diff --git a/crates/base/test_cases/trace-context-callee/index.ts b/crates/base/test_cases/trace-context-callee/index.ts
new file mode 100644
index 000000000..90b6009f4
--- /dev/null
+++ b/crates/base/test_cases/trace-context-callee/index.ts
@@ -0,0 +1,8 @@
+Deno.serve((req: Request) =>
+  Response.json({
+    traceparent: req.headers.get("traceparent"),
+    tracestate: req.headers.get("tracestate"),
+    baggage: req.headers.get("baggage"),
+    custom: req.headers.get("x-app-traceparent"),
+  })
+);
diff --git a/crates/base/test_cases/trace-context-caller/index.ts b/crates/base/test_cases/trace-context-caller/index.ts
new file mode 100644
index 000000000..6f76decc9
--- /dev/null
+++ b/crates/base/test_cases/trace-context-caller/index.ts
@@ -0,0 +1,53 @@
+import { createClient } from "npm:@supabase/supabase-js@2.112.3";
+
+const traceparent = "00-b1e669305668dc82c96cc31ce2e6cd99-1a5b74eca03f769f-01";
+
+Deno.serve(async (req: Request) => {
+  const origin = new URL(req.url).origin;
+  const client = createClient(origin, "test-key", {
+    auth: {
+      persistSession: false,
+      autoRefreshToken: false,
+      detectSessionInUrl: false,
+    },
+  });
+
+  const results = await Promise.all(
+    ["fetch", "request", "invoke"].flatMap((method) =>
+      ["present", "empty", "absent"].map(async (state) => {
+        const headers: Record<string, string> = {
+          // Exercise case-insensitive detection and an unrelated header.
+          TraceParent: traceparent,
+          "x-app-traceparent": traceparent,
+          baggage: "caller=value",
+        };
+        if (state !== "absent") {
+          headers.TraceState = state === "empty" ? "" : "caller=value";
+        }
+
+        let data;
+        if (method === "invoke") {
+          const result = await client.functions.invoke("callee", {
+            headers,
+            body: { hello: "world" },
+          });
+          if (result.error) throw result.error;
+          data = result.data;
+        } else {
+          const url = `${origin}/callee`;
+          const response = method === "request"
+            ? await fetch(new Request(url, { headers }))
+            : await fetch(url, { headers });
+          if (!response.ok) throw new Error(await response.text());
+          data = await response.json();
+        }
+        return { method, state, ...data };
+      })
+    ),
+  );
+
+  // Automatic propagation must still work when no context is supplied.
+  const automatic = await fetch(`${origin}/callee`);
+  if (!automatic.ok) throw new Error(await automatic.text());
+  return Response.json({ results, automatic: await automatic.json() });
+});
diff --git a/crates/base/test_cases/trace-context-main/index.ts b/crates/base/test_cases/trace-context-main/index.ts
new file mode 100644
index 000000000..18262a65f
--- /dev/null
+++ b/crates/base/test_cases/trace-context-main/index.ts
@@ -0,0 +1,20 @@
+Deno.serve(async (req: Request) => {
+  const name = new URL(req.url).pathname.split("/").pop();
+  const isCaller = name === "caller" || name === "caller-untraced";
+  if (!isCaller && name !== "callee") {
+    return new Response("Not found", { status: 404 });
+  }
+
+  const worker = await EdgeRuntime.userWorkers.create({
+    servicePath: `./test_cases/trace-context-${isCaller ? "caller" : "callee"}`,
+    memoryLimitMb: 150,
+    workerTimeoutMs: 60_000,
+    cpuTimeSoftLimitMs: 60_000,
+    cpuTimeHardLimitMs: 60_000,
+    otelConfig: {
+      tracing_enabled: name !== "caller-untraced",
+      propagators: ["TraceContext", "Baggage"],
+    },
+  });
+  return await worker.fetch(req);
+});
diff --git a/crates/base/tests/integration_tests.rs b/crates/base/tests/integration_tests.rs
index 0cc559a6c..c41bac686 100644
--- a/crates/base/tests/integration_tests.rs
+++ b/crates/base/tests/integration_tests.rs
@@ -4271,6 +4271,87 @@ async fn test_brotli_async() {
