softwaremill · jonoabroad · Aug 11, 2026 · Aug 11, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -12,7 +12,7 @@ sttp-ai is a Scala library providing a non-official client wrapper for OpenAI, C
 - Native Gemini (Google) API support via the Interactions API with dedicated module
 - OpenAI-compatible API support (Ollama, Grok, OpenRouter, etc.)
 - Streaming support for all major effect systems
-- Cross-platform: Scala 2.13.18 and Scala 3.3.8
+- Cross-platform: Scala 2.12.20, 2.13.18 and Scala 3.3.8
 - Agent loop tools loadable from [MCP](https://modelcontextprotocol.io) servers (`mcp` module, Scala 3 only, via [chimp](https://github.com/softwaremill/chimp)), in addition to manually defined `AgentTool`s
 
 ## Development Commands
@@ -137,7 +137,7 @@ Each streaming module (`streaming/{effect-system}/`) provides extensions for **a
   - Claude: Requires `ANTHROPIC_API_KEY`, minimal token usage
   - Gemini: Requires `GEMINI_API_KEY`, minimal token usage
   - Auto-skip if API key not set
-- **Cross-building**: sbt-projectmatrix for Scala 2.13.18 & 3.3.8
+- **Cross-building**: sbt-projectmatrix for Scala 2.12.20, 2.13.18 & 3.3.8
 
 ## Client Implementation Patterns
 

diff --git a/README.md b/README.md
@@ -40,7 +40,7 @@ Add the following dependency:
 // For streaming support, add one or more (these modules are shared across OpenAI, Claude, and Gemini):
 "com.softwaremill.sttp.ai" %% "fs2" % "0.8.0"    // cats-effect/fs2
 "com.softwaremill.sttp.ai" %% "zio" % "0.8.0"    // ZIO
-"com.softwaremill.sttp.ai" %% "akka" % "0.8.0"   // Akka Streams (Scala 2.13 only)
+"com.softwaremill.sttp.ai" %% "akka" % "0.8.0"   // Akka Streams (Scala 2.12/2.13 only)
 "com.softwaremill.sttp.ai" %% "pekko" % "0.8.0"  // Pekko Streams
 "com.softwaremill.sttp.ai" %% "ox" % "0.8.0"    // Ox direct-style (Scala 3 only)
 ```
@@ -55,12 +55,12 @@ Add the following dependency:
 // For streaming support, add one or more (these modules are shared across OpenAI, Claude, and Gemini):
 "com.softwaremill.sttp.ai" %% "fs2" % "0.8.0"    // cats-effect/fs2
 "com.softwaremill.sttp.ai" %% "zio" % "0.8.0"    // ZIO
-"com.softwaremill.sttp.ai" %% "akka" % "0.8.0"   // Akka Streams (Scala 2.13 only)
+"com.softwaremill.sttp.ai" %% "akka" % "0.8.0"   // Akka Streams (Scala 2.12/2.13 only)
 "com.softwaremill.sttp.ai" %% "pekko" % "0.8.0"  // Pekko Streams
 "com.softwaremill.sttp.ai" %% "ox" % "0.8.0"    // Ox direct-style (Scala 3 only)
 ```
 
-sttp-openai is available for Scala 2.13 and Scala 3
+sttp-openai is available for Scala 2.12, 2.13 and Scala 3
 
 Then head to the [documentation](https://sttp-ai.softwaremill.com) for usage examples: OpenAI, Claude, and Gemini clients, streaming, structured outputs, tool calling, and the agent loop — plus the `agent-testkit` module for testing agents without calling a paid API.
 

diff --git a/agent-testkit/src/main/scala/sttp/ai/core/agent/testing/RecordedInteractions.scala b/agent-testkit/src/main/scala/sttp/ai/core/agent/testing/RecordedInteractions.scala
@@ -52,7 +52,10 @@ trait RecordedInteractions {
   /** The tools offered on any request, deduplicated by name. Note that the agent loop withholds tools on the last allowed iteration, so
     * with `maxIterations(1)` no tools are ever offered and this is empty.
     */
-  final def offeredTools: Seq[OfferedTool] = requests.flatMap(_.toolsOffered).distinctBy(_.name)
+  final def offeredTools: Seq[OfferedTool] = {
+    val seen = scala.collection.mutable.Set.empty[String]
+    requests.flatMap(_.toolsOffered).filter(t => seen.add(t.name))
+  }
 
