Narrative:

At XA46 received cpdlc (controller pilot data link communications) clearance to deviate +/- 25 NM for weather. At approximately XB05; [we] returned to centerline to split two thunderstorm cells; but verbalized; 'we're back on course; but don't report back on course as we're still needing to deviate.' I do not recall if PNF (pilot not flying) acknowledged my comment; but I believed he understood. At about XB07; PNF workload suddenly became high as we had just crossed the fir (flight information region) and he was accepting the ZZZZ login and PNF said; 'I'll make the position report.' he apparently also reported 'back on course;' though he did not recall for certain and the PF (pilot flying) did not hear any comment to indicate that he had. There was crew change at approximately XB11; the captain was briefed that we had clearance to deviate; and the other PNF left the cockpit.at approximately XB20 we again deviated for weather as we believed we were cleared to do so and ZZZZ almost immediately sent us a message; 'I show you 3 NM right off course.' we again asked for clearance to deviate +/- 25 NM and received it; but upon checking the ATC downlink log; it became obvious that we had indeed reported 'back on course;' so it was our error. It was a minor deviation that was caught immediately; but it was a deviation.fatigue was an issue as both the PNF and PF were tired due to circadian issues. Ph was about 30 hours since full night's sleep but with about 2-3 hours sleep during 4.5 hour FAR117 prescribed break. Primary issue was imprecise communication and lack of verification leading to PF and PNF not being on the same page regarding the current clearance.

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Original NASA ASRS Text

Title: B787 First Officer reported deviating off course in cruise without clearance.

Narrative: At XA46 received CPDLC (Controller Pilot Data Link Communications) clearance to deviate +/- 25 NM for weather. At approximately XB05; [we] returned to centerline to split two thunderstorm cells; but verbalized; 'We're back on course; but don't report back on course as we're still needing to deviate.' I do not recall if PNF (Pilot Not Flying) acknowledged my comment; but I believed he understood. At about XB07; PNF workload suddenly became high as we had just crossed the FIR (Flight Information Region) and he was accepting the ZZZZ login and PNF said; 'I'll make the position report.' He apparently also reported 'Back on course;' though he did not recall for certain and the PF (Pilot Flying) did not hear any comment to indicate that he had. There was crew change at approximately XB11; the Captain was briefed that we had clearance to deviate; and the other PNF left the cockpit.At approximately XB20 we again deviated for weather as we believed we were cleared to do so and ZZZZ almost immediately sent us a message; 'I show you 3 NM right off course.' We again asked for clearance to deviate +/- 25 NM and received it; but upon checking the ATC downlink log; it became obvious that we had indeed reported 'back on course;' so it was our error. It was a minor deviation that was caught immediately; but it was a deviation.Fatigue was an issue as both the PNF and PF were tired due to circadian issues. PH was about 30 hours since full night's sleep but with about 2-3 hours sleep during 4.5 hour FAR117 prescribed break. Primary issue was imprecise communication and lack of verification leading to PF and PNF not being on the same page regarding the current clearance.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.