fix(translation): encode data URI images as Anthropic base64 sources

crates/switchyard-translation/src/codecs/anthropic/buffered.rs

@@ -908,9 +908,13 @@ fn encode_one_anthropic_block(block: &ContentBlock) -> Vec<Value> {
             })]
         }
         ContentBlock::Image { source } => vec![match source {
-            ImageSource::Url { url, .. } => {
-                json!({"type": "image", "source": {"type": "url", "url": url}})
-            }
+            ImageSource::Url { url, .. } => match split_base64_data_uri(url) {
+                Some((media_type, data)) => json!({
+                    "type": "image",
+                    "source": {"type": "base64", "media_type": media_type, "data": data},
+                }),
+                None => json!({"type": "image", "source": {"type": "url", "url": url}}),
+            },
             ImageSource::Base64 { media_type, data } => json!({
                 "type": "image",
                 "source": {
@@ -990,6 +994,22 @@ fn encode_one_anthropic_tool_result_block(block: &ContentBlock) -> Vec<Value> {
     }
 }
 
+// Splits `data:<media type>[;<parameter>...];base64,<payload>`, the form OpenAI-compatible
+// clients use for inline images. A percent-encoded payload has to be decoded before it is
+// base64, and a data URI with no media type leaves Anthropic's `media_type` with nothing to
+// carry, so both keep the handling they had before and are relayed as-is.
+fn split_base64_data_uri(url: &str) -> Option<(&str, &str)> {
+    let (metadata, data) = url.strip_prefix("data:")?.split_once(',')?;
+    let parameters = metadata.strip_suffix(";base64")?;
+    let media_type = parameters
+        .split_once(';')
+        .map_or(parameters, |(media_type, _)| media_type);
+    if media_type.is_empty() || data.contains('%') {
+        return None;
+    }
+    Some((media_type, data))
+}
+
 // Anthropic requires `tool_use.input` to be object-shaped, while OpenAI and
 // Responses commonly carry function-call arguments as JSON strings.
 fn anthropic_tool_input(arguments: &Value) -> Value {

crates/switchyard-translation/tests/request_translation.rs

@@ -1581,6 +1581,60 @@ fn anthropic_unmappable_output_format_is_rejected_under_strict_policy() -> TestR
     Ok(())
 }
 
+// Verifies inline `data:` images become Anthropic base64 sources, since Anthropic's URL
+// source rejects anything that is not an http(s) link. A percent-encoded payload is not raw
+// base64, so it keeps the URL source it had before rather than reaching Anthropic mislabelled.
+#[test]
+fn openai_data_uri_images_translate_to_anthropic_sources() -> TestResult {
+    let engine = TranslationEngine::default();
+    let cases = [
+        (
+            "base64 data URI",
+            json!({"url": "data:image/png;base64,aW1hZ2U=", "detail": "high"}),
+            json!({"type": "base64", "media_type": "image/png", "data": "aW1hZ2U="}),
+        ),
+        (
+            "data URI carrying extra parameters",
+            json!({"url": "data:image/jpeg;charset=binary;base64,aW1hZ2U="}),
+            json!({"type": "base64", "media_type": "image/jpeg", "data": "aW1hZ2U="}),
+        ),
+        (
+            "percent-encoded data URI",
+            json!({"url": "data:image/png;base64,YQ%3D%3D"}),
+            json!({"type": "url", "url": "data:image/png;base64,YQ%3D%3D"}),
+        ),
+    ];
+
+    for (case, image_url, expected_source) in cases {
+        let body = json!({
+            "model": "claude-sonnet-4-20250514",
+            "messages": [{
+                "role": "user",
+                "content": [
+                    {"type": "text", "text": "What color is this?"},
+                    {"type": "image_url", "image_url": image_url}
+                ]
+            }]
+        });
+
+        let output = engine
+            .translate_request(
+                WireFormat::OpenAiChat,
+                WireFormat::AnthropicMessages,
+                &body,
+                &TranslationPolicy::default(),
+            )?
+            .body;
+
+        assert_eq!(
+            output["messages"][0]["content"][1],
+            json!({"type": "image", "source": expected_source}),
+            "{case}"
+        );
+    }
+    Ok(())
+}
+
 // Verifies Anthropic receives its supported schema subset without mutating the neutral contract.
 #[test]
 fn openai_schema_constraints_are_removed_from_anthropic_output_format() -> TestResult {