   /** All (toolName, result) pairs fed back to the model, from the final history, in order. */
   final def toolResultsSent: Seq[(String, String)] =

diff --git a/build.sbt b/build.sbt
@@ -3,7 +3,7 @@ import com.softwaremill.Publish.{ossPublishSettings, updateDocs}
 import com.softwaremill.SbtSoftwareMillCommon.commonSmlBuildSettings
 import com.softwaremill.UpdateVersionInDocs
 
-val scala2 = List("2.13.18")
+val scala2 = List("2.13.18", "2.12.20")
 val scala3 = List("3.3.8")
 
 def dependenciesFor(version: String)(deps: (Option[(Long, Long)] => ModuleID)*): Seq[ModuleID] =
@@ -13,7 +13,10 @@ lazy val commonSettings = commonSmlBuildSettings ++ ossPublishSettings ++ Seq(
   organization := "com.softwaremill.sttp.ai",
   // Suppress ScalaTest Assertion unused value warnings in tests
   Test / scalacOptions += "-Wconf:msg=unused value of type org.scalatest.Assertion:silent",
-  Test / scalacOptions += "-Wconf:msg=discarded non-Unit value of type org.scalatest.Assertion:silent"
+  Test / scalacOptions += "-Wconf:msg=discarded non-Unit value of type org.scalatest.Assertion:silent",
+  // 2.12 has no `scala.annotation.unused` to suppress warnings per-site (see sttp.ai.core.compat.unused), so silence the category there;
+  // 2.13 keeps full unused checking
+  scalacOptions ++= (if (scalaVersion.value.startsWith("2.12")) Seq("-Wconf:msg=never used:silent") else Seq.empty)
 )
 
 lazy val root = (project in file("."))

diff --git a/claude/src/main/scala/sttp/ai/claude/json/ClaudeManualCodecs.scala b/claude/src/main/scala/sttp/ai/claude/json/ClaudeManualCodecs.scala
@@ -99,7 +99,7 @@ object ClaudeManualCodecs {
         // (e.g. a property-less object, since `properties` is a required field; or union types) falls back to `CustomRaw`. Caveat:
         // a decode -> re-encode round trip through this codec is therefore lossy whenever the original schema had structure `Custom`
         // can't express, even though the decode step itself reports no error.
-        case _ => c.as[Tool.Custom].orElse(decodeCustomRaw(c))
+        case _ => c.as[Tool.Custom].left.flatMap(_ => decodeCustomRaw(c))
       }
     ),
     Encoder.instance {

diff --git a/core/src/main/scala-2.12/sttp/ai/core/compat/unused.scala b/core/src/main/scala-2.12/sttp/ai/core/compat/unused.scala
@@ -0,0 +1,6 @@
+package sttp.ai.core.compat
+
+/** Cross-version stand-in for `scala.annotation.unused`, which does not exist on Scala 2.12: inert here, aliased to the real annotation in
+  * the scala-2.13 and scala-3 source trees.
+  */
+class unused extends scala.annotation.StaticAnnotation
diff --git a/core/src/main/scala-2.13/sttp/ai/core/compat/package.scala b/core/src/main/scala-2.13/sttp/ai/core/compat/package.scala
@@ -0,0 +1,6 @@
+package sttp.ai.core
+
+/** See the scala-2.12 `unused` stand-in: on 2.13 the real annotation exists, so alias it. */
+package object compat {
+  type unused = scala.annotation.unused
+}
diff --git a/core/src/main/scala-3/sttp/ai/core/compat/compat.scala b/core/src/main/scala-3/sttp/ai/core/compat/compat.scala
@@ -0,0 +1,4 @@
+package sttp.ai.core.compat
+
+/** See the scala-2.12 `unused` stand-in: on Scala 3 the real annotation exists, so alias it. */
+type unused = scala.annotation.unused
diff --git a/core/src/main/scala/sttp/ai/core/agent/AgentInterceptor.scala b/core/src/main/scala/sttp/ai/core/agent/AgentInterceptor.scala
@@ -1,6 +1,6 @@
 package sttp.ai.core.agent
 