   );
 }
 
+async fn assert_fetch_trace_context(tracing_enabled: bool) {
+  init_otel();
+  let caller = if tracing_enabled {
+    "caller"
+  } else {
+    "caller-untraced"
+  };
+
+  integration_test_with_server_flag!(
+    ServerFlags::default(),
+    "./test_cases/trace-context-main",
+    NON_SECURE_PORT,
+    caller,
+    None,
+    Some(
+      reqwest::Client::new()
+        .get(format!("http://localhost:{NON_SECURE_PORT}/{caller}"))
+        .header(
+          "traceparent",
+          "00-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-bbbbbbbbbbbbbbbb-01",
+        )
+        .header("tracestate", "upstream=value")
+        .header("baggage", "tenant=test")
+    ),
+    None::<Tls>,
+    (|resp| async {
+      let resp = resp.unwrap();
+      let status = resp.status();
+      let body = resp.text().await.unwrap();
+      assert_eq!(status.as_u16(), 200, "{body}");
+      let data: serde_json::Value = serde_json::from_str(&body).unwrap();
+      let results = data["results"].as_array().unwrap();
+      assert_eq!(results.len(), 9);
+      for result in results {
+        assert_eq!(
+          result["traceparent"],
+          "00-b1e669305668dc82c96cc31ce2e6cd99-1a5b74eca03f769f-01",
+          "{result}",
+        );
+        assert_eq!(result["custom"], result["traceparent"]);
+        match result["state"].as_str().unwrap() {
+          "present" => assert_eq!(result["tracestate"], "caller=value"),
+          "empty" => assert_eq!(result["tracestate"], ""),
+          "absent" => assert!(result["tracestate"].is_null()),
+          state => panic!("unexpected state: {state}"),
+        }
+        assert_eq!(result["baggage"], "caller=value");
+      }
+      let automatic = &data["automatic"];
+      if tracing_enabled {
+        let traceparent = automatic["traceparent"].as_str().unwrap();
+        let parts: Vec<_> = traceparent.split('-').collect();
+        assert_eq!(parts.len(), 4);
+        assert_eq!(parts[0], "00");
+        assert_eq!(parts[1], "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+        assert_eq!(parts[2].len(), 16);
+        assert!(parts[2].bytes().all(|byte| byte.is_ascii_hexdigit()));
+        assert_ne!(parts[2], "0000000000000000");
+        assert_eq!(parts[3], "01");
+      } else {
+        assert!(automatic["traceparent"].is_null());
+        assert!(automatic["tracestate"].is_null());
+        assert!(automatic["baggage"].is_null());
+      }
+    }),
+    TerminationToken::new()
+  );
+}
+
+#[tokio::test]
+#[serial]
+async fn test_fetch_preserves_explicit_trace_context() {
+  assert_fetch_trace_context(true).await;
+}
+
+#[tokio::test]
+#[serial]
+async fn test_fetch_preserves_explicit_trace_context_without_tracing() {
+  assert_fetch_trace_context(false).await;
+}
+
 async fn assert_rate_limit_error(
   resp: Result<reqwest::Response, reqwest::Error>,
 ) {
diff --git a/vendor/deno_fetch/26_fetch.js b/vendor/deno_fetch/26_fetch.js
index 516e2cd26..72a2e4cff 100644
--- a/vendor/deno_fetch/26_fetch.js
+++ b/vendor/deno_fetch/26_fetch.js
@@ -376,6 +376,15 @@ function fetch(input, init = { __proto__: null }) {
       if (span) {
         const context = ContextManager.active();
         for (const propagator of new SafeArrayIterator(PROPAGATORS)) {
+          // Explicit trace context belongs to the caller. Appending another
+          // traceparent makes it invalid, and injecting the active tracestate
+          // would associate state from a different trace with that context.
+          if (
+            requestObject.headers.has("traceparent") &&
+            ArrayPrototypeIncludes(propagator.fields(), "traceparent")
+          ) {
+            continue;
+          }
           propagator.inject(context, requestObject.headers, {
             set(carrier, key, value) {
               carrier.append(key, value);