diff --git a/docs-site/src/content/docs/reference/proxy-formats.md b/docs-site/src/content/docs/reference/proxy-formats.md
index 8975c944cf2..7cec1af766f 100644
--- a/docs-site/src/content/docs/reference/proxy-formats.md
+++ b/docs-site/src/content/docs/reference/proxy-formats.md
@@ -81,6 +81,11 @@ With `stream: true`, the response is `text/event-stream`. The bridge emits Respo
 With `stream: false` or no `stream`, the same adapter events are collected into one Responses JSON
 object. Both forms preserve the selected model, output items, terminal status, and usage.
 
+When a provider filters or truncates a response, an unfinished tool call remains `incomplete`
+in both JSON and SSE. Partial output is preserved, and the bridge does not emit an argument
+completion event for that open call. Calls already completed keep their status. This preserves
+the provider outcome; client retry behavior for incomplete responses is unchanged.
+
 On the pending `dev` implementation for #4112, a final upstream HTTP 413 on this surface
 is classified as `invalid_request_error` / `context_length_exceeded`. Non-streaming callers
 retain HTTP 413 with a JSON `error`; streaming callers retain the terminal SSE failure.
diff --git a/src/bridge.ts b/src/bridge.ts
index 20e7c3fe09f..3f5f529d3fa 100644
--- a/src/bridge.ts
+++ b/src/bridge.ts
@@ -1293,7 +1293,10 @@ export function bridgeToResponsesSSE(
               if (currentReasoning) closeCurrentReasoning();
               if (currentRawReasoning) closeCurrentRawReasoning();
               flushHiddenRawReasoning();
-              if (currentToolCall) closeCurrentToolCall();
+              if (currentToolCall) {
+                if (isTruncatedStopReason(event.stopReason)) failCurrentToolCall();
+                else closeCurrentToolCall();
+              }
               if (currentWebSearch) closeCurrentWebSearch("completed", []);
               releasePendingWebSources();
               // Redacted-only turns (or hidden thinking without a trailing signature event) still
@@ -2055,7 +2058,10 @@ function buildResponseJSONWithBudget(
   // must one left open by a stream that stopped without any terminal at all. That case previously
   // fell through to "completed", handing back a function_call whose arguments were half-written
   // JSON, inside a turn also marked completed.
-  if (currentToolCallId) flushToolCall(errorEvent || incompleteEvent || !sawTerminal ? "incomplete" : "completed");
+  if (currentToolCallId) {
+    flushToolCall(errorEvent || incompleteEvent || !sawTerminal || isTruncatedStopReason(rawStopReason)
+      ? "incomplete" : "completed");
+  }
   if (batchKiroRedacted) {
     // pushOutput reserves the item itself and releases the retained raw blob it replaces.
     pushOutput({
diff --git a/structure/adapters/registry.md b/structure/adapters/registry.md
index b0fab66633e..86c630aacb4 100644
--- a/structure/adapters/registry.md
+++ b/structure/adapters/registry.md
@@ -57,6 +57,10 @@ so the schema is not something a user can fix from configuration (issue #2673).
 
 > Decision record: [ADR-0093](../decisions/ADR-0093-moonshot-ref-with-siblings-normalization.md)
 
