From cb839125e0e7d9273329f60ea4c2ea217149c759 Mon Sep 17 00:00:00 2001
From: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>
Date: Mon, 30 Mar 2026 16:50:45 +0000
Subject: [PATCH] fix: scope version parsing to the openmetrics media type in
 Accept header

parseOpenMetricsVersion was splitting the entire Accept header by ";"
which meant version parameters from other media types (e.g. text/plain;
version=0.0.4) could be incorrectly attributed to openmetrics-text.

Now splits by "," first to isolate individual media types, finds the
openmetrics-text entry, then parses only its parameters for version.
Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>
---
 .../expositionformats/ExpositionFormats.java  | 19 +++++++-----
 .../ExpositionFormatsTest.java                | 31 +++++++++++++++++++
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/ExpositionFormats.java b/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/ExpositionFormats.java
index 10556859d..3863b5b74 100644
--- a/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/ExpositionFormats.java
+++ b/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/ExpositionFormats.java
@@ -108,14 +108,19 @@ private static String parseOpenMetricsVersion(@Nullable String acceptHeader) {
     if (acceptHeader == null) {
       return null;
     }
-    for (String mediaType : acceptHeader.split(";")) {
-      String[] tokens = mediaType.split("=");
-      if (tokens.length == 2) {
-        String key = tokens[0].trim();
-        String value = tokens[1].trim();
-        if (key.equals("version")) {
-          return value;
+    for (String mediaType : acceptHeader.split(",")) {
+      if (mediaType.contains("application/openmetrics-text")) {
+        for (String param : mediaType.split(";")) {
+          String[] tokens = param.split("=");
+          if (tokens.length == 2) {
+            String key = tokens[0].trim();
+            String value = tokens[1].trim();
+            if (key.equals("version")) {
+              return value;
+            }
+          }
         }
+        return null;
       }
     }
     return null;
diff --git a/prometheus-metrics-exposition-textformats/src/test/java/io/prometheus/metrics/expositionformats/ExpositionFormatsTest.java b/prometheus-metrics-exposition-textformats/src/test/java/io/prometheus/metrics/expositionformats/ExpositionFormatsTest.java
index 66e49b385..e0782251e 100644
--- a/prometheus-metrics-exposition-textformats/src/test/java/io/prometheus/metrics/expositionformats/ExpositionFormatsTest.java
+++ b/prometheus-metrics-exposition-textformats/src/test/java/io/prometheus/metrics/expositionformats/ExpositionFormatsTest.java
@@ -231,6 +231,37 @@ void testOM2ContentNegotiationDisabled() {
     assertThat(writer3).isInstanceOf(OpenMetrics2TextFormatWriter.class);
   }
 
+  @Test
+  void testOM2ContentNegotiationMultiTypeAcceptHeaderWithoutOMVersion() {
+    // When the Accept header has multiple media types and only text/plain has a version,
+    // the openmetrics type should be treated as having no version (use OM1).
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(true).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer =
+        formats.findWriter("application/openmetrics-text, text/plain; version=0.0.4");
+    assertThat(writer).isInstanceOf(OpenMetricsTextFormatWriter.class);
+  }
+
+  @Test
+  void testOM2ContentNegotiationMultiTypeAcceptHeaderWithOMVersion() {
+    // When the Accept header has multiple media types and openmetrics has version=2.0.0,
+    // the OM2 writer should be selected.
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(true).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer =
+        formats.findWriter(
+            "application/openmetrics-text; version=2.0.0, text/plain; version=0.0.4");
+    assertThat(writer).isInstanceOf(OpenMetrics2TextFormatWriter.class);
+  }
+
   @Test
   void testCounterComplete() throws IOException {
     String openMetricsText =