-import scala.annotation.unused
+import sttp.ai.core.compat.unused
 
 /** Middleware for the agent loop. Interceptors wrap iterations, LLM calls, and tool executions (onion-style, like sttp backend wrappers),
   * and can steer the loop via [[decide]].

diff --git a/core/src/main/scala/sttp/ai/core/agent/LoopAgent.scala b/core/src/main/scala/sttp/ai/core/agent/LoopAgent.scala
@@ -14,7 +14,7 @@ private[agent] class LoopAgent[F[_], In, Out](
     extends Agent[F, In, Out] {
   import LoopAgent.{ContinueLoop, Finished, IterationOutcome}
 
-  private val toolMap = config.userTools.map(t => t.name -> t).toMap
+  private val toolMap: Map[String, AgentTool[F, _]] = config.userTools.map(t => (t.name, t: AgentTool[F, _])).toMap
 
   private val interceptor: AgentInterceptor[F] = AgentInterceptor.compose(config.interceptors)
 

diff --git a/core/src/main/scala/sttp/ai/core/http/RetryingBackend.scala b/core/src/main/scala/sttp/ai/core/http/RetryingBackend.scala
@@ -121,7 +121,7 @@ object RetryingBackend {
   private def retryAfter(response: Response[_]): Option[FiniteDuration] =
     response
       .header(HeaderNames.RetryAfter)
-      .flatMap(_.trim.toLongOption)
+      .flatMap(s => scala.util.Try(s.trim.toLong).toOption)
       .filter(seconds => seconds >= 0 && seconds <= MaxRetryAfter.toSeconds)
       .map(_.seconds)
 

diff --git a/core/src/test/scala/sttp/ai/core/agent/AgentInterceptorComposeSpec.scala b/core/src/test/scala/sttp/ai/core/agent/AgentInterceptorComposeSpec.scala
@@ -55,7 +55,7 @@ class AgentInterceptorComposeSpec extends AnyFlatSpec with Matchers {
     val second = new AgentInterceptor[Identity] {
       override def decide(s: AgentRunState): LoopDecision = LoopDecision.FinishNow(FinishReason.BudgetExceeded, "second")
     }
-    val composed = AgentInterceptor.compose(Seq(AgentInterceptor.noop[Identity], first, second))
+    val composed = AgentInterceptor.compose[Identity](Seq(AgentInterceptor.noop[Identity], first, second))
     composed.decide(state()) shouldBe LoopDecision.FinishNow(FinishReason.Custom("deadline"), "first")
   }
 

diff --git a/core/src/test/scala/sttp/ai/core/http/RetryingBackendSpec.scala b/core/src/test/scala/sttp/ai/core/http/RetryingBackendSpec.scala
@@ -19,7 +19,7 @@ class RetryingBackendSpec extends AnyFlatSpec with Matchers {
     DefaultSyncBackend.stub.whenAnyRequest.thenRespondF { _ =>
       val i = attempts.getAndIncrement()
       val (code, headers) = responses(math.min(i, responses.size - 1))
-      ResponseStub.adjust("body", code, headers)
+      ResponseStub.adjust("body", code, headers.toList)
     }
 
   private def noHeaders(codes: StatusCode*): Seq[(StatusCode, Seq[Header])] = codes.map(_ -> Seq.empty[Header])

diff --git a/docs/adr/0004-scala-2-12-cross-building.md b/docs/adr/0004-scala-2-12-cross-building.md
@@ -0,0 +1,50 @@
+# 4. Cross-build the Scala 2 modules for Scala 2.12
+
+Date: 2026-08-12
+
+## Status
+
+Proposed.
+
+## Context
+
+Some consumers run large Scala 2.12 codebases where a 2.13 migration is a multi-month project, but
+adopting an AI client library is needed now. sttp-ai publishes only 2.13 and 3 artifacts, while every
+dependency the build pins (sttp client4, tapir-apispec-docs, circe, circe-generic-extras, sttp-apispec,
+pekko, akka 2.6, scalatest) already publishes `_2.12` artifacts at those exact versions — so the gap is
+in this build, not the ecosystem. Existing 2.13 and 3 support must not regress, in behaviour or in
+compiler-warning hygiene.
+
+## Decision
+
+Add Scala 2.12.20 to the `scala2` build-matrix row, cross-building every module that already builds for
+2.13: `core`, `openai`, `claude`, `gemini`, `agent-testkit`, and the fs2/zio/pekko/akka streaming
+modules. The Scala 3-only modules (`mcp`, `ox`, `examples`, `docs`) and the Scala Native targets are
+unchanged.
+
+Shared sources are kept cross-compatible rather than duplicated:
+
+* 2.13-only stdlib calls are replaced with equivalents available on both (`String.toLongOption`,
+  `Either.orElse`, `Seq.distinctBy`).
+* `sttp.ai.core.compat.unused` papers over `scala.annotation.unused`, which does not exist on 2.12:
+  an alias to the real annotation on 2.13/3, an inert annotation on 2.12, with the unused-warning
+  message silenced on 2.12 rows only so 2.13 keeps full checking.
+* Immutable collections are materialised where 2.12's default `scala.collection.Seq` is not accepted
+  (sttp's `multipartBody`, pekko/akka `Source`), and type ascriptions added where 2.12's weaker
+  inference fails.
+* `Attachment` references `assistants.Tool` fully qualified: 2.12 resolves same-package members ahead
+  of explicit imports ([scala/bug#4695](https://github.com/scala/bug/issues/4695), fixed in 2.13), which
+  silently bound the wrong `Tool` type there.
+
+## Consequences
+
+* 2.12 users get the full Scala 2 surface, including the akka streaming module that exists for them.
+* Shared sources are restricted to the 2.12-compatible subset of the standard library; the 2.12 CI rows
+  enforce this on every PR, so the constraint is discovered at review time, not by consumers.
+* Dependency updates must keep `_2.12` artifacts available. When a pinned dependency drops 2.12, that is
+  the exit criterion: the 2.12 row is removed in the next breaking release rather than holding the
+  dependency back.
+* The import-precedence difference is a silent-miscompilation hazard unique to 2.12 (code compiles with a
+  different meaning, not an error); the cross-version test suite is the guard against it.
+* Per-site unused-warning suppression is unavailable on 2.12; that warning category is silenced row-wide
+  there as the least-bad option.
diff --git a/docs/index.md b/docs/index.md
@@ -19,7 +19,7 @@ sttp-ai uses [sttp client](https://github.com/softwaremill/sttp) to describe req
 
 ## Why sttp-ai?
 
-sttp-ai implements the native APIs of all three major providers — OpenAI, Claude, and Gemini — in one library, rather than funnelling everything through an OpenAI-compatibility shim. You bring your own effect system: cats-effect, ZIO, Akka/Pekko Streams, direct-style [Ox](https://github.com/softwaremill/ox), or plain blocking calls. Structured-output schemas and tool definitions are derived from case classes instead of written by hand, and the built-in agent loop gives you typed tools and typed results, with tools loadable from MCP servers. Cross-built for Scala 2.13 and Scala 3, with Scala Native support (Scala 3) for the core and provider modules.
+sttp-ai implements the native APIs of all three major providers — OpenAI, Claude, and Gemini — in one library, rather than funnelling everything through an OpenAI-compatibility shim. You bring your own effect system: cats-effect, ZIO, Akka/Pekko Streams, direct-style [Ox](https://github.com/softwaremill/ox), or plain blocking calls. Structured-output schemas and tool definitions are derived from case classes instead of written by hand, and the built-in agent loop gives you typed tools and typed results, with tools loadable from MCP servers. Cross-built for Scala 2.12, 2.13 and Scala 3, with Scala Native support (Scala 3) for the core and provider modules.
 
 ```{eval-rst}
 .. toctree::

diff --git a/docs/quickstart.md b/docs/quickstart.md
@@ -10,7 +10,7 @@ Add the following dependency:
 // For streaming support, add one or more (these modules are shared across OpenAI, Claude, and Gemini):
 "com.softwaremill.sttp.ai" %% "fs2" % "@VERSION@"    // cats-effect/fs2
 "com.softwaremill.sttp.ai" %% "zio" % "@VERSION@"    // ZIO
-"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.13 only)
+"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.12/2.13 only)
 "com.softwaremill.sttp.ai" %% "pekko" % "@VERSION@"  // Pekko Streams
 "com.softwaremill.sttp.ai" %% "ox" % "@VERSION@"     // Ox direct-style (Scala 3 only)
 ```
@@ -48,7 +48,7 @@ Add the following dependency:
 // For streaming support, add one or more (these modules are shared across OpenAI, Claude, and Gemini):
 "com.softwaremill.sttp.ai" %% "fs2" % "@VERSION@"    // cats-effect/fs2
 "com.softwaremill.sttp.ai" %% "zio" % "@VERSION@"    // ZIO
-"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.13 only)
+"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.12/2.13 only)
 "com.softwaremill.sttp.ai" %% "pekko" % "@VERSION@"  // Pekko Streams
 "com.softwaremill.sttp.ai" %% "ox" % "@VERSION@"     // Ox direct-style (Scala 3 only)
 ```
@@ -87,7 +87,7 @@ Add the following dependency:
 // For streaming support, add one or more (these modules are shared across OpenAI, Claude, and Gemini):
 "com.softwaremill.sttp.ai" %% "fs2" % "@VERSION@"    // cats-effect/fs2
 "com.softwaremill.sttp.ai" %% "zio" % "@VERSION@"    // ZIO
-"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.13 only)
+"com.softwaremill.sttp.ai" %% "akka" % "@VERSION@"   // Akka Streams (Scala 2.12/2.13 only)
 "com.softwaremill.sttp.ai" %% "pekko" % "@VERSION@"  // Pekko Streams
 "com.softwaremill.sttp.ai" %% "ox" % "@VERSION@"     // Ox direct-style (Scala 3 only)
 ```
@@ -111,4 +111,4 @@ object GeminiHello:
 
 See [Gemini API basics](gemini/basics.md) for more.
 
-sttp-ai is available for Scala 2.13 and Scala 3.
+sttp-ai is available for Scala 2.12, 2.13 and Scala 3.
diff --git a/gemini/src/test/scala/sttp/ai/gemini/unit/GeminiClientSerializationSpec.scala b/gemini/src/test/scala/sttp/ai/gemini/unit/GeminiClientSerializationSpec.scala
@@ -69,7 +69,7 @@ class GeminiClientSerializationSpec extends AnyFlatSpec with Matchers with Eithe
   it should "preserve replayed function_call arguments and function_result results verbatim" in {
     val arguments = parse("""{"city":"Paris","unit":null}""").value
     val result = parse("""{"temp":20,"error":null}""").value
-    val request = InteractionRequest(
+    val request: InteractionRequest = InteractionRequest(
       model = testModel,
       input = InteractionInput.StepsInput(
         List(Step.FunctionCall("c1", "get_weather", arguments), Step.FunctionResult("c1", "get_weather", result))

diff --git a/openai/src/main/scala/sttp/ai/openai/OpenAI.scala b/openai/src/main/scala/sttp/ai/openai/OpenAI.scala
@@ -194,8 +194,8 @@ class OpenAI(
       .multipartBody {
         import imageEditsConfig._
         val imageParts = image match {
-          case singleImage :: Nil => Seq(multipartFile("image", singleImage))
-          case _                  => image.map(img => multipartFile("image[]", img))
+          case singleImage :: Nil => List(multipartFile("image", singleImage))
+          case _                  => image.map(img => multipartFile("image[]", img)).toList
         }
         imageParts ++ Seq(
           Some(multipart("prompt", prompt)),
@@ -212,7 +212,7 @@ class OpenAI(
           responseFormat.map(format => multipart("response_format", ResponseFormat.asString(format))),
           stream.map(s => multipart("stream", s.toString)),
           user.map(u => multipart("user", u))
-        ).flatten
+        ).flatten.toList
       }
       .response(asJson_parseErrors[ImageResponse])
 
@@ -274,7 +274,7 @@ class OpenAI(
           size.map(s => multipart("size", s.value)),
           responseFormat.map(format => multipart("response_format", ResponseFormat.asString(format))),
           user.map(multipart("user", _))
-        ).flatten
+        ).flatten.toList
       }
       .response(asJson_parseErrors[ImageResponse])
 
@@ -739,7 +739,7 @@ class OpenAI(
           responseFormat.map(format => multipart("response_format", asJson(format))),
           temperature.map(i => multipart("temperature", i.toString)),
           language.map(multipart("language", _))
-        ).flatten
+        ).flatten.toList
       }
       .response(asJson_parseErrors[AudioResponse])
 
@@ -814,7 +814,7 @@ class OpenAI(
           responseFormat.map(format => multipart("response_format", ResponseFormat.asString(format))),
           temperature.map(t => multipart("temperature", t.toString)),
           language.map(lang => multipart("language", lang.value))
-        ).flatten
+        ).flatten.toList
       }
       .response(asJson_parseErrors[AudioResponse])
 

diff --git a/openai/src/main/scala/sttp/ai/openai/requests/completions/chat/SchemaSupport.scala b/openai/src/main/scala/sttp/ai/openai/requests/completions/chat/SchemaSupport.scala
@@ -54,7 +54,7 @@ object SchemaSupport {
         .map(_.flatMap(_.asString).toSet)
         .getOrElse(Set.empty)
 
-      val state = value.toList.foldRight(FolderState(Nil, addAdditionalProperties = false, Nil)) { case ((k, v), acc) =>
+      val state: FolderState = value.toList.foldRight(FolderState(Nil, addAdditionalProperties = false, Nil)) { case ((k, v), acc) =>
         if (k == "properties") {
           // The container map's own entries are parameter NAMES (which may themselves be "properties", "type", "required", ...), not
           // schema keywords: fold each entry's schema VALUE, but never fold the container object itself through `onObject` (F6 - doing so

diff --git a/openai/src/main/scala/sttp/ai/openai/requests/completions/chat/message/Attachment.scala b/openai/src/main/scala/sttp/ai/openai/requests/completions/chat/message/Attachment.scala
@@ -1,5 +1,5 @@
 package sttp.ai.openai.requests.completions.chat.message
 
-import sttp.ai.openai.requests.assistants.Tool
+import sttp.ai.openai.requests.assistants
 
-case class Attachment(fileId: Option[String] = None, tools: Option[Seq[Tool]] = None)
+case class Attachment(fileId: Option[String] = None, tools: Option[Seq[assistants.Tool]] = None)
diff --git a/streaming/akka/src/test/scala/sttp/ai/openai/streaming/akka/AkkaClientSpec.scala b/streaming/akka/src/test/scala/sttp/ai/openai/streaming/akka/AkkaClientSpec.scala
@@ -110,7 +110,7 @@ class AkkaClientSpec extends AsyncFlatSpec with Matchers with EitherValues {
     val events = (eventsToProcess :+ emptyEvent) :+ DoneEvent
 
     val delimiter = "\n\n"
-    val streamedResponse = Source(events)
+    val streamedResponse = Source(events.toList)
       .map(_.toString + delimiter)
       .map(ByteString(_))
 
@@ -126,7 +126,7 @@ class AkkaClientSpec extends AsyncFlatSpec with Matchers with EitherValues {
     val events = (eventsToProcess :+ DoneEvent) ++ eventsToProcess
 
     val delimiter = "\n\n"
-    val streamedResponse = Source(events)
+    val streamedResponse = Source(events.toList)
       .map(_.toString + delimiter)
       .map(ByteString(_))
 

diff --git a/streaming/pekko/src/test/scala/sttp/ai/openai/streaming/pekko/PekkoClientSpec.scala b/streaming/pekko/src/test/scala/sttp/ai/openai/streaming/pekko/PekkoClientSpec.scala
@@ -110,7 +110,7 @@ class PekkoClientSpec extends AsyncFlatSpec with Matchers with EitherValues {
     val events = (eventsToProcess :+ emptyEvent) :+ DoneEvent
 
     val delimiter = "\n\n"
-    val streamedResponse = Source(events)
+    val streamedResponse = Source(events.toList)
       .map(_.toString + delimiter)
       .map(ByteString(_))
 
@@ -126,7 +126,7 @@ class PekkoClientSpec extends AsyncFlatSpec with Matchers with EitherValues {
     val events = (eventsToProcess :+ DoneEvent) ++ eventsToProcess
 
     val delimiter = "\n\n"
-    val streamedResponse = Source(events)
+    val streamedResponse = Source(events.toList)
       .map(_.toString + delimiter)
       .map(ByteString(_))