+## Truncated tool finalization
+
+The bridge keeps an open function, custom, or tool-search call incomplete when an adapter ends with a recognized truncated stop reason. Streaming emits no argument/input completion frame for that open call, and buffered JSON applies the same status. A call already closed by its own tool-call end retains its completed state. The response remains incomplete, partial output is preserved, and truncated compaction never replaces history.
+
 Chat helper admission in `src/server/responses/core.ts` follows the
 [deferred stored-main contract](../providers/openai-tiers.md): only a needed Direct OpenAI helper
 claims stored main, after terminal vision, routed vision and search exclusions.
diff --git a/tests/adapters/bridge-nonstreaming-terminal.test.ts b/tests/adapters/bridge-nonstreaming-terminal.test.ts
index 7fe0a705abb..de6bfb37c90 100644
--- a/tests/adapters/bridge-nonstreaming-terminal.test.ts
+++ b/tests/adapters/bridge-nonstreaming-terminal.test.ts
@@ -310,3 +310,71 @@ describe("Command Code finishReason error is a failure, not a stop", () => {
     expect((json.output as { type: string }[]).some(o => o.type === "compaction")).toBe(false);
   });
 });
+
+
+describe("truncated done preserves open tool integrity (#4312)", () => {
+  const cases = [
+    ["refusal", "content_filter"],
+    ["content_filter", "content_filter"],
+    ["max_tokens", "max_output_tokens"],
+    ["length", "max_output_tokens"],
+  ] as const;
+  for (const [stopReason, reason] of cases) {
+    for (const kind of ["function_call", "custom_tool_call", "tool_search_call"] as const) {
+      test(`${stopReason}: open ${kind} stays incomplete in both projections`, async () => {
+        const options = {
+          freeformToolNames: kind === "custom_tool_call" ? new Set(["fixture"]) : undefined,
+          toolSearchToolNames: kind === "tool_search_call" ? new Set(["fixture"]) : undefined,
+        };
+        const args = kind === "custom_tool_call" ? "partial input" : '{"arg":"unfinished';
+        const events: AdapterEvent[] = [
+          { type: "text_delta", text: "partial answer" },
+          { type: "tool_call_start", id: "call_fixture", name: "fixture" },
+          { type: "tool_call_delta", arguments: args },
+          { type: "done", stopReason },
+        ];
+        async function* source(): AsyncGenerator<AdapterEvent> { yield* events; }
+        const text = await new Response(bridgeToResponsesSSE(
+          source(), "fixture/model", undefined, options.freeformToolNames, options.toolSearchToolNames,
+        )).text();
+        const frames = text.split("\n\n").flatMap(frame => {
+          const data = frame.split("\n").find(line => line.startsWith("data: "))?.slice(6);
+          return data && data !== "[DONE]" ? [JSON.parse(data)] : [];
+        });
+        expect(terminalEventNames(text)).toEqual(["response.incomplete"]);
+        expect(frames.some(frame => frame.type === "response.function_call_arguments.done"
+          || frame.type === "response.custom_tool_call_input.done")).toBe(false);
+        const terminal = frames.find(frame => frame.type === "response.incomplete");
+        const buffered = buildResponseJSON(events, "fixture/model", options);
+        for (const response of [terminal.response, buffered]) {
+          expect(response.status).toBe("incomplete");
+          expect(response.incomplete_details.reason).toBe(reason);
+          const call = response.output.find((item: { type: string }) => item.type === kind);
+          expect(call.status).toBe("incomplete");
+          if (kind === "function_call") expect(call.arguments).toBe(args);
+          if (kind === "custom_tool_call") expect(call.input).toBe(args);
+          expect(response.output.find((item: { type: string }) => item.type === "message")
+            .content[0].text).toBe("partial answer");
+        }
+        const itemDone = frames.find(frame => frame.type === "response.output_item.done" && frame.item.type === kind);
+        expect(itemDone.item.status).toBe("incomplete");
+      });
+    }
+    test(`${stopReason}: a previously completed call stays completed`, async () => {
+      const events: AdapterEvent[] = [
+        { type: "tool_call_start", id: "call_complete", name: "fixture" },
+        { type: "tool_call_delta", arguments: '{"arg":"complete"}' },
+        { type: "tool_call_end", id: "call_complete" },
+        { type: "done", stopReason },
+      ];
+      const buffered = buildResponseJSON(events, "fixture/model");
+      expect(buffered.status).toBe("incomplete");
+      const output = buffered.output as { type: string; status?: string }[];
+      expect(output.find(item => item.type === "function_call")?.status).toBe("completed");
+      const text = await sseText(events);
+      expect(terminalEventNames(text)).toEqual(["response.incomplete"]);
+      expect(text).toContain("event: response.function_call_arguments.done");
+      expect(text).toContain('"arguments":"{\\"arg\\":\\"complete\\"}","status":"completed"');
+    });
+  }
+});