlank()) { "name must not be blank" }
         require(key.isNotBlank()) { "key must not be blank" }
     }
+
+    /**
+     * Redacts the secret [key]. Without this override any log line, exception message, or
+     * debugger that stringifies the credential would expose the key; this emits `key=***`
+     * instead while keeping the non-secret [name] for diagnostics. Identity equality is
+     * unaffected.
+     */
+    override fun toString(): String = "NamedKeyCredential(name=$name, key=***)"
 }
diff --git a/sdk-core/src/test/kotlin/org/dexpace/sdk/core/http/auth/CredentialTest.kt b/sdk-core/src/test/kotlin/org/dexpace/sdk/core/http/auth/CredentialTest.kt
index 150d5fff..cad9f7ac 100644
--- a/sdk-core/src/test/kotlin/org/dexpace/sdk/core/http/auth/CredentialTest.kt
+++ b/sdk-core/src/test/kotlin/org/dexpace/sdk/core/http/auth/CredentialTest.kt
@@ -11,7 +11,10 @@ import org.dexpace.sdk.core.http.common.HttpHeaderName
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
+import kotlin.test.assertFalse
+import kotlin.test.assertNotEquals
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 
 class CredentialTest {
     // ----------------- KeyCredential -----------------
@@ -54,6 +57,31 @@ class CredentialTest {
         assertEquals("k", (cred as KeyCredential).apiKey)
     }
 
+    @Test
+    fun `KeyCredential toString redacts the apiKey but keeps non-secret fields`() {
+        val cred = KeyCredential("super-secret-key", prefix = "SharedAccessKey")
+        val rendered = cred.toString()
+        assertFalse(rendered.contains("super-secret-key"), "toString must not contain the raw apiKey")
+        assertTrue(rendered.contains("apiKey=***"), "toString must redact the apiKey")
+        assertTrue(rendered.contains("SharedAccessKey"), "toString should keep the non-secret prefix for diagnostics")
+        assertTrue(
+            rendered.contains(HttpHeaderName.AUTHORIZATION.toString()),
+            "toString should keep the non-secret headerName for diagnostics",
+        )
+    }
+
+    @Test
+    fun `KeyCredential toString redacts the apiKey with default fields`() {
+        val cred = KeyCredential("another-secret")
+        val rendered = cred.toString()
+        assertFalse(rendered.contains("another-secret"), "toString must not contain the raw apiKey")
+        assertTrue(rendered.contains("apiKey=***"), "toString must redact the apiKey")
+        assertTrue(
+            rendered.contains(HttpHeaderName.AUTHORIZATION.toString()),
+            "toString should keep the default headerName for diagnostics",
+        )
+    }
+
     // ----------------- NamedKeyCredential -----------------
 
     @Test
@@ -92,4 +120,30 @@ class CredentialTest {
         val cred: Credential = NamedKeyCredential("acct", "secret")
         assertEquals("acct", (cred as NamedKeyCredential).name)
     }
+
+    @Test
+    fun `NamedKeyCredential toString redacts the key but keeps the name`() {
+        val cred = NamedKeyCredential("acct-name", "super-secret-key")
+        val rendered = cred.toString()
+        assertFalse(rendered.contains("super-secret-key"), "toString must not contain the raw key")
+        assertTrue(rendered.contains("key=***"), "toString must redact the key")
+        assertTrue(rendered.contains("acct-name"), "toString should keep the non-secret name for diagnostics")
+    }
+
+    @Test
+    fun `NamedKeyCredential keeps identity equality - redaction is toString-only`() {
+        // These are reference types (not data classes): equality is identity-based and the
+        // redacting toString override must not change that. Same instance equals itself;
+        // distinct instances with the same fields are not equal.
+        val cred = NamedKeyCredential("acct", "secret")
+        assertEquals(cred, cred)
+        assertNotEquals<NamedKeyCredential>(cred, NamedKeyCredential("acct", "secret"))
+    }
+
+    @Test
+    fun `KeyCredential keeps identity equality - redaction is toString-only`() {
+        val cred = KeyCredential("secret-key")
+        assertEquals(cred, cred)
+        assertNotEquals<KeyCredential>(cred, KeyCredential("secret-key"))
+    }